Rapid Mitochondrial DNA Segregation in Primate Preimplantation Embryos Precedes Somatic and Germline Bottleneck

Abstract

SUMMARY The timing and mechanisms of mitochondrial DNA (mtDNA) segregation and transmission in mammals are poorly understood. Genetic bottleneck in female germ cells has been proposed as the main phenomenon responsible for rapid intergenerational segregation of heteroplasmic mtDNA. We demonstrate here that mtDNA segregation occurs during primate preimplantation embryogenesis resulting in partitioning of mtDNA variants between daughter blastomeres. A substantial shift toward homoplasmy occurred in fetuses and embryonic stem cells (ESCs) derived from these heteroplasmic embryos. We also observed a wide range of heteroplasmic mtDNA variants distributed in individual oocytes recovered from these fetuses. Thus, we present here evidence for a previously unknown mtDNA segregation and bottleneck during preimplantation embryo development, suggesting that return to the homoplasmic condition can occur during development of an individual organism from the zygote to birth, without a passage through the germline.

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.…”

“…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). 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.…”

“…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.…”

Assisted reproductive technologies to prevent human mitochondrial disease transmission

“…Some fetuses and some stem cell lines derived from the embryos exhibited skewed mtDNA ratios, although there was no evidence of preferential mtDNA selection. This suggested that both variants functioned equally, even though the two mtDNA those of sequences were as different from each other as they were from some other primate species 33 .…”

“…When Lee et al 33 generated macaque oocytes that were heteroplasmic (50:50) for two subspecies mtDNA variants, the resulting embryos underwent marked partitioning of the mtDNA between different blastomeres and to some extent between trophectoderm and inner cell mass (ICM) in the resulting blastocysts. Some fetuses and some stem cell lines derived from the embryos exhibited skewed mtDNA ratios, although there was no evidence of preferential mtDNA selection.…”