Supplementary Material for: Novel double MYH7/MYBPC3 variants in a Chinese family of Hypertrophic Cardiomyopathy with Early-onset and Sudden Death

Background: Hypertrophic cardiomyopathy (HCM) is a common inherited heart condition. Traditional genetic testing is typically conducted on the proband only, with family members undergoing Sanger sequencing, which may overlook other pathogenic variants. This study explores the gene sequencing strategy in a three-generation family based on genetic carrier status and examines the relationship between phenotypic characteristics and genotype. Methods: High-throughput second-generation sequencing was performed on the proband to analyze HCM-related pathogenic genes. Subsequently, the identified pathogenic variants were validated by Sanger sequencing in the proband and family members. Clinical, electrocardiographic, and echocardiographic assessments were conducted for family members. Results: Second-generation sequencing of the proband (III7) revealed a pathogenic variant MYBPC3-P453Lfs. Initially, no HCM-related pathogenic variants were detected in another patient (III11), prompting additional sequencing of III11, which identified the MYH7-G823E pathogenic variant. Both patients had severe left ventricular outflow tract obstruction. Sanger sequencing showed that five family members carried both mutations. Among them, three died suddenly before age 40, one required an implantable cardioverter-defibrillator for arrhythmias, and one developed HCM before adulthood. Cardiac magnetic resonance imaging (MRI) of patients carrying both mutations showed myocardial fibrosis of 32.75%, significantly higher than the 6.98% observed in patients carrying only one mutation. Conclusion: In families with varying HCM phenotypes, second-generation sequencing should be considered for all members. In this family, carrying one variant led to outflow tract obstruction, while carrying both variants resulted in severe disease, including sudden death and early onset. Cardiac MRI is crucial for assessing the severity of the disease within the family.