{"gene":"ACTA2","run_date":"2026-04-28T17:12:37","timeline":{"discoveries":[{"year":2015,"finding":"The R258C mutation in smooth muscle α-actin (ACTA2) destabilizes actin filaments, making them more susceptible to cofilin-mediated severing; smooth muscle tropomyosin offers little protection from cofilin cleavage of R258C filaments (unlike WT). Profilin binds more tightly to R258C monomers, increasing the G-actin pool. In an in vitro motility assay, smooth muscle myosin moves R258C filaments more slowly than WT, and under loaded conditions small myosin ensembles cannot generate force on R258C actin-tropomyosin filaments. The mutation acts allosterically, affecting multiple regions of the monomer beyond the mutated residue.","method":"Baculovirus expression of mutant SM α-actin; TIRF microscopy of single filament growth; cofilin severing assay; profilin binding assay; in vitro motility assay with SM myosin; loaded motility assay","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 — multiple in vitro reconstitution assays with purified components and functional mutagenesis in a single rigorous study","pmids":["26153420"],"is_preprint":false},{"year":2021,"finding":"The R149C mutation in ACTA2 causes increased retention of mutant SM α-actin in the chaperonin-containing t-complex polypeptide (CCT) folding complex, reducing the amount of mutant actin available relative to WT in smooth muscle cells. This is associated with disorganized and reduced SM α-actin filaments, enhanced nucleation by formin, and decreased interaction of mutant filaments with SM myosin in vitro. The enhanced CCT retention of mutant actin likely minimizes pathogenic effects on SMC function and may explain reduced penetrance of aortic disease in R149C carriers.","method":"CRISPR/Cas9 knock-in mouse model; in vitro motility assays; TIRF microscopy polymerization studies; co-immunoprecipitation of actin with CCT; western blot quantification of actin levels in SMCs; aortic contraction assays","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1–2 — multiple orthogonal methods including in vitro reconstitution, TIRF, co-IP, and in vivo mouse model in a single study","pmids":["34600884"],"is_preprint":false},{"year":2025,"finding":"The ACTA2 R179H pathogenic variant causes a dramatic phenotypic switch in human iPSC-derived smooth muscle cells (SMCs) from a contractile to a synthetic state, associated with increased proliferation and migration and decreased contractility. CRISPR adenine base editing (ABE8e-SpCas9-VRQR) to correct this variant restored normal SMC contractile phenotype in iPSC-derived SMCs and, when delivered via AAV9 in humanized R179H mice, rescued widespread smooth muscle dysfunction including aortic dilation/dissection, bladder enlargement, gut dilation, and hydronephrosis.","method":"iPSC-derived SMC differentiation; proliferation, migration, and contractility assays; adenine base editing; AAV9 systemic delivery in humanized knock-in mice; phenotypic assessment of multiple organ systems","journal":"Circulation","confidence":"High","confidence_rationale":"Tier 1–2 — in vitro reconstitution with iPSC-SMCs plus in vivo rescue in humanized mouse model using orthogonal methods","pmids":["40378078"],"is_preprint":false},{"year":2025,"finding":"ACTA2 R179C/+ smooth muscle cells fail to fully differentiate and maintain stem cell-like features including increased migration and elevated glycolytic flux compared to WT SMCs. After carotid artery injury, Acta2-SMC-R179C/+ mice develop moyamoya-like intraluminal SMC accumulation and occlusive lesions with neurological symptoms. Boosting mitochondrial respiration with nicotinamide riboside (NR) drives SMC differentiation, reduces migration of mutant SMCs, and prevents MMD-like lesions and strokes after injury.","method":"SMC-specific knock-in mouse model; carotid artery injury model; metabolic flux analysis; migration assays; NR pharmacological treatment; histology; neurological phenotyping","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 2 — clean SMC-specific KI mouse model with carotid injury, multiple phenotypic readouts, and pharmacological rescue","pmids":["40603847"],"is_preprint":false},{"year":2013,"finding":"Purine-rich element binding protein B (Purβ) represses ACTA2 transcription by binding cooperatively to the sense strand of a polypurine element (inverted MCAT cis-element) in the ACTA2 promoter/enhancer. The Purβ homodimer possesses three separate single-stranded DNA-binding modules formed by inter- and intramolecular subdomain interactions. Knockdown of Purβ in mouse embryo fibroblasts promoted myofibroblast-like morphology, increased ACTA2 actin isoform expression, and increased cell migration, confirming that Purβ suppresses myofibroblast differentiation by repressing ACTA2 transcription.","method":"shRNA knockdown of Purβ in MEFs; promoter-reporter assays; recombinant Purβ truncation mutants; biochemical ssDNA binding assays; biophysical analyses; co-immunoprecipitation of Purβ with YBX1","journal":"Biochemistry","confidence":"High","confidence_rationale":"Tier 1–2 — cell-based loss-of-function with defined transcriptional readout, combined with biochemical reconstitution and mutagenesis in one study","pmids":["23724822"],"is_preprint":false},{"year":2016,"finding":"Electrostatic and hydrophobic interactions mediate high-affinity binding of Purβ to the purine-rich ssDNA MCAT element in the Acta2 promoter. Specific basic residues R267 (intermolecular subdomain), K82 and R159 (intramolecular subdomains) are essential for Purβ repressor function in Acta2 promoter-reporter assays; R267A mutation reduced both ssDNA binding affinity and interaction with the Acta2 corepressor Y-box-binding protein 1 (YBX1).","method":"Quantitative ssDNA binding assays with monovalent salt/detergent titration; pH titration; site-directed mutagenesis; Acta2 promoter-reporter assays in fibroblasts; co-immunoprecipitation","journal":"Biochemistry","confidence":"High","confidence_rationale":"Tier 1–2 — mutagenesis combined with biochemical binding assays and cell-based transcriptional assays, identifying specific residues","pmids":["27064749"],"is_preprint":false},{"year":2013,"finding":"ACTA2 (α-smooth muscle actin) is required for myofibroblast motility and contraction in hepatic stellate cells (activated myofibroblasts). Knockdown of Acta2 (without affecting cytoplasmic actin isoforms) significantly reduced cellular motility and contraction of activated hepatic stellate cells, and was associated with reduced ERK1/2 phosphorylation, indicating that the Acta2 cytoskeleton participates in signaling as well as structural functions during wound healing.","method":"siRNA/shRNA knockdown of Acta2 in activated hepatic stellate cells; cell motility assay; collagen gel contraction assay; western blot for ERK1/2 phosphorylation; in vivo liver injury models","journal":"PloS one","confidence":"Medium","confidence_rationale":"Tier 2 — clean KD with multiple defined functional readouts (motility, contraction, signaling) in a single study","pmids":["24204762"],"is_preprint":false},{"year":2013,"finding":"In lung adenocarcinoma cells, ACTA2 regulates expression of c-MET and FAK, which selectively influence metastatic potential. Knockdown of ACTA2 using shRNA/siRNA markedly impaired in vitro migration, invasion, clonogenicity, and transendothelial penetration, and significantly reduced metastatic potential in vivo without altering tumorigenic potential. ACTA2 depletion was accompanied by loss of mesenchymal characteristics.","method":"shRNA/siRNA knockdown of ACTA2 in lung adenocarcinoma cell lines; in vitro migration/invasion/transendothelial assays; in vivo xenograft metastasis assay; western blot for c-MET and FAK","journal":"Clinical cancer research","confidence":"Medium","confidence_rationale":"Tier 2 — loss-of-function with multiple in vitro and in vivo functional readouts and downstream signaling endpoint","pmids":["23995859"],"is_preprint":false},{"year":2016,"finding":"EGFR/HER2 dimerization induces ACTA2 expression in breast cancer cells through a JAK2/STAT1 signaling pathway. STAT1 inhibition (fludarabine) or JAK2 inhibition (AG490) reduced basal ACTA2 expression, while STAT1 overexpression increased it. ACTA2 knockdown suppressed