{"gene":"MYBPC3","run_date":"2026-06-10T05:19:52","timeline":{"discoveries":[{"year":2006,"finding":"PKA phosphorylation of cMyBP-C accelerates cross-bridge cycling kinetics and the stretch activation response in skinned myocardium. Ablation of cMyBP-C mimics PKA treatment (accelerated force redevelopment and stretch activation), but PKA has no additional effect in cMyBP-C null myocardium, indicating that PKA acts specifically through cMyBP-C phosphorylation to enhance contractile kinetics.","method":"Skinned ventricular myocardium from wild-type and cMyBP-C null mice; PKA treatment; mechanical measurements of force redevelopment rate and stretch activation response","journal":"Circulation research","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — clean genetic ablation combined with pharmacological PKA treatment and direct mechanical assay, with rigorous null-mouse controls","pmids":["16973906"],"is_preprint":false},{"year":2008,"finding":"PKA-mediated phosphorylation of cMyBP-C increases the proximity of myosin cross-bridges to actin (displaces cross-bridges radially/azimuthally from thick filaments) in resting myocardium, as demonstrated by an ~50% increase in the equatorial x-ray diffraction intensity ratio I(11)/I(10). This effect is absent in cMyBP-C null myocardium, confirming cMyBP-C acts as a tether that restrains cross-bridges and is relieved upon phosphorylation.","method":"Synchrotron low-angle x-ray diffraction of skinned trabeculae from wild-type and cMyBP-C null mice; PKA treatment","journal":"Circulation research","confidence":"High","confidence_rationale":"Tier 1 / Strong — synchrotron x-ray structural measurement combined with genetic null model and pharmacological manipulation, multiple orthogonal readouts","pmids":["18599866"],"is_preprint":false},{"year":2011,"finding":"The N-terminal domains of cMyBP-C bind to F-actin via subdomain 1, and 3D reconstruction by electron microscopy showed that cMyBP-C density covers a broad portion of actin subdomain 1, extending tangentially toward the pointed end. The binding location would sterically interfere with myosin head binding to actin and could modulate tropomyosin position, providing a structural basis for thin-filament regulation by cMyBP-C.","method":"Negative-stain electron microscopy and 3D helical reconstruction of F-actin decorated with bacterially expressed N-terminal cMyBP-C fragments; molecular fitting with atomic Ig-domain structure","journal":"Journal of molecular biology","confidence":"High","confidence_rationale":"Tier 1 / Strong — direct structural reconstruction with recombinant fragments, molecular docking, and multiple salt conditions as controls","pmids":["21601575"],"is_preprint":false},{"year":2015,"finding":"The HCM-associated cMyBP-C L348P variant (M-domain leucine→proline) causes a larger displacement of tropomyosin on the thin filament—from the blocked position to approximately the open position—compared with wild-type fragments that shift tropomyosin only to the closed position. This enhanced tropomyosin shift is phosphorylation-independent, explaining the gain-of-function Ca²⁺-sensitization conferred by L348P.","method":"Electron microscopy and 3D reconstruction of thin filaments decorated with wild-type or L348P cMyBP-C N-terminal fragments; phosphorylated vs. unphosphorylated comparisons","journal":"Journal of molecular and cellular cardiology","confidence":"High","confidence_rationale":"Tier 1 / Moderate — direct structural reconstruction with mutant versus wild-type fragments and phosphorylation controls in a single rigorous study","pmids":["26718724"],"is_preprint":false},{"year":2013,"finding":"The E258K missense HCM mutation in cMyBP-C abolishes the N-terminal cMyBP-C interaction with myosin S2 (detected by yeast two-hybrid), while simultaneously increasing affinity between N-terminal cMyBP-C and actin. Expression of E258K cMyBP-C in murine engineered cardiac tissue lacking endogenous cMyBP-C caused accelerated contractile kinetics and severely reduced twitch force, phenocopying cMyBP-C ablation with respect to kinetics but additionally impairing force.","method":"Adenoviral expression in cMyBP-C null murine engineered cardiac tissue; yeast two-hybrid for protein interaction mapping; contractile force measurements","journal":"The Journal of general physiology","confidence":"High","confidence_rationale":"Tier 1–2 / Moderate — yeast two-hybrid interaction mapping combined with functional reconstitution in null-background tissue, multiple orthogonal readouts","pmids":["23980194"],"is_preprint":false},{"year":2014,"finding":"The C-terminal C10 domain of cMyBP-C binds the light meromyosin (LMM) region of the myosin heavy chain; the HCM-associated 25-bp deletion mutant (cMyBP-CᶜC10mut) loses this interaction as demonstrated by co-sedimentation and protein cross-linking assays. cMyBP-CᶜC10mut fails to incorporate into the sarcomere C-zone (immunofluorescence and subcellular fractionation), and its adenoviral expression in adult rat cardiomyocytes reduces fractional shortening and sarcomere shortening without altering Ca²⁺ transients.","method":"Co-sedimentation and protein cross-linking assays; subcellular fractionation; immunofluorescence; adenoviral expression in adult rat cardiomyocytes; contractile and Ca²⁺ transient measurements","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — multiple orthogonal biochemical and cell-biological methods (co-sedimentation, cross-linking, fractionation, imaging, functional assay) in a single study","pmids":["25583989"],"is_preprint":false},{"year":2014,"finding":"An N-terminal fragment of cMyBP-C generated by post-MI proteolysis (C0-C1f) incorporated into permeabilized human left ventricular myocardium decreases maximal Ca²⁺-activated force and myofilament Ca²⁺ sensitivity, and increases cross-bridge cycling kinetics and tension cost. These effects occur through direct interaction of C0-C1f with actin and α-tropomyosin, identifying a mechanism by which MI-generated fragments impair cardiac contractility.","method":"Recombinant human C0-C1f incorporated into permeabilized human LV myocardium; mechanical property measurements at two sarcomere lengths; biochemical binding assays for actin and tropomyosin interactions","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — in vitro reconstitution in human myocardium with recombinant fragment, functional mechanical assays, and direct binding assays for mechanism","pmids":["24509847"],"is_preprint":false},{"year":2019,"finding":"Stepwise depletion of cMyBP-C causes dose-dependent augmentation of myosin contractility. cMyBP-C depletion enhances the myosin disordered-relaxed (DRX) state that enables ATP hydrolysis and thin-filament interaction, while reducing the super-relaxed (SRX) conformation. A dilated cardiomyopathy missense variant (F764L) in myosin normalizes the increased contractility from cMyBP-C depletion. The myosin ATPase inhibitor MYK-461 rescues relaxation deficits and restores normal contractility in MYBPC3-mutant mouse and human cardiomyocytes.","method":"Genetic and biochemical approaches in mouse and human cardiomyocytes; myosin conformation assays; pharmacological rescue with MYK-461; stepwise cMyBP-C depletion models","journal":"Science translational medicine","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — multiple orthogonal genetic and biochemical methods combined with pharmacological rescue in both mouse and human cardiomyocytes","pmids":["30674652"],"is_preprint":false},{"year":2017,"finding":"MYBPC3 mutations causing HCM are associated with a significantly reduced super-relaxed (SRX) myosin state in human LV tissue. MYBPC3-mutant samples showed fewer myosin heads in the SRX and shorter ATP turnover lifetime compared to donors. A positive correlation between cMyBP-C expression level and the proportion of SRX myosin heads indicates that cMyBP-C modulates and maintains the SRX state.","method":"Single-nucleotide turnover assay for SRX measurement in human myectomy samples; genotyping of HCM patients; quantitative comparison of MYBPC3-mutant vs. non-mutant vs. donor samples","journal":"PloS one","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct functional SRX assay on human tissue with genotype stratification; single lab, two orthogonal measurements (SRX proportion and ATP turnover lifetime)","pmids":["28658286"],"is_preprint":false},{"year":2012,"finding":"Protein kinase D (PKD) binds to cMyBP-C in contracting cardiomyocytes (shown by immunoprecipitation) and phosphorylates it at Ser315. In permeabilized ventricular myocytes from wild-type mice, PKD increased both myofilament Ca²⁺ sensitivity and maximal Ca²⁺-activated tension (Tmax); in cMyBP-C knockout myocytes, PKD still increased Ca²⁺ sensitivity but failed to increase Tmax, demonstrating that cMyBP-C is specifically required for PKD-mediated Tmax enhancement.","method":"Immunoprecipitation in contracting cardiomyocytes; permeabilized myocyte force measurements in wild-type and cMyBP-C knockout mice; PKD treatment","journal":"American journal of physiology. Heart and circulatory physiology","confidence":"High","confidence_rationale":"Tier 1–2 / Moderate — co-IP binding assay combined with functional reconstitution using null-mouse controls, identifying specific phosphorylation site and functional consequence","pmids":["22636676"],"is_preprint":false},{"year":2014,"finding":"Phosphorylation of cMyBP-C at specific serine residues has site-specific effects on contractile mechanics: the DAD phosphomimetic mutation (Asp273-Ala282-Asp302) causes ~50% reduction in tension, decreased stiffness, altered cross-bridge kinetics (decreased 2πc, increased 2πb), while SAS (Ser273-Ala282-Ser302) and ADA mutations cause minimal changes. The non-phosphorylatable Ala at position 282 combined with phosphomimetic Asp at 273 and/or 302 is particularly detrimental.","method":"Sinusoidal analysis of papillary and trabecular muscle fibers from cMyBP-C null mice transgenically expressing site-specific phosphorylation mutants; pCa-tension studies","journal":"Biophysical journal","confidence":"High","confidence_rationale":"Tier 1 / Moderate — reconstitution in null-mouse background with defined phosphorylation mutants, rigorous mechanical analysis with multiple readouts","pmids":["24606935"],"is_preprint":false},{"year":2004,"finding":"Heterozygous cMyBP-C null mice with ~50% reduction in cMyBP-C protein develop asymmetric septal hypertrophy with fibrosis at 10–11 months, recapitulating a key feature of human FHC. Homozygous null mice develop eccentric LV hypertrophy with decreased fractional shortening at 3–4 months and markedly impaired relaxation after 9 months, establishing that haploinsufficiency of cMyBP-C is sufficient to cause the HCM-like phenotype.","method":"Targeted deletion of Mybpc3 exons 1–2 in mice; echocardiography; morphological and molecular analyses at multiple ages in het and hom mice","journal":"Cardiovascular research","confidence":"High","confidence_rationale":"Tier 2 / Strong — clean