Different laboratory and muscle biopsy findings in a family with an m.8851T>C mutation in the mitochondrial MTATP6 gene.

We report the second known family with a very rare, maternally inherited missense m.8851T>C mutation in the mitochondrial MTATP6 gene. A failure to thrive, microcephaly, psychomotor retardation and hypotonia were present in a 3-year-old girl with a high mtDNA mutation load (87-97%). Ataxia and Leigh syndrome were subsequently documented in a neurological examination and brain MRI. A muscle biopsy demonstrated decreased ATP synthase and an accumulation of succinate dehydrogenase products, indicating mitochondrial myopathy. Her 36-year-old mother (68% blood heteroplasmy) developed peripheral neuropathy and muscle weakness at the age of 22 years. Our findings extend the clinical and laboratory phenotype associated with the m.8851T>C mutation.