Transcription driven somatic DNA methylation within the imprinted Gnas cluster.

Stuti Mehta,Martin Fray,Charlotte Tibbit,Christine M Williamson,Jo Peters,Simon Ball,Colin Beechey,Osman El-Maarri

doi:10.1371/journal.pone.0117378

Abstract

Differential marking of genes in female and male gametes by DNA methylation is essential to genomic imprinting. In female gametes transcription traversing differentially methylated regions (DMRs) is a common requirement for de novo methylation at DMRs. At the imprinted Gnas cluster oocyte specific transcription of a protein-coding transcript, Nesp, is needed for methylation of two DMRs intragenic to Nesp, namely the Nespas-Gnasxl DMR and the Exon1A DMR, thereby enabling expression of the Gnas transcript and repression of the Gnasxl transcript. On the paternal allele, Nesp is repressed, the germline DMRs are unmethylated, Gnas is repressed and Gnasxl is expressed. Using mutant mouse models, we show that on the paternal allele, ectopic transcription of Nesp traversing the intragenic Exon1A DMR (which regulates Gnas expression) results in de novo methylation of the Exon1A DMR and de-repression of Gnas just as on the maternal allele. However, unlike the maternal allele, methylation on the mutant paternal allele occurs post-fertilisation, i.e. in somatic cells. This, to our knowledge is the first example of transcript/transcription driven DNA methylation of an intragenic CpG island, in somatic tissues, suggesting that transcription driven de novo methylation is not restricted to the germline in the mouse. Additionally, Gnasxl is repressed on a paternal chromosome on which Nesp is ectopically expressed. Thus, a paternally inherited Gnas cluster showing ectopic expression of Nesp is “maternalised” in terms of Gnasxl and Gnas expression. We show that these mice have a phenotype similar to mutants with two expressed doses of Gnas and none of Gnasxl.

Highlights

Genomic imprinting, which results in two genetically identical genes showing distinct expression patterns according to parental origin, has traditionally been a useful model system for PLOS ONE | DOI:10.1371/journal.pone.0117378 February 6, 2015Transcription Driven Somatic DNA Methylation at the Gnas Cluster studying epigenetic modification and processes
Our results show that on the paternal allele, Nesp expression traversing through the Exon1A differentially methylated regions (DMRs) results in acquisition of de novo methylation at the Exon1A DMR, just as it does on the maternal allele
The Exon1A DMR is completely methylated when Nesp is transcribed through the Gnas cluster on the paternal allele

Summary

Introduction

Genomic imprinting, which results in two genetically identical genes showing distinct expression patterns according to parental origin, has traditionally been a useful model system for PLOS ONE | DOI:10.1371/journal.pone.0117378 February 6, 2015Transcription Driven Somatic DNA Methylation at the Gnas Cluster studying epigenetic modification and processes. The Gnas cluster is well conserved between man and mouse, and contains a number of maternally, paternally and biallelically expressed transcripts. Nesp is maternally expressed and codes for the neuroendocrine secretory protein, NESP55 [3,5] It originates furthest upstream and transcribes through the entire length of the cluster (Fig. 1). Codes for extra large forms of Gsα and gives rise to a number of different protein variants [7]. These comprise XLαs, an N-terminally extended form XXLαs and in neural tissues a C-terminally truncated form XLN1 [8]. Nespas is transcribed in a direction opposite to all above transcripts, and covers the promoter region of Nesp alone (Fig. 1) [10,11]

Methods

Results

Conclusion