Identification of a methylation imprint mark within the mouse Gnas locus

The imprinted mouse gene Gnas produces the G protein alpha-subunit Gs alpha and several other gene products by using alternative promoters and first e xons. G(s)alpha is maternally expressed in some tissues and biallelically e xpressed in most other tissues, while the gene products NESP55 and XL alpha s are maternally and paternally expressed, respectively. We investigated t he mechanisms of Gnas imprinting. The G(s)alpha promoter and first exon are not methylated on either allele. A further upstream region (approximately from positions -3400 to -939 relative to the G(s)alpha translational start site) is methylated only on the maternal allele in all adult somatic tissue s and in early postimplantation development. Within this region lies a four th promoter and first exon (exon 1A) that generates paternal-specific mRNAs of unknown function. Exon 1A and G(s)alpha mRNAs have similar expression p atterns, making competition between their promoters unlikely. Differential methylation in this region is established during gametogenesis, being prese nt in oocytes and absent in spermatozoa, and is maintained in preimplantati on E3.5d blastocysts. Therefore, this region is a methylation imprint mark. In contrast, differential methylation of the NESP55 and XL alpha s promote r regions (Nesp and Gnasxl) is not established during gametogenesis. The me thylation imprint mark that we identified may be important for the tissue-s pecific imprinting of G(s)alpha.