BackgroundWe have previously described two distinct mutations in the β-myosin heavy chain gene with markedly different clinical presentations and outcome: The 908Ieu→valmutation was associated with a low disease penetrance and a benign prognosis. In contrast, the 403Arg→Glnmutation in a Caucasian kindred was associated with a 100% disease penetrance and high incidence of sudden cardiac death. Recently, another mutation, 606val→Met, has been reported to be associated with “near normal survival” and offered as evidence for the benign nature of neutral charge substitutions.Methods and ResultsWe report (1) a large kindred (245 family members at risk of inheriting the disease gene) with a 256Gly→Glumutation characterized by a similar disease penetrance in adults and in children (56% and 60%, respectively) and a cumulative sudden cardiac death rate of only 2% at 50 years of age, (2) a kindred with the 606Val→Metmutation with four sudden cardiac deaths in eight affected individuals, and (3) a Korean kindred with the 403Arg→Ginmutation. Although the disease occurred early and was associated with a high prevalence of myocardial ischemia in both of our kindreds with the 403Arg→Ginmutation, no sudden cardiac death or syncope has occurred in the Korean kindred. Furthermore, in the Caucasian kindred, all patients had nonobstructive hypertrophic cardiomyopathy, but most of the patients in the Korean kindred had left ventricular outflow obstruction.ConclusionsThe conclusions are as follows: (1) Although several sudden cardiac deaths are sufflicient to establish that a mutation is malignant, study of a large kindred is necessary to be certain that a mutation is benign. To date, only the 908Leu→Valand the 256Gly→Glumutations satisfy this requirement. (2) The 256Gly→Glumutation demonstrates that not all mutations that result in a charge change are malignant. (3) Conversely, the 606Val→Metmutation is malignant in some kindreds; hence, despite the absence of a charge change, minor substitutions in critical regions of 3-myosin heavy chain protein may also have serious consequences. (4) The diverse ethnic origins of the two 403Arg→Ginkindreds provide evidence suggesting that the identical mutation occurred independently and was associated with different genetic backgrounds. Their distinct phenotypes underline the importance of modifying genes and nongenetic factors.