The GNAS gene spans over 20-kilobase pair and contains thirteen exons and codifies the α-subunit of the stimulatory G protein (Gsα) (Kozasa et al., 1988).

The GNAS locus produces multiple gene products as it has four alternative first exons (NESP55 (Ischia et al., 1997), XLαs (Kehlenbach et al., 1994), A/B (Ishikawa et al., 1990; Swaroop et al., 1991) and E1-Gsα) that splice onto a common exons 2 to 13. These first alternative exons lie within CpG islands and are differently imprinted, while to increase its complexity this locus also has an antisense transcript to NESP55, referred as NESPas (Hayward and Bonthron, 2000) (Figure 1).
Exon A/B or exon 1A, located 2.5 kb centromeric from Gsα exon 1, splices onto common exons 2-13, and is methylated on the maternal allele. In this case, because there is no consensus AUG translational start site in exon A/B, it is thought that the resulting transcript is not translated (Ishikawa et al., 1990; Liu et al., 2000). It has been suggested that this region has a negative regulatory cis-acting element that suppresses the paternal Gsα allele in a tissue specific manner (i.e. renal proximal tubules) (Williamson et al., 2004; Liu et al., 2005).
XLαs alternative first exon, is located about 35 kb centromeric from Gsα exon 1, join exons 2-13 leading a transcript that encodes the extra large protein (XLαs), an isoform of Gsα with similar functions but slightly longer, and its promoter is imprinted on the maternal allele (Hayward et al., 1998b).
Finally, the farthest alternative exon (49kb centromeric from exon 1), together with the other common exons 2-13, makes the transcript encoding the protein NESP55, chromogranin-like protein that is expressed mostly in neuroendocrine tissues and only from the maternal allele, due to methylation on the paternal allele (Hayward et al., 1998b).
Regarding GNAS gene transcripts, by different splicing of exon 3 and/or use of two 5splice sites of exon 4, two long (Gsα-L) and two short (Gsα-S) transcript variants are created, which contain alternatively exon 3 and/or a CAG sequence, respectively (Figure 2) (Bray et al., 1986; Robishaw et al., 1986; Kozasa et al., 1988). It is not methylated on either allele (Kozasa et al., 1988; Hayward et al., 1998a; Peters et al., 1999; Liu et al., 2005).

No pseudogenes have been identified.

The Gsα protein has 394 aminoacids with a mass of about 46 kDa. Gα-subunits contain two domains: a GTPase domain that is involved in the binding and hydrolysis of GTP and a helical domain.
The α subunit guanine nucleotide pocket consists of five distinct, highly conserved stretches (G1-G5). The G1, G4 and G5 regions are important for the binding of GTP while the G2 and G3 regions determine the intrinsic GTPase activity of the α subunit. The GDP-bound form binds tightly to bg and is inactive, whereas the GTP-bound form dissociates from bg and serves as a regulator of effector proteins. The receptor molecules cause the activation of G proteins by affecting several steps of the GTP cycle, resulting in the facilitation of the exchange of GTP for GDP on the α subunit (Lania et al., 2001; Cherfils and Chabre, 2003).

GNAS is biallelically expressed in most tissues studied (Hayward et al., 1998a; Hayward et al., 1998b; Zheng et al., 2001; Mantovani et al., 2004); however, in some tissues (thyroid, renal proximal tubule, pituitary and ovaries) primarily maternal expression is observed leading to a parental-of-origin effect (Davies and Hughes, 1993; Campbell et al., 1994; Hayward et al., 2001; Weinstein, 2001; Mantovani et al., 2002; Germain-Lee et al., 2002; Liu et al., 2003) (Yu et al., 1998).

