Deficient activity of guanine nucleotide regulatory protein in erythrocytes from patients with pseudohypoparathyroidism

GNAS locus: bone related diseases and mouse models

GNASis a complex locus characterized by multiple transcripts and an imprinting effect. It orchestrates a variety of physiological processes via numerous signaling pathways. Human diseases associated with the GNAS gene encompass fibrous dysplasia (FD), Albright's Hereditary Osteodystrophy (AHO), parathyroid hormone(PTH) resistance, and Progressive Osseous Heteroplasia (POH), among others. To facilitate the study of the GNAS locus and its associated diseases, researchers have developed a range of mouse models. In this review, we will systematically explore the GNAS locus, its related signaling pathways, the bone diseases associated with it, and the mouse models pertinent to these bone diseases.

PTH, FGF-23, Klotho and Vitamin D as regulators of calcium and phosphorus: Genetics, epigenetics and beyond

The actions of several bone-mineral ion regulators, namely PTH, FGF23, Klotho and 1,25(OH)2 vitamin D (1,25(OH)2D), control calcium and phosphate metabolism, and each of these molecules has additional biological effects related to cell signaling, metabolism and ultimately survival. Therefore, these factors are tightly regulated at various levels - genetic, epigenetic, protein secretion and cleavage. We review the main determinants of mineral homeostasis including well-established genetic and post-translational regulators and bring attention to the epigenetic mechanisms that affect the function of PTH, FGF23/Klotho and 1,25(OH)2D. Clinically relevant epigenetic mechanisms include methylation of cytosine at CpG-rich islands, histone deacetylation and micro-RNA interference. For example, sporadic pseudohypoparathyroidism type 1B (PHP1B), a disease characterized by resistance to PTH actions due to blunted intracellular cAMP signaling at the PTH/PTHrP receptor, is associated with abnormal methylation at the GNAS locus, thereby leading to reduced expression of the stimulatory G protein α-subunit (Gsα). Post-translational regulation is critical for the function of FGF-23 and such modifications include glycosylation and phosphorylation, which regulate the cleavage of FGF-23 and hence the proportion of available FGF-23 that is biologically active. While there is extensive data on how 1,25(OH)2D and the vitamin D receptor (VDR) regulate other genes, much more needs to be learned about their regulation. Reduced VDR expression or VDR mutations are the cause of rickets and are thought to contribute to different disorders. Epigenetic changes, such as increased methylation of the VDR resulting in decreased expression are associated with several cancers and infections. Genetic and epigenetic determinants play crucial roles in the function of mineral factors and their disorders lead to different diseases related to bone and beyond.

Pseudo-hypoparathyroïdie et ses variants

Les pseudohypoparathyroïdies (PHP) sont des maladies rares, caractérisées par une résistance à l’action rénale de la parathormone. Le défaut génétique est localisé au locus GNAS, qui code la sous-unité alpha stimulatrice des protéines G (Gαs). Ce locus est le siège de régulations complexes, épissage alternatif et empreinte parentale éteigant de façon tissu-spécifique l’expression de l’allèle paternel. Des mutations hétérozygotes perte de fonction, des épimutations responsables d’une perte d’expression sont associées à un large spectre pathologique : PHP1A, PHP1B, ossification hétérotopique, ostéodystophie, obésité, retard de croissance in utero, etc., dont les mécanismes restent encore incomplètement connus.

A novel deletion involving the first GNAS exon encoding Gsα causes PHP1A without methylation changes at exon A/B

Individuals affected by pseudohypoparathyroidism type 1A (PHP1A) display hyperphosphatemia and hypocalcemia despite elevated PTH levels, as well as features of Albright Hereditary Osteodystrophy (AHO). PHP1A is caused by variants involving the maternal GNAS exons 1-13 encoding the stimulatory G protein α-subunit (Gsα). MLPA and aCGH analysis led in a male PHP1A patient to identification of a de novo 1284-bp deletion involving GNAS exon 1. This novel variant overlaps with a previously identified 1438-bp deletion in another PHP1A patient (ref. Li et al. (2020) [13], patient 2) that extends from the exon 1 promoter into the up-stream intronic region. This latter deletion is associated with reduced methylation at GNAS exon A/B, i.e. the differentially methylated region (DMR) that is demethylated in most pseudohypoparathyroidism type 1B (PHP1B) patients. In contrast, genomic DNA from our patient revealed no evidence for an epigenetic GNAS defect as determined by MS-MLPA and pyrosequencing. These findings thus reduce the region, which, in addition to other nucleotide sequences telomeric of exon A/B, may undergo histone modifications or interacts with transcription factors and possibly as-yet unknown proteins that are required for establishing the maternal methylation imprints at this site. Taken together, nucleotide deletions or changes within an approximately 1300-bp region telomeric of exon A/B could be a cause of PHP1B variants with complete or incomplete loss-of-methylation at the exon A/B DMR. In addition, when investigating patients with suspected PHP1A, MLPA should be considered to search for structural abnormalities within this difficult to analyze genomic region comprising GNAS exon 1.

