Epidemiology and genetics of hypertrophic cardiomyopathy

Background: Hypertrophic cardiomyopathy (HCM) is a heart muscle disorder and is known to be inherited as an autosomal dominant trait. Mutations in several sarcomeric, cytoskeletal and mitochondrial genes have been reported in HCM. Though many cases of HCM are being identified, there is limited data regarding the epidemiology and genetics of HCM in India.
Aim: Therefore the present study is envisaged at identifying the epidemiological variables in HCM and fitting a probability model assuming dominant mode of inheritance in HCM, which may in turn shed light on the heterogeneity of this complex disorder.
Materials AND Methods: The 127 HCM cases were divided into subtypes based on pattern of hypertrophy. Chi square analysis, odds ratio, probability, relative frequency, penetrance and heritability estimates were calculated apart from epidemiological variables.
Results: The HCM subtypes revealed the heterogeneous nature of the condition suggesting that the genes/mutations involved in their pathogenesis are different and this is supported by distinctive differences observed in their probability, heritability and penetrance estimates apart from epidemiological variables. An increased male preponderance was observed with the sex ratio being 3.7:1. The age at onset was found to be more than a decade early in familial cases (30 ± 10 yrs) compared to non familial cases (44 ± 14 yrs). Chi square analysis revealed obstructive HCM to be following autosomal dominant mode of inheritance where as non-obstructive HCM was significantly deviating. The level of deviation was significantly high for the middle onset group compared to early and late onset groups, therefore this group may be considered as an admixture wherein genes/gene modifiers and environmental variables may be contributing to the heterogeneity and this is further supported by odds ratio.
Conclusions: The study thus brings out the complexity of HCM and suggests that modes of inheritance other than autosomal dominant may be encountered in a subset of HCM especially in asymmetric septal hypertrophy, apical, concentric and mid cavity obstruction subsets and hence a mixed model of inheritance is the best fit for such complex disorders.

Keywords
Hypertrophic cardiomyopathy, inheritance, penetrance, heritability, relative estimates.