MYBPC3 Human

What is the structural organization of the MYBPC3 gene and protein?

MYBPC3 is located on chromosome 11p11.2 in humans and exceeds 21,000 base pairs in size. The gene contains 35 exons, with 34 being coding exons . The resulting protein, cardiac MyBP-C, is composed of 11 globular domains: 8 with homology to immunoglobulin I and 3 with fibronectin III domains . This structural arrangement is critical for the protein's function in regulating cardiac contractility.

What is the primary function of cardiac MyBP-C in sarcomere physiology?

Cardiac MyBP-C functions as a regulatory "brake" on actin-myosin-mediated contraction . The protein influences myosin conformations during both contraction and relaxation phases of the cardiac cycle. Normal cardiac MyBP-C helps maintain proper balance between the active state that enables ATP hydrolysis and thin filament interactions, and the super-relaxed conformation associated with energy conservation . This regulatory role is critical for normal cardiac function and energy homeostasis.

How prevalent are MYBPC3 mutations in hypertrophic cardiomyopathy cohorts?

MYBPC3 mutations represent the most common genetic cause of familial hypertrophic cardiomyopathy. In the Sarcomeric Human Cardiomyopathy Registry (SHaRe) study of 4,756 genotyped HCM patients, 1,316 patients (approximately 28%) were identified with pathogenic MYBPC3 variants . Among these patients, 80% carried truncating variants (234 unique variants) while 20% had nontruncating variants (22 unique variants) .

What experimental models provide the most valuable insights into MYBPC3 function?

Researchers typically employ a multi-model approach combining:

• Mouse models: Genetically modified mice (including knockout and knock-in models) provide in vivo systems to study physiological effects of MYBPC3 variants .

• Human cardiac tissue: Myectomy samples from HCM patients undergoing surgical treatment allow direct examination of protein expression and sarcomere structure .

• Isolated cardiomyocytes: Both rodent and human cardiomyocytes can be studied to assess contractile properties and calcium handling .

• Recombinant protein studies: For biochemical characterization and structural analyses of normal and mutant proteins .

Cross-species validation is particularly important, as MYBPC3 effects can vary between species despite conserved biological processes .

How can researchers effectively characterize MYBPC3 variant pathogenicity?

A comprehensive approach to variant characterization should include:

• Genetic segregation analysis: Tracking variant co-segregation with disease phenotype in families .

• Protein expression studies: Determining whether variants affect protein levels through haploinsufficiency or dominant-negative mechanisms .

• Functional assays: Measuring protein incorporation into myofilaments and degradation rates .

• Myosin conformation assessment: Evaluating how variants affect the equilibrium between different myosin states .

• Contractility measurements: Direct testing of contractile parameters in isolated cardiomyocytes .

For nontruncating variants, researchers should particularly investigate protein domain localization, as pathogenic missense variants cluster in specific functional domains (C3, C6, and C10) .

What techniques allow measurement of MYBPC3 effects on myosin conformations?

Researchers can assess myosin conformational changes using:

• Single-molecule fluorescence resonance energy transfer (FRET) to detect conformational changes in myosin heads.

• In vitro motility assays to measure actin-myosin interaction kinetics.

• Biochemical assays of myosin ATPase activity with and without MYBPC3.

• Super-resolution microscopy to visualize sarcomere structural changes.

These techniques revealed that MYBPC3 mutations disrupt normal myosin states, increasing the proportion of myosin heads available for contraction while reducing those in the super-relaxed conformation .

Is there evidence for location-dependent effects of MYBPC3 truncating mutations?

No significant correlation exists between truncating variant location and disease severity. The SHaRe study analyzed truncating MYBPC3 variants by quartiles from 5' to 3' location and found no statistically significant differences in:

• Maximum wall thickness

• Age-adjusted left atrial diameter

• Composite adverse event rates

This supports the haploinsufficiency model, where reduction in functional protein (regardless of mutation position) is the primary disease mechanism rather than dominant-negative effects from truncated proteins .

How do phenotypes differ between truncating and nontruncating MYBPC3 mutations?

Despite differences in age of onset and LVOT obstruction, both variant types show similar hypertrophy severity and adverse event rates, suggesting similar disease mechanisms . Nontruncating variants cluster predominantly in the C3, C6, and C10 domains (82% of pathogenic missense variants) .

