Alternative promoter and 5' exon generate a novel Gs alpha mRNA

Presentation of pseudohypoparathyroidism and pseudopseudohypoparathyroidism with skin lesions: Case reports and review

We report three cases of patients with pseudohypoparathyroidism or pseudopseudohypoparathyroidism. These diseases are considered GNAS inactivating mutation syndromes that are characterized by a diversity of alterations among which a particular phenotype and specific endocrine or ossification abnormalities may be found. These patients may present with hard cutaneous nodules, which can represent osteoma cutis. The presence of these lesions in pediatric patients should prompt the dermatologist's consideration of this group of diseases when reaching a diagnosis. A multidisciplinary team of pediatricians, endocrinologists, geneticists, and dermatologists should carefully evaluate these patients.

Heterotopic ossification: Mechanistic insights and clinical challenges

Bone formation is exquisitely controlled both spatially and temporally. Heterotopic ossification (HO) is pathological bone formation in soft tissues that often leads to deleterious outcomes. Inherited genetic forms of HO can be life-threatening and can happen as early as in infancy. However, there is currently no effective treatment for HO as the underlying cellular and molecular mechanisms have not been completely elucidated. Trauma-induced non-genetic forms of HO often occur as a common complication after surgeries or accidents, and the location of HO occurrence largely determines the symptom and outcome. While it has been difficult to determine the complicated factors causing HO, recent advancement in identifying cellular and molecular mechanism causing the genetic forms of HO may provide important insights in all HO. Here in this review, we summarize recent studies on HO to provide a current status of both clinical options of HO treatments and mechanical understanding of HO.

Heterotrimeric G proteins in the control of parathyroid hormone actions

Parathyroid hormone (PTH) is a key regulator of skeletal physiology and calcium and phosphate homeostasis. It acts on bone and kidney to stimulate bone turnover, increase the circulating levels of 1,25 dihydroxyvitamin D and calcium and inhibit the reabsorption of phosphate from the glomerular filtrate. Dysregulated PTH actions contribute to or are the cause of several endocrine disorders. This calciotropic hormone exerts its actions via binding to the PTH/PTH-related peptide receptor (PTH1R), which couples to multiple heterotrimeric G proteins, including G_s and G_q/11 Genetic mutations affecting the activity or expression of the alpha-subunit of G_s, encoded by the GNAS complex locus, are responsible for several human diseases for which the clinical findings result, at least partly, from aberrant PTH signaling. Here, we review the bone and renal actions of PTH with respect to the different signaling pathways downstream of these G proteins, as well as the disorders caused by GNAS mutations.

Pseudohypoparathyroidism: one gene, several syndromes

Pseudohypoparathyroidism (PHP) and pseudopseudohypoparathyroidism (PPHP) are caused by mutations and/or epigenetic changes at the complex GNAS locus on chromosome 20q13.3 that undergoes parent-specific methylation changes at several sites. GNAS encodes the alpha-subunit of the stimulatory G protein (Gsα) and several splice variants thereof. Heterozygous inactivating mutations involving the maternal GNAS exons 1-13 cause PHP type Ia (PHP1A). Because of much reduced paternal Gsα expression in certain tissues, such as the proximal renal tubules, thyroid, and pituitary, there is little or no Gsα protein in the presence of maternal GNAS mutations, thus leading to PTH-resistant hypocalcemia and hyperphosphatemia. When located on the paternal allele, the same or similar GNAS mutations are the cause of PPHP. Besides biochemical abnormalities, patients affected by PHP1A show developmental abnormalities, referred to as Albrights hereditary osteodystrophy (AHO). Some, but not all of these AHO features are encountered also in patients affected by PPHP, who typically show no laboratory abnormalities. Autosomal dominant PHP type Ib (AD-PHP1B) is caused by heterozygous maternal deletions within GNAS or STX16, which are associated with loss-of-methylation (LOM) at exon A/B alone or at all maternally methylated GNAS exons. LOM at exon A/B and the resulting biallelic expression of A/B transcripts reduces Gsα expression, thus leading to hormonal resistance. Epigenetic changes at all differentially methylated GNAS regions are also observed in sporadic PHP1B, the most frequent disease variant, which remains unresolved at the molecular level, except for rare cases with paternal uniparental isodisomy or heterodisomy of chromosome 20q (patUPD20q).

GNAS Spectrum of Disorders

The GNAS complex locus encodes the alpha-subunit of the stimulatory G protein (Gsα), a ubiquitous signaling protein mediating the actions of many hormones, neurotransmitters, and paracrine/autocrine factors via generation of the second messenger cAMP. GNAS gives rise to other gene products, most of which exhibit exclusively monoallelic expression. In contrast, Gsα is expressed biallelically in most tissues; however, paternal Gsα expression is silenced in a small number of tissues through as-yet-poorly understood mechanisms that involve differential methylation within GNAS. Gsα-coding GNAS mutations that lead to diminished Gsα expression and/or function result in Albright's hereditary osteodystrophy (AHO) with or without hormone resistance, i.e., pseudohypoparathyroidism type-Ia/Ic and pseudo-pseudohypoparathyroidism, respectively. Microdeletions that alter GNAS methylation and, thereby, diminish Gsα expression in tissues in which the paternal Gsα allele is normally silenced also cause hormone resistance, which occurs typically in the absence of AHO, a disorder termed pseudohypoparathyroidism type-Ib. Mutations of GNAS that cause constitutive Gsα signaling are found in patients with McCune-Albright syndrome, fibrous dysplasia of bone, and different endocrine and non-endocrine tumors. Clinical features of these diseases depend significantly on the parental allelic origin of the GNAS mutation, reflecting the tissue-specific paternal Gsα silencing. In this article, we review the pathogenesis and the phenotypes of these human diseases.

G(olf) in the basal ganglia

All neurotransmitter and hormone receptors that stimulate adenylyl cyclase are thought to do so via the alpha subunit of the guanine nucleotide binding (G) protein G(s). The basal ganglia contain a well-characterized dopamine-stimulated adenylyl cyclase and D1 dopamine receptors coupled to G(s) are thought to mediate this activity. We have found using immunohistochemistry, immunoblotting, and cholera toxin-dependent ADP ribosylation that the rat basal ganglia contain very high levels of a G(salpha)-like protein; however, it is distinct from the G(s) in other brain regions. Furthermore, in situ hybridization and Northern blot studies showed that the striatum contains remarkably low levels of G(salpha) mRNA. G(olf) is a G protein recently cloned from olfactory sensory neurons which can also stimulate adenylyl cyclase. We have now discovered high levels of G(olf) mRNA expression in the striatum, nucleus accumbens, and olfactory tubercle. Northern blot analyses indicate that in the striatum, G(olf) transcripts are approximately 10-fold more abundant than G(salpha) transcripts. Thus G(olf) is not an olfactory neuronspecific G protein. It is also the major stimulatory G protein in the basal ganglia, where it may couple D 1 dopamine receptors to adenylyl cyclase.

