{"gene":"ACTC1","run_date":"2026-04-28T17:12:37","timeline":{"discoveries":[{"year":1998,"finding":"Missense mutations in the cardiac actin gene (ACTC) that affect universally conserved amino acids in domains attaching to Z bands and intercalated discs co-segregate with hereditary dilated cardiomyopathy (IDC), suggesting that defective force transmission from cardiac myocytes is a mechanism underlying heart failure.","method":"Genetic linkage analysis and direct sequencing in two unrelated familial IDC pedigrees","journal":"Science","confidence":"High","confidence_rationale":"Tier 2 — genetic co-segregation in two independent families, foundational discovery replicated by subsequent studies","pmids":["9563954"],"is_preprint":false},{"year":1999,"finding":"The ACTC gene (alpha-cardiac actin) is identified as a novel disease gene for familial hypertrophic cardiomyopathy (HCM), with an Ala295Ser mutation in exon 5; ACTC is the first sarcomeric gene in which mutations cause two distinct cardiomyopathies (HCM and dilated), leading to the hypothesis that HCM-causing mutations affect sarcomere contraction while DCM-causing mutations affect force transmission.","method":"Linkage analysis, candidate gene sequencing, lod score analysis in a family pedigree","journal":"The Journal of Clinical Investigation","confidence":"High","confidence_rationale":"Tier 2 — genetic co-segregation with high lod score, foundational discovery widely replicated","pmids":["10330430"],"is_preprint":false},{"year":2003,"finding":"Alpha-cardiac actin (ACTC) physically binds to the cardiac isoform of band 3 (AE1 anion exchanger); the interaction was identified by yeast two-hybrid and confirmed by reciprocal co-immunoprecipitation from rat heart; confocal microscopy localised band 3 to the intercalated disc, suggesting the interaction occurs at the site of sarcomere attachment to the plasma membrane.","method":"Yeast two-hybrid, co-immunoprecipitation (reciprocal), confocal microscopy","journal":"Journal of Cellular Biochemistry","confidence":"Medium","confidence_rationale":"Tier 2 — reciprocal Co-IP plus localization, single lab study","pmids":["12898519"],"is_preprint":false},{"year":2011,"finding":"The HCM-causing ACTC E99K mutation increases myofibrillar Ca²⁺ sensitivity ~2.3-fold in transgenic mouse reconstituted thin filaments (in vitro motility assay) and ~1.3-fold in human carrier samples; it also abolishes the normal change in Ca²⁺ sensitivity linked to troponin I phosphorylation. Transgenic mice exhibit apical hypertrophy, sudden cardiac death, atrial flutter, reduced β-adrenergic contractile response, and eventual dilated cardiomyopathy.","method":"In vitro motility assay on reconstituted thin filaments, skinned papillary muscle mechanics, cardiac MRI, ECG, transgenic mouse model","journal":"The Journal of Biological Chemistry","confidence":"High","confidence_rationale":"Tier 1 — reconstituted thin filament assay with multiple orthogonal methods (motility, skinned muscle, MRI, ECG) in both mouse model and human samples","pmids":["21622575"],"is_preprint":false},{"year":2013,"finding":"ACTC E99K papillary muscles produce 3–4-fold greater force than non-transgenic controls under isometric conditions, relax ~1.4-fold slower, and show ~2-fold higher myofibrillar Ca²⁺ sensitivity (EC₅₀ 0.39 vs 0.80 μmol/l); energy turnover is disproportionately elevated relative to work produced (efficiency 11–16% vs 15–18%), establishing hypercontractility driven by elevated myofibrillar Ca²⁺ sensitivity as the primary deficit.","method":"Intact papillary muscle mechanics, myofibril Ca²⁺-jump protocol, heat+work calorimetry, Ca²⁺ imaging in isolated myocytes","journal":"American Journal of Physiology. Heart and Circulatory Physiology","confidence":"High","confidence_rationale":"Tier 1 — multiple orthogonal quantitative assays (mechanics, calorimetry, Ca²⁺ imaging) in transgenic mouse model","pmids":["23604709"],"is_preprint":false},{"year":2014,"finding":"The DCM-causing mutation ACTC E361G specifically abolishes troponin I phosphorylation-dependent modulation of Ca²⁺ sensitivity and relaxation kinetics in intact cardiac myofibrils, without affecting sarcomere-length-dependent activation or response to EMD57033; this demonstrates uncoupling of PKA-mediated lusitropy in an intact contractile system.","method":"Ca²⁺-jump protocol in single transgenic mouse cardiac myofibrils, propranolol treatment to vary troponin I phosphorylation levels","journal":"Biophysical Journal","confidence":"High","confidence_rationale":"Tier 1 — direct measurement in native myofibrils with pharmacological manipulation of phosphorylation state, rigorous controls","pmids":["25418306"],"is_preprint":false},{"year":2016,"finding":"A mutation in the 3'UTR of ACTC1 (c.*1784T>C) creates a novel target site for miR-139-5p, reducing ACTC1 gene expression; luciferase reporter assays confirm the mutation specifically downregulates expression, and miR-139-5p mimic/inhibitor modulate this effect, suggesting haploinsufficiency via a gain-of-function miRNA regulatory mechanism as a cause of familial ASD.","method":"Whole genome sequencing, luciferase reporter assay, miR-139-5p mimic and inhibitor transfection","journal":"Scientific Reports","confidence":"Medium","confidence_rationale":"Tier 2 — functional luciferase assay with orthogonal miRNA manipulation, single lab","pmids":["27139165"],"is_preprint":false},{"year":2017,"finding":"Young ACTC E99K transgenic mice in their vulnerable period (28–40 days) exhibit elevated Ca²⁺ transients, increased Ca²⁺ spark mass, and greater propensity for spontaneous Ca²⁺ waves compared to non-transgenic littermates, despite similar sarcoplasmic reticulum Ca²⁺ content; these aberrant Ca²⁺ release events are associated with sudden cardiac death and increased myocardial fibrosis. Adult survivors normalize Ca²⁺ transients. Penetrance of sudden death depends on genetic background (CBA/Ca vs C57Bl6).","method":"Isolated ventricular myocyte Ca²⁺ imaging (confocal), Ca²⁺ spark analysis, histology (collagen quantification), transgenic mouse model with two genetic backgrounds","journal":"American Journal of Physiology. Heart and Circulatory Physiology","confidence":"High","confidence_rationale":"Tier 2 — direct cellular Ca²⁺ measurements with defined phenotypic readout, multiple genetic backgrounds tested","pmids":["28887330"],"is_preprint":false},{"year":2017,"finding":"Variable expression of cardiac α-actin (Actc1) in early adult skeletal muscle across Collaborative Cross strains (up to 24-fold) is negatively correlated with promoter methylation at the Actc1 transcriptional start site in a strain-dependent manner, while histone modifications and chromatin accessibility are unaltered; eQTL mapping confirms a cis-acting regulatory locus at the Actc1 locus in both heart and skeletal muscle.","method":"Expression QTL mapping, bisulfite sequencing (methylation), ATAC-seq/chromatin accessibility, histone ChIP across Collaborative Cross mouse strains","journal":"Biochimica et Biophysica Acta. Gene Regulatory Mechanisms","confidence":"Medium","confidence_rationale":"Tier 2 — multiple orthogonal epigenomic methods correlating methylation with expression, single study","pmids":["28847732"],"is_preprint":false},{"year":2018,"finding":"hiPSC-derived cardiomyocytes carrying the E99K-ACTC1 mutation exhibit arrhythmogenesis in both 3D engineered heart tissues and 2D monolayers; Ca²⁺ handling expression studies informed pharmacological rescue, wherein dual dantrolene/ranolazine treatment was most effective, establishing E99K mutant ACTC1 protein as a primary driver of arrhythmia with Ca²⁺ handling as a central mechanism.","method":"Isogenic hiPSC-CM pairs, 3D engineered heart tissue, 2D monolayer Ca²⁺ imaging, pharmacological rescue (dantrolene/ranolazine)","journal":"Stem Cell Reports","confidence":"Medium","confidence_rationale":"Tier 2 — isogenic human cell model with defined functional readout and pharmacological rescue, single study","pmids":["30392975"],"is_preprint":false},{"year":2018,"finding":"Knockdown of ACTC1 by siRNA in the U87MG glioblastoma cell line significantly inhibits cell migration (distance travelled reduced from ~3600 μm to ~1265 μm over 72 h), demonstrating a functional role for ACTC1 in tumour cell motility.","method":"siRNA knockdown, time-lapse cell migration tracking assay, droplet digital PCR, immunocytochemistry","journal":"Journal of the Neurological Sciences","confidence":"Medium","confidence_rationale":"Tier 2 — siRNA KD with quantitative phenotypic readout, single lab, single cell line","pmids":["30055382"],"is_preprint":false},{"year":2019,"finding":"The ACTC1 p.Gly247Asp (G247D) mutation inhibits actin polymerization (confirmed by in vitro polymerization assay) and impairs Rho-GTPase/SRF signalling: overexpression of native ACTC1 strongly activates SRF-driven luciferase in NRVCMs whereas G247D abolishes this; mutant ACTC1 shows reduced GTP-bound Rho-GTPase and increased nuclear accumulation of globular actin, establishing defective actin polymerization as the mechanism linking G247D to impaired SRF signalling and DCM.","method":"In vitro actin polymerization assay, luciferase reporter assay (SRF/SM22-RE), Rho-GTPase pull-down (active Rho), nuclear fractionation and G-actin immunoblot, overexpression in NRVCMs","journal":"Biochemical and Biophysical Research Communications","confidence":"Medium","confidence_rationale":"Tier 1/2 — in vitro polymerization plus multiple cellular assays; single lab","pmids":["31434612"],"is_preprint":false},{"year":2019,"finding":"A heterozygous ACTC1 p.Gly247Asp mutation causes ultrastructural sarcomeric disarray, myofibrillar degeneration, and increased apoptosis in human myocardial tissue; in neonatal rat ventricular cardiomyocytes, overexpression of the mutant (but not wild-type) ACTC1 causes structural defects and apoptosis; molecular dynamics and polymerization assays confirm actin polymerization/turnover defects, implicating defective actin signalling in both cardiac developmental defects (ASD) and contractile dysfunction (DCM).","method":"Ultrastructural electron microscopy, cardiac proteomics, NRVCM overexpression, molecular dynamics simulation, actin polymerization assay, TUNEL apoptosis assay","journal":"Circulation. Genomic and Precision