How do MYBPC3 variants affect protein integration and degradation?

Pathogenic variants have different effects on protein behavior:

• C10 domain variants: Failed to incorporate into myofilaments with accelerated degradation rates (~90% faster than wild-type).

• C3 and C6 domain variants: Normal myofilament incorporation with degradation rates similar to wild-type .

These findings suggest domain-specific mechanisms, where C10 mutations likely cause haploinsufficiency through protein instability, while C3/C6 mutations may impair protein function despite normal localization and stability .

Is haploinsufficiency or a dominant-negative effect the primary disease mechanism in MYBPC3-associated HCM?

Evidence strongly supports haploinsufficiency as the primary mechanism:

• Human myectomy samples from patients with truncating MYBPC3 mutations show no detectable truncated MyBP-C protein .

• Both truncation and missense mutations result in reduced levels of full-length protein .

• Stepwise loss of cMyBPC results in reciprocal augmentation of myosin contractility in experimental models .

The absence of detectable truncated protein argues against incorporation into myofibers and any dominant-negative effect. Instead, reduced full-length protein content (approximately 50% in heterozygous mutations) appears sufficient to cause disease .

How do MYBPC3 mutations alter cardiac contractility at the molecular level?

MYBPC3 mutations disrupt the normal equilibrium of myosin conformations:

• Increase the proportion of myosin heads in the active state that enables ATP hydrolysis and thin filament interactions.

• Reduce the proportion of myosin heads in the super-relaxed conformation associated with energy conservation .

These changes result in hypercontractility (increased force generation) and impaired relaxation (diastolic dysfunction), the hallmark functional abnormalities in HCM . The disruption of normal myosin states also increases energy consumption, contributing to energetic inefficiency in the myocardium.

What transcriptomic changes are associated with MYBPC3 mutations in cardiomyocytes?

Single-cell transcriptomic profiling of MYBPC3-associated HCM has revealed that:

• Differential gene expression patterns vary between species (mouse, feline, human) despite similar phenotypes.

• Core biological processes affected are conserved across species despite differences in specific genes.

• Mechanical and electrical forces experienced by cardiomyocytes likely differ across species due to scaling effects of heart size, contributing to species-specific transcriptomic responses .

This suggests that MYBPC3 mutations trigger adaptive responses that are context-dependent and influenced by heart size, wall tension, and electrical properties unique to each species .

What evidence supports myosin inhibition as a therapeutic strategy for MYBPC3-associated HCM?

Preclinical studies demonstrate that direct attenuation of myosin function can normalize the hypercontractility resulting from MYBPC3 mutations:

• MYK-461, a pharmacologic inhibitor of myosin ATPase, rescued relaxation deficits and restored normal contractility in both mouse and human cardiomyocytes with MYBPC3 mutations .

• A dilated cardiomyopathy-causing myosin missense variant (F764L) that directly attenuates myosin function normalized the increased contractility from cMyBPC depletion in experimental models .

These findings provide mechanistic support for myosin inhibition as a targeted therapeutic approach for MYBPC3-associated HCM .

How might variability in phenotypic expression among patients with identical MYBPC3 mutations be explained?

Founder populations with identical MYBPC3 mutations show similar variance in disease expression (mean SD of maximum wall thickness: 5.96±0.79 mm) compared to patients with non-founder mutations (SD: 5.98 mm) . This suggests that the marked phenotypic variability in MYBPC3-associated HCM is predominantly influenced by additional factors:

• Background genetic modifiers (polygenic influences)

• Epigenetic factors

• Environmental exposures and lifestyle factors

• Age-related penetrance patterns

This observation has important implications for risk stratification and personalized management approaches, as genetic testing alone may not predict disease severity.

What methodological considerations are important when comparing MYBPC3 research across different species?

When translating findings between species, researchers should consider:

• Scale-dependent factors: Heart size directly affects wall tension and myocardial oxygen consumption through the Law of Laplace .

• Heart rate differences: Heart rate inversely correlates with mammalian size and affects electrical conduction times .

• Mechanical and electrical forces: Individual cells experience different forces based on species-specific cardiac architecture .

• Experimental standardization: When comparing species, identical mechanical strain or electrical stimulation conditions should be applied to isolated cardiomyocytes .

These considerations explain why differentially expressed genes associated with MYBPC3 dysfunction vary across species, even while core biological processes remain conserved .