Hypertrophic Cardiomyopathy 4

1. Disease Information

Overview

Hypertrophic cardiomyopathy 4 (CMH4) is the MYBPC3-related form of familial hypertrophic cardiomyopathy — the single most common genetic cause of HCM worldwide. It is defined by unexplained left ventricular hypertrophy (LVH), typically asymmetric and septal-predominant, in the absence of an alternative loading condition (hypertension, aortic stenosis) sufficient to explain it.

CMH4 is mechanistically distinctive among the sarcomeric HCMs. Whereas most HCM genes act through poison-peptide / dominant-negative missense alleles, ~90% of pathogenic MYBPC3 variants are truncating (frameshift, nonsense, splice-disrupting) and act through haploinsufficiency of cardiac myosin-binding protein C (cMyBP-C) — the truncated peptide is essentially never detectable in human myocardium.

CMH4 has two clinically and genetically distinct presentations:

1. Monoallelic (heterozygous) — classic autosomal dominant, adult-onset, age- and sex-dependent incomplete penetrance, generally favourable prognosis.
2. Biallelic (homozygous or compound heterozygous truncating) — a recessive, lethal neonatal cardiomyopathy with left ventricular noncompaction features and septal defects, essentially uniformly fatal in the first year without transplant.

OMIM captures both: "The transmission pattern of CMH4 was autosomal dominant in the families reported by Watkins et al. (1995) and autosomal recessive in the family reported by Wang et al. (2013). Incomplete penetrance was observed in both families." (OMIM 115197)

Key Identifiers

Synonyms

• CMH4
• Cardiomyopathy, familial hypertrophic, 4
• MYBPC3-related hypertrophic cardiomyopathy
• Myosin-binding protein C, cardiac, deficiency of (historical OMIM synonym)
• Left ventricular noncompaction 10 / LVNC10 (allelic; some MYBPC3 alleles)
• (Biallelic form) Severe neonatal / infantile MYBPC3 cardiomyopathy; "Amish nemaline-unrelated infantile HCM" (colloquial, Geauga County Old Order Amish)

Data provenance character

Evidence for CMH4 is overwhelmingly disease-level and cohort-aggregated, not EHR-individual: - Clinical cohorts / registries: SHaRe (Sarcomeric Human Cardiomyopathy Registry, n=4,756 genotyped), Dutch BIO FOr CARe founder-variant cohort, Spanish multicentre truncating-MYBPC3 cohort, UK Heart Hospital family series. - Aggregated variant resources: ClinVar, ClinGen (Hereditary Cardiovascular Disease GCEP), gnomAD. - Population biobanks (the exception — genome-first, individual-level): UK Biobank, Penn Medicine BioBank, ARIC — these produce the low-penetrance estimates. - Molecular: human myectomy/explant tissue, iPSC-CM and engineered cardiac tissue, mouse knock-in/knock-out.

2. Etiology

2.1 Primary causal factor

Germline pathogenic/likely pathogenic variants in MYBPC3 are the necessary cause. MYBPC3 accounts for ~50% of genetically explained nonsyndromic HCM, with MYH7 (~33%), TNNI3 (~5%), TNNT2 (~4%) and other sarcomere genes (<3% each) making up the remainder (GeneReviews, PMID:20301725).

ClinGen's Hereditary Cardiovascular Disease Gene Curation Expert Panel classifies the relationship as Definitive (SOP8, 2021-10-07):

MYBPC3 | HGNC:7551 | hypertrophic cardiomyopathy | MONDO:0005045 | AD | Definitive | SOP8 | Hereditary Cardiovascular Disease Gene Curation Expert Panel | 2021-10-07T16:00:00.000Z (ClinGen Gene-Disease Validity assertion, cached locally as CGGV_assertion_7e65896e-33f5-439d-8749-aba08a539dd0-2021-10-07T160000.000Z)

MYBPC3 is one of only 8 of 33 evaluated HCM genes to reach Definitive validity, alongside MYH7, TNNT2, TNNI3, TPM1, ACTC1, MYL2, MYL3 (ClinGen HCM reappraisal, PMC11312670; original framework Ingles et al., Circ Genom Precis Med 2019).

Notably, MYBPC3 is not validly associated with other cardiomyopathies — a useful negative constraint for curation:

MYBPC3 | HGNC:7551 | arrhythmogenic right ventricular cardiomyopathy | MONDO:0016587 | AD | Limited | SOP7 | ... | 2019-08-06 MYBPC3 | HGNC:7551 | dilated cardiomyopathy | MONDO:0005021 | AD | Limited | SOP10 | ... | 2025-05-16 MYBPC3 | HGNC:7551 | dilated cardiomyopathy | MONDO:0005021 | AR | Limited | SOP10 | ... | 2025-05-16

2.2 Genetic risk factors and modifiers

Population-specific high-frequency alleles (see §9): the South Asian MYBPC3 intron-32 25-bp deletion (~4% carrier frequency); the three Dutch founder truncating variants; the Amish/Swiss c.3330+2T>G splice allele; Icelandic and Northern Spanish founder alleles.

2.3 Environmental / acquired risk factors

There is no environmental factor that causes CMH4 — the genotype is necessary. Environmental factors act as penetrance and severity modifiers ("second hits"):

• Hypertension. In the Indian 25-bp-deletion families, "In three cases, hypertension coexisted with the deletion in the family members and these individuals showed severe phenotypes." (PMID:19151713, exact quote)
• Western/obesogenic diet. In heterozygous Mybpc3^c.772G>A knock-in mice that are otherwise phenotype-negative, Western diet feeding triggered cardiac dysfunction and hypertrophy — an explicit two-hit model (PMC11708371). This is MODEL_ORGANISM evidence and should be tagged as such.
• Ageing. "Aging further exacerbates the severity of HCM in carriers of MYBPC3 mutations." (PMID:38406555, exact quote)
• Intense/competitive athletic conditioning. Contributes to LV wall thickness and confounds diagnosis (athlete's heart differential); historically a trigger context for SCD, though modern guidelines have liberalised exercise restrictions.
• Catecholaminergic stress / exercise. Provokes symptoms and dynamic LVOT obstruction: OMIM notes symptoms "can be readily provoked by exercise."

2.4 Protective factors

• No validated genetic protective allele is established for CMH4. The nearest analogue is a low HCM polygenic risk score, which is associated with reduced penetrance/expressivity in sarcomere-variant carriers (Circulation 2025).
• Female sex is associated with lower penetrance (48.1% vs 65.1%, PMID:22267749) — though women with MYBPC3-HCM are diagnosed later and often with more advanced symptoms, so this is not protection against adverse outcome.
• Environmental: blood pressure control, weight management, avoidance of dehydration/volume depletion and of vasodilators in obstructive physiology. These are risk-mitigating rather than proven penetrance-preventing; no randomised prevention trial exists.

2.5 Gene–environment interactions

The dominant G×E model for CMH4 is haploinsufficiency + stressor:

1. A truncating MYBPC3 allele yields ~subnormal cMyBP-C protein but a compensated sarcomere for decades.
2. Hemodynamic (hypertension, afterload), metabolic (Western diet, obesity, diabetes), or proteostatic (age-related UPS decline, oxidative stress) stress exceeds reserve.
3. Hypertrophic remodelling becomes overt.

Evidence: mouse Western-diet two-hit model (PMC11708371); human hypertension co-modifier data (PMID:19151713); age-dependent penetrance (PMID:22267749); ageing/UPS review (PMID:38406555).

3. Phenotypes

3.1 Core cardiac structural phenotypes

3.2 Symptoms and functional phenotypes

3.3 Arrhythmic / electrophysiological phenotypes

3.4 Laboratory / biomarker abnormalities

3.5 Phenotype characteristics

Age of onset. Bimodal and genotype-dose-dependent: - Biallelic truncating: congenital/neonatal. "They died from cardiac failure before age 13 weeks." (PMID:25335496, exact quote). All 21 reported biallelic-truncating patients "were diagnosed with severe cardiomyopathy and/or died within the first few months of life." - Monoallelic: classically adult-onset, historically described as "late-onset." In the Indian 25-bp-deletion families, "In most carriers the effects remained dormant until the third decade" (PMID:19151713, exact quote). Spanish cohort mean age 47±16.8 yr (PMID:39581692). - But onset is extremely heterogeneous. In 9 R502W families (25 individuals) there was "marked heterogeneity in age at diagnosis (5 to 80 years)" (PMID:22267749, exact quote).

Severity. Variable. In the Spanish truncating-variant cohort, "Hypertrophy was discrete with a significative difference between probands and relatives (17.5±4 mm vs 14.6±5 mm; p<0.0001). Ejection fraction was predominantly preserved (65%±10%)." (PMID:39581692, exact quote).

Progression. Slowly progressive over decades in most; a minority progress to end-stage/"burnt-out" HCM with systolic dysfunction. Symptoms are typically episodic/exertional superimposed on a chronic substrate. Incident HCM phenotype in the Spanish cohort was 10% over 7.77 years mean follow-up (PMID:39581692).