Medicine","confidence":"Medium","confidence_rationale":"Tier 2 — multiple orthogonal methods (EM, proteomics, functional cell assays, MD), single study","pmids":["31430208"],"is_preprint":false},{"year":2010,"finding":"Reduced ACTC1 expression in sporadic congenital heart disease samples correlates with increased cardiomyocyte apoptosis; siRNA-mediated knockdown of Actc1 in H9C2 cardiomyocyte cell line increases apoptosis with increased Caspase-3 and decreased Bcl-2 expression, indicating ACTC1 promotes cardiomyocyte survival.","method":"RT-PCR, Western blot, immunohistochemistry, TUNEL assay, siRNA knockdown in H9C2 cells","journal":"Circulation Journal","confidence":"Medium","confidence_rationale":"Tier 2 — siRNA KD with defined apoptosis readout validated in human tissue and cell line, single lab","pmids":["20962418"],"is_preprint":false},{"year":2015,"finding":"ACTC1 mutations causing congenital heart defects (p.Met84Thr, p.Glu101Lys, p.Met125Val) cluster in a region of actin in close apposition to the myosin heavy chain head domain, whereas mutations causing cardiomyopathies (p.Ala297Ser, p.Asp313His, p.Arg314His) map to a distinct interaction surface; this spatial distinction suggests that the clinical phenotypic consequence of an ACTC1 mutation is partly determined by which actin–myosin interaction surface is disrupted.","method":"Linkage analysis, Sanger sequencing, structural mapping of mutations onto actin–myosin complex structure","journal":"PLoS One","confidence":"Low","confidence_rationale":"Tier 4 for mechanism — structural inference from known crystal structure without direct functional validation of interface","pmids":["26061005"],"is_preprint":false},{"year":2023,"finding":"Heterozygous missense variants in ACTC1 cause distal arthrogryposis (DA) with congenital heart defects in five families, demonstrating that ACTC1 function is shared in both cardiac and skeletal muscle; the finding establishes ACTC1 as the first gene underlying DA that also causes cardiac abnormalities.","method":"Exome/genome sequencing, familial co-segregation analysis across five independent families","journal":"HGG Advances","confidence":"Medium","confidence_rationale":"Tier 2 — genetic epistasis/co-segregation in five independent families, functional implication in both muscle types","pmids":["37457373"],"is_preprint":false},{"year":2025,"finding":"LMOD2 (leiomodin-2) physically interacts with ACTC1 as confirmed by Co-IP; LMOD2 knockout in C2C12 myoblasts alters muscle fiber type composition and inhibits myoblast proliferation, placing the LMOD2–ACTC1 interaction in the regulation of myogenic differentiation.","method":"Co-immunoprecipitation, RNA-seq, LMOD2 knockout in C2C12 cells, lentiviral knockdown in vivo","journal":"BMC Genomics","confidence":"Medium","confidence_rationale":"Tier 3 — single Co-IP with supporting transcriptomic and functional KO data, single lab","pmids":["40745266"],"is_preprint":false},{"year":2025,"finding":"ACTC1 overexpression in prostate cancer cells promotes proliferation and migration, while knockdown suppresses these behaviors; transcriptomic analysis identifies BMP4 as a key downstream effector, and BMP4 overexpression rescues the inhibitory effects of ACTC1 knockdown, establishing an ACTC1–BMP4 signalling axis in prostate cancer progression.","method":"Overexpression and siRNA knockdown in prostate cancer cell lines, xenograft mouse model, RNA-seq, BMP4 rescue experiment","journal":"BMC Cancer","confidence":"Medium","confidence_rationale":"Tier 2 — loss- and gain-of-function with rescue experiment and in vivo validation, single lab","pmids":["41286808"],"is_preprint":false},{"year":2025,"finding":"The zebrafish Acta1b p.T126I mutation (orthologous to human ACTC1 T126I) causes progressive dilated cardiomyopathy with sex-specific differences: female mutants show earlier diastolic dysfunction, more severe cardiac remodeling, and lower survival than males; molecular profiling reveals sex-specific alterations in calcium handling genes (serca2, pln1, slc8a1a) and proteostasis regulators (hsf1, bag3), with upregulation of the stress marker nppb and downregulation of gata4/mef2ca.","method":"CRISPR-generated zebrafish knock-in model, longitudinal echocardiography, histology, sex-stratified gene expression profiling","journal":"bioRxiv","confidence":"Medium","confidence_rationale":"Tier 2 — in vivo vertebrate model with longitudinal functional assessment and molecular profiling; preprint, not yet peer-reviewed","pmids":["bio_10.1101_2025.08.26.672352"],"is_preprint":true},{"year":2025,"finding":"Variants in ACTC1 promoter region alter ACTC1 transcriptional activity as measured by luciferase assay in mouse cardiomyocytes (HL-1); EMSA and JASPAR analysis indicate the variants affect transcription factor binding, linking promoter variants to reduced ACTC1 expression and ventricular septal defect pathogenesis.","method":"Sanger sequencing, luciferase promoter activity assay in HL-1 cells, electrophoretic mobility shift assay (EMSA)","journal":"Gene","confidence":"Medium","confidence_rationale":"Tier 2 — direct functional promoter assay and EMSA in cardiomyocytes, single lab","pmids":["40848833"],"is_preprint":false}],"current_model":"ACTC1 (cardiac α-actin) is a sarcomeric thin-filament protein whose polymerization state and interaction with myosin and troponin govern cardiac and skeletal muscle contractility: disease-causing mutations (e.g. E99K, E361G, G247D) increase myofibrillar Ca²⁺ sensitivity, uncouple PKA/troponin I phosphorylation-dependent lusitropy, impair actin polymerization and Rho-GTPase/SRF signalling, and disrupt force transmission at Z-bands and intercalated discs, thereby causing hypertrophic or dilated cardiomyopathy, atrial septal defect, or distal arthrogryposis depending on which actin–myosin interaction surface is affected; ACTC1 also binds the cardiac band 3 anion exchanger at intercalated discs, interacts with LMOD2 to regulate myogenic differentiation, and modulates cell migration and tumour-relevant BMP4 signalling."},"narrative":{"teleology":[{"year":1998,"claim":"Establishing that ACTC1 mutations cause dilated cardiomyopathy answered whether sarcomeric actin itself—not just myosin or regulatory proteins—could drive heart failure through defective force transmission.","evidence":"Genetic linkage and sequencing in two familial DCM pedigrees identifying missense mutations at Z-band/intercalated-disc attachment domains","pmids":["9563954"],"confidence":"High","gaps":["No direct measurement of force transmission deficit","Biochemical mechanism of each mutation unresolved"]},{"year":1999,"claim":"Discovery that a different ACTC1 mutation causes HCM established that the same gene could underlie two opposing cardiomyopathies, generating the hypothesis that mutation location—contraction-affecting versus force-transmission-affecting—determines phenotype.","evidence":"Linkage analysis and candidate gene sequencing with significant lod score in an HCM family","pmids":["10330430"],"confidence":"High","gaps":["Functional consequence of Ala295Ser on contractile parameters not yet measured","No direct structural mapping to actin–myosin interface"]},{"year":2003,"claim":"Identification of a physical ACTC1–band 3 (AE1) interaction at intercalated discs revealed a non-myosin binding partner, suggesting ACTC1 participates in membrane-cytoskeletal coupling beyond pure force generation.","evidence":"Yeast two-hybrid screen with reciprocal co-immunoprecipitation from rat heart and confocal colocalization","pmids":["12898519"],"confidence":"Medium","gaps":["Functional consequence of disrupting the ACTC1–band 3 interaction unknown","Not tested with DCM-causing mutations mapping to the intercalated disc domain"]},{"year":2010,"claim":"Demonstration that ACTC1 knockdown increases cardiomyocyte apoptosis established a pro-survival role beyond contractile function, linking reduced ACTC1 expression to congenital heart disease pathogenesis.","evidence":"siRNA knockdown in H9C2 cells with TUNEL, caspase-3 and Bcl-2 readouts, correlated with reduced ACTC1 in human CHD tissue","pmids":["20962418"],"confidence":"Medium","gaps":["Signalling pathway from ACTC1 loss to apoptosis not delineated","Single cell line (H9C2) used"]},{"year":2011,"claim":"Quantitative measurement of the E99K mutation's effect on Ca²⁺ sensitivity and troponin I phosphorylation-dependent regulation in reconstituted thin filaments and transgenic mice provided the first direct mechanistic explanation for HCM-associated hypercontractility.","evidence":"In vitro motility assay on reconstituted thin filaments, skinned papillary muscle mechanics, MRI and ECG in transgenic mice, human carrier samples","pmids":["21622575"],"confidence":"High","gaps":["Structural basis of E99K's effect on troponin I phosphorylation coupling unresolved","Mechanism of transition from hypertrophy to dilation not addressed"]},{"year":2013,"claim":"Calorimetric measurement of the E99K muscle showed that hypercontractility is accompanied by reduced contractile efficiency, establishing energetic maladaptation as a feature of ACTC1-driven HCM.","evidence":"Intact papillary muscle mechanics, myofibril Ca²⁺-jump protocol, heat+work calorimetry in transgenic mice","pmids":["23604709"],"confidence":"High","gaps":["Whether energy inefficiency is a universal feature of ACTC1 HCM mutations or specific to E99K","Mitochondrial compensation mechanisms not examined"]},{"year":2014,"claim":"Demonstration that the DCM-causing E361G mutation specifically uncouples PKA/troponin I phosphorylation-dependent lusitropy without altering sarcomere-length sensitivity distinguished the DCM mechanism from the HCM mechanism at the same molecular level—thin-filament regulation.","evidence":"Ca²⁺-jump protocol in single transgenic mouse myofibrils with propranolol-modulated troponin I phosphorylation","pmids":["25418306"],"confidence":"High","gaps":["Whether this uncoupling is generalizable to other DCM-causing ACTC1 mutations","Structural basis for selective loss of phosphorylation sensitivity not resolved"]},{"year":2016,"claim":"A 3′UTR variant creating a miR-139-5p binding site demonstrated that ACTC1 haploinsufficiency via post-transcriptional regulation can cause atrial septal defect, expanding the mutational mechanism beyond coding-region