Hypertrophic Cardiomyopathy 9

1. Disease Information

1.1 Overview

Hypertrophic cardiomyopathy 9 (CMH9) is the OMIM disease-series designation for hypertrophic cardiomyopathy (HCM) attributed to heterozygous variants in TTN (titin) on chromosome 2q31.2. MONDO defines it purely by gene attribution, not by any distinguishing clinical or mechanistic feature:

"Any hypertrophic cardiomyopathy in which the cause of the disease is a mutation in the TTN gene." — MONDO:0013412 definition (MONDO:patterns/disease_series_by_gene)

The entity originates from Satoh et al. (1999), who screened TTN in 82 HCM probands negative for the then-known sarcomere genes and found one heterozygous c.2219G>T, p.Arg740Leu substitution (PMID:10462489, Biochem Biophys Res Commun 262:411‑7):

"A G to T transversion in codon 740, from CGC to CTC, replacing Arginine with Leucine was found in a patient. This mutation was not found in more than 500 normal chromosomes and increased the binding affinity of titin to alpha‑actitin [sic] in the yeast two‑hybrid assay. These observations suggest that the titin mutation may cause HCM in this patient via altered affinity to alpha‑actinin." (HUMAN_CLINICAL + IN_VITRO; note the authors' own hedge)

There is no CMH9-specific clinical syndrome. Reported patients present as ordinary nonsyndromic HCM. No TTN-specific hypertrophy pattern, age of onset, arrhythmic profile, or treatment response has ever been described.

1.2 Key identifiers

Ontology suggestion for the KB disease_term: MONDO:0013412 (exact). Do not substitute MONDO:0005045 (hypertrophic cardiomyopathy) — that is the umbrella entity.

1.3 Synonyms

CMH9; cardiomyopathy, familial hypertrophic, 9 (or type 9); hypertrophic cardiomyopathy type 9; TTN hypertrophic cardiomyopathy; hypertrophic cardiomyopathy caused by mutation in TTN. (All are EXACT synonyms in MONDO.)

1.4 Provenance of information

Disease-level aggregated resources only (OMIM, MONDO, ClinGen, ClinVar, GeneReviews) plus a handful of small primary case series. No EHR/registry-derived, patient-level CMH9 cohort exists. Because the entity is defined by gene attribution rather than a distinguishable phenotype, any EHR case-finding for "CMH9" would in practice retrieve generic HCM cases (I42.1/I42.2) plus a TTN genotype.

2. Etiology

2.1 Disease causal factors

Claimed cause: heterozygous rare missense variation in TTN — a disputed causal claim.

Three lines of causal claim exist, all thin:

1. Z‑repeat missense (p.Arg740Leu) — one proband, 1999 (PMID:10462489). Never replicated, never segregated.
2. M‑line/A‑band‑transition Ig‑domain missense — two variants in 96 sarcomere‑negative Japanese familial HCM probands, plus a medaka fish mutant (PMID:31628103).
3. Titin‑truncating variants (TTNtv) — the DCM mechanism — explicitly not supported for HCM (see §4.4).

Counter-evidence is stronger than the positive evidence:

• Herman et al. 2012, N Engl J Med 366:619‑28 (PMID:22335739): "the frequency of TTN mutations was significantly higher among subjects with dilated cardiomyopathy (54 of 203 [27%]) than among subjects with hypertrophic cardiomyopathy (3 of 231 [1%], P=3×10⁻¹⁶) or controls (7 of 249 [3%], P=9×10⁻¹⁴)." Note HCM (1%) was below controls (3%). Also: "Mutations associated with dilated cardiomyopathy were overrepresented in the titin A‑band but were absent from the Z‑disk and M‑band regions of titin" — i.e., precisely the two regions the CMH9 hypotheses invoke. (HUMAN_CLINICAL)
• Bos et al. 2006, Mol Genet Metab 88:78‑85 (PMID:16352453): 389 unrelated HCM patients, targeted analysis of the HCM‑associated TTN exons (2, 3, 4, 14) — "No TTN mutations were detected." (HUMAN_CLINICAL; negative replication)
• Wang et al. 2017, Can J Cardiol 33:1292‑7 (PMID:28822653): 529 Chinese HCM vs 307 controls — "We identified 13 and 8 TTNtv in patients with HCM (13 of 529 [2.5%]) and controls (8 of 307 [2.6%]) … The prevalence of TTNtv in patients with HCM and in healthy controls was comparable (P = 0.895)." (HUMAN_CLINICAL)
• ClinGen HCM GCEP reappraisal (Hespe, Waddell, Asatryan et al., JACC 2025;85(7):727‑740, doi:10.1016/j.jacc.2024.12.010; preprint PMID:39132495 / PMC11312670): TTN scored 1.2 genetic + 5.5 experimental = 6.7 points → Limited. Panel narrative: "the majority are missense without functional data or located in an exon with low percent spliced in (PSI) cardiac tissue"; "no excess TTN variants were noted in cases compared to controls in 2 studies"; "evidence for TTN variants causing HCM remained limited, rather than being reclassified as disputed." (OTHER / expert-panel curation)

Interpretation for the KB: model TTN with relationship_type: DISPUTED, and record the mechanism nodes at mechanism_confidence: HYPOTHETICAL.