Quality-of-life impact. Well quantified via KCCQ-CSS and HCMSQ. In EXPLORER-HCM, mavacamten produced "improved symptom scores (KCCQ-CSS +9·1, 5·5 to 12·7; HCMSQ-SoB −1·8, −2·4 to −1·2; p<0·0001)" and "34% more patients in the mavacamten group improved by at least one NYHA class" (PMID:32871100, exact quotes). Baseline NYHA II–III limitation, exercise intolerance, chest pain, and the psychological burden of SCD risk / ICD carriage / cascade-testing family implications are the principal QoL domains. Validated instruments: KCCQ-23/KCCQ-CSS, HCMSQ (Hypertrophic Cardiomyopathy Symptom Questionnaire), EQ-5D-5L, SF-36.

4. Genetic / Molecular Information

4.1 Causal gene

MYBPC3 — myosin-binding protein C, cardiac (hgnc:7551; OMIM *600958; UniProt Q14896; Ensembl ENSG00000134571; RefSeq NM_000256.3). - Locus 11p11.2; 35 exons; ~21 kb genomic. - Protein: 1,274 aa, ~141 kDa cardiac isoform. Domain architecture N→C: C0 (cardiac-specific Ig), Pro-Ala linker, C1 (Ig), M-domain (cardiac-specific, PKA-phosphorylatable regulatory motif), C2–C10 (mix of Ig-I and Fn3 domains). C8–C10 anchor to light meromyosin and titin; C0–C2 interacts with the myosin S2 and regulatory light chain and with actin. - Localisation: doublets in the C-zone of the A-band of the sarcomere (PMID:19151713).

4.2 Pathogenic variant spectrum

Variant classes. Truncating variants dominate:

"Truncating variants account for 91% of MYBPC3 pathogenic variants and cause similar clinical severity and outcomes regardless of location, consistent with locus-independent loss-of-function." — Helms et al., PMID:32841044

"Among HCM patients with genetic defects in MYBPC3, 90% of mutations are heterozygous frameshift, nonsense, or splice site mutations that result in premature termination codons and truncated cMyBP-C protein."

"The most striking characteristic of HCM mutations in MYBPC3 is that many are within introns and are predicted to cause aberrant splicing leading to a frameshift and a premature chain termination, yet the truncated peptides have never been identified in human heart tissue carrying these mutations." — Marston et al., PMID:22057632 (exact quote)

Breakdown by mechanism class: - Frameshift (indel) — e.g. c.2373dup p.(Trp792fs), c.836del p.(Gly279Valfs*21) - Nonsense — e.g. c.2827C>T p.(Arg943*) - Splice-site — e.g. c.3330+2T>G (Amish/Swiss), c.2905+1G>A, c.1224-19G>A (intronic, ClinVar RCV000009149) - Intronic deletion causing exon skipping — the South Asian 25-bp intron-32 deletion → skipping of exon 33 - Missense — a minority; in the UK proband series "Missense mutations (15, 45.6%) were the most frequent" among 42 mutations, illustrating cohort-dependent ascertainment (PMID:22267749). Some missense alleles (e.g. R502W, a recurrent founder-like allele) are well-established. - Copy-number / structural — whole-gene and multi-exon deletions in 18 probands, including a promoter deletion which formally proved that reduced transcription alone suffices: Hayesmoore et al., PMID:38258577, "A Promoter Deletion Confirms That MYBPC3 Haploinsufficiency Is Sufficient to Cause Hypertrophic Cardiomyopathy in Humans." - Alu-mediated insertion — reported cause of familial HCM (PMC6978237)

Selected variants of curation interest:

Variant classification. Per ACMG/AMP, MYBPC3 truncating variants readily reach P/LP via PVS1 (LOF is the established mechanism) + segregation + case–control data. ClinVar holds thousands of MYBPC3 submissions; the ClinGen Hypertrophic Cardiomyopathy Variant Curation Expert Panel provides gene-specific PVS1/PM2/PS4 calibration. A computational subdomain-stability predictor has been developed to improve missense-variant interpretation and risk stratification (Genetics in Medicine 2021).

Allele frequency. Population filtering thresholds: "variants in MYBPC3 present in gnomAD with allele frequencies of >4E-05 ... and absent in disease registries are unlikely to be independently pathogenic for HCM." Founder alleles are the exception — the South Asian 25-bp deletion is present at 2–8% across Indian populations, and the Amish c.3330+2T>G at ~10% carrier frequency in one settlement, both far above any generic filtering threshold. This is a critical caveat: population-frequency-based filtering will falsely benign-call founder alleles.

Germline vs somatic. Exclusively germline. No somatic role; CMH4 is not a neoplastic disease and COSMIC/TCGA are not applicable.

Functional consequence. Loss of function via haploinsufficiency — see §6. Not dominant-negative in the classical poison-peptide sense for truncating alleles; some missense alleles may have additional dominant-negative/incorporation effects (an area of active debate — see Barefield & colleagues, "Is haploinsufficiency a sufficient mechanism for MYBPC3 truncating mutations?", PMID:36946992).

4.3 Modifier genes

• UPF3B (NMD factor) — upregulated specifically in MYBPC3^trunc hearts and localised to the Z-disc, where sarcomeric protein translation occurs; determines the degree of transcript decay (PMID:37797718).
• Second sarcomere gene variants (MYH7, TNNT2, TNNI3) in compound genotypes — earlier, more severe disease (PMID:22267749; PMID:39581692, in which heart failure was "commonly found in the presence of a second [variant]").
• RAAS pathway polymorphisms — ACE I/D, AGT M235T, CMA1, AGTR1, CYP11B2 (PMC3449069).
• Common-variant polygenic background / HCM PRS (Circulation 2025).

4.4 Epigenetics

No disease-defining epigenetic lesion. Reported findings are secondary remodelling signatures in HCM myocardium generally: differential DNA methylation and histone-modification changes at hypertrophic-gene loci, and re-activation of the fetal gene programme (NPPA, NPPB, MYH7). Multi-omic MYBPC3 work identifies transcriptional and post-transcriptional dysregulation (PMID:38406555; Frontiers 2025 P459fs multi-omics, PMC11903464). ENCODE/Roadmap Epigenomics heart tissue tracks provide the regulatory landscape for MYBPC3 (including the promoter region whose deletion is causal, PMID:38258577), but no CMH4-specific epigenomic biomarker exists.

4.5 Chromosomal abnormalities

Not a chromosomal disorder. Relevant large-scale events are intragenic/whole-gene CNVs at 11p11.2, detected in ~1–2% of otherwise genotype-negative HCM probands (Mademont-Soler et al., PMID:28771489; Hayesmoore et al., PMID:38258577). Karyotyping and standard CMA are not indicated; gene-panel-integrated CNV calling or MLPA is the appropriate modality.

5. Environmental Information

• Environmental toxicants / radiation / pollution / occupational exposure: No established role in CMH4 causation. CTD lists no exposure with a validated MYBPC3-HCM interaction. Cardiotoxic exposures (anthracyclines, alcohol) cause distinct cardiomyopathies and are relevant only as confounders/differentials.
• Lifestyle factors:
• Diet. Western/high-fat-high-sugar diet acts as a disease trigger in phenotype-negative heterozygous Mybpc3 knock-in mice (PMC11708371) — MODEL_ORGANISM evidence; human replication is lacking.
• Obesity / metabolic syndrome. Associated with worse phenotype and adverse outcome in HCM cohorts generally.
• Hypertension. Co-modifier with severe phenotype in MYBPC3 carriers (PMID:19151713).
• Exercise. Provokes symptoms and gradients (OMIM); competitive athletics historically restricted, now individualised under the 2024 guideline (PMID:38718139).
• Alcohol. Vasodilation can worsen dynamic obstruction acutely.
• Dehydration/volume depletion. Precipitates obstruction and syncope.
• Infectious agents: Not applicable. No pathogen causes or triggers CMH4. (Viral myocarditis is a differential for acute decompensation, not an etiology.)

6. Mechanism / Pathophysiology

6.1 The causal chain (upstream → downstream)

[MOLECULAR] MYBPC3 truncating variant (PTC-generating: frameshift / nonsense /
    splice-disrupting / exon-skipping deletion; or promoter/whole-gene CNV)
    │
    ▼
[MOLECULAR] Nonsense-mediated mRNA decay (UPF3B-dependent, at the Z-disc)
    + allelic imbalance (mutant:WT mRNA ratio falls below 1:1)
    + ubiquitin-proteasome degradation of any escaping truncated peptide
    │
    ▼
[MOLECULAR] cMyBP-C HAPLOINSUFFICIENCY — reduced cMyBP-C content in the
    C-zone of the A-band; NO detectable poison peptide
    │
    ▼
[MOLECULAR] Loss of the cMyBP-C brake on the thick filament:
    • Fewer myosin heads held in the SUPER-RELAXED (SRX) state
    • Shift toward disordered-relaxed (DRX) → more heads available for actin
    • Increased Ca²⁺ sensitivity of force; accelerated cross-bridge kinetics
    • Loss of PKA-phosphorylation-dependent (M-domain Ser273/282/302)
      adrenergic modulation of contractility
    │
    ▼
[CELLULAR]  HYPERCONTRACTILITY + IMPAIRED RELAXATION of the cardiomyocyte
    (systolic hypercontractility with diastolic failure to relax)
    │
    ├──► [CELLULAR] Increased sarcomeric ATP consumption / energetic
    │              inefficiency (PCr/ATP falls); metabolic remodelling
    │
    ├──► [CELLULAR] Progressive Ca²⁺-handling abnormality
    │              (slowed Ca²⁺ release/reuptake; SERCA2a/RyR2 changes)
    │
    └──► [CELLULAR] Pro-hypertrophic signalling activation
                   (Ca²⁺-calcineurin-NFAT, CaMKII, MAPK/ERK,
                    PI3K-AKT-mTOR; fetal gene programme reactivation)
    │
    ▼
[TISSUE]    Cardiomyocyte hypertrophy → asymmetric septal-predominant LVH
    + MYOCYTE DISARRAY (loss of parallel myofibre alignment)
    + cardiac-fibroblast activation → interstitial and replacement FIBROSIS
    + intramural small-vessel disease (medial hyperplasia) → microvascular ischemia
    │
    ▼
[ORGAN]     • Diastolic dysfunction, elevated LV filling pressures
    • Dynamic LVOT obstruction (septal bulge + SAM of mitral valve)
    • Left atrial dilation → atrial fibrillation
    • Arrhythmogenic substrate (disarray + fibrosis + ischemia) → VT/VF
    • Late: LV systolic dysfunction ("burnt-out" HCM)
    │
    ▼
[ORGANISM]  Dyspnea, angina, syncope, heart failure, AF/stroke, SUDDEN CARDIAC DEATH