changes.","evidence":"Luciferase reporter assay with miR-139-5p mimic/inhibitor in transfected cells, whole-genome sequencing of ASD family","pmids":["27139165"],"confidence":"Medium","gaps":["In vivo miR-139-5p levels in developing septum not measured","No animal model confirming ASD from 3′UTR-mediated ACTC1 reduction"]},{"year":2017,"claim":"Age-dependent aberrant Ca²⁺ release (sparks, waves) in young E99K mice during a vulnerable window linked the elevated myofibrillar Ca²⁺ sensitivity to arrhythmogenesis and sudden cardiac death, with genetic background modifying penetrance.","evidence":"Confocal Ca²⁺ imaging in isolated ventricular myocytes, Ca²⁺ spark analysis, histological fibrosis quantification across two inbred backgrounds","pmids":["28887330"],"confidence":"High","gaps":["Molecular basis for age-dependent normalization of Ca²⁺ transients in survivors unknown","Modifier genes underlying background-dependent penetrance not identified"]},{"year":2019,"claim":"Showing that the G247D mutation inhibits actin polymerization and abolishes Rho-GTPase/SRF signalling established a non-contractile signalling axis disrupted in ACTC1-linked DCM and congenital heart defects.","evidence":"In vitro actin polymerization assay, SRF luciferase reporter, Rho-GTPase pull-down, nuclear G-actin fractionation in NRVCMs","pmids":["31434612","31430208"],"confidence":"Medium","gaps":["Whether SRF pathway disruption is a common mechanism in other ACTC1 DCM mutations","In vivo rescue by restoring SRF signalling not attempted"]},{"year":2023,"claim":"Identification of ACTC1 variants causing distal arthrogryposis with congenital heart defects in five families demonstrated that ACTC1 is required in skeletal as well as cardiac muscle, broadening its disease spectrum beyond cardiomyopathy.","evidence":"Exome/genome sequencing with co-segregation analysis across five independent families","pmids":["37457373"],"confidence":"Medium","gaps":["Functional consequence of DA-causing variants on actin polymerization or myosin interaction not tested","Skeletal muscle biopsy data not available for most families"]},{"year":2025,"claim":"Promoter variants reducing ACTC1 transcription were directly validated, and LMOD2 was identified as a physical ACTC1 interactor regulating myogenic differentiation, while an ACTC1–BMP4 axis was established in prostate cancer, extending ACTC1 functions beyond sarcomeric contraction.","evidence":"Luciferase promoter assay and EMSA in HL-1 cardiomyocytes; Co-IP of LMOD2–ACTC1 with LMOD2 KO in C2C12; overexpression/knockdown with BMP4 rescue in prostate cancer xenografts","pmids":["40848833","40745266","41286808"],"confidence":"Medium","gaps":["LMOD2–ACTC1 interaction awaits reciprocal Co-IP and domain mapping","BMP4 mechanism downstream of ACTC1 not delineated","Relevance of ACTC1 in non-muscle tissues in vivo remains preliminary"]},{"year":null,"claim":"Key unresolved questions include the high-resolution structural basis for mutation-specific phenotype determination (HCM versus DCM versus congenital defect), the signalling pathways linking ACTC1 loss to apoptosis, the in vivo relevance of ACTC1 in non-muscle contexts, and whether therapeutic restoration of Ca²⁺ sensitivity or SRF signalling can prevent disease progression.","evidence":"","pmids":[],"confidence":"Low","gaps":["No cryo-EM or crystal structure of mutant ACTC1 in thin filament context","No gene therapy or allele-specific silencing study for ACTC1 cardiomyopathy","Mechanism linking ACTC1 to septal morphogenesis not established beyond expression correlation"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0005198","term_label":"structural molecule activity","supporting_discovery_ids":[0,1,3,4,5]},{"term_id":"GO:0008092","term_label":"cytoskeletal protein binding","supporting_discovery_ids":[2,11,16]}],"localization":[{"term_id":"GO:0005856","term_label":"cytoskeleton","supporting_discovery_ids":[0,1,3,4,11,12]},{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[2]}],"pathway":[{"term_id":"R-HSA-397014","term_label":"Muscle contraction","supporting_discovery_ids":[0,1,3,4,5,15]},{"term_id":"R-HSA-1643685","term_label":"Disease","supporting_discovery_ids":[0,1,3,7,12,15]},{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[11,17]},{"term_id":"R-HSA-5357801","term_label":"Programmed Cell Death","supporting_discovery_ids":[12,13]},{"term_id":"R-HSA-1266738","term_label":"Developmental Biology","supporting_discovery_ids":[6,14,15]}],"complexes":["cardiac sarcomeric thin filament"],"partners":["SLC4A1","LMOD2","TNNC1","TNNI3","MYH7"],"other_free_text":[]},"mechanistic_narrative":"ACTC1 encodes the principal thin-filament actin of the cardiac sarcomere, where its polymerization and interaction with myosin, troponin, and Z-band/intercalated-disc scaffolds govern contraction, relaxation, and force transmission. Disease-causing missense mutations alter distinct functional surfaces: mutations affecting the actin–myosin interface (e.g., M84T, E101K) cause congenital heart defects including atrial septal defect, whereas mutations at force-transmission domains cause dilated cardiomyopathy, and mutations that increase myofibrillar Ca²⁺ sensitivity (e.g., E99K) produce hypertrophic cardiomyopathy with hypercontractility, impaired lusitropy, aberrant Ca²⁺ release, and arrhythmogenesis [PMID:9563954, PMID:10330430, PMID:21622575, PMID:25418306, PMID:23604709]. Heterozygous ACTC1 variants also cause distal arthrogryposis with cardiac defects, establishing a shared requirement in cardiac and skeletal muscle [PMID:37457373]. Polymerization-defective mutations (e.g., G247D) impair Rho-GTPase/SRF signalling and promote cardiomyocyte apoptosis, while reduced ACTC1 expression—whether through promoter variants, 3′UTR miRNA gain-of-function, or epigenetic silencing—is linked to congenital heart disease and septal defects [PMID:31434612, PMID:20962418, PMID:27139165, PMID:40848833]."},"prefetch_data":{"uniprot":{"accession":"P68032","full_name":"Actin, alpha cardiac muscle 1","aliases":["Alpha-cardiac actin"],"length_aa":377,"mass_kda":42.0,"function":"Actins are highly conserved proteins that are involved in various types of cell motility and are ubiquitously expressed in all eukaryotic cells","subcellular_location":"Cytoplasm, cytoskeleton","url":"https://www.uniprot.org/uniprotkb/P68032/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/ACTC1","classification":"Not Classified","n_dependent_lines":1,"n_total_lines":1208,"dependency_fraction":0.0008278145695364238},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[{"gene":"ACTG1","stoichiometry":0.2},{"gene":"CAPZB","stoichiometry":0.2},{"gene":"CTTN","stoichiometry":0.2}],"url":"https://opencell.sf.czbiohub.org/search/ACTC1","total_profiled":1310},"omim":[{"mim_id":"620265","title":"CONGENITAL MYOPATHY 2B, SEVERE INFANTILE, AUTOSOMAL RECESSIVE; CMYO2B","url":"https://www.omim.org/entry/620265"},{"mim_id":"620093","title":"ACTIN MATURATION PROTEASE; ACTMAP","url":"https://www.omim.org/entry/620093"},{"mim_id":"619222","title":"SUPPRESSOR OF CANCER CELL INVASION; SCAI","url":"https://www.omim.org/entry/619222"},{"mim_id":"617135","title":"L3MBTL HISTONE METHYL-LYSINE-BINDING PROTEIN 4; L3MBTL4","url":"https://www.omim.org/entry/617135"},{"mim_id":"615396","title":"LEFT VENTRICULAR NONCOMPACTION 10; LVNC10","url":"https://www.omim.org/entry/615396"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Tissue enriched","tissue_distribution":"Detected in many","driving_tissues":[{"tissue":"heart muscle","ntpm":10840.1}],"url":"https://www.proteinatlas.org/search/ACTC1"},"hgnc":{"alias_symbol":["CMD1R"],"prev_symbol":["ACTC"]},"alphafold":{"accession":"P68032","domains":[{"cath_id":"3.30.420.40","chopping":"9-139_341-374","consensus_level":"medium","plddt":94.7867,"start":9,"end":374},{"cath_id":"3.30.420.40","chopping":"144-181_274-337","consensus_level":"medium","plddt":97.6754,"start":144,"end":337},{"cath_id":"3.90.640.10","chopping":"183-267","consensus_level":"high","plddt":97.1568,"start":183,"end":267}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/P68032","model_url":"https://alphafold.ebi.ac.uk/files/AF-P68032-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-P68032-F1-predicted_aligned_error_v6.png","plddt_mean":95.38},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=ACTC1","jax_strain_url":"https://www.jax.org/strain/search?query=ACTC1"},"sequence":{"accession":"P68032","fasta_url":"https://rest.uniprot.org/uniprotkb/P68032.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/P68032/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/P68032"}},"corpus_meta":[{"pmid":"30392975","id":"PMC_30392975","title":"Isogenic Pairs of hiPSC-CMs with Hypertrophic Cardiomyopathy/LVNC-Associated ACTC1 E99K Mutation Unveil Differential Functional Deficits.","date":"2018","source":"Stem cell reports","url":"https://pubmed.ncbi.nlm.nih.gov/30392975","citation_count":55,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"21622575","id":"PMC_21622575","title":"Molecular mechanism of the E99K mutation in cardiac actin (ACTC Gene) that causes apical hypertrophy in man and mouse.","date":"2011","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/21622575","citation_count":54,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"31430208","id":"PMC_31430208","title":"Cardiac α-Actin (ACTC1) Gene Mutation Causes Atrial-Septal Defects Associated With Late-Onset Dilated Cardiomyopathy.","date":"2019","source":"Circulation. 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PKA-dependent regulation.\",\n      \"method\": \"Ca2+-jump protocol in single transgenic mouse heart myofibrils, propranolol treatment to modulate troponin I phosphorylation levels\",\n      \"journal\": \"Biophysical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — direct myofibril mechanistic assay with mutagenesis and pharmacological controls in intact contractile system\",\n      \"pmids\": [\"25418306\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"ACTC1 E99K mutation causes hypercontractility through higher myofibrillar Ca2+ sensitivity (EC50 reduced ~2-fold) with slower relaxation, but does not alter the amplitude or time course of Ca2+ transients in myocytes, and substantially increases energy cost per unit work.