Role of mitochondrial DNA replication during differentiation of reprogrammed stem cells

Abstract

Mitochondrial DNA (mtDNA) is a 16.6 kb genome that encodes for 13 of the 100+ subunits of the electron transfer chain (ETC), whilst the other subunits are encoded by chromosomal DNA. The ETC is responsible for the generation of the majority of cellular ATP through the process of oxidative phosphorylation (OXPHOS). mtDNA is normally inherited from the population present in the mature oocyte just prior to fertilisation. However, following somatic cell nuclear transfer (SCNT), mtDNA can be transmitted from both the donor cell and the recipient oocyte. This heteroplasmic transmission of mtDNA is a random event and does not appear to be related to the amount of mtDNA contributed by the donor cell. The distribution of mtDNA is randomly segregated between blastomeres and differentiating tissues, and therefore the mtDNA complement transmitted to offspring tissue cannot be predicted. mtDNA divergence between the cytoplast and the donor cell in intra-and inter-specific crosses favours a slightly more diverse mtDNA haplotype. However, this is limited as interspecies SCNT (iSCNT) genetic divergence contributes to developmental failure. SCNT embryos demonstrate a plethora of aberrantly reprogrammed characteristics including the uncoordinated regulation of the mtDNA replication factors. This results in increased mtDNA copy number during preimplantation development and propagates the replication of donor cell mtDNA. These failures are likely to be a consequence of incompatible nuclear-and mtDNA -encoded proteins interacting within the ETC thus reducing ATP production. The outcomes would be similar to the severely debilitating or even fatal mtDNA diseases associated with genetic rearrangements to mtDNA or mtDNA depletion type syndromes and have serious implications for any form of karyoplast transfer approach. The only method to overcome the problems of heteroplasmy in SCNT embryos is to completely deplete the donor cell of its mtDNA prior to SCNT.

Mitochondrial DNA copy number is regulated in a tissue specific manner by DNA methylation of the nuclear-encoded DNA polymerase gamma A

“…During neural differentiation of mouse (15) and human (63) ESCs, mtDNA copy number reduces, which allows progenitor cells to establish the ‘mtDNA set point’, from which the mitochondrial complement may be expanded in a cell specific manner (64). The ‘mtDNA set point’ was established in divergent ESCs although, during downstream differentiation, there were cell line specific differences in mtDNA copy number, PolgA expression and DNA methylation profiles for PolgA when compared with non-divergent controls.…”

Random Mutagenesis, Clonal Events, and Embryonic or Somatic Origin Determine the mtDNA Variant Type and Load in Human Pluripotent Stem Cells

“…For instance, mtDNA haplogroups appear to affect cellular function. Work on mouse ESCs has shown that in both undifferentiated and differentiating cells, the mitochondrial haplogroup has a significant impact on the expression of genes involved in pluripotency and differentiation, and does consequently influence the capacity of the cells to differentiate (Kelly and St John, 2010, Kelly et al., 2013). In the human, recent work in the context of mitochondrial replacement in oocytes indicated that some haplogroups can modify the growth dynamics of hESCs, resulting in a growth advantage that can lead to a culture takeover (Kang et al., 2016a).…”

Mitochondrial DNA Haplotypes Define Gene Expression Patterns in Pluripotent and Differentiating Embryonic Stem Cells

“…During early mammalian development, there are significant reductions in mtDNA copy number as mtDNA passes from the fertilized oocyte into the inner cell mass and into its derivatives, embryonic stem cells (ESCs) [15, 16], which establishes the mtDNA set point. The mtDNA set point ensures that undifferentiated cells have a small population of mtDNA, which is then expanded in a cell‐specific manner later during differentiation in order that specialized cells can meet their specific needs for OXPHOS‐derived ATP [15, 17, 18]. Consequently, undifferentiated ESCs rely predominantly on glycolysis to produce ATP [19, 20], although not exclusively [20, 21].…”