Placental phenotypes associated with abnormal genomic imprinting on distal mouse chromosome 7

Imprinted genes are expressed either from the maternal or paternal allele during development and tend to be found in clusters throughout the mammalian genome, suggesting they may be regulated by long-range mechanisms. Many of them have important roles in placental development. The Beckwith-Wiedemann Syndrome (BWS) region on human chromosome 11p15.5 contains two imprinted subdomains each regulated by their own differentially methylated regions, known as imprinting centres (IC1 and IC2). These two imprinted subdomains are separated by an evolutionarily conserved region of about 300 kilobases. Distal mouse chromosome 7 (MMU7) shares syntenic homology with the human BWS region. Since the mechanisms by which imprinting occurs are unclear, we sought to characterize this region further using two mouse lines carrying deletions within the BWS imprinted region. The first mouse line, called DelTel7/IC2KO, allows us to dissect out the role of imprinting centre 2 in the silencing of imprinted genes. We demonstrate that all of the distal MMU7 imprinted genes implicated in placental function are silenced by IC2 and the noncoding RNA Kcnq1ot1. The second mouse line, called Del7AI, allows us to determine whether placental imprinting is perturbed when the region between IC1 and IC2 is deleted. We found that maternal inheritance of Del7AI leads to partial loss of the gene Ascl2, and we show that this affects all three layers of the mature mouse placenta. We found that paternal inheritance of Del7AI leads to partial loss of Ascl2 imprinting. Detailed investigation of the underlying mechanisms of imprinting and phenotypes in these mouse lines provides us with new fundamental insights into placental biology and the regulation of gene expression by imprinting centres on distal mouse chromosome 7.