\",\n      \"method\": \"Papillary muscle mechanics, myofibril Ca2+-jump assay, Ca2+ transient imaging, work-loop calorimetry in transgenic mouse model\",\n      \"journal\": \"American journal of physiology. Heart and circulatory physiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — multiple orthogonal methods in intact muscle and isolated myofibrils confirming mechanism\",\n      \"pmids\": [\"23604709\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"The ACTC1 G247D mutation impairs actin polymerization/turnover, reduces GTP-bound Rho-GTPase, increases nuclear localization of globular actin, and thereby inhibits SRF (serum response factor) signaling in neonatal rat cardiomyocytes, providing a molecular mechanism linking actin polymerization defects to DCM.\",\n      \"method\": \"In vitro actin polymerization assay, luciferase reporter assay (SM22-response element), Rho-GTPase activity assay, immunofluorescence for actin localization in NRVCMs, molecular dynamics simulation\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods in single study; mechanistic pathway established in vitro\",\n      \"pmids\": [\"31434612\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"The ACTC1 G247D mutation causes sarcomeric disarray, myofibrillar degeneration, increased apoptosis, and increased extracellular matrix proteins in human cardiac tissue and neonatal rat cardiomyocytes, with confirmed actin polymerization/turnover defects affecting contractility.\",\n      \"method\": \"Ultrastructural electron microscopy of myocardial tissue, cardiac proteomics, neonatal rat ventricular cardiomyocyte overexpression, molecular dynamics, actin polymerization assays\",\n      \"journal\": \"Circulation. Genomic and precision medicine\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal structural and functional assays in single study with patient tissue validation\",\n      \"pmids\": [\"31430208\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"In the ACTC1 E99K mouse model of HCM, young transgenic mice exhibit aberrant Ca2+ waves and increased Ca2+ spark mass with similar sarcoplasmic reticulum Ca2+ content compared to controls, linking increased myofilament Ca2+ sensitivity to arrhythmogenic Ca2+ release and sudden cardiac death; penetrance of sudden cardiac death depends on genetic background.\",\n      \"method\": \"Isolated ventricular myocyte Ca2+ transient and spark imaging, collagen quantification, strain comparison of C57Bl6 vs CBA/Ca backgrounds\",\n      \"journal\": \"American journal of physiology. Heart and circulatory physiology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct cellular Ca2+ imaging with genetic background epistasis in transgenic mouse model\",\n      \"pmids\": [\"28887330\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"ACTC1 E99K mutation in isogenic hiPSC-CMs causes arrhythmogenesis in both 2D and 3D engineered heart tissues; dual dantrolene/ranolazine treatment was most effective at rescuing Ca2+ handling phenotypes, implicating aberrant Ca2+ handling as the primary pharmacologically addressable mechanism.\",\n      \"method\": \"Isogenic hiPSC-CM pairs, 3D engineered heart tissues, 2D monolayers, Ca2+ handling expression studies, pharmacological rescue with dantrolene and ranolazine\",\n      \"journal\": \"Stem cell reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — isogenic pairs with multiple functional readouts and pharmacological rescue in human cell model\",\n      \"pmids\": [\"30392975\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Alpha-cardiac actin (ACTC1) physically binds to the cardiac isoform of band 3 (AE1) anion exchanger; the interaction was detected by yeast two-hybrid and confirmed by reciprocal co-immunoprecipitation from rat heart tissue, with band 3 localizing to intercalated discs by confocal microscopy.\",\n      \"method\": \"Yeast two-hybrid assay, reciprocal co-immunoprecipitation from rat heart, confocal microscopy\",\n      \"journal\": \"Journal of cellular biochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — reciprocal Co-IP from native tissue plus yeast two-hybrid; localization data provided\",\n      \"pmids\": [\"12898519\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"siRNA knockdown of ACTC1 in the H9C2 cardiomyocyte cell line increases apoptosis, associated with increased Caspase-3 and decreased Bcl-2 expression, establishing a role for ACTC1 in cardiomyocyte survival signaling.\",\n      \"method\": \"siRNA knockdown in H9C2 cells, TUNEL assay, Western blotting for Caspase-3 and Bcl-2\",\n      \"journal\": \"Circulation journal\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — siRNA loss-of-function with defined apoptotic readout and molecular marker validation\",\n      \"pmids\": [\"20962418\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"A gain-of-function mutation (c.*1784T>C) in the ACTC1 3'UTR creates a new target site for miR-139-5p; functional luciferase assay demonstrates that this mutation downregulates ACTC1 expression, and miR-139-5p mimic further reduces expression while miR-139-5p inhibitor rescues it, identifying miR-139-5p as a negative regulator of ACTC1 via this 3'UTR site.\",\n      \"method\": \"Luciferase reporter assay with 3'UTR constructs, miR-139-5p mimic and inhibitor transfection\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct functional validation of UTR mutation with miRNA gain- and loss-of-function experiments\",\n      \"pmids\": [\"27139165\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"ACTC1 knockdown by siRNA in U87MG glioblastoma cells significantly reduces cell migration distance (from ~3600 μm to ~1265 μm over 72 h), establishing a role for ACTC1 in cancer cell migration.\",\n      \"method\": \"siRNA knockdown, time-lapse cell migration tracking assay, immunocytochemistry, ddPCR\",\n      \"journal\": \"Journal of the neurological sciences\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — siRNA knockdown with quantitative migration phenotype in single study\",\n      \"pmids\": [\"30055382\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Structural analysis of ACTC1 mutations causing congenital heart defects (M84T, E101K, M125V) maps them to the actin surface in close contact with myosin heavy chain head domain, whereas cardiomyopathy-causing mutations (A297S, D313H, R314H) localize to a distinct interaction surface, suggesting that the clinical consequence of ACTC1 mutation depends on the actin-myosin interaction interface affected.\",\n      \"method\": \"Linkage analysis, missense mutation mapping onto published actin-myosin crystal structure, segregation analysis\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 4 — structural inference based on published crystal structure without direct biochemical validation of the interaction\",\n      \"pmids\": [\"26061005\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"LMOD2 physically interacts with ACTC1 (confirmed by Co-IP) and together they regulate myogenic differentiation; LMOD2 knockout alters muscle fiber types and suppresses muscle contraction-related gene expression in C2C12 cells.\",\n      \"method\": \"Co-immunoprecipitation, RNA-seq, siRNA knockdown, protein prediction, C2C12 cell differentiation assays, lentivirus-mediated knockdown in vivo\",\n      \"journal\": \"BMC genomics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — Co-IP interaction confirmed with functional KO phenotype; single study\",\n      \"pmids\": [\"40745266\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Actc1 expression in early adult skeletal muscle varies up to 24-fold across mouse strains and negatively correlates with promoter methylation at the transcriptional start site, while histone modifications and chromatin accessibility are unchanged, identifying DNA methylation as a key regulatory mechanism of ACTC1 expression.\",\n      \"method\": \"Expression QTL mapping in Collaborative Cross strains, bisulfite methylation analysis, histone ChIP, ATAC-seq, RT-qPCR\",\n      \"journal\": \"Biochimica et biophysica acta. Gene regulatory mechanisms\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple epigenomic methods across many strains with consistent correlation; mechanistic link between methylation and expression\",\n      \"pmids\": [\"28847732\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"ACTC1 promotes prostate cancer cell proliferation and migration; ACTC1 knockdown suppresses these malignant behaviors and reduces BMP4 expression, while BMP4 overexpression rescues the inhibitory effects of ACTC1 knockdown, placing ACTC1 upstream of BMP4 in a tumor-promoting pathway.\",\n      \"method\": \"siRNA knockdown, overexpression, xenograft experiments, transcriptomic analysis, rescue with BMP4 overexpression\",\n      \"journal\": \"BMC cancer\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — loss-of-function with rescue experiment establishing pathway position; single study\",\n      \"pmids\": [\"41286808\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Variants at ACTC1 residues Gly57 and Glu101 alter ATP binding or putative protein-protein interaction surfaces (protein structure analysis), and in vivo zebrafish validation confirms their pathogenicity and impact on cranial tissue development, expanding ACTC1 function to extracardiac developmental contexts.\",\n      \"method\": \"Protein structure analysis, in vivo zebrafish morphology assay\",\n      \"journal\": \"Clinical genetics\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — structural modeling plus zebrafish in vivo without detailed biochemical mechanistic validation\",\n      \"pmids\": [\"40421724\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"ACTC1 promoter variants found in VSD patients alter transcriptional activity in mouse cardiomyocytes (HL-1 cells) and affect binding of transcription factors as demonstrated by EMSA, suggesting that reduced ACTC1 transcription contributes to VSD pathogenesis.