POINT MUTATIONS IN THE MITOCHONDRIAL TRNA(LYS) GENE - IMPLICATIONS FOR PATHOGENESIS AND MECHANISM

MERRF (myoclonic epilepsy with ragged-red fibers) is a severe, multisy stem disorder characterized by myoclonus, seizures, progressive cerebe llar syndrome, muscle weakness, and the presence of ragged-red fibers in the muscle biopsy. MERRF is associated with heteroplasmic point mut ations, either A8344G or T8356C, in the gene encoding the mitochondria l tRNA(Lys). The human rho degrees cell system was utilized to examine the phenotypic consequences of these mutations, and to investigate th eir molecular genetic causes. Wild-type and mutant transmitochondrial cell lines harboring a pathogenic point mutation at either A8344G or T 8356C in the human mitochondrial tRNA(Lys) gene were isolated and exam ined. Mitochondrial transformants containing 100% mutated mitochondria l DNAs (mtDNAs) exhibited severe defects in respiratory chain activity , in the rates of protein synthesis, and in the steady-state levels of mitochondrial translation products as compared with mitochondrial tra nsformants containing 100% wild-type mtDNAs. In addition, both mutant cell lines exhibited the presence of aberrant mi tochondrial translati on products. These results demonstrate that two different mtDNA point mutations in tRNA(Lys) result in fundamentally identical defects at th e cellular level, and that these specific protein synthesis abnormalit ies contribute to the pathogenesis of MERRE.