rs193922385 — MYBPC3 Arg177Cys

MYBPC3 Arg177Cys — A Rare Cardiac Variant of Uncertain Significance

The MYBPC3 gene encodes cardiac myosin-binding protein C (cMyBP-C)¹¹ cardiac myosin-binding protein C (cMyBP-C)
A thick-filament associated protein embedded in the sarcomere — the basic contractile unit of heart muscle cells — at regular 43 nm intervals along the myosin backbone, the most important structural protein in the heart's contractile machinery. cMyBP-C serves double duty: it stabilizes the ordered arrangement of myosin filaments and acts as a phosphorylation-dependent regulator of contraction and relaxation rate. When MYBPC3 is damaged — either truncated by frameshift or splice mutations, or subtly altered by missense changes — the heart's pump function can deteriorate over years or decades, eventually producing hypertrophic cardiomyopathy (HCM) or, less commonly, dilated cardiomyopathy (DCM).

The rs193922385 variant changes a single nucleotide (G→A on the plus strand; c.529C>T on the coding strand, since MYBPC3 is transcribed from the minus strand), substituting a cysteine for the arginine at position 177 of the mature protein. This specific amino acid change — p.Arg177Cys²² p.Arg177Cys
Also written R177C; Arginine (positively charged, polar) replaced by Cysteine (sulfur-containing, can form disulfide bonds) — a chemically significant change — has been observed in individuals with HCM and DCM across multiple clinical sequencing programs, but its role in causing disease, versus being an incidental bystander, remains unresolved.

The Mechanism

Arg177 falls within the C1 immunoglobulin-like domain³³ C1 immunoglobulin-like domain
cMyBP-C contains 8 Ig-like domains (C0–C5, C8, C10) plus 3 fibronectin-type III domains; the N-terminal C0–C1–C2 region interacts with actin thin filaments and myosin-S2, making it central to crossbridge regulation of cMyBP-C, a region that directly participates in binding to the myosin-S2 subfragment and F-actin thin filaments. Replacing Arg177 with cysteine is predicted by computational tools to be structurally disruptive — the introduction of a free sulfhydryl group in a buried domain position could destabilize the Ig fold or create aberrant disulfide bridges. Whether this translates into a functionally deficient protein, a poison-polypeptide incorporated into the sarcomere with dominant-negative effects, or a tolerated variant with no appreciable impact on cMyBP-C function is unknown: no functional studies have been published⁴⁴ no functional studies have been published
Multiple ClinVar submitters including Mayo Clinic Laboratories, All of Us Research Program, and Color Diagnostics explicitly note the absence of published functional data for this variant.

An important clue comes from a different variant at the same position: p.Arg177His (a histidine substitution) is found in approximately 1.3% of individuals of African ancestry⁵⁵ p.Arg177His (a histidine substitution) is found in approximately 1.3% of individuals of African ancestry
Reported by LabCorp Genetics in ClinVar; this frequency is ~130 times higher than Arg177Cys, strongly implying Arg177 tolerates substitution at population scale. This does not prove Arg177Cys is benign — different substitutions at the same residue can have very different structural consequences — but it does indicate that position 177 is not absolutely conserved across vertebrates.

The Evidence

MYBPC3 is the most frequently mutated gene in HCM⁶⁶ most frequently mutated gene in HCM
Accounting for 40-50% of all HCM mutations; over 350 individual variants have been identified. Pathogenic MYBPC3 variants — primarily truncating mutations — produce haploinsufficiency, leaving the sarcomere with insufficient functional cMyBP-C and triggering compensatory hypertrophy. Missense variants like Arg177Cys are biologically distinct: they produce a full-length mutant protein that may or may not interfere with sarcomeric structure.

The Arg177Cys variant sits in a gray zone for current classification standards. Its overall allele frequency in gnomAD is approximately 0.009%⁷⁷ 0.009%
14 of 148,808 alleles in gnomAD v4 genomes; very similar frequency in TopMed at 30/264,690 alleles. A statistical framework published by Whiffin and colleagues⁸⁸ statistical framework published by Whiffin and colleagues
Using ExAC/gnomAD to derive maximum tolerable allele frequencies for dominant cardiomyopathy genes suggests that variants with AF above approximately 0.004% are unlikely to be independently pathogenic in dominant HCM — Arg177Cys is very close to this threshold, creating genuine ambiguity.

Clinical classification across 13 ClinVar submitters (as of April 2026) shows 12 submissions of Uncertain Significance and 1 of Likely Benign⁹⁹ 12 submissions of Uncertain Significance and 1 of Likely Benign
Major contributors include GeneDx, Invitae, Mayo Clinic, Color Diagnostics, Ambry Genetics, and Laboratory for Molecular Medicine — representing the leading cardiac genetic testing laboratories. Many individuals in whom this variant was identified also carried additional pathogenic variants in other sarcomere genes, making it difficult to attribute disease causation to Arg177Cys specifically. The variant failed to segregate with disease in at least one family.

Family studies with adequate numbers of informative meioses — and functional characterization in cardiomyocyte models — are needed to resolve the classification. Until then, Arg177Cys should be treated as a variant requiring cardiac surveillance rather than a confirmed disease- causing mutation.

Practical Actions

The 2024 AHA/ACC Guideline for Management of Hypertrophic Cardiomyopathy¹⁰¹⁰ 2024 AHA/ACC Guideline for Management of Hypertrophic Cardiomyopathy
The first major HCM guideline revision in over a decade; PMID 38718139 recommends that individuals who are gene-positive and phenotype-negative (no current signs of cardiomyopathy) undergo periodic cardiac surveillance: echocardiography, electrocardiography, and clinical assessment every 1–2 years in children and adolescents, and every 3–5 years in adults who remain asymptomatic. This framework applies to carriers of confirmed pathogenic variants; for a VUS like Arg177Cys, the appropriate surveillance interval should be determined in consultation with a cardiologist or clinical geneticist who can weigh the full clinical context.

Genetic counselling is appropriate given the VUS status — classification may change as functional studies are performed or additional segregation data becomes available. First-degree relatives (parents, siblings, children) may benefit from cascade testing with genetic counselling, recognising that a positive result for the same VUS does not itself confirm pathogenicity.

Interactions

MYBPC3 missense VUS carriers who also carry a second sarcomeric gene variant (in MYH7, TNNT2, TNNI3, or other MYBPC3 loci) face compound genetic burden. Several individuals with Arg177Cys in the literature also carried additional variants in MYH7 or VCL, and in those cases disease attribution to Arg177Cys alone is particularly uncertain. If a second sarcomeric variant is present, the compound burden may be the primary driver of clinical phenotype. This assessment requires expert interpretation and is outside the scope of interpretation of this single- variant result.