Deletion of a silencer element in lgf2 results in loss of imprinting independent of H19

Igf2 and H19 are closely linked, reciprocally imprinted genes on mouse dist al chromosome 7. The paternally expressed Igf2 encodes a potent fetal growt h factor and the maternally expressed H19 encodes a non-coding RNA (refs 1, 2). Shared endoderm-specific enhancers 3' to H19 are necessary for transcri ption of the maternal copy of H19 and the paternal copy of Igf2 (ref. 3), a chromatin boundary upstream of H19 preventing access of the enhancers to t he maternal Igf2 promoters(4-8). Mesoderm-specific control elements have no t been identified, and the role of differentially methylated regions (DMRs) in Igf2 has not been addressed. Two DMRs in Igf2 are methylated on the act ive paternal allele, suggesting that they contain silencers(9-12). Here we have deleted the DMR1 region in Igf2. Maternal transmission of the deletion results in biallelic expression of Igf2 in most mesodermally derived tissu es without altering H19 imprinting or expression. Paternal or maternal tran smission leads to continued postnatal transcription of Igf2, in contrast to the wild-type allele, which is silenced soon after birth. These results re veal a mesodermal silencer, which may be regulated by methylation and which has a major role in H19-independent expression and imprinting control of I gf2. Our results establish a new mechanistic principle for imprinted genes whereby epigenetically regulated silencers interact with enhancers to contr ol expression, and suggest a new mechanism for loss of imprinting (LOI) of Igf2, which may be important in a number of diseases.