Mesencephalic complex I deficiency does not correlate with parkinsonism in mitochondrial DNA maintenance disorders.

Genetic evidence from recessively inherited Parkinson's disease has indicated a clear causative role for mitochondrial dysfunction in Parkinson's disease. This role has long been discussed based on findings that toxic inhibition of mitochondrial respiratory complex I caused parkinsonism and that tissues of patients with Parkinson's disease show complex I deficiency. Disorders of mitochondrial DNA maintenance are a common cause of inherited neurodegenerative disorders, and lead to mitochondrial DNA deletions or depletion and respiratory chain defect, including complex I deficiency. However, parkinsonism associates typically with defects of catalytic domain of mitochondrial DNA polymerase gamma. Surprisingly, however, not all mutations affecting DNA polymerase gamma manifest as parkinsonism, but, for example, spacer region mutations lead to spinocerebellar ataxia and/or severe epilepsy. Furthermore, defective Twinkle helicase, a close functional companion of DNA polymerase gamma in mitochondrial DNA replication, results in infantile-onset spinocerebellar ataxia, epilepsy or adult-onset mitochondrial myopathy, but not typically parkinsonism. Here we sought for clues for this specificity in the neurological manifestations of mitochondrial DNA maintenance disorders by studying mesencephalic neuropathology of patients with DNA polymerase gamma or Twinkle defects, with or without parkinsonism. We show here that all patients with mitochondrial DNA maintenance disorders had neuronopathy in substantia nigra, most severe in DNA polymerase gamma-associated parkinsonism. The oculomotor nucleus was also affected, but less severely. In substantia nigra, all patients had a considerable decrease of respiratory chain complex I, but other respiratory chain enzymes were not affected. Complex I deficiency did not correlate with parkinsonism, age, affected gene or inheritance. We conclude that the cell number in substantia nigra correlated well with parkinsonism in DNA polymerase gamma and Twinkle defects. However, complex I defect is a general consequence of mitochondrial DNA maintenance defects, and does not explain manifestation of parkinsonism or degree of mesencephalic cell death in patients with mitochondrial DNA maintenance disorders.

Functional analysis of H. sapiens DNA polymerase gamma spacer mutation W748S with and without common variant E1143G.

Mitochondrial DNA polymerase, POLG, is the sole DNA polymerase found in animal mitochondria. In humans, POLGalpha W748S in cis with an E1143G mutation has been linked to a new type of recessive ataxia, MIRAS, which is the most common inherited ataxia in Finland. We investigated the biochemical phenotypes of the W748S amino acid change, using recombinant human POLG. We measured processive and non-processive DNA polymerase activity, DNA binding affinity, enzyme processivity, and subunit interaction with recombinant POLGbeta. In addition, we studied the effects of the W748S and E1143G mutations in primary human cell cultures using retroviral transduction. Here, we examined cell viability, mitochondrial DNA copy number, and products of mitochondrial translation. Our results indicate that the W748S mutant POLGalpha does not exhibit a clear biochemical phenotype, making it indistinguishable from wild type POLGalpha and as such, fail to replicate previously published results. Furthermore, results from the cell models were concurrent with the findings from patients, and support our biochemical findings.

Mitochondrial recessive ataxia syndrome mimicking dominant spinocerebellar ataxia.

We studied the genetic background of a family with SCA, showing dominant inheritance and anticipation. Muscle histology, POLG1 gene sequence, neuropathology and mitochondrial DNA analyses in a mother and a son showed typical findings for a mitochondrial disorder, and both were shown to be homozygous for a recessive POLG1 mutation, underlying mitochondrial recessive ataxia syndrome, MIRAS. The healthy father was a heterozygous carrier for the same mutation. Recessively inherited MIRAS mutations should be tested in dominantly inherited SCAs cases of unknown cause, as the high carrier frequency of MIRAS may result in two independent introductions of the mutant allele in the family and thereby mimic dominant inheritance.