Loss of XLαs (extra-large αs) imprinting results in early postnatal hypoglycemia and lethality in a mouse model of pseudohypoparathyroidism Ib

Maternal deletion of the NESP55 differentially methylated region (DMR) (delNESP55/ASdel3-4(m), delNAS(m)) from the GNAS locus in humans causes autosomal dominant pseudohypoparathyroidism type Ib (AD-PHP-Ib(delNASm)), a disorder of proximal tubular parathyroid hormone (PTH) resistance associated with loss of maternal GNAS methylation imprints. Mice carrying a similar, maternally inherited deletion of the Nesp55 DMR (ΔNesp55(m)) replicate these Gnas epigenetic abnormalities and show evidence for PTH resistance, yet these mice demonstrate 100% mortality during the early postnatal period. We investigated whether the loss of extralarge αs (XLαs) imprinting and the resultant biallelic expression of XLαs are responsible for the early postnatal lethality in ΔNesp55(m) mice. First, we found that ΔNesp55(m) mice are hypoglycemic and have reduced stomach-to-body weight ratio. We then generated mice having the same epigenetic abnormalities as the ΔNesp55(m) mice but with normalized XLαs expression due to the paternal disruption of the exon giving rise to this Gnas product. These mice (ΔNesp55(m)/Gnasxl(m+/p-)) showed nearly 100% survival up to postnatal day 10, and a substantial number of them lived to adulthood. The hypoglycemia and reduced stomach-to-body weight ratio observed in 2-d-old ΔNesp55(m) mice were rescued in the ΔNesp55(m)/Gnasxl(m+/p-) mice. Surviving double-mutant animals had significantly reduced Gαs mRNA levels and showed hypocalcemia, hyperphosphatemia, and elevated PTH levels, thus providing a viable model of human AD-PHP-Ib. Our findings show that the hypoglycemia and early postnatal lethality caused by the maternal deletion of the Nesp55 DMR result from biallelic XLαs expression. The double-mutant mice will help elucidate the pathophysiological mechanisms underlying AD-PHP-Ib.

Parathyroid hormone signaling via Gαs is selectively inhibited by an NH(2)-terminally truncated Gαs: implications for pseudohypoparathyroidism

Pseudohypoparathyroid patients have resistance predominantly to parathyroid hormone (PTH), and here we have examined the ability of an alternative Gαs-related protein to inhibit Gαs activity in a hormone-selective manner. We tested whether the GNAS exon A/B-derived NH(2)-terminally truncated (Tr) αs protein alters stimulation of adenylate cyclase by the PTH receptor (PTHR1), the thyroid-stimulating hormone (TSH) receptor (TSHR), the β(2)-adrenergic receptor (β(2)AR), or the AVP receptor (V2R). HEK293 cells cotransfected with receptor and full-length (FL) Gαs ± Tr αs protein expression vectors were stimulated with agonists (PTH [10(-7) to 10(-9)  M], TSH [1 to 100 mU], isoproterenol [10(-6) to 10(-8)  M], or AVP [10(-6) to 10(-8)  M]). Following PTH stimulation, HEK293 cells cotransfected with PTHR1 + FL Gαs + Tr αs had a significantly lower cAMP response than those transfected with only PTHR1 + FL Gαs. Tr αs also exerted an inhibitory effect on the cAMP levels stimulated by TSH via the TSHR but had little or no effect on isoproterenol or AVP acting via β(2)AR or V2R, respectively. These differences mimic the spectrum of hormone resistance in pseudohypoparathyroidism type 1a (PHP-1a) and type 1b (PHP-1b) patients. In opossum kidney (OK) cells, endogenously expressing the PTHR1 and β(2)AR, the exogenous expression of Tr αs at a level similar to endogenous FL Gαs resulted in blunting of the cAMP response to PTH, whereas that to isoproterenol was unaltered. A pseudopseudohypoparathyroid patient with Albright hereditary osteodystrophy harbored a de novo paternally inherited M1I Gαs mutation. Similar maternally inherited mutations at the initiation codon have been identified previously in PHP-1a patients. The M1I αs mutant (lacking the first 59 amino acids of Gαs) blunted the increase in cAMP levels stimulated via the PTHR1 in both HEK293 and OK cells similar to the Tr αs protein. Thus NH(2)-terminally truncated forms of Gαs may contribute to the pathogenesis of pseudohypoparathyroidism by inhibiting the activity of Gαs itself in a GPCR selective manner.

A novel aspect of GNAS imprinting: higher maternal expression of Gαs in human lymphoblasts, peripheral blood mononuclear cells, mammary adipose tissue, and heart

The human GNAS gene is imprinted in a tissue-specific manner, being expressed primarily from the maternal allele in pituitary, thyroid, renal proximal tubules, and gonads, but is supposed to be biallelically expressed with an equal allelic expression in most other tissues. We analysed allelic expression of Gαs using Pyrosequencing. By genotyping the GNAS T393C polymorphism we quantified mRNA transcripts in lymphoblasts (Ly, n=11), peripheral blood mononuclear cells (PBMC, n=18), mammary adipose tissue (MAT, n=23) and heart tissue (HT, n=44). Allelic expression analysis revealed an unequal allelic expression (ratio maternal/total×100±SEM: 55.7±1% (95% CI 53.4-58.1%) in Ly, 56.1±0.8 (95% CI 54.5-57.7%) in PBMC, 54.5±0.8% (95% CI 53-56.1%) in MAT and 54.1±0.6% (95% CI 53-55.3%) in HT). Maternal ratio differed significantly from the mean (p<0.0001). This phenomenon may be a general feature existing in all tissues.

Effects of deficiency of the G protein Gsα on energy and glucose homeostasis

G(s)α is a ubiquitously expressed G protein α-subunit that couples receptors to the generation of intracellular cyclic AMP. The G(s)α gene GNAS is a complex gene that undergoes genomic imprinting, an epigenetic phenomenon that leads to differential expression from the two parental alleles. G(s)α is imprinted in a tissue-specific manner, being expressed primarily from the maternal allele in a small number of tissues. Albright hereditary osteodystrophy is a monogenic obesity disorder caused by heterozygous G(s)α mutations but only when the mutations are maternally inherited. Studies in mice indicate a similar parent-of-origin effect on energy and glucose metabolism, with maternal but not paternal mutations leading to obesity, reduced sympathetic nerve activity and energy expenditure, glucose intolerance and insulin resistance, with no primary effect on food intake. These effects result from G(s)α imprinting leading to severe G(s)α deficiency in one or more regions of the central nervous system, and are associated with a specific defect in melanocortins to stimulate sympathetic nerve activity and energy expenditure.

Diseases associated with genomic imprinting

Genomic imprinting is the phenomenon where the expression of a locus differs between the maternally and paternally inherited alleles. Typically, this manifests as transcriptional silencing of one of the alleles, although many genes are imprinted in a tissue- or isoform-specific manner. Diseases associated with imprinted genes include various cancers, disorders of growth and metabolism, and disorders in neurodevelopment, cognition, and behavior, including certain major psychiatric disorders. In many cases, the disease phenotypes associated with dysfunction at particular imprinted loci can be understood in terms of the evolutionary processes responsible for the origin of imprinting. Imprinted gene expression represents the outcome of an intragenomic evolutionary conflict, where natural selection favors different expression strategies for maternally and paternally inherited alleles. This conflict is reasonably well understood in the context of the early growth effects of imprinted genes, where paternally inherited alleles are selected to place a greater demand on maternal resources than are maternally inherited alleles. Less well understood are the origins of imprinted gene expression in the brain, and their effects on cognition and behavior. This chapter reviews the genetic diseases that are associated with imprinted genes, framed in terms of the evolutionary pressures acting on gene expression at those loci. We begin by reviewing the phenomenon and evolutionary origins of genomic imprinting. We then discuss diseases that are associated with genetic or epigenetic defects at particular imprinted loci, many of which are associated with abnormalities in growth and/or feeding behaviors that can be understood in terms of the asymmetric pressures of natural selection on maternally and paternally inherited alleles. We next described the evidence for imprinted gene effects on adult cognition and behavior, and the possible role of imprinted genes in the etiology of certain major psychiatric disorders. Finally, we conclude with a discussion of how imprinting, and the evolutionary-genetic conflicts that underlie it, may enhance both the frequency and morbidity of certain types of diseases.

