High mitochondrial DNA T8993G mutation (<90%) without typical features of Leigh's and NARP syndromes.

Neuropathy, ataxia, and retinitis pigmentosa (NARP) syndrome and maternally inherited Leigh's syndrome have been associated with T8993G point mutations in the mitochondrial adenosine triphosphatase 6 gene. Typically, NARP syndrome is characterized by developmental delay, seizures, dementia, retinitis pigmentosa, ataxia, sensory neuropathy, and proximal weakness. Usually, there is a correlation between the percentage of mutated mitochondrial DNA and clinical severity, and when mutated mitochondrial DNA is > 90%, it is often seen with Leigh's syndrome. We now report a family with mitochondrial DNA T8993G mutation in eight living members, five with mutant mitochondrial DNA >90% and one with 20% mutant mitochondrial DNA. However, their clinical features include variable combinations of seizures, behavior problems, learning disability, mental retardation, sensorineural deafness, cerebellar ataxia, and proximal muscle weakness. No retinitis pigmentosa was found in all eight living members, including a 56-year-old grandmother. Only one dead female relative was diagnosed with Leigh's syndrome on the neuropathologic examination at age 22 years, when she died of an accident. High mitochondrial DNA T8993G mutation is not always associated with typical features of Leigh's and NARP syndromes.