A complex pheotype in a girl with a novel heterozygous missense variant (p.Ile56Phe) of the GNAS gene

BACKGROUND

GNAS is a complex gene that encodes Gsα, a signaling protein that triggers a complex network of pathways. Heterozygous inactivating mutations in Gsα-coding GNAS exons cause hormonal resistance; on the contrary, activating mutations in Gsα result in constitutive cAMP stimulation. Recent research has described a clinical condition characterized by both gain and loss of Gsα function, due to a heterozygous de novo variant of the maternal GNAS allele.

PATIENTS AND METHODS

We describe a girl with a complex combination of clinical signs and a new heterozygous GNAS variant. For the molecular analysis of GNAS gene, DNA samples of the proband and her parents were extracted from their peripheral blood samples. In silico analysis was performed to predict the possible in vivo effect of the detected novel genetic variant. The activity of Gsα protein was in vitro analyzed from samples of erythrocyte membranes, recovered from heparinized blood samples.

RESULTS

We found a new heterozygous missense c.166A > T-(p.Ile56Phe) GNAS variant in exon 2, inherited from the mother that determined a reduced activity of 50% of Gsα protein function. The analysis of her parents showed a 20-25% reduction in Gsα protein activity in the mother and a normal function in the father. Clinically our patient presented a multisystemic disorder characterized by hyponatremia compatible with a nephrogenic syndrome of inappropriate antidiuresis, subclinical hyperthyroidism, subclinical hypercortisolism, precocious thelarche and pubarche and congenital bone abnormalities.

CONCLUSIONS

This is the first time that the new variant c.166A > T (p.Ile56Phe) on exon 2 of GNAS gene, originated on maternal allele, has been described as probable cause of a multisystemic disorder. Although the mutation is associated with a reduced activity of the function of Gsα protein, this unusual phenotype on the contrary suggests a mild functional gain.

New insights into thyroid dysfunction in patients with inactivating parathyroid hormone/parathyroid hormone-related protein signalling disorder (the hormonal and ultrasound aspects): One-centre preliminary results

OBJECTIVE

This study aimed to present the spectrum of thyroid dysfunction, including hormonal and ultrasound aspects, in a cohort of paediatric and adult patients diagnosed with inactivating parathyroid hormone (PTH)/PTH-related protein signalling disorders 2 and 3 (iPPSD).

METHODS

The medical records of 31 patients from 14 families diagnosed with iPPSD between 1980 and 2021 in a single tertiary unit were retrospectively analysed. Biochemical, hormonal, molecular, and ultrasonographic parameters were assessed.

RESULTS

In total, 28 patients from 13 families were diagnosed with iPPSD2 (previously pseudohypoparathyroidism [PHP], PHP1A, and pseudo-PHP) at a mean age of 12.2 years (ranging from infancy to 48 years), and three patients from one family were diagnosed with iPPSD3 (PHP1B). Thyroid dysfunction was diagnosed in 21 of the 28 (75%) patients with iPPSD2. Neonatal screening detected congenital hypothyroidism (CH) in 4 of the 20 (20%) newborns. The spectrum of thyroid dysfunction included: CH, 3/21 (14.2%); CH and autoimmune thyroiditis with nodular goitre, 1/21 (4.8%); subclinical hypothyroidism, 10/21 (47.6%); subclinical hypothyroidism and nodular goitre, 1/21 (4.8%); primary hypothyroidism, 4/21 (19%); and autoimmune thyroiditis (Hashimoto and Graves' disease), 2/21 (9.6%). Thyroid function was normal in 7 of the 28 (25%) patients with iPPSD2 and in all patients with iPPSD3. Ultrasound evaluation of the thyroid gland revealed markedly inhomogeneous echogenicity and structure in all patients with thyroid dysfunction. Goitre was found in three patients.

CONCLUSION

The spectrum of thyroid dysfunction in iPPSD ranges from CH to autoimmune thyroiditis and nodular goitre. Ultrasonography of the thyroid gland may reveal an abnormal thyroid parenchyma.

High-throughput Molecular Analysis of Pseudohypoparathyroidism 1b Patients Reveals Novel Genetic and Epigenetic Defects

CONTEXT

Patients with pseudohypoparathyroidism type 1b (PHP1b) show disordered imprinting of the maternal GNAS allele or paternal uniparental disomy (UPD). Genetic deletions in STX16 or in upstream exons of GNAS are present in many familial but not sporadic cases.