genetic model with dose-dependent phenotypic characterization at multiple time points, multiple functional and histological readouts","pmids":["15249187"],"is_preprint":false},{"year":2009,"finding":"In human myectomy samples, MYBPC3 mutations (both truncating and missense) cause a significant reduction (~24%) in full-length cMyBP-C protein in myofibrils compared with donor and non-MYBPC3 mutant samples, without any detectable truncated peptides. This provides direct evidence that haploinsufficiency (not dominant-negative poison peptide) is the primary disease mechanism for MYBPC3-mutation HCM.","method":"Quantitative immunoblotting of myofibrillar fractions from human myectomy samples; RT-PCR for mRNA quantification; comparison of mutation-positive vs. mutation-negative vs. donor samples","journal":"Circulation research","confidence":"High","confidence_rationale":"Tier 2 / Strong — quantitative protein analysis in human tissue, multiple mutation types tested, replicated finding across 9 MYBPC3-mutant samples with appropriate controls","pmids":["19574547"],"is_preprint":false},{"year":2012,"finding":"Mybpc3 knock-in heterozygous mice (mimicking human HCM) exhibit increased myofilament Ca²⁺ sensitivity and diastolic dysfunction without LV hypertrophy, demonstrating that myofilament Ca²⁺ sensitization and impaired relaxation are early phenotypic consequences of Mybpc3 mutations that precede the development of hypertrophy.","method":"Skinned ventricular trabeculae force-Ca²⁺ measurements; intact myocyte Ca²⁺ transient analysis; in vivo echocardiography and Doppler in heterozygous knock-in mice at 10 weeks","journal":"Journal of molecular and cellular cardiology","confidence":"High","confidence_rationale":"Tier 1–2 / Moderate — genetic mouse model with multiple orthogonal functional assays from isolated protein to whole organ; directly shows sequence of events","pmids":["22465693"],"is_preprint":false},{"year":2006,"finding":"In vivo LV systolic and diastolic function are compromised under basal conditions in cMyBP-C null mice. Beta-adrenergic stimulation significantly improves ejection fraction and contractile indices in wild-type but not cMyBP-C null mice. Adaptation to chronic pressure overload (aortic banding) is also diminished in cMyBP-C null mice, demonstrating that cMyBP-C is required for β-adrenergic inotropic reserve and pressure-overload adaptation.","method":"In vivo pressure-volume relations; echocardiography; β-adrenergic treatment; aortic banding in cMyBP-C null and wild-type mice","journal":"American journal of physiology. Heart and circulatory physiology","confidence":"High","confidence_rationale":"Tier 2 / Moderate — clean genetic null model with in vivo pressure-volume analysis and multiple functional perturbations","pmids":["17122190"],"is_preprint":false},{"year":2018,"finding":"MYBPC3 truncation mutations in human HCM myectomy samples result in 40% reduction in MyBP-C content and reduced MyBP-C phosphorylation (quantified by mass spectrometry) without production of truncated protein, and enhance maximal myofilament sliding velocity specifically within the C-zone of thick filaments as demonstrated by single-particle TIRF microscopy. Analytical modeling confirmed that the 40% MyBP-C reduction is sufficient to explain the enhanced sliding velocity.","method":"Quantitative mass spectrometry on human myectomy samples; TIRF microscopy single-particle sliding assay with native thick and thin filaments; analytical modeling","journal":"Journal of molecular and cellular cardiology","confidence":"High","confidence_rationale":"Tier 1–2 / Moderate — direct functional assay with native human proteins using TIRF, quantitative proteomics, and analytical modeling providing mechanistic convergence","pmids":["30550750"],"is_preprint":false},{"year":2019,"finding":"iPSC-derived cardiomyocytes carrying MYBPC3 premature termination codon mutations activate nonsense-mediated decay (NMD) and show aberrant Ca²⁺ handling with prolonged decay kinetics and elevated diastolic Ca²⁺ without structural abnormalities. Specific pharmacological inhibition of NMD reversed the molecular phenotype and normalized Ca²⁺ handling, establishing chronic NMD activation as a direct molecular mechanism linking MYBPC3 PTC mutations to HCM pathogenesis.","method":"Isogenic hiPSC-CMs with MYBPC3 PTC mutations; NMD pathway inhibition; Ca²⁺ handling measurements; transcriptome analysis","journal":"Circulation","confidence":"High","confidence_rationale":"Tier 1–2 / Moderate — isogenic human cell model with pharmacological rescue and molecular pathway identification, multiple orthogonal methods","pmids":["30586709"],"is_preprint":false},{"year":2020,"finding":"Heterozygous MYBPC3 frameshift mutations cause allelic loss-of-function (reduced wild-type mRNA) but protein-level compensation maintaining normal MyBP-C stoichiometry through a previously uncharacterized mechanism involving reduced MyBP-C synthesis rate but slower MyBP-C degradation. Homozygous mutant iPSCMs showed contractile dysregulation, whereas heterozygous iPSCMs with compensated MyBP-C protein levels had normal contractile function. Stable isotope labeling measured MyBP-C synthesis and degradation rates.","method":"Patient-derived and genome-engineered iPSC-CMs; stable isotope labeling to measure synthesis and degradation rates; contractile function measurements; RNA-Seq","journal":"JCI insight","confidence":"High","confidence_rationale":"Tier 1–2 / Moderate — stable isotope pulse-chase for protein turnover in isogenic human cell lines with multiple orthogonal methods; novel mechanistic finding with quantitative rigor","pmids":["31877118"],"is_preprint":false},{"year":2020,"finding":"Acute removal of cMyBP-C N-terminal domains (C0–C7) in situ using a cut-and-paste genetic/protein engineering approach reduces myofilament Ca²⁺ sensitivity and increases cross-bridge cycling rate (ktr) at submaximal Ca²⁺, and induces auto-oscillatory (SPOC) contractions. Ligation of recombinant C0–C7 rescues pCa50 and ktr and abolishes oscillations; phosphorylated C0–C7 does not fully rescue these effects, revealing a role for cMyBP-C N-terminal domains in damping sarcomere-driven contractile oscillations.","method":"Spy-C mouse model with TEV protease cleavage site and SpyTag; detergent-permeabilized cardiomyocytes; recombinant C0–C7 ligation via SpyCatcher-SpyTag; force-Ca²⁺ and ktr measurements","journal":"Circulation research","confidence":"High","confidence_rationale":"Tier 1 / Moderate — novel reconstitution approach enabling acute domain removal and replacement in situ with functional readouts and phosphorylation-dependent controls","pmids":["32078438"],"is_preprint":false},{"year":2020,"finding":"Nontruncating MYBPC3 pathogenic variants cluster in the C3, C6, and C10 domains. C10-domain missense variants cause failure of MyBP-C to incorporate into myofilaments with ~90% accelerated degradation (measured in rat ventricular myocytes), while C3 and C6 variants incorporate normally with normal degradation rates, demonstrating that a subset of missense variants cause loss-of-function through a distinct myofilament incorporation failure mechanism.","method":"Expression of missense mutant MyBP-C in rat ventricular myocytes; myofilament localization assay; degradation rate measurements; domain mapping in a large HCM cohort","journal":"Circulation. Genomic and precision medicine","confidence":"Medium","confidence_rationale":"Tier 2–3 / Moderate — direct cellular localization and degradation measurements with multiple domain mutants; single lab with multiple variants tested","pmids":["32841044"],"is_preprint":false},{"year":2016,"finding":"In engineered heart tissue from Mybpc3 knock-out mouse cardiomyocytes reconstituted with viral expression, missense cMyBP-C (c.1591G>C) incorporates correctly into the sarcomere whereas truncated cMyBP-C does not; both mutations fail to restore force to wild-type levels. This demonstrates different pathogenic mechanisms: haploinsufficiency for truncating variants and sarcomere poisoning/incorrect incorporation for the missense variant.","method":"AAV-mediated expression in cMyBP-C null engineered heart tissue; immunofluorescence for sarcomere incorporation; isometric force measurements at multiple Ca²⁺ concentrations","journal":"Journal of molecular and cellular cardiology","confidence":"High","confidence_rationale":"Tier 1–2 / Moderate — functional reconstitution in null background with multiple mutation types, direct sarcomere incorporation imaging combined with mechanical assays","pmids":["27108529"],"is_preprint":false},{"year":2022,"finding":"MYBPC3 is expressed in cardiac fibroblasts (not only cardiomyocytes), and CRISPR-mediated disruption of Mybpc3 in NIH3T3 fibroblasts activates the NF-κB signaling pathway, increases TGF-β1 expression, promotes HIF-1α-driven aerobic glycolysis, and accelerates fibroblast activation and cardiac fibrosis in a R495Q mutant pig model.","method":"Cytosine base editing in pigs (R495Q mutation); CRISPR disruption of Mybpc3 in NIH3T3 fibroblasts; NF-κB and TGF-β1 pathway analysis; glycolysis pathway measurements","journal":"Cell death & disease","confidence":"Medium","confidence_rationale":"Tier 2–3 / Moderate — in vitro CRISPR loss-of-function with pathway analysis and in vivo pig model; novel finding of expression in non-cardiomyocyte cell type","pmids":["36357371"],"is_preprint":false},{"year":2011,"finding":"The ubiquitin-proteasome system (UPS) is activated early in postnatal cardiac hypertrophy in Mybpc3 knock-in and knock-out mice. In aged knock-in mice (expressing truncated cMyBP-C) but not knock-out mice, UPS degradation capacity is specifically impaired (fourfold higher Ub(G76V)-GFP accumulation), while autophagy-lysosome pathway markers are elevated in both models, suggesting defective ALP as a common mechanism and specific UPS impairment when truncated cMyBP-C is present.","method":"Ubiquitinated protein levels; proteasomal activity assays; Ub(G76V)-GFP reporter mice; ALP marker analysis in Mybpc3-KI and KO mice","journal":"Basic research in cardiology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vivo reporter mouse model with pharmacological and biochemical readouts, parallel comparison of two genetic models revealing mutation-specific vs. common mechanisms","pmids":["22189562"],"is_preprint":false},{"year":2017,"finding":"Autophagic flux is blunted in hearts of aged Mybpc3 knock-in mice and in human HCM myectomies with MYBPC3 mutations, associated with Akt-mTORC1 pathway activation. Treatment with rapamycin or caloric restriction (activating autophagy) partially rescued cardiomyopathy and restored autophagic flux in knock-in mice.","method":"LC3-II protein level measurement; autophagic flux assay; flow cytometry; rapamycin treatment; caloric restriction; echocardiography in Mybpc3-KI mice","journal":"Circulation. Heart failure","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic mouse model combined with pharmacological rescue and human tissue