cell motility in vitro and reduced lung metastatic nodules in vivo.","method":"HER2 overexpression/siRNA knockdown; STAT1 inhibitor/JAK2 inhibitor treatment; STAT1 overexpression; ACTA2 shRNA; in vitro motility assays; in vivo lung metastasis model","journal":"Oncotarget","confidence":"Medium","confidence_rationale":"Tier 2 — multiple genetic and pharmacological perturbations connecting HER2/JAK2/STAT1 to ACTA2 and functional metastasis readouts","pmids":["28881584"],"is_preprint":false},{"year":2015,"finding":"TGF-β2-induced epithelial-mesenchymal transition (EMT) in lens epithelial cells is associated with increased histone H4 acetylation specifically at the ACTA2 promoter region, driving ACTA2 expression. The histone deacetylase inhibitor trichostatin A (TSA) suppresses EMT and reduces ACTA2 levels by epigenetically reducing this promoter histone acetylation.","method":"TGF-β2 treatment of FHL124 lens epithelial cells; chromatin immunoprecipitation (ChIP) for acetylated histone H4 at ACTA2 promoter; RT-PCR and western blot for ACTA2; TSA treatment; cell migration assay","journal":"Eye (London, England)","confidence":"Medium","confidence_rationale":"Tier 2 — ChIP directly demonstrated histone acetylation at ACTA2 promoter with functional TSA intervention","pmids":["25853442"],"is_preprint":false},{"year":2022,"finding":"Deletion of Acta2 in cardiac fibroblasts does not prevent myofibroblast differentiation or impair post-myocardial infarction cardiac repair in mice. Loss of Acta2 causes compensatory transcriptional upregulation of other actin isoforms (especially Actg2 and Acta1), maintaining total filamentous actin and actin levels. Myocardin-related transcription factor-A (MRTF-A) is critical for myofibroblast differentiation but not for the compensatory upregulation of non-Acta2 isoforms, indicating functional redundancy among actin isoforms in cardiac myofibroblasts.","method":"Tamoxifen-inducible cardiac fibroblast-specific Acta2 knockout mice; post-MI cardiac function and histology; proliferation, migration, and contractility assays; RT-qPCR for actin isoforms; MRTF-A deletion","journal":"Journal of molecular and cellular cardiology","confidence":"High","confidence_rationale":"Tier 2 — clean conditional KO with multiple functional readouts and mechanistic follow-up identifying compensatory isoforms and transcription factor dependency","pmids":["36007455"],"is_preprint":false},{"year":2020,"finding":"Downregulation of ACTA2 in neural stem cells (NSCs) inhibits migration by hindering actin filament polymerization via increased RhoA expression and decreased Rac1 expression, placing ACTA2 upstream of the RhoA/Rac1 balance in regulating NSC actin dynamics and migration.","method":"siRNA knockdown of ACTA2 in primary NSCs; migration assay; immunostaining; RT-PCR; RhoA and Rac1 expression analysis","journal":"Stem cells international","confidence":"Medium","confidence_rationale":"Tier 2–3 — KD with migration phenotype and mechanistic pathway placement via RhoA/Rac1, single study","pmids":["32508931"],"is_preprint":false},{"year":2018,"finding":"Deletion of ACTA2 in mice promotes angiotensin II-induced progressive aortic lumen dilation, increased apoptosis (elevated Bax/Bcl-2 ratio), and phenotypic modulation of VSMCs (increased osteopontin expression, decreased α-SMA), demonstrating that ACTA2 is required to maintain VSMC contractile phenotype and resist AngII-induced aortic pathology.","method":"ACTA2 knockout mice; AngII osmotic minipump infusion; ultrasound aortic diameter measurement; TUNEL apoptosis assay; RT-qPCR and western blot for OPN, Bax/Bcl-2, α-SMA; histopathology","journal":"Journal of thoracic disease","confidence":"Medium","confidence_rationale":"Tier 2 — in vivo KO with multiple functional readouts under AngII challenge","pmids":["30233845"],"is_preprint":false},{"year":2015,"finding":"ACTA2 mutations impaired migration velocity and reduced contractility in VSMC-like cells transdifferentiated from patient fibroblasts, while ACTA2 and FBN1 DN variant cells showed decreased SMAD2 phosphorylation, placing ACTA2 upstream of TGFβ/SMAD signaling in VSMCs.","method":"Fibroblast-to-VSMC transdifferentiation protocol; migration velocity assay; collagen gel contraction assay; western blot for pSMAD2; comparison across ACTA2, MYH11, SMAD3, FBN1 variant patient cells","journal":"Human molecular genetics","confidence":"Medium","confidence_rationale":"Tier 2 — patient-derived VSMC-like cells with functional assays; limitation is that it is a patient variant cell model, not isogenic","pmids":["34244757"],"is_preprint":false},{"year":2015,"finding":"3D molecular modeling of the actin filament revealed that residue R179 is positioned at the interface between the two strands of filamentous actin; the R179H mutation is predicted to destabilize inter-strand bundling, providing a structural explanation for the severe phenotype associated with this mutation.","method":"Actin three-dimensional molecular modeling; histopathological analysis of cerebral arteries from ACTA2 R179H patient (massive intimal SMC proliferation, medial fibromuscular proliferation, arterial fibrosis)","journal":"Acta neuropathologica communications","confidence":"Low","confidence_rationale":"Tier 4 for structural claim — computational modeling only without experimental validation; histopathology supports phenotype but not the molecular mechanism directly","pmids":["26637293"],"is_preprint":false},{"year":2022,"finding":"Transient angiotensin II infusion causes sustained (memory) downregulation of Acta2 mRNA and protein in aortic tissue beyond the period of AngII treatment, associated with increased H3K27me3 at nuclei of aortic sections and sustained downregulation of the Acta2 transcriptional activator Myocardin (Myocd), indicating epigenetic silencing of ACTA2 as a vascular memory mechanism.","method":"Mouse AngII infusion with post-treatment follow-up; RNAseq profiling of aortic tissue; RT-qPCR and western blot validation; immunohistochemistry for H3K27me3; human aortic VSMC cell culture model","journal":"Frontiers in cardiovascular medicine","confidence":"Medium","confidence_rationale":"Tier 2 — RNAseq plus IHC for H3K27me3 in vivo, replicated in cell culture; epigenetic mechanism identified but not fully dissected mechanistically","pmids":["35360022"],"is_preprint":false},{"year":1990,"finding":"The human vascular smooth muscle actin gene (ACTA2/ACTSA) was mapped to chromosome 10q22-q24 by Southern blot analysis of somatic cell hybrids and confirmed by in situ hybridization, placing it on a different chromosome from the other four actin genes examined at the time.","method":"Southern blot analysis of rodent-human somatic cell hybrids; in situ hybridization","journal":"The Japanese journal of human genetics","confidence":"High","confidence_rationale":"Tier 2 — direct chromosomal assignment by two independent methods (somatic cell hybrids and in situ hybridization)","pmids":["2398629"],"is_preprint":false},{"year":2024,"finding":"Missense variants in ACTA2 (G148R and R179H) injected as mRNA into one-cell stage zebrafish embryos caused significantly reduced cardiac shortening fractions, thinner myocardial walls, and decreased total cell number and proliferating cell numbers in the endothelial and myocardial regions compared to WT, demonstrating that pathogenic ACTA2 variants impair cardiac morphological development in vivo.","method":"Zebrafish mRNA injection of ACTA2 WT, G148R, and R179H; measurement of cardiac shortening fraction; histopathological evaluation at 72 hpf; quantification of proliferating cells (endothelial and myocardial)","journal":"Journal of human genetics","confidence":"Medium","confidence_rationale":"Tier 2 — in vivo zebrafish overexpression model with multiple cardiac readouts; single study","pmids":["38316882"],"is_preprint":false},{"year":2023,"finding":"In aganglionic segments of Hirschsprung disease (HSCR), ACTA2 expression is abnormally elevated in circular smooth muscle cells beginning at embryonic day E15.5, and siRNA-mediated knockdown of Acta2 weakens the contraction ability of intestinal smooth muscle cells, establishing that elevated ACTA2 causes hyperactive contraction contributing to smooth muscle spasm in aganglionic bowel.","method":"Immunohistochemistry