OBJECTIVE

Characterization of epigenetic and genetic defects in patients with PHP1b.

DESIGN AND PATIENTS

DNA from 84 subjects, including 26 subjects with sporadic PHP1b, 27 affected subjects and 17 unaffected and/or obligate gene carriers from 12 PHP1b families, 11 healthy individuals, and 3 subjects with PHP1a was subjected to quantitative pyrosequencing of GNAS differentially methylated regions (DMRs), microarray analysis, and microsatellite haplotype analysis.

SETTING

Academic medical center.

MAIN OUTCOME MEASUREMENTS

Molecular pathology of PHP1b.

RESULTS

Healthy subjects, unaffected family members and obligate carriers of paternal PHP1b alleles, and subjects with PHP1a showed normal methylation of all DMRs. All PHP1b subjects showed loss of methylation (LOM) at the exon A/B DMR. Affected members of 9 PHP1b kindreds showed LOM only at the exon A/B DMR, which was associated with a 3-kb deletion of STX16 exons 4 through 6 in 7 families and a novel deletion of STX16 and adjacent NEPEPL1 in 1 family. A novel NESP deletion was found in 1 of 2 other families with more extensive methylation defects. One sporadic PHP1b had UPD of 20q, 2 had 3-kb STX16 deletions, and 5 had apparent epigenetic mosaicism.

CONCLUSIONS

We found diverse patterns of defective methylation and identified novel or previously known mutations in 9 of 12 PHP1b families.

PTH resistance

Parathyroid hormone (PTH), which is primarily regulated by extracellular calcium changes, controls calcium and phosphate homeostasis. Different diseases are derived from PTH deficiency (hypoparathyroidism), excess (hyperparathyroidism) and resistance (pseudohypoparathyroidism, PHP). Pseudohypoparathyroidism was historically classified into subtypes according to the presence or not of inherited PTH resistance associated or not with features of Albright's hereditary osteodystrophy and deep and progressive ectopic ossifications. The growing knowledge on the PTH/PTHrP signaling pathway showed that molecular defects affecting different members of this pathway determined distinct, yet clinically related disorders, leading to the proposal of a new nomenclature and classification encompassing all disorders, collectively termed inactivating PTH/PTHrP signaling disorders (iPPSD).

A Distinct Variant of Pseudohypoparathyroidism (PHP) First Characterized Some 41 Years Ago Is Caused by the 3-kbSTX16 Deletion

In 1980, Farfel and colleagues (NEJM, 1980;303:237-42) provided first evidence for two distinct variants of pseudohypoparathyroidism (PHP) that present with hypocalcemia and impaired parathyroid hormone (PTH)-stimulated urinary cAMP and phosphate excretion, either in the presence or absence of Albright's hereditary osteodystrophy (AHO). An "abnormal allele" and an "unexpressed allele" were considered as underlying defects, predictions that turned out to be correct for both forms of PHP. Patients affected by the first variant (now referred to as PHP1A) were later shown to be carriers of inactivating mutations involving the maternal GNAS exons encoding Gsα. Patients affected by the second variant (now referred to as PHP1B) were shown in the current study to carry a maternal 3-kb STX16 deletion, the most frequent cause of autosomal dominant PHP1B, which is associated with loss of methylation at GNAS exon A/B that reduces or abolishes maternal Gsα expression. However, the distinct maternal mutations leading to either PHP1A or PHP1B are disease-causing only because paternal Gsα expression in the proximal renal tubules is silenced, ie, "unexpressed." Our findings resolve at the molecular level carefully conducted investigations reported some 41 years ago that had provided first clues for the existence of two distinct PHP variants. © 2021 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Pseudohypoparathyroidism, acrodysostosis, progressive osseous heteroplasia: different names for the same spectrum of diseases?

Pseudohypoparathyroidism (PHP), the first known post-receptorial hormone resistance, derives from a partial deficiency of the α subunit of the stimulatory G protein (Gsα), a key component of the PTH/PTHrP signaling pathway. Since its first description, different studies unveiled, beside the molecular basis for PHP, the existence of different subtypes and of diseases in differential diagnosis associated with genetic alterations in other genes of the PTH/PTHrP pathway. The clinical and molecular overlap among PHP subtypes and with different but related disorders make both differential diagnosis and genetic counseling challenging. Recently, a proposal to group all these conditions under the novel term "inactivating PTH/PTHrP signaling disorders (iPPSD)" was promoted and, soon afterwards, the first international consensus statement on the diagnosis and management of these disorders has been published. This review will focus on the major and minor features characterizing PHP/iPPSDs as a group and on the specificities as well as the overlap associated with the most frequent subtypes.

