A near homoplasmic T8993G mtDNA mutation in a patient with atypic Leigh syndrome not present in the mother's tissues.

We report an interesting case of a T8993G mutation in the muscle mtDNA of a 2-year-old girl who presented with myoclonia, developmental delay, increased lactate concentration in CSF and cerebral MRI abnormalities without retinopathy, suggesting an atypical form of Leigh syndrome. Search for mutant molecules in the blood and skin fibroblasts of the patient's healthy mother as well as in the blood of the also unaffected patient's sister was unsuccessful. This "sporadic' case of 8993 mtDNA mutation may be due either to a de novo mutation arising during oogenesis (or early embryogenesis) or to a mutation pre-existing in oocytes in a few mtDNA molecules and selected through a narrow bottleneck mechanism. Whatever the mechanism involved, this observation illustrates a complete shift of the mtDNA type in only one generation: from 0 to nearly 100% of mtDNA molecules with the T8993G mutation.