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Neurogenetics

Cerebellar ataxia is a common finding in mitochondrial encephalomyopathies, usually being part of more or less complex clinical pictures caused by the multisystem involvement due to impaired oxidative phosphorylation. The clinical manifestation of a T to G point mutation at nucleotide 8,993 in the ATPase 6 gene of the mitochondrial DNA (mtDNA) first described in maternal inherited NARP syndrome (neurogenic muscle weakness ataxia, and retinitis pigmentosa) depends on the tissue specific heteroplasmic variation of this mutation and may vary markedly ranging from LEIGH syndrome (>90%), to milder symptoms as ataxia and retinitis (60-90%), or even to no symptoms at all (<60%).
We describe an 11 years old girl with nonfamilial cerebellar ataxia, slight mental retardation and asymptomatic retinopathy harbouring the T8,993G point mutation. Detailed molecular analysis of mtDNA from different tissues (blood, muscle, fibroblasts) resulted in variable heteroplasmic proportions of the mutant population (55-85%), preferentially present in muscle tissue. These data correspond to pronounced cerebellar atrophy on MRI and to elevated lactate levels in cerebral spinal fluid and cerebral-cerebellar parenchyma determined by in vivo proton MR spectroscopy of the brain. In contrast, biochemical investigation of the respiratory chain in muscle was normal.
We conclude that mtDNA defects are important causes of cerebellar atactic syndromes especially in cases of multisystem involvement.



Headings
T8,993G mitochondrial DNA point mutation
ATPase 6
cerebral ataxia
mental retardation
retinitis pigmentosa

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