confirmation; multiple autophagy readouts","pmids":["29021349"],"is_preprint":false},{"year":2023,"finding":"The MYBPC3:c772G>A (E258K) founder mutation reduces cMyBP-C expression (haploinsufficiency) and causes faster cross-bridge cycling and higher energy cost of tension generation in ventricular myofibrils and permeabilized strips. Compensatory slowing of excitation-contraction coupling (prolonged action potentials, slower Ca²⁺ transients) preserves twitch duration despite faster sarcomere kinetics.","method":"Mechanical studies in single myofibrils and permeabilized strips; Ca²⁺ transient measurement in native and hiPSC-derived cardiomyocytes; intact trabeculae; hiPSC-EHTs; tissue clearing/optical microscopy; in silico modeling","journal":"Circulation research","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — multiple orthogonal experimental systems (human tissue, hiPSC-CMs, EHTs, in silico) converging on a single mechanistic model with quantitative rigor","pmids":["36744470"],"is_preprint":false},{"year":2021,"finding":"Among 44 nontruncating MYBPC3 variants, ~50% of HCM-linked variants showed alterations in RNA splicing or protein stability detectable by a cell-based minigene assay and protein stability assay, both with high specificity for HCM pathogenicity (100% and 94%, respectively). This demonstrates that protein haploinsufficiency, driven by splicing errors or protein instability, is a common pathomechanism even for nontruncating variants.","method":"Cell-based minigene splicing assay; protein stability assay; cosegregation and population genetics classification of 44 variants","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2–3 / Moderate — systematic functional assays on multiple variants with clinical classification as reference standard; single lab","pmids":["34097875"],"is_preprint":false},{"year":2023,"finding":"cMyBP-C haploinsufficiency in human engineered cardiac tissue (ECT) causes a progressive phenotype: initially hypercontractile, progressing to hypocontractility with impaired relaxation and severe force depression in homozygous null ECTs over 6 weeks. Ca²⁺-handling abnormalities develop progressively and are more pronounced with complete cMyBP-C absence, suggesting that while the primary effect may relate to myosin crossbridge orientation, the contractile phenotype is Ca²⁺-mediated.","method":"CRISPR-Cas9 heterozygous and homozygous MYBPC3 frameshift in human iPSCs; cardiac micropatterns and ECTs; longitudinal contractile, Ca²⁺-handling, and Ca²⁺-sensitivity measurements; RNA-seq","journal":"The Journal of general physiology","confidence":"High","confidence_rationale":"Tier 1–2 / Moderate — isogenic human cell models at two doses with longitudinal functional analysis and transcriptomics; multiple orthogonal readouts","pmids":["36893011"],"is_preprint":false}],"current_model":"cMyBP-C (encoded by MYBPC3) is a thick-filament-associated sarcomeric protein whose N-terminal domains (C0–C7) interact with both myosin S2 and actin/tropomyosin to restrain myosin cross-bridge proximity to actin and damp contractile oscillations; PKA (and PKD) phosphorylation of specific serine residues in its regulatory M-domain relieves this tethering, displacing cross-bridges toward actin to accelerate force development and cross-bridge cycling kinetics, while also maintaining the myosin super-relaxed state to preserve energetic efficiency. MYBPC3 truncating mutations cause haploinsufficiency—reduction of full-length cMyBP-C without detectable truncated protein—leading to dose-dependent augmentation of myosin contractility, increased Ca²⁺ sensitivity, impaired relaxation, and, in some contexts, chronic nonsense-mediated decay activation and progressive Ca²⁺-handling abnormalities; compensatory mechanisms including reduced MyBP-C degradation can partially restore protein stoichiometry in early disease, and protein level modulation (via gene therapy or myosin ATPase inhibitors) rescues the contractile defects."},"narrative":{"mechanistic_narrative":"MYBPC3 encodes cardiac myosin-binding protein C (cMyBP-C), a thick-filament-associated sarcomeric protein that acts as a mechanical brake on myosin cross-bridges to tune the rate and economy of cardiac contraction [PMID:16973906, PMID:18599866]. Its N-terminal domains tether cross-bridges close to the thick filament backbone—genetic ablation or PKA phosphorylation displaces cross-bridges radially toward actin, accelerating force redevelopment and the stretch-activation response, with PKA acting specifically through cMyBP-C [PMID:16973906, PMID:18599866]. The N-terminal region binds F-actin at a site that would sterically interfere with myosin head binding and modulate tropomyosin position, providing a structural basis for thin-filament regulation [PMID:21601575], while acute removal of the C0–C7 domains reduces Ca²⁺ sensitivity, raises cross-bridge cycling rate, and triggers auto-oscillatory contractions that recombinant C0–C7 ligation reverses, defining cMyBP-C as a damper of sarcomere oscillations [PMID:32078438]. Beyond cross-bridge tethering, cMyBP-C maintains the energy-sparing super-relaxed (SRX) myosin state and restrains the disordered-relaxed (DRX) state, with cMyBP-C level correlating with the SRX fraction [PMID:30674652, PMID:28658286]. Phosphorylation by PKA and by PKD at Ser315 relieves tethering and modulates Ca²⁺ sensitivity and maximal tension in a site-specific manner [PMID:22636676, PMID:24606935]. The protein is anchored in the sarcomere C-zone through C-terminal C10-domain binding to the light meromyosin region of the myosin heavy chain [PMID:25583989]. MYBPC3 mutations cause hypertrophic cardiomyopathy primarily through haploinsufficiency: truncating and many nontruncating variants reduce full-length cMyBP-C in human myocardium without detectable truncated peptide, producing increased myofilament Ca²⁺ sensitivity, enhanced myosin contractility and sliding velocity, and impaired relaxation that precede hypertrophy [PMID:15249187, PMID:19574547, PMID:22465693, PMID:30550750]. Distinct loss-of-function routes include premature-termination-codon-triggered nonsense-mediated decay [PMID:30586709], splicing or protein-stability defects [PMID:34097875], and failure of C10-domain missense mutants to incorporate into myofilaments with accelerated degradation [PMID:25583989, PMID:32841044], whereas certain missense variants such as E258K disrupt the myosin-S2 interaction and increase actin affinity [PMID:23980194, PMID:36744470]. Disease progression in human engineered tissue evolves from an early hypercontractile state to hypocontractility with Ca²⁺-handling abnormalities [PMID:36893011]. Restoring protein level or inhibiting myosin ATPase with MYK-461 rescues the contractile defect [PMID:30674652].","teleology":[{"year":2004,"claim":"Established that loss of cMyBP-C dosage, rather than a poison peptide, is sufficient to produce HCM-like disease, framing haploinsufficiency as the candidate mechanism.","evidence":"Targeted Mybpc3 deletion in mice with dose-dependent phenotyping by echocardiography and histology","pmids":["15249187"],"confidence":"High","gaps":["Mouse phenotype timing may not map to human disease course","Does not resolve the molecular events linking protein loss to hypertrophy"]},{"year":2006,"claim":"Resolved how PKA enhances contractile kinetics by showing it acts specifically through cMyBP-C rather than other substrates.","evidence":"Skinned myocardium from WT and cMyBP-C null mice with PKA treatment and force/stretch-activation assays","pmids":["16973906"],"confidence":"High","gaps":["Did not visualize the structural change underlying accelerated kinetics","Did not map the responsible phosphosites"]},{"year":2006,"claim":"Demonstrated that cMyBP-C is required for β-adrenergic inotropic reserve and adaptation to pressure overload in the intact heart.","evidence":"In vivo pressure-volume and echocardiography with β-agonist and aortic banding in null mice","pmids":["17122190"],"confidence":"High","gaps":["Whole-organ readout does not isolate the sarcomeric mechanism","Compensatory remodeling could confound interpretation"]},{"year":2008,"claim":"Provided the structural basis for cross-bridge tethering by showing PKA phosphorylation radially displaces cross-bridges toward actin, an effect absent in null myocardium.","evidence":"Synchrotron low-angle x-ray diffraction of skinned trabeculae from WT and null mice with PKA treatment","pmids":["18599866"],"confidence":"High","gaps":["Resting-state measurement does not establish the dynamic effect during cycling","Specific domain mediating tethering not defined here"]},{"year":2009,"claim":"Provided direct human evidence that MYBPC3 mutations reduce full-length protein without truncated peptide, confirming haploinsufficiency as the primary disease mechanism in patients.","evidence":"Quantitative immunoblotting and RT-PCR of human myectomy samples versus donor controls","pmids":["19574547"],"confidence":"High","gaps":["Did not establish the degradation/quality-control route eliminating truncated peptides","~24% reduction smaller than expected for full allelic loss"]},{"year":2011,"claim":"Mapped the actin-binding interface of the N-terminal domains, explaining how cMyBP-C could occlude myosin binding and reposition tropomyosin.","evidence":"Negative-stain EM and 3D helical reconstruction of F-actin decorated with recombinant N-terminal fragments","pmids":["21601575"],"confidence":"High","gaps":["Reconstruction used isolated fragments, not full-length protein in situ","Functional consequence of tropomyosin shift inferred structurally"]},{"year":2011,"claim":"Connected protein-quality-control pathways to disease, revealing UPS impairment specific to truncated-protein-expressing models and shared autophagy defects.","evidence":"Ub(G76V)-GFP reporter mice, proteasome activity and ALP marker assays in Mybpc3 KI and KO mice","pmids":["22189562"],"confidence":"Medium","gaps":["Causal link between proteostasis defect and contractile phenotype not established","Findings in mouse models only"]},{"year":2012,"claim":"Identified PKD as a second kinase that binds and phosphorylates cMyBP-C at Ser315, with cMyBP-C specifically required for PKD-driven maximal tension enhancement.","evidence":"Co-IP in contracting cardiomyocytes and permeabilized myocyte force assays in WT and KO mice","pmids":["22636676"],"confidence":"High","gaps":["Single Co-IP for the interaction without reciprocal/structural validation","Physiological context for PKD signaling to cMyBP-C unclear"]},{"year":2012,"claim":"Showed that Ca²⁺ sensitization and impaired relaxation are early consequences of Mybpc3 mutation that precede hypertrophy, ordering the disease cascade.","evidence":"Skinned trabeculae force-Ca²⁺, intact myocyte Ca²⁺ transients, and echocardiography in heterozygous knock-in mice","pmids":["22465693"],"confidence":"High","gaps":["Knock-in expresses truncated allele, distinct from pure haploinsufficiency","Did not define the molecular trigger of Ca²⁺ sensitization"]},{"year":2013,"claim":"Defined a missense mechanism distinct from haploinsufficiency by showing E258K abolishes the myosin-S2 interaction while increasing actin affinity, impairing both kinetics and force.","evidence":"Yeast two-hybrid interaction mapping and adenoviral expression in cMyBP-C null engineered cardiac tissue","pmids":["23980194"],"confidence":"High","gaps":["Yeast two-hybrid interaction not confirmed in native myocardium","Relationship to later haploinsufficiency finding for E258K not reconciled here"]},{"year":2014,"claim":"Localized sarcomere anchoring to the C-terminal C10 domain via binding to light meromyosin and showed an HCM deletion disrupts incorporation and contractility.","evidence":"Co-sedimentation, cross-linking, fractionation, immunofluorescence, and contractile assays in adult rat cardiomyocytes","pmids":["25583989"],"confidence":"High","gaps":["Effect on Ca²⁺ transients excluded but downstream signaling not examined","Overexpression in rat cells may not reflect endogenous stoichiometry"]},{"year":2014,"claim":"Demonstrated that proteolytic N-terminal fragments produced post-MI act through actin and tropomyosin to depress force and Ca²⁺ sensitivity, identifying a fragment-driven contractile pathology.","evidence":"Recombinant human C0-C1f incorporated into permeabilized human LV myocardium with mechanical and binding assays","pmids":["24509847"],"confidence":"High","gaps":["Endogenous abundance of these fragments in vivo not quantified here","Mechanism specific to ischemic context"]},{"year":2014,"claim":"Resolved site-specific roles of M-domain phosphorylation, showing combinations like the DAD phosphomimetic markedly alter tension and cross-bridge kinetics.","evidence":"Sinusoidal analysis of muscle fibers from null mice transgenically expressing phosphosite mutants","pmids":["24606935"],"confidence":"High","gaps":["Phosphomimetics approximate but do not equal true phosphorylation","In vivo regulation of individual sites not addressed"]},{"year":2015,"claim":"Explained gain-of-function Ca²⁺ sensitization for an M-domain HCM variant by showing L348P shifts tropomyosin further toward the open position, phosphorylation-independently.","evidence":"EM 3D reconstruction of thin filaments decorated with WT vs L348P N-terminal fragments","pmids":["26718724"],"confidence":"High","gaps":["Isolated fragment study, not full-length protein in sarcomere","Direct tension consequences not measured here"]},{"year":2016,"claim":"Contrasted pathogenic mechanisms in a null background, showing truncated protein fails to incorporate (haploinsufficiency) while a missense variant incorporates yet still depresses force (poisoning).","evidence":"AAV expression in cMyBP-C null engineered heart tissue with immunofluorescence and force measurements","pmids":["27108529"],"confidence":"High","gaps":["Viral expression levels may not match endogenous","Single missense variant tested"]},{"year":2017,"claim":"Linked cMyBP-C to the energetic super-relaxed myosin state by showing MYBPC3-mutant human tissue has reduced SRX, with SRX fraction correlating with cMyBP-C level.","evidence":"Single-nucleotide turnover SRX assay on genotyped human myectomy samples","pmids":["28658286"],"confidence":"Medium","gaps":["Correlative, single-lab measurement","Causal direction between cMyBP-C loss and SRX reduction not isolated"]},{"year":2017,"claim":"Implicated blunted autophagic flux with Akt-mTORC1 activation in disease and showed autophagy-activating interventions partially rescue cardiomyopathy.","evidence":"Autophagy markers/flux assays and rapamycin/caloric-restriction rescue in KI mice plus human myectomy confirmation","pmids":["29021349"],"confidence":"Medium","gaps":["Whether autophagy defect is cause or consequence unresolved","Rescue only partial"]},{"year":2018,"claim":"Tied reduced human MyBP-C content directly to enhanced contractile output by showing 40% reduction accelerates myosin sliding velocity specifically in the thick-filament C-zone.","evidence":"Quantitative mass spectrometry and single-particle TIRF sliding assays on native human filaments with analytical modeling","pmids":["30550750"],"confidence":"High","gaps":["In vitro motility may not capture intact-cell regulation","Reduced phosphorylation contribution not fully separated from reduced content"]},{"year":2019,"claim":"Unified the contractile phenotype with myosin conformation, showing cMyBP-C depletion shifts myosin from SRX toward DRX dose-dependently and that MYK-461 rescues the defect.","evidence":"Genetic/biochemical depletion, myosin conformation assays, and MYK-461 rescue in mouse and human cardiomyocytes","pmids":["30674652"],"confidence":"High","gaps":["Direct structural mechanism of cMyBP-C maintaining SRX not resolved","Long-term in vivo efficacy of inhibitor not addressed here"]},{"year":2019,"claim":"Established chronic NMD activation as a direct molecular route from premature-termination-codon mutations to aberrant Ca²⁺ handling, reversible by NMD inhibition.","evidence":"Isogenic hiPSC-CMs with MYBPC3 PTCs, NMD inhibition, Ca²⁺ handling, and transcriptome analysis","pmids":["30586709"],"confidence":"High","gaps":["NMD inhibition not therapeutically selective in vivo","Link between Ca²⁺ abnormality and structural disease not established"]},{"year":2020,"claim":"Revealed protein-level compensation in heterozygous cells via slower MyBP-C degradation that maintains stoichiometry and preserves contractile function despite allelic loss.","evidence":"Stable isotope labeling of synthesis/degradation, contractile assays, and RNA-seq in patient-derived and engineered iPSC-CMs","pmids":["31877118"],"confidence":"High","gaps":["Whether compensation persists with aging/stress unclear","Mechanism stabilizing residual protein not identified"]},{"year":2020,"claim":"Defined a distinct loss-of-function route for C10-domain missense variants through failed myofilament incorporation and accelerated degradation, separating them from C3/C6 variants.","evidence":"Expression of missense mutants in rat ventricular myocytes with localization and degradation assays plus cohort domain mapping","pmids":["32841044"],"confidence":"Medium","gaps":["Single-lab cellular assay","Functional contractile consequence of these variants not measured here"]},{"year":2020,"claim":"Defined the N-terminal domains as dampers of sarcomere oscillations through an acute in-situ removal-and-replacement strategy with phosphorylation-dependent rescue.","evidence":"Spy-C mouse with TEV-cleavable C0–C7 and SpyTag/SpyCatcher ligation in permeabilized cardiomyocytes with force-Ca²⁺/ktr assays","pmids":["32078438"],"confidence":"High","gaps":["Acute removal differs from chronic mutation context","Phosphorylated C0–C7 incomplete rescue not fully explained"]},{"year":2021,"claim":"Generalized haploinsufficiency to nontruncating variants by showing ~half alter splicing or protein stability with high specificity for HCM pathogenicity.","evidence":"Cell-based minigene splicing and protein stability assays across 44 classified variants","pmids":["34097875"],"confidence":"Medium","gaps":["Minigene assays may not reflect native splicing","Functional contractile validation not performed for each variant"]},{"year":2022,"claim":"Extended MYBPC3 biology beyond cardiomyocytes by showing fibroblast expression and that its disruption activates NF-κB/TGF-β1/HIF-1α-driven fibrosis pathways.","evidence":"CRISPR disruption in NIH3T3 fibroblasts with pathway analysis and an R495Q base-edited pig model","pmids":["36357371"],"confidence":"Medium","gaps":["Fibroblast role not validated in human tissue","Mechanism connecting cMyBP-C loss to NF-κB activation unknown"]},{"year":2023,"claim":"Demonstrated that the E258K founder mutation produces haploinsufficiency with faster, energetically costly cross-bridge cycling, partly offset by compensatory slowing of excitation-contraction coupling.","evidence":"Myofibril/permeabilized-strip mechanics, Ca²⁺ transients, hiPSC-CMs/EHTs, imaging, and in silico modeling","pmids":["36744470"],"confidence":"High","gaps":["Reconciliation with earlier S2-interaction loss for E258K not detailed","Durability of EC-coupling compensation in disease progression unclear"]},{"year":2023,"claim":"Mapped the temporal progression of haploinsufficiency from early hypercontractility to hypocontractility with progressive Ca²⁺-handling abnormalities in human engineered tissue.","evidence":"Isogenic heterozygous/homozygous MYBPC3 frameshift iPSC ECTs with longitudinal contractile, Ca²⁺, and RNA-seq analyses","pmids":["36893011"],"confidence":"High","gaps":["Engineered tissue maturation differs from adult myocardium","Trigger converting hyper- to hypocontractility not defined"]},{"year":null,"claim":"How cMyBP-C structurally maintains the super-relaxed myosin state, and what molecular trigger converts early hypercontractility into progressive Ca²⁺-handling dysfunction and hypocontractility, remain unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No atomic-resolution model of cMyBP-C maintaining SRX in situ","Causal driver of the hyper-to-hypocontractile transition undefined","Role of fibroblast-expressed cMyBP-C in human disease unconfirmed"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0008092","term_label":"cytoskeletal protein binding","supporting_discovery_ids":[2,5,6]},{"term_id":"GO:0098772","term_label":"molecular function regulator 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Ablation of cMyBP-C mimics PKA treatment (accelerated force redevelopment and stretch activation), but PKA has no additional effect in cMyBP-C null myocardium, indicating that PKA acts specifically through cMyBP-C phosphorylation to enhance contractile kinetics.\",\n      \"method\": \"Skinned ventricular myocardium from wild-type and cMyBP-C null mice; PKA treatment; mechanical measurements of force redevelopment rate and stretch activation response\",\n      \"journal\": \"Circulation research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — clean genetic ablation combined with pharmacological PKA treatment and direct mechanical assay, with rigorous null-mouse controls\",\n      \"pmids\": [\"16973906\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"PKA-mediated phosphorylation of cMyBP-C increases the proximity of myosin cross-bridges to actin (displaces cross-bridges radially/azimuthally from thick filaments) in resting myocardium, as demonstrated by an ~50% increase in the equatorial x-ray diffraction intensity ratio I(11)/I(10). This effect is absent in cMyBP-C null myocardium, confirming cMyBP-C acts as a tether that restrains cross-bridges and is relieved upon phosphorylation.