Variation in germline mtDNA heteroplasmy is determined prenatally but modified during subsequent transmission

Abstract

A genetic bottleneck explains the marked changes in mitochondrial DNA (mtDNA) heteroplasmy observed during the transmission of pathogenic mutations, but the precise timing remains controversial, and it is not clear whether selection plays a role. These issues are critically important for the genetic counseling of prospective mothers, and developing treatments aimed at disease prevention. By studying mice transmitting a heteroplasmic single base-pair deletion in the mitochondrial tRNAMet gene, we show that mammalian mtDNA heteroplasmy levels are principally determined prenatally within the developing female germ line. Although we saw no evidence of mtDNA selection prenatally, skewed heteroplasmy levels were observed in the offspring of the next generation, consistent with purifying selection. High percentage levels of the tRNAMet mutation were linked to a compensatory increase in overall mitochondrial RNAs, ameliorating the biochemical phenotype, and explaining why fecundity is not compromised.

Tissue- and cell-type–specific manifestations of heteroplasmic mtDNA 3243A>G mutation in human induced pluripotent stem cell-derived disease model

“…This ES cell shift was suggested to be the result of strong mtDNA bottleneck occurring upon epiblast specification in the few cells directed to somatic lineages. After ES cell stage, mtDNA heteroplasmy levels did not vary considerably between different ES cell clones or in different solid tissues in primates, whereas in oocytes mtDNA segregation appeared to be random in mice and monkeys (25,46). As previously suggested that reprogramming takes iPSC mitochondria back to an epiblast-like stage (31, 49), our results further suggest that also the mtDNA bottleneck is reproduced in vitro during reprogramming.…”

“…This phenomenon could be explained by the existence of mitochondrial genetic bottleneck (42,43), involving decreased mtDNA copy number, selective amplification of a subset of mtDNA molecules, or preferential amplification of an mtDNA type. However, the exact stage in embryogenesis when this bottleneck occurs has been under debate (25,34,47,48). The biphasic shift in our iPSCs mimics closely the results seen in primate heteroplasmic ES cells, which had shifted toward homoplasmy at early passage (46).…”

“…Rapid shifts in mitochondrial heteroplasmy have been observed to occur in germ line (i.e., stem cells) in cows (42,43), in human patients (44,45), in mice with heteroplasmic neutral or potentially pathogenic mtDNA variants (18,25,34), and in primates (46). This phenomenon could be explained by the existence of mitochondrial genetic bottleneck (42,43), involving decreased mtDNA copy number, selective amplification of a subset of mtDNA molecules, or preferential amplification of an mtDNA type.…”

“…Heteroplasmic mice carrying two normal mtDNA variants (18) or randomly occurring mutations isolated from somatic tissues (19)(20)(21)(22) have been generated by cytoplast fusion to zygotes. Further, random mtDNA mutagenesis has been induced by manipulating nuclear-encoded mtDNA maintenance genes (23)(24)(25)(26). However, transgenic mice, carrying human disease-associated mt-tRNA mutations, do not exist.…”

Segregation of Naturally Occurring Mitochondrial DNA Variants in a Mini-Pig Model

“…The mtDNA mutator mouse demonstrated the differential purification between synonymous and nonsynonymous mtDNA variants within the mouse germline (Stewart et al 2008). Moreover, the extent of heteroplasmy was primarily determined in the oocyte population with the level of mtDNA variant modified in the next generation (Freyer et al 2012). Oocyte metabolism could cope with slightly inefficient OXPHOS and, because of the mitochondrial genetic bottleneck in the germline (Cree et al 2008), different levels of mtDNA variants can be found among the oocyte population (Frederiksen et al 2006).…”