A recent study of identical twins with HCM found significant variation in the expression of hypertrophic cardiomyopathy, even between twins carrying the same HCM-causing genetic mutation. 

This paper concludes that epigenetics (things which influence expression of DNA like behavioral and environmental factors) greatly influence the expression of HCM genetic mutations. 

The researchers studied 11 pairs of twins with HCM. 9 of the twin pairs had known HCM sarcomere mutations while 2 of the pairs had HCM of unknown cause. The siblings were followed for a time frame of between 5 to 14 years.

Researchers compared left ventricular wall thickness, left atrial size and left ventricular ejection fraction. Differences were found in the left ventricular wall thickness of all 11 pairs of twins, while left atrial size was similar in 3 of the 9 twin pairs who carried HCM mutations. Left ventricular ejection fraction was different in 4 of 7 twin pairs. 

The researchers theorize that similarities in left atrial size may be due to impaired ventricular relaxation directly tied to sarcomere dysfunction. In contrast, environmental factors yield more influence over ventricular function.

HCM epigenetics is a field ripe for research. As HCM patients, we hope that it will yield actionable data in the near future.