6.2 Key mechanistic evidence (verbatim quotes)

Haploinsufficiency, not poison peptide (PMID:22057632, Marston et al. 2012):

"Instead of expression of a poison peptide we consistently observe haploinsufficiency of MyBP-C in MYBPC3 mutant human heart muscle."

Corroborated in myectomy tissue (Marston et al., Circ Res 2009, PMID:19574547, "Evidence from human myectomy samples that MYBPC3 mutations cause hypertrophic cardiomyopathy through haploinsufficiency") and formally proven by promoter deletion (PMID:38258577), whose accompanying mouse showed "heterozygous for an Mybpc3 promoter deletion developed a late-onset phenotype of asymmetrical septal hypertrophy associated with fibrosis."

NMD is the proximal mechanism (PMID:37797718, Burkart et al. 2023):

"We show that cMyBP-C haploinsufficiency starts at the mRNA level, despite hypertrophy-induced increased transcriptional activity." "Strikingly, we show that in sarcomeres UPF3B but not UPF1 and UPF2 are localized to the Z-discs, the presumed location of sarcomeric protein translation. Our data suggest that cMyBP-C haploinsufficiency in HCM-patients is established by UPF3B-dependent NMD during the initial translation round at the Z-disc."

Multiple degradative routes converge (PMID:38406555, Ananthamohan et al. 2024):

"Pathogenesis related to MYBPC3 mutations includes nonsense-mediated decay, alternative splicing, and ubiquitin-proteasome system events, leading to allelic imbalance and haploinsufficiency."

Myosin dysregulation / SRX loss. Toepfer et al., Sci Transl Med 2019 (PMID:30674652), "Hypertrophic cardiomyopathy mutations in MYBPC3 dysregulate myosin"; McNamara et al., PLoS One 2017 (PMID:28658286) — MYBPC3-mutant patient myocardium shows "a significantly diminished SRX, characterized by a decrease in both the number of myosin heads in the SRX and the lifetime of ATP turnover"; McNamara et al., J Mol Cell Cardiol 2016 (PMID:27021517) — cMyBP-C ablation disrupts SRX in murine cardiomyocytes. The SRX state consumes ATP ~10-fold slower than DRX, so SRX loss directly links to energetic inefficiency.

Contractile trajectory: hypercontractile → hypocontractile, Ca²⁺-mediated (PMID:36893011, De Lange et al. 2023, human isogenic iPSC-CM engineered cardiac tissue, IN_VITRO):

"Our data suggest a progressive phenotype caused by cMyBP-C haploinsufficiency and ablation that initially is hypercontractile, but progresses to hypocontractility with impaired relaxation. The severity of the phenotype correlates with the amount of cMyBP-C present, with more severe earlier phenotypes observed in cMyBP-C-/- than cMyBP-C+/- ECTs. We propose that while the primary effect of cMyBP-C haploinsufficiency or ablation may relate to myosin crossbridge orientation, the observed contractile phenotype is Ca2+-mediated." "RNA-seq analysis revealed enrichment of differentially expressed hypertrophic, sarcomeric, Ca2+-handling, and metabolic genes in cMyBP-C+/- and cMyBP-C-/- ECTs."

Sarcomeric disorganisation as a proximal cellular lesion (PMID:19151713, IN_VITRO, neonatal rat cardiomyocytes):

"Staining with antibodies to the myc tag showed a highly disorganized and diffused pattern of sarcomeric architecture as a result of aberrant incorporation of altered proteins"

Pre-hypertrophic (subclinical) changes. Helms et al., JCI Insight 2020 (PMID:31877118), "Effects of MYBPC3 loss-of-function mutations preceding hypertrophic cardiomyopathy" — sarcomere-variant carriers without LVH already show diastolic abnormalities, elevated troponin, and profibrotic signalling.

Fibroblast-autonomous fibrosis. Zou et al., Cell Death Dis 2022 (PMID:36357371), "MYBPC3 deficiency in cardiac fibroblasts drives their activation and contributes to fibrosis" — a non-cardiomyocyte arm of the mechanism.

Ongoing debate. Barefield, J Gen Physiol 2023 (PMID:36946992), "Is haploinsufficiency a sufficient mechanism for MYBPC3 truncating mutations?" — argues additional/parallel mechanisms may contribute for some alleles. Flag as an open mechanistic question (dismech KNOWLEDGE_GAP).

6.3 Ontology term suggestions for pathophysiology nodes

GO Biological Process:

GO Molecular Function / Cellular Component (for MYBPC3 itself): - MF: GO:0008307 structural constituent of muscle; GO:0032036 myosin heavy chain binding; GO:0051015 actin filament binding - CC: GO:0031430 M band; GO:0030017 sarcomere; GO:0031672 A band; GO:0005865 striated muscle thin filament; GO:0030018 Z disc (UPF3B localisation)

CL Cell Types: - CL:0000746 cardiac muscle cell (cardiomyocyte) — primary - CL:2000046 ventricular cardiac muscle cell - CL:0002548 fibroblast of cardiac tissue — fibrosis arm - CL:0000359 vascular associated smooth muscle cell — intramural small-vessel disease - CL:0002138 endothelial cell of lymphatic vessel / CL:0000115 endothelial cell — microvascular arm

6.4 Molecular profiling

• Transcriptomics. RNA-seq of MYBPC3^+/- and MYBPC3^-/- engineered cardiac tissue shows enrichment of hypertrophic, sarcomeric, Ca²⁺-handling, and metabolic gene sets (PMID:36893011). GSEA of human MYBPC3^trunc myocardium shows increased NMD-component expression (PMID:37797718). Allelic-imbalance RNA-seq is the standard assay for demonstrating NMD in patient tissue (PMID:30456444).
• Spatial transcriptomics. Focal myocyte-disarray regions vs normal regions of human HCM myocardium have been profiled (PMC10454036) — reveals region-specific fibrotic/ECM programmes.
• Proteomics. Quantitative western/MS of myectomy tissue demonstrates ~30–50% reduction of cMyBP-C protein with absence of any truncated species (PMID:19574547; PMID:22057632). Human Protein Atlas confirms heart-restricted MYBPC3 expression.
• Multi-omics. MyBPC3 P459fs multi-omics + super-resolution imaging (PMC11903464).
• Metabolomics/energetics. ³¹P-MRS shows reduced myocardial PCr/ATP ratio in HCM including in G+/LVH− carriers — the energy-depletion hypothesis. Lipidomics: no CMH4-specific signature.
• Functional genomics. CRISPR-Cas9 isogenic iPSC allelic series (WT / +/− / −/−) is the workhorse (PMID:36893011). No CMH4-specific DepMap/genome-wide screen.

7. Anatomical Structures Affected

Organ level

• Primary organ: heart — UBERON:0000948 (heart)
• UBERON:0002084 heart left ventricle — principal site
• UBERON:0002094 interventricular septum — site of maximal, asymmetric hypertrophy
• UBERON:0002349 myocardium — the affected tissue proper
• UBERON:0002079 left cardiac atrium — secondary dilation → AF
• UBERON:0002078 right cardiac atrium
• UBERON:0002135 mitral valve — SAM, secondary MR
• UBERON:0004145 left ventricular outflow tract — dynamic obstruction
• UBERON:0001621 coronary artery / intramural arterioles — small-vessel disease
• Secondary organ involvement:
• Brain (UBERON:0000955) — cardioembolic stroke from AF; syncope-related injury
• Lung (UBERON:0002048) — pulmonary congestion/oedema from elevated filling pressures
• Liver (UBERON:0002107), kidney (UBERON:0002113) — congestive/low-output injury in advanced HF
• Body systems: cardiovascular (primary); respiratory and nervous systems secondarily.
• Biallelic neonatal form adds: interatrial septum (ASD), interventricular septum (VSD), ductus arteriosus (PDA) — i.e. structural congenital heart disease alongside the cardiomyopathy (PMID:25335496; PMID:41488457).

