Identification of a Methylation Imprint Mark within the Mouse Gnas Locus

Abstract

The imprinted mouse gene Gnas produces the G protein ␣-subunit G S ␣ and several other gene products by using alternative promoters and first exons. G S ␣ is maternally expressed in some tissues and biallelically expressed in most other tissues, while the gene products NESP55 and XL␣s are maternally and paternally expressed, respectively. We investigated the mechanisms of Gnas imprinting. The G S ␣ 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 ␣ translational start site) is methylated only on the maternal allele in all adult somatic tissues and in early postimplantation development. Within this region lies a fourth promoter and first exon (exon 1A) that generates paternal-specific mRNAs of unknown function. Exon 1A and G S ␣ mRNAs have similar expression patterns, making competition between their promoters unlikely. Differential methylation in this region is established during gametogenesis, being present in oocytes and absent in spermatozoa, and is maintained in preimplantation E3.5d blastocysts. Therefore, this region is a methylation imprint mark. In contrast, differential methylation of the NESP55 and XL␣s promoter regions (Nesp and Gnasxl) is not established during gametogenesis. The methylation imprint mark that we identified may be important for the tissue-specific imprinting of G S ␣.

Alternative Gnas gene products have opposite effects on glucose and lipid metabolism

“…NESP55 and XL␣s are oppositely imprinted: NESP55 is expressed from the maternal allele, and its promoter is DNA-methylated on the paternal allele, whereas XL␣s is paternally expressed, and its promoter region is methylated on the maternal allele (6,10). Exon 1A, another alternative first exon located just upstream of the G s ␣ promoter, is methylated on the maternal allele and generates paternal-specific untranslated transcripts (11,12). Loss of exon 1A imprinting leads to pseudohypoparathyroidism type 1B, a syndrome of isolated parathyroid hormone resistance (12).…”

Identification of the control region for tissue-specific imprinting of the stimulatory G protein α-subunit

“…Nesp and Gnasxl are oppositely imprinted; the Nesp promoter is DNA methylated on the paternal allele and transcriptionally active on the maternal allele, whereas Gnasxl is methylated on the maternal allele and active on the paternal allele (4,5). Nesp is not a primary imprinting center because its imprinting is not established until after implantation (6). Paternal-specific antisense transcripts (Nespas) generated from the Gnasxl promoter region may be important for Nesp imprinting (7,8).…”

“…Just upstream of the G s ␣ promoter is a differentially methylated region (DMR) that is methylated on the maternal allele and contains a promoter and first exon (exon 1A), which generates paternal-specific mRNAs of unknown function (6,15) (Fig. 1 A).…”

“…(Throughout this article, we use the term G s ␣ imprinting to refer to allele-specific differences in G s ␣ expression in specific tissues, not the underlying epigenetic marks, which are presumably not tissue-specific.) G s ␣ imprinting is not associated with DNA methylation of its promoter but is associated with allele-specific differences in histone methylation (6,14). Heterozygous G s ␣-inactivating mutations in Albright hereditary osteodystrophy also lead to parathyroid hormone (PTH), TSH, and gonadotropin resistance (pseudohypoparathyroidism type 1A, PHP1A) when present on the maternal allele (2) because of the loss of G s ␣ expression from the active maternal allele in hormone target tissues.…”

Hypoparathyroidism and pseudohypoparathyroidism

“…In the majority of tissues, exon 1A is methylated but the repressor is not expressed, and as a result the G s α is expressed biallelically. In PHP1b, the methylation of this exon is absent in the maternal allele, allowing the repressor to link to both alleles and suppress the G s α expression in the renal proximal tubules, raising the G s α deficiency and PTH resistance (26,29,34), which normally only express the maternal allele. These individuals do not present AHO, because the defective imprinting does not have an effect in the G s α expression of the majority of the tissues where it is expressed biallelically.…”