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GNAS (guanine nucleotide-binding protein, α stimulating) is a complex imprinted locus coding, besides the α-stimulatory subunit of the G protein, the paternally (extra-large, antisense and A/B) and maternally (neuroendocrine secretory protein) transcripts. Heterozygous mutations in the coding sequence of GNAS produce dominant phenotypes (combination of resistances to hormones signaling through G-protein-coupled receptors, osteodystrophy and obesity) that depend on the parental origin of the mutated allele. Likewise, alterations in the methylation at promoters of GNAS transcripts, associated or not with deletions of imprinting control regions in the nearby STX16 gene or within GNAS, prompt resistance to parathormone when affecting the maternal allele. Therefore, imprinting of GNAS is the determining factor for the variability of the phenotype. Knowledge of the various phenotypes is necessary for genetic counseling as well as an appropriate therapeutic balance between regular follow-up, prevention of disease complications and iatrogeny.

Parental allele-specific expression of imprinted genes is mediated by imprinting control regions (ICRs) that are constitutively marked by DNA methylation imprints on the maternal or paternal allele. Mono-allelic DNA methylation is strictly required for the process of imprinting and has to be faithfully maintained during the entire life-span. While the regulation of DNA methylation itself is well understood, the mechanisms whereby the opposite allele remains unmethylated are unclear. Here, we show that in the mouse, at maternally methylated ICRs, the paternal allele, which is constitutively associated with H3K4me2/3, is marked by default by H3K27me3 when these ICRs are transcriptionally inactive, leading to the formation of a bivalent chromatin signature. Our data suggest that at ICRs, chromatin bivalency has a protective role by ensuring that DNA on the paternal allele remains unmethylated and protected against spurious and unscheduled gene expression. Moreover, they provide the proof of concept that, beside pluripotent cells, chromatin bivalency is the default state of transcriptionally inactive CpG island promoters, regardless of the developmental stage, thereby contributing to protect cell identity.

Summary Data from the literature indicate that genomic imprint marks are disturbed in human pluripotent stem cells (PSCs). GNAS is an imprinted locus that produces one biallelic (Gsα) and four monoallelic (NESP55, GNAS-AS1, XLsα, and A/B) transcripts due to differential methylation of their promoters (DMR). To document imprinting at the GNAS locus in PSCs, we studied GNAS locus DMR methylation and transcript (NESP55, XLsα, and A/B) expression in human embryonic stem cells (hESCs) and human induced pluripotent stem cells (hiPSCs) derived from two human fibroblasts and their progenies. Results showed that (1) methylation at the GNAS locus DMRs is DMR and cell line specific, (2) changes in allelic transcript expression can be independent of a change in allele-specific DNA methylation, and (3) interestingly, methylation at A/B DMR is correlated with A/B transcript expression. These results indicate that these models are valuable to study the mechanisms controlling GNAS methylation, factors involved in transcript expression, and possibly mechanisms involved in the pathophysiology of pseudohypoparathyroidism type 1B., Highlights • GNAS locus methylation is DMR and cell line specific in human pluripotent stem cells • Allelic transcript expression can be independent of allele-specific DNA methylation • A/B transcript expression, a key for PHP1B, is correlated with A/B DMR methylation, GNAS, a complex imprinted locus, produces biallelic (Gsα) and monoallelic (NESP55, GNAS-AS1, XLsα, and A/B) transcripts. In this article, Silve and colleagues show that hESCs and hiPSCs are valuable models to study mechanisms controlling GNAS methylation, regulation of transcript expression, and possibly mechanisms involved in the pathophysiology of PHP1B, a constellation of rare diseases caused by GNAS methylation defects.

Epigenetic modifications are crucial for maintaining and faithfully transmitting the identity of each cell type during cell division. During mammalian germ cell development, the acquisition of the ability to form a totipotent zygote is associated with extensive epigenetic reprogramming that affects all major developmental processes, including genomic imprinting, X-inactivation, retroelement silencing and gene expression. The existing epigenetic patterns are first erased during primordial germ cell development, followed by acquisition of a germline-specific epigenetic signature that can be eventually transmitted to and interpreted by the progeny. A better characterisation of the underlying mechanisms is relevant for both fundamental and clinical research dealing with epigenetic inheritance, epigenetic control of mammalian development and regenerative medicine. In this review we present and discuss recent advances on the nature, mechanisms and consequences of resetting the epigenetic pattern during primordial germ cell formation and (re)acquiring a new set of epigenetic marks at later stages of germline development.

GNAS (guanine nucleotide-binding protein, α stimulating) is a complex imprinted locus coding, besides the α-stimulatory subunit of the G protein, the paternally (extra-large, antisense and A/B) and maternally (neuroendocrine secretory protein) transcripts. Heterozygous mutations in the coding sequence of GNAS produce dominant phenotypes (combination of resistances to hormones signaling through G-protein-coupled receptors, osteodystrophy and obesity) that depend on the parental origin of the mutated allele. Likewise, alterations in the methylation at promoters of GNAS transcripts, associated or not with deletions of imprinting control regions in the nearby STX16 gene or within GNAS, prompt resistance to parathormone when affecting the maternal allele. Therefore, imprinting of GNAS is the determining factor for the variability of the phenotype. Knowledge of the various phenotypes is necessary for genetic counseling as well as an appropriate therapeutic balance between regular follow-up, prevention of disease complications and iatrogeny.

Pseudohypoparathyroidism (PHP) type Ia is a rare maternally transmitted disease due to maternal loss-of-function mutations of GNAS, the gene encoding Galphas, the alpha-stimulatory subunit of the G protein. Affected individuals display hormonal resistance (mainly PTH and TSH resistance) and Albright hereditary osteodystrophy. PHP type Ib (PHP-Ib), usually defined by isolated renal resistance to PTH and sometimes mild TSH resistance, is due to a maternal loss of GNAS exon A/B methylation, leading to decreased Galphas expression in specific tissues.We report a girl with obvious Albright osteodystrophy features, PTH resistance, normal Galphas bioactivity in red blood cells, yet no loss-of-function mutation in the GNAS coding sequence (exons 1-13). The methylation analysis of the four GNAS differentially methylated regions, i.e. NESP, AS, XL, and A/B, revealed broad methylation changes at all differentially methylated regions, including GNAS exon A/B, leading to a paternal epigenotype on both alleles.This observation suggests that: 1) the decreased expression of Galphas due to GNAS epimutations is not restricted to the renal tubule but may affect nonimprinted tissues like bone; 2) PHP-Ib is a heterogeneous disorder that should lead to studying GNAS epigenotype in patients with PHP and no mutation in GNAS exons 1-13, regardless of their physical features.