Molecular Definition of Pseudohypoparathyroidism Variants

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 differentially methylated regions (DMRs). GNAS encodes the alpha-subunit of the stimulatory G protein (Gsα) and several splice variants thereof. PHP type Ia (PHP1A) is caused by heterozygous inactivating mutations involving the maternal exons 1-13. Heterozygosity of these maternal GNAS mutations cause PTH-resistant hypocalcemia and hyperphosphatemia because paternal Gsα expression is suppressed in certain organs thus leading to little or no Gsα protein in the proximal renal tubules and other tissues. Besides biochemical abnormalities, PHP1A patients show developmental abnormalities, referred to as Albright's hereditary osteodystrophy (AHO). Some, but not all of these AHO features are encountered also in patients affected by PPHP, who carry paternal Gsα-specific mutations and 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 at the A/B DMR alone or at all maternally methylated GNAS exons. Loss of methylation of exon A/B and the resulting biallelic expression of A/B transcript reduces Gsα expression thus leading to hormonal resistance. Epigenetic changes at all differentially methylated GNAS regions are also observed in sporadic PHP1B, which is the most frequent PHP1B variant. However, this disease variant remains unresolved at the molecular level, except for rare cases with paternal uniparental isodisomy or heterodisomy of chromosome 20q (patUPD20q).

Shortened Fingers and Toes: GNAS Abnormalities are Not the Only Cause

The PTH/PTHrP receptor (PTHR1) mediates the actions of parathyroid hormone (PTH) and PTH-related peptide (PTHrP) by coupling this G protein-coupled receptor (GPCR) to the alpha-subunit of the heterotrimeric stimulatory G protein (Gsα) and thereby to the formation of cAMP. In growth plates, PTHrP-dependent activation of the cAMP/PKA second messenger pathway prevents the premature differentiation of chondrocytes into hypertrophic cells resulting in delayed growth plate closure. Heterozygous mutations in GNAS, the gene encoding Gsα, lead to a reduction in cAMP levels in growth plate chondrocytes that is sufficient to cause shortening of metacarpals and/or -tarsals, i. e. typical skeletal aspects of Albright's Hereditary Osteodystrophy (AHO). However, heterozygous mutations in other genes, including those encoding PTHrP, PRKAR1A, PDE4D, and PDE3A, can lead to similar or even more pronounced acceleration of skeletal maturation that is particularly obvious in hands and feet, and reduces final adult height. Genetic mutations other than those resulting in Gsα haploinsufficiency thus reduce intracellular cAMP levels in growth plate chondrocytes to a similar extent and thereby accelerate skeletal maturation.

Resistance to GHRH but Not to PTH in a 15-Year-Old Boy With Pseudohypoparathyroidism 1A

Pseudohypoparathyroidism 1A (PHP1A) consists of signs of Albright hereditary osteodystrophy (AHO) and multiple, variable hormonal resistances. Elevated PTH levels are the biochemical hallmark of the disease. Short stature in PHP1A may be caused by a form of accelerated chondrocyte differentiation leading to premature growth plate closure, possibly in combination with GH deficiency in some patients. Treatment of short stature with recombinant growth hormone (rhGH) in pediatric patients may improve final height if started during childhood. The 10 11/12-year-old boy with clinical signs of AHO presented for evaluation of short stature [height standard deviation score (SDS) −2.72]. Clinically his mother was affected by AHO as well. A heterozygous mutation c.505G>A (p.E169K) in exon 6 of the GNAS gene confirmed a diagnosis of PHP1A in the boy. However, hormonal assessment was unremarkable except for low serum IGF-1 (SDS −2.67). On follow-up, GH deficiency due to GHRH resistance was suspected and confirmed by clonidine and arginine stimulation tests. Treatment with rhGH (0.035 mg/kg) for 2 years resulted in catch-up growth (height SDS −1.52). At age 15 years the PTH levels and bone age of the patient remain within the normal range. In patients with PHP1A, short stature is caused by the effects of G_s-α deficiency on the growth plate. However, resistance to GHRH and the resulting GH deficiency might also contribute. Recombinant GH treatment increases growth in these patients. Diagnostic workup for GH deficiency as a factor contributing to short stature is recommended even in the absence of other hormonal resistances.

Pseudohypoparathyroidism

Pseudohypoparathyroidism (PHP) refers to a heterogeneous group of uncommon, yet related metabolic disorders that are characterized by impaired activation of the Gsα/cAMP/PKA signaling pathway by parathyroid hormone (PTH) and other hormones that interact with Gsa-coupled receptors. Proximal renal tubular resistance to PTH and thus hypocalcemia and hyperphosphatemia, frequently in presence of brachydactyly, ectopic ossification, early-onset obesity, or short stature are common features of PHP. Registries and large cohorts of patients are needed to conduct clinical and genetic research, to improve the still limited knowledge regarding the underlying disease mechanisms, and allow the development of novel therapies.

