Analysis of parent-specific gene expression in early mouse embryos and embryonic stem cells using high-resolution two-dimensional electrophoresis of proteins

Genomic imprinting is an important genetic mechanism in mammals whereby certain genes are epigenetically modified and their expression altered according to their parental origin. The most important consequence of this is the requirement for both a maternal and a paternal genome for normal development to proceed to term. Although there are many instances of specific phenotypes (in the mouse) and diseases (in humans) resulting from imbalances in the parental chromosomes, it is only in the past few years that some of the imprinted genes responsible have been identified. It is however unclear what proportion of the genome is imprinted, particularly in the early embryo. To address the question to what extent parent-specific gene expression occurs in the early embryo and with a possible view to identifying new imprinted genes, the protein profiles of parthenogenetic and normal blastocysts were compared using the technique of high-resolution two-dimensional electrophoresis. The protein profiles of parthenogenetic, androgenetic and normal embryonic stem cells were also compared. Hence parent-specific gene expression was examined in embryonic and extraembryonic lineages of the early embryo. Approximately 1000 polypeptides were examined in each of the analyses, however no parent-specific differences were observed for any of these polypeptides. From this result, it is concluded that expression of genes encoding these polypeptides is identical from the parental chromosomes. These findings have important implications for estimates of the number of imprinted genes in the genome and for the interpretation of phenotypes of parthenogenetic and androgenetic embryos.