in HSCR patients and Ednrb-/- mice; siRNA knockdown of Acta2 in intestinal SMCs; contractility assay; developmental staging in Ednrb-/- mice","journal":"Pediatric surgery international","confidence":"Medium","confidence_rationale":"Tier 2–3 — siRNA knockdown with contractility readout plus in vivo disease model; single study","pmids":["37278766"],"is_preprint":false},{"year":2014,"finding":"RHOA knockdown significantly downregulates ACTA2 gene expression in both osteoblast-like and osteoclast-like human cells, placing RHOA upstream of ACTA2 in a bone cell regulatory pathway.","method":"siRNA knockdown of RHOA and ARHGEF3 in Saos-2 and hFOB 1.19 osteoblast-like cells and osteoclast-like cells; microarray followed by RT-qPCR validation","journal":"PloS one","confidence":"Low","confidence_rationale":"Tier 3 — single knockdown with transcriptional readout; no direct functional consequence of ACTA2 changes measured","pmids":["24840563"],"is_preprint":false},{"year":2024,"finding":"In a yeast model system, heterozygous expression of TAAD-associated ACTA2 missense variants in Saccharomyces cerevisiae (whose actin is highly conserved with human SM α-actin at the affected residues) causes abnormal actin cytoskeleton organization and mitochondrial distribution, consistent with a dominant-negative pathogenic mechanism of ACTA2 variants on actin function.","method":"Yeast expression of mutant ACTA2 alleles in heterozygous condition; spot test growth assay; fluorescence microscopy of actin cytoskeleton and mitochondria","journal":"European journal of human genetics","confidence":"Medium","confidence_rationale":"Tier 2 — orthologous yeast model with cytoskeletal and organelle readouts; mechanistic inference of dominant-negative mechanism supported by multiple variants","pmids":["38486025"],"is_preprint":false}],"current_model":"ACTA2-encoded smooth muscle α-actin is a major component of the contractile apparatus of vascular and visceral smooth muscle cells, where it polymerizes into stable filaments that interact with smooth muscle myosin to generate force; disease-causing mutations (e.g., R258C, R179H, R149C) disrupt filament stability, myosin interaction, and/or CCT-chaperone folding, shifting SMCs from a contractile to a proliferative/synthetic phenotype that drives aneurysm, dissection, and occlusive vascular disease, while its transcription is repressed by Purβ binding to a purine-rich ssDNA element in the promoter and activated via histone H4 acetylation downstream of TGFβ signaling."},"narrative":{"teleology":[{"year":1990,"claim":"Establishing the genomic location of ACTA2 on chromosome 10q22-q24 distinguished it from the other human actin genes and enabled subsequent genetic studies linking it to disease.","evidence":"Southern blot analysis of somatic cell hybrids and in situ hybridization","pmids":["2398629"],"confidence":"High","gaps":["No functional characterization accompanied the mapping"]},{"year":2013,"claim":"Purβ was identified as a transcriptional repressor of ACTA2 that binds cooperatively to a purine-rich ssDNA element in the ACTA2 promoter, establishing a key mechanism controlling myofibroblast differentiation and ACTA2 expression levels.","evidence":"shRNA knockdown in MEFs; promoter-reporter assays; recombinant Purβ truncation and binding assays; confirmed by mutagenesis of specific Purβ residues (R267, K82, R159) in 2016","pmids":["23724822","27064749"],"confidence":"High","gaps":["How Purβ/YBX1 repression is regulated during SMC differentiation in vivo is not defined","Whether Purβ binding is modulated by chromatin context is unclear"]},{"year":2013,"claim":"Direct demonstration that ACTA2 is required for myofibroblast contractility and motility — not just a marker — showed it participates in both mechanical and signaling (ERK1/2) functions in activated hepatic stellate cells.","evidence":"siRNA/shRNA knockdown in activated hepatic stellate cells; contraction and migration assays; ERK1/2 phosphorylation assessment","pmids":["24204762"],"confidence":"Medium","gaps":["Whether ERK1/2 regulation is direct or secondary to cytoskeletal remodeling is unknown","Isoform specificity of signaling effect not tested"]},{"year":2015,"claim":"Reconstitution of the R258C mutation revealed multiple allosteric effects on actin filament stability, cofilin severing, profilin binding, and myosin force generation, establishing the first molecular mechanism for how a single ACTA2 mutation causes vascular disease.","evidence":"Baculovirus-expressed mutant SM α-actin; TIRF microscopy; cofilin severing assay; profilin binding; loaded in vitro motility with SM myosin","pmids":["26153420"],"confidence":"High","gaps":["In vivo consequences of R258C not tested in animal model in this study","Whether tropomyosin isoform identity affects protection remains open"]},{"year":2015,"claim":"TGFβ-induced ACTA2 transcription was linked to histone H4 acetylation at the ACTA2 promoter, revealing an epigenetic activation mechanism during EMT.","evidence":"ChIP for acetyl-H4 at ACTA2 promoter in TGFβ2-treated lens epithelial cells; TSA treatment","pmids":["25853442"],"confidence":"Medium","gaps":["Specific histone acetyltransferase responsible not identified","Whether this mechanism operates in vascular SMCs not tested"]},{"year":2018,"claim":"Whole-animal Acta2 knockout demonstrated that loss of ACTA2 predisposes to angiotensin II-induced aortic dilation, VSMC apoptosis, and phenotypic modulation, directly linking ACTA2 to aortic structural integrity under hemodynamic stress.","evidence":"ACTA2 knockout mice with AngII infusion; ultrasound; TUNEL; RT-qPCR and western blot","pmids":["30233845"],"confidence":"Medium","gaps":["Whether aortic pathology is cell-autonomous to SMCs or involves adventitial/immune contributions not resolved","Compensatory actin isoform expression not assessed"]},{"year":2021,"claim":"The R149C mutation was shown to be retained in the CCT chaperonin, reducing the pool of mutant actin incorporated into filaments and explaining the reduced disease penetrance of this variant compared to R258C and R179H.","evidence":"CRISPR knock-in mouse; co-IP of actin with CCT; TIRF polymerization; in vitro motility; aortic contraction assays","pmids":["34600884"],"confidence":"High","gaps":["Whether CCT retention is a generalizable quality-control mechanism for other ACTA2 variants is untested","Structural basis of enhanced CCT binding not determined"]},{"year":2022,"claim":"Conditional deletion of Acta2 in cardiac fibroblasts revealed functional redundancy among actin isoforms: compensatory upregulation of Actg2 and Acta1 preserved myofibroblast function and post-MI repair, resolving whether ACTA2 is essential for all myofibroblast contexts.","evidence":"Tamoxifen-inducible cardiac fibroblast-specific KO; post-MI functional assessment; RT-qPCR for actin isoforms; MRTF-A deletion","pmids":["36007455"],"confidence":"High","gaps":["Whether similar compensation occurs in vascular SMCs is unknown","Mechanism triggering compensatory isoform upregulation not identified"]},{"year":2022,"claim":"Transient AngII exposure was found to cause sustained epigenetic silencing of Acta2 via H3K27me3 and downregulation of Myocardin, establishing a 'vascular memory' mechanism that maintains SMC phenotypic modulation long after the stimulus is removed.","evidence":"Mouse AngII infusion with follow-up; RNAseq; IHC for H3K27me3; human VSMC culture","pmids":["35360022"],"confidence":"Medium","gaps":["Specific H3K27 methyltransferase (e.g. EZH2) acting at the Acta2 locus not identified","Whether this epigenetic mark is reversible in vivo not tested"]},{"year":2025,"claim":"CRISPR base editing correction of R179H in iPSC-derived SMCs and AAV9 delivery in humanized mice rescued multiorgan smooth muscle dysfunction, providing the first gene-therapy proof of concept and confirming R179H causes a global contractile-to-synthetic switch.","evidence":"iPSC-SMC differentiation; adenine base editing; AAV9 systemic delivery in R179H knock-in mice; aortic, bladder, gut, kidney phenotyping","pmids":["40378078"],"confidence":"High","gaps":["Long-term durability and off-target editing effects not assessed","Whether correction after disease onset can reverse established pathology is