Making sense of imprinting the mouse and human IGF2R loci.

The mouse and human IGF2R genes are similar in terms of expression pattern, gene structure and organization. Both genes have features that are common to imprinted genes. These common features are allele-specific methylation and replication asynchrony, plus the ability to restrict expression to one parental allele in diploid cells despite the presence of two functional parental alleles. In inbred laboratory mice Igf2r is initially expressed from both parental chromosomes in preimplantation embryos, it then shows maternal-specific monoallelic expression in all tissues of the postimplantation embryo and adult. The human gene is similarly monoallelically expressed in preterm postimplantation embryonic tissues (preimplantation embryos have not been examined). The behaviour of the human gene then diverges from that observed in inbred mice because it shows biallelic expression in term embryonic tissues and in the adult. An extra difference displayed by the human gene is that monoallelic expression is polymorphic and only occurs in 50% of individuals. The mechanism of IGF2R imprinting will be discussed with relevance to these similarities and differences between the mouse and human genes.