2.2 Risk factors

Genetic. - TTN rare missense variation — disputed (above). - Background rare-variant burden is the confounder. Titin is the largest human protein (~34,350 aa canonical; up to ~35,991 aa inferred‑complete isoform; ~364 exons in the meta‑transcript NM_001267550). Every genome carries multiple rare TTN missense alleles; ~2–3% of unselected individuals carry a truncating allele (3% of controls in PMID:22335739). Any sufficiently large candidate-gene screen will therefore find rare TTN variants irrespective of causality — the core methodological problem with CMH9. - Real HCM risk factors (parent phenotype): pathogenic variants in the 8 definitive sarcomere genes — MYBPC3 (~50% of genotype‑positive), MYH7 (~33%), TNNI3 (~5%), TNNT2 (~4%), then TPM1, ACTC1, MYL2, MYL3 (<3% each) (GeneReviews, PMID:20301725). The 2025 ClinGen reappraisal recognises 29 genes at moderate/strong/definitive for HCM or isolated LVH (MYBPC3, MYH7, TNNT2, TNNI3, TNNC1, TPM1, ACTC1, MYL2, MYL3, ACTN2, CSRP3, FHOD3, FLNC, PRKAG2, PLN, DES, FHL1, LAMP2, GLA, CACNA1C, TTR, PTPN11, RAF1, RIT1, and others). - Polygenic background modulates penetrance and expressivity in HCM generally; low-penetrance sarcomere variants contribute additive risk (Circulation 2025, "Low Penetrance Sarcomere Variants Contribute to Additive Risk in Hypertrophic Cardiomyopathy"). - Modifier claim specific to TTN: TTNtv may be an outcome modifier rather than a cause — see §4.5.

Environmental / demographic (parent phenotype; none TTN‑specific). - Age (penetrance is age-dependent; typical onset adolescence–early adulthood). - Male sex (over-represented in HCM cohorts; adverse events occur earlier in male TTN carriers in the DCM setting, PMID:22335739). - Intense competitive athletic training — a trigger for arrhythmic events and a differential-diagnosis confounder ("athlete's heart"), not an initiating cause. - Hypertension and aortic stenosis are phenocopy causes of LVH, not CMH9 risk factors. - No toxin, infectious, occupational, or dietary exposure has ever been linked to CMH9. Not applicable / no data.

2.3 Protective factors

No CMH9-specific protective genetic or environmental factor has been reported. For HCM generally: avoidance of burst/extreme exertion and of dehydration/volume depletion in LVOT-obstructive physiology; blood-pressure control; avoidance of pure vasodilators and high-dose diuretics in obstructive disease. These are management rather than validated primary prevention. No protective allele is documented in gnomAD-scale data.

2.4 Gene–environment interactions

No CMH9-specific GxE data. In the broader titin field, the best-characterised GxE is in DCM, not HCM: TTNtv carriers show a stress- or exposure-dependent phenotype (alcohol, peripartum, chemotherapy, atrial fibrillation, hypertension; e.g. Nat Cardiovasc Res 2024, "Titin truncating variants, cardiovascular risk factors and the risk of atrial fibrillation and heart failure"), and heterozygous Ttn-truncation mice are normal at baseline but decompensate under angiotensin II/isoproterenol or transverse aortic constriction (PMID:26504781; MODEL_ORGANISM). Do not transfer this to CMH9 — it is DCM biology.

3. Phenotypes

No phenotype in this list is TTN-specific; all are inherited from the HCM parent phenotype, with the exception of the diastolic-dysfunction emphasis that comes from the medaka model. Frequencies below are HCM-cohort figures (GeneReviews PMID:20301725; 2024 AHA/ACC guideline PMID:38718139) and should be curated as parent-phenotype frequencies, not CMH9 frequencies.

Quality-of-life impact. No CMH9-specific QoL data. In HCM generally, exertional dyspnea, chest pain and fatigue drive impairment; the SEQUOIA‑HCM aficamten trial used the Kansas City Cardiomyopathy Questionnaire (KCCQ) and pVO₂ as functional endpoints, with ~60% of aficamten-treated vs 24% of placebo patients improving NYHA class. Additional QoL burdens: exercise restriction counselling, ICD-related anxiety, and cascade-screening implications for relatives. Instruments in use: KCCQ, SF‑36, EQ‑5D, HCM Symptom Questionnaire (HCMSQ).

Severity/progression descriptors (HCM parent). Severity: variable, from lifelong asymptomatic to end-stage HF. Progression: slow and progressive with an episodic arrhythmic overlay. Penetrance 50–62% in P/LP heterozygotes, gene-dependent (~32% MYL3 to ~69% ACTC1) (PMID:20301725). None of these figures has ever been measured for TTN carriers.

4. Genetic / Molecular Information

4.1 Causal gene

TTN (titin), HGNC:12403, OMIM *188840, 2q31.2, UniProt Q8WZ42. Encodes the giant sarcomeric filament spanning the half-sarcomere from Z‑disc to M‑band (>1 µm), 3–4.2 MDa depending on splice isoform (N2B, N2BA, novex isoforms). Functional segments: Z‑disc (Z‑repeats binding α‑actinin, telethonin/TCAP anchoring), I‑band (elastic spring: tandem Ig, N2B/N2BA, PEVK), A‑band (super-repeats binding myosin/MyBP‑C), M‑band (titin kinase domain, MURF1/MURF2 binding, obscurin/myomesin interface).

GO annotations to use: GO:0008307 structural constituent of muscle; GO:0042805 actinin binding; GO:0031430 M band; GO:0030018 Z disc; GO:0030016 myofibril; GO:0055003 cardiac myofibril assembly; GO:0016567 protein ubiquitination; GO:0014898 cardiac muscle hypertrophy in response to stress.

4.2 The reported CMH9 alleles

Critical caveat on the R740L ClinVar record: its "Pathogenic" label is a 0‑star OMIM-derived assertion from 1999 predating ACMG/AMP criteria. Under ACMG/AMP 2015 rules the variant would today, at best, be a VUS: PS3 is weak (yeast two‑hybrid interaction assay, not a disease-relevant functional readout), PM2 support was "absent from 500 chromosomes" (not gnomAD-scale), and there is no PP1 segregation and no PS4 case-control enrichment. gnomAD frequency is not displayed on the ClinVar record and could not be retrieved programmatically for this report — flag as a gap to fill before curating any allele-frequency claim.