Parathyroid hormone resistance syndromes - Inactivating PTH/PTHrP signaling disorders (iPPSDs)

Metabolic disorders caused by impairments of the Gsα/cAMP/PKA pathway affecting the signaling of PTH/PTHrP lead to features caused by non-responsiveness of target organs, in turn leading to manifestations similar to the deficiency of the hormone itself. Pseudohypoparathyroidism (PHP) and related disorders derive from a defect of the α subunit of the stimulatory G protein (Gsα) or of downstream effectors of the same pathway, such as the PKA regulatory subunit 1A and the phosphodiesterase type 4D. The increasing knowledge on these diseases made the actual classification of PHP outdated as it does not include related conditions such as acrodysostosis (ACRDYS) or progressive osseous heteroplasia (POH), so that a new nomenclature and classification has been recently proposed grouping these disorders under the term "inactivating PTH/PTHrP signaling disorder" (iPPSD). This review will focus on the pathophysiology, clinical and molecular aspects of these rare, heterogeneous but closely related diseases.

Diagnosis and management of pseudohypoparathyroidism and related disorders: first international Consensus Statement

This Consensus Statement covers recommendations for the diagnosis and management of patients with pseudohypoparathyroidism (PHP) and related disorders, which comprise metabolic disorders characterized by physical findings that variably include short bones, short stature, a stocky build, early-onset obesity and ectopic ossifications, as well as endocrine defects that often include resistance to parathyroid hormone (PTH) and TSH. The presentation and severity of PHP and its related disorders vary between affected individuals with considerable clinical and molecular overlap between the different types. A specific diagnosis is often delayed owing to lack of recognition of the syndrome and associated features. The participants in this Consensus Statement agreed that the diagnosis of PHP should be based on major criteria, including resistance to PTH, ectopic ossifications, brachydactyly and early-onset obesity. The clinical and laboratory diagnosis should be confirmed by a molecular genetic analysis. Patients should be screened at diagnosis and during follow-up for specific features, such as PTH resistance, TSH resistance, growth hormone deficiency, hypogonadism, skeletal deformities, oral health, weight gain, glucose intolerance or type 2 diabetes mellitus, and hypertension, as well as subcutaneous and/or deeper ectopic ossifications and neurocognitive impairment. Overall, a coordinated and multidisciplinary approach from infancy through adulthood, including a transition programme, should help us to improve the care of patients affected by these disorders.

Multiple hormone resistance and alterations of G-protein-coupled receptors signaling

Metabolic disorders deriving from the non-responsiveness of target organs to hormones, which manifest clinically similar to the deficiency of a given hormone itself, derive from molecular alterations affecting specific hormone receptors. Pseudohypoparathyroidism (PHP) and related disorders exemplify an unusual form of hormone resistance as the underlying molecular defect is a partial deficiency of the α subunit of the stimulatory G protein (Gsα), a key regulator of cAMP signaling pathway, or, as more recently described, of downstream effector proteins of the same pathway, such as PKA regulatory subunit 1A (R1A) and phosphodyestarase type 4D (PDE4D). In this group of diseases, resistance to hormones such as PTH, TSH, gonadotropins and GHRH may be variably present, so that the clinical and molecular overlap among these different but related disorders represents a challenge for endocrinologists as to differential diagnosis and genetic counseling. This review will describe the presenting features of multiple resistance in PHP and related disorders, focusing on both our current understanding and future challenges.

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).

Pseudohypoparathyroidism Type 1A-Subclinical Hypothyroidism and Rapid Weight Gain as Early Clinical Signs: A Clinical Review of 10 Cases

OBJECTIVE

To evaluate the clinical signs and symptoms that would help clinicians to consider pseudohypoparathyroidism (PHP) type 1A as a diagnosis in a child.

METHODS

A retrospective review of the medical records of children diagnosed by erythrocyte Gsα activity and/or GNAS1 gene study and followed-up for PHP type 1A. Clinical and biochemical parameters along with epidemiological data were extracted and analyzed. Weight gain during infancy and early childhood was calculated as change in weight standard deviation score (SDS), using the French growth reference values. An upward gain in weight ≥0.67 SDS during these periods was considered indicative of overweight and/or obesity.

RESULTS

Ten cases of PHP type 1A were identified (mean age 41.1 months, range from 4 to 156 months). In children aged ≤2 years, the commonest clinical features were round lunar face, obesity (70%), and subcutaneous ossifications (60%). In older children, brachydactyly was present in 60% of cases. Seizures occurred in older children (3 cases). Short stature was common at all ages. Subclinical hypothyroidism was present in 70%, increased parathormone (PTH) in 83%, and hyperphosphatemia in 50%. Only one case presented with hypocalcemia. Erythrocyte Gsα activity tested in seven children was reduced; GNAS1 gene testing was performed in 9 children. Maternal transmission was the most common (six patients). In three other cases, the mutations were de novo, c.585delGACT in exon 8 (case 2) and c.344C>TP115L in exon 5 (cases 6&7).

