Bioenergetics of mitochondrial diseases associated with mtDNA mutations

This mini-review summarizes our present view of the biochemical alterations associated with mitochondrial DNA (mtDNA) point mutations. Mitochondrial cytopathies caused by mutations of mtDNA are well-known genetic and clinical entities, but the biochemical pathogenic mechanisms are often obscure. Leber's hereditary optic neuropathy (LHON) is due to three main mutations in genes for complex I subunits. Even if the catalytic activity of complex I is maintained except in cells carrying the 3460/ND1 mutation, in all cases there is a change in sensitivity to complex I inhibitors and an impairment of mitochondrial respiration, eliciting the possibility of generation of reactive oxygen species (ROS) by the complex. Neurogenic muscle weakness, Ataxia and Retinitis Pigmentosa (NARP), is due to a mutation in the ATPase-6 gene. In NARP patients ATP synthesis is strongly depressed to an extent proportional to the mutation load; nevertheless, ATP hydrolysis and ATP-driven proton translocation are not affected. It is suggested that the NARP mutation affects the ability of the enzyme to couple proton transport to ATP synthesis. A point mutation in subunit III of cytochrome c oxidase is accompanied by a syndrome resembling MELAS: however, no major biochemical defect is found, if we except an enhanced production of ROS. The mechanism of such enhancement is at present unknown. In this review, we draw attention to a few examples in which the overproduction of ROS might represent a common step in the induction of clinical phenotypes and/or in the progression of several human pathologies associated with mtDNA point mutations.