REGIONAL HETEROGENEITY OF MTDNA HETEROPLASMY IN PARKINSONIAN BRAIN

There is increasing evidence for a role of defects of mitochondrial DN A in the etiology of neurodegenerative disorders such as Parkinson's a nd Alzheimer's disease as well as in normal aging. In several studies a biochemical defect of complex I of the respiratory chain (NADH dehyd rogenase, EC 1.6.5.3) has been found in the substantia nigra of Parkin sonian brains. Thus, mutations of mitochondrial genes encoding subunit s of complex I could contribute to the pathogenesis of Parkinson's dis ease. A heteroplasmic G5460A mutation affecting the ND2 subunit of NAD H dehydrogenase was detected in several brains of patients with idiopa thic Parkinson's disease. Since this mutation is heteroplasmic we were interested in the distribution of mutated and wild-type mitochondrial DNA in different brain areas. Relative levels of mutated DNA were qua ntified in a large number of anatomical regions using DNA extracted fr om formalin-fixed and paraffin-embedded brain tissue. DNA was amplifie d by the polymerase chain reaction and digested employing the restrict ion enzyme HphI. The proportion of mutated DNA was determined by laser densitometry. In addition, genotype-phenotype analyses were performed on sections of the substantia nigra with the aid of an automated imag e analysis system. Ratios of mutant to wild-type DNA varied between 44 % and 98%. However, there was no systematic relationship between mutat ed DNA ratios and ontogenetically related brain areas suggesting that the observed regional heterogeneity of mitochondrial DNA heteroplasmy is most likely due to random segregation during development. Therefore , tissue-specific differences in the sensitivity to pathogenic effects of the ND2(5460) mutation or the influence of additional susceptibili ty genes may be envisioned.