Objective: The Gnas transcription unit located within an imprinting region encodes several proteins, including the G-protein a-subunit, Gsa, its isoform XLas and their variant truncated neural forms GsaN1 and XLN1. Gsa and GsaN1 are expressed predominantly from the maternally derived allele in some tissues, whereas XLas and XLN1 are expressed exclusively from the paternally derived allele. The relative contribution of full-length Gsa and XLas, and truncated forms GsaN1 and XLN1 to phenotype is unknown. The edematous-small point mutation (Oed-Sml) in exon 6 of Gnas lies downstream of GsaN1 and XLN1, but affects full-length Gsa and XLas, allowing us to address the role of full-length Gsa and XLas. The aim of this study was therefore to determine the metabolic phenotypes of Oed and Sml mice, and to correlate phenotypes with affected transcripts. Methods: Mice were fed standard or high-fat diets and weighed regularly. Fat mass was determined by DEXA analysis. Indirect calorimetry was used to measure metabolic rate. Glucose was measured in tolerance tests and biochemical parameters in fasted plasma samples. Histological analysis of fat and liver was carried out post mortem. Results: Oed mice are obese on either diet and have a reduced metabolic rate. Sml mice are lean and are resistant to a high-fat diet and have an increased metabolic rate. Conclusion: Adult Oed and Sml mice have opposite metabolic phenotypes. On maternal inheritance, the obese Oed phenotype can be attributed to non-functional full-length Gsa. In contrast, on paternal inheritance, Sml mice were small and resistant to the development of obesity on a high-fat diet, effects that can be attributed to mutant XLas. Thus, the neural isoforms, GsaN1 and XLN1, do not appear to play a role in these metabolic phenotypes.

Genomic imprinting results in parent-of-origin-dependent monoallelic gene expression. Early work showed that distal mouse chromosome 2 is imprinted, as maternal and paternal duplications of the region (with corresponding paternal and maternal deficiencies) give rise to different anomalous phenotypes with early postnatal lethalities. Newborns with maternal duplication (MatDp(dist2)) are long, thin and hypoactive whereas those with paternal duplication (PatDp(dist2)) are chunky, oedematous, and hyperactive. Here we focus on PatDp(dist2). Loss of expression of the maternally expressed Gnas transcript at the Gnas cluster has been thought to account for the PatDp(dist2) phenotype. But PatDp(dist2) also have two expressed doses of the paternally expressed Gnasxl transcript. Through the use of targeted mutations, we have generated PatDp(dist2) mice predicted to have 1 or 2 expressed doses of Gnasxl, and 0, 1 or 2 expressed doses of Gnas. We confirm that oedema is due to lack of expression of imprinted Gnas alone. We show that it is the combination of a double dose of Gnasxl, with no dose of imprinted Gnas, that gives rise to the characteristic hyperactive, chunky, oedematous, lethal PatDp(dist2) phenotype, which is also hypoglycaemic. However PatDp(dist2) mice in which the dosage of the Gnasxl and Gnas is balanced (either 2∶2 or 1∶1) are neither dysmorphic nor hyperactive, have normal glucose levels, and are fully viable. But PatDp(dist2) with biallelic expression of both Gnasxl and Gnas show a marked postnatal growth retardation. Our results show that most of the PatDp(dist2) phenotype is due to overexpression of Gnasxl combined with loss of expression of Gnas, and suggest that Gnasxl and Gnas may act antagonistically in a number of tissues and to cause a wide range of phenotypic effects. It can be concluded that monoallelic expression of both Gnasxl and Gnas is a requirement for normal postnatal growth and development.

GNAS/Gnas encodes Gsα that is mainly biallelically expressed but shows imprinted expression in some tissues. In Albright Hereditary Osteodystrophy (AHO) heterozygous loss of function mutations of GNAS can result in ectopic ossification that tends to be superficial and attributable to haploinsufficiency of biallelically expressed Gsα. Oed-Sml is a point missense mutation in exon 6 of the orthologous mouse locus Gnas. We report here both the late onset ossification and occurrence of benign cutaneous fibroepithelial polyps in Oed-Sml. These phenotypes are seen on both maternal and paternal inheritance of the mutant allele and are therefore due to an effect on biallelically expressed Gsα. The ossification is confined to subcutaneous tissues and so resembles the ossification observed with AHO. Our mouse model is the first with both subcutaneous ossification and fibroepithelial polyps related to Gsα deficiency. It is also the first mouse model described with a clinically relevant phenotype associated with a point mutation in Gsα and may be useful in investigations of the mechanisms of heterotopic bone formation. Together with earlier results, our findings indicate that Gsα signalling pathways play a vital role in repressing ectopic bone formation.