Role of mitochondrial DNA replication during differentiation of reprogrammed stem cells.

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.