A TRANSFER-RNA SUPPRESSOR MUTATION IN HUMAN MITOCHONDRIA

Mitochondrial mutations are associated with a wide spectrum of human d iseases(1,2). A common class of point mutations affects tRNA genes, an d mutations in the tRNA-leu(UUR) gene (MTTL1) are the most frequently detected. In earlier studies, we showed that lung carcinoma cybrid cel ls containing high levels (greater than 95%) of mutated mtDNA from a p atient with the pathological nucleotide pair (np) 3243 tRNA-leu(UUR) m utation can remain genotypically stable over time, and exhibit severe defects in mitochondrial respiratory metabolism(3,4). From such a cybr id containing 99% mutated mtDNA, we have isolated a spontaneous deriva tive that retains mutant mtDNA at this level but which has nevertheles s reverted to the wild-type phenotype, based on studies of respiration , growth in selective media, mitochondrial protein synthesis and bioge nesis of mitochondrial membrane complexes. The cells are heteroplasmic for a novel anticodon mutation in tRNA-leu(CUN) at np 12300, predicte d to generate a suppressor tRNA capable of decoding UUR leucine codons . The suppressor mutation represents approximately 10% of the total mt DNA, but was undetectable in a muscle biopsy sample taken from the ori ginal patient or in the parental cybrid. These results indicate that t he primary biochemical defect in cells with high levels of np 3243 mut ated mtDNA is the inability to translate UUR leucine codons.