Random mitotic segregation of mitochondrial DNA in MELAS syndrome.

We describe the heterogeneity of clinical features and molecular genetic characteristics of the probands and other members in two families with mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes (MELAS) syndrome. A point mutation at the 3243rd nucleotide position of mtDNA was found only in some of the maternal lineage members of the two families. Furthermore, the proportions of mutant mtDNA were varied and found only in some tissues of the individuals. Intriguingly, in some subjects, the mutant mtDNA was found in blood cells or hair follicles but was absent in muscles. The data do not support the notion of a selective advantage of wild-type mtDNA to rapidly replicating cells. We suggest that a rapid replicative segregation may occur in early embryogenesis.