4.3 Variant classes and functional consequences

• Type: all CMH9 claims are missense. This matters: the disease-validated TTN mechanism (DCM) is truncating (nonsense/frameshift/canonical splice) with haploinsufficiency/poison-peptide effects in high‑PSI A‑band exons (Roberts et al. 2015, Sci Transl Med, PMID:25589632).
• Proposed functional consequence #1 (gain of function / altered binding): R740L increases titin–α‑actinin affinity (~40%) — an unusual "too‑tight" gain-of-binding, not loss of function.
• Proposed functional consequence #2 (enhanced degradation): M‑line‑proximal Ig missense increases MURF1 binding and ubiquitin-mediated titin turnover — effectively a localised loss of titin protein.
• These two proposals are mechanistically incompatible with each other and implicate opposite ends of the molecule. They should be curated as two separate, non-merged mechanistic_hypotheses groups.
• Germline in all cases. Somatic TTN variation is irrelevant here (TTN is a well-known false-positive "long gene" hit in tumour mutation datasets — do not cite COSMIC/TCGA for this entity).

4.4 Why truncating variants are excluded

TTNtv frequencies: DCM 27% vs HCM 1% vs controls 3% (PMID:22335739); HCM 2.5% vs controls 2.6% (PMID:28822653). PSI/exon-usage analysis (PMID:25589632) established that only constitutively expressed (high‑PSI) A‑band TTNtv are DCM-relevant — "the most common genetic cause of DCM in ambulant patients in the community." The ClinGen panel found the only truncating-type HCM case evidence to be one A‑band frameshift (PSI 100%) and one I‑band termination (PSI 100%) — two isolated observations against a high background rate.

4.5 Modifier hypothesis (a separable claim)

Wang et al. 2017 (PMID:28822653): among 529 HCM patients, TTNtv carriers had cardiovascular death in 3/13 (23.1%) vs 39/516 (7.6%) in non-carriers, adjusted HR 6.88 (95% CI 2.04–23.20; P=0.002). Authors: "Our study suggests that TTNtv might be a genetic modifier of HCM and confers increased risk for cardiovascular death." Single cohort, 3 events, unreplicated. Curate as CONTROVERSY, not as a causal mechanism. Note the logical structure: TTN can be simultaneously (a) not an HCM gene and (b) a prognostic modifier within HCM.

4.6 Modifier genes, epigenetics, chromosomal abnormalities

• Modifier genes for CMH9: none identified. No data.
• Epigenetics: no CMH9-specific methylation/histone data. Generic HCM myocardium shows DNA methylation and ncRNA remodelling, but nothing titin- or CMH9-specific is published. No data.
• Chromosomal abnormalities: not a mechanism here. Large TTN CNVs are rare; GeneReviews states "large deletions and duplications are not a major cause of nonsyndromic HCM." Chromosomal microarray/karyotype have no role.

5. Environmental Information

• Environmental factors: none established for CMH9. Not applicable.
• Lifestyle factors: relevant only as symptom/arrhythmia modifiers of the HCM phenotype (exertion, dehydration, alcohol, stimulants). For titin biology generally, mechanical/haemodynamic load is the physiologically meaningful "environment" — titin is a load-sensing molecule — but no CMH9 load-interaction study exists.
• Infectious agents: Not applicable. No infectious trigger implicated.

6. Mechanism / Pathophysiology

6.1 Causal chain — as claimed, with confidence labels

[HYPOTHETICAL, MOLECULAR]
Rare heterozygous TTN missense variant in a sarcomere-negative HCM patient
   │  (ascertainment: residual, gene-elusive HCM — candidate-gene design)
   ├──► HYPOTHESIS A (Z-disc):  ↑ titin Z-repeat binding to α-actinin (~40%, Y2H)
   │        └─► perturbed Z-disc assembly / Z-disc mechanosensing  [UNBRIDGED]
   │                └─► ??? ─────────────────────────┐
   └──► HYPOTHESIS B (M-line):  ↑ titin Ig-domain binding to MURF1  │
    └─► ↑ ubiquitin-mediated titin degradation              │
 └─► M-line disassembly, fewer myofibrils,          │
     stiffer (N2B-shifted) titin isoforms  [fish]   │
          └─► ??? ────────────────────────────────► │
                                                    ▼
                          [TISSUE] Hypertrophic remodeling of the LV
                          (wall thickening + myocyte disarray +
                           impaired diastolic filling)
                                    │
                                    ▼
                    Generic HCM downstream cascade
            (see kb/modules/cardiomyopathy_maladaptive_remodeling)

The step marked ??? is entirely unbridged in both hypotheses. No intermediate signalling has been demonstrated linking either binding perturbation to a hypertrophic transcriptional program in human cardiomyocytes.

6.2 Hypothesis A — altered Z‑disc / α‑actinin interaction

Titin's N‑terminus spans the Z‑disc; the central Z‑repeats bind the C‑terminal calmodulin-like domain of α‑actinin‑2 (ACTN2), anchoring the filament and creating a mechanosensing hot spot. Arg740 lies in this Z‑repeat region. The 1999 claim is that a ~40% affinity increase alters Z‑disc assembly or mechanotransduction.