unknown"]},{"year":2025,"claim":"R179C mutant SMCs were shown to maintain stem cell-like features with elevated glycolysis, and boosting mitochondrial respiration with nicotinamide riboside drove differentiation and prevented moyamoya-like occlusive lesions, linking ACTA2 mutation to metabolic reprogramming as a druggable disease mechanism.","evidence":"SMC-specific knock-in mice; carotid artery injury; metabolic flux analysis; NR treatment; neurological phenotyping","pmids":["40603847"],"confidence":"High","gaps":["Whether metabolic shift is a direct consequence of defective actin filaments or secondary to impaired differentiation signaling is unresolved","Efficacy in human patients unknown"]},{"year":null,"claim":"No high-resolution experimental structure of disease-mutant SM α-actin filaments has been determined, and the precise structural basis by which individual mutations differentially affect filament inter-strand contacts, myosin binding interfaces, and chaperonin recognition remains unresolved.","evidence":"","pmids":[],"confidence":"Low","gaps":["Cryo-EM or crystal structures of mutant SM α-actin filaments needed","Genotype-phenotype correlation across the full spectrum of >50 known ACTA2 variants not systematically addressed at the biochemical level","Whether pharmacological stabilization of mutant filaments can serve as therapy is untested"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0005198","term_label":"structural molecule activity","supporting_discovery_ids":[0,1,6,18]},{"term_id":"GO:0008092","term_label":"cytoskeletal protein binding","supporting_discovery_ids":[0,1]}],"localization":[{"term_id":"GO:0005856","term_label":"cytoskeleton","supporting_discovery_ids":[0,1,6,11,20]}],"pathway":[{"term_id":"R-HSA-397014","term_label":"Muscle contraction","supporting_discovery_ids":[0,1,6,18]},{"term_id":"R-HSA-1643685","term_label":"Disease","supporting_discovery_ids":[2,3,12,13]},{"term_id":"R-HSA-74160","term_label":"Gene expression (Transcription)","supporting_discovery_ids":[4,5,9,15]}],"complexes":["actin-tropomyosin filament"],"partners":["MYH11","PURB","YBX1","CFL1","PFN1","CCT"],"other_free_text":[]},"mechanistic_narrative":"ACTA2 encodes smooth muscle α-actin, the principal actin isoform of the contractile apparatus in vascular and visceral smooth muscle cells (SMCs), where it polymerizes into filaments that interact with smooth muscle myosin to generate contractile force [PMID:26153420, PMID:24204762]. Disease-causing missense mutations (R258C, R179H, R149C) destabilize filaments, impair myosin-driven motility, alter chaperonin (CCT) folding, and shift SMCs from a contractile to a synthetic/proliferative state, causing thoracic aortic aneurysm and dissection, moyamoya-like cerebrovascular disease, and multiorgan smooth muscle dysfunction [PMID:26153420, PMID:34600884, PMID:40378078, PMID:40603847]. In cardiac fibroblasts, Acta2 deletion is compensated by upregulation of other actin isoforms (Actg2, Acta1), indicating functional redundancy in myofibroblast contexts [PMID:36007455]. Transcription of ACTA2 is repressed by Purβ binding to a purine-rich single-stranded element in the promoter and is activated through histone H4 acetylation downstream of TGFβ signaling and by Myocardin, with sustained silencing achievable via H3K27me3 epigenetic marks [PMID:23724822, PMID:25853442, PMID:35360022]."},"prefetch_data":{"uniprot":{"accession":"P62736","full_name":"Actin, aortic smooth muscle","aliases":["Alpha-actin-2","Cell growth-inhibiting gene 46 protein"],"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/P62736/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/ACTA2","classification":"Not Classified","n_dependent_lines":0,"n_total_lines":1208,"dependency_fraction":0.0},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/ACTA2","total_profiled":1310},"omim":[{"mim_id":"620475","title":"THROMBOCYTOPENIA 8, WITH DYSMORPHIC FEATURES AND DEVELOPMENTAL DELAY; THC8","url":"https://www.omim.org/entry/620475"},{"mim_id":"620093","title":"ACTIN MATURATION PROTEASE; ACTMAP","url":"https://www.omim.org/entry/620093"},{"mim_id":"619656","title":"LOEYS-DIETZ SYNDROME 6; LDS6","url":"https://www.omim.org/entry/619656"},{"mim_id":"619365","title":"MEGACYSTIS-MICROCOLON-INTESTINAL HYPOPERISTALSIS SYNDROME 4; MMIHS4","url":"https://www.omim.org/entry/619365"},{"mim_id":"614042","title":"MOYAMOYA DISEASE 5; MYMY5","url":"https://www.omim.org/entry/614042"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Actin filaments","reliability":"Approved"}],"tissue_specificity":"Tissue enhanced","tissue_distribution":"Detected in all","driving_tissues":[{"tissue":"blood vessel","ntpm":10509.9},{"tissue":"smooth muscle","ntpm":7564.0}],"url":"https://www.proteinatlas.org/search/ACTA2"},"hgnc":{"alias_symbol":["ACTSA"],"prev_symbol":[]},"alphafold":{"accession":"P62736","domains":[{"cath_id":"3.30.420.40","chopping":"9-139_341-374","consensus_level":"medium","plddt":94.8638,"start":9,"end":374},{"cath_id":"3.30.420.40","chopping":"145-181_274-337","consensus_level":"medium","plddt":97.573,"start":145,"end":337},{"cath_id":"3.90.640.10","chopping":"183-267","consensus_level":"high","plddt":97.0654,"start":183,"end":267}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/P62736","model_url":"https://alphafold.ebi.ac.uk/files/AF-P62736-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-P62736-F1-predicted_aligned_error_v6.png","plddt_mean":95.38},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=ACTA2","jax_strain_url":"https://www.jax.org/strain/search?query=ACTA2"},"sequence":{"accession":"P62736","fasta_url":"https://rest.uniprot.org/uniprotkb/P62736.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/P62736/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/P62736"}},"corpus_meta":[{"pmid":"19409525","id":"PMC_19409525","title":"Mutations in smooth muscle alpha-actin (ACTA2) cause coronary artery disease, stroke, and Moyamoya disease, along with thoracic aortic disease.","date":"2009","source":"American journal of human genetics","url":"https://pubmed.ncbi.nlm.nih.gov/19409525","citation_count":424,"is_preprint":false},{"pmid":"20734336","id":"PMC_20734336","title":"De novo ACTA2 mutation causes a novel syndrome of multisystemic smooth muscle dysfunction.","date":"2010","source":"American journal of medical genetics. 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Part A","url":"https://pubmed.ncbi.nlm.nih.gov/36607831","citation_count":3,"is_preprint":false},{"pmid":"40785968","id":"PMC_40785968","title":"Human Amniotic Epithelial Stem Cell Exosomes Regulate Chondrocyte Ferroptosis through ACTA2-AS1-Targeted Binding to ACSL4 for Osteoarthritis Intervention.","date":"2025","source":"Research (Washington, D.C.)","url":"https://pubmed.ncbi.nlm.nih.gov/40785968","citation_count":3,"is_preprint":false},{"pmid":"38044429","id":"PMC_38044429","title":"A highly penetrant ACTA2 mutation of thoracic aortic disease.","date":"2023","source":"Journal of cardiothoracic surgery","url":"https://pubmed.ncbi.nlm.nih.gov/38044429","citation_count":3,"is_preprint":false},{"pmid":"40378078","id":"PMC_40378078","title":"Genomic Editing of a Pathogenic Sequence Variant in ACTA2 Rescues Multisystemic Smooth Muscle Dysfunction Syndrome in Mice.","date":"2025","source":"Circulation","url":"https://pubmed.ncbi.nlm.nih.gov/40378078","citation_count":3,"is_preprint":false},{"pmid":"32452246","id":"PMC_32452246","title":"Aneurysmal Dilatation of Ductus Arteriosus and Pulmonary Artery in Association With ACTA2 Mutation.","date":"2020","source":"World journal for pediatric & congenital heart surgery","url":"https://pubmed.ncbi.nlm.nih.gov/32452246","citation_count":3,"is_preprint":false},{"pmid":"40603847","id":"PMC_40603847","title":"Immature Acta2R179C/+ smooth muscle cells cause moyamoya-like cerebrovascular lesions in mice prevented by boosting OXPHOS.","date":"2025","source":"Nature communications","url":"https://pubmed.ncbi.nlm.nih.gov/40603847","citation_count":2,"is_preprint":false},{"pmid":"35248443","id":"PMC_35248443","title":"Triple bypass for multisystem smooth muscle dysfunction syndrome due to Arg179His ACTA2 mutation.","date":"2022","source":"Journal of stroke and cerebrovascular diseases : the official journal of National Stroke