The role of GNAS and other imprinted genes in the development of obesity

Genomic imprinting is an epigenetic phenomenon affecting a small number of genes, which leads to differential expression from the two parental alleles. Imprinted genes are known to regulate fetal growth and a 'kinship' or 'parental conflict' model predicts that paternally and maternally expressed imprinted genes promote and inhibit fetal growth, respectively. In this review we examine the role of imprinted genes in postnatal growth and metabolism, with an emphasis on the GNAS/Gnas locus. GNAS is a complex imprinted locus with multiple oppositely imprinted gene products, including the G-protein alpha-subunit G(s)alpha that is expressed primarily from the maternal allele in some tissues and the G(s)alpha isoform XLalphas that is expressed only from the paternal allele. Maternal, but not paternal, G(s)alpha mutations lead to obesity in Albright hereditary osteodystrophy. Mouse studies show that this phenomenon is due to G(s)alpha imprinting in the central nervous system leading to a specific defect in the ability of central melanocortins to stimulate sympathetic nervous system activity and energy expenditure. In contrast mutation of paternally expressed XLalphas leads to opposite metabolic effects in mice. Although these findings conform to the 'kinship' model, the effects of other imprinted genes on body weight regulation do not conform to this model.

Genetics of pseudohypoparathyroidism: bases for proper genetic counselling

Pseudohypoparathyroidism (PHP) is characterized by hypocalcemia and hyperphosphatemia due to resistance to parathyroid hormone (PTH). Patients with PHP-Ia often show additional hormone resistance and characteristic physical features that are collectively termed Albright's hereditary osteodystrophy (AHO). These features are also present in pseudopseudohypoparathyroidism (PPHP), but patients with this disorder do not show hormone resistance. PHP-Ib patients, on the other hand, predominantly show renal PTH resistance and lack features of AHO. From the genetic point of view, PHP-I is caused by defects in the GNAS gene or in the 5' region of this gene locus. PHP-Ia is caused by heterozygous inactivating mutations in any of the 13 exons codifying the alpha subunit of the stimulatory guanine nucleotide-binding protein (Gsα), while PHP-Ib is due to alterations in the methylation pattern of the 5' regions of the locus, usually associated with upstream microdeletions that are maternally transmitted. The imprinting pattern that affects the GNAS locus has important implications for the inheritance pattern and consequently for appropriate genetic counselling.

GNAS defects identified by stimulatory G protein alpha-subunit signalling studies in platelets

CONTEXT

GNAS is an imprinted region that gives rise to several transcripts, antisense transcripts, and noncoding RNAs, including transcription of RNA encoding the alpha-subunit of the stimulatory G protein (Gsalpha). The complexity of the GNAS cluster results in ubiquitous genomic imprints, tissue-specific Gsalpha expression, and multiple genotype-phenotype relationships. Phenotypes resulting from genetic and epigenetic abnormalities of the GNAS region include Albright's hereditary osteodystrophy, pseudohypoparathyroidism types Ia (PHPIa) and Ib (PHPIb), and pseudopseudohypoparathyroidism (PPHP).

OBJECTIVE

The aim was to study the complex GNAS pathology by a functional test as an alternative to the generally used but labor-intensive erythrocyte complementation assay.

DESIGN AND PATIENTS

We report the first platelet-based diagnostic test for Gsalpha hypofunction, supported by clinical, biochemical, and molecular data for six patients with PHPIa or PPHP and nine patients with PHPIb. The platelet test is based on the inhibition of platelet aggregation by cAMP, produced after Gsalpha stimulation.

RESULTS

Platelets are easily accessible, and platelet aggregation responses were found to reflect Gsalpha signaling defects in patients, in concordance with the patient's phenotype and genotype. Gsalpha hypofunction in PHPIa and PPHP patients with GNAS mutations was clearly detected by this method. Mildly decreased or normal Gsalpha function was detected in patients with PHPIb with either an overall or exon 1A-only epigenetic defect, respectively. Platelet Gsalpha expression was reduced in both PHPIb patient groups, whereas XLalphas was up-regulated only in PHPIb patients with the broad epigenetic defect.

CONCLUSION

The platelet-based test is a novel tool for establishing the diagnosis of Gsalpha defects, which may otherwise be quite challenging.

New mutation type in pseudohypoparathyroidism type Ia

CONTEXT

The GNAS gene encodes the alpha-subunit of the stimulatory G proteins, which play a crucial role in intracellular signal transduction of peptide and neurotransmitter receptors. Heterozygous inactivating maternally inherited mutations of GNAS (including translation initiation mutations, amino acid substitutions, nonsense mutations, splice site mutations and small insertions or deletions) lead to a phenotype in which Albright hereditary osteodystrophy is associated with pseudohypoparathyroidism type Ia.

OBJECTIVE

We sought to identify the molecular defect in a patient who was thought to have PHP-Ia.

METHODS AND RESULTS

The GNAS gene of a 5-year-old boy with brachydactily, mental retardation, pseudohypoparathyroidism and congenital hypothyroidism was investigated. We found a heterozygous inversion of exon 2 and part of intron 1 of de novo origin. Molecular studies of cDNA from blood RNA demonstrated that both the normal and the mutant variants were stable and that new splice-sites were generated.

CONCLUSION

This report demonstrates the first evidence for an inversion at the GNAS gene responsible of pseudohypoparathyroidism type Ia.

A disruptive mutation in exon 3 of the GNAS gene with albright hereditary osteodystrophy, normocalcemic pseudohypoparathyroidism, and selective long transcript variant Gsalpha-L deficiency

OBJECTIVE

The GNAS gene encodes the alpha-subunit of stimulatory G proteins, which play a crucial role in intracellular signal transduction of peptide and neurotransmitter receptors. In addition to transcript variants that differ in their first exon due to different promoters, there are two long (Gsalpha-L) and two short (Gsalpha-S) splice variants, created by alternative splicing. Heterozygous inactivating maternally inherited mutations of GNAS lead to a phenotype in which Albright hereditary osteodystrophy is associated with pseudohypoparathyroidism type Ia.

METHODS AND RESULTS

The GNAS gene of a 10-yr-old girl with brachymetacarpia, mental retardation, normocalcemic pseudohypoparathyroidism, and hypothyroidism was investigated. We found a heterozygous insertion of an adenosine in exon 3 altering codon 85 and leading to a frame shift inducing a stop codon in exon 4. Molecular studies of cDNA from blood RNA demonstrated normal, biallelic expression of Gsalpha-S transcripts, whereas expression of Gsalpha-L transcripts from the maternal allele was reduced. Immunoblot analysis revealed a reduced Gsalpha-L protein level to about 50%, whereas the protein level of Gsalpha-S was unaltered. Furthermore, the Gsalpha protein activity in erythrocyte membranes was diminished to about 75% of normal. Both the reduced activity and the mutation were also found in the mother and the affected younger brother.

CONCLUSION

This report demonstrates the first evidence for a pathogenic mutation in exon 3 of the GNAS gene. The mutation is associated with a phenotype of Albright hereditary osteodystrophy and pseudohypoparathyroidism type Ia due to selective deficiency of Gsalpha-L and a partial reduction of Gsalpha activity.