Evidence base: one yeast two-hybrid experiment. No cardiac cell, tissue, or animal data for this variant. Independent support that Z‑disc/α‑actinin‑2 disruption can cause human myocardial mechanical dysfunction exists (e.g., Circ Heart Fail/PMC10572656, "Disruption of Z‑Disc Function Promotes Mechanical Dysfunction in Human Myocardium: Evidence for a Dual Myofilament Modulatory Role by Alpha‑Actinin 2"), and ACTN2 itself is a ClinGen-recognised HCM gene — but that supports the pathway's plausibility, not this variant's causality. Notably, Herman 2012 found DCM-associated truncations were absent from the Z‑disc region, and Bos 2006 found nothing at all when directly resequencing the Z‑disc-encoding exons in 389 HCM patients.

GO: GO:0042805 actinin binding (modifier INCREASED); GO:0030018 Z disc; GO:0055003 cardiac myofibril assembly.

6.3 Hypothesis B — titin/MURF1 signalling and enhanced titin turnover

The titin M‑band harbours the titin kinase domain, now understood to be a catalytically inactive pseudokinase scaffold that recruits the E3 ubiquitin ligases MURF1 (TRIM63) and MURF2, coupling sarcomeric mechanics to ubiquitin-dependent turnover and myofibril trophicity (Bogomolovas et al., Open Biol 2014;4:140041, doi:10.1098/rsob.140041). TRIM63 is itself a recognised HCM-associated gene.

The medaka non‑spring heart (nsh) mutant carries an Ig-domain missense (D23186V, exon 204) at the M‑line/A‑band transition (PMID:31628103, Dis Model Mech 12:dmm041103):

"The nsh homozygotes had fewer myofibrils, disrupted sarcomeres and expressed pathologically stiffer titin isoforms. In addition, the nsh heterozygotes showed M‑line disassembly that is similar to the pathological changes found in HCM." (MODEL_ORGANISM)

"Screening of mutations in 96 unrelated patients with familial HCM, who had no previously implicated mutations in known sarcomeric gene candidates, identified two mutations in Ig domains close to the M‑line region of titin. In vitro studies revealed that the mutations found both in medaka fish and in familial HCM increased binding of titin to muscle‑specific ring finger protein 1 (MURF1) and enhanced titin degradation by ubiquitination. These findings implicate an impaired interaction between titin and MURF1 as a novel mechanism underlying the pathogenesis of HCM." (IN_VITRO + MODEL_ORGANISM)

Mechanistic details from the full text: atrial systolic/diastolic velocities fell from ~183 µm/s (WT) to ~64 µm/s (nsh); N2B (stiff) isoform expression increased, i.e., reduced elasticity and increased passive stiffness — a biophysically coherent route to diastolic dysfunction; MURF1/MURF2 catalysed multi-ubiquitination of the titin fragment in vitro; mutant constructs showed lower steady-state protein levels. The authors state a key limitation: "since the medaka TTN gene is very large, we were unable to clone the full-length cDNA (~15 kb) to perform the rescue experiments" — the model lacks genetic rescue. The human arm presents family pedigrees, but ClinGen's independent assessment of the total case-level evidence remained Limited.

GO: GO:0016567 protein ubiquitination (INCREASED); GO:0030239 myofibril assembly (DECREASED); GO:0031430 M band; GO:0061077 chaperone-mediated protein folding (n/a); GO:0043161 proteasome-mediated ubiquitin-dependent protein catabolic process.

6.4 Downstream (inherited from the HCM parent phenotype, not TTN-specific)

Increased sarcomeric energetic cost and Ca²⁺ sensitisation → myocyte hypertrophy and disarray → interstitial and replacement fibrosis → microvascular dysfunction/ischemia → impaired relaxation and elevated filling pressures → LVOT obstruction (± SAM of the mitral valve) → arrhythmogenic substrate → AF, VT, SCD, and in a minority the end-stage systolic ("burnt-out") phase. This is exactly the chain captured by the dismech module cardiomyopathy_maladaptive_remodeling; a conforms_to link from CMH9 is not warranted because the disease-specific edges into that chain are hypothetical.

6.5 Cell types, compartments, metabolism, immunity

• Cell types: CL:0000746 cardiac muscle cell (cardiomyocyte); secondarily CL:0002548 fibroblast of cardiac tissue (fibrotic remodelling), CL:0002144 capillary endothelial cell (microvascular dysfunction).
• Subcellular: GO:0030017 sarcomere; GO:0030018 Z disc; GO:0031430 M band; GO:0030016 myofibril; GO:0005634 nucleus (titin N‑terminus/Z‑disc mechanosignalling relays, e.g., MLP/CSRP3 shuttling); GO:0005739 mitochondrion (energetic stress, downstream).
• Metabolic changes: no CMH9-specific metabolomics. HCM generally shows impaired myocardial energetics (reduced PCr/ATP by ³¹P‑MRS), a shift toward glucose utilisation, and increased ATP cost of tension. No titin-specific data.
• Immune involvement: none primary. Sterile inflammation accompanies fibrotic remodelling but is not disease-defining. Not applicable as a mechanism.
• Tissue damage mechanisms: myocyte hypertrophy/disarray, interstitial + replacement fibrosis, microvascular ischemia. Generic.
• Molecular profiling (transcriptomics/proteomics/metabolomics/lipidomics/single-cell/spatial/CRISPR screens): No CMH9-specific dataset exists in GEO, ArrayExpress, PRIDE, MetaboLights, HCA, or DepMap. Titin exon-usage/PSI resources from the DCM field (PMID:25589632; Inferring disease course from differential exon usage in the wide titinopathy spectrum, PMC11514934) are methodologically relevant for interpreting TTN variants but are not CMH9 data. This is a genuine and complete gap.

