Extreme variability of clinical symptoms among sibs in a MELAS family correlated with heteroplasmy for the mitochondrial A3243G mutation

In a family with mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes with extremely varying clinical expression, we have identified the A3243G heteroplasmic point mutation in mitochondrial DNA. The degree of severity of the clinical symptoms in the various family members was reflected in the relative quantity of mutated mitochondrial DNA in different tissues. The biochemical activity of complex I of the respiratory chain in muscle was decreased in some members of this family.