Tissue-specific reactivation of gene expression at an imprinted locus

Genomic imprinting is the phenomenon where the expression pattern of an allele at a locus differs depending on the allele's parent of origin. In most cases, one of the two alleles is transcriptionally silent. Recent empirical work has shown some genes to be imprinted in a tissue-specific manner, where the silenced allele becomes reactivated in particular cell lineages during development. Here I describe an evolutionary model of tissue-specific transcriptional reactivation. The model describes the relationships among various inclusive fitness functions and phenotypic effects necessary for natural selection to favor the epigenetic reprogramming required for this sort of reactivation, and makes predictions regarding the nature and magnitude of phenotypic and fitness consequences of mutations in particular somatic tissues. In particular, if an imprinted gene is reactivated in one of two tissues that interact in producing a particular phenotype, expression of the gene in those two tissues is expected to have opposite phenotypic effects. The model predicts that in some cases, mutations affecting the silenced allele at an imprinted locus may be phenotypically more severe than those affecting the expressed allele. These predictions are contrasted with those of an alternative explanation for reactivation: protection against deleterious recessive somatic mutations. The inclusive-fitness model of reactivation indicates that the intragenomic conflicts present in the parental germ lines and developing embryo persist though adult life, and can have complex effects on phenotypes and patterns of gene expression in somatic tissues.

Imprinting the Gnas locus

Gnas is an enigmatic and rather complex imprinted gene locus. A single transcription unit encodes three, and possibly more, distinct proteins. These are determined by overlapping transcripts from alternative promoters with different patterns of imprinting. The canonical Gnas transcript codes for Gsalpha, a highly conserved signalling protein and an essential intermediate in growth, differentiation and homeostatic pathways. Monoallelic expression of Gnas is highly tissue-restricted. The alternative transcripts encode XLalphas, an unusual variant of Gsalpha, and the chromogranin-like protein Nesp55. These transcripts are expressed specifically from the paternal and maternal chromosomes, respectively. Their existence in the Gnas locus might imply functional connections amongst them or with Gsalpha. In this review, we consider how imprinting of Gnas was discovered, the phenotypic consequences of mutations in each of the gene products, both in the mouse and human, and provide some conjectures to explain why this elaborate imprinted locus has evolved in this manner in mammals.

Mammalian G proteins and their cell type specific functions

Heterotrimeric G proteins are key players in transmembrane signaling by coupling a huge variety of receptors to channel proteins, enzymes, and other effector molecules. Multiple subforms of G proteins together with receptors, effectors, and various regulatory proteins represent the components of a highly versatile signal transduction system. G protein-mediated signaling is employed by virtually all cells in the mammalian organism and is centrally involved in diverse physiological functions such as perception of sensory information, modulation of synaptic transmission, hormone release and actions, regulation of cell contraction and migration, or cell growth and differentiation. In this review, some of the functions of heterotrimeric G proteins in defined cells and tissues are described.

A novel STX16 deletion in autosomal dominant pseudohypoparathyroidism type Ib redefines the boundaries of a cis-acting imprinting control element of GNAS

A unique heterozygous 3-kb microdeletion within STX16, a closely linked gene centromeric of GNAS, was previously identified in multiple unrelated kindreds as a cause of autosomal dominant pseudohypoparathyroidism type Ib (AD-PHP-Ib). We now report a novel heterozygous 4.4-kb microdeletion in a large kindred with AD-PHP-Ib. Affected individuals from this kindred share an epigenetic defect that is indistinguishable from that observed in patients with AD-PHP-Ib who carry the 3-kb microdeletion in the STX16 region (i.e., an isolated loss of methylation at GNAS exon A/B). The novel 4.4-kb microdeletion overlaps with the previously identified deletion by 1,286 bp and, similar to the latter deletion, removes several exons of STX16 (encoding syntaxin-16). Because these microdeletions lead to AD-PHP-Ib only after maternal transmission, we analyzed expression of this gene in lymphoblastoid cells of affected individuals with the 3-kb or the 4.4-kb microdeletion, an individual with a NESP55 deletion, and a healthy control. We found that STX16 mRNA was expressed in all cases from both parental alleles. Thus, STX16 is apparently not imprinted, and a loss-of-function mutation in one allele is therefore unlikely to be responsible for this disorder. Instead, the region of overlap between the two microdeletions likely harbors a cis-acting imprinting control element that is necessary for establishing and/or maintaining methylation at GNAS exon A/B, thus allowing normal G alpha(s) expression in the proximal renal tubules. In the presence of either of the two microdeletions, parathyroid hormone resistance appears to develop over time, as documented in an affected individual who was diagnosed at birth with the 4.4-kb deletion of STX16 and who had normal serum parathyroid hormone levels until the age of 21 mo.

GNAS locus and pseudohypoparathyroidism

Pseudohypoparathyroidism (PHP) is characterized by hypocalcemia and hyperphosphatemia due to resistance to parathyroid hormone (PTH). Patients with PHP-Ia often present with additional hormonal resistance and show characteristic physical features that are collectively termed Albright's hereditary osteodystrophy (AHO). These features are also present in pseudopseudohypoparathyroidism (PPHP), but patients affected by this disorder do not show hormone resistance. PHP-Ib patients, on the other hand, present predominantly with renal PTH resistance and lack any features of AHO. Most of these PHP forms are caused by defects in GNAS (20q13.3), an imprinted gene locus with multiple transcriptional units. PHP-Ia and PPHP are caused by heterozygous inactivating mutations in those exons of GNAS encoding the alpha subunit of the stimulatory guanine nucleotide-binding protein (Gsalpha), and the autosomal dominant form of PHP-Ib (AD-PHP-Ib) is caused by heterozygous mutations disrupting a long-range imprinting control element of GNAS. Expressed nearly in all cells, Gsalpha plays essential roles in a multitude of physiological processes. Its expression in renal proximal tubules occurs predominantly from the maternal allele, and this tissue- and parent-specific imprinting of Gsalpha is an important determinant of hormone resistance in kindreds with PHP-Ia/PPHP and AD-PHP-Ib.

Genetics of pituitary tumors: Focus on G-protein mutations

In recent years the demonstration that human pituitary adenomas are monoclonal in origin has provided further evidence that pituitary neoplasia arise from the replication of a single mutated cell in which growth advantage results from either activation of proto-oncogenes or inactivation of tumor suppressor genes. While common oncogenes, such as Ras, are only exceptionally involved, the only mutations identified in a significant proportion of pituitary tumors, and particular in GH-secreting adenomas, occur in the Gsalpha gene (GNAS1) and cause constitutive activation of the cAMP pathway (gsp oncogene). Moreover, pituitary tumors overexpress hypothalamic releasing hormones, growth factors, and their receptors as well as cyclins involved in cell cycle progression. As far as the role of tumor suppressor genes in pituitary tumorigenesis is concerned, reduced expression of these genes seems to frequently occur in pituitary tumors as a consequence of abnormal methylation processes. Although the only mutational change so far identified in pituitary tumors is the gsp oncogene, this oncogene is not associated with a clear phenotype in patients bearing positive tumors. Mechanisms able to counteract the cAMP pathway, such as high sensitivity to somatostatin, and induction of genes with opposite actions, such as phosphodiesterases, CREB end ICER, or instability of mutant Gsalpha, have been proposed to account for the lack of genotype/phenotype relationships.

