DNA methylation of nuclear donor cells and cloning efficiency in cattle

Somatic cell nuclear transfer (SCNT) efficiency remains low, largely due to incomplete epigenetic reprogramming of donor-cell nucleus by the enucleated oocyte during early embryonic development. In this study, we investigated DNA methylation patterns across candidate genomic regions in six bovine skin fibroblast lines, including three classified as high- and three as low-efficiency lines based on a large number of embryo transfers. Repetitive elements, including α-Satellite and Satellite I DNA, did not differ significantly in methylation between groups. Likewise, no significant differences in methylation were observed in the METTL14 region. In the XIST DMR1, methylation patterns varied substantially among animals, largely due to the aberrant methylation profile observed in one cell line from the low-efficiency group, highlighting inter-individual epigenetic variation among animals. In Oct4 region 1, methylation levels tended to be higher in the high-efficiency group. The H19/IGF2 ICR showed differences between groups of fibroblast lines, with a higher proportion of unmethylated alleles in the low-efficiency group. The IGF2R/AIRN ICR exhibited higher methylation levels in the high-efficiency group. Because IGF2R/AIRN is not imprinted in fibroblasts, this differential methylation pattern may represent an additional epigenetic barrier to oocyte-mediated nuclear reprogramming. These findings identify the IGF2R/AIRN ICR as a candidate locus associated with SCNT efficiency and suggest that donor-cell epigenetic features may influence nuclear reprogramming following nuclear transfer. Understanding how distinct genomic regions respond to reprogramming may provide further insight into the molecular mechanisms the low efficiency of SCNT.