Association","url":"https://pubmed.ncbi.nlm.nih.gov/35248443","citation_count":2,"is_preprint":false},{"pmid":"37886459","id":"PMC_37886459","title":"Augmenting Mitochondrial Respiration in Immature Smooth Muscle Cells with an ACTA2 Pathogenic Variant Mitigates Moyamoya-like Cerebrovascular Disease.","date":"2023","source":"Research square","url":"https://pubmed.ncbi.nlm.nih.gov/37886459","citation_count":2,"is_preprint":false},{"pmid":"35420309","id":"PMC_35420309","title":"Clinical and neuroimaging features of a familial pathogenic ACTA2 variant as a model of a vascular neurocristopathy.","date":"2022","source":"Neuroradiology","url":"https://pubmed.ncbi.nlm.nih.gov/35420309","citation_count":2,"is_preprint":false},{"pmid":"32464348","id":"PMC_32464348","title":"Expanding the cerebrovascular phenotype of the p.R258H variant in ACTA2 related hereditary thoracic aortic disease (HTAD).","date":"2020","source":"Journal of the neurological sciences","url":"https://pubmed.ncbi.nlm.nih.gov/32464348","citation_count":2,"is_preprint":false},{"pmid":"28343608","id":"PMC_28343608","title":"Management of Bilateral Axillary Aneurysms, Threatened Limb, and Diffuse Vasculopathy in a Patient With ACTA2 Mutation.","date":"2017","source":"The Canadian journal of cardiology","url":"https://pubmed.ncbi.nlm.nih.gov/28343608","citation_count":2,"is_preprint":false},{"pmid":"19586843","id":"PMC_19586843","title":"[Cloning and expression analysis of porcine ACTA2 gene and its association with production traits].","date":"2009","source":"Yi chuan = Hereditas","url":"https://pubmed.ncbi.nlm.nih.gov/19586843","citation_count":2,"is_preprint":false},{"pmid":"34546411","id":"PMC_34546411","title":"Nonsurgical treatment of cerebral ischemia associated with ACTA2 cerebral arteriopathy: a case report and literature review.","date":"2021","source":"Child's nervous system : ChNS : official journal of the International Society for Pediatric Neurosurgery","url":"https://pubmed.ncbi.nlm.nih.gov/34546411","citation_count":1,"is_preprint":false},{"pmid":"37672683","id":"PMC_37672683","title":"Aortic Dissection and a Previously Unreported ACTA2 Missense Variant Mutation in a Young Patient: A Case Report.","date":"2023","source":"Pediatric and developmental pathology : the official journal of the Society for Pediatric Pathology and the Paediatric Pathology Society","url":"https://pubmed.ncbi.nlm.nih.gov/37672683","citation_count":1,"is_preprint":false},{"pmid":"38486025","id":"PMC_38486025","title":"A yeast based assay establishes the pathogenicity of novel missense ACTA2 variants associated with aortic aneurysms.","date":"2024","source":"European journal of human genetics : EJHG","url":"https://pubmed.ncbi.nlm.nih.gov/38486025","citation_count":1,"is_preprint":false},{"pmid":"33299681","id":"PMC_33299681","title":"ACTA2 Mutation: Microsurgeon Beware!","date":"2020","source":"Plastic and reconstructive surgery. Global open","url":"https://pubmed.ncbi.nlm.nih.gov/33299681","citation_count":1,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":49682,"output_tokens":6054,"usd":0.119928},"stage2":{"model":"claude-opus-4-6","input_tokens":9669,"output_tokens":2946,"usd":0.182993},"total_usd":0.302921,"stage1_batch_id":"msgbatch_01TqNM6p6FsoMBT2jzQTwKBr","stage2_batch_id":"msgbatch_01Ya473eco8sTHbvRymQaU8o","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2015,\n      \"finding\": \"The R258C mutation in smooth muscle α-actin (ACTA2) destabilizes actin filaments, making them more susceptible to cofilin-mediated severing; smooth muscle tropomyosin offers little protection from cofilin cleavage of R258C filaments (unlike WT). Profilin binds more tightly to R258C monomers, increasing the G-actin pool. In an in vitro motility assay, smooth muscle myosin moves R258C filaments more slowly than WT, and under loaded conditions small myosin ensembles cannot generate force on R258C actin-tropomyosin filaments. The mutation acts allosterically, affecting multiple regions of the monomer beyond the mutated residue.\",\n      \"method\": \"Baculovirus expression of mutant SM α-actin; TIRF microscopy of single filament growth; cofilin severing assay; profilin binding assay; in vitro motility assay with SM myosin; loaded motility assay\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — multiple in vitro reconstitution assays with purified components and functional mutagenesis in a single rigorous study\",\n      \"pmids\": [\"26153420\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"The R149C mutation in ACTA2 causes increased retention of mutant SM α-actin in the chaperonin-containing t-complex polypeptide (CCT) folding complex, reducing the amount of mutant actin available relative to WT in smooth muscle cells. This is associated with disorganized and reduced SM α-actin filaments, enhanced nucleation by formin, and decreased interaction of mutant filaments with SM myosin in vitro. The enhanced CCT retention of mutant actin likely minimizes pathogenic effects on SMC function and may explain reduced penetrance of aortic disease in R149C carriers.\",\n      \"method\": \"CRISPR/Cas9 knock-in mouse model; in vitro motility assays; TIRF microscopy polymerization studies; co-immunoprecipitation of actin with CCT; western blot quantification of actin levels in SMCs; aortic contraction assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — multiple orthogonal methods including in vitro reconstitution, TIRF, co-IP, and in vivo mouse model in a single study\",\n      \"pmids\": [\"34600884\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"The ACTA2 R179H pathogenic variant causes a dramatic phenotypic switch in human iPSC-derived smooth muscle cells (SMCs) from a contractile to a synthetic state, associated with increased proliferation and migration and decreased contractility. CRISPR adenine base editing (ABE8e-SpCas9-VRQR) to correct this variant restored normal SMC contractile phenotype in iPSC-derived SMCs and, when delivered via AAV9 in humanized R179H mice, rescued widespread smooth muscle dysfunction including aortic dilation/dissection, bladder enlargement, gut dilation, and hydronephrosis.\",\n      \"method\": \"iPSC-derived SMC differentiation; proliferation, migration, and contractility assays; adenine base editing; AAV9 systemic delivery in humanized knock-in mice; phenotypic assessment of multiple organ systems\",\n      \"journal\": \"Circulation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — in vitro reconstitution with iPSC-SMCs plus in vivo rescue in humanized mouse model using orthogonal methods\",\n      \"pmids\": [\"40378078\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"ACTA2 R179C/+ smooth muscle cells fail to fully differentiate and maintain stem cell-like features including increased migration and elevated glycolytic flux compared to WT SMCs. After carotid artery injury, Acta2-SMC-R179C/+ mice develop moyamoya-like intraluminal SMC accumulation and occlusive lesions with neurological symptoms. Boosting mitochondrial respiration with nicotinamide riboside (NR) drives SMC differentiation, reduces migration of mutant SMCs, and prevents MMD-like lesions and strokes after injury.\",\n      \"method\": \"SMC-specific knock-in mouse model; carotid artery injury model; metabolic flux analysis; migration assays; NR pharmacological treatment; histology; neurological phenotyping\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean SMC-specific KI mouse model with carotid injury, multiple phenotypic readouts, and pharmacological rescue\",\n      \"pmids\": [\"40603847\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Purine-rich element binding protein B (Purβ) represses ACTA2 transcription by binding cooperatively to the sense strand of a polypurine element (inverted MCAT cis-element) in the ACTA2 promoter/enhancer. The Purβ homodimer possesses three separate single-stranded DNA-binding modules formed by inter- and intramolecular subdomain interactions. Knockdown of Purβ in mouse embryo fibroblasts promoted myofibroblast-like morphology, increased ACTA2 actin isoform expression, and increased cell migration, confirming that Purβ suppresses myofibroblast differentiation by repressing ACTA2 transcription.