ALTERED PROPERTIES OF MITOCHONDRIAL ATP-SYNTHASE IN PATIENTS WITH A T-]G MUTATION IN THE ATPASE 6(SUBUNIT A) GENE AT POSITION-8993 OF MTDNA

A family is described with a T-->G mutation at position 8993 of mtDNA. This mutation is located in the ATPase 6 gene of mtDNA which encodes subunit a of the ATP-synthase complex (FlFo-ATPase). Clinically, the p atients showed severe infantile lactate acidosis and encephalomyopathy in a form that was different from the classical Leigh syndrome. In 3 affected boys, ranging in age from 3 months to 8 years, the mutation w as found in 95-99% of the mtDNA population. The clinical symptoms corr elated with the mtDNA heteroplasmy and in the healthy mother 50% of th e mtDNA was mutated. The rate of mitochondrial ATP production by cultu red skin fibroblasts containing 99% of mutated mtDNA was about 2-fold lower than that in normal fibroblasts. Native electrophoresis of the m itochondrial enzyme complexes revealed instability of the FlFo-ATPase in all the tissues of the patient that were investigated (heart, muscl e, kidney, liver). Only a small portion of the ATP-synthase complex wa s present in the complete, intact form (620 kDa). Incomplete forms of the enzyme were present as subcomplexes with approx. molecular weights of 460, 390 and 150 kDa, respectively, which differed in the content of F-l and F-o subunits. Immunochemical analysis of the subunits of th e FlFo-ATPase further revealed a markedly decreased content of the F-o subunit b in mitochondria from muscle and heart, and an increased con tent of the F-o subunit c in muscle mitochondria, respectively. These results indicate that in this family the T-->G point mutation at posit ion 8993 in the mitochondrial ATPase 6 gene is accompanied by structur al instability and altered assembly of the enzyme complex, that are bo th most likely due to changes in the properties of subunit a of the me mbrane sector part of the ATP-synthase.