rs397516407 — MYL2 Glu163Ala

Pathogenic missense variant in the regulatory myosin light chain gene; heterozygous carriers face substantially elevated risk of hypertrophic cardiomyopathy and warrant cardiac evaluation and cascade screening of first-degree relatives

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MYL2 Glu163Ala — A Sarcomere Fault at the Heart of Contraction

Every heartbeat depends on myosin motors pulling actin filaments with exquisite precision. The regulatory myosin light chain — encoded by MYL2 — wraps around the myosin neck like a molecular clamp, stiffening the lever arm and fine-tuning the speed and force of each contraction stroke. rs397516407 (c.488A>C, p.Glu163Ala) replaces a negatively charged glutamic acid with a neutral, non-polar alanine at position 163 of this 166-amino-acid protein — [a non-conservative substitution | glutamic acid carries a negative charge; alanine is neutral and non-polar; this difference can disrupt calcium-sensitive conformational switches] in the C-terminal EF-hand-like domain. The variant is absent from all large population databases (gnomAD, 1000 Genomes) and appears exclusively in hypertrophic cardiomyopathy (HCM) families, earning a "likely pathogenic / pathogenic" classification from multiple clinical laboratories including Invitae and the Laboratory for Molecular Medicine (ClinVar VCV000043480).

The Mechanism

MYL2 belongs to the [EF-hand superfamily | calcium-binding proteins with a characteristic helix-loop-helix fold; the MYL2 C-terminal EF-hand modulates how the protein responds to intracellular calcium changes during systole and diastole]. Position 163 sits within the fourth EF-hand pair, adjacent to Asp166 — a residue whose substitution (D166V) has independently been shown to impair calcium binding affinity and disrupt sarcomeric force generation. [Sheikh et al., 2015 | Sheikh F, Lyon RC, Chen J. Functions of myosin light chain-2 (MYL2) in cardiac muscle and disease. Gene. 2015;569(1):14-20. https://pubmed.ncbi.nlm.nih.gov/26074085/11 https://pubmed.ncbi.nlm.nih.gov/26074085/]

When the MYL2–myosin interaction is perturbed by a missense in this domain, myosin heads spend more time in the force-generating (on) state relative to the resting (off) state — a hallmark of sarcomeric HCM. The result is hypercontractility, impaired relaxation, and a remodelling cascade: the ventricular wall thickens, becomes stiffer, and can obstruct outflow. Over years this [diastolic dysfunction | impaired filling of the heart between beats, even while pumping strength is preserved or increased] drives breathlessness, fatigue, and elevated arrhythmia risk.

The Evidence

MYL2 variants account for 1–3% of all familial HCM cases — a small fraction, but with disproportionate clinical impact because MYL2 mutations can cause both benign and malignant phenotypes. [Flavigny et al., 1998 | Flavigny J et al. Identification of two novel mutations in the ventricular regulatory myosin light chain gene (MYL2) associated with familial and classical forms of hypertrophic cardiomyopathy. J Mol Med. 1998;76:208–214. https://pubmed.ncbi.nlm.nih.gov/9535554/22 https://pubmed.ncbi.nlm.nih.gov/9535554/]

The pathogenic p.Glu163Ala substitution (along with the related p.Glu163Gly at the same codon) is absent from gnomAD across all ancestries, strongly supporting disease causation rather than benign polymorphism. An adjacent variant, p.Gly162Glu (one residue N-terminal to Glu163), was studied by [Renaudin et al., 2018 | Renaudin P et al. A Novel Missense Mutation p.Gly162Glu of the Gene MYL2 Involved in Hypertrophic Cardiomyopathy: A Pedigree Analysis of a Proband. Mol Diagn Ther. 2018;22:219–223. https://pubmed.ncbi.nlm.nih.gov/29549657/33 https://pubmed.ncbi.nlm.nih.gov/29549657/] in a 27-member pedigree: 12 of 16 carriers developed overt HCM; zero of 11 non-carriers had cardiomyopathy. This near-perfect segregation at the codon 162–163 region is direct functional evidence that mutations here are causally linked to HCM.

MYL2 penetrance is variable and context-dependent. A large meta-analysis of cascade-screened families found that pooled penetrance across sarcomere genes was 57% (95% CI 52–63%), with mean age at HCM diagnosis of 38 years. [Pelliccia et al., 2024 | Meta-Analysis of Penetrance and Systematic Review on Transition to Disease in Genetic Hypertrophic Cardiomyopathy. Circulation. 2024. https://pubmed.ncbi.nlm.nih.gov/37929589/44 https://pubmed.ncbi.nlm.nih.gov/37929589/] Importantly, penetrance in MYL2 carriers rises sharply with co-existing hypertrophy triggers: in a Dutch cohort carrying MYL2 p.Glu22Lys, penetrance reached 89% when hypertension or obesity was present, versus 36% in carriers without additional risk factors. [Claes et al., 2016 | https://pubmed.ncbi.nlm.nih.gov/26497160/55 https://pubmed.ncbi.nlm.nih.gov/26497160/] Controlling modifiable risk factors is therefore a meaningful lever for gene-positive individuals.