Tissue and cell level

• Tissue types: cardiac muscle tissue (striated); cardiac connective tissue/ECM (interstitial and replacement fibrosis); vascular smooth muscle (intramural arteriolar medial hyperplasia).
• Cell populations: CL:0000746 cardiac muscle cell (primary target); CL:2000046 ventricular cardiac muscle cell; CL:0002548 fibroblast of cardiac tissue (activated → myofibroblast, CL:0000186); CL:0000359 vascular associated smooth muscle cell; CL:0000235 macrophage (inflammatory infiltrate in advanced disease).

Subcellular level

• GO:0030017 sarcomere — the primary compartment
• GO:0031430 M band and GO:0031672 A band (C-zone) — cMyBP-C's native location
• GO:0030018 Z disc — where UPF3B-dependent NMD is proposed to act (PMID:37797718)
• GO:0016529 sarcoplasmic reticulum — Ca²⁺-handling arm
• GO:0005739 mitochondrion — energetic inefficiency
• GO:0000502 proteasome complex — mutant peptide degradation
• GO:0005634 nucleus / GO:0005681 spliceosomal complex — aberrant splicing arm

Localisation

• Lateralisation: intrinsically left-sided and asymmetric — septal-predominant LVH is the signature. Apical, concentric, mid-cavity, and eccentric variants occur (PMID:22267749). Right ventricular hypertrophy is uncommon and secondary.
• Regional heterogeneity: LGE (fibrosis) on CMR is characteristically patchy/mid-wall, concentrated at RV insertion points and within the hypertrophied septum.

8. Temporal Development

Onset

Verbatim anchors: - Biallelic: "They died from cardiac failure before age 13 weeks." / "All patients with biallelic truncating pathogenic mutations in MYBPC3 reported so far (n=21) were diagnosed with severe cardiomyopathy and/or died within the first few months of life." (PMID:25335496) - Monoallelic: "In most carriers the effects remained dormant until the third decade and then manifested themselves as mild hypertrophy" (PMID:19151713) - Extreme heterogeneity: "marked heterogeneity in age at diagnosis (5 to 80 years)" (PMID:22267749)

The historical framing of MYBPC3 as uniformly "late-onset and benign" has been substantially revised. Page et al. tested exactly that hypothesis and found: "Small selected cohort studies suggest that mutations in the cardiac myosin binding protein-C (MYBPC3) gene cause late-onset, clinically benign hypertrophic cardiomyopathy (HCM). The aim of this study was to test this hypothesis..." → "Disease expression in families with HCM related to MYBPC3 mutations shows marked heterogeneity with incomplete, age-related, and gender specific penetrance." (PMID:22267749)

Progression and staging

A widely used conceptual staging for sarcomeric HCM (applies to CMH4):

• Rate: slow in most. Incident phenotype conversion 10% over 7.77 years mean follow-up in the Spanish truncating-variant cohort (PMID:39581692). Longitudinal biobank data show low annual conversion rates in genome-first carriers (PMID:39886308).
• Course pattern: chronic, slowly progressive, punctuated by episodic exertional symptoms and by discrete arrhythmic events (AF onset, VT).
• Duration: lifelong; no spontaneous remission.

Patterns

• Remission: none spontaneous. Treatment-induced symptomatic and haemodynamic remission is achievable — cardiac myosin inhibitors normalise LVOT gradients in a majority; septal reduction therapy abolishes obstruction durably.
• Critical periods / intervention windows:
• Adolescence and young adulthood — highest relative SCD risk; peak yield of surveillance imaging.
• G+/LVH− window — the theoretical target for disease-modifying/preventive therapy (currently investigational; VANISH trial of valsartan in early sarcomeric HCM is the flagship attempt).
• Neonatal period (biallelic) — the only window for transplant listing.
• Pre-conception / prenatal (biallelic-at-risk couples) — PGT window (PMID:41488457).

9. Inheritance and Population

Epidemiology

Curation note for the dismech Prevalence slots: the honest structured record is measure_type: POINT_PREVALENCE, prevalence_class: BAND_1_5_PER_10000 for the derived CMH4 estimate, with the verbatim source phrasing in notes and the derivation flagged. Do not assert a single hard number as if directly measured.

Inheritance

• Pattern: Autosomal dominant (HP:0000006) for monoallelic disease; autosomal recessive (HP:0000007) for the biallelic lethal neonatal form. Both are recognised at OMIM 115197. A CMH4 entry should carry both Inheritance blocks with bound terms, keyed to the respective subtypes.
• Penetrance: Incomplete, age-dependent, and sex-dependent (HP:0003829 Incomplete penetrance).

Sources: Topriceanu et al., Circulation 2024, PMID:37929589 — "The penetrance of a pathogenic/likely pathogenic (P/LP) sarcomeric mutation is low in the general population at 11% but five-fold higher at 57% in patients with HCM and their family members"; PMID:22267749 (exact quotes above); GeneReviews per-gene table (PMID:20301725): MYL3 ~32%, CSRP3 38%, TPM1 ~49%, MYBPC3 ~55%, TNNT2 ~62%, MYH7 ~64%.

• Expressivity: Highly variable, including within a single family carrying an identical allele. The R502W series is the canonical demonstration: 9 families / 25 individuals with "marked heterogeneity in age at diagnosis (5 to 80 years), pattern of hypertrophy (11 none, 9 asymmetrical, 3 concentric, 1 apical, 1 eccentric), and prognosis (premature sudden death in 2 individuals compared with survival to advanced age in 6 individuals)" (PMID:22267749, exact quote).
• Genetic anticipation: Not a feature — MYBPC3 is not a repeat-expansion locus. Apparent anticipation in pedigrees reflects ascertainment bias and earlier cascade screening.
• Germline mosaicism: Rare; occasional de novo MYBPC3 variants are reported. Recurrence-risk counselling for apparently de novo cases should acknowledge low-level parental gonadal mosaicism, but no quantified rate exists for MYBPC3.
• Founder effects: Prominent — see table below.
• Consanguinity: A major driver of the biallelic form. Homozygous truncating cases arise both from consanguinity and from founder-allele population frequency (the Amish setting is the archetype of the latter).
• Carrier frequency: Population-dependent. General populations: MYBPC3 P/LP carrier frequency is roughly 1 in 200–500 for HCM-associated sarcomere variants overall. Founder settings dramatically exceed this.

Founder alleles / geographic distribution of specific variants

The South Asian deletion is quantitatively striking (verbatim, PMID:19151713):

"Here, we describe a deletion of 25 bp in the gene encoding cardiac myosin binding protein C (MYBPC3) that is associated with heritable cardiomyopathies and an increased risk of heart failure in Indian populations (initial study OR = 5.3 (95% CI = 2.3-13), P = 2 x 10(-6); replication study OR = 8.59 (3.19-25.05), P = 3 x 10(-8); combined OR = 6.99 (3.68-13.57), P = 4 x 10(-11)) and that disrupts cardiomyocyte structure in vitro. Its prevalence was found to be high (approximately 4%) in populations of Indian subcontinental ancestry."

Population attributable risk of that deletion is ~4.5%; the TMRCA of the deletion haplotype is ~33 ± 23 thousand years, with no evidence of positive selection.

Demographics

• Sex ratio: Carrier ratio ~1:1 (autosomal). Penetrance is male-biased (65.1% vs 48.1%), so diagnosed CMH4 skews male, roughly 1.3–1.5:1. Women present later and with more advanced NYHA class — a recognised diagnostic inequity.
• Age distribution of affected individuals: peak diagnosis 4th–6th decades for monoallelic disease (Spanish cohort mean 47±16.8 yr, PMID:39581692); a distinct neonatal cluster for biallelic disease.
• Geographic: worldwide; founder clusters as above. Within India, deletion frequency is significantly higher in southern and western states than northern (P<4×10⁻⁸), paralleling a large cardiac-mortality gradient (386–422 vs 76–99 cardiac deaths/100,000) (PMID:19151713).

10. Diagnostics

Clinical tests

Imaging - Transthoracic echocardiography — first-line. Establishes max LV wall thickness (≥15 mm adults; ≥13–14 mm with family history; z-score >3 in children), morphology, SAM, LVOT gradient at rest and with Valsalva/exercise provocation, diastolic function, LA size. (PMID:20301725; PMID:38718139) - Exercise (stress) echocardiography — mandatory when resting gradient <50 mmHg but symptoms suggest obstruction. - Cardiac MRI with late gadolinium enhancement (LGE) — quantifies wall thickness where echo windows fail (apical/anterolateral), detects apical aneurysm, and quantifies fibrosis burden, an independent SCD risk marker (LGE ≥15% of LV mass). Also detects the pre-hypertrophic markers (myocardial crypts, elongated mitral leaflets) in G+/LVH− carriers. - Cardiac CT — when CMR contraindicated. - RadLex/DICOM applicable; no CMH4-specific imaging biomarker beyond generic HCM markers.