Autosomal dominant pseudohypoparathyroidism type Ib is associated with a heterozygous microdeletion that likely disrupts a putative imprinting control element of GNAS

Patients with pseudohypoparathyroidism type Ib (PHP-Ib) have hypocalcemia and hyperphosphatemia due to renal parathyroid hormone (PTH) resistance, but lack physical features of Albright hereditary osteodystrophy. PHP-Ib is thus distinct from PHP-Ia, which is caused by mutations in the GNAS exons encoding the G protein alpha subunit. However, an imprinted autosomal dominant form of PHP-Ib (AD-PHP-Ib) has been mapped to a region of chromosome 20q13.3 containing GNAS. Furthermore, loss of methylation at a differentially methylated region (DMR) of this locus, exon A/B, has been observed thus far in all investigated sporadic PHP-Ib cases and the affected members of multiple AD-PHP-Ib kindreds. We now report that affected members and obligate gene carriers of 12 unrelated AD-PHP-Ib kindreds and four apparently sporadic PHP-Ib patients, but not healthy controls, have a heterozygous approximately 3-kb microdeletion located approximately 220 kb centromeric of GNAS exon A/B. The deleted region, which is flanked by two direct repeats, includes three exons of STX16, the gene encoding syntaxin-16, for which no evidence of imprinting could be found. Affected individuals carrying the microdeletion show loss of exon A/B methylation but no epigenetic abnormalities at other GNAS DMRs. We therefore postulate that this microdeletion disrupts a putative cis-acting element required for methylation at exon A/B, and that this genetic defect underlies the renal PTH resistance in AD-PHP-Ib.

Altered CTX-catalyzed and endogenous [32P]ADP-ribosylation of stimulatory G protein alphas isoforms in postmortem bipolar affective disorder temporal cortex

Reports of elevated Gs alpha subunit (alpha(s)) immunolabeling and cAMP-mediated hyper-functionality in autopsied cerebral cortical brain regions from bipolar affective disorder (BD) patients suggest signal transduction abnormalities occur in this disorder. Because covalent modification of alpha(s) can affect its turnover and levels, we determined whether CTX-catalyzed and endogenous [(32)P] adenosine diphosphate (ADP)-ribosylation of alpha(s) isoforms are altered in temporal and occipital cortical regions, which show elevated alpha(s) levels in BD as compared to nonpsychiatric subjects. Reduced CTX-catalyzed [(32)P]ADP-ribosylated alpha(s-S) and endogenous [(32)P]ADP-ribosylation of a 39-kDa alpha(s)-like protein were found in BD temporal cortex compared to controls. These findings suggest that clearance of these alpha(s) isoforms through ADP-ribosylation may be decreased in BD temporal cortex. Although no differences were observed in mean levels of endogenous and CTX-catalyzed [(32)P]ADP-ribosylation of alpha(s-L) in BD temporal cortex, alpha(s-L) immunolabeling was elevated significantly and correlated inversely with the degree of endogenous [(32)P]ADP-ribosylation of this subunit. In addition, endogenous [(32)P]ADP-ribosylation of an exogenous substrate, myelin basic protein, was similar in BD and comparison subject temporal cortex. Taken together, these observations suggest that elevations of alpha(s) in BD brain are more likely related to factors affecting the disposition or availability of alpha(s) to this posttranslational enzymatic modification.

The pseudohypoparathyroidism type lb locus is linked to a region including GNAS1 at 20q13.3

Pseudohypoparathyroidism (PHP) is characterized by biochemical hypoparathyroidism with elevated parathyroid hormone levels owing to reduced target tissue responsiveness to parathyroid hormone. Patients with PHP la have somatic defects termed Albright's hereditary osteodystrophy (AHO) and exhibit resistance to additional hormones because of heterozygous mutations in the GNAS1 gene that lead to a generalized deficiency of the a subunit of Gs, the heterotrimeric G protein that couples receptors to adenylyl cyclase. By contrast, patients with PHP 1b lack AHO and have selective parathyroid hormone (PTH) resistance, presumably because of an imprinting defect that impairs expression of G(s)alpha in the proximal renal tubule. Although an epigenetic defect in GNAS1 has been identified in subjects with PHP1b, the genetic defect is unknown. To define the genetic defect in PHP 1b, we performed a genome-wide linkage analysis in five multi-generational PHP lb families. Of the 408 polymorphic microsatellite markers examined, markers located on chromosome 20q13.3, the region containing GNAS1, demonstrated linkage to PHP lb. Fine-mapping and multipoint linkage analysis of this region demonstrated linkage to a 5.7-cM region between 907rep2 and the telomere. Haplotype analysis established that affected individuals shared a 5-cM region including part of the GNAS1 gene to the telomere. Our data confirm that PHP1b is linked to a region that includes GNAS1, and further refine the locus, although the primary genetic mutation(s) that causes defective imprinting of GNAS1 remains undefined.

Receptor-mediated adenylyl cyclase activation through XLalpha(s), the extra-large variant of the stimulatory G protein alpha-subunit

XLalpha(s), the large variant of the stimulatory G protein alpha subunit (Gsalpha), is derived from GNAS1 through the use of an alternative first exon and promoter. Gs(alpha) and XLalpha(s) have distinct amino-terminal domains, but are identical over the carboxyl-terminal portion encoded by exons 2-13. XLalpha(s) can mimic some functions of Gs(alpha), including betagamma interaction and adenylyl cyclase stimulation. However, previous attempts to demonstrate coupling of XLalpha(s) to typically Gs-coupled receptors have not been successful. We now report the generation of murine cell lines that carry homozygous disruption of Gnas exon 2, and are therefore null for endogenous XLalpha(s) and Gs(alpha) (Gnas(E2-/E2-)). Gnas(E2-/E2-) cells transfected with plasmids encoding XLalpha(s) and different heptahelical receptors, including the beta2-adrenergic receptor and receptors for PTH, TSH, and CRF, showed agonist-mediated cAMP accumulation that was indistinguishable from that observed with cells transiently coexpressing Gs(alpha) and these receptors. Our findings thus indicate that XLalpha(s) is capable of functionally coupling to receptors that normally act via Gs(alpha).

The p66Shc longevity gene is silenced through epigenetic modifications of an alternative promoter

The mammal Shc locus encodes three overlapping isoforms (46, 52, and 66 kDa) that differ in the length of their N-terminal regions. p46/p52Shc and p66Shc have been implicated, respectively, in the cytoplasmic propagation of growth and apoptogenic signals. Levels of p66Shc expression correlate with life span duration in mice. p46Shc and p52Shc are ubiquitously expressed, whereas p66Shc is expressed in a cell lineage-specific fashion. However, the mechanisms underlying the regulation of Shc protein expression are unknown. Here we report the identification of two alternative promoters, driving the transcription of two mRNAs coding for p46/p52Shc and p66Shc. We show that treatment with an inhibitor of histone deacetylases or with a demethylating agent results in induction of p66Shc expression in cells that normally do not express this isoform but leaves the levels of the two other isoforms unchanged. Moreover, analysis of the methylation pattern of the p66Shc promoter in a panel of primary and immortalized human cells showed inverse correlation between p66Shc expression and methylation density of its promoter. These results identify histone deacetylation and cytosine methylation as the mechanisms underlying p66Shc silencing in nonexpressing cells.