Practical Actions

Heterozygous carriers should have baseline and serial cardiac evaluation: echocardiography or cardiac MRI to detect left ventricular hypertrophy, 12-lead ECG and 24–48 hour Holter monitoring for arrhythmia, and cardiology review. Consistent with the 2024 AHA/ACC guideline for HCM management, cascade testing of all first-degree relatives is strongly recommended when a pathogenic or likely pathogenic sarcomere variant is confirmed. [AHA/ACC 2024 | https://pubmed.ncbi.nlm.nih.gov/38718139/66 https://pubmed.ncbi.nlm.nih.gov/38718139/] Blood pressure should be kept in the optimal range: hypertension substantially amplifies penetrance in MYL2 carriers.

Interactions

The Glu163 codon also has a second clinically relevant alternate allele: T>C on the plus strand produces p.Glu163Gly (ClinVar VCV000181421), classified as likely pathogenic by GeneDx. Compound heterozygosity at this locus (Glu163Ala + Glu163Gly) would be mechanistically significant but is not reported in the literature — carriers of either allele should be assessed individually by a cardiologist. MYL2 pathogenic variants interact additively with hypertension (the most important co-modifier in published cohorts) and with other sarcomere-gene variants; anyone carrying a second sarcomere mutation is at substantially higher risk and warrants expedited specialist referral.

Genotype Interpretations

What each possible genotype means for this variant.

TT Non-Carrier Normal

No MYL2 Glu163Ala variant detected

You carry two copies of the common reference allele at rs397516407 and do not carry the MYL2 Glu163Ala pathogenic variant. The vast majority of people share this genotype — this variant is essentially absent from all large population databases. Your myosin regulatory light chain at this position follows the typical sequence. This does not rule out other forms of HCM, which has many genetic causes and can also arise without an identifiable mutation.

GT Carrier Carrier

One copy of MYL2 Glu163Ala — heightened HCM risk requiring cardiac evaluation

MYL2 encodes the regulatory myosin light chain, a 166-amino acid protein that wraps around the myosin neck to stabilize the lever arm and regulate myosin ATPase cycling. Position 163 lies within the C-terminal EF-hand-like calcium-sensing domain, and substituting the charged glutamic acid with neutral alanine disrupts the conformational dynamics that keep myosin heads in the resting (off) state between beats. The result is a chronically hypercontractile sarcomere — the mechanistic seed of left ventricular hypertrophy and diastolic dysfunction.

Penetrance in MYL2 carriers is not fixed. A Dutch study of 38 MYL2 p.Glu22Lys carriers found penetrance rose from 36% in carriers without modifying factors to 89% in carriers with hypertension or obesity. This means managing blood pressure is not merely good general advice for you — it is a gene-specific disease-modifier that can materially reduce the probability that your variant causes overt HCM.

An adjacent codon variant (p.Gly162Glu, studied by Renaudin et al. 2018) showed near- perfect disease segregation in a 27-member family: 12/16 carriers developed HCM, 0/11 non-carriers did. This spatial proximity strongly supports pathogenicity of mutations at the 162–163 region.

The 2024 AHA/ACC HCM guideline recommends cascade genetic testing for all first-degree relatives when a P/LP sarcomere variant is identified. Each first-degree relative has a 50% prior probability of carrying the same variant. Those who test positive warrant the same serial cardiac surveillance described below.

GG Homozygous Homozygous

Two copies of MYL2 Glu163Ala — extremely rare, expected severe cardiac involvement

Biallelic pathogenic variants in sarcomere genes have been described most extensively for MYBPC3 and MYH7, where homozygous or compound heterozygous carriers typically present with severe, early-onset HCM — often in childhood or early adulthood — with higher rates of outflow tract obstruction, adverse remodelling, and sudden cardiac death. For MYL2, a novel frameshift variant study (PMID 32453731) showed that recessive (biallelic) MYL2 deficiency causes a distinct, severe phenotype. Homozygosity for a missense variant at the pathogenic Glu163 site would be expected to produce severe perturbation of regulatory light chain function in both alleles simultaneously, resulting in a uniformly hypercontractile sarcomere.

Given the extreme rarity of this genotype, clinical management should be guided by a specialist HCM centre. SCD risk stratification with cardiac MRI (for late gadolinium enhancement), ambulatory ECG monitoring, and exercise testing is indicated. Cascade testing of all first-degree relatives is essential — each parent is expected to be heterozygous.