A recent paper by researchers in Australia, published this week in Circulation: Cardiovascular Genetics, found more than one rare HCM gene in 4% of patients in a 758 patient sample group.
The researchers found that those patients with multiple rare HCM genes tended to present with HCM at a younger age, were more likely to experience cardiac arrest or death from other causes, and were more likely to require a heart transplant.
In general, few patients have multiples of mutations commonly associated with HCM. See this Canadian study from April of this year which found that multiple mutations were less prevalent and harmful than previously thought.
HCM Beat 