\",\n      \"method\": \"Synchrotron low-angle x-ray diffraction of skinned trabeculae from wild-type and cMyBP-C null mice; PKA treatment\",\n      \"journal\": \"Circulation research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — synchrotron x-ray structural measurement combined with genetic null model and pharmacological manipulation, multiple orthogonal readouts\",\n      \"pmids\": [\"18599866\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"The N-terminal domains of cMyBP-C bind to F-actin via subdomain 1, and 3D reconstruction by electron microscopy showed that cMyBP-C density covers a broad portion of actin subdomain 1, extending tangentially toward the pointed end. The binding location would sterically interfere with myosin head binding to actin and could modulate tropomyosin position, providing a structural basis for thin-filament regulation by cMyBP-C.\",\n      \"method\": \"Negative-stain electron microscopy and 3D helical reconstruction of F-actin decorated with bacterially expressed N-terminal cMyBP-C fragments; molecular fitting with atomic Ig-domain structure\",\n      \"journal\": \"Journal of molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — direct structural reconstruction with recombinant fragments, molecular docking, and multiple salt conditions as controls\",\n      \"pmids\": [\"21601575\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"The HCM-associated cMyBP-C L348P variant (M-domain leucine→proline) causes a larger displacement of tropomyosin on the thin filament—from the blocked position to approximately the open position—compared with wild-type fragments that shift tropomyosin only to the closed position. This enhanced tropomyosin shift is phosphorylation-independent, explaining the gain-of-function Ca²⁺-sensitization conferred by L348P.\",\n      \"method\": \"Electron microscopy and 3D reconstruction of thin filaments decorated with wild-type or L348P cMyBP-C N-terminal fragments; phosphorylated vs. unphosphorylated comparisons\",\n      \"journal\": \"Journal of molecular and cellular cardiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — direct structural reconstruction with mutant versus wild-type fragments and phosphorylation controls in a single rigorous study\",\n      \"pmids\": [\"26718724\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"The E258K missense HCM mutation in cMyBP-C abolishes the N-terminal cMyBP-C interaction with myosin S2 (detected by yeast two-hybrid), while simultaneously increasing affinity between N-terminal cMyBP-C and actin. Expression of E258K cMyBP-C in murine engineered cardiac tissue lacking endogenous cMyBP-C caused accelerated contractile kinetics and severely reduced twitch force, phenocopying cMyBP-C ablation with respect to kinetics but additionally impairing force.\",\n      \"method\": \"Adenoviral expression in cMyBP-C null murine engineered cardiac tissue; yeast two-hybrid for protein interaction mapping; contractile force measurements\",\n      \"journal\": \"The Journal of general physiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Moderate — yeast two-hybrid interaction mapping combined with functional reconstitution in null-background tissue, multiple orthogonal readouts\",\n      \"pmids\": [\"23980194\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"The C-terminal C10 domain of cMyBP-C binds the light meromyosin (LMM) region of the myosin heavy chain; the HCM-associated 25-bp deletion mutant (cMyBP-CᶜC10mut) loses this interaction as demonstrated by co-sedimentation and protein cross-linking assays. cMyBP-CᶜC10mut fails to incorporate into the sarcomere C-zone (immunofluorescence and subcellular fractionation), and its adenoviral expression in adult rat cardiomyocytes reduces fractional shortening and sarcomere shortening without altering Ca²⁺ transients.\",\n      \"method\": \"Co-sedimentation and protein cross-linking assays; subcellular fractionation; immunofluorescence; adenoviral expression in adult rat cardiomyocytes; contractile and Ca²⁺ transient measurements\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — multiple orthogonal biochemical and cell-biological methods (co-sedimentation, cross-linking, fractionation, imaging, functional assay) in a single study\",\n      \"pmids\": [\"25583989\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"An N-terminal fragment of cMyBP-C generated by post-MI proteolysis (C0-C1f) incorporated into permeabilized human left ventricular myocardium decreases maximal Ca²⁺-activated force and myofilament Ca²⁺ sensitivity, and increases cross-bridge cycling kinetics and tension cost. These effects occur through direct interaction of C0-C1f with actin and α-tropomyosin, identifying a mechanism by which MI-generated fragments impair cardiac contractility.\",\n      \"method\": \"Recombinant human C0-C1f incorporated into permeabilized human LV myocardium; mechanical property measurements at two sarcomere lengths; biochemical binding assays for actin and tropomyosin interactions\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — in vitro reconstitution in human myocardium with recombinant fragment, functional mechanical assays, and direct binding assays for mechanism\",\n      \"pmids\": [\"24509847\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Stepwise depletion of cMyBP-C causes dose-dependent augmentation of myosin contractility. cMyBP-C depletion enhances the myosin disordered-relaxed (DRX) state that enables ATP hydrolysis and thin-filament interaction, while reducing the super-relaxed (SRX) conformation. A dilated cardiomyopathy missense variant (F764L) in myosin normalizes the increased contractility from cMyBP-C depletion. The myosin ATPase inhibitor MYK-461 rescues relaxation deficits and restores normal contractility in MYBPC3-mutant mouse and human cardiomyocytes.\",\n      \"method\": \"Genetic and biochemical approaches in mouse and human cardiomyocytes; myosin conformation assays; pharmacological rescue with MYK-461; stepwise cMyBP-C depletion models\",\n      \"journal\": \"Science translational medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — multiple orthogonal genetic and biochemical methods combined with pharmacological rescue in both mouse and human cardiomyocytes\",\n      \"pmids\": [\"30674652\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"MYBPC3 mutations causing HCM are associated with a significantly reduced super-relaxed (SRX) myosin state in human LV tissue. MYBPC3-mutant samples showed fewer myosin heads in the SRX and shorter ATP turnover lifetime compared to donors. A positive correlation between cMyBP-C expression level and the proportion of SRX myosin heads indicates that cMyBP-C modulates and maintains the SRX state.\",\n      \"method\": \"Single-nucleotide turnover assay for SRX measurement in human myectomy samples; genotyping of HCM patients; quantitative comparison of MYBPC3-mutant vs. non-mutant vs. donor samples\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct functional SRX assay on human tissue with genotype stratification; single lab, two orthogonal measurements (SRX proportion and ATP turnover lifetime)\",\n      \"pmids\": [\"28658286\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Protein kinase D (PKD) binds to cMyBP-C in contracting cardiomyocytes (shown by immunoprecipitation) and phosphorylates it at Ser315. In permeabilized ventricular myocytes from wild-type mice, PKD increased both myofilament Ca²⁺ sensitivity and maximal Ca²⁺-activated tension (Tmax); in cMyBP-C knockout myocytes, PKD still increased Ca²⁺ sensitivity but failed to increase Tmax, demonstrating that cMyBP-C is specifically required for PKD-mediated Tmax enhancement.\",\n      \"method\": \"Immunoprecipitation in contracting cardiomyocytes; permeabilized myocyte force measurements in wild-type and cMyBP-C knockout mice; PKD treatment\",\n      \"journal\": \"American journal of physiology. Heart and circulatory physiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Moderate — co-IP binding assay combined with functional reconstitution using null-mouse controls, identifying specific phosphorylation site and functional consequence\",\n      \"pmids\": [\"22636676\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Phosphorylation of cMyBP-C at specific serine residues has site-specific effects on contractile mechanics: the DAD phosphomimetic mutation (Asp273-Ala282-Asp302) causes ~50% reduction in tension, decreased stiffness, altered cross-bridge kinetics (decreased 2πc, increased 2πb), while SAS (Ser273-Ala282-Ser302) and ADA mutations cause minimal changes. The non-phosphorylatable Ala at position 282 combined with phosphomimetic Asp at 273 and/or 302 is particularly detrimental.\",\n      \"method\": \"Sinusoidal analysis of papillary and trabecular muscle fibers from cMyBP-C null mice transgenically expressing site-specific phosphorylation mutants; pCa-tension studies\",\n      \"journal\": \"Biophysical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — reconstitution in null-mouse background with defined phosphorylation mutants, rigorous mechanical analysis with multiple readouts\",\n      \"pmids\": [\"24606935\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"Heterozygous cMyBP-C null mice with ~50% reduction in cMyBP-C protein develop asymmetric septal hypertrophy with fibrosis at 10–11 months, recapitulating a key feature of human FHC. Homozygous null mice develop eccentric LV hypertrophy with decreased fractional shortening at 3–4 months and markedly impaired relaxation after 9 months, establishing that haploinsufficiency of cMyBP-C is sufficient to cause the HCM-like phenotype.\",\n      \"method\": \"Targeted deletion of Mybpc3 exons 1–2 in mice; echocardiography; morphological and molecular analyses at multiple ages in het and hom mice\",\n      \"journal\": \"Cardiovascular research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — clean genetic model with dose-dependent phenotypic characterization at multiple time points, multiple functional and histological readouts\",\n      \"pmids\": [\"15249187\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"In human myectomy samples, MYBPC3 mutations (both truncating and missense) cause a significant reduction (~24%) in full-length cMyBP-C protein in myofibrils compared with donor and non-MYBPC3 mutant samples, without any detectable truncated peptides. This provides direct evidence that haploinsufficiency (not dominant-negative poison peptide) is the primary disease mechanism for MYBPC3-mutation HCM.\",\n      \"method\": \"Quantitative immunoblotting of myofibrillar fractions from human myectomy samples; RT-PCR for mRNA quantification; comparison of mutation-positive vs. mutation-negative vs. donor samples\",\n      \"journal\": \"Circulation research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — quantitative protein analysis in human tissue, multiple mutation types tested, replicated finding across 9 MYBPC3-mutant samples with appropriate controls\",\n      \"pmids\": [\"19574547\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Mybpc3 knock-in heterozygous mice (mimicking human HCM) exhibit increased myofilament Ca²⁺ sensitivity and diastolic dysfunction without LV hypertrophy, demonstrating that myofilament Ca²⁺ sensitization and impaired relaxation are early phenotypic consequences of Mybpc3 mutations that precede the development of hypertrophy.