CONCLUSION

Based on our results, PHP type 1A should be considered in toddlers presenting with round face, rapid weight gain, subcutaneous ossifications, and subclinical hypothyroidism. In older children, moderate mental retardation, brachydactyly, afebrile seizures, short stature, and thyroid-stimulating hormone resistance are the most suggestive features.

From pseudohypoparathyroidism to inactivating PTH/PTHrP signalling disorder (iPPSD), a novel classification proposed by the EuroPHP network

OBJECTIVE

Disorders caused by impairments in the parathyroid hormone (PTH) signalling pathway are historically classified under the term pseudohypoparathyroidism (PHP), which encompasses rare, related and highly heterogeneous diseases with demonstrated (epi)genetic causes. The actual classification is based on the presence or absence of specific clinical and biochemical signs together with an in vivo response to exogenous PTH and the results of an in vitro assay to measure Gsa protein activity. However, this classification disregards other related diseases such as acrodysostosis (ACRDYS) or progressive osseous heteroplasia (POH), as well as recent findings of clinical and genetic/epigenetic background of the different subtypes. Therefore, the EuroPHP network decided to develop a new classification that encompasses all disorders with impairments in PTH and/or PTHrP cAMP-mediated pathway.

DESIGN AND METHODS

Extensive review of the literature was performed. Several meetings were organised to discuss about a new, more effective and accurate way to describe disorders caused by abnormalities of the PTH/PTHrP signalling pathway.

RESULTS AND CONCLUSIONS

After determining the major and minor criteria to be considered for the diagnosis of these disorders, we proposed to group them under the term 'inactivating PTH/PTHrP signalling disorder' (iPPSD). This terminology: (i) defines the common mechanism responsible for all diseases; (ii) does not require a confirmed genetic defect; (iii) avoids ambiguous terms like 'pseudo' and (iv) eliminates the clinical or molecular overlap between diseases. We believe that the use of this nomenclature and classification will facilitate the development of rationale and comprehensive international guidelines for the diagnosis and treatment of iPPSDs.

Pseudohypoparathyroidism and Gsα-cAMP-linked disorders: current view and open issues

Pseudohypoparathyroidism exemplifies an unusual form of hormone resistance as the underlying molecular defect is a partial deficiency of the α subunit of the stimulatory G protein (Gsα), a key regulator of the cAMP signalling pathway, rather than of the parathyroid hormone (PTH) receptor itself. Despite the first description of this disorder dating back to 1942, later findings have unveiled complex epigenetic alterations in addition to classic mutations in GNAS underpining the molecular basis of the main subtypes of pseudohypoparathyroidism. Moreover, mutations in PRKAR1A and PDE4D, which encode proteins crucial for Gsα-cAMP-mediated signalling, have been found in patients with acrodysostosis. As acrodysostosis, a disease characterized by skeletal malformations and endocrine disturbances, shares clinical and molecular characteristics with pseudohypoparathyroidism, making a differential diagnosis and providing genetic counselling to patients and families is a challenge for endocrinologists. Accumulating data on the genetic and clinical aspects of this group of diseases highlight the limitation of the current classification system and prompt the need for a new definition as well as for new diagnostic and/or therapeutic algorithms. This Review discusses both the current understanding and future challenges for the clinical and molecular diagnosis, classification and treatment of pseudohypoparathyroidism.

Guanine nucleotide-binding protein α subunit hypofunction in children with short stature and disproportionate shortening of the 4th and 5th metacarpals

BACKGROUND

GNAS encodes the α subunit of the stimulatory G protein (Gsα). Maternal inherited Gsα mutations cause pseudohypoparathyroidism type Ia (PHP-Ia), associated with shortening of the 4th and 5th metacarpals.

AIMS

Here we investigated the Gsα pathway in short patients with distinct shortening of the 4th and 5th metacarpals.

METHODS

In 571 children with short stature and 4 patients with PHP-Ia metacarpal bone lengths were measured. In identified patients we analysed the Gsα protein function in platelets, performed GNAS sequencing, and epigenetic analysis of four significant differentially methylated regions.

RESULTS

In 51 patients (8.9%) shortening of the 4th and 5th metacarpals was more pronounced than their height deficit. No GNAS coding mutations were identified in 20 analysed patients, except in 2 PHP-Ia patients. Gsα activity was reduced in all PHP-Ia patients and in 25% of the analysed patients. No significant methylation changes were identified.