\",\n      \"method\": \"Luciferase reporter assay in HL-1 cardiomyocytes, EMSA, Sanger sequencing of patient samples, JASPAR database analysis\",\n      \"journal\": \"Gene\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — functional promoter assay with EMSA in cardiac cells; mechanistic link between transcription factor binding and ACTC1 expression established\",\n      \"pmids\": [\"40848833\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"In a zebrafish model carrying the orthologous ACTC1 p.T126I (Acta1b p.T126I) DCM mutation, mutants develop progressive dilated cardiomyopathy with sex-specific differences: female mutants show earlier diastolic dysfunction, more severe remodeling, and lower survival, associated with sex-specific alterations in calcium handling genes (serca2, pln1, slc8a1a) and proteostasis regulators (hsf1, bag3).\",\n      \"method\": \"Zebrafish transgenic model, longitudinal cardiac phenotyping by echocardiography, gene expression profiling, sex-stratified analysis\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — in vivo zebrafish model with multiple functional and molecular readouts; preprint, not yet peer-reviewed\",\n      \"pmids\": [\"bio_10.1101_2025.08.26.672352\"],\n      \"is_preprint\": true\n    }\n  ],\n  \"current_model\": \"ACTC1 (cardiac α-actin) is a core thin filament component that interacts with myosin to generate sarcomeric force; disease-causing mutations (E99K, E361G, G247D) primarily act by increasing myofibrillar Ca2+ sensitivity and/or uncoupling PKA/troponin I phosphorylation-dependent modulation of relaxation, while the G247D DCM mutation additionally impairs actin polymerization, reduces Rho-GTPase activity, and inhibits SRF transcriptional signaling, collectively establishing that ACTC1 mutations disrupt both the mechanical and regulatory aspects of cardiac contractility through distinct mechanisms depending on their location in the actin-myosin or actin-troponin interaction interfaces.\"\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\n```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 1998,\n      \"finding\": \"Missense mutations in the cardiac actin gene (ACTC) that affect universally conserved amino acids in domains attaching to Z bands and intercalated discs co-segregate with hereditary dilated cardiomyopathy (IDC), suggesting that defective force transmission from cardiac myocytes is a mechanism underlying heart failure.\",\n      \"method\": \"Genetic linkage analysis and direct sequencing in two unrelated familial IDC pedigrees\",\n      \"journal\": \"Science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic co-segregation in two independent families, foundational discovery replicated by subsequent studies\",\n      \"pmids\": [\"9563954\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"The ACTC gene (alpha-cardiac actin) is identified as a novel disease gene for familial hypertrophic cardiomyopathy (HCM), with an Ala295Ser mutation in exon 5; ACTC is the first sarcomeric gene in which mutations cause two distinct cardiomyopathies (HCM and dilated), leading to the hypothesis that HCM-causing mutations affect sarcomere contraction while DCM-causing mutations affect force transmission.\",\n      \"method\": \"Linkage analysis, candidate gene sequencing, lod score analysis in a family pedigree\",\n      \"journal\": \"The Journal of Clinical Investigation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic co-segregation with high lod score, foundational discovery widely replicated\",\n      \"pmids\": [\"10330430\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Alpha-cardiac actin (ACTC) physically binds to the cardiac isoform of band 3 (AE1 anion exchanger); the interaction was identified by yeast two-hybrid and confirmed by reciprocal co-immunoprecipitation from rat heart; confocal microscopy localised band 3 to the intercalated disc, suggesting the interaction occurs at the site of sarcomere attachment to the plasma membrane.\",\n      \"method\": \"Yeast two-hybrid, co-immunoprecipitation (reciprocal), confocal microscopy\",\n      \"journal\": \"Journal of Cellular Biochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal Co-IP plus localization, single lab study\",\n      \"pmids\": [\"12898519\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"The HCM-causing ACTC E99K mutation increases myofibrillar Ca²⁺ sensitivity ~2.3-fold in transgenic mouse reconstituted thin filaments (in vitro motility assay) and ~1.3-fold in human carrier samples; it also abolishes the normal change in Ca²⁺ sensitivity linked to troponin I phosphorylation. Transgenic mice exhibit apical hypertrophy, sudden cardiac death, atrial flutter, reduced β-adrenergic contractile response, and eventual dilated cardiomyopathy.\",\n      \"method\": \"In vitro motility assay on reconstituted thin filaments, skinned papillary muscle mechanics, cardiac MRI, ECG, transgenic mouse model\",\n      \"journal\": \"The Journal of Biological Chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstituted thin filament assay with multiple orthogonal methods (motility, skinned muscle, MRI, ECG) in both mouse model and human samples\",\n      \"pmids\": [\"21622575\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"ACTC E99K papillary muscles produce 3–4-fold greater force than non-transgenic controls under isometric conditions, relax ~1.4-fold slower, and show ~2-fold higher myofibrillar Ca²⁺ sensitivity (EC₅₀ 0.39 vs 0.80 μmol/l); energy turnover is disproportionately elevated relative to work produced (efficiency 11–16% vs 15–18%), establishing hypercontractility driven by elevated myofibrillar Ca²⁺ sensitivity as the primary deficit.\",\n      \"method\": \"Intact papillary muscle mechanics, myofibril Ca²⁺-jump protocol, heat+work calorimetry, Ca²⁺ imaging in isolated myocytes\",\n      \"journal\": \"American Journal of Physiology. Heart and Circulatory Physiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — multiple orthogonal quantitative assays (mechanics, calorimetry, Ca²⁺ imaging) in transgenic mouse model\",\n      \"pmids\": [\"23604709\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"The DCM-causing mutation ACTC E361G specifically abolishes troponin I phosphorylation-dependent modulation of Ca²⁺ sensitivity and relaxation kinetics in intact cardiac myofibrils, without affecting sarcomere-length-dependent activation or response to EMD57033; this demonstrates uncoupling of PKA-mediated lusitropy in an intact contractile system.\",\n      \"method\": \"Ca²⁺-jump protocol in single transgenic mouse cardiac myofibrils, propranolol treatment to vary troponin I phosphorylation levels\",\n      \"journal\": \"Biophysical Journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — direct measurement in native myofibrils with pharmacological manipulation of phosphorylation state, rigorous controls\",\n      \"pmids\": [\"25418306\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"A mutation in the 3'UTR of ACTC1 (c.*1784T>C) creates a novel target site for miR-139-5p, reducing ACTC1 gene expression; luciferase reporter assays confirm the mutation specifically downregulates expression, and miR-139-5p mimic/inhibitor modulate this effect, suggesting haploinsufficiency via a gain-of-function miRNA regulatory mechanism as a cause of familial ASD.\",\n      \"method\": \"Whole genome sequencing, luciferase reporter assay, miR-139-5p mimic and inhibitor transfection\",\n      \"journal\": \"Scientific Reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — functional luciferase assay with orthogonal miRNA manipulation, single lab\",\n      \"pmids\": [\"27139165\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Young ACTC E99K transgenic mice in their vulnerable period (28–40 days) exhibit elevated Ca²⁺ transients, increased Ca²⁺ spark mass, and greater propensity for spontaneous Ca²⁺ waves compared to non-transgenic littermates, despite similar sarcoplasmic reticulum Ca²⁺ content; these aberrant Ca²⁺ release events are associated with sudden cardiac death and increased myocardial fibrosis. Adult survivors normalize Ca²⁺ transients. Penetrance of sudden death depends on genetic background (CBA/Ca vs C57Bl6).\",\n      \"method\": \"Isolated ventricular myocyte Ca²⁺ imaging (confocal), Ca²⁺ spark analysis, histology (collagen quantification), transgenic mouse model with two genetic backgrounds\",\n      \"journal\": \"American Journal of Physiology. Heart and Circulatory Physiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — direct cellular Ca²⁺ measurements with defined phenotypic readout, multiple genetic backgrounds tested\",\n      \"pmids\": [\"28887330\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Variable expression of cardiac α-actin (Actc1) in early adult skeletal muscle across Collaborative Cross strains (up to 24-fold) is negatively correlated with promoter methylation at the Actc1 transcriptional start site in a strain-dependent manner, while histone modifications and chromatin accessibility are unaltered; eQTL mapping confirms a cis-acting regulatory locus at the Actc1 locus in both heart and skeletal muscle.\",\n      \"method\": \"Expression QTL mapping, bisulfite sequencing (methylation), ATAC-seq/chromatin accessibility, histone ChIP across Collaborative Cross mouse strains\",\n      \"journal\": \"Biochimica et Biophysica Acta. Gene Regulatory Mechanisms\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal epigenomic methods correlating methylation with expression, single study\",\n      \"pmids\": [\"28847732\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"hiPSC-derived cardiomyocytes carrying the E99K-ACTC1 mutation exhibit arrhythmogenesis in both 3D engineered heart tissues and 2D monolayers; Ca²⁺ handling expression studies informed pharmacological rescue, wherein dual dantrolene/ranolazine treatment was most effective, establishing E99K mutant ACTC1 protein as a primary driver of arrhythmia with Ca²⁺ handling as a central mechanism.\",\n      \"method\": \"Isogenic hiPSC-CM pairs, 3D engineered heart tissue, 2D monolayer Ca²⁺ imaging, pharmacological rescue (dantrolene/ranolazine)\",\n      \"journal\": \"Stem Cell Reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — isogenic human cell model with defined functional readout and pharmacological rescue, single study\",\n      \"pmids\": [\"30392975\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Knockdown of ACTC1 by siRNA in the U87MG glioblastoma cell line significantly inhibits cell migration (distance travelled reduced from ~3600 μm to ~1265 μm over 72 h), demonstrating a functional role for ACTC1 in tumour cell motility.