\",\n      \"method\": \"Skinned ventricular trabeculae force-Ca²⁺ measurements; intact myocyte Ca²⁺ transient analysis; in vivo echocardiography and Doppler in heterozygous knock-in mice at 10 weeks\",\n      \"journal\": \"Journal of molecular and cellular cardiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Moderate — genetic mouse model with multiple orthogonal functional assays from isolated protein to whole organ; directly shows sequence of events\",\n      \"pmids\": [\"22465693\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"In vivo LV systolic and diastolic function are compromised under basal conditions in cMyBP-C null mice. Beta-adrenergic stimulation significantly improves ejection fraction and contractile indices in wild-type but not cMyBP-C null mice. Adaptation to chronic pressure overload (aortic banding) is also diminished in cMyBP-C null mice, demonstrating that cMyBP-C is required for β-adrenergic inotropic reserve and pressure-overload adaptation.\",\n      \"method\": \"In vivo pressure-volume relations; echocardiography; β-adrenergic treatment; aortic banding in cMyBP-C null and wild-type mice\",\n      \"journal\": \"American journal of physiology. Heart and circulatory physiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — clean genetic null model with in vivo pressure-volume analysis and multiple functional perturbations\",\n      \"pmids\": [\"17122190\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"MYBPC3 truncation mutations in human HCM myectomy samples result in 40% reduction in MyBP-C content and reduced MyBP-C phosphorylation (quantified by mass spectrometry) without production of truncated protein, and enhance maximal myofilament sliding velocity specifically within the C-zone of thick filaments as demonstrated by single-particle TIRF microscopy. Analytical modeling confirmed that the 40% MyBP-C reduction is sufficient to explain the enhanced sliding velocity.\",\n      \"method\": \"Quantitative mass spectrometry on human myectomy samples; TIRF microscopy single-particle sliding assay with native thick and thin filaments; analytical modeling\",\n      \"journal\": \"Journal of molecular and cellular cardiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Moderate — direct functional assay with native human proteins using TIRF, quantitative proteomics, and analytical modeling providing mechanistic convergence\",\n      \"pmids\": [\"30550750\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"iPSC-derived cardiomyocytes carrying MYBPC3 premature termination codon mutations activate nonsense-mediated decay (NMD) and show aberrant Ca²⁺ handling with prolonged decay kinetics and elevated diastolic Ca²⁺ without structural abnormalities. Specific pharmacological inhibition of NMD reversed the molecular phenotype and normalized Ca²⁺ handling, establishing chronic NMD activation as a direct molecular mechanism linking MYBPC3 PTC mutations to HCM pathogenesis.\",\n      \"method\": \"Isogenic hiPSC-CMs with MYBPC3 PTC mutations; NMD pathway inhibition; Ca²⁺ handling measurements; transcriptome analysis\",\n      \"journal\": \"Circulation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Moderate — isogenic human cell model with pharmacological rescue and molecular pathway identification, multiple orthogonal methods\",\n      \"pmids\": [\"30586709\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Heterozygous MYBPC3 frameshift mutations cause allelic loss-of-function (reduced wild-type mRNA) but protein-level compensation maintaining normal MyBP-C stoichiometry through a previously uncharacterized mechanism involving reduced MyBP-C synthesis rate but slower MyBP-C degradation. Homozygous mutant iPSCMs showed contractile dysregulation, whereas heterozygous iPSCMs with compensated MyBP-C protein levels had normal contractile function. Stable isotope labeling measured MyBP-C synthesis and degradation rates.\",\n      \"method\": \"Patient-derived and genome-engineered iPSC-CMs; stable isotope labeling to measure synthesis and degradation rates; contractile function measurements; RNA-Seq\",\n      \"journal\": \"JCI insight\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Moderate — stable isotope pulse-chase for protein turnover in isogenic human cell lines with multiple orthogonal methods; novel mechanistic finding with quantitative rigor\",\n      \"pmids\": [\"31877118\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Acute removal of cMyBP-C N-terminal domains (C0–C7) in situ using a cut-and-paste genetic/protein engineering approach reduces myofilament Ca²⁺ sensitivity and increases cross-bridge cycling rate (ktr) at submaximal Ca²⁺, and induces auto-oscillatory (SPOC) contractions. Ligation of recombinant C0–C7 rescues pCa50 and ktr and abolishes oscillations; phosphorylated C0–C7 does not fully rescue these effects, revealing a role for cMyBP-C N-terminal domains in damping sarcomere-driven contractile oscillations.\",\n      \"method\": \"Spy-C mouse model with TEV protease cleavage site and SpyTag; detergent-permeabilized cardiomyocytes; recombinant C0–C7 ligation via SpyCatcher-SpyTag; force-Ca²⁺ and ktr measurements\",\n      \"journal\": \"Circulation research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — novel reconstitution approach enabling acute domain removal and replacement in situ with functional readouts and phosphorylation-dependent controls\",\n      \"pmids\": [\"32078438\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Nontruncating MYBPC3 pathogenic variants cluster in the C3, C6, and C10 domains. C10-domain missense variants cause failure of MyBP-C to incorporate into myofilaments with ~90% accelerated degradation (measured in rat ventricular myocytes), while C3 and C6 variants incorporate normally with normal degradation rates, demonstrating that a subset of missense variants cause loss-of-function through a distinct myofilament incorporation failure mechanism.\",\n      \"method\": \"Expression of missense mutant MyBP-C in rat ventricular myocytes; myofilament localization assay; degradation rate measurements; domain mapping in a large HCM cohort\",\n      \"journal\": \"Circulation. Genomic and precision medicine\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 / Moderate — direct cellular localization and degradation measurements with multiple domain mutants; single lab with multiple variants tested\",\n      \"pmids\": [\"32841044\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"In engineered heart tissue from Mybpc3 knock-out mouse cardiomyocytes reconstituted with viral expression, missense cMyBP-C (c.1591G>C) incorporates correctly into the sarcomere whereas truncated cMyBP-C does not; both mutations fail to restore force to wild-type levels. This demonstrates different pathogenic mechanisms: haploinsufficiency for truncating variants and sarcomere poisoning/incorrect incorporation for the missense variant.\",\n      \"method\": \"AAV-mediated expression in cMyBP-C null engineered heart tissue; immunofluorescence for sarcomere incorporation; isometric force measurements at multiple Ca²⁺ concentrations\",\n      \"journal\": \"Journal of molecular and cellular cardiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Moderate — functional reconstitution in null background with multiple mutation types, direct sarcomere incorporation imaging combined with mechanical assays\",\n      \"pmids\": [\"27108529\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"MYBPC3 is expressed in cardiac fibroblasts (not only cardiomyocytes), and CRISPR-mediated disruption of Mybpc3 in NIH3T3 fibroblasts activates the NF-κB signaling pathway, increases TGF-β1 expression, promotes HIF-1α-driven aerobic glycolysis, and accelerates fibroblast activation and cardiac fibrosis in a R495Q mutant pig model.\",\n      \"method\": \"Cytosine base editing in pigs (R495Q mutation); CRISPR disruption of Mybpc3 in NIH3T3 fibroblasts; NF-κB and TGF-β1 pathway analysis; glycolysis pathway measurements\",\n      \"journal\": \"Cell death & disease\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 / Moderate — in vitro CRISPR loss-of-function with pathway analysis and in vivo pig model; novel finding of expression in non-cardiomyocyte cell type\",\n      \"pmids\": [\"36357371\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"The ubiquitin-proteasome system (UPS) is activated early in postnatal cardiac hypertrophy in Mybpc3 knock-in and knock-out mice. In aged knock-in mice (expressing truncated cMyBP-C) but not knock-out mice, UPS degradation capacity is specifically impaired (fourfold higher Ub(G76V)-GFP accumulation), while autophagy-lysosome pathway markers are elevated in both models, suggesting defective ALP as a common mechanism and specific UPS impairment when truncated cMyBP-C is present.\",\n      \"method\": \"Ubiquitinated protein levels; proteasomal activity assays; Ub(G76V)-GFP reporter mice; ALP marker analysis in Mybpc3-KI and KO mice\",\n      \"journal\": \"Basic research in cardiology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vivo reporter mouse model with pharmacological and biochemical readouts, parallel comparison of two genetic models revealing mutation-specific vs. common mechanisms\",\n      \"pmids\": [\"22189562\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Autophagic flux is blunted in hearts of aged Mybpc3 knock-in mice and in human HCM myectomies with MYBPC3 mutations, associated with Akt-mTORC1 pathway activation. Treatment with rapamycin or caloric restriction (activating autophagy) partially rescued cardiomyopathy and restored autophagic flux in knock-in mice.\",\n      \"method\": \"LC3-II protein level measurement; autophagic flux assay; flow cytometry; rapamycin treatment; caloric restriction; echocardiography in Mybpc3-KI mice\",\n      \"journal\": \"Circulation. Heart failure\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic mouse model combined with pharmacological rescue and human tissue confirmation; multiple autophagy readouts\",\n      \"pmids\": [\"29021349\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"The MYBPC3:c772G>A (E258K) founder mutation reduces cMyBP-C expression (haploinsufficiency) and causes faster cross-bridge cycling and higher energy cost of tension generation in ventricular myofibrils and permeabilized strips. Compensatory slowing of excitation-contraction coupling (prolonged action potentials, slower Ca²⁺ transients) preserves twitch duration despite faster sarcomere kinetics.