Association of Cardiomyopathy With MYBPC3 D389V and MYBPC3Δ25bpIntronic Deletion in South Asian Descendants.

Importance: The genetic variant MYBPC3Δ25bp occurs in 4% of South Asian descendants, with an estimated 100 million carriers worldwide. MYBPC3 Δ25bp has been linked to cardiomyopathy and heart failure. However, the high prevalence of MYBPC3Δ25bp suggests that other stressors act in concert with MYBPC3Δ25bp. Objective: To determine whether there are additional genetic factors that contribute to the cardiomyopathic expression of MYBPC3Δ25bp. Design, Setting, andParticipants: South Asian individuals living in the United States were screened for MYBPC3Δ25bp, and a subgroup was clinically evaluated using electrocardiograms and echocardiograms at Loyola University, Chicago, Illinois, between January 2015 and July 2016. Main Outcomes and Measures: Next-generation sequencing of 174 cardiovascular disease genes was applied to identify additional modifying gene mutations and correlate genotype-phenotype parameters. Cardiomyocytes derived from human-induced pluripotent stem cells were established and examined to assess the role of MYBPC3Δ25bp. Results: In this genotype-phenotype study, individuals of South Asian descent living in the United States from both sexes (36.23% female) with a mean population age of 48.92 years (range, 18-84 years) were recruited. Genetic screening of 2401 US South Asian individuals found an MYBPC3Δ25bpcarrier frequency of 6%. A higher frequency of missense TTN variation was found in MYBPC3Δ25bp carriers compared with noncarriers, identifying distinct genetic backgrounds within the MYBPC3Δ25bp carrier group. Strikingly, 9.6% of MYBPC3Δ25bp carriers also had a novel MYBPC3 variant, D389V. Family studies documented D389V was in tandem on the same allele as MYBPC3Δ25bp, and D389V was only seen in the presence of MYBPC3Δ25bp. In contrast to MYBPC3Δ25bp, MYBPC3Δ25bp/D389V was associated with hyperdynamic left ventricular performance (mean [SEM] left ventricular ejection fraction, 66.7 [0.7%]; left ventricular fractional shortening, 36.6 [0.6%]; P < .03) and stem cell-derived cardiomyocytes exhibited cellular hypertrophy with abnormal Ca2+ transients. Conclusions and Relevance: MYBPC3Δ25bp/D389V is associated with hyperdynamic features, which are an early finding in hypertrophic cardiomyopathy and thought to reflect an unfavorable energetic state. These findings support that a subset of MYBPC3Δ25bp carriers, those with D389V, account for the increased risk attributed to MYBPC3Δ25bp.

Hypertrophic cardiomyopathy clinical phenotype is independent of gene mutation and mutation dosage.

Over 1,500 gene mutations are known to cause hypertrophic cardiomyopathy (HCM). Previous studies suggest that cardiac ß-myosin heavy chain (MYH7) gene mutations are commonly associated with a more severe phenotype, compared to cardiac myosin binding protein-C (MYBPC3) gene mutations with milder phenotype, incomplete penetrance and later age of onset. Compound mutations can worsen the phenotype. This study aimed to validate these comparative differences in a large cohort of individuals and families with HCM. We performed genome-phenome correlation among 80 symptomatic HCM patients, 35 asymptomatic carriers and 35 non-carriers, using an 18-gene clinical diagnostic HCM panel. A total of 125 mutations were identified in 14 genes. MYBPC3 and MYH7 mutations contributed to 50.0% and 24.4% of the HCM patients, respectively, suggesting that MYBPC3 mutations were the most frequent cause of HCM in our cohort. Double mutations were found in only nine HCM patients (7.8%) who were phenotypically indistinguishable from single-mutation carriers. Comparisons of clinical parameters of MYBPC3 and MYH7 mutants were not statistically significant, but asymptomatic carriers had high left ventricular ejection fraction and diastolic dysfunction when compared to non-carriers. The presence of double mutations increases the risk for symptomatic HCM with no change in severity, as determined in this study subset. The pathologic effects of MYBPC3 and MYH7 were found to be independent of gene mutation location. Furthermore, HCM pathology is independent of protein domain disruption in both MYBPC3 and MYH7. These data provide evidence that MYBPC3 mutations constitute the preeminent cause of HCM and that they are phenotypically indistinguishable from HCM caused by MYH7 mutations.

High-Throughput Diagnostic Assay for a Highly Prevalent Cardiomyopathy-Associated MYBPC3 Variant.

A 25-basepair deletion variant of MYBPC3 occurs at high frequency in individuals of South Asian descent and is estimated to affect 55 million people worldwide, carrying an increased likelihood of cardiomyopathy. Since this variant is prevalent and severe in this subpopulation, quick and affordable screening to provide risk-assessment to guide treatment for these patients is critical. An RNaseH qPCR assay was developed to quickly and specifically diagnose the presence of the 25-basepair deletion variant in MYBPC3. RNAseH-blocked nucleotide primers were designed to identify the presence or absence of the wild type MYBPC3 allele or the genomic sequence containing the 25-basepair deletion. Using this assay, three blinded operators were able to accurately determine the genotype from human genomic DNA samples from blood and saliva using a qPCR thermocycler. Furthermore, positive variant subjects were examined by both electrocardiography and echocardiography for the presence of cardiomyopathy. A simple, robust assay was established, verified and validated that can be automated to detect the presence of the highly prevalent 25-basepair deletion MYBPC3 variant using both blood and saliva samples. The assay will provide quick and accurate prescreening of individuals at high risk for cardiomyopathies and allow for better clinical identification of 25-basepair deletion MYBPC3 carriers in large cohort epidemiological studies.