Suppression of a mitochondrial tRNA gene mutation phenotype associated with changes in the nuclear background

We previously have characterized a pathogenic mtDNA mutation in the tRNAAsn gene. This mutation (G5703A) was associated with a severe mitochondrial protein synthesis defect and a reduction in steady-state levels of tRNAAsn. We now show that, although transmitochondrial cybrids harboring homoplasmic levels of the mutation do not survive in galactose medium, several galactose-resistant clones could be obtained. These cell lines had restored oxidative phosphorylation function and 2-fold higher steady-state levels of tRNAAsn when compared with the parental mutant cell line. The revertant lines contained apparently homoplasmic levels of the mutation and no other detectable alteration in the tRNAAsn gene. To investigate the origin of the suppression, we transferred mtDNA from the revertants (143B/206 TK-) to a different nuclear background (143B/207 TK-, 8AGr). These new transmitochondrial cybrids became defective once again in oxidative phosphorylation and regained galactose sensitivity. However, galactose-resistant clones could also be obtained by growing the 8AGr transmitochondrial cybrids under selection. Because the original rate of reversion was higher than that expected by a classic second site nuclear mutation, and because of the aneuploid features of these cell lines, we searched for the presence of chromosomal alterations that could be associated with the revertant phenotype. These studies, however, did not reveal any gross changes. Our results suggest that modulation of the dosage or expression of unknown nuclear-coded factor(s) can compensate for a pathogenic mitochondrial tRNA gene mutation, suggesting new strategies for therapeutic intervention.