\",\n      \"method\": \"Mechanical studies in single myofibrils and permeabilized strips; Ca²⁺ transient measurement in native and hiPSC-derived cardiomyocytes; intact trabeculae; hiPSC-EHTs; tissue clearing/optical microscopy; in silico modeling\",\n      \"journal\": \"Circulation research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — multiple orthogonal experimental systems (human tissue, hiPSC-CMs, EHTs, in silico) converging on a single mechanistic model with quantitative rigor\",\n      \"pmids\": [\"36744470\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Among 44 nontruncating MYBPC3 variants, ~50% of HCM-linked variants showed alterations in RNA splicing or protein stability detectable by a cell-based minigene assay and protein stability assay, both with high specificity for HCM pathogenicity (100% and 94%, respectively). This demonstrates that protein haploinsufficiency, driven by splicing errors or protein instability, is a common pathomechanism even for nontruncating variants.\",\n      \"method\": \"Cell-based minigene splicing assay; protein stability assay; cosegregation and population genetics classification of 44 variants\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 / Moderate — systematic functional assays on multiple variants with clinical classification as reference standard; single lab\",\n      \"pmids\": [\"34097875\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"cMyBP-C haploinsufficiency in human engineered cardiac tissue (ECT) causes a progressive phenotype: initially hypercontractile, progressing to hypocontractility with impaired relaxation and severe force depression in homozygous null ECTs over 6 weeks. Ca²⁺-handling abnormalities develop progressively and are more pronounced with complete cMyBP-C absence, suggesting that while the primary effect may relate to myosin crossbridge orientation, the contractile phenotype is Ca²⁺-mediated.\",\n      \"method\": \"CRISPR-Cas9 heterozygous and homozygous MYBPC3 frameshift in human iPSCs; cardiac micropatterns and ECTs; longitudinal contractile, Ca²⁺-handling, and Ca²⁺-sensitivity measurements; RNA-seq\",\n      \"journal\": \"The Journal of general physiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Moderate — isogenic human cell models at two doses with longitudinal functional analysis and transcriptomics; multiple orthogonal readouts\",\n      \"pmids\": [\"36893011\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"cMyBP-C (encoded by MYBPC3) is a thick-filament-associated sarcomeric protein whose N-terminal domains (C0–C7) interact with both myosin S2 and actin/tropomyosin to restrain myosin cross-bridge proximity to actin and damp contractile oscillations; PKA (and PKD) phosphorylation of specific serine residues in its regulatory M-domain relieves this tethering, displacing cross-bridges toward actin to accelerate force development and cross-bridge cycling kinetics, while also maintaining the myosin super-relaxed state to preserve energetic efficiency. MYBPC3 truncating mutations cause haploinsufficiency—reduction of full-length cMyBP-C without detectable truncated protein—leading to dose-dependent augmentation of myosin contractility, increased Ca²⁺ sensitivity, impaired relaxation, and, in some contexts, chronic nonsense-mediated decay activation and progressive Ca²⁺-handling abnormalities; compensatory mechanisms including reduced MyBP-C degradation can partially restore protein stoichiometry in early disease, and protein level modulation (via gene therapy or myosin ATPase inhibitors) rescues the contractile defects.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"MYBPC3 encodes cardiac myosin-binding protein C (cMyBP-C), a thick-filament-associated sarcomeric protein that acts as a mechanical brake on myosin cross-bridges to tune the rate and economy of cardiac contraction [#0, #1]. Its N-terminal domains tether cross-bridges close to the thick filament backbone—genetic ablation or PKA phosphorylation displaces cross-bridges radially toward actin, accelerating force redevelopment and the stretch-activation response, with PKA acting specifically through cMyBP-C [#0, #1]. The N-terminal region binds F-actin at a site that would sterically interfere with myosin head binding and modulate tropomyosin position, providing a structural basis for thin-filament regulation [#2], while acute removal of the C0–C7 domains reduces Ca²⁺ sensitivity, raises cross-bridge cycling rate, and triggers auto-oscillatory contractions that recombinant C0–C7 ligation reverses, defining cMyBP-C as a damper of sarcomere oscillations [#18]. Beyond cross-bridge tethering, cMyBP-C maintains the energy-sparing super-relaxed (SRX) myosin state and restrains the disordered-relaxed (DRX) state, with cMyBP-C level correlating with the SRX fraction [#7, #8]. Phosphorylation by PKA and by PKD at Ser315 relieves tethering and modulates Ca²⁺ sensitivity and maximal tension in a site-specific manner [#9, #10]. The protein is anchored in the sarcomere C-zone through C-terminal C10-domain binding to the light meromyosin region of the myosin heavy chain [#5]. MYBPC3 mutations cause hypertrophic cardiomyopathy primarily through haploinsufficiency: truncating and many nontruncating variants reduce full-length cMyBP-C in human myocardium without detectable truncated peptide, producing increased myofilament Ca²⁺ sensitivity, enhanced myosin contractility and sliding velocity, and impaired relaxation that precede hypertrophy [#11, #12, #13, #15]. Distinct loss-of-function routes include premature-termination-codon-triggered nonsense-mediated decay [#16], splicing or protein-stability defects [#25], and failure of C10-domain missense mutants to incorporate into myofilaments with accelerated degradation [#5, #19], whereas certain missense variants such as E258K disrupt the myosin-S2 interaction and increase actin affinity [#4, #24]. Disease progression in human engineered tissue evolves from an early hypercontractile state to hypocontractility with Ca²⁺-handling abnormalities [#26]. Restoring protein level or inhibiting myosin ATPase with MYK-461 rescues the contractile defect [#7].\",\n  \"teleology\": [\n    {\n      \"year\": 2004,\n      \"claim\": \"Established that loss of cMyBP-C dosage, rather than a poison peptide, is sufficient to produce HCM-like disease, framing haploinsufficiency as the candidate mechanism.\",\n      \"evidence\": \"Targeted Mybpc3 deletion in mice with dose-dependent phenotyping by echocardiography and histology\",\n      \"pmids\": [\"15249187\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mouse phenotype timing may not map to human disease course\", \"Does not resolve the molecular events linking protein loss to hypertrophy\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Resolved how PKA enhances contractile kinetics by showing it acts specifically through cMyBP-C rather than other substrates.\",\n      \"evidence\": \"Skinned myocardium from WT and cMyBP-C null mice with PKA treatment and force/stretch-activation assays\",\n      \"pmids\": [\"16973906\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not visualize the structural change underlying accelerated kinetics\", \"Did not map the responsible phosphosites\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Demonstrated that cMyBP-C is required for β-adrenergic inotropic reserve and adaptation to pressure overload in the intact heart.\",\n      \"evidence\": \"In vivo pressure-volume and echocardiography with β-agonist and aortic banding in null mice\",\n      \"pmids\": [\"17122190\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whole-organ readout does not isolate the sarcomeric mechanism\", \"Compensatory remodeling could confound interpretation\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Provided the structural basis for cross-bridge tethering by showing PKA phosphorylation radially displaces cross-bridges toward actin, an effect absent in null myocardium.\",\n      \"evidence\": \"Synchrotron low-angle x-ray diffraction of skinned trabeculae from WT and null mice with PKA treatment\",\n      \"pmids\": [\"18599866\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Resting-state measurement does not establish the dynamic effect during cycling\", \"Specific domain mediating tethering not defined here\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Provided direct human evidence that MYBPC3 mutations reduce full-length protein without truncated peptide, confirming haploinsufficiency as the primary disease mechanism in patients.\",\n      \"evidence\": \"Quantitative immunoblotting and RT-PCR of human myectomy samples versus donor controls\",\n      \"pmids\": [\"19574547\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not establish the degradation/quality-control route eliminating truncated peptides\", \"~24% reduction smaller than expected for full allelic loss\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Mapped the actin-binding interface of the N-terminal domains, explaining how cMyBP-C could occlude myosin binding and reposition tropomyosin.\",\n      \"evidence\": \"Negative-stain EM and 3D helical reconstruction of F-actin decorated with recombinant N-terminal fragments\",\n      \"pmids\": [\"21601575\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Reconstruction used isolated fragments, not full-length protein in situ\", \"Functional consequence of tropomyosin shift inferred structurally\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Connected protein-quality-control pathways to disease, revealing UPS impairment specific to truncated-protein-expressing models and shared autophagy defects.\",\n      \"evidence\": \"Ub(G76V)-GFP reporter mice, proteasome activity and ALP marker assays in Mybpc3 KI and KO mice\",\n      \"pmids\": [\"22189562\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Causal link between proteostasis defect and contractile phenotype not established\", \"Findings in mouse models only\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Identified PKD as a second kinase that binds and phosphorylates cMyBP-C at Ser315, with cMyBP-C specifically required for PKD-driven maximal tension enhancement.\",\n      \"evidence\": \"Co-IP in contracting cardiomyocytes and permeabilized myocyte force assays in WT and KO mice\",\n      \"pmids\": [\"22636676\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Single Co-IP for the interaction without reciprocal/structural validation\", \"Physiological context for PKD signaling to cMyBP-C unclear\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Showed that Ca²⁺ sensitization and impaired relaxation are early consequences of Mybpc3 mutation that precede hypertrophy, ordering the disease cascade.\",\n      \"evidence\": \"Skinned trabeculae force-Ca²⁺, intact myocyte Ca²⁺ transients, and echocardiography in heterozygous knock-in mice\",\n      \"pmids\": [\"22465693\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Knock-in expresses truncated allele, distinct from pure haploinsufficiency\", \"Did not define the molecular trigger of Ca²⁺ sensitization\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Defined a missense mechanism distinct from haploinsufficiency by showing E258K abolishes the myosin-S2 interaction while increasing actin affinity, impairing both kinetics and force.