CONCLUSIONS

Our findings suggest that patients with short stature and distinct metacarpal bone shortening could be part of the wide variety of PHP/PPHP, therefore it was worthwhile analysing the Gsα protein function and GNAS gene in these patients in order to further elucidate the phenotype and genotype of Gsα dysfunction.

A positive genotype-phenotype correlation in a large cohort of patients with Pseudohypoparathyroidism Type Ia and Pseudo-pseudohypoparathyroidism and 33 newly identified mutations in the GNAS gene

Maternally inherited inactivating GNAS mutations are the most common cause of parathyroid hormone (PTH) resistance and Albright hereditary osteodystrophy (AHO) leading to pseudohypoparathyroidism type Ia (PHPIa) due to Gsα deficiency. Paternally inherited inactivating mutations lead to isolated AHO signs characterizing pseudo-pseudohypoparathyroidism (PPHP). Mutations are distributed throughout the Gsα coding exons of GNAS and there is a lack of genotype–phenotype correlation. In this study, we sequenced exon 1–13 of GNAS in a large cohort of PHPIa- and PPHP patients and identified 58 different mutations in 88 patients and 27 relatives. Thirty-three mutations including 15 missense mutations were newly discovered. Furthermore, we found three hot spots: a known hotspot (p.D190MfsX14), a second at codon 166 (p.R166C), and a third at the exon 5 acceptor splice site (c.435 + 1G>A), found in 15, 5, and 4 unrelated patients, respectively. Comparing the clinical features to the molecular genetic data, a significantly higher occurrence of subcutaneous calcifications in patients harboring truncating versus missense mutations was demonstrated. Thus, in the largest cohort of PHPIa patients described to date, we extend the spectrum of known GNAS mutations and hot spots and demonstrate for the first time a correlation between the genetic defects and the expression of a clinical AHO-feature.

Different pattern of epigenetic changes of the GNAS gene locus in patients with pseudohypoparathyroidism type Ic confirm the heterogeneity of underlying pathomechanisms in this subgroup of pseudohypoparathyroidism and the demand for a new classification of GNAS-related disorders

CONTEXT

Disorders characterized by PTH resistance are grouped within the term pseudohypoparathyroidism type I (PHPI). Most subtypes of this disease are caused by genetic or epigenetic changes of the GNAS locus leading to deficiency of the α-subunit of stimulatory G proteins (Gsα). Because the in vitro measured Gsα protein activity is normal in pseudohypoparathyroidism Ic (PHPIc), it had previously been postulated that this subtype is caused by impairment of distinct components of the G protein-signaling pathway. However, recently, pathogenic GNAS mutations in a subset of PHPIc patients were found.

OBJECTIVE

To clarify the underlying pathogenic mechanism of GNAS exon 1-13 mutation-negative PHPIc cases by investigating the differentially methylated regions of GNAS for epigenetic abnormalities.

PATIENTS AND METHODS

The methylation pattern of GNAS exons A/B, AS, XL, and NESP from blood-derived leukocytes of 26 PHPIc patients was assessed by pyrosequencing of bisulfite-converted DNA.

RESULTS

Six patients presented with three different patterns of epigenetic changes. One patient had an exclusive loss of methylation of exon A/B associated with a STX16 deletion; four patients had an additional loss of methylation in XL and AS and a gain of methylation in NESP; and one patient presented with partial GNAS methylation changes concerning all differentially methylated regions.

CONCLUSIONS

Our results confirm that PHPIc is a heterogeneous entity caused in part by impaired Gsα function, not only due to mutations, but also due to abnormal imprinting of GNAS. However, in the majority of cases of PHPIc, the underlying etiopathogenesis remains elusive.

Quantitative analysis of methylation defects and correlation with clinical characteristics in patients with pseudohypoparathyroidism type I and GNAS epigenetic alterations

CONTEXT

Pseudohypoparathyroidism type I (PHP-I) includes two main subtypes, PHP-Ia and -Ib. About 70% of PHP-Ia patients, who show Albright hereditary osteodystrophy (AHO) associated with resistance toward multiple hormones (PTH/TSH/GHRH/gonadotropins), carry heterozygous mutations in the α-subunit of the stimulatory G protein (Gsα) exons 1-13, encoded by the guanine nucleotide binding-protein α-stimulating activity polypeptide 1 (GNAS), whereas the majority of PHP-Ib patients, who classically display hormone resistance limited to PTH and TSH with no AHO sign, have methylation defects in the imprinted GNAS cluster. Recently methylation defects have been detected also in patients with PHP and different degrees of AHO, indicating a molecular overlap between the two forms.