\",\n      \"method\": \"shRNA knockdown of Purβ in MEFs; promoter-reporter assays; recombinant Purβ truncation mutants; biochemical ssDNA binding assays; biophysical analyses; co-immunoprecipitation of Purβ with YBX1\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — cell-based loss-of-function with defined transcriptional readout, combined with biochemical reconstitution and mutagenesis in one study\",\n      \"pmids\": [\"23724822\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Electrostatic and hydrophobic interactions mediate high-affinity binding of Purβ to the purine-rich ssDNA MCAT element in the Acta2 promoter. Specific basic residues R267 (intermolecular subdomain), K82 and R159 (intramolecular subdomains) are essential for Purβ repressor function in Acta2 promoter-reporter assays; R267A mutation reduced both ssDNA binding affinity and interaction with the Acta2 corepressor Y-box-binding protein 1 (YBX1).\",\n      \"method\": \"Quantitative ssDNA binding assays with monovalent salt/detergent titration; pH titration; site-directed mutagenesis; Acta2 promoter-reporter assays in fibroblasts; co-immunoprecipitation\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — mutagenesis combined with biochemical binding assays and cell-based transcriptional assays, identifying specific residues\",\n      \"pmids\": [\"27064749\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"ACTA2 (α-smooth muscle actin) is required for myofibroblast motility and contraction in hepatic stellate cells (activated myofibroblasts). Knockdown of Acta2 (without affecting cytoplasmic actin isoforms) significantly reduced cellular motility and contraction of activated hepatic stellate cells, and was associated with reduced ERK1/2 phosphorylation, indicating that the Acta2 cytoskeleton participates in signaling as well as structural functions during wound healing.\",\n      \"method\": \"siRNA/shRNA knockdown of Acta2 in activated hepatic stellate cells; cell motility assay; collagen gel contraction assay; western blot for ERK1/2 phosphorylation; in vivo liver injury models\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — clean KD with multiple defined functional readouts (motility, contraction, signaling) in a single study\",\n      \"pmids\": [\"24204762\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"In lung adenocarcinoma cells, ACTA2 regulates expression of c-MET and FAK, which selectively influence metastatic potential. Knockdown of ACTA2 using shRNA/siRNA markedly impaired in vitro migration, invasion, clonogenicity, and transendothelial penetration, and significantly reduced metastatic potential in vivo without altering tumorigenic potential. ACTA2 depletion was accompanied by loss of mesenchymal characteristics.\",\n      \"method\": \"shRNA/siRNA knockdown of ACTA2 in lung adenocarcinoma cell lines; in vitro migration/invasion/transendothelial assays; in vivo xenograft metastasis assay; western blot for c-MET and FAK\",\n      \"journal\": \"Clinical cancer research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — loss-of-function with multiple in vitro and in vivo functional readouts and downstream signaling endpoint\",\n      \"pmids\": [\"23995859\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"EGFR/HER2 dimerization induces ACTA2 expression in breast cancer cells through a JAK2/STAT1 signaling pathway. STAT1 inhibition (fludarabine) or JAK2 inhibition (AG490) reduced basal ACTA2 expression, while STAT1 overexpression increased it. ACTA2 knockdown suppressed cell motility in vitro and reduced lung metastatic nodules in vivo.\",\n      \"method\": \"HER2 overexpression/siRNA knockdown; STAT1 inhibitor/JAK2 inhibitor treatment; STAT1 overexpression; ACTA2 shRNA; in vitro motility assays; in vivo lung metastasis model\",\n      \"journal\": \"Oncotarget\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple genetic and pharmacological perturbations connecting HER2/JAK2/STAT1 to ACTA2 and functional metastasis readouts\",\n      \"pmids\": [\"28881584\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"TGF-β2-induced epithelial-mesenchymal transition (EMT) in lens epithelial cells is associated with increased histone H4 acetylation specifically at the ACTA2 promoter region, driving ACTA2 expression. The histone deacetylase inhibitor trichostatin A (TSA) suppresses EMT and reduces ACTA2 levels by epigenetically reducing this promoter histone acetylation.\",\n      \"method\": \"TGF-β2 treatment of FHL124 lens epithelial cells; chromatin immunoprecipitation (ChIP) for acetylated histone H4 at ACTA2 promoter; RT-PCR and western blot for ACTA2; TSA treatment; cell migration assay\",\n      \"journal\": \"Eye (London, England)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — ChIP directly demonstrated histone acetylation at ACTA2 promoter with functional TSA intervention\",\n      \"pmids\": [\"25853442\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Deletion of Acta2 in cardiac fibroblasts does not prevent myofibroblast differentiation or impair post-myocardial infarction cardiac repair in mice. Loss of Acta2 causes compensatory transcriptional upregulation of other actin isoforms (especially Actg2 and Acta1), maintaining total filamentous actin and actin levels. Myocardin-related transcription factor-A (MRTF-A) is critical for myofibroblast differentiation but not for the compensatory upregulation of non-Acta2 isoforms, indicating functional redundancy among actin isoforms in cardiac myofibroblasts.\",\n      \"method\": \"Tamoxifen-inducible cardiac fibroblast-specific Acta2 knockout mice; post-MI cardiac function and histology; proliferation, migration, and contractility assays; RT-qPCR for actin isoforms; MRTF-A deletion\",\n      \"journal\": \"Journal of molecular and cellular cardiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean conditional KO with multiple functional readouts and mechanistic follow-up identifying compensatory isoforms and transcription factor dependency\",\n      \"pmids\": [\"36007455\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Downregulation of ACTA2 in neural stem cells (NSCs) inhibits migration by hindering actin filament polymerization via increased RhoA expression and decreased Rac1 expression, placing ACTA2 upstream of the RhoA/Rac1 balance in regulating NSC actin dynamics and migration.\",\n      \"method\": \"siRNA knockdown of ACTA2 in primary NSCs; migration assay; immunostaining; RT-PCR; RhoA and Rac1 expression analysis\",\n      \"journal\": \"Stem cells international\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — KD with migration phenotype and mechanistic pathway placement via RhoA/Rac1, single study\",\n      \"pmids\": [\"32508931\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Deletion of ACTA2 in mice promotes angiotensin II-induced progressive aortic lumen dilation, increased apoptosis (elevated Bax/Bcl-2 ratio), and phenotypic modulation of VSMCs (increased osteopontin expression, decreased α-SMA), demonstrating that ACTA2 is required to maintain VSMC contractile phenotype and resist AngII-induced aortic pathology.\",\n      \"method\": \"ACTA2 knockout mice; AngII osmotic minipump infusion; ultrasound aortic diameter measurement; TUNEL apoptosis assay; RT-qPCR and western blot for OPN, Bax/Bcl-2, α-SMA; histopathology\",\n      \"journal\": \"Journal of thoracic disease\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — in vivo KO with multiple functional readouts under AngII challenge\",\n      \"pmids\": [\"30233845\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"ACTA2 mutations impaired migration velocity and reduced contractility in VSMC-like cells transdifferentiated from patient fibroblasts, while ACTA2 and FBN1 DN variant cells showed decreased SMAD2 phosphorylation, placing ACTA2 upstream of TGFβ/SMAD signaling in VSMCs.\",\n      \"method\": \"Fibroblast-to-VSMC transdifferentiation protocol; migration velocity assay; collagen gel contraction assay; western blot for pSMAD2; comparison across ACTA2, MYH11, SMAD3, FBN1 variant patient cells\",\n      \"journal\": \"Human molecular genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — patient-derived VSMC-like cells with functional assays; limitation is that it is a patient variant cell model, not isogenic\",\n      \"pmids\": [\"34244757\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"3D molecular modeling of the actin filament revealed that residue R179 is positioned at the interface between the two strands of filamentous actin; the R179H mutation is predicted to destabilize inter-strand bundling, providing a structural explanation for the severe phenotype associated with this mutation.