Abstract 14479: Compound Heterozygosity With Two Novel Mutations in MYBPC3 as a Cause of Severe and Early-Onset Hypertrophic Cardiomyopathy

Case presentation: A 8-year-old male was diagnosed with severe hypertrophic cardiomyopathy (HCM) after a cardiac arrest from which he was successfully defibrillated. Maximal wall thickness was 23 mm (z-score 6.36) on echocardiography. An implantable cardioverter defibrillator (ICD) was implanted and atenolol was started. After 4 ventricular fibrillation-terminating ICD shocks in the following 2 months, further treatment is under consideration. A genetic test identified the likely pathogenic intronic variant (c.1927+337G>T) in the MYBPC3 gene. An additional variant in the same gene (p.Arg817Gly), of uncertain significance but potentially relevant for the phenotype, was also found. His 10-year-old sister was also diagnosed with severe HCM, genetic test identified the two previously referred variants and ICD was also implanted. In the 4-year-old one sister, who is apparently healthy, only the likely pathogenic intronic variant (c.1927+337G>T) was found. Regarding their parents, both with no evidence of cardiomyopathy, c.1927+337G>T variant was identified in the father, while p.Arg817Gly was present in the mother.

Discussion: This clinical case is noteworthy as an example of compound heterozygosity in which each parent donates one alternate allele, being these alleles located at different loci within the same gen, which emphasizes the importance of family genetic testing. The variant of uncertain significance p.Arg817Gly appears to be a disease modifier, causing the phenotype to be expressed early and severely in the presence of the probably pathogenic intronic variant c.1927+337G>T in MYBPC3. This deep intronic variant has not been described previously but was found in thirteen index cases referred for HCM and a history of familial sudden death at an early age, being overrepresented in the cohort of HCM in the north of Spain. Being a carrier of both these two novel variants appears to cause an extremely aggressive childhood-onset phenotype.