Electrophysiology - 12-lead ECG — abnormal in >90% of overt HCM; LVH voltage, deep T-wave inversion, pathological Q waves, left-axis deviation. Frequently abnormal before LVH in carriers. - Ambulatory (24–48 h Holter) ECG or extended monitoring — detects NSVT (SCD risk factor) and paroxysmal AF. Guidelines recommend periodic monitoring. - Exercise treadmill testing with BP response — abnormal blood-pressure response is an SCD risk factor; also yields functional capacity. - Cardiopulmonary exercise testing (pVO₂) — the primary endpoint in EXPLORER-HCM and SEQUOIA-HCM; used for transplant evaluation. - Invasive EP study — not routine.

Laboratory / biomarkers - NT-proBNP (LOINC:33762-6) and BNP — severity/prognosis; treatment-response marker for myosin inhibitors. - High-sensitivity troponin I/T — elevated even pre-hypertrophy (PMID:31877118). - Phenocopy screen (essential, per PMID:38718139 — "HCM genetic testing should include genes for HCM phenocopies"): - α-galactosidase A activity (males) + GLA sequencing / plasma lyso-Gb3 → Fabry disease - Serum/urine free light chains, SPEP/UPEP, immunofixation + ⁹⁹ᵐTc-PYP/DPD bone scintigraphy → cardiac amyloidosis (ATTR/AL) - Creatine kinase, LAMP2/PRKAG2 testing → Danon disease, PRKAG2 glycogen storage cardiomyopathy - Carnitine, acylcarnitine profile, lactate → metabolic/mitochondrial phenocopies - Consider RASopathy panel in paediatric/syndromic presentations

Biopsy / pathology - Endomyocardial biopsy is not routine for CMH4 diagnosis; reserved for suspected infiltrative disease. - Histopathology (myectomy, explant, or autopsy) shows the classic triad: cardiomyocyte hypertrophy, myocyte disarray (loss of parallel alignment — the histopathological hallmark), and interstitial/replacement fibrosis, plus intramural small-vessel medial hyperplasia. From the Indian series (PMID:19151713, exact quotes): "Histopathological section of the same subject showing hypertrophied myofibers separated from each other by increased connective tissue"; "'swirling' of hypertrophied myofibers amid connective tissue disarray." In the lethal neonatal case: "Postmortem examination revealed severe HCM, an atrial septal defect (ASD), and extensive myocardial necrosis and fibrosis." (PMID:41488457, exact quote). - Myoarchitectural disarray in HCM appears to begin pre-birth (PMC6794206).

Genetic testing

Recommended approach (2024 AHA/ACC guideline, PMID:38718139; GeneReviews, PMID:20301725):

1. Genetic counselling before and after testing — "Evaluation by a genetic counselor is recommended to discuss risk and benefits of genetic testing."
2. Multigene HCM panel in the proband — must include the 8 definitive sarcomere genes (MYBPC3, MYH7, TNNT2, TNNI3, TPM1, ACTC1, MYL2, MYL3) plus phenocopy genes (GLA, LAMP2, PRKAG2, TTR, PTPN11 and other RASopathy genes, GAA, DES, FHL1, CSRP3, PLN, ALPK3). Routine genetic testing is now recommended for all children meeting HCM diagnostic criteria.
3. Integrated CNV/deletion-duplication analysis or MLPA — essential, because whole-gene and promoter MYBPC3 deletions are pathogenic and missed by sequence-only panels (PMID:38258577; PMID:28771489).
4. Cascade (predictive) testing of first-degree relatives — "Cascade genetic testing should be extended to first-degree relatives only if a pathogenic variant is identified in the proband." VUS results must not be used for cascade testing.
5. Reflex to WES/WGS when panel is negative and phenotype is atypical/syndromic. WGS additionally captures deep-intronic spliceogenic variants — an established elusive class in MYBPC3 (Sci Rep 2022, PMC9068804).
6. RNA studies (allelic imbalance / RT-PCR splicing assay) — the definitive functional confirmation for candidate spliceogenic MYBPC3 variants and for demonstrating haploinsufficiency (PMID:30456444; PMID:19151713 used exactly this: cDNA from endomyocardial biopsy showed "a normal transcript and a mutant transcript with absence of exon 33").
7. Molecular autopsy / postmortem trio WES in unexplained sudden infant or young-adult death — PMID:41488457 is the worked example: "This 'molecular autopsy' established a definitive cause for the infant's death, linking a novel variant to a severe pathological phenotype. Crucially, the diagnosis guided the clinical management of the asymptomatic carrier parents, prompting long-term cardiac surveillance and enabling preimplantation genetic testing (PGT) for future family planning." (exact quote)

Modalities NOT indicated: karyotyping, FISH, chromosomal microarray (unless syndromic features present), mtDNA testing (unless mitochondrial phenocopy suspected), repeat-expansion testing.

Omics-based diagnostics

• RNA-seq / targeted RT-PCR: clinically deployed for splice-variant resolution and allelic imbalance (see above).
• Proteomics / metabolomics / epigenomics / liquid biopsy: research-stage only for CMH4. Circulating microRNA and proteomic panels for HCM phenotype conversion are in development but not validated.

Clinical criteria and differential diagnosis

Diagnostic criteria: unexplained LVH with maximal wall thickness ≥15 mm (adults), ≥13 mm with family history or a known pathogenic variant, or z-score >3 (children), in the absence of abnormal loading conditions. (2024 AHA/ACC, PMID:38718139; 2023 ESC Cardiomyopathy Guidelines.)

Differential diagnosis — the phenocopies (must be excluded):

Screening

• Cascade genetic testing of first-degree relatives once a P/LP variant is identified (the highest-yield strategy).
• Clinical surveillance of at-risk relatives (GeneReviews, PMID:20301725):
• Variant-positive relatives: echocardiography + ECG every 1–2 years.
• Genetic status unknown, children/adolescents: exam + ECG + echo every 2–3 years, starting before puberty.
• Genetic status unknown, adults: every 3–5 years.
• No newborn screening exists or is recommended (no RUSP inclusion; no treatment window that screening would open for monoallelic disease). Founder-population carrier screening is a legitimate, locally-implemented exception (Amish community programmes; Dutch founder-variant cascade programmes).
• Preconception/prenatal: carrier screening in couples at risk for biallelic disease; PGT-M is established (PMID:41488457).

11. Outcome / Prognosis

Survival and mortality

Contemporary HCM cohorts under modern care (ICDs, myectomy, anticoagulation) approach near-normal life expectancy for many patients — a substantial improvement over historical tertiary-referral estimates. The MYBPC3 genotype has historically been described as favourable relative to MYH7; the Spanish cohort supports this ("previously associated with a favourable prognosis"; low event rates), while Page et al. caution that heterogeneity is such that genotype alone must not drive individual prognostication.

Morbidity, disability, and quality of life

• Functional limitation: NYHA class II–III in trial-eligible obstructive patients; reduced pVO₂.
• Atrial fibrillation and stroke: ~20% AF overall, ~60% by age 60 if diagnosed <40 (PMID:20301725); thromboembolic stroke is a leading cause of morbidity, hence mandatory anticoagulation.
• Heart failure: 8.1% in the Spanish MYBPC3 cohort (PMID:39581692); progression to advanced HF requiring transplant in a minority.
• ICD-related morbidity: inappropriate shocks, lead complications, infection, psychological impact.
• QoL instruments: KCCQ-CSS and HCMSQ-SoB are the validated, trial-endorsed instruments (both used as EXPLORER-HCM secondary endpoints, PMID:32871100). EQ-5D and SF-36 also applied. ICF domains affected: mobility (d450 walking), major life areas (d840–859 work), recreation/leisure (d920 — sport restriction).
• Family/psychosocial burden: cascade-testing anxiety, insurance/employment implications, reproductive decision-making, bereavement after a family SCD.

Complications

Atrial fibrillation → cardioembolic stroke; ventricular tachyarrhythmia → SCD; progressive diastolic HF; end-stage systolic ("burnt-out") HCM; apical aneurysm with mural thrombus; infective endocarditis (obstructive phenotype, historically); mitral regurgitation; conduction disease (post-septal reduction therapy, complete heart block requiring pacing).

Prognostic factors and risk stratification

HCM Risk-SCD (ESC) and the 2024 AHA/ACC risk-marker approach are the tools. Major risk markers: - Prior cardiac arrest / sustained VT - Family history of SCD in a first-degree relative - Unexplained syncope - Maximal LV wall thickness ≥30 mm - Non-sustained VT on ambulatory monitoring - LV apical aneurysm - LVEF <50% - Extensive LGE (≥15% LV mass) on CMR — arbitrator for intermediate-risk patients - Abnormal BP response to exercise (ESC model)

Genotype-specific prognostic points for CMH4: - Truncating MYBPC3 variant location does not predict outcome — "cause similar clinical severity and outcomes regardless of location, consistent with locus-independent loss-of-function" (PMID:32841044). Do not build a domain-based risk model for truncating alleles. - Compound/complex genotype predicts worse outcome (PMID:22267749; PMID:39581692). - Biallelic status is categorically prognostic (lethal neonatal). - Male sex and age ≥40 predict phenotype expression (PMID:22267749). - Genotype-positive status itself (vs genotype-negative HCM) is associated with earlier onset and higher event rates in SHaRe.

Prognostic biomarkers: NT-proBNP, hs-troponin, LGE burden, LA volume index, global longitudinal strain.

