Hypertrophic cardiomyopathy in identical twins: a case report with mismatch between genotype and phenotype.

Background: Hypertrophic cardiomyopathy (HCM) is the most common genetic cardiomyopathy; however, it is a very rare condition among twins. A mismatch between the phenotype and clinical course has been identified, even in the presence of apparently identical genotypes. Case summary: A pair of male monozygotic-diamniotic twins of 17 years old with a mismatch in HCM clinical expression is informed. Both twins have the same pathogenic variant [c.1816G>A (p.Val606Met)] located in MYH7, a gene that encodes an essential component in the contraction of cardiac muscle. The twins presented obstructive basal anteroseptal vs. non-obstructive medium apical HCM and different clinical courses (non-fatal sudden cardiac death vs. asymptomatic). It is hypothesized that twin-to-twin transfusion syndrome could be implied in the different phenotypes due to prenatal history, but other genetic and epigenetic factors can be implied. Discussion: Despite carrying the same pathogenic variant, the phenotypical presentation and clinical course vary widely, despite a similar risk profile, suggesting the influence of genetic/epigenetic and environmental factors on disease progression.