Abstract
Most humans carry a mixed population of mitochondrial DNA (mtDNA heteroplasmy) affecting ~1–2% of molecules, but rapid percentage shifts occur over one generation leading to severe mitochondrial diseases. A decrease in the amount of mtDNA within the developing female germ line appears to play a role, but other sub-cellular mechanisms have been implicated. Establishing an in vitro model of early mammalian germ cell development from embryonic stem cells, here we show that the reduction of mtDNA content is modulated by oxygen and reaches a nadir immediately before germ cell specification. The observed genetic bottleneck was accompanied by a decrease in mtDNA replicating foci and the segregation of heteroplasmy, which were both abolished at higher oxygen levels. Thus, differences in oxygen tension occurring during early development likely modulate the amount of mtDNA, facilitating mtDNA segregation and contributing to tissue-specific mutation loads.
Highlights
Most humans carry a mixed population of mitochondrial DNA affecting ~1–2% of molecules, but rapid percentage shifts occur over one generation leading to severe mitochondrial diseases
In order to investigate the segregation of the heteroplasmic ND1 variant during the early stages of primordial germ cells (PGCs) development, primordial germ cell-like cells (PGCLCs) were generated using the two isogenic Embryonic stem cells (ESCs) subclones (WT and ND1, Fig. 1b)[21] that sequentially expressed the BV followed by the SC transgenic reporter genes (Fig. 1c and S1g, h)
The observed reduction in mitochondrial DNA (mtDNA) content at 3% oxygen levels closely resembles the mtDNA genetic bottleneck measured in single cells in mice in vivo[10,11,12], and suggests that low oxygen tension contributes to the formation of a mtDNA genetic bottleneck
Summary
Most humans carry a mixed population of mitochondrial DNA (mtDNA heteroplasmy) affecting ~1–2% of molecules, but rapid percentage shifts occur over one generation leading to severe mitochondrial diseases. The total amount of mtDNA within the embryo remains constant during pre-implantation development[10,11,14], there is ongoing mtDNA turnover in pre-implantation embryos[15,16] This means that the measured reduction of mtDNA within individual cells could be due to several mechanisms, including a reduction of the replication rate of mtDNA and an increase in the destruction of mtDNA by autophagy[16]. Resolving these issues at a cellular level will be challenging in living organisms. We developed an in vitro model of the critical time point during early germ cell development, allowing us to study mitochondria and mtDNA at the cellular and sub-cellular level during and immediately before germ cell specification
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