12. Treatment

There is currently no approved disease-modifying therapy specific to CMH4; management targets the downstream physiology (obstruction, hypercontractility, arrhythmia, heart failure). Gene-replacement therapy is the first genuinely genotype-directed approach and is in early clinical trials.

12.1 Pharmacotherapy

Drugs to AVOID in obstructive CMH4: pure vasodilators (nitrates, dihydropyridine CCBs, hydralazine), high-dose diuretics, positive inotropes (digoxin, dobutamine) — all increase the dynamic gradient.

EXPLORER-HCM efficacy (verbatim, PMID:32871100):

"45 (37%) of 123 patients on mavacamten versus 22 (17%) of 128 on placebo met the primary endpoint (difference +19·4%, 95% CI 8·7 to 30·1; p=0·0005). Patients on mavacamten had greater reductions than those on placebo in post-exercise LVOT gradient (-36 mm Hg, 95% CI -43·2 to -28·1; p<0·0001), greater increase in pVO2 (+1·4 mL/kg per min, 0·6 to 2·1; p=0·0006)" "Safety and tolerability were similar to placebo. Treatment-emergent adverse events were generally mild."

SEQUOIA-HCM (aficamten), Maron MS et al., N Engl J Med 2024;390(20):1849–1861, PMID:38739079: 282 patients randomised at 101 centres; pVO₂ improved by a least-squares mean difference of +1.74 mL/kg/min (p=0.000002); all 10 secondary endpoints met; serious TEAEs 5.6% (aficamten) vs 9.3% (placebo). Efficacy extends to mildly symptomatic patients (Eur Heart J 2025) and improves disease/symptom burden (JACC 2024).

Pharmacogenomics. Both myosin inhibitors are CYP2C19-metabolised (mavacamten predominantly; also CYP3A4/2C9). CYP2C19 poor metabolisers require dose reduction and more intensive echo monitoring for LVEF decline — mavacamten labelling carries CYP2C19-genotype-relevant dosing guidance, and this is one of the few genuine pharmacogenomic considerations in HCM care (PharmGKB/CPIC-relevant). Concomitant strong CYP2C19/CYP3A4 inhibitors are contraindicated or require dose adjustment. There is no MYBPC3-genotype-directed drug selection at present.

12.2 Advanced therapeutics

Gene therapy — the flagship CMH4-specific programme.

Preclinical (Greer-Short et al., Nat Commun 2025, PMID:40038304, exact quotes):

"Loss-of-function mutations in Myosin Binding Protein C3, MYBPC3, are the most common genetic cause of HCM, with the majority of mutations resulting in haploinsufficiency. To restore cardiac MYBPC3, we use an adeno-associated virus (AAV9) vector and engineer an optimized expression cassette with a minimal promoter and cis-regulatory elements (TN-201) to enhance packaging efficiency and cardiomyocyte expression." "Rather than simply preventing cardiac dysfunction preclinically, we demonstrate in a symptomatic MYBPC3-deficient murine model the ability of AAV gene therapy to reverse cardiac hypertrophy and systolic dysfunction, improve diastolic dysfunction, and prolong survival. Dose-ranging efficacy studies exhibit restoration of wild-type MYBPC3 protein levels and saturation of cardiac improvement at the clinically relevant dose of 3E13 vg/kg, outperforming a previously published construct."

Clinical — MyPEAK-1 (NCT05836259), Phase 1b/2, open-label dose-escalation of single IV TN-201 in symptomatic adults with MYBPC3-associated HCM: - First-in-human results: Desai MY et al., Cardiovasc Res 2025;121(17):2628–2631, PMID:41206746 — "First-in-human study of TN-201, an AAV9 gene replacement therapy in MYBPC3-associated hypertrophic cardiomyopathy." - Interim data presented at AHA Scientific Sessions 2025: 3 patients at 3E13 vg/kg (Cohort 1, ≥1 yr follow-up) and 3 at 6E13 vg/kg (Cohort 2). Reported dose-dependent transgene RNA expression, increasing cMyBP-C protein at one year, with biomarkers stable or improved (Tenaya press release, 8 Nov 2025). - ⚠️ On 7 November 2025 the FDA placed MyPEAK-1 on clinical hold. This must be recorded alongside the efficacy signal. TN-201 holds EMA PRIME designation. - Eligibility gating: pre-existing anti-AAV9 neutralising antibodies exclude patients; seroeligibility in this population was assessed by Desai MY et al., Front Med 2025;12:1635586, PMID:41020222.

Suggested annotation: therapeutic_modality: GENE_THERAPY; treatment_term NCIT:C15238 Gene Therapy.

Other advanced modalities: - Gene editing (base/prime editing) and allele-specific silencing — preclinical only for MYBPC3. - ASO / siRNA — not applicable to CMH4's dominant mechanism. Because the lesion is loss of protein via NMD, knockdown strategies are mechanistically wrong; the ASO paradigms in the dismech antisense_oligonucleotide_therapy module do not map here. (A theoretical NMD-inhibition or exon-skipping-to-restore-frame approach has been proposed but is not in trials.) - Cell therapy / immunotherapy — not applicable. - Targeted therapy — cardiac myosin inhibitors are the targeted therapy class for HCM (targeting the downstream hypercontractility rather than the gene).

12.3 Surgical and interventional

12.4 Supportive and rehabilitative

• Symptom-directed care, volume management, sleep-apnoea screening and treatment.
• Cardiac rehabilitation / structured moderate exercise — the 2024 guideline liberalised exercise recommendations; moderate-intensity recreational exercise is now considered beneficial and safe for most patients, with shared decision-making for higher intensities. NCIT:C15315 Rehabilitation.
• Weight management, BP control, alcohol moderation. NCIT:C15447 Dietary Intervention; modality BEHAVIORAL.
• Psychological support, particularly around ICD carriage and family SCD.
• Genetic counselling (NCIT:C15240) is a formal component of care, not an adjunct.

12.5 Experimental / trials

12.6 Treatment strategy / algorithm

Confirmed CMH4 (MYBPC3 P/LP variant + phenotype)
│
├── Asymptomatic, no obstruction ──► Surveillance + SCD risk stratification
│                                     ± ICD if high risk; lifestyle counselling
│
├── Symptomatic, OBSTRUCTIVE (gradient ≥50 mmHg rest or provoked)
│      Step 1: beta-blocker  →  Step 2: verapamil/diltiazem (if BB-intolerant)
│      Step 3: add disopyramide  OR  cardiac myosin inhibitor
│              (mavacamten / aficamten) — with serial echo LVEF monitoring
│      Step 4: septal reduction therapy (myectomy preferred; ASA if
│              surgically unsuitable) at an experienced centre
│
├── Symptomatic, NON-OBSTRUCTIVE
│      Beta-blocker / CCB; diuretics for congestion; treat AF;
│      evaluate for advanced HF therapies if LVEF falls
│
├── ATRIAL FIBRILLATION ──► Anticoagulate (mandatory) + rate/rhythm control ± ablation
│
├── HIGH SCD RISK ──► ICD
│
└── END-STAGE (LVEF <50%) ──► GDMT + transplant evaluation
               (stop myosin inhibitor)

Reference: 2024 AHA/ACC/AMSSM/HRS/PACES/SCMR Guideline, PMID:38718139; 2023 ESC Cardiomyopathy Guidelines.

Personalised medicine. Genotype currently drives family screening and reproductive counselling, not drug choice. TN-201, if it clears the clinical hold, would be the first genotype-restricted therapy (MYBPC3 haploinsufficiency + AAV9-seronegative). CYP2C19 genotype is the only actionable pharmacogene.

13. Prevention

Primary prevention (preventing disease occurrence)

• The disease itself cannot be prevented in a variant carrier with current tools — no therapy is proven to prevent phenotype conversion. This is the central unmet need; VANISH (valsartan) is the flagship attempt.
• Preventing transmission: genetic counselling, prenatal diagnosis, and preimplantation genetic testing for monogenic disease (PGT-M) — explicitly enacted in PMID:41488457, where molecular autopsy diagnosis enabled "preimplantation genetic testing (PGT) for future family planning."
• Modifiable-risk-factor control (hypertension, obesity, metabolic syndrome) is biologically plausible as penetrance mitigation given the two-hit data, but is not proven to prevent conversion in humans. Curate this as a mechanistic hypothesis, not an established preventive.
• Immunization: not applicable to CMH4 pathogenesis. (Routine influenza/COVID/pneumococcal vaccination is standard care for anyone with structural heart disease, to prevent decompensation.)

Secondary prevention (early detection)

• Cascade genetic testing of first-degree relatives — the single highest-yield intervention. Extends only when a P/LP variant is found in the proband (PMID:38718139).
• Serial clinical surveillance of at-risk relatives at the intervals given in §10.
• Founder-population screening programmes: the Dutch founder-variant cascade programme and Amish community screening (10% carrier frequency in Geauga County) are the two operationalised examples. In South Asia, the 25-bp deletion's ~4% carrier frequency has been proposed for population genotyping: "genotyping could be used for the identification of persons at risk of heart failure among South Asians and could be accompanied by advice for a lower-risk lifestyle" (PMID:19151713, exact quote) — this remains a proposal, not implemented policy.
• No newborn screening; not on the RUSP. The biallelic neonatal form is the only presentation with a theoretical NBS rationale, and it fails the treatability criterion outside transplant-capable settings.
• Pre-participation athlete screening (ECG-inclusive, as in the Italian and some European models) detects HCM incidentally; its cost-effectiveness remains contested.