Gs(alpha) mutations and imprinting defects in human disease

Gs is the ubiquitously expressed heterotrimeric G protein that couples receptors to the effector enzyme adenylyl cyclase and is required for receptor-stimulated intracellular cAMP generation. Activated receptors promote the exchange of GTP for GDP on the Gs alpha-subunit (Gs(alpha)), resulting in Gs activation; an intrinsic GTPase activity of Gs(alpha) deactivates Gs by hydrolyzing bound GTP to GDP. Mutations of Gs(alpha) residues involved in the GTPase reaction that lead to constitutive activation are present in endocrine tumors, fibrous dysplasia of bone, and McCune-Albright syndrome. Heterozygous loss-of-function mutations lead to Albright hereditary osteodystrophy (AHO), a disease characterized by short stature, obesity, and skeletal defects, and are sometimes associated with progressive osseous heteroplasia. Maternal transmission of Gs(alpha) mutations leads to AHO plus resistance to several hormones (e.g., parathyroid hormone) that activate Gs in their target tissues (pseudohypoparathyroidism type IA), while paternal transmission leads only to the AHO phenotype (pseudopseudohypoparathyroidism). Studies in both mice and humans demonstrate that Gs(alpha) is imprinted in a tissue-specific manner, being expressed primarily from the maternal allele in some tissues and biallelically expressed in most other tissues. This likely explains why multihormone resistance occurs only when Gs(alpha) mutations are inherited maternally. The Gs(alpha) gene GNAS1 has at least four alternative promoters and first exons, leading to the production of alternative gene products including Gs(alpha), XL alphas (a novel Gs(alpha) isoform expressed only from the paternal allele), and NESP55 (a chromogranin-like protein expressed only from the maternal allele). The fourth alternative promoter and first exon (exon 1A) located just upstream of the Gs(alpha) promoter is normally methylated on the maternal allele and is transcriptionally active on the paternal allele. In patients with parathyroid hormone resistance but without AHO (pseudohypoparathyroidism type IB), the exon 1A promoter region is unmethylated and transcriptionally active on both alleles. This GNAS1 imprinting defect is predicted to decrease Gs(alpha) expression in tissues where Gs(alpha) is normally imprinted and therefore to lead to renal parathyroid hormone resistance.

Endocrine manifestations of stimulatory G protein alpha-subunit mutations and the role of genomic imprinting

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 metabolism.

Galphas transcripts are biallelically expressed in the human kidney cortex: implications for pseudohypoparathyroidism type 1b

Pseudohypoparathyroid type 1b patients are characterized by renal resistance to PTH in the absence of Albright's hereditary osteodystrophy or other endocrine abnormalities. Kindred studies have suggested that the cause of this resistance is a specific decrease in Galphas activity in renal proximal tubules due to paternal imprinting of Galphas. To test this, allelic expression of Galphas was analyzed in human fetal kidney cortex samples by RT-PCR assays. The results showed that, in contrast to the parent-specific expression of exon 1A and XLalphas (paternal) or NESP (maternal) mRNAs, Galphas transcripts are biallelically expressed in human kidney cortex. These data implicate abnormal imprinting of alternative regions within the GNAS1 locus as a more likely cause of pseudohypoparathyroid type 1b.

A GNAS1 imprinting defect in pseudohypoparathyroidism type IB

Pseudohypoparathyroidism type IB (PHPIB) is characterized by renal resistance to parathyroid hormone (PTH) and the absence of other endocrine or physical abnormalities. Familial PHPIB has been mapped to 20q13, near GNAS1, which encodes G(s)alpha, the G protein alpha-subunit required for receptor-stimulated cAMP generation. However, G(s)alpha function is normal in blood cells from PHPIB patients, ruling out mutations within the G(s)alpha coding region. In mice G(s)alpha is expressed only from the maternal allele in renal proximal tubules (the site of PTH action) but is biallelically expressed in most other tissues. Studies in patients with Albright hereditary osteodystrophy suggest a similar G(s)alpha imprinting pattern in humans. Here we identify a region upstream of the G(s)alpha promoter that is normally methylated on the maternal allele and unmethylated on the paternal allele, but that is unmethylated on both alleles in all 13 PHPIB patients studied. Within this region is an alternative promoter and first exon (exon 1A), generating transcripts that are normally expressed only from the paternal allele, but that are biallelically expressed in PHPIB patients. Therefore, PHPIB is associated with a paternal-specific imprinting pattern of the exon 1A region on both alleles, which may lead to decreased G(s)alpha expression in renal proximal tubules. We propose that loss of exon 1A imprinting is the cause of PHPIB.

Identification of a methylation imprint mark within the mouse Gnas locus

The imprinted mouse gene Gnas produces the G protein alpha-subunit G(S)alpha and several other gene products by using alternative promoters and first exons. G(S)alpha is maternally expressed in some tissues and biallelically expressed in most other tissues, while the gene products NESP55 and XLalphas are maternally and paternally expressed, respectively. We investigated the mechanisms of Gnas imprinting. The G(S)alpha promoter and first exon are not methylated on either allele. A further upstream region (approximately from positions -3400 to -939 relative to the G(S)alpha translational start site) is methylated only on the maternal allele in all adult somatic tissues and in early postimplantation development. Within this region lies a fourth promoter and first exon (exon 1A) that generates paternal-specific mRNAs of unknown function. Exon 1A and G(S)alpha mRNAs have similar expression patterns, making competition between their promoters unlikely. Differential methylation in this region is established during gametogenesis, being present in oocytes and absent in spermatozoa, and is maintained in preimplantation E3. 5d blastocysts. Therefore, this region is a methylation imprint mark. In contrast, differential methylation of the NESP55 and XLalphas promoter regions (Nesp and Gnasxl) is not established during gametogenesis. The methylation imprint mark that we identified may be important for the tissue-specific imprinting of G(S)alpha.

Variable imprinting of the heterotrimeric G protein G(s) alpha-subunit within different segments of the nephron

The heterotrimeric G protein G(s) is required for hormone-stimulated intracellular cAMP generation because it couples hormone receptors to the enzyme adenylyl cyclase. Hormones that activate G(s) in the kidney include parathyroid hormone, glucagon, calcitonin, and vasopressin. Recently, it has been demonstrated that the G(s)alpha gene is imprinted in a tissue-specific manner, leading to preferential expression of G(s)alpha from the maternal allele in some tissues. In the kidney, G(s)alpha is imprinted in the proximal tubule but not in more distal nephron segments, such as the thick ascending limb or collecting duct. This most likely explains why in both humans and mice heterozygous mutations in the maternal allele lead to parathyroid hormone resistance in the proximal tubule whereas mutations in the paternal allele do not. In contrast, heterozygous mutations have little effect on vasopressin action in the collecting ducts. In mice with heterozygous null G(s)alpha mutations (both those with mutations on the maternal or paternal allele), expression of the Na-K-2Cl cotransporter was decreased in the thick ascending limb, suggesting that its expression is regulated by cAMP. The G(s)alpha genes also generate alternative, oppositely imprinted transcripts encoding XLalphas, a G(s)alpha isoform with a long NH(2)-terminal extension, and NESP55, a chromogranin-like neurosecretory protein. The role, if any, of these proteins in renal physiology is unknown.

