Endocrine Manifestations of Stimulatory G Protein α-Subunit Mutations and the Role of Genomic Imprinting

Abstract

The heterotrimeric G protein G(s) couples hormone receptors (as well as other receptors) to the effector enzyme adenylyl cyclase and is therefore required for hormone-stimulated intracellular cAMP generation. Receptors activate G(s) by promoting exchange of GTP for GDP on the G(s) alpha-subunit (G(s)alpha) while an intrinsic GTPase activity of G(s)alpha that hydrolyzes bound GTP to GDP leads to deactivation. Mutations of specific G(s)alpha residues (Arg(201) or Gln(227)) that are critical for the GTPase reaction lead to constitutive activation of G(s)-coupled signaling pathways, and such somatic mutations are found in endocrine tumors, fibrous dysplasia of bone, and the McCune-Albright syndrome. Conversely, heterozygous loss-of-function mutations may lead to Albright hereditary osteodystrophy (AHO), a disease characterized by short stature, obesity, brachydactyly, sc ossifications, and mental deficits. Similar mutations are also associated with progressive osseous heteroplasia. Interestingly, paternal transmission of GNAS1 mutations leads to the AHO phenotype alone (pseudopseudohypoparathyroidism), while maternal transmission leads to AHO plus resistance to several hormones (e.g., PTH, TSH) that activate G(s) in their target tissues (pseudohypoparathyroidism type IA). Studies in G(s)alpha knockout mice demonstrate that G(s)alpha is imprinted in a tissue-specific manner, being expressed primarily from the maternal allele in some tissues (e.g., renal proximal tubule, the major site of renal PTH action), while being biallelically expressed in most other tissues. Disrupting mutations in the maternal allele lead to loss of G(s)alpha expression in proximal tubules and therefore loss of PTH action in the kidney, while mutations in the paternal allele have little effect on G(s)alpha expression or PTH action. G(s)alpha has recently been shown to be also imprinted in human pituitary glands. The G(s)alpha gene GNAS1 (as well as its murine ortholog Gnas) has at least four alternative promoters and first exons, leading to the production of alternative gene products including G(s)alpha, XLalphas (a novel G(s)alpha isoform that is expressed only from the paternal allele), and NESP55 (a chromogranin-like protein that is expressed only from the maternal allele). A fourth alternative promoter and first exon (exon 1A) located approximately 2.5 kb upstream of the G(s)alpha promoter is normally methylated on the maternal allele and transcriptionally active on the paternal allele. In patients with isolated renal resistance to PTH (pseudohypoparathyroidism type IB), the exon 1A promoter region has a paternal-specific imprinting pattern on both alleles (unmethylated, transcriptionally active), suggesting that this region is critical for the tissue-specific imprinting of G(s)alpha. The GNAS1 imprinting defect in pseudohypoparathyroidism type IB is predicted to decrease G(s)alpha expression in renal proximal tubules. Studies in G(s)alpha knockout mice also demonstrate that this gene is critical in the regulation of lipid and glucose metab...

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

“…We have proposed that XL␣s may normally inhibit sympathetic activity in the central nervous system, and that XL␣s deficiency therefore leads to increased sympathetic activity (19). Whatever the mechanism, the role of XL␣s in metabolic regulation appears to be speciesspecific, because pseudopseudohypoparathyroidism patients with paternal GNAS mutations that disrupt XL␣s expression do not develop a similar phenotype (1). The presence of obesity in both E1 mϪ/ϩ and E1 ϩ/pϪ mice provides further evidence that the obesity in AHO patients results from G s ␣ deficiency.…”

“…Heterozygous G s ␣ inactivating mutations lead to Albright hereditary osteodystrophy (AHO), a syndrome characterized by obesity and skeletal and neurobehavioral defects (1). Patients who inherit AHO maternally also develop resistance to parathyroid hormone (PTH), thyroidstimulating hormone (TSH), and gonadotropins, a condition known as pseudohypoparathyroidism type 1A.…”

“…1A) (1). Promoters for 55-kDa neuroendocrinespecific protein (NESP55), a chromogranin-like protein, and extralarge G s ␣ (XL␣s), a neuroendocrine-specific G s ␣ isoform with a long N-terminal extension encoded by its alternative first exon, are located 47 and 35 kb upstream of the G s ␣ promoter, respectively (6).…”

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

“…We have proposed that tissue-specific G s ␣ imprinting results from cis-acting negative regulatory elements within the 1A DMR that are both tissue-specific and DNA methylation-sensitive (and, therefore, do not suppress G s ␣ expression from the maternal allele) (2,15). Our model predicts that maternal 1A deletion would not affect G s ␣ expression, whereas paternal 1A deletion would relieve paternal G s ␣ imprinting leading to G s ␣ overexpression in tissues, such as proximal tubules, where G s ␣ is normally imprinted.…”

“…In PHP1B, in which patients develop renal PTH resistance but not Albright hereditary osteodystrophy, maternal imprinting (methylation) of the 1A DMR is lost (15). We have suggested that the 1A DMR has cis-acting negative regulatory elements (e.g., silencers or insulators) for the G s ␣ promoter that are both methylation-sensitive (and, hence, suppress G s ␣ only on the paternal allele) and tissue-specific (perhaps because of tissue-specific expression of trans-acting factors) (2,15). This model predicts that loss of maternal 1A methylation in PHP1B leads to biallelic loss of G s ␣ expression in renal proximal tubules and renal PTH resistance, but it has little effect in most other tissues where normally the cis-acting elements do not affect G s ␣ promoter activity.…”