Imprinting at the mouse Gnas cluster

• Main Author: Mehta, Stuti Rushikant
• Published: University of Oxford 2012
• Subjects: 572.88
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.600233

At the imprinted mouse Gnas locus, expression of Nesp in the growing oocyte is required for methylation of the downstream Nespas-Gnasxl DMR and ExonlA DMR; resulting in silencing of Gnasxl and ExonlA, and expression of Gnas on the maternal allele. On the paternal allele, the somatic Nesp promoter is silenced and Gnasxl and Gnas show the opposite pattern of expression. Using mutants in which Nesp is ectopically expressed on the paternal allele, I show that Nesp expression correlates with gain of DNA methylation at the ExonlA DMR, and with de-repression of Gnas; just as it does on the maternal allele. This DNA methylation is acquired post-fertilisation on the paternal allele, as opposed to the maternal allele where the ExoniA DMR is methylated in the growing oocyte. My subsequent work was focussed on understanding how Nesp expression is silenced on the paternal allele. I investigated if RNA interference is involved in silencing of Nesp, the developmental profile of expression of transcripts at the Gnas cluster, and the acquisition of DNA methylation and histone marks at the Nesp DMR. Nesp is expressed predominantly from the maternal allele at 6.5dpc, before the Nesp DMR is fully methylated on the paternal allele. The maternally inherited Nesp DMR is tri-methylated at H3K4 and H3K27 between the ICM and the 10.5dpc stages of development. Preferential enrichment of H3K4me3 and H3K27me3 at the maternally inherited Nesp DMR may playa role in establishing monoallelic expression of Nesp. Expression of Nespas, a noncoding RNA on the paternal allele protects the Nesp DMR from gaining H3K4me3 and H3K27me3 marks in cis. My work supports an emerging model of establishment of imprinted expression at the Gnas cluster, in which imprinting is regulated by concerted expression of two RNAs: a noncoding RNA (Nespas) on the paternal allele and, a protein coding RNA (Nesp) on the maternal allele.