6 THE EFFECTS OF DEPLETING DONOR CELL MITOCHONDRIAL DNA ON CATTLE EMBRYOS DERIVED FROM SOMATIC CELL NUCLEAR TRANSFER

Abstract

Although somatic cell nuclear transfer (SCNT) is a valuable tool for producing animals for agricultural and research purposes, the resultant mixing of mitochondrial DNA (mtDNA) from the donor cell and recipient oocyte (heteroplasmy) affects embryo development and offspring survival and health. The aim of this study was to determine the effects of depleting donor cells of their mtDNA before SCNT on embryo development. mtDNA was depleted from cattle fibroblasts using 2′,3′-dideoxycytidine. mtDNA copy number in cells depleted for 30 days (0.85 ± 0.05) was significantly decreased when compared with nondepleted cells (150.12 ± 29.90; P < 0.0001, ANOVA). Moreover, mtDNA copy number in depleted cells could not be replenished after depletion for 30 days. Depleted cells and nondepleted cells were used as donor cells for SCNT. Somatic cell nuclear transfer embryos were produced by electrofusion of a single donor cell with an enucleated cow oocyte. Reconstructed oocytes were chemically activated and cultured for 7 days (nontreated embryos). Another cohort of embryos was treated with Trichostatin A (TSA), to enhance reprogramming, by activating reconstructed oocytes and culturing them in the presence of 50 nM TSA for up to 10 h. The embryos were then cultured in the absence of TSA. In nontreated groups, the fusion rates of depleted cells (78.0 ± 0.8%) were significantly lower than those of nondepleted cells (92.1 ± 1.4%; P < 0.05). No positive effect on fusion rates was found after TSA treatment. The blastocyst rate for SCNT embryos derived from depleted cells (18.7 ± 4.9%) was significantly lower than the nondepleted group (32.5 ± 3.1%; P < 0.05). Trichostatin A treatment increased blastocyst rates for SCNT embryos derived from depleted cells (32.5 ± 5.3%) to levels equivalent to those of nondepleted cells but did not have any beneficial effect on SCNT embryos derived from nondepleted cells. We have analysed blastocysts for the presence of donor cell mtDNA by high resolution melting analysis. Four out of 10 SCNT blastocysts derived from nondepleted cells were heteroplasmic, whereas others had no donor cell mtDNA. However, all 10 analysed SCNT blastocysts derived from depleted cells were homoplasmic as they harboured only oocyte mtDNA. From RNA sequencing results, TSA treatment of SCNT blastocysts derived from depleted cells increased the expression of key developmental transcription regulators and decreased expression of the mtDNA-specific replication factors, which is essential for embryo development. In conclusion, homoplasmic SCNT embryos were successfully produced by using mtDNA depleted donor cells. Trichostatin A treatment enhanced nuclear reprogramming efficiency in SCNT embryos derived from depleted cells. This work was supported by MitoStock Pty. Ltd., Australia.