Imprinted expression of the canine IGF2R, in the absence of an anti-sense transcript or promoter methylation.

Imprinted genes are epigenetically modified in a parent of origin-dependent manner, and as a consequence, are differentially expressed. Although the evolution of genomic imprinting is a subject of intense debate, imprinted genes have been studied primarily in mice and humans and in a small number of marsupial mammals. Comparative studies involving rodents and primates are of limited value, as they belong to the same superordinal group of eutherian mammals (Euarchontoglires). On the other hand, comparisons involving marsupials may not be informative, due to phylogenetic distance. Canis familiaris belongs to Laurasiatheria, a sister-group of Euarchontoglires, and should prove useful in comparative studies of imprinted genes. Using RT-PCR we demonstrate monoallelic expression of the canine IGF2R in several tissues, including uterus and umbilical cord. In the case of umbilical cord, we identify the expressed allele as maternally derived. The canine IGF2R is thus an imprinted gene. Using bisulfite sequencing, we show that the canine IGF2R resembles the imprinted mouse Igf2r in having a CpG island in intron 2 that is hemi-methylated. However, it differs from the mouse gene in that maintenance of the monoallelic expression of canine IGF2R does not require expression of an anti-sense transcript from the paternally derived allele, or methylation of the repressed IGF2R promoter. In these two important features, the imprinted canine gene resembles the imprinted opossum IGF2R. Our data suggest that these features were properties of the ancestral imprinted IGF2R and that the anti-sense transcript (Air) and promoter methylation observed in mouse are derived features of the mouse Igf2r locus.