7. Anatomical Structures Affected

Organ level - Primary: heart — UBERON:0000948; specifically left ventricle UBERON:0002084 and interventricular septum UBERON:0002094 (asymmetric septal hypertrophy). - Secondary: left atrium UBERON:0002079 (dilation, AF substrate); mitral valve UBERON:0002135 (SAM, regurgitation); cardiac conduction system UBERON:0004146; pulmonary circulation (post-capillary pulmonary hypertension); systemic embolic targets — brain UBERON:0000955 (cardioembolic stroke). - Body systems: cardiovascular primary. Note that titin is also expressed in skeletal muscle UBERON:0001134 — skeletal myopathy is not a feature of CMH9 as reported, but is central to other titinopathies (HMERF, LGMD R10, Salih myopathy) under the shared MONDO parent autosomal dominant titinopathy (MONDO:0100494). No skeletal-muscle phenotype has been described in CMH9 patients.

Tissue / cell level - Cardiac muscle tissue UBERON:0001133 / myocardium UBERON:0002349. - CL:0000746 cardiac muscle cell — the primary affected population; CL:0002548 cardiac fibroblast; CL:0002144 capillary endothelial cell.

Subcellular — GO:0030017 sarcomere, GO:0030018 Z disc, GO:0031430 M band, GO:0030016 myofibril.

Localization / lateralization — Bilateral in the sense of biventricular potential, but the phenotype is characteristically left-sided and regionally asymmetric (basal anteroseptal predominance; apical, midventricular, and concentric variants occur). Right ventricular involvement is possible but secondary. Use HP:0001670 (asymmetric septal hypertrophy) to capture asymmetry.

8. Temporal Development

Onset. No CMH9-specific onset data (n≈4 reported probands). HCM parent phenotype: onset typically adolescence or early adulthood, but ranges from infancy to the eighth decade; onset is insidious, detected on screening ECG/echo or after a symptomatic/arrhythmic event. HPO onset terms: HP:0003581 Adult onset / HP:0011462 Young adult onset / HP:0003621 Juvenile onset (choose per case; for CMH9 the honest annotation is variable/unknown).

Progression / staging. Recognised HCM stages: (1) genotype‑positive/phenotype‑negative (subclinical; may show impaired relaxation, ECG changes, crypts before hypertrophy); (2) classic phenotype with preserved EF ± obstruction; (3) adverse remodelling with AF, progressive fibrosis, worsening diastolic failure; (4) end-stage/"burnt-out" with systolic dysfunction (~8% of cohorts) requiring advanced therapies. Rate is slow and variable over decades, punctuated by episodic arrhythmic events. Duration: chronic, lifelong (HP:0003679 progressive; HP:0003676 progressive disorder).

Patterns. No spontaneous remission. Treatment-induced symptomatic remission is achievable (myectomy, alcohol septal ablation, cardiac myosin inhibitors) but does not reverse the genotype; regression of hypertrophy with myosin inhibitors is partial and drug-dependent. Critical periods: adolescence/young adulthood (peak SCD risk, athletic exposure) and the period around phenotype conversion in genotype-positive relatives — the rationale for serial screening every 1–2 years (see §13).

9. Inheritance and Population

9.1 Epidemiology

CMH9-specific: unquantifiable. Cumulative published cases ≈ 4 probands worldwide (1 Japanese in 1999; 2 Japanese in 2019; 1 Chinese family in 2025). Appropriate prevalence_class: CASES_IN_LITERATURE with measure_type: CASES_IN_LITERATURE and a note that the entity's validity is disputed. Do not assign a numeric rate.

HCM parent phenotype (for context only): - Classic estimate 1 in 500 (~200 per 100,000) from echocardiographic screening (CARDIA). - Contemporary US administrative-claims estimate (JACC: Advances 2025, "Epidemiology of Hypertrophic Cardiomyopathy in the United States From 2016 to 2023"): 1 in 327, ~832,956 US cases — higher than classic estimates, reflecting ascertainment and awareness. - Range across literature 1:500–1:3,000 depending on method; genotype-based estimates (P/LP variant carriage) suggest higher latent carriage with incomplete penetrance.

9.2 Genetic parameters

• Inheritance: autosomal dominant — HP:0000006. Recorded as such in ClinGen's TTN‑HCM assertion and asserted by OMIM. But note: the index CMH9 observation was a single proband with no segregation data; support for dominance in this specific entity is PARTIAL, inherited from HCM generally rather than demonstrated for TTN.
• Penetrance: unknown for CMH9. HCM overall: 50–62% in P/LP heterozygotes, age-dependent, gene-specific (~32% MYL3 to ~69% ACTC1) (PMID:20301725). Population-based genotype-first studies show substantially lower penetrance than clinic-ascertained estimates.
• Expressivity: highly variable with marked intrafamilial variability (HCM generally).
• Anticipation: Not applicable — no repeat expansion mechanism.
• Germline mosaicism: possible in principle for HCM (parental gonadal mosaicism reported); no CMH9 report.
• Founder effects: none for CMH9. (Contrast: founder TTNtv exist for DCM, e.g. TTN:c.12478del in Slovenia — DCM, not HCM.)
• Consanguinity: no role established; dominant mechanism.
• Carrier frequency: not a meaningful parameter for a dominant, disputed entity. gnomAD frequency of R740L was not retrievable for this report — explicit gap.

9.3 Population demographics

• Affected populations: the three primary reports are East Asian (2 Japanese cohorts, 1 Chinese family). This almost certainly reflects ascertainment by the research groups involved, not a genuine ancestry effect, and should be curated as such rather than as a population-prevalence claim.
• Geographic distribution: none established. No variant shows geographic clustering.
• Sex ratio: no CMH9 data. HCM cohorts are male-predominant (~60:40) with women diagnosed later and often more symptomatic.
• Age distribution: no CMH9 data; HCM parent as in §8.