\",\n      \"evidence\": \"Yeast two-hybrid interaction mapping and adenoviral expression in cMyBP-C null engineered cardiac tissue\",\n      \"pmids\": [\"23980194\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Yeast two-hybrid interaction not confirmed in native myocardium\", \"Relationship to later haploinsufficiency finding for E258K not reconciled here\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Localized sarcomere anchoring to the C-terminal C10 domain via binding to light meromyosin and showed an HCM deletion disrupts incorporation and contractility.\",\n      \"evidence\": \"Co-sedimentation, cross-linking, fractionation, immunofluorescence, and contractile assays in adult rat cardiomyocytes\",\n      \"pmids\": [\"25583989\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Effect on Ca²⁺ transients excluded but downstream signaling not examined\", \"Overexpression in rat cells may not reflect endogenous stoichiometry\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Demonstrated that proteolytic N-terminal fragments produced post-MI act through actin and tropomyosin to depress force and Ca²⁺ sensitivity, identifying a fragment-driven contractile pathology.\",\n      \"evidence\": \"Recombinant human C0-C1f incorporated into permeabilized human LV myocardium with mechanical and binding assays\",\n      \"pmids\": [\"24509847\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Endogenous abundance of these fragments in vivo not quantified here\", \"Mechanism specific to ischemic context\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Resolved site-specific roles of M-domain phosphorylation, showing combinations like the DAD phosphomimetic markedly alter tension and cross-bridge kinetics.\",\n      \"evidence\": \"Sinusoidal analysis of muscle fibers from null mice transgenically expressing phosphosite mutants\",\n      \"pmids\": [\"24606935\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Phosphomimetics approximate but do not equal true phosphorylation\", \"In vivo regulation of individual sites not addressed\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Explained gain-of-function Ca²⁺ sensitization for an M-domain HCM variant by showing L348P shifts tropomyosin further toward the open position, phosphorylation-independently.\",\n      \"evidence\": \"EM 3D reconstruction of thin filaments decorated with WT vs L348P N-terminal fragments\",\n      \"pmids\": [\"26718724\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Isolated fragment study, not full-length protein in sarcomere\", \"Direct tension consequences not measured here\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Contrasted pathogenic mechanisms in a null background, showing truncated protein fails to incorporate (haploinsufficiency) while a missense variant incorporates yet still depresses force (poisoning).\",\n      \"evidence\": \"AAV expression in cMyBP-C null engineered heart tissue with immunofluorescence and force measurements\",\n      \"pmids\": [\"27108529\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Viral expression levels may not match endogenous\", \"Single missense variant tested\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Linked cMyBP-C to the energetic super-relaxed myosin state by showing MYBPC3-mutant human tissue has reduced SRX, with SRX fraction correlating with cMyBP-C level.\",\n      \"evidence\": \"Single-nucleotide turnover SRX assay on genotyped human myectomy samples\",\n      \"pmids\": [\"28658286\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Correlative, single-lab measurement\", \"Causal direction between cMyBP-C loss and SRX reduction not isolated\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Implicated blunted autophagic flux with Akt-mTORC1 activation in disease and showed autophagy-activating interventions partially rescue cardiomyopathy.\",\n      \"evidence\": \"Autophagy markers/flux assays and rapamycin/caloric-restriction rescue in KI mice plus human myectomy confirmation\",\n      \"pmids\": [\"29021349\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether autophagy defect is cause or consequence unresolved\", \"Rescue only partial\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Tied reduced human MyBP-C content directly to enhanced contractile output by showing 40% reduction accelerates myosin sliding velocity specifically in the thick-filament C-zone.\",\n      \"evidence\": \"Quantitative mass spectrometry and single-particle TIRF sliding assays on native human filaments with analytical modeling\",\n      \"pmids\": [\"30550750\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vitro motility may not capture intact-cell regulation\", \"Reduced phosphorylation contribution not fully separated from reduced content\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Unified the contractile phenotype with myosin conformation, showing cMyBP-C depletion shifts myosin from SRX toward DRX dose-dependently and that MYK-461 rescues the defect.\",\n      \"evidence\": \"Genetic/biochemical depletion, myosin conformation assays, and MYK-461 rescue in mouse and human cardiomyocytes\",\n      \"pmids\": [\"30674652\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct structural mechanism of cMyBP-C maintaining SRX not resolved\", \"Long-term in vivo efficacy of inhibitor not addressed here\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Established chronic NMD activation as a direct molecular route from premature-termination-codon mutations to aberrant Ca²⁺ handling, reversible by NMD inhibition.\",\n      \"evidence\": \"Isogenic hiPSC-CMs with MYBPC3 PTCs, NMD inhibition, Ca²⁺ handling, and transcriptome analysis\",\n      \"pmids\": [\"30586709\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"NMD inhibition not therapeutically selective in vivo\", \"Link between Ca²⁺ abnormality and structural disease not established\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Revealed protein-level compensation in heterozygous cells via slower MyBP-C degradation that maintains stoichiometry and preserves contractile function despite allelic loss.\",\n      \"evidence\": \"Stable isotope labeling of synthesis/degradation, contractile assays, and RNA-seq in patient-derived and engineered iPSC-CMs\",\n      \"pmids\": [\"31877118\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether compensation persists with aging/stress unclear\", \"Mechanism stabilizing residual protein not identified\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Defined a distinct loss-of-function route for C10-domain missense variants through failed myofilament incorporation and accelerated degradation, separating them from C3/C6 variants.\",\n      \"evidence\": \"Expression of missense mutants in rat ventricular myocytes with localization and degradation assays plus cohort domain mapping\",\n      \"pmids\": [\"32841044\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single-lab cellular assay\", \"Functional contractile consequence of these variants not measured here\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Defined the N-terminal domains as dampers of sarcomere oscillations through an acute in-situ removal-and-replacement strategy with phosphorylation-dependent rescue.\",\n      \"evidence\": \"Spy-C mouse with TEV-cleavable C0–C7 and SpyTag/SpyCatcher ligation in permeabilized cardiomyocytes with force-Ca²⁺/ktr assays\",\n      \"pmids\": [\"32078438\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Acute removal differs from chronic mutation context\", \"Phosphorylated C0–C7 incomplete rescue not fully explained\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Generalized haploinsufficiency to nontruncating variants by showing ~half alter splicing or protein stability with high specificity for HCM pathogenicity.\",\n      \"evidence\": \"Cell-based minigene splicing and protein stability assays across 44 classified variants\",\n      \"pmids\": [\"34097875\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Minigene assays may not reflect native splicing\", \"Functional contractile validation not performed for each variant\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Extended MYBPC3 biology beyond cardiomyocytes by showing fibroblast expression and that its disruption activates NF-κB/TGF-β1/HIF-1α-driven fibrosis pathways.\",\n      \"evidence\": \"CRISPR disruption in NIH3T3 fibroblasts with pathway analysis and an R495Q base-edited pig model\",\n      \"pmids\": [\"36357371\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Fibroblast role not validated in human tissue\", \"Mechanism connecting cMyBP-C loss to NF-κB activation unknown\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Demonstrated that the E258K founder mutation produces haploinsufficiency with faster, energetically costly cross-bridge cycling, partly offset by compensatory slowing of excitation-contraction coupling.\",\n      \"evidence\": \"Myofibril/permeabilized-strip mechanics, Ca²⁺ transients, hiPSC-CMs/EHTs, imaging, and in silico modeling\",\n      \"pmids\": [\"36744470\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Reconciliation with earlier S2-interaction loss for E258K not detailed\", \"Durability of EC-coupling compensation in disease progression unclear\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Mapped the temporal progression of haploinsufficiency from early hypercontractility to hypocontractility with progressive Ca²⁺-handling abnormalities in human engineered tissue.\",\n      \"evidence\": \"Isogenic heterozygous/homozygous MYBPC3 frameshift iPSC ECTs with longitudinal contractile, Ca²⁺, and RNA-seq analyses\",\n      \"pmids\": [\"36893011\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Engineered tissue maturation differs from adult myocardium\", \"Trigger converting hyper- to hypocontractility not defined\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How cMyBP-C structurally maintains the super-relaxed myosin state, and what molecular trigger converts early hypercontractility into progressive Ca²⁺-handling dysfunction and hypocontractility, remain unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No atomic-resolution model of cMyBP-C maintaining SRX in situ\", \"Causal driver of the hyper-to-hypocontractile transition undefined\", \"Role of fibroblast-expressed cMyBP-C in human disease unconfirmed\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0008092\", \"supporting_discovery_ids\": [2, 5, 6]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [0, 1, 7, 18]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005856\", \"supporting_discovery_ids\": [5, 20]},\n      {\"term_id\": \"GO:0005861\", \"supporting_discovery_ids\": [5, 15, 20]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-397014\", \"supporting_discovery_ids\": [0, 1, 7, 18]},\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [11, 12, 16, 26]}\n    ],\n    \"complexes\": [\"sarcomere thick filament (C-zone)\"],\n    \"partners\": [\"MYH7\", \"ACTC1\", \"TPM1\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":10,"faith_total":10,"faith_pct":100.0}}