Cytoplasmatic membrane-associated

Heterotrimeric G proteins are membrane bound GTPases that are linked to seven-transmembrane domain receptors (Kleuss and Krause, 2003). Each G protein contains an alpha-, beta- and gamma-subunit and is bound to GDP in the "off" state (Olate and Allende, 1991). Ligand-receptor binding results in detachment of the G protein, switching it to an "on" state and permitting Gα activation of second messenger signalling cascades (Cabrera-Vera et al., 2003). Gsα mediates the simulation of adenylate cyclase regulated by various peptide hormones (PTH, TSH, gonadotropins, ACTH, GHRH, ADH, glucagon, calcitonin, among others) (Spiegel, 1999; Spiegel and Weinstein, 2004). Gsα-subunits contain two domains: a GTPase domain that is involved in the binding and hydrolysis of GTP and a helical domain that buries the GTP within the core of the protein (Cabrera-Vera et al., 2003).
Exon 5 is thought to codify the highly conserved domain of Gsa that interacts with adenylate cyclase, while exon 13 is responsible for the interaction with the receptor (Pennington, 1994).

There are several types of Gα proteins; Gsα, Gqα, Gi/oα and G12/13α (Riobo and Manning, 2005). Members of Gsα bind directly to adenylyl cyclase and stimulate its activity, whereas their effects on ion channel activity are restricted to selected cell types; Gi/oα are involved in adenylyl cyclase inhibition, ion channel modulation and phosphatase activation. Finally, G12/13α family is implicated in processes of determination and cell proliferation. Subunits of the Gq/11 class are putative mediators of phospholipase C activation (Landis et al., 1989; Lania et al., 2001).