10. Diagnostics

There is no CMH9-specific diagnostic test or pathway. Diagnosis is (1) diagnose HCM, then (2) genotype — and the TTN finding, if any, is currently not reportable as causal.

10.1 Clinical tests (HCM parent; 2024 AHA/ACC guideline, PMID:38718139)

10.2 Genetic testing

• Recommended approach (2024 AHA/ACC): HCM-focused multigene panel in the proband, with evaluation by a genetic counsellor before and after testing; panels should include phenocopy genes (GLA, LAMP2, PRKAG2, TTR, PTPN11/RASopathies, FHL1, DES, CACNA1C, ACTN2, FLNC, FHOD3, ALPK3). Diagnostic yield ~30% in unselected HCM, ~60% in familial cases.
• Cascade testing: offered to first-degree relatives only when a P/LP variant is identified in the proband. Genotype-positive/phenotype-negative relatives get serial imaging; ICDs are not indicated for them, and they may participate in competitive sport.
• WES/WGS: second-line when panels are negative and a syndromic/phenocopy diagnosis is suspected; WGS adds little for TTN interpretation given the background variant burden.
• Single-gene testing: appropriate only for a known familial variant.
• CMA / karyotype / FISH: not indicated — large CNVs are not a significant cause of nonsyndromic HCM.
• mtDNA testing: consider for maternally inherited LVH (MT‑TI and other mt‑tRNA variants).
• Repeat expansion testing: not applicable, except Friedreich ataxia (FXN GAA) in the syndromic differential.
• The TTN-specific interpretive rule: a rare TTN variant on an HCM panel should not be reported as an established cause of the patient's hypertrophy. For any TTN variant, evaluate exon PSI (percent-spliced-in) and domain location — but note that even high‑PSI TTNtv are DCM-, not HCM-, associated.

10.3 Omics-based diagnostics

RNA-seq on blood/muscle can resolve splice-impact of candidate TTN variants and is the one omics modality with real utility here. Proteomics, metabolomics, epigenomics, liquid biopsy: no established diagnostic role. Not applicable.

10.4 Clinical criteria and differential diagnosis

Diagnostic criterion (2024 AHA/ACC): maximal LV wall thickness ≥15 mm in any myocardial segment (or ≥13 mm with a positive family history or positive genotype), by any imaging modality, not explained solely by abnormal loading conditions. Pediatric: z-score >3 (or >2 with family history/genotype).

Differential (all must be excluded): - Hypertensive heart disease, aortic stenosis — loading-condition LVH. - Athlete's heart — concentric mild LVH with increased LV cavity size, normal diastolic function, regression with deconditioning. - Cardiac amyloidosis (ATTR/AL) — low voltage with LVH, apical sparing on strain, positive PYP scan. - Fabry disease (GLA) — low α‑Gal A, prominent inferolateral LGE, extracardiac features. - Danon disease (LAMP2), PRKAG2 glycogen storage — pre-excitation, conduction disease. - RASopathies (Noonan/PTPN11, RAF1, RIT1; Costello) — dysmorphology, pulmonary valve stenosis. - Pompe disease, Friedreich ataxia, desminopathy, mitochondrial cardiomyopathy, Timothy syndrome (CACNA1C).

10.5 Screening

• Cascade clinical screening of first-degree relatives with ECG + echo, repeated every 1–2 years (more often in adolescence), regardless of genotype availability.
• Newborn screening: not applicable.
• Population carrier screening: not applicable for a dominant, low-penetrance, disputed entity.

11. Outcome / Prognosis

No CMH9-specific outcome data exist. All figures below are for HCM overall and must be curated as parent-phenotype prognosis.

• Mortality: contemporary HCM-related mortality in specialist centres is ~0.5–1%/year, far below the 2–4%/year of older referral-bias-era series. HCM remains a leading cause of SCD in young athletes (5–14% of cases).
• Survival: with modern management, life expectancy in many HCM patients approaches that of the general population; the end-stage phase (~8% with systolic dysfunction) carries substantially worse survival and may require transplant.
• Composite burden: SHaRe registry (Ho et al., Circulation 2018;138:1387‑1398, PMID:30297972; 4,591 patients, 2,763 genotyped, mean follow-up 5.4±6.9 y) established that sarcomere-variant-positive patients have earlier onset and a higher lifetime burden of adverse events (arrhythmias, HF, AF, stroke, death) than sarcomere-negative patients. This is directly relevant framing for CMH9: the reported CMH9 patients are by definition sarcomere-negative on established genes, which is the lower-risk SHaRe stratum — another reason not to assume a distinct severe TTN-HCM phenotype.
• Morbidity/disability: exercise intolerance, AF (up to ~60% by age 60 in early-diagnosed patients) with elevated stroke risk requiring anticoagulation irrespective of CHA₂DS₂‑VASc, ICD-related morbidity, and activity restriction.
• QoL instruments: KCCQ, SF‑36, EQ‑5D, HCMSQ. No CMH9-specific QoL study.
• Complications: SCD/VT, AF and cardioembolic stroke, progressive HF, infective endocarditis (rare, with SAM/MR), pregnancy-related decompensation (maternal mortality is nonetheless low, 0.2%, GeneReviews PMID:20301725).
• Prognostic factors (HCM): maximal wall thickness, LV apical aneurysm, extensive LGE on CMR, unexplained syncope, NSVT, family history of SCD, abnormal BP response to exercise, LVEF <50%, left atrial size — all integrated in the ESC HCM Risk‑SCD calculator and the AHA/ACC risk-marker approach.
• TTN-specific prognostic claim: the single unreplicated finding that TTNtv carriage predicts cardiovascular death within HCM (adjusted HR 6.88) (PMID:28822653). Treat as hypothesis-generating.