OBJECTIVES

The objectives of the study were to collect patients with the following characteristics: clinical PHP-I (with or without AHO), no mutation in Gsα coding sequence, but the presence of GNAS methylation alterations and to investigate the existence of correlations between the degree of the epigenetic defect and the severity of the disease.

PATIENTS AND METHODS

We quantified GNAS methylation alterations by both PCR-pyrosequencing and methylation specific-multiplex ligation-dependent probe amplification assay in genomic DNA from 63 patients with PHP-I and correlated these findings with clinical parameters (age at diagnosis; calcium, phosphorus, PTH, TSH levels; presence or absence of each AHO sign).

RESULTS

By both approaches, the degree of the imprinting defect did not correlate with the onset of the disease, the severity of endocrine resistances, or with the presence/absence of specific AHO signs.

CONCLUSIONS

Similar molecular alterations may lead to a broad spectrum of diseases, from isolated PTH resistance to complete PHP-Ia, and the degree of methylation alterations does not reflect or anticipate the severity and the type of different PHP/AHO manifestations.

Clinical review: Pseudohypoparathyroidism: diagnosis and treatment

CONTEXT

The term pseudohypoparathyroidism (PHP) indicates a group of heterogeneous disorders whose common feature is represented by impaired signaling of various hormones (primarily PTH) that activate cAMP-dependent pathways via Gsα protein. The two main subtypes of PHP, PHP type Ia, and Ib (PHP-Ia, PHP-Ib) are caused by molecular alterations within or upstream of the imprinted GNAS gene, which encodes Gsα and other translated and untranslated products.

EVIDENCE ACQUISITION

A PubMed search was used to identify the available studies (main query terms: pseudohypoparathyroidism; Albright hereditary osteodystrophy; GNAS; GNAS1; progressive osseous heteroplasia). The most relevant studies until February 2011 have been included in the review.

EVIDENCE SYNTHESIS AND CONCLUSIONS

Despite the first description of this disorder dates back to 1942, recent findings indicating complex epigenetic alterations beside classical mutations at the GNAS complex gene, pointed out the limitation of the actual classification of the disease, resulting in incorrect genetic counselling and diagnostic procedures, as well as the gap in our actual knowledge of the pathogenesis of these disorders. This review will focus on PHP type I, in particular its diagnosis, classification, treatment, and underlying molecular alterations.

Gsα activity is reduced in erythrocyte membranes of patients with psedohypoparathyroidism due to epigenetic alterations at the GNAS locus

In pseudohypoparathyroidism (PHP), PTH resistance results from impairment of signal transduction of G protein-coupled receptors caused by a deficiency of the Gsα-cAMP signaling cascade due to diminished Gsα activity in maternally imprinted tissues. In PHP-Ia, inactivating mutations of the GNAS gene lead to haploinsufficiency in some tissues with biallelic expression, so in addition to PHP, Albright's hereditary osteodystrophy (AHO) is also present. In PHP-Ib, caused by methylation defects at the GNAS locus, diminished Gsα activity was thought to be limited to maternally imprinted tissues, such as the renal proximal tubule and the thyroid, leading to a lack of AHO. Recently, we demonstrated methylation defects in patients with AHO signs, indicating a connection between epigenetic changes and AHO. Our objective was to determine Gsα activity in erythrocyte membranes in patients with epigenetic defects at the GNAS locus compared to normal controls and patients with inactivating GNAS mutations. Gsα activity and expression, mutation of the GNAS locus, and methylation status were studied in patients with PHP and mild signs of AHO (PHP-Ia: 12; PHP-Ib: 17, of which 8 had some features of AHO). Then, we statistically compared the Gsα activity of the different PHP subtypes. Patients with methylation defects at the GNAS locus show a significant decrease in erythrocyte Gsα activity compared to normal controls (PHP-Ib versus controls, p < .001). This was significantly lower in patients with AHO signs (PHP-Ib + mild-AHO versus PHP-Ib, p < .05). Our research shows that PHP-Ia and PHP-Ib classification is not only overlapped genetically, as reported, but also in terms of Gsα activity. Reduced expression of GNAS due to methylation defects could downregulate Gsα activity in other tissues beyond those described and could also be causative of AHO.

Functional characterization of GNAS mutations found in patients with pseudohypoparathyroidism type Ic defines a new subgroup of pseudohypoparathyroidism affecting selectively Gsα-receptor interaction

Pseudohypoparathyroidism type Ia (PHPIa) is caused by GNAS mutations leading to deficiency of the α-subunit of stimulatory G proteins (Gsα) that mediate signal transduction of G protein-coupled receptors via cAMP. PHP type Ic (PHPIc) and PHPIa share clinical features of Albright hereditary osteodystrophy (AHO); however, in vitro activity of solubilized Gsα protein is normal in PHPIc but reduced in PHPIa