Tertiary prevention (preventing complications in affected individuals)

• ICD for SCD prevention in high-risk patients — the definitive tertiary preventive.
• Anticoagulation for AF → stroke prevention (mandatory, CHA₂DS₂-VASc-independent in HCM).
• Septal reduction therapy to prevent progressive HF from chronic obstruction.
• Avoidance of dehydration, vasodilators, and inotropes in obstructive physiology.
• AF screening with periodic ambulatory monitoring.
• Blood-pressure and weight control to limit remodelling.

Risk stratification

HCM Risk-SCD (ESC) and the 2024 AHA/ACC marker-based approach (see §11), with CMR-LGE as the arbitrator for intermediate-risk patients.

Counselling

• Formal genetic counselling before and after testing (guideline Class I).
• Reproductive counselling covering AD 50% transmission risk, and — crucially for CMH4 — the recessive biallelic risk when both partners carry truncating MYBPC3 variants, which is a real scenario in founder populations and consanguineous families. This is the single most consequential CMH4-specific counselling point.
• Counselling must convey incomplete, age- and sex-dependent penetrance: a positive predictive test does not mean certain disease (population penetrance 11–18%; family-based ~55%).

Public health / environmental

• Community AED placement and CPR training in schools and sports venues.
• Founder-population health education (Amish, Dutch, South Asian diaspora).
• No environmental intervention applies.

14. Other Species / Natural Disease

Taxonomy and natural disease

Domestic cat — Felis catus, NCBITaxon:9685 is the outstanding natural model. Feline HCM is the most common feline heart disease and is genuinely MYBPC3-driven, making it a true comparative-pathology counterpart rather than an induced model.

Breeds (VBO-relevant): - Maine Coon — MYBPC3 p.A31P (c.91G>C, exon 3; Ala→Pro, predicted conformational change). Restricted to Maine Coons. - Ragdoll — MYBPC3 p.R820W (C>T; Arg→Trp). Restricted to Ragdolls. - A third variant, A74T, is a widely distributed polymorphism of uncertain significance.

Genotype–phenotype in cats (directly parallel to the human allelic-dose relationship):

"HCM was most prevalent in Maine Coon homozygotes for the A31P mutation and the penetrance increased with age. The penetrance of the heterozygote genotype was lower (0.08) compared to the P/P genotype (0.58) in Maine Coon."

Transmission is autosomal dominant with incomplete penetrance — "the mutation does not appear to behave as a simple dominant trait, but rather as a dominant trait with incomplete penetrance." Age-dependent penetrance and homozygote-severity gradient are exactly the human pattern.

Sources: Longeri et al., J Vet Intern Med 2013; OMIA:002951; OMIA:002952; Maine Coon p.A31P clinical significance, PMC3044103; Feline HCM advances 2025, PMC11946439.

Veterinary importance: Feline HCM causes congestive heart failure, aortic thromboembolism ("saddle thrombus") — a feline-specific complication with no direct human counterpart in HCM — and sudden death. Genotype testing (UC Davis VGL, LabGenVet) is used in breeding programmes to reduce allele frequency. This is a live example of population-level allele management.

Orthologous genes

Comparative biology

• Conservation: the C0–C10 domain architecture, the cardiac-specific C0 domain and PKA-phosphorylatable M-domain, and the C-zone A-band localisation are conserved across mammals. The thick-filament SRX regulatory mechanism is deeply conserved.
• Comparative pathology: cat and human share asymmetric septal hypertrophy, myocyte disarray, interstitial fibrosis, LVOT obstruction with SAM, diastolic dysfunction, atrial enlargement/thrombosis, arrhythmia, and sudden death. Differences: cats develop aortic thromboembolism far more readily; feline HCM is more often diagnosed by auscultation of a dynamic murmur; the human LVNC/septal-defect biallelic phenotype has no clear feline analogue.
• Mouse divergence: heterozygous Mybpc3 mice are largely phenotype-negative without a second hit, unlike humans — a genuine HUMAN_MODEL_MISMATCH for the dismech schema.

Transmission

Not zoonotic. No cross-species transmission. CMH4 is a germline Mendelian disorder; the feline disease is an independent, convergent MYBPC3 disorder, not transmitted between species.

15. Model Organisms

15.1 Mouse (Mus musculus, NCBITaxon:10090) — the primary in vivo model

15.2 Human iPSC and engineered tissue — the leading in vitro system

• CRISPR-Cas9 isogenic MYBPC3 allelic series in human iPSCs (WT / cMyBP-C^+/- / cMyBP-C^-/-), differentiated to cardiomyocytes and assembled into cardiac micropatterns and engineered cardiac tissues (ECTs) — PMID:36893011. This is the highest-fidelity human system currently available and reproduces the allelic-dose severity gradient, the haploinsufficiency-in-3D phenomenon ("While heterozygous frame shifts did not alter cMyBP-C protein levels in 2-D cardiomyocytes, cMyBP-C+/- ECTs were haploinsufficient" — exact quote), and the hypercontractile→hypocontractile trajectory.
• Patient-derived iPSC lines from Dutch founder-variant families with variable HCM severity — enables isogenic-vs-background comparison of modifiers (Stem Cell Res 2025).
• Neonatal rat ventricular cardiomyocytes with adenoviral WT vs mutant cMyBP-C — the classic sarcomere-disorganisation readout (PMID:19151713).
• Human myectomy / explant tissue — the gold standard for demonstrating haploinsufficiency, allelic imbalance, and absent truncated protein (PMID:19574547; PMID:22057632; PMID:37797718; PMID:30456444).
• Cardiac fibroblast cultures — for the fibroblast-autonomous fibrosis arm (PMID:36357371).

15.3 Other systems

• Zebrafish (Danio rerio, NCBITaxon:7955) — mybpc3 morphant/mutant work exists for cardiac development; less used for HCM modelling than mouse or iPSC.
• Drosophila — a Human Disease Model report for CMH4 exists (FlyBase FBhh0000414), but Drosophila lacks a true cMyBP-C ortholog with cardiac-specific C0/M-domain regulation; utility is limited to generic sarcomere biology.
• C. elegans, yeast — not applicable.

15.4 Phenotype recapitulation and limitations

Recapitulated well: - Allelic-dose severity gradient (het mild/absent → homozygous severe) — mouse, iPSC-ECT, cat - Haploinsufficiency without poison peptide — human tissue, mouse, iPSC-ECT - Late-onset asymmetric septal hypertrophy with fibrosis — promoter-deletion mouse - Contractile hypercontractility → hypocontractility trajectory — iPSC-ECT - Ca²⁺-handling deterioration — iPSC-ECT - SRX disruption — mouse KO, human tissue - Rescue by MYBPC3 restoration — mouse gene therapy

Not recapitulated (HUMAN_MODEL_MISMATCH candidates for the dismech entry): 1. Heterozygous mice are phenotype-negative without a second hit, whereas ~55% of human heterozygotes develop HCM in family-based studies. The mouse underestimates dominant penetrance. (Conversely, this is arguably a faithful model of the 11–18% population penetrance — the mismatch is really about which human population is the referent, which is itself a curatable question.) 2. Myocyte disarray — the human histopathological hallmark — is poorly reproduced in mice. 3. Dynamic LVOT obstruction with SAM, the dominant clinical problem, does not occur in mice (small, differently shaped LV; different mitral apparatus). 4. Atrial fibrillation and thromboembolic stroke are not modelled in mice. 5. Sudden cardiac death from VT/VF in a structurally HCM heart is not reliably reproduced. 6. iPSC-CMs are immature — fetal-like sarcomeres, negative force-frequency relationship, low mitochondrial density. Time-in-culture (2 wk vs 6 wk in PMID:36893011) is itself a variable, which is why that study's progressive phenotype is interpretable but not directly age-mappable to human decades. 7. The biallelic neonatal LVNC + septal-defect phenotype — septal defects have not been convincingly modelled. 8. Species differences in cMyBP-C phosphorylation stoichiometry and in the alpha/beta myosin heavy-chain ratio (mouse ventricle is α-MHC-dominant, human is β-MHC-dominant) limit direct translation of contractile measurements.

15.5 Research applications

Mechanism of haploinsufficiency and NMD; SRX/thick-filament regulation; Ca²⁺-handling; energetics; fibrosis; preclinical gene-therapy dose-finding and efficacy (the direct enabler of TN-201, PMID:40038304); myosin-inhibitor pharmacology; G×E (diet, exercise, hypertension) second-hit modelling.

15.6 Model resources

MGI (mouse; Mybpc3 MGI:1338871), IMPC, IMSR, JAX, MMRRC, EMMA (mouse strains); RGD (rat); ZFIN (zebrafish); Cellosaurus/hPSCreg (iPSC lines); OMIA (feline natural disease); Alliance of Genome Resources (cross-species integration).

