Multiple mitochondrial DNA deletions are associated with clinically heterogeneous disorders transmitted as mendelian traits. Dominant missense mutations were found in the gene encoding the heart and skeletal muscle-specific isoform of the adenine nucleotide translocator (ANT1) in families with autosomal dominant progressive external opthalmoplegia and in a sporadic patient. We herein report on a sporadic patient who presented with hypertrophic cardiomyopathy, mild myopathy with exercise intolerance and lactic acidosis but no ophthalmoplegia. A muscle biopsy showed the presence of numerous ragged-red fibers, and Southern blot analysis disclosed multiple deletions of muscle mitochondrial DNA. Molecular analysis revealed a C to A homozygous mutation at nucleotide 368 of the ANT1 gene. The mutation converted a highly conserved alanine into an aspartic acid at codon 123 and was absent in 500 control individuals. This is the first report of a recessive mutation in the ANT1 gene. The clinical and biochemical features are different from those found in dominant ANT1 mutations, resembling those described in ANT1 knockout mice. No ATP uptake was measured in proteoliposomes reconstituted with protein extracts from the patient's muscle. The equivalent mutation in AAC2, the yeast ortholog of human ANT1, resulted in a complete loss of transport activity and in the inability to rescue the severe Oxidative Phosphorylation phenotype displayed by WB-12, an AAC1/AAC2 defective strain. Interestingly, exposure to reactive oxygen species (ROS) scavengers dramatically increased the viability of the WB-12 transformant, suggesting that increased redox stress is involved in the pathogenesis of the disease and that anti-ROS therapy may be beneficial to patients.

The human POLG gene encodes the catalytic subunit of mitochondrial DNA polymerase γ (pol γ). Mutations in pol γ are associated with a spectrum of disease phenotypes including autosomal dominant and recessive forms of progressive external ophthalmoplegia, spino-cerebellar ataxia and epilepsy, and Alpers–Huttenlocher hepatocerebral poliodystrophy. Multiple deletions, or depletion of mtDNA in affected tissues, are the molecular hallmarks of pol γ mutations. To shed light on the pathogenic mechanisms leading to these phenotypes, we have introduced in MIP1, the yeast homologue of POLG, two mutations equivalent to the human Y955C and G268A mutations, which are associated with dominant and recessive PEO, respectively. Both mutations induced the generation of petite colonies, carrying either rearranged (ρ⁻) or no (ρ⁰) mtDNA. Mutations in genes that control the mitochondrial supply of deoxynucleotides (dNTP) affect the mtDNA integrity in both humans and yeast. To test whether the manipulation of the dNTP pool can modify the effects of pol γ mutations in yeast, we have overexpressed a dNTP checkpoint enzyme, ribonucleotide reductase, RNR1, or deleted its inhibitor, SML1. In both mutant strains, the petite mutability was dramatically reduced. The same result was obtained by exposing the mutant strains to dihydrolipoic acid, an anti-oxidant agent. Therefore, an increase of the mitochondrial dNTP pool and/or a decrease of reactive oxygen species can prevent the mtDNA damage induced by pol γ mutations in yeast and, possibly, in humans.