Organization and Control of Imprinted Genes: The Common Features

Abstract

During mammalian development, the two parental genomes are not functionally equivalent. This was first demonstrated in the early 1980s by the pronuclear transfer experiments of Surani and Solter, which demonstrated that both parental genomes are essential for normal development (McGrath and Solter 1984; Surani et al. 1984). Embryos that contain two paternal genomes (androgenetic) show very poor embryonic development. Gynogenetic embryos, which contain two maternal genomes, show exceptionally poor extraembryonic tissue development. In both instances, lethality occurs by mid-gestation. These results suggest that the parental genomes play complementary roles, involving differential (monoallelic) expression of essential genes in embryonic development. In recent years, a variety of techniques have been used to identify an increasing number of mouse and human genes that are differentially expressed depending on their parental origin. For example, polymorphisms in a given gene may be used to assess the parental origin of an expressed allele. Such genes, which “remember” which parent they are inherited from, have been termed imprinted genes.

Other work with mice containing uniparental duplications of sub-chromosomal regions (both copies derived from one parent) showed that duplication of some domains results in a phenotype which is often embryonic-lethal (Cattanach and Kirk 1985). This indicates that essential genes in these regions are imprinted. Ten such domains have been identified, located on mouse chromosomes 2, 6, 7, 11, 12, and 17. Some imprinted genes are clustered within these domains and may be controlled by common regulatory elements. Other imprinted genes may have their own imprinting regulatory elements and...