Appendix A — Consolidated Ontology Term Suggestions

Disease: MONDO:0007268 hypertrophic cardiomyopathy 4 (parent MONDO:0005045)

Gene: hgnc:7551 MYBPC3 (lowercase prefix per dismech convention)

HPO (all verified against sqlite:obo:hp): HP:0001639 Hypertrophic cardiomyopathy · HP:0001712 Left ventricular hypertrophy · HP:0001670 Asymmetric septal hypertrophy · HP:0002094 Dyspnea · HP:0001681 Angina pectoris · HP:0001962 Palpitations · HP:0001279 Syncope · HP:0001635 Congestive heart failure · HP:0005110 Atrial fibrillation · HP:0004308 Ventricular arrhythmia · HP:0001695 Cardiac arrest · HP:0001645 Sudden cardiac death · HP:0011675 Arrhythmia (Additional terms to verify before use: HP:0031573, HP:0025168, HP:0011664, HP:0001631, HP:0001629, HP:0001643, HP:0001508, HP:0011968, HP:0001653, HP:0003115, HP:0031185, HP:0003829, HP:0000006, HP:0000007)

GO BP (verified): GO:0006936 muscle contraction · GO:0060048 cardiac muscle contraction · GO:0030049 muscle filament sliding · GO:0000381 regulation of alternative mRNA splicing, via spliceosome · GO:0043161 proteasome-mediated ubiquitin-dependent protein catabolic process · GO:0006402 mRNA catabolic process · GO:0032781 positive regulation of ATP-dependent activity (To verify: GO:0000184 NMD, GO:0003300 cardiac muscle hypertrophy, GO:0045214 sarcomere organization, GO:0031430 M band, GO:0030017 sarcomere, GO:0030018 Z disc)

CL (verified): CL:0000746 cardiac muscle cell · CL:0002548 fibroblast of cardiac tissue · CL:0000057 fibroblast

UBERON (verified): UBERON:0000948 heart · UBERON:0002084 heart left ventricle · UBERON:0002094 interventricular septum · UBERON:0002349 myocardium · UBERON:0002078 right cardiac atrium

CHEBI (verified): CHEBI:6904 metoprolol · CHEBI:9948 verapamil · CHEBI:4657 disopyramide (Mavacamten and aficamten are absent from the local CHEBI adapter — use NCIT.)

NCIT (verified): NCIT:C174901 Mavacamten · NCIT:C179072 Aficamten · NCIT:C80435 Implantable Cardioverter-Defibrillator Placement · NCIT:C93238 Implantable Cardioverter-Defibrillator · NCIT:C15246 Heart Transplantation (To verify: NCIT:C15986 Pharmacotherapy, NCIT:C15238 Gene Therapy, NCIT:C15329 Surgical Procedure, NCIT:C15240 Genetic Counseling, NCIT:C15315 Rehabilitation, NCIT:C49236 Therapeutic Procedure)

Candidate dismech conforms_to module targets: - cardiomyopathy_maladaptive_remodeling#Ventricular Remodeling — the structural/contractile cardiomyopathy final common pathway - fibrotic_response#Mesenchymal Cell Activation — the cardiac-fibroblast arm (PMID:36357371) - thrombogenesis#Coagulation Cascade Activation and Thrombin-Driven Fibrin Formation — the AF→cardioembolic-stroke arm (indirect; consider whether it belongs on this entry or on a comorbidity entry) - Note: cardiac_ion_channel_repolarization is not appropriate — CMH4 is a structural/contractile cardiomyopathy in a structurally abnormal heart, not an inherited channelopathy in a structurally normal heart. The arrhythmic substrate here is disarray + fibrosis + ischemia, not a repolarization defect. - antisense_oligonucleotide_therapy is not applicable (see §12.2).

Appendix B — Evidence Register

Verified locally (abstract/full text present in references_cache/; snippets above are exact substrings)

Identified and bibliographically verified via NCBI eutils; abstracts NOT locally cached — fetch with just fetch-reference PMID:xxxxx and re-verify any snippet before committing to the KB

Non-PMID / structured and grey sources

OMIM 115197, OMIM *600958 · ClinGen HCM GCEP reappraisal (PMC11312670) · ClinVar RCV000009149, RCV000009152 · Orphanet ORPHA:155, ORPHA:217569 · OMIA:000515-9685, OMIA:002951-9685, OMIA:002952-9685 · ClinicalTrials.gov NCT05836259, NCT03470545, NCT05186818, NCT05767346, NCT04349072, NCT01912534 · Tenaya Therapeutics investor releases (AHA 2025 interim data; 7 Nov 2025 FDA clinical hold) · Cytokinetics investor releases (Myqorzo/aficamten FDA approval 19 Dec 2025).

Appendix C — Notable Gaps and Open Questions (candidate dismech discussions)

Sources

• OMIM 115197 — CARDIOMYOPATHY, FAMILIAL HYPERTROPHIC, 4; CMH4
• OMIM *600958 — MYOSIN-BINDING PROTEIN C, CARDIAC; MYBPC3
• MGI / Disease Ontology DOID:0110310
• GeneReviews — Nonsyndromic Hypertrophic Cardiomyopathy Overview (NBK1768)
• ClinGen — Evaluating the Clinical Validity of Hypertrophic Cardiomyopathy Genes
• ClinGen HCM Gene Reappraisal (PMC11312670)
• Ingles et al., Circ Genom Precis Med 2019 — HCM gene clinical validity
• Meta-Analysis of Penetrance… Genetic HCM, Circulation 2024 · ACC summary
• Low Penetrance Sarcomere Variants Contribute to Additive Risk in HCM, Circulation
• Longitudinal Evaluation of Genetic HCM Penetrance in an Academic Biobank (PMC11780075)
• Dutch MYBPC3 founder mutation outcomes, Circ Cardiovasc Genet 2017
• BIO FOr CARe Dutch founder cohort (PMC8160056) · PubMed 33532905
• RAAS polymorphisms in MYBPC3-related HCM (PMC3449069)
• Amish MYBPC3 c.3330+2T>G homozygosity, Heart 2008 (PMID 18467358)
• The «Amish» c.3330+2T>G is an ancient Swiss mutation (PMID 36162733)
• Icelandic MYBPC3 founder mutation carriers (PMC7174027)
• Northern Spain truncating MYBPC3 cohort (PMC10137663)
• Spatial and Functional Distribution of MYBPC3 Pathogenic Variants, Circ Genom Precis Med 2020 · PubMed 32841044
• A Promoter Deletion Confirms MYBPC3 Haploinsufficiency Is Sufficient…, Circ Genom Precis Med
• Effects of MYBPC3 loss-of-function mutations preceding HCM, JCI Insight · PMC7098724
• MYBPC3 mutations and reduced super-relaxed state, PLOS One 2017 · PMC5489194
• cMyBP-C phosphorylation regulates the super-relaxed state, PNAS
• MYBPC3 deficiency in cardiac fibroblasts drives fibrosis (PMC9649783)
• Spatial transcriptomics of myocyte disarray in human HCM (PMC10454036)
• Myoarchitectural disarray of HCM begins pre-birth (PMC6794206)
• MyBPC3 P459fs multi-omics and super-resolution imaging (PMC11903464)
• ALU transposition induces familial HCM (PMC6978237)
• Elusive spliceogenic MYBPC3 variant, Sci Rep 2022 (PMC9068804)
• Computational prediction of MYBPC3 subdomain stability, Genet Med 2021
• Mybpc3 c.2373InsG hetero/homozygous mouse characterization, J Mol Cell Cardiol 2023
• Western diet two-hit Mybpc3 mouse model (PMC11708371)
• Targeted Mybpc3 knock-out mice with mitral valve abnormalities (PMC4725593)
• AAV9-mediated MYBPC3 gene therapy (TN-201), Nat Commun 2025
• First-in-human TN-201 MyPEAK-1, Cardiovasc Res 2025
• Tenaya — MyPEAK-1 interim clinical data, AHA 2025 · GlobeNewswire
• 2024 AHA/ACC/AMSSM/HRS/PACES/SCMR HCM Guideline, Circulation · JACC · PubMed 38718139 · ACC Key Points
• SEQUOIA-HCM primary results, NEJM 2024 · Aficamten in mildly symptomatic oHCM, Eur Heart J 2025 · Disease and symptom burden, JACC 2024
• Cytokinetics — FDA approval of MYQORZO (aficamten), Dec 2025 · US availability, Jan 2026 · AJMC · TCTMD
• Prevalence of clinically apparent HCM in Germany (PMC5933727) · Stable rates of obstructive HCM (PMC8770922)
• Orphanet — Rare hypertrophic cardiomyopathy (ORPHA:217569) · Familial isolated HCM (ORPHA:155)
• MYBPC3 variants in domestic cats (A31P, A74T, R820W), J Vet Intern Med 2013 · OMIA:000515-9685 · OMIA:002951-9685 · OMIA:002952-9685 · Maine Coon p.A31P clinical significance (PMC3044103) · Feline HCM advances (PMC11946439) · UC Davis VGL Maine Coon HCM test
• iPSC lines from Dutch MYBPC3 founder families, Stem Cell Res 2025
• ClinVar RCV000009149 (MYBPC3 c.1224-19G>A / Familial HCM 4) · RCV000009152 (c.3330+2T>G / HCM 4)
• MalaCards — Cardiomyopathy, Familial Hypertrophic, 4 · FlyBase Human Disease Model FBhh0000414