Characterization of alpha(s)-immunoreactive ADP-ribosylated proteins in postmortem human brain

ADP-ribosylation of the stimulatory G protein alpha subunit, alpha(s), has been demonstrated in a number of different mammalian tissues. However, little is known about the occurrence and role of this process in modifying alpha(s) levels/function in human brain. In the present study, endogenous and cholera toxin (CTX)-catalyzed [32P]ADP-ribosylated products were characterized in postmortem human temporal cortex by (1) immunoprecipitation with alpha(s) antisera (RM/1), (2) comparisons of immunoblots and autoradiograms of the [32P]ADP-ribosylated products, and (3) limited protease digestion. Of the three major endogenous [32P]ADP-ribosylated products (48, 45, and 39 kDa) in postmortem brain, the 48-kDa and 45-kDa bands were clearly identified as alpha(s-L) (long isoform) and alpha(s-S) (short isoform), respectively. RM/1 immunoprecipitated the 39-kDa [32P]ADP-ribosylated protein, and overlays of immunoblots and autoradiograms showed that this product corresponded to an alpha(s)-like-immunoreactive protein. Furthermore, limited protease digestion of the 39-kDa endogenous [32P]ADP-ribosylated band generated peptide fragments similar to both endogenous and CTX-catalyzed [32P]ADP-ribosylated alpha(s-S). Two major CTX-catalyzed [32P]ADP-ribosylated products were also identified as alpha(s-L) (52 kDa) and alpha(s-S) (45 kDa). These findings clearly demonstrate that alpha(s) is a substrate for endogenous and CTX-catalyzed [32P]ADP-ribosylation in postmortem human brain. Furthermore, a lower molecular weight alpha(s)-like immunoreactive protein is also expressed in human brain and is a substrate for endogenous but not CTX-catalyzed [32P]ADP-ribosylation.

The Role of Genomic Imprinting of Galpha in the Pathogenesis of Albright Hereditary Osteodystrophy

Albright hereditary osteodystrophy (AHO) is caused by heterozygous inactivating mutations of the gene encoding the alpha-subunit of the G protein Gs. The Gsalpha gene is a complex gene that uses various alternative promoters and produces various protein products. Recently, it has been shown that this gene is imprinted in a tissue-specific manner. The role of tissue-specific imprinting of Gsalpha in the pathogenesis of AHO is discussed.

Bidirectional imprinting of a single gene: GNAS1 encodes maternally, paternally, and biallelically derived proteins

The GNAS1 gene encodes the alpha subunit of the guanine nucleotide-binding protein Gs, which couples signaling through peptide hormone receptors to cAMP generation. GNAS1 mutations underlie the hormone resistance syndrome pseudohypoparathyroidism type Ia (PHP-Ia), so the maternal inheritance displayed by PHP-Ia has raised suspicions that GNAS1 is imprinted. Despite this suggestion, in most tissues Gsalpha is biallelically encoded. In contrast, the large G protein XLalphas, also encoded by GNAS1, is paternally derived. Because the inheritance of PHP-Ia predicts the existence of maternally, rather than paternally, expressed transcripts, we have investigated the allelic origin of other mRNAs derived from GNAS1. We find this gene to be remarkable in the complexity of its allele-specific regulation. Two upstream promoters, each associated with a large coding exon, lie only 11 kb apart, yet show opposite patterns of allele-specific methylation and monoallelic transcription. The more 5' of these exons encodes the neuroendocrine secretory protein NESP55, which is expressed exclusively from the maternal allele. The NESP55 exon is 11 kb 5' to the paternally expressed XLalphas exon. The transcripts from these two promoters both splice onto GNAS1 exon 2, yet share no coding sequences. Despite their structural unrelatedness, the encoded proteins, of opposite allelic origin, both have been implicated in regulated secretion in neuroendocrine tissues. Remarkably, maternally (NESP55), paternally (XLalphas), and biallelically (Gsalpha) derived proteins all are produced by different patterns of promoter use and alternative splicing of GNAS1, a gene showing simultaneous imprinting in both the paternal and maternal directions.

Targeted disruption of Gnas in embryonic stem cells

Mutations in the gene encoding the stimulatory G protein of adenylyl cyclase (G alpha(s)) are present in subjects with Albright hereditary osteodystrophy, a syndrome of characteristic developmental defects and, in some patients, resistance to multiple hormones that stimulate cAMP accumulation (pseudohypoparathyroidism type Ia). As the first step in generating a model of Albright hereditary osteodystrophy, the gene encoding G alpha(s) (Gnas) was disrupted in mouse embryonic stem (ES) cells by homologous recombination. Northern blot analysis and immunoblot analysis demonstrated that steady-state levels of G alpha(s) messenger RNA and G alpha(s) protein in targeted ES cells were approximately 50% of levels in untargeted ES cells. In response to 10 microM forskolin and to various concentrations of isoproterenol (0.1-3.0 microM), cAMP accumulation was reduced in the G alpha(s) knockout ES cell lines, relative to wild-type ES cells and to five of six ES cell lines with randomly integrated targeting vector. These results support the role of G alpha(s) haploinsufficiency in reducing the ability of hormones to generate cAMP in subjects with pseudohypoparathyroidism type Ia. The targeted disruption of Gnas in mouse ES cells establishes an in vitro system for further studies of the role of G alpha(s) and cAMP coupled signal transduction in differentiation and development.

Molecular cloning and characterization of NESP55, a novel chromogranin-like precursor of a peptide with 5-HT1B receptor antagonist activity

The chromogranins comprise a class of acidic proteins that are secreted from large dense core vesicles and expressed in neuronal and endocrine tissues. We describe here the molecular characterization of NESP55 (neuroendocrine secretory protein of Mr 55,000), a novel member of the chromogranins. Several NESP55 cDNA clones were isolated from bovine chromaffin cell libraries. The cDNA sequence of NESP55 totals 1499 nucleotides. All of the clones that were isolated contained in their 3'-untranslated mRNA a sequence that was homologous to exon 2 of the G-protein Gsalpha. The open reading frame encodes for an acidic and hydrophilic protein of 241 amino acids with a predicted molecular mass of 27,494 Da. An antiserum directed against the C terminus of NESP55 labeled a band of Mr 55,000 with an acidic pI ranging from 4.4 to 5.2 in one- and two-dimensional immunoblots of secretory proteins from chromaffin granules. NESP55 is localized within the cell to the large dense secretory vesicles and is expressed, apart from the adrenal medulla, in the anterior and posterior pituitary and various regions of the brain. For the physiological function, one interesting factor has emerged. NESP55 is proteolytically processed within the chromaffin granule to smaller peptides that might be physiologically active. One tetrapeptide, Leu-Ser-Ala-Leu (LSAL), present in the NESP55 sequence and flanked by arginine residues suitable for cleavage by prohormone convertases, has been identified recently as an endogenous antagonist of the serotonergic 5-HT1B receptor subtype. Alterations in the serotonergic system are thought to play an important role in mental disorders, especially depression, and might be related to abnormal ethanol consumption. It is tempting to speculate that increased expression of NESP55 or its proteolytically derived peptide LSAL might contribute to the pathophysiology of the serotonergic transmission.

Clinical implications of genetic defects in G proteins. The molecular basis of McCune-Albright syndrome and Albright hereditary osteodystrophy

Inactivating and activating mutations in the gene encoding G alpha s (GNAS1) are known to be the basis for 2 well-described contrasting clinical disorders, Albright hereditary osteodystrophy (AHO) and McCune-Albright syndrome (MAS). AHO is an autosomal dominant disorder due to germline mutations in GNAS1 that decrease expression or function of G alpha s protein. Loss of G alpha s function leads to tissue resistance to multiple hormones whose receptors couple to G alpha s. By contrast, MAS results from postzygotic somatic mutations in GNAS1 that lead to enhanced function of G alpha s protein. Acquisition of the activating mutation early in life leads to a more generalized distribution of the mosaicism and is associated with the classic clinical triad of polyostotic fibrous dysplasia, endocrine hyperfunction, and café au lait skin lesions described in MAS. Acquisition of a similar activating mutation in GNAS1 later in life presumably accounts for the restricted distribution of the gsp oncogene, and is associated with the development of isolated lesions (for example, fibrous dysplasia, pituitary or thyroid tumors) without other manifestations of MAS. Tissues that are affected by loss of G alpha s function in AHO are also affected by gain of G alpha s function in MAS, thus identifying specific tissues in which the second messenger cAMP plays a dominant role in cell growth, proliferation, or function. Further investigations of the functions of G alpha s and other members of the GTPase binding protein family will provide more insight into the pathogenesis and clinical manifestations of human disease.

