YEAST AS A TOOL FOR IN VIVO STUDY OF HUMAN MITOCHONDRIAL TRNA MUTATIONS

The mitochondrial genome (mt DNA) is a small circular chromosome localized in mitochondria, the energy producing cell organelles, and contains the structural genes necessary for the synthesis of the enzyme complexes which perform the oxidative phosphorylation (OXPHOS), the most important enegy producing pathway in the cell. The mitochondrial genome also contains genes encoding RNA molecules (ribosomal RNA, rRNA and transfer RNA,tRNA) necessary to translate its structural genes into proteins. The diseases caused by mtDNA mutations constitute nowadays a very important group of hereditary diseases, often characterized by progressive degeneration of the nervous system and of the skeletal muscle system caused by defective energy production. A significant proportion of these pathologies is caused by mutations in mt tRNAs. However mt DNA often also harbours base substitutions devoid of pathological significance which are not easily distinguished from mutations causing severe diseases. This distinction is very important to predict the effects of a certain base substitution. To evaluate the pathogenicity of mt tRNA mutations we propose to use yeast, a microorganism in which the study of the OXPHOS functionality is very easy. We have introduced into yeast mtDNA some base substitutions equivalent to those which cause mitochondrial diseases, and we have observed that these mutations produce a severe OXPHOS defect in yeast, while non pathogenetic mutations have no relevant effects. Moreover we have observed that increased expression of some nuclear genes can rescue the respiratory defect. This latter observation can open interesting perspectives of gene therapy of mitochondrial diseases , for which up to now no effective treatment exists.