MYL2 Human

What is the molecular structure and function of MYL2 in human cardiac tissue?

MYL2 is an ~19kDa sarcomeric protein (166 amino acids) belonging to the EF-hand calcium binding protein superfamily . It forms part of the CTER subfamily (calmodulin, troponin C, essential and regulatory light chains of myosin) and contains two pairs of EF-hand calcium binding domains arranged in a globular structure . MYL2 functions as a regulatory light chain in the myosin neck/tail region . In cardiac tissue, MYL2 regulates cross-bridge cycling kinetics and calcium-dependent muscle contraction, with essential roles in maintaining proper myosin lever arm conformation during force generation .

What are the major isoforms of MYL2 and their tissue distribution patterns?

MYL2 exists as three major isoforms encoded by distinct genes in mammalian striated muscle:

Each isoform has a distinct developmental expression pattern, contributing to the functional specialization of different muscle types . In humans, the MYL2 gene (encoding the ventricular isoform) is located on chromosome 12, while in mice it's found on chromosome 5 .

How is MYL2 phosphorylation regulated in the heart?

In the adult heart, MYL2 function is regulated primarily through phosphorylation, which displays a specific expression pattern (high in epicardium and low in endocardium) across the myocardium . This transmural gradient is critical for proper cardiac contractile mechanics. The phosphorylation state is dynamically controlled by the balanced activities of myosin light chain kinase (MLCK) and myosin light chain phosphatase (MLCP). This regulatory mechanism directly influences cross-bridge cycling kinetics, calcium sensitivity of the contractile apparatus, and cardiac torsion mechanics .

What developmental functions does MYL2 serve in cardiogenesis?

Genetic loss-of-function studies in mice have demonstrated that cardiac isoforms of MYL2 (MLC-2v and MLC-2a) are essential for proper cardiac contractile function during early embryogenesis . Research using zebrafish models shows that knocking down myl2b (the zebrafish homolog of human MYL2) leads to aberrant cardiac structures, confirming its importance in heart formation . The precise timing and localization of MYL2 expression during development is critical for establishing chamber-specific contractile properties necessary for proper cardiac morphogenesis and function .

What molecular mechanisms underlie MYL2 variant pathogenicity in cardiomyopathies?

MYL2 variants contribute to cardiac pathology through several mechanisms:

• Structural disruption: Variants affecting the EF-hand domains can alter protein stability and sarcomere organization .

• Altered lever arm mechanics: Variants like p.D166H affect critical regions that interact with the myosin lever arm, changing its steric conformation during cross-bridge cycling .

• Protein degradation: Certain frameshift variants (e.g., p.Pro144Argfs*57) result in protein products that undergo active proteasomal degradation .

• Phosphorylation disruption: Some variants may alter the normal phosphorylation pattern essential for proper contractile function .

• Protein-protein interaction alterations: Variants can disrupt critical interactions between MYL2 and other sarcomeric components, particularly the myosin heavy chain .

How do inheritance patterns differ between MYL2 variant types?

The inheritance patterns of MYL2-associated cardiac disorders vary based on the variant type:

• Missense variants (e.g., p.D166H, p.Gly162Arg) typically follow dominant inheritance patterns, causing hypertrophic cardiomyopathy (HCM) with variable expressivity and adult-onset presentation .

• Frameshift variants (e.g., p.Pro144Argfs*57) can follow recessive inheritance patterns, where heterozygous carriers remain unaffected while homozygotes develop severe infantile-onset HCM with early mortality .

This dichotomy suggests fundamentally different molecular mechanisms: dominant variants likely exert gain-of-function or dominant-negative effects, while recessive variants typically cause loss-of-function that can be compensated by the wild-type allele in heterozygotes .

What is the relationship between MYL2 variants and specific cardiac phenotypes?

MYL2 variants are associated with distinct cardiac phenotypes:

• Hypertrophic cardiomyopathy (HCM): Most commonly associated with MYL2 variants, with varying degrees of ventricular hypertrophy .

• Restrictive physiology: The novel p.D166H variant shows high penetrance for restrictive filling patterns .

• Atrial fibrillation (AF): Certain variants like p.D166H show strong association with AF development .

• Infantile-onset HCM: Homozygous frameshift variants can cause severe early-onset disease with mitral valve dysplasia and mortality before one year of age .

• Congenital heart disease (CHD): Both p.Ile158Thr and p.Val146Met variants have been associated with CHD development in Han Chinese populations .

How does MYL2 phosphorylation influence cardiac biomechanics?

MYL2 phosphorylation creates a transmural gradient (epicardium to endocardium) that directly contributes to:

• Cardiac torsion: The phosphorylation gradient helps establish the twisting motion of the heart during contraction, optimizing ejection fraction .

• Cross-bridge cycling kinetics: Phosphorylation alters the attachment/detachment rates of myosin heads to actin filaments .

• Calcium sensitivity: Phosphorylated MYL2 modifies the calcium response of the contractile apparatus .