12. Treatment

No CMH9-specific, genotype-directed therapy exists. Management is standard HCM management (2024 AHA/ACC/AMSSM/HRS/PACES/SCMR guideline, PMID:38718139).

12.1 Pharmacotherapy

Avoid: pure vasodilators (nitrates, dihydropyridines), high-dose diuretics, digoxin, and positive inotropes in obstructive physiology.

Pharmacogenomics: mavacamten is a CYP2C19 substrate — dosing and titration are CYP2C19-phenotype-informed (poor metabolisers require dose caps); aficamten has a less CYP2C19-dependent profile. No TTN-genotype-directed pharmacogenomics exists. PharmGKB/CPIC have no CMH9 entry.

12.2 Advanced therapeutics

• Gene therapy: active for MYBPC3-related HCM (AAV9 gene replacement, e.g., TN‑201 and related programs). Nothing for TTN — and TTN's ~100 kb coding sequence makes conventional AAV gene replacement structurally impossible. Base/prime editing of specific TTN alleles is conceivable but entirely preclinical. Not applicable to CMH9 today.
• Cell therapy, RNA therapies (ASO/siRNA), targeted therapy, immunotherapy: none for CMH9. (Note: TTN exon-skipping ASO strategies are being explored preclinically for TTN-DCM, not HCM.)

12.3 Surgical / interventional

• Septal myectomy (extended transaortic) — gold standard for drug-refractory obstructive HCM in experienced centres; NCIT:C15329 Surgical Procedure.
• Alcohol septal ablation — catheter alternative for selected anatomy/comorbidity; NCIT:C49236 Therapeutic Procedure.
• ICD implantation — primary prevention per risk stratification; secondary prevention after arrest/sustained VT; NCIT:C50040? (use NCIT:C49236 with a device descriptor; therapeutic_modality: DEVICE).
• Catheter ablation for AF; cardiac transplantation (NCIT:C15289 Organ Transplantation) for end-stage disease.

12.4 Supportive, rehabilitative, and lifestyle

Symptom-directed care; moderate-intensity exercise is now endorsed (a change from historical blanket restriction), with shared decision-making for competitive/high-intensity sport; cardiac rehabilitation (NCIT:C15315 Rehabilitation); genetic counselling (NCIT:C15240); pre-conception and prenatal counselling.

12.5 Experimental / trials

No trial has ever enrolled by TTN genotype in HCM. Relevant HCM trials: SEQUOIA‑HCM (NCT05186818, aficamten, completed), MAPLE‑HCM (aficamten monotherapy vs metoprolol), ACACIA‑HCM (non-obstructive HCM), ODYSSEY‑HCM, VALOR‑HCM (mavacamten vs septal reduction therapy), and the MYBPC3 gene-therapy programs. Any clinical_trials block for CMH9 must be annotated as parent-phenotype trials, not CMH9-specific.

12.6 Treatment strategy

Algorithm: confirm HCM and exclude phenocopies → risk-stratify for SCD (ICD decision) → if obstructive and symptomatic: beta-blocker → ±verapamil/disopyramide → cardiac myosin inhibitor (mavacamten or aficamten, with echo surveillance and REMS) → septal reduction therapy if refractory. If non-obstructive: symptom-directed HF therapy, AF management, transplant evaluation at end stage. Genotype currently informs family screening, not drug choice — and a TTN variant informs neither.

13. Prevention

• Primary prevention (preventing the disease): not possible — germline, dominant. No modifiable exposure initiates CMH9.
• Secondary prevention (early detection): the core intervention. Cascade clinical screening (ECG + echo) of first-degree relatives every 1–2 years; cascade genetic testing only if a P/LP variant is identified in the proband. A rare TTN variant does not meet this bar and should not be used to include or exclude relatives from surveillance — the practically important consequence of the Limited classification.
• Tertiary prevention (preventing complications): ICD for high SCD risk; anticoagulation for any AF; endocarditis awareness; avoidance of dehydration/vasodilators in obstruction; blood-pressure control; individualised exercise prescription; pregnancy planning and monitoring.
• Immunization: not disease-specific; standard influenza/COVID/pneumococcal vaccination as for any cardiac patient.
• Genetic screening: PGD/PGT‑M and prenatal diagnosis are technically available for a known P/LP familial variant — not appropriate for a Limited-validity TTN variant.
• Risk stratification: ESC HCM Risk‑SCD model; AHA/ACC major risk markers; CMR‑LGE burden.
• Counselling: genetic counselling before and after testing is a guideline recommendation (NCIT:C15240 Genetic Counseling); counselling for CMH9 specifically must convey uncertain gene–disease validity.
• Public health / environmental interventions: pre-participation athletic screening programs (contested cost-effectiveness), AED availability at sporting venues. Not CMH9-specific.

14. Other Species / Natural Disease

• Taxonomy: naturally occurring HCM is well described in domestic cat, Felis catus (NCBITaxon:9685) — the single most important spontaneous animal model of HCM — and less commonly in dog (Canis lupus familiaris, NCBITaxon:9615), pig, and some non-human primates.
• Breeds (VBO): Maine Coon and Ragdoll cats are the classic HCM breeds, with MYBPC3 founder variants (A31P in Maine Coon; R820W in Ragdoll) — see OMIA. Sphynx and British Shorthair have breed-associated HCM without a fully defined gene.
• Critically: feline HCM is a MYBPC3 story, not a TTN story. There is no OMIA entry, and no naturally occurring animal disease, attributed to TTN in hypertrophic cardiomyopathy. TTN variants in animals are associated with DCM (e.g., reports in Doberman-type DCM genetics remain contested) and with muscular phenotypes.
• Orthologues: mouse Ttn (NCBI Gene 22138, MGI:98864); rat Ttn (NCBI Gene 84005); zebrafish ttn.1/ttn.2; medaka ttn. Titin's Z‑disc α‑actinin interface, A‑band super-repeats, and M‑band kinase/MURF1 module are deeply conserved across vertebrates — the premise on which the medaka model rests.
• Comparative pathology: the medaka nsh mutant reproduces hypertrophic myocardium, diastolic dysfunction, sarcomeric disarray, and M‑line disassembly — a genuine cross-species mechanistic parallel, but in a two-chambered fish heart without the coronary microcirculation, fibrotic remodelling, or LVOT obstruction that dominate human HCM.
• Zoonotic potential / transmission: not applicable.

