rs397516394 — TPM1 Met281Val

TPM1 Met281Val — A Tropomyosin Variant of Uncertain Significance in HCM

When a genetic test returns a "variant of uncertain significance" (VUS) in a cardiomyopathy gene, it sits in one of medicine's most difficult grey zones: clinically actionable enough to flag, not conclusive enough to act on alone. rs397516394 — a missense substitution that converts methionine to valine at position 281 of cardiac alpha-tropomyosin (encoded by TPM1) — is precisely this kind of result. It appears on HCM gene panels because TPM1 is a well-established HCM gene and position 281 is in a biologically meaningful region. Yet as of 2026, five independent clinical laboratories have evaluated this variant and all five reached the same conclusion: uncertain significance¹¹ uncertain significance
Variants of uncertain significance (VUS) cannot be classified as pathogenic or benign because available evidence is insufficient; they are neither reassuring nor alarming in isolation and require clinical context and family studies for interpretation.

The Mechanism

Cardiac alpha-tropomyosin is a 284-amino acid coiled-coil protein that runs along the length of the actin thin filament in cardiac muscle. Its function is regulatory: at diastolic (low) calcium concentrations, tropomyosin occupies the "blocked" position that sterically prevents myosin from binding actin. When calcium rises during a heartbeat, troponin pulls tropomyosin aside to expose myosin-binding sites, enabling contraction. When this system is disrupted — either by increased flexibility or altered calcium sensitivity — the consequence is cardiac hypertrophy, diastolic dysfunction, and arrhythmia risk, the hallmarks of hypertrophic cardiomyopathy²² hypertrophic cardiomyopathy
HCM affects ~1:500 people and is the most common inherited cardiac condition; it is the leading cause of sudden cardiac death in people under 35.

Position 281 (methionine) sits near the C-terminus of tropomyosin, a region involved in head-to-tail overlap with adjacent tropomyosin dimers along the actin filament. An adjacent substitution at this residue — Met281Thr, a different amino acid swap at the same codon — has been studied in mechanistic assays as a dilated cardiomyopathy (DCM) variant³³ dilated cardiomyopathy (DCM) variant
DCM is the opposite of HCM: the ventricle becomes dilated and hypocontractile rather than thick and hypercontractile; DCM-associated tropomyosin mutations reduce Ca²⁺ sensitivity whereas HCM mutations increase it. The Met→Val substitution at rs397516394 changes the amino acid from a flexible sulfur-containing side chain to a small branched hydrophobic residue; the net structural impact on tropomyosin flexibility and calcium sensitivity has not been reported in published functional studies.

The Evidence

The evidence base for rs397516394 is thin and indirect:

ClinVar classification: Five clinical laboratories⁴⁴ Five clinical laboratories
ClinVar VCV000043448; submitters include Ambry Genetics, GeneDx, Laboratory for Molecular Medicine (Mass General Brigham), Labcorp Genetics (Invitae), and Color Diagnostics have independently classified Met281Val as uncertain significance. In silico analysis by multiple submitters suggested the variant may be tolerated (PolyPhen-2 prediction of low pathogenicity). The only pointer toward clinical relevance is the observation that "other variants that disrupt this residue have been determined to be pathogenic" — meaning the Met281 position has functional importance, but the Val substitution specifically has not been proven to share that importance.

Population frequency: The G allele (Val) appears in approximately 6 per million alleles in gnomAD exomes — not absent, but consistent with a disease-relevant rare variant. By comparison, confirmed pathogenic TPM1 variants like E180G are absent from gnomAD entirely.

Gene context: Walsh et al., 2017⁵⁵ Walsh et al., 2017
Walsh R et al. Reassessment of Mendelian gene pathogenicity using 7,855 cardiomyopathy cases and 60,706 reference samples; Genet Med 2017 applied rigorous statistical standards to cardiomyopathy genes and found that many rare missense variants in sarcomeric genes previously treated as pathogenic do not meet evidence thresholds. TPM1 missense VUSs — including variants at positions where other substitutions cause disease — are a well-recognized challenge in clinical HCM genetics.

The Gupte et al. benchmark: Mechanistic work on seven TPM1 variants⁶⁶ Mechanistic work on seven TPM1 variants
Gupte et al. J Biol Chem 2015 — examined Ca²⁺ sensitivity of human β-cardiac myosin ATPase; HCM-associated TPM1 mutations showed hypersensitivity, DCM mutations showed hyposensitivity included M281T (the adjacent threonine substitution). Whether Met281Val follows the same mechanistic pattern has not been published. The absence of functional data is a key reason the variant remains unclassified.

Clinical observations: The variant has been identified in individuals undergoing HCM genetic testing — but given the prevalence of HCM (~1:500) and the high background rate of VUS findings in cardiomyopathy panels (~15% of probands per Alfares et al.), coincidental ascertainment without causation cannot be excluded. No published family co-segregation data, functional studies, or case series specifically characterizing Met281Val have been identified.

Practical Actions

A TPM1 VUS result has specific implications for clinical management — different from both a confirmed pathogenic finding and a benign result. Current HCM guidelines (AHA/ACC and ESC) recommend against making diagnostic or treatment decisions based on a VUS alone. However, the presence of a VUS in a disease-relevant gene warrants cardiac surveillance and family investigation to gather additional evidence for future reclassification.

If this result appears in the setting of clinical HCM (confirmed cardiac imaging findings), it strengthens but does not prove the genetic basis. If this result appears incidentally in someone without HCM features, it requires surveillance rather than immediate intervention.

Interactions

Met281Val is in the same gene and biological context as confirmed pathogenic TPM1 variants: E180G (rs104894502) at position 180 and D175N (rs104894503) at position 175. All three are ultra-rare missense changes in cardiac alpha-tropomyosin; the confirmed variants both increase calcium sensitivity through reduced protein rigidity. Whether Met281Val shares this mechanism or the opposite (DCM-type hyposensitivity, as seen with Met281Thr) is a key open question. Other sarcomeric HCM genes — MYBPC3 (rs36211723 and others), MYH7, TNNT2 — contribute independently to HCM risk and are not captured by this result.