A cis-acting control region is required exclusively for the tissue-specific imprinting of Gnas.

Genomic imprinting brings about allele-specific silencing according to parental origin. Silencing is controlled by cis-acting regulatory regions that are differentially marked during gametogenesis and can act over hundreds of kilobases to silence many genes. Two candidate imprinting control regions (ICRs) have been identified at the compact imprinted Gnas cluster on distal mouse chromosome 2, one at exon 1A upstream of Gnas itself and one covering the promoters for Gnasxl and the antisense Nespas (ref. 8). This imprinted cluster is complex, containing biallelic, maternally and paternally expressed transcripts that share exons. Gnas itself is mainly biallelically expressed but is weakly paternally repressed in specific tissues. Here we show that a paternally derived targeted deletion of the germline differentially methylated region at exon 1A abolishes tissue-specific imprinting of Gnas. This rescues the abnormal phenotype of mice with a maternally derived Gnas mutation. Imprinting of alternative transcripts, Nesp, Gnasxl and Nespas (ref. 13), in the cluster is unaffected. The results establish that the differentially methylated region at exon 1A contains an imprinting control element that specifically regulates Gnas and comprises a characterized ICR for a gene that is only weakly imprinted in a minority of tissues. There must be a second ICR regulating the alternative transcripts.

The imprinted oedematous-small mutation on mouse chromosome 2 identifies new roles for Gnas and Gnasxl in development.

The Gnas locus is highly complex and encodes several oppositely imprinted and alternatively spliced transcripts. Gnas itself encodes Gsalpha, which is involved in endocrine function and bone development, but the roles for the other transcripts have not been established. Here we describe a mouse mutation that provides further biological functions for the Gnas locus. The mutation Oed-Sml, induced by ethylnitrosourea (ENU), has been mapped to the distal chromosome 2 imprinting region that includes Gnas. The mutation displays two distinct phenotypes dependent on parental origin. When the mutation is maternally transmitted, a microcardia with gross edema (Oed) results. By contrast, when the mutation is paternally transmitted, a growth retardation (Sml) is seen that becomes evident within 5 days of birth. Here we show Oed-Sml to be a point mutation in Gnas exon 6, resulting in a valine to glutamate substitution at residue 159 (V159E). Both maternal- and paternal-specific transcripts derive from this missense mutation. The maternally expressed mutant Gnas transcript is the candidate for Oed and the paternally expressed mutant Gnasxl transcript is the candidate for Sml. We propose a new role for Gnas in heart growth and a role for Gnasxl in postnatal growth. These findings potentially have implications for human Albright hereditary osteodystrophy, a condition caused by mutations in GNAS.

Alternative non-coding splice variants of Nespas, an imprinted gene antisense to Nesp in the Gnas imprinting cluster.

The Gnas locus on mouse Chr 2 represents a unique cluster of overlapping imprinted genes. Three of these in the order Nesp--Gnasxl--Gnas are transcribed in the sense direction with Nesp having maternal-specific expression, Gnasxl having paternal expression, and Gnas as being biallelically expressed in most tissues. A fourth imprinted gene, Nespas, is paternally expressed, lies antisense to Nesp, and expresses an unspliced transcript. Large unspliced antisense transcripts are emerging as a feature of imprinted gene clusters, and such non-coding RNAs may have a cis-regulatory function. Here we show that, in addition to an unspliced form of Nepas, we can detect five alternatively spliced forms of Nespas up to 1.4 kb in length that are non-coding. The splice variants are paternally expressed; they start approximately 2 kb upstream of Gnasxl in a region of maternal methylation and end 2.5 kb beyond the ATG of Nesp. These variants do not correspond to exons of the human antisense transcript although they start in the same region; the Nespas transcript, like its human counterpart, is spliced in various alternative patterns. The identification of a set of small spliced imprinted transcripts in the human and now in the mouse suggests that these antisense transcripts are functionally important.