rs199474703 — MYL3 Arg94His (R94H)

MYL3 Arg94His — A High-Penetrance Sarcomeric Variant for Hypertrophic Cardiomyopathy

Your heart's pumping power comes from millions of sarcomeres¹¹ sarcomeres
The sarcomere is the basic contractile unit of cardiac muscle. Each sarcomere contains an interdigitating lattice of thick (myosin) and thin (actin) filaments that slide past each other to produce contraction working in concert. The MYL3 gene encodes the essential myosin light chain (ELC), a structural component of the myosin thick filament that stabilizes the lever arm region and regulates the force and speed of cardiac muscle contraction. The Arg94His substitution — a single amino acid change replacing arginine with histidine at position 94 of the ELC — disrupts this regulation in a way that causes the heart muscle to thicken abnormally, a condition called hypertrophic cardiomyopathy (HCM).

HCM is the most common inherited cardiac disease, affecting approximately 1 in 500 people overall. However, MYL3 mutations are a rare cause of HCM — responsible for less than 1% of all sarcomeric HCM cases. The Arg94His variant in particular has been documented in a small number of families worldwide and is classified as pathogenic by eight independent clinical laboratories²² pathogenic by eight independent clinical laboratories
ClinVar VCV000031777: submissions from LabCorp Genetics, Women's Health and Genetics/LabCorp, Ambry Genetics, GeneDx, Mass General Brigham, NIH All of Us, Color Diagnostics, and CHEO Genetics.

The Mechanism

The MYL3 ELC wraps around the myosin heavy chain lever arm³³ lever arm
The lever arm is the portion of myosin that amplifies the small conformational change at the catalytic domain into a large stroke movement that pulls actin filaments. Light chains stabilize this lever arm against mechanical stress. Arginine-94 sits in a functionally critical region of the ELC. Replacing the positively charged arginine with histidine (which carries a smaller, uncharged side chain at physiological pH) is predicted to disrupt local protein folding and the ELC's interaction with the myosin heavy chain, altering the super relaxed state of myosin — a low-energy resting state that is disrupted in HCM-associated ELC mutations. The net result is hypercontractility of the sarcomere, leading over time to pathological left ventricular hypertrophy, diastolic dysfunction, and in some cases left ventricular outflow tract obstruction.

The Evidence

The primary evidence for MYL3 Arg94His comes from a Japanese family study⁴⁴ Japanese family study
Nomura et al., J Cardiol, 2016 (N=7 family members, 5 affected; followed by registry screening of 600 HCM patients) that used whole-exome sequencing combined with bioinformatic filtering to identify the variant. Among carriers in that family, disease penetrance was 88% — very high for an HCM gene. All clinically affected carriers showed asymmetric septal hypertrophy with a maximum left ventricular wall thickness of 18±3mm without outflow obstruction. Two additional carriers were found in a registry of 600 HCM patients, supporting that this is a genuine pathogenic variant rather than a private family mutation.

By contrast, other MYL3 variants (such as the p.Val79Ile variant⁵⁵ p.Val79Ile variant
Andersen et al., Biochem Res Int, 2012 — nine heterozygous carriers in a Danish family) show much lower penetrance (~40%) and late onset. This variability is consistent with the broader GeneReviews HCM data⁶⁶ GeneReviews HCM data
Cirino et al., updated 2025 — MYL3 overall penetrance ~32%, the lowest among sarcomeric HCM genes when all variants are pooled. The Arg94His variant appears to be at the higher end of MYL3 penetrance, based on the Nomura family data, though the evidence is limited to a small number of families. Eight independent clinical laboratories have independently classified it as pathogenic or likely pathogenic, providing strong multi-submitter validation.

In terms of population frequency, Arg94His is extremely rare: gnomAD v4 exomes identified only 7 T alleles among over 730,000 alleles examined globally, exclusively in European and Latino ancestry groups.

Practical Actions

For carriers of MYL3 Arg94His, the central priorities are: (1) confirm the diagnosis with a specialist, (2) establish regular cardiac surveillance, and (3) arrange cascade testing for first-degree relatives.

Unlike many common HCM genes (MYBPC3, MYH7) where genotype-specific prognosis is well-characterized, MYL3 Arg94His has been observed in relatively few families. Standard HCM management — echocardiography every 1–2 years, ambulatory ECG monitoring, exercise testing, and SCD risk stratification — applies. Drug therapy (beta-blockers, calcium channel blockers) addresses symptoms; septal reduction therapy (myectomy or alcohol ablation) is reserved for refractory obstructive cases. The newer selective cardiac myosin inhibitor mavacamten is approved for obstructive HCM and may be relevant if outflow obstruction develops.

Interactions

MYL3 variants have been observed to interact with other sarcomeric HCM genes. Individuals carrying two sarcomeric pathogenic variants (compound or digenic) show substantially worse outcomes — a hazard ratio of 7.5 for adverse events such as cardiac transplantation or ICD placement⁷⁷ hazard ratio of 7.5 for adverse events such as cardiac transplantation or ICD placement
Cirino et al., GeneReviews, 2025 — based on registry data across sarcomeric HCM genes compared to single-variant carriers. This means that if a carrier of MYL3 Arg94His also carries a pathogenic variant in MYBPC3, MYH7, TNNT2, or another sarcomeric gene, clinical severity is likely to be substantially greater.

MYL3 is an essential light chain (encoded by chromosome 3), distinct from the regulatory light chain encoded by MYL2. Variants in ACTC1 (cardiac actin, rs193922680) affect the thin filament partner of myosin and can produce overlapping HCM phenotypes through a different molecular mechanism.