• Force generation: The phosphorylation state directly impacts the force-generating capacity of cardiac muscle .

Computational models incorporating this phosphorylation gradient have helped elucidate the relationship between molecular modifications and whole-organ biomechanics, revealing how disruptions can lead to cardiac dysfunction .

What animal models are most effective for studying MYL2 function and pathogenic variants?

Several animal models have proven valuable for MYL2 research:

Each model offers complementary advantages for investigating basic mechanisms and evaluating variant pathogenicity .

What techniques are optimal for assessing MYL2 variant stability and degradation?

Several approaches are used to study MYL2 variant stability and degradation:

• Protein expression analysis: Western blotting of tissue samples or transfected cells expressing wild-type versus mutant MYL2 can reveal differences in steady-state levels .

• Proteasome inhibition studies: Treatment with proteasome inhibitors can rescue the degradation of unstable variants, as demonstrated with the MYL2-fs variant (p.Pro144Argfs*57) .

• Immunohistochemistry: Analysis of ventricular muscle samples can show marked reduction of MYL2 expression in patients with degradation-prone variants compared to controls .

• In vitro overexpression: Transfection of cells with wild-type versus mutant MYL2 constructs allows comparison of protein levels and localization patterns .

• Structural modeling: Computational analysis of variants can predict stability changes, as seen with the p.Val146Met variant which altered inter-domain distances (40.4Å to 43.1Å) .

What genetic screening approaches are most effective for identifying MYL2 variants in patient populations?

Optimal genetic screening approaches for MYL2 variants include:

• Targeted gene panels: Next-generation sequencing panels focusing on cardiomyopathy-associated genes (including MYL2) provide cost-effective screening with high coverage depth. For example, a 56-gene panel successfully identified the novel p.D166H variant .

• Exome sequencing: Broader coverage enables identification of novel variants in complex cases, as demonstrated in the discovery of the homozygous frameshift variant p.Pro144Argfs*57 .

• Variant classification pipeline:

  • Population frequency filtering (e.g., gnomAD database)

  • In silico prediction tools (PROVEAN, PolyPhen-2)

  • Segregation analysis in families

  • American College of Medical Genetics and Genomics (ACMG) guidelines application

• Confirmatory testing: Sanger sequencing for variant verification followed by functional studies for variants of uncertain significance .

What in vitro approaches best evaluate the functional consequences of MYL2 variants?

Several complementary approaches effectively evaluate MYL2 variant function:

• Protein localization studies: Visualization of MYL2 variants through immunofluorescence or tagged constructs reveals disruptions in sarcomeric integration .

• Protein expression analysis: Western blotting of cells transfected with wild-type versus mutant constructs quantifies expression differences, as seen with the significantly upregulated p.Ile158Thr variant .

• Structural modeling: In silico analysis predicts functional impacts, such as the p.D166H variant's effect on lever arm conformation during cross-bridge cycling .

• Protein-protein interaction studies: Co-immunoprecipitation and binding assays evaluate how variants affect interactions with myosin heavy chains and other sarcomeric components .

• Rescue experiments: Complementation studies in model systems (e.g., zebrafish) reveal whether mutant proteins can restore normal function, as demonstrated when wild-type but not mutant MYL2 rescued morpholino-induced cardiac defects .

How can genotype-phenotype correlations in MYL2 variants inform clinical management?

Understanding genotype-phenotype correlations for MYL2 variants has important clinical implications:

• Risk stratification: Certain variants (like p.D166H) associate with higher incidence of restrictive physiology, atrial fibrillation, and poor clinical outcomes, warranting more aggressive monitoring and intervention .

• Family screening: Knowledge of inheritance patterns (dominant vs. recessive) guides family screening approaches and genetic counseling .

• Early intervention: Identification of variants associated with infantile-onset disease (e.g., p.Pro144Argfs*57) enables proactive monitoring in affected families .

• Clinical trial eligibility: Genotype-specific therapeutic approaches may emerge as understanding of molecular mechanisms advances .

• Precision medicine: Different variant mechanisms (degradation vs. dysfunction) may eventually guide selection of tailored therapeutic strategies .

What computational frameworks best predict phenotypic severity of novel MYL2 variants?

Integrated computational frameworks for predicting MYL2 variant severity include:

• Multi-algorithm consensus approach: Combining predictions from PROVEAN, PolyPhen-2, SIFT, and MutationTaster improves accuracy over any single tool .

• Structural impact analysis: Homology modeling and molecular dynamics simulations predict functional consequences of amino acid substitutions, as shown for variants affecting EF-hand domains .

• Conservation analysis: Evaluation of evolutionary conservation at variant sites helps assess functional importance (e.g., p.D166H affects the last and highly conserved amino acid of the protein) .

• Integrative scoring systems: Frameworks that combine multiple lines of evidence (population frequency, conservation, structural predictions) provide more robust variant classification .

These computational approaches serve as valuable screening tools for prioritizing variants for functional validation and clinical correlation studies .