\",\n      \"method\": \"Actin three-dimensional molecular modeling; histopathological analysis of cerebral arteries from ACTA2 R179H patient (massive intimal SMC proliferation, medial fibromuscular proliferation, arterial fibrosis)\",\n      \"journal\": \"Acta neuropathologica communications\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 4 for structural claim — computational modeling only without experimental validation; histopathology supports phenotype but not the molecular mechanism directly\",\n      \"pmids\": [\"26637293\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Transient angiotensin II infusion causes sustained (memory) downregulation of Acta2 mRNA and protein in aortic tissue beyond the period of AngII treatment, associated with increased H3K27me3 at nuclei of aortic sections and sustained downregulation of the Acta2 transcriptional activator Myocardin (Myocd), indicating epigenetic silencing of ACTA2 as a vascular memory mechanism.\",\n      \"method\": \"Mouse AngII infusion with post-treatment follow-up; RNAseq profiling of aortic tissue; RT-qPCR and western blot validation; immunohistochemistry for H3K27me3; human aortic VSMC cell culture model\",\n      \"journal\": \"Frontiers in cardiovascular medicine\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — RNAseq plus IHC for H3K27me3 in vivo, replicated in cell culture; epigenetic mechanism identified but not fully dissected mechanistically\",\n      \"pmids\": [\"35360022\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1990,\n      \"finding\": \"The human vascular smooth muscle actin gene (ACTA2/ACTSA) was mapped to chromosome 10q22-q24 by Southern blot analysis of somatic cell hybrids and confirmed by in situ hybridization, placing it on a different chromosome from the other four actin genes examined at the time.\",\n      \"method\": \"Southern blot analysis of rodent-human somatic cell hybrids; in situ hybridization\",\n      \"journal\": \"The Japanese journal of human genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — direct chromosomal assignment by two independent methods (somatic cell hybrids and in situ hybridization)\",\n      \"pmids\": [\"2398629\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Missense variants in ACTA2 (G148R and R179H) injected as mRNA into one-cell stage zebrafish embryos caused significantly reduced cardiac shortening fractions, thinner myocardial walls, and decreased total cell number and proliferating cell numbers in the endothelial and myocardial regions compared to WT, demonstrating that pathogenic ACTA2 variants impair cardiac morphological development in vivo.\",\n      \"method\": \"Zebrafish mRNA injection of ACTA2 WT, G148R, and R179H; measurement of cardiac shortening fraction; histopathological evaluation at 72 hpf; quantification of proliferating cells (endothelial and myocardial)\",\n      \"journal\": \"Journal of human genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — in vivo zebrafish overexpression model with multiple cardiac readouts; single study\",\n      \"pmids\": [\"38316882\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"In aganglionic segments of Hirschsprung disease (HSCR), ACTA2 expression is abnormally elevated in circular smooth muscle cells beginning at embryonic day E15.5, and siRNA-mediated knockdown of Acta2 weakens the contraction ability of intestinal smooth muscle cells, establishing that elevated ACTA2 causes hyperactive contraction contributing to smooth muscle spasm in aganglionic bowel.\",\n      \"method\": \"Immunohistochemistry in HSCR patients and Ednrb-/- mice; siRNA knockdown of Acta2 in intestinal SMCs; contractility assay; developmental staging in Ednrb-/- mice\",\n      \"journal\": \"Pediatric surgery international\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — siRNA knockdown with contractility readout plus in vivo disease model; single study\",\n      \"pmids\": [\"37278766\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"RHOA knockdown significantly downregulates ACTA2 gene expression in both osteoblast-like and osteoclast-like human cells, placing RHOA upstream of ACTA2 in a bone cell regulatory pathway.\",\n      \"method\": \"siRNA knockdown of RHOA and ARHGEF3 in Saos-2 and hFOB 1.19 osteoblast-like cells and osteoclast-like cells; microarray followed by RT-qPCR validation\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — single knockdown with transcriptional readout; no direct functional consequence of ACTA2 changes measured\",\n      \"pmids\": [\"24840563\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"In a yeast model system, heterozygous expression of TAAD-associated ACTA2 missense variants in Saccharomyces cerevisiae (whose actin is highly conserved with human SM α-actin at the affected residues) causes abnormal actin cytoskeleton organization and mitochondrial distribution, consistent with a dominant-negative pathogenic mechanism of ACTA2 variants on actin function.\",\n      \"method\": \"Yeast expression of mutant ACTA2 alleles in heterozygous condition; spot test growth assay; fluorescence microscopy of actin cytoskeleton and mitochondria\",\n      \"journal\": \"European journal of human genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — orthologous yeast model with cytoskeletal and organelle readouts; mechanistic inference of dominant-negative mechanism supported by multiple variants\",\n      \"pmids\": [\"38486025\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"ACTA2-encoded smooth muscle α-actin is a major component of the contractile apparatus of vascular and visceral smooth muscle cells, where it polymerizes into stable filaments that interact with smooth muscle myosin to generate force; disease-causing mutations (e.g., R258C, R179H, R149C) disrupt filament stability, myosin interaction, and/or CCT-chaperone folding, shifting SMCs from a contractile to a proliferative/synthetic phenotype that drives aneurysm, dissection, and occlusive vascular disease, while its transcription is repressed by Purβ binding to a purine-rich ssDNA element in the promoter and activated via histone H4 acetylation downstream of TGFβ signaling.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"ACTA2 encodes smooth muscle α-actin, the principal actin isoform of the contractile apparatus in vascular and visceral smooth muscle cells (SMCs), where it polymerizes into filaments that interact with smooth muscle myosin to generate contractile force [PMID:26153420, PMID:24204762]. Disease-causing missense mutations (R258C, R179H, R149C) destabilize filaments, impair myosin-driven motility, alter chaperonin (CCT) folding, and shift SMCs from a contractile to a synthetic/proliferative state, causing thoracic aortic aneurysm and dissection, moyamoya-like cerebrovascular disease, and multiorgan smooth muscle dysfunction [PMID:26153420, PMID:34600884, PMID:40378078, PMID:40603847]. In cardiac fibroblasts, Acta2 deletion is compensated by upregulation of other actin isoforms (Actg2, Acta1), indicating functional redundancy in myofibroblast contexts [PMID:36007455]. Transcription of ACTA2 is repressed by Purβ binding to a purine-rich single-stranded element in the promoter and is activated through histone H4 acetylation downstream of TGFβ signaling and by Myocardin, with sustained silencing achievable via H3K27me3 epigenetic marks [PMID:23724822, PMID:25853442, PMID:35360022].\",\n  \"teleology\": [\n    {\n      \"year\": 1990,\n      \"claim\": \"Establishing the genomic location of ACTA2 on chromosome 10q22-q24 distinguished it from the other human actin genes and enabled subsequent genetic studies linking it to disease.