The development of a bovine interspecies model for the analysis of genomic imprinting in normal and nuclear transfer derived fetuses

The advent of somatic cell nuclear transfer in cattle has provided the opportunity for researchers to generate genetically identical animals as well as animals that possess precise genetic modifications for agriculture and biomedical purposes. However, in spite of the revolutionary impact this technology presents, problems remain which hinder the production of healthy animals on a consistent basis. Research on cloned mice implicates improper reprogramming of epigenetic modifications and genomic imprinting for the low pregnancy rates and high incidence of abnormalities that are manifested in cloned animals; however, a systematic and comprehensive analysis of nuclear reprogramming in cloned cattle remains undone. The purpose of this research is to assess and characterize the patterns of genomic imprinting in normal and nuclear transfer derived bovine fetuses. To facilitate the identification of imprinted genes in the bovine, a Bos gaurus/Bos taurus interspecies model has been incorporated to maximize the genetic heterozygosity that exists between the alleles of putative imprinted genes for allelic discrimination and parental inheritance.
The sequence of twenty-six genes, previously reported as imprinted in mice and humans, was analyzed in Bos gaurus (Gaur) and Bos taurus (Angus) cattle for the presence of single nucleotide polymorphisms (SNP). SNPs were detected in the Gene trap locus 2 (GTL2), Insulin like growth factor 2 (IGF2), Wilms tumor 1 (WT1) and the X chromosome inactivation specific transcript (XIST). Allelic expression analysis in interspecies hybrids indicated maternal genomic imprinting at the IGF2 and XIST loci, paternal genomic imprinting at the GTL2 locus and no imprinting at the WT1 locus. Analysis in cloned hybrids indicated fidelity of allelic expression at the IGF2 and GTL2 loci, however disruption of imprinting was observed at the XIST locus in the placenta of clones. These results are the largest identification of imprinted genes in the bovine and the first identification of the disruption of an imprinted gene in an animal derived from somatic cell nuclear transfer.