Both germinal and somatic, activating and inactivating, genetic and epigenetic alterations have been described at GNAS locus associated with different entities.
Activating mutations: Mutations at Arg201 or Gln227 inhibits the GTPase activity, maintaining Gsα in its active form. The mutant Gsα protein carrying these activating mutations is termed the gsp oncogene (Landis et al., 1989).
In McCune-Albright syndrome, the somatic mutation at Arg201, leading to its change into cysteine or histidine (even serine or glycine), occurs in early embryogenesis, resulting in widespread tissue distribution of abnormalities. The post zygotic mutation is responsible for the mosaic pattern of tissue distribution and the extreme variability of clinical changes (Weinstein et al., 1991).
Endocrine and non-endocrine tumors: Somatic mutations of Arg201 or Gln227 have been identified in human growth hormone-secreting pituitary adenomas, (Landis et al., 1989; Landis et al., 1990), ACTH-secreting pituitary adenomas (Williamson et al., 1995; Riminucci et al., 2002), nonfunctioning pituitary tumors (Tordjman et al., 1993), thyroid tumors (Suarez et al., 1991), Leydig cell tumor (Libe et al., 2012), ovarian granulosa cell tumors (Kalfa et al., 2006a), renal cell carcinoma (Kalfa et al., 2006b), hepatocellular carcinoma (Nault et al., 2012) and myelodysplastic syndromes (Bejar et al., 2011). The mutation at codon 201 (Arg into Cys or His) is more frequent that the mutation at 227 (Gln into Arg, His, Lys or Leu).
Fibrous dysplasia of the bone: Fibrous dysplasia (FD) is a benign intramedullary osteofibrous lesion that may involve either one (monostotic FD) or several (polyostotic FD) bones. FD may occur in isolation or as part of the McCune-Albright syndrome or within Mazabrauds syndrome. Some cases of FD have been found to have a somatic GNAS mutation, mainly R201C and R201H (Riminucci et al., 1997), though R201S (Candeliere et al., 1997) and Q227L (Idowu et al., 2007) has also been reported.
Inactivating mutations: The first reports of germ-line inactivating Gsα mutations were reported in 1990 (Patten et al., 1990; Weinstein et al., 1990). Latter on, many different mutations have been described in literature and summarized in the Human Gene Mutation Database at the Institute of Medical Genetics in Cardiff (www.hgmd.cf.ac.uk) as a cause of a hormonal disorder coupled to Gsα activity characterized by PTH renal resistance called Pseudohypoparathyroidism (PHP).
Mutation types include translation initiation mutations, amino acid substitutions, nonsense mutations, inversions, splice site mutations, insertions or deletions (even intragenic or encompassing the whole gene). Mutations are distributed throughout the Gsα coding region. Although each mutation is usually associated to a single kindred, a mutational hot-spot involving 20% of all mutations so far described has been identified within exon 7 (Weinstein et al., 1992; Yu et al., 1995; Yokoyama et al., 1996; Ahmed et al., 1998; Mantovani et al., 2000; Aldred and Trembath, 2000). It is a 4 bp deletion which coincides with a defined consensus sequence for arrest of DNA polymerase a, a region known to be prone to sporadic deletion mutations (Krawczak and Cooper, 1991; Yu et al., 1995). In most cases it has been found as a de novo mutation, thus representing a recurring new mutation rather than a founder effect.
As mentioned above, in some tissues paternal GNAS allele is silenced, leading to a parental-of-origin effect. In case of maternally inherited mutation, AHO is associated with end-organ resistance to the Gsα-mediated action of different hormones, primarily PTH, TSH, gonadotropin, and GHRH. AHO with endocrinopathy is then termed pseudohypoparathyroidism type Ia (PHP-Ia; MIM: 103580) or pseudohypoparathyroidism type Ic (PHP-Ic; MIM: 612462). In contrast, AHO due to paternally inherited mutation transmission lacks biochemical evidence of hormone resistance and is designated as pseudopseudohypoparathyroidism (PPHP; MIM 612463) (Davies and Hughes, 1993; Campbell et al., 1994; Weinstein, 2001; Weinstein et al., 2004) (see below for further details).
An intriguing missense mutation (Iiri et al., 1994; Nakamoto et al., 1996) localized within the highly conserved G5 region of the Gsα, has been identified in two unrelated males who presented with AHO, PTH resistance and testotoxicosis (Iiri et al., 1994). This substitution (A366S) leads to constitutive activation of adenylyl cyclase by causing accelerated release of GDP, thus increasing the fraction of active GTP-bound Gsα. However, while this mutant protein is stable at the reduced temperature of the testis, it is thermolabile at 37°C, resulting in reduced Gsα activity in almost tissues and AHO phenotype.
Progressive Osseous Heteroplasia (POH; MIM: 166350) is defined by cutaneous ossification, characteristically presenting during childhood, that progresses to involve subcutaneous and deep connective tissues, including muscle and fascia, in the absence of multiple features of Albright hereditary osteodystrophy (AHO) or hormone resistance (Kaplan et al., 1994). Most cases of POH are caused by heterozygous paternally-inherited inactivating mutations of GNAS (Shore et al., 2002; Adegbite et al., 2008).
Epigenetic alterations: Loss of methylation at GNAS exon A/B, sometimes combined with epigenetic defects at other GNAS differentially methylated regions has been associated with pseudohypoparathyroidism type Ib (PHP-Ib; MIM: 603233). The familial form of the disease has been shown to be mostly associated with an exon A/B-only methylation defect and a heterozygous 3-kb or 4.4-kb deletion mutation within the closely linked STX16 gene (Bastepe et al., 2003; Linglart et al., 2005), although four families of AD-PHP-Ib associated with NESP55 and NESPas deletions have also been described, the latter leading to the loss of all maternal GNAS imprints (Bastepe et al., 2005; Chillambhi et al., 2010; Richard et al., 2012). The exon A/B region is known as an imprinting control region and is believed to be critical for the tissue-specific imprinting of Gsα in the renal proximal tubules (Weinstein et al., 2001). The sporadic form of PHP-Ib show complete loss of methylation at the NESPas, XLαs and A/B regions, and no other changes in cis- or trans-acting elements have been found to explain this loss of methylation. In the scientific literature six cases have been described in which there is an association between the complete loss of methylation and partial or complete paternal isodisomy of chromosome 20q covering the GNAS locus (Bastepe et al., 2001; Bastepe et al., 2010; Fernandez-Rebollo et al., 2010). And on the other hand, it has been recently published a new trait of inheritance, an autosomal recessive form, explaining the molecular mechanism underlying the sporadic PHP-Ib in five families (Fernandez-Rebollo et al., 2011).