\",\n      \"method\": \"siRNA knockdown, time-lapse cell migration tracking assay, droplet digital PCR, immunocytochemistry\",\n      \"journal\": \"Journal of the Neurological Sciences\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — siRNA KD with quantitative phenotypic readout, single lab, single cell line\",\n      \"pmids\": [\"30055382\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"The ACTC1 p.Gly247Asp (G247D) mutation inhibits actin polymerization (confirmed by in vitro polymerization assay) and impairs Rho-GTPase/SRF signalling: overexpression of native ACTC1 strongly activates SRF-driven luciferase in NRVCMs whereas G247D abolishes this; mutant ACTC1 shows reduced GTP-bound Rho-GTPase and increased nuclear accumulation of globular actin, establishing defective actin polymerization as the mechanism linking G247D to impaired SRF signalling and DCM.\",\n      \"method\": \"In vitro actin polymerization assay, luciferase reporter assay (SRF/SM22-RE), Rho-GTPase pull-down (active Rho), nuclear fractionation and G-actin immunoblot, overexpression in NRVCMs\",\n      \"journal\": \"Biochemical and Biophysical Research Communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1/2 — in vitro polymerization plus multiple cellular assays; single lab\",\n      \"pmids\": [\"31434612\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"A heterozygous ACTC1 p.Gly247Asp mutation causes ultrastructural sarcomeric disarray, myofibrillar degeneration, and increased apoptosis in human myocardial tissue; in neonatal rat ventricular cardiomyocytes, overexpression of the mutant (but not wild-type) ACTC1 causes structural defects and apoptosis; molecular dynamics and polymerization assays confirm actin polymerization/turnover defects, implicating defective actin signalling in both cardiac developmental defects (ASD) and contractile dysfunction (DCM).\",\n      \"method\": \"Ultrastructural electron microscopy, cardiac proteomics, NRVCM overexpression, molecular dynamics simulation, actin polymerization assay, TUNEL apoptosis assay\",\n      \"journal\": \"Circulation. Genomic and Precision Medicine\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (EM, proteomics, functional cell assays, MD), single study\",\n      \"pmids\": [\"31430208\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Reduced ACTC1 expression in sporadic congenital heart disease samples correlates with increased cardiomyocyte apoptosis; siRNA-mediated knockdown of Actc1 in H9C2 cardiomyocyte cell line increases apoptosis with increased Caspase-3 and decreased Bcl-2 expression, indicating ACTC1 promotes cardiomyocyte survival.\",\n      \"method\": \"RT-PCR, Western blot, immunohistochemistry, TUNEL assay, siRNA knockdown in H9C2 cells\",\n      \"journal\": \"Circulation Journal\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — siRNA KD with defined apoptosis readout validated in human tissue and cell line, single lab\",\n      \"pmids\": [\"20962418\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"ACTC1 mutations causing congenital heart defects (p.Met84Thr, p.Glu101Lys, p.Met125Val) cluster in a region of actin in close apposition to the myosin heavy chain head domain, whereas mutations causing cardiomyopathies (p.Ala297Ser, p.Asp313His, p.Arg314His) map to a distinct interaction surface; this spatial distinction suggests that the clinical phenotypic consequence of an ACTC1 mutation is partly determined by which actin–myosin interaction surface is disrupted.\",\n      \"method\": \"Linkage analysis, Sanger sequencing, structural mapping of mutations onto actin–myosin complex structure\",\n      \"journal\": \"PLoS One\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 4 for mechanism — structural inference from known crystal structure without direct functional validation of interface\",\n      \"pmids\": [\"26061005\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Heterozygous missense variants in ACTC1 cause distal arthrogryposis (DA) with congenital heart defects in five families, demonstrating that ACTC1 function is shared in both cardiac and skeletal muscle; the finding establishes ACTC1 as the first gene underlying DA that also causes cardiac abnormalities.\",\n      \"method\": \"Exome/genome sequencing, familial co-segregation analysis across five independent families\",\n      \"journal\": \"HGG Advances\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis/co-segregation in five independent families, functional implication in both muscle types\",\n      \"pmids\": [\"37457373\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"LMOD2 (leiomodin-2) physically interacts with ACTC1 as confirmed by Co-IP; LMOD2 knockout in C2C12 myoblasts alters muscle fiber type composition and inhibits myoblast proliferation, placing the LMOD2–ACTC1 interaction in the regulation of myogenic differentiation.\",\n      \"method\": \"Co-immunoprecipitation, RNA-seq, LMOD2 knockout in C2C12 cells, lentiviral knockdown in vivo\",\n      \"journal\": \"BMC Genomics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — single Co-IP with supporting transcriptomic and functional KO data, single lab\",\n      \"pmids\": [\"40745266\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"ACTC1 overexpression in prostate cancer cells promotes proliferation and migration, while knockdown suppresses these behaviors; transcriptomic analysis identifies BMP4 as a key downstream effector, and BMP4 overexpression rescues the inhibitory effects of ACTC1 knockdown, establishing an ACTC1–BMP4 signalling axis in prostate cancer progression.\",\n      \"method\": \"Overexpression and siRNA knockdown in prostate cancer cell lines, xenograft mouse model, RNA-seq, BMP4 rescue experiment\",\n      \"journal\": \"BMC Cancer\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — loss- and gain-of-function with rescue experiment and in vivo validation, single lab\",\n      \"pmids\": [\"41286808\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"The zebrafish Acta1b p.T126I mutation (orthologous to human ACTC1 T126I) causes progressive dilated cardiomyopathy with sex-specific differences: female mutants show earlier diastolic dysfunction, more severe cardiac remodeling, and lower survival than males; molecular profiling reveals sex-specific alterations in calcium handling genes (serca2, pln1, slc8a1a) and proteostasis regulators (hsf1, bag3), with upregulation of the stress marker nppb and downregulation of gata4/mef2ca.\",\n      \"method\": \"CRISPR-generated zebrafish knock-in model, longitudinal echocardiography, histology, sex-stratified gene expression profiling\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — in vivo vertebrate model with longitudinal functional assessment and molecular profiling; preprint, not yet peer-reviewed\",\n      \"pmids\": [\"bio_10.1101_2025.08.26.672352\"],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Variants in ACTC1 promoter region alter ACTC1 transcriptional activity as measured by luciferase assay in mouse cardiomyocytes (HL-1); EMSA and JASPAR analysis indicate the variants affect transcription factor binding, linking promoter variants to reduced ACTC1 expression and ventricular septal defect pathogenesis.\",\n      \"method\": \"Sanger sequencing, luciferase promoter activity assay in HL-1 cells, electrophoretic mobility shift assay (EMSA)\",\n      \"journal\": \"Gene\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct functional promoter assay and EMSA in cardiomyocytes, single lab\",\n      \"pmids\": [\"40848833\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"ACTC1 (cardiac α-actin) is a sarcomeric thin-filament protein whose polymerization state and interaction with myosin and troponin govern cardiac and skeletal muscle contractility: disease-causing mutations (e.g. E99K, E361G, G247D) increase myofibrillar Ca²⁺ sensitivity, uncouple PKA/troponin I phosphorylation-dependent lusitropy, impair actin polymerization and Rho-GTPase/SRF signalling, and disrupt force transmission at Z-bands and intercalated discs, thereby causing hypertrophic or dilated cardiomyopathy, atrial septal defect, or distal arthrogryposis depending on which actin–myosin interaction surface is affected; ACTC1 also binds the cardiac band 3 anion exchanger at intercalated discs, interacts with LMOD2 to regulate myogenic differentiation, and modulates cell migration and tumour-relevant BMP4 signalling.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"ACTC1 encodes cardiac α-actin, a principal thin filament protein of the sarcomere that directly engages myosin to generate contractile force in the heart. Disease-causing missense mutations act through at least two distinct mechanisms: HCM-associated mutations such as E99K increase myofibrillar Ca²⁺ sensitivity (∼2-fold EC50 reduction), slow relaxation, raise energy cost per unit work, and promote arrhythmogenic Ca²⁺ waves, while simultaneously abolishing the normal troponin I phosphorylation-dependent modulation of Ca²⁺ sensitivity [PMID:21622575, PMID:23604709, PMID:28887330]; DCM-associated mutations such as E361G selectively uncouple PKA-mediated regulation without affecting length-dependent activation [PMID:25418306], and G247D impairs actin polymerization, reduces Rho-GTPase activity, inhibits SRF transcriptional signaling, and causes sarcomeric disarray and apoptosis [PMID:31434612, PMID:31430208]. Mutations in ACTC1 cause familial hypertrophic cardiomyopathy, dilated cardiomyopathy, and congenital heart defects including ventricular septal defects, with the clinical phenotype determined by whether the mutation disrupts the actin–myosin, actin–troponin, or actin polymerization interface [PMID:26061005, PMID:40848833].\",\n  \"teleology\": [\n    {\n      \"year\": 2003,\n      \"claim\": \"Establishing that cardiac α-actin has binding partners beyond the sarcomeric machinery, ACTC1 was shown to physically interact with the cardiac isoform of the band 3 anion exchanger at intercalated discs, raising the possibility that ACTC1 participates in membrane-associated signaling or ion transport scaffolding.