Segregation of mitochondrial DNAs carrying a pathogenic point mutation (tRNA(leu3243)) in cybrid cells

To investigate the mechanism of segregation of mitochondrial DNAs (mtDNAs) carrying a pathogenic point mutation in the tRNA(leu(UUR)) gene (A to G at position 3243) cytoplasts, derived from a heteroplasmic myoblast clone, were fused to rhoo cells to create cybrid cells carrying different proportions of mutant and wild-type mtDNAs. Although the individual myoblasts used as fusion partners contained predominantly mutant mtDNAs (mean proportion 0.77, range 0.46-0.94), the majority (56%) of the cybrid clones isolated after growth in selective medium were homoplasmic wild-type, indicating preferential replication of wild-type mtDNAs. Long-term culture of heteroplasmic cybrid clones in non-selective medium produced no change in the mean proportion of wild-type mtDNAs but an increase in population variance, consistent with a purely stochastic segregation mechanism. These results are at variance with a previous report suggesting a replicative advantage for mtDNAs carrying the tRNA(leu3243) point mutation.