Selection of a mtDNA sequence variant in hepatocytes of heteroplasmic miceis not due to differences in respiratory chain function or efficiency of replication

We have previously constructed lines of heteroplasmic mice from two inbred strains (NZB/BinJ and BALB/c) to investigate the mechanisms of segregation of mtDNA sequence variants. Analysis of the segregation behaviour of mtDNA in several tissues showed that the NZB genotype was invariably selected in liver/kidney and the BALB genotype in blood/spleen. Segregation was not sig nificant in post-mitotic tissues. Here we have investigated this novel patt ern of mtDNA segregation in isolated hepatocytes to determine the mechanism of selection. Polarographic measurements of respiratory chain function sho wed no difference between mitochondria containing either 0 or 91-97% NZB mt DNAs on a BALB nuclear background. Single-cell PCR analysis of mtDNA in iso lated hepatocytes demonstrated that most hepatocytes eventually fix the NZB genotype. The rate of selection was constant with time and independent of the initial genotype frequency. Based on a mtDNA replication rate of 9.4 da ys, NZB mtDNA has an similar to 14% selective advantage over BALB mtDNA; ho wever, in vivo pulse labelling with BrdU demonstrated that this was not bas ed on efficiency of replication. Surprisingly, when hepatocytes were cultur ed in vitro, the majority of independent colonies selected BALB mtDNA, even if they were nearly fixed for the NZB mtDNA genotype when initially plated . These data suggest that selection for NZB mtDNA in the liver of these mic e is not based on respiratory chain function at the cellular or organellar level, or a simple replicative advantage, but on a factor(s) involved with mtDNA maintenance.