\",\n      \"evidence\": \"Yeast two-hybrid screen, reciprocal co-immunoprecipitation from rat heart tissue, and confocal microscopy localizing band 3 to intercalated discs\",\n      \"pmids\": [\"12898519\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Functional consequence of the ACTC1–band 3 interaction on ion transport or signaling is unknown\", \"Not replicated in independent studies\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Loss-of-function experiments revealed that ACTC1 is required for cardiomyocyte survival, as its knockdown activates caspase-3-dependent apoptosis, suggesting that ACTC1 deficiency itself—not only missense mutations—can damage the myocardium.\",\n      \"evidence\": \"siRNA knockdown in H9C2 cells with TUNEL assay and Western blot for Caspase-3 and Bcl-2\",\n      \"pmids\": [\"20962418\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanism connecting actin loss to apoptotic signaling is undefined\", \"Validated only in an immortalized cell line, not primary cardiomyocytes\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"The central mechanistic question for ACTC1 HCM—whether mutations cause hypercontractility via Ca²⁺ sensitivity or altered force generation—was answered: the E99K mutation increases thin filament Ca²⁺ sensitivity 2.3-fold and abolishes troponin I phosphorylation-dependent modulation, directly linking myofilament Ca²⁺ sensitization to HCM pathogenesis.\",\n      \"evidence\": \"In vitro motility assay on reconstituted thin filaments, skinned papillary muscle preparations, and transgenic mouse phenotyping by MRI/ECG\",\n      \"pmids\": [\"21622575\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis for how E99K alters the actin–troponin interface was not resolved\", \"Whether all HCM-causing ACTC1 mutations share this mechanism was unknown\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Building on the Ca²⁺ sensitivity finding, it was demonstrated that E99K hypercontractility does not arise from altered Ca²⁺ transients but from myofilament-level changes that also slow relaxation and substantially increase energetic cost per unit work, explaining how HCM mutations can cause both diastolic dysfunction and metabolic stress.\",\n      \"evidence\": \"Papillary muscle mechanics, myofibril Ca²⁺-jump assay, Ca²⁺ transient imaging, and work-loop calorimetry in transgenic mice\",\n      \"pmids\": [\"23604709\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether increased energy cost causally contributes to pathological remodeling was not tested\", \"Long-term metabolic consequences in vivo remain uncharacterized\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"The mechanism of DCM-causing ACTC1 mutations was distinguished from HCM mutations: E361G selectively uncouples PKA/troponin I phosphorylation-dependent Ca²⁺ sensitivity modulation without affecting length-dependent activation, establishing that DCM and HCM mutations perturb different regulatory axes of the same contractile apparatus.\",\n      \"evidence\": \"Ca²⁺-jump protocol in single myofibrils from transgenic mouse hearts with propranolol-mediated phosphorylation modulation\",\n      \"pmids\": [\"25418306\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether the E361G defect is sufficient to explain DCM or whether secondary remodeling is required was not resolved\", \"Direct structural evidence for altered troponin–actin contact is lacking\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Structural mapping showed that congenital heart defect mutations (M84T, E101K, M125V) cluster on the actin surface contacting the myosin head, whereas cardiomyopathy mutations localize to a distinct surface, providing a structural rationale for genotype–phenotype correlations.\",\n      \"evidence\": \"Linkage/segregation analysis combined with mutation mapping onto published actin–myosin crystal structure\",\n      \"pmids\": [\"26061005\"],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No direct biochemical validation of altered protein–protein interaction at these sites\", \"Structural inference based on static crystal structure without dynamic or functional confirmation\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"The arrhythmogenic mechanism downstream of Ca²⁺ sensitization was identified: E99K mice develop aberrant Ca²⁺ waves and increased spark mass without altered SR content, directly linking myofilament Ca²⁺ hypersensitivity to sudden cardiac death risk, with genetic background modifying penetrance.\",\n      \"evidence\": \"Isolated ventricular myocyte Ca²⁺ spark/transient imaging across two inbred mouse strain backgrounds\",\n      \"pmids\": [\"28887330\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Specific modifier genes responsible for background-dependent penetrance are unidentified\", \"Whether Ca²⁺ wave suppression would prevent sudden death was not tested\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"At the epigenetic level, ACTC1 expression was shown to be regulated by promoter DNA methylation at the transcriptional start site, with up to 24-fold expression variation across mouse strains, independent of histone modifications or chromatin accessibility.\",\n      \"evidence\": \"Expression QTL mapping across Collaborative Cross mouse strains, bisulfite sequencing, histone ChIP, ATAC-seq\",\n      \"pmids\": [\"28847732\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether DNA methylation causally drives expression differences (e.g., via demethylation experiments) was not directly tested\", \"Relevance to human ACTC1 regulation is inferred, not demonstrated\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Human disease relevance of the E99K Ca²⁺ mechanism was confirmed in isogenic hiPSC-cardiomyocytes, where arrhythmogenesis was pharmacologically rescued by combined dantrolene/ranolazine treatment, validating aberrant Ca²⁺ handling as a druggable target.\",\n      \"evidence\": \"Isogenic hiPSC-CM pairs in 2D monolayers and 3D engineered heart tissues with pharmacological rescue\",\n      \"pmids\": [\"30392975\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"In vivo efficacy of dual drug treatment was not assessed\", \"Whether other HCM-causing ACTC1 mutations respond similarly is unknown\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"A second DCM mechanism was uncovered for the G247D mutation: impaired actin polymerization reduces GTP-Rho and SRF transcriptional signaling, and causes sarcomeric disarray, myofibrillar degeneration, and increased apoptosis, showing that some ACTC1 mutations act upstream of gene regulatory networks rather than solely at the contractile level.\",\n      \"evidence\": \"In vitro actin polymerization assays, Rho-GTPase activity assays, SRF-dependent luciferase reporter, electron microscopy of patient tissue, neonatal rat cardiomyocyte overexpression\",\n      \"pmids\": [\"31434612\", \"31430208\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether SRF pathway disruption is a primary driver of DCM or secondary to actin depolymerization is unresolved\", \"In vivo animal model for G247D is lacking\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"ACTC1 function was expanded to extracardiac contexts: promoter variants reducing ACTC1 transcription were linked to ventricular septal defects, ACTC1 was shown to promote prostate cancer proliferation/migration upstream of BMP4, and LMOD2 was identified as a physical interaction partner involved in myogenic differentiation.\",\n      \"evidence\": \"Luciferase reporter and EMSA in HL-1 cells for promoter variants; siRNA/overexpression with BMP4 rescue in prostate cancer cells and xenografts; Co-IP and RNA-seq for LMOD2 interaction in C2C12 cells\",\n      \"pmids\": [\"40848833\", \"41286808\", \"40745266\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"ACTC1–BMP4 axis mechanism is undefined beyond epistasis\", \"LMOD2 interaction validated only by single Co-IP without reciprocal pull-down\", \"In vivo validation of promoter variants in cardiac development is lacking\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include the high-resolution structural basis by which specific ACTC1 mutations differentially alter actin–troponin versus actin–myosin interfaces, whether the SRF signaling defect of polymerization-impaired mutants is a universal feature of DCM-causing ACTC1 variants, and whether pharmacological Ca²⁺ desensitization or combined ion channel modulation can prevent disease progression in vivo across different mutation classes.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No high-resolution cryo-EM or crystal structure of mutant ACTC1 in complex with troponin or myosin\", \"No systematic comparison of all known ACTC1 mutations in a single in vivo model\", \"In vivo therapeutic trials targeting Ca²⁺ sensitivity or SRF signaling for ACTC1 mutations are absent\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0005198\", \"supporting_discovery_ids\": [0, 1, 2, 3, 4]},\n      {\"term_id\": \"GO:0008092\", \"supporting_discovery_ids\": [0, 1, 7, 12]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005856\", \"supporting_discovery_ids\": [0, 2, 3, 4]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-397014\", \"supporting_discovery_ids\": [0, 1, 2, 5, 6]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [3, 9]},\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [0, 1, 3, 4, 5, 11]},\n      {\"term_id\": \"R-HSA-5357801\", \"supporting_discovery_ids\": [4, 8]}\n    ],\n    \"complexes\": [\n      \"sarcomeric thin filament\"\n    ],\n    \"partners\": [\n      \"LMOD2\",\n      \"SLC4A1\",\n      \"TNNI3\",\n      \"BMP4\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\n```json\n{\n  \"mechanistic_narrative\": \"ACTC1 encodes the principal thin-filament actin of the cardiac sarcomere, where its polymerization and interaction with myosin, troponin, and Z-band/intercalated-disc scaffolds govern contraction, relaxation, and force transmission. Disease-causing missense mutations alter distinct functional surfaces: mutations affecting the actin–myosin interface (e.g., M84T, E101K) cause congenital heart defects including atrial septal defect, whereas mutations at force-transmission domains cause dilated cardiomyopathy, and mutations that increase myofibrillar Ca²⁺ sensitivity (e.g., E99K) produce hypertrophic cardiomyopathy with hypercontractility, impaired lusitropy, aberrant Ca²⁺ release, and arrhythmogenesis [PMID:9563954, PMID:10330430, PMID:21622575, PMID:25418306, PMID:23604709]. Heterozygous ACTC1 variants also cause distal arthrogryposis with cardiac defects, establishing a shared requirement in cardiac and skeletal muscle [PMID:37457373]. Polymerization-defective mutations (e.g., G247D) impair Rho-GTPase/SRF signalling and promote cardiomyocyte apoptosis, while reduced ACTC1 expression—whether through promoter variants, 3′UTR miRNA gain-of-function, or epigenetic silencing—is linked to congenital heart disease and septal defects [PMID:31434612, PMID:20962418, PMID:27139165, PMID:40848833].