\",\n      \"evidence\": \"Southern blot analysis of somatic cell hybrids and in situ hybridization\",\n      \"pmids\": [\"2398629\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No functional characterization accompanied the mapping\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Purβ was identified as a transcriptional repressor of ACTA2 that binds cooperatively to a purine-rich ssDNA element in the ACTA2 promoter, establishing a key mechanism controlling myofibroblast differentiation and ACTA2 expression levels.\",\n      \"evidence\": \"shRNA knockdown in MEFs; promoter-reporter assays; recombinant Purβ truncation and binding assays; confirmed by mutagenesis of specific Purβ residues (R267, K82, R159) in 2016\",\n      \"pmids\": [\"23724822\", \"27064749\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How Purβ/YBX1 repression is regulated during SMC differentiation in vivo is not defined\", \"Whether Purβ binding is modulated by chromatin context is unclear\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Direct demonstration that ACTA2 is required for myofibroblast contractility and motility — not just a marker — showed it participates in both mechanical and signaling (ERK1/2) functions in activated hepatic stellate cells.\",\n      \"evidence\": \"siRNA/shRNA knockdown in activated hepatic stellate cells; contraction and migration assays; ERK1/2 phosphorylation assessment\",\n      \"pmids\": [\"24204762\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether ERK1/2 regulation is direct or secondary to cytoskeletal remodeling is unknown\", \"Isoform specificity of signaling effect not tested\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Reconstitution of the R258C mutation revealed multiple allosteric effects on actin filament stability, cofilin severing, profilin binding, and myosin force generation, establishing the first molecular mechanism for how a single ACTA2 mutation causes vascular disease.\",\n      \"evidence\": \"Baculovirus-expressed mutant SM α-actin; TIRF microscopy; cofilin severing assay; profilin binding; loaded in vitro motility with SM myosin\",\n      \"pmids\": [\"26153420\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vivo consequences of R258C not tested in animal model in this study\", \"Whether tropomyosin isoform identity affects protection remains open\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"TGFβ-induced ACTA2 transcription was linked to histone H4 acetylation at the ACTA2 promoter, revealing an epigenetic activation mechanism during EMT.\",\n      \"evidence\": \"ChIP for acetyl-H4 at ACTA2 promoter in TGFβ2-treated lens epithelial cells; TSA treatment\",\n      \"pmids\": [\"25853442\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Specific histone acetyltransferase responsible not identified\", \"Whether this mechanism operates in vascular SMCs not tested\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Whole-animal Acta2 knockout demonstrated that loss of ACTA2 predisposes to angiotensin II-induced aortic dilation, VSMC apoptosis, and phenotypic modulation, directly linking ACTA2 to aortic structural integrity under hemodynamic stress.\",\n      \"evidence\": \"ACTA2 knockout mice with AngII infusion; ultrasound; TUNEL; RT-qPCR and western blot\",\n      \"pmids\": [\"30233845\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether aortic pathology is cell-autonomous to SMCs or involves adventitial/immune contributions not resolved\", \"Compensatory actin isoform expression not assessed\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"The R149C mutation was shown to be retained in the CCT chaperonin, reducing the pool of mutant actin incorporated into filaments and explaining the reduced disease penetrance of this variant compared to R258C and R179H.\",\n      \"evidence\": \"CRISPR knock-in mouse; co-IP of actin with CCT; TIRF polymerization; in vitro motility; aortic contraction assays\",\n      \"pmids\": [\"34600884\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether CCT retention is a generalizable quality-control mechanism for other ACTA2 variants is untested\", \"Structural basis of enhanced CCT binding not determined\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Conditional deletion of Acta2 in cardiac fibroblasts revealed functional redundancy among actin isoforms: compensatory upregulation of Actg2 and Acta1 preserved myofibroblast function and post-MI repair, resolving whether ACTA2 is essential for all myofibroblast contexts.\",\n      \"evidence\": \"Tamoxifen-inducible cardiac fibroblast-specific KO; post-MI functional assessment; RT-qPCR for actin isoforms; MRTF-A deletion\",\n      \"pmids\": [\"36007455\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether similar compensation occurs in vascular SMCs is unknown\", \"Mechanism triggering compensatory isoform upregulation not identified\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Transient AngII exposure was found to cause sustained epigenetic silencing of Acta2 via H3K27me3 and downregulation of Myocardin, establishing a 'vascular memory' mechanism that maintains SMC phenotypic modulation long after the stimulus is removed.\",\n      \"evidence\": \"Mouse AngII infusion with follow-up; RNAseq; IHC for H3K27me3; human VSMC culture\",\n      \"pmids\": [\"35360022\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Specific H3K27 methyltransferase (e.g. EZH2) acting at the Acta2 locus not identified\", \"Whether this epigenetic mark is reversible in vivo not tested\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"CRISPR base editing correction of R179H in iPSC-derived SMCs and AAV9 delivery in humanized mice rescued multiorgan smooth muscle dysfunction, providing the first gene-therapy proof of concept and confirming R179H causes a global contractile-to-synthetic switch.\",\n      \"evidence\": \"iPSC-SMC differentiation; adenine base editing; AAV9 systemic delivery in R179H knock-in mice; aortic, bladder, gut, kidney phenotyping\",\n      \"pmids\": [\"40378078\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Long-term durability and off-target editing effects not assessed\", \"Whether correction after disease onset can reverse established pathology is unknown\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"R179C mutant SMCs were shown to maintain stem cell-like features with elevated glycolysis, and boosting mitochondrial respiration with nicotinamide riboside drove differentiation and prevented moyamoya-like occlusive lesions, linking ACTA2 mutation to metabolic reprogramming as a druggable disease mechanism.\",\n      \"evidence\": \"SMC-specific knock-in mice; carotid artery injury; metabolic flux analysis; NR treatment; neurological phenotyping\",\n      \"pmids\": [\"40603847\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether metabolic shift is a direct consequence of defective actin filaments or secondary to impaired differentiation signaling is unresolved\", \"Efficacy in human patients unknown\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"No high-resolution experimental structure of disease-mutant SM α-actin filaments has been determined, and the precise structural basis by which individual mutations differentially affect filament inter-strand contacts, myosin binding interfaces, and chaperonin recognition remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"Cryo-EM or crystal structures of mutant SM α-actin filaments needed\", \"Genotype-phenotype correlation across the full spectrum of >50 known ACTA2 variants not systematically addressed at the biochemical level\", \"Whether pharmacological stabilization of mutant filaments can serve as therapy is untested\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0005198\", \"supporting_discovery_ids\": [0, 1, 6, 18]},\n      {\"term_id\": \"GO:0008092\", \"supporting_discovery_ids\": [0, 1]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005856\", \"supporting_discovery_ids\": [0, 1, 6, 11, 20]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-397014\", \"supporting_discovery_ids\": [0, 1, 6, 18]},\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [2, 3, 12, 13]},\n      {\"term_id\": \"R-HSA-74160\", \"supporting_discovery_ids\": [4, 5, 9, 15]}\n    ],\n    \"complexes\": [\n      \"actin-tropomyosin filament\"\n    ],\n    \"partners\": [\n      \"MYH11\",\n      \"PURB\",\n      \"YBX1\",\n      \"CFL1\",\n      \"PFN1\",\n      \"CCT\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}