15. Model Organisms

Overall model-system verdict: CMH9 has no mammalian genetic model, no genetic rescue in the one model that exists, and no human-cell model. In dismech terms, this warrants a HUMAN_MODEL_MISMATCH discussion alongside the KNOWLEDGE_GAP: evidence exists (medaka, yeast two-hybrid), but its translational validity to human HCM is precisely the unresolved question.

16. Synthesis: Knowledge Gaps and the Experiments That Would Settle Them

Open question 1 (KNOWLEDGE_GAP): Is TTN a hypertrophic cardiomyopathy gene at all, or should CMH9 be retired as a disease entity?

What is missing is a specific evidence class: large-cohort, region-stratified case-control burden testing of rare TTN missense variation (Z‑disc / I‑band / A‑band / M‑band strata) in HCM against gnomAD-scale controls, plus segregation in multiplex families. Note that truncating-variant burden has already been tested and is negative — the burden question that remains open is missense-specific, and it has not been properly asked. Proposed experiments:

1. Region-stratified rare TTN missense burden test in a large sarcomere-negative HCM cohort vs population reference, stratified by domain and PSI. Supports CMH9 if a regional excess emerges; refutes it if the burden matches population expectation.
2. Segregation analysis of p.Arg740Leu, p.Ser30186Ala, p.Asp30994Asn (and now p.Arg6745Cys) in extended pedigrees. This is the evidence class most conspicuously absent from the founding report.
3. Isogenic human iPSC-cardiomyocyte modelling of the reported alleles: hypertrophic growth, sarcomere organisation, titin turnover/half-life, passive stiffness, and relaxation kinetics vs isogenic controls — replacing yeast two-hybrid and fish data with human cardiac cell data.

Open question 2 (CONTROVERSY): If TTN does not cause HCM, do TTNtv nonetheless modify outcome within established HCM? The two claims are logically separable and the evidence points in opposite directions: prevalence data argue against causation, while the same cohort reports adjusted HR 6.88 for cardiovascular death (PMID:28822653; 3 deaths among 13 carriers, unreplicated). Resolution requires replication in an independent, ancestrally distinct HCM cohort with adequate event numbers — e.g., within SHaRe or a national registry with linked TTN sequencing.

Curation guard (Named Entity Confusion risk — high). TTN's dominant, well-validated cardiomyopathy association is with dilated cardiomyopathy (MONDO:0005021; ClinGen Definitive; TTNtv in ~25% of familial DCM). That literature is an order of magnitude larger than the HCM literature and is trivially easy to import by mistake — including via search engines and deep-research tools that will happily return DCM content for a "TTN cardiomyopathy" query. Every claim in a CMH9 entry must be checked against the question: does this source's cohort have hypertrophic, or dilated, cardiomyopathy?

Reference list (with evidence-source classification)

Unresolved retrieval gaps in this report (flagged rather than guessed): (1) gnomAD v4 allele frequency for TTN p.Arg740Leu could not be retrieved programmatically (gnomAD is a client-rendered app; ClinVar shows no frequency) — verify via the gnomAD GraphQL API or browser before curating any frequency claim; (2) the full-text TTN paragraph of the final JACC 2025 reappraisal was accessible only via the medRxiv/PMC preprint — verify wording against the published version before quoting it as an evidence snippet; (3) whether the two 2019 M‑line Ig variants segregated in their families is asserted in that paper's pedigrees but was judged insufficient by ClinGen — read the primary figures before making a segregation claim either way.

Sources: - OMIM #613765 — CMH9 · OMIM *188840 — TTN · Clinical Synopsis 613765 - GTR: Hypertrophic cardiomyopathy 9 (C1861065) · ClinVar VCV000012649 · ClinVar RCV000013484 - ClinGen HCM gene reappraisal (JACC 2025) · preprint PMC11312670 · ClinGen summary page · GenCC TTN–HCM submission - Evaluating the Clinical Validity of HCM Genes (Circ Genom Precis Med 2019) · SHaRe: Genotype and Lifetime Burden of Disease in HCM - Titin/MURF1 medaka + human HCM study (PMC6899042) · PMID 28822653 · Titin-truncated mouse TAC model · Titin kinase pseudokinase / MuRF1 scaffold · α-actinin-2 Z-disc dysfunction in human myocardium - GeneReviews: Nonsyndromic HCM Overview (NBK1768) · 2024 AHA/ACC HCM Guideline · ACC Key Points - US HCM epidemiology 2016–2023 (JACC Advances) · Orphanet: rare hypertrophic cardiomyopathy (ORPHA:217569) · Orphanet: familial isolated HCM (ORPHA:155) - Cytokinetics: FDA approval of MYQORZO (aficamten) · TCTMD coverage · Aficamten: First Approval (PMID 41941083) - TTN A Case Study, Int J Gen Med 2025 (PMID 39895828) · ICD-10-CM I42.1 · ICD-10-CM I42.2 · ICD-11 HCM block