\",\n  \"teleology\": [\n    {\n      \"year\": 1998,\n      \"claim\": \"Establishing that ACTC1 mutations cause dilated cardiomyopathy answered whether sarcomeric actin itself—not just myosin or regulatory proteins—could drive heart failure through defective force transmission.\",\n      \"evidence\": \"Genetic linkage and sequencing in two familial DCM pedigrees identifying missense mutations at Z-band/intercalated-disc attachment domains\",\n      \"pmids\": [\"9563954\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No direct measurement of force transmission deficit\", \"Biochemical mechanism of each mutation unresolved\"]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"Discovery that a different ACTC1 mutation causes HCM established that the same gene could underlie two opposing cardiomyopathies, generating the hypothesis that mutation location—contraction-affecting versus force-transmission-affecting—determines phenotype.\",\n      \"evidence\": \"Linkage analysis and candidate gene sequencing with significant lod score in an HCM family\",\n      \"pmids\": [\"10330430\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Functional consequence of Ala295Ser on contractile parameters not yet measured\", \"No direct structural mapping to actin–myosin interface\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Identification of a physical ACTC1–band 3 (AE1) interaction at intercalated discs revealed a non-myosin binding partner, suggesting ACTC1 participates in membrane-cytoskeletal coupling beyond pure force generation.\",\n      \"evidence\": \"Yeast two-hybrid screen with reciprocal co-immunoprecipitation from rat heart and confocal colocalization\",\n      \"pmids\": [\"12898519\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Functional consequence of disrupting the ACTC1–band 3 interaction unknown\", \"Not tested with DCM-causing mutations mapping to the intercalated disc domain\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Demonstration that ACTC1 knockdown increases cardiomyocyte apoptosis established a pro-survival role beyond contractile function, linking reduced ACTC1 expression to congenital heart disease pathogenesis.\",\n      \"evidence\": \"siRNA knockdown in H9C2 cells with TUNEL, caspase-3 and Bcl-2 readouts, correlated with reduced ACTC1 in human CHD tissue\",\n      \"pmids\": [\"20962418\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Signalling pathway from ACTC1 loss to apoptosis not delineated\", \"Single cell line (H9C2) used\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Quantitative measurement of the E99K mutation's effect on Ca²⁺ sensitivity and troponin I phosphorylation-dependent regulation in reconstituted thin filaments and transgenic mice provided the first direct mechanistic explanation for HCM-associated hypercontractility.\",\n      \"evidence\": \"In vitro motility assay on reconstituted thin filaments, skinned papillary muscle mechanics, MRI and ECG in transgenic mice, human carrier samples\",\n      \"pmids\": [\"21622575\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of E99K's effect on troponin I phosphorylation coupling unresolved\", \"Mechanism of transition from hypertrophy to dilation not addressed\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Calorimetric measurement of the E99K muscle showed that hypercontractility is accompanied by reduced contractile efficiency, establishing energetic maladaptation as a feature of ACTC1-driven HCM.\",\n      \"evidence\": \"Intact papillary muscle mechanics, myofibril Ca²⁺-jump protocol, heat+work calorimetry in transgenic mice\",\n      \"pmids\": [\"23604709\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether energy inefficiency is a universal feature of ACTC1 HCM mutations or specific to E99K\", \"Mitochondrial compensation mechanisms not examined\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Demonstration that the DCM-causing E361G mutation specifically uncouples PKA/troponin I phosphorylation-dependent lusitropy without altering sarcomere-length sensitivity distinguished the DCM mechanism from the HCM mechanism at the same molecular level—thin-filament regulation.\",\n      \"evidence\": \"Ca²⁺-jump protocol in single transgenic mouse myofibrils with propranolol-modulated troponin I phosphorylation\",\n      \"pmids\": [\"25418306\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether this uncoupling is generalizable to other DCM-causing ACTC1 mutations\", \"Structural basis for selective loss of phosphorylation sensitivity not resolved\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"A 3′UTR variant creating a miR-139-5p binding site demonstrated that ACTC1 haploinsufficiency via post-transcriptional regulation can cause atrial septal defect, expanding the mutational mechanism beyond coding-region changes.\",\n      \"evidence\": \"Luciferase reporter assay with miR-139-5p mimic/inhibitor in transfected cells, whole-genome sequencing of ASD family\",\n      \"pmids\": [\"27139165\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"In vivo miR-139-5p levels in developing septum not measured\", \"No animal model confirming ASD from 3′UTR-mediated ACTC1 reduction\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Age-dependent aberrant Ca²⁺ release (sparks, waves) in young E99K mice during a vulnerable window linked the elevated myofibrillar Ca²⁺ sensitivity to arrhythmogenesis and sudden cardiac death, with genetic background modifying penetrance.\",\n      \"evidence\": \"Confocal Ca²⁺ imaging in isolated ventricular myocytes, Ca²⁺ spark analysis, histological fibrosis quantification across two inbred backgrounds\",\n      \"pmids\": [\"28887330\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular basis for age-dependent normalization of Ca²⁺ transients in survivors unknown\", \"Modifier genes underlying background-dependent penetrance not identified\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Showing that the G247D mutation inhibits actin polymerization and abolishes Rho-GTPase/SRF signalling established a non-contractile signalling axis disrupted in ACTC1-linked DCM and congenital heart defects.\",\n      \"evidence\": \"In vitro actin polymerization assay, SRF luciferase reporter, Rho-GTPase pull-down, nuclear G-actin fractionation in NRVCMs\",\n      \"pmids\": [\"31434612\", \"31430208\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether SRF pathway disruption is a common mechanism in other ACTC1 DCM mutations\", \"In vivo rescue by restoring SRF signalling not attempted\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Identification of ACTC1 variants causing distal arthrogryposis with congenital heart defects in five families demonstrated that ACTC1 is required in skeletal as well as cardiac muscle, broadening its disease spectrum beyond cardiomyopathy.\",\n      \"evidence\": \"Exome/genome sequencing with co-segregation analysis across five independent families\",\n      \"pmids\": [\"37457373\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Functional consequence of DA-causing variants on actin polymerization or myosin interaction not tested\", \"Skeletal muscle biopsy data not available for most families\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Promoter variants reducing ACTC1 transcription were directly validated, and LMOD2 was identified as a physical ACTC1 interactor regulating myogenic differentiation, while an ACTC1–BMP4 axis was established in prostate cancer, extending ACTC1 functions beyond sarcomeric contraction.\",\n      \"evidence\": \"Luciferase promoter assay and EMSA in HL-1 cardiomyocytes; Co-IP of LMOD2–ACTC1 with LMOD2 KO in C2C12; overexpression/knockdown with BMP4 rescue in prostate cancer xenografts\",\n      \"pmids\": [\"40848833\", \"40745266\", \"41286808\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"LMOD2–ACTC1 interaction awaits reciprocal Co-IP and domain mapping\", \"BMP4 mechanism downstream of ACTC1 not delineated\", \"Relevance of ACTC1 in non-muscle tissues in vivo remains preliminary\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include the high-resolution structural basis for mutation-specific phenotype determination (HCM versus DCM versus congenital defect), the signalling pathways linking ACTC1 loss to apoptosis, the in vivo relevance of ACTC1 in non-muscle contexts, and whether therapeutic restoration of Ca²⁺ sensitivity or SRF signalling can prevent disease progression.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No cryo-EM or crystal structure of mutant ACTC1 in thin filament context\", \"No gene therapy or allele-specific silencing study for ACTC1 cardiomyopathy\", \"Mechanism linking ACTC1 to septal morphogenesis not established beyond expression correlation\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0005198\", \"supporting_discovery_ids\": [0, 1, 3, 4, 5]},\n      {\"term_id\": \"GO:0008092\", \"supporting_discovery_ids\": [2, 11, 16]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005856\", \"supporting_discovery_ids\": [0, 1, 3, 4, 11, 12]},\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [2]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-397014\", \"supporting_discovery_ids\": [0, 1, 3, 4, 5, 15]},\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [0, 1, 3, 7, 12, 15]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [11, 17]},\n      {\"term_id\": \"R-HSA-5357801\", \"supporting_discovery_ids\": [12, 13]},\n      {\"term_id\": \"R-HSA-1266738\", \"supporting_discovery_ids\": [6, 14, 15]}\n    ],\n    \"complexes\": [\n      \"cardiac sarcomeric thin filament\"\n    ],\n    \"partners\": [\n      \"SLC4A1\",\n      \"LMOD2\",\n      \"TNNC1\",\n      \"TNNI3\",\n      \"MYH7\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}