Molecular analysis of the multiple Golf alpha subunit mRNAs in the rat brain

The alpha subunit of GTP-binding protein Golf (G alpha olf), identified in the olfactory epithelium, in which it is supposed to mediate odorant-generated adenylyl cyclase activations, is much more expressed in the striatum than G alpha s, the classical stimulatory G protein alpha subunit (Hervé et al., J Neurosci., 13 (1993) 2237-2248). Four species of G alpha olf messenger RNA (mRNA) were observed by Northern blot in the rat striatum. Analysis of striatal G alpha olf cDNA clones demonstrated that despite extensive variations in their 5' and 3' untranslated regions, these four G alpha olf mRNAs encode the same G alpha olf polypeptide. One of the four mRNA species, which was selectively observed in the striatum, is generated by a novel promoter whose activity was barely detectable in the olfactory epithelium. Surprisingly, this promoter generates an unexpectedly high proportion of transcripts in which the first intron is unspliced. The retention of intron was found to abolish the translation of G alpha olf mRNA in the reticulocyte lysate system, suggesting that it could be involved in the down regulation of G alpha olf expression in the striatum. Furthermore, a new polyadenylation site with a non canonical sequence, AATACA, was found to be responsible of the two shortest forms of G alpha olf mRNA. In conclusion, we have shown that the G alpha olf proteins present in the striatum and the olfactory epithelium are identical and that multiple variations in the untranslated parts of the mRNAs could affect G alpha olf expression.

Melatonin receptors couple through a cholera toxin-sensitive mechanism to inhibit cyclic AMP in the ovine pituitary

The nature of melatonin receptor-G-protein coupling in ovine pars tuberalis (PT) cells of the pituitary was addressed using cholera (CTX) and pertussis (PTX) toxins. ADP-ribosylation of ovine PT membrane proteins using 32P-NAD in the presence of CTX radiolabelled several substrates including 44, 51, and 60 kD proteins. Each were clearly distinct from the 40 kD substrate radiolabelled in the presence of PTX. Acute incubation of PT membranes with either toxin reduced the number of high affinity binding sites for 125I-MEL, although the magnitude of the inhibition was much greater for CTX (56%) than for PTX (20%). A CTX-sensitive component also mediates the inhibition of forskolin-stimulated cyclic AMP accumulation as pre-treatment of PT cells with CTX (5 micrograms/ml) for 16 h blocked this response. Gs alpha is a major substrate for ADP-ribosylation by CTX, and 16 h pre-treatment of PT cells with CTX (5 micrograms/ml) caused a down-regulation of Gs alpha. Northern analysis showed only one major transcript of Gs alpha of about 2 kb, which would encompass all of the known splice variants of the Gs gene. Screening of a cDNA library from ovine PT for Gs-related genes and sequencing of clones, combined with RT-PCR of PT mRNA, revealed no novel products. On this basis it is concluded that the CTX substrate is unlikely to be a novel splice variant or related gene product of the Gs class of G-protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Overexpression of Gs alpha subunit in thyroid tumors bearing a mutated Gs alpha gene

Point mutations occurring at codon 201 of the gene coding for the alpha subunit of the stimulatory G protein impair intrinsic GTPase activity and lead to a constitutive activation of adenylate cyclase. We have examined thyroid follicular and papillary carcinomas and follicular adenomas and found samples that bear this mutation at codon 201 of the Gs alpha gene. Both mutation-positive and mutation-negative tissue samples were investigated for the level of Gs alpha expression relative to a pool of normal thyroid tissue, using immunoblotting against two (mid-region-specific and C-end-specific) antipeptide antibodies. Using 8000 g and 100,000 g membrane fractions of homogenized tissues we have demonstrated that the Gs alpha proteins in normal ad neoplastic thyroid tissues are represented by three isoforms: 43 kDa, 45 kDa and 52 kDa. We have quantified and compared the amount of Gs alpha protein and find it is overexpressed in mutation-bearing tissue samples.

XL alpha s is a new type of G protein

The GTP-binding proteins are well-known regulators of cellular functions, including vesicular transport. Cholera toxin, which is known to catalyse ADP-ribosylation of the alpha s subunit of heterotrimeric G proteins, stimulates secretory vesicle formation from the trans-Golgi network. Here we describe a new cholera toxin target, an 'extra large' G protein (XL alpha s; M(r) 92K) which consists of a new 51K XL-portion linked to a G alpha s truncated at the amino terminus. XL alpha s is specifically associated with the trans-Golgi network and occurs selectively in cells containing both the regulated and the constitutive pathway of protein secretion. Hence, XL alpha s may mediate the effects of cholera toxin on secretory vesicle formation.

Developmental changes in Gs and G(olf) proteins and adenylyl cyclases in mouse brain membranes

Guanine nucleotide-binding (G) proteins, Gs and G(olf) mediate the increase in cAMP formation through the activation of adenylyl cyclases. The developmental profiles of Gs, G(olf) and adenylyl were determined in mouse striatum and whole brain using immunobloting with specific antisera. Gs and the 115 kDa and 150 kDa adenylyl cyclases were present at the earliest age tested, embryonic day (E) 14.5 G(olf) and the 160 kDa adenylyl cyclase emerged in parallel, postnatally; during this period the increase in the relative abundance of the 150 kDa was observed. Gpp[NH]p activated Gs/G(olf) in a dose dependent manner, with a smaller response observed in embryos compared to adults. Mn2+ and forskolin activated the adenylyl cyclases and this activation increased during development. At E 14.5, maximal activation with Mn2+ and forskolin elicited a similar increase in cAMP levels, but from postnatal day 1, a nearly two fold higher response was obtained with forskolin compared to Mn2+; at the same time the 160 kDa adenylyl cyclase was detected. These data suggest that the appearance of certain forms of stimulatory G proteins was developmentally correlated with the expression of specific adenylyl cyclases.

Two alpha 1b-adrenergic receptor mRNAS expressed in Sprague-Dawley rat liver have distinct 5'-regions

Previous studies have revealed two size classes of alpha 1b-adrenergic receptor mRNAs, 3.3 kb and 2.7 kb, in the Sprague Dawley rat that are transcribed from a single gene and are expressed in approximately equal amounts in liver. Only the 2.7 kb mRNA is expressed in heart. Both alpha 1b-adrenergic receptor mRNAs appear to share extensive regions of homology, therefore, we used oligonucleotide-directed ribonuclease H mapping to detect sequence differences between the two transcripts. Initial experiments using oligo (dT)-directed RNase H hydrolysis indicated that the two mRNAs have poly [A+] tails of identical length. By using region-specific cDNA probes, we determined that the sequence difference between the two alpha 1b-adrenergic receptor mRNAs lies in the 5' end, upstream from the known initiator AUG in the 2.7 kb transcript. In addition, results from ribonuclease protection assays and Northern blot analysis in which an oligonucleotide was used as the probe suggested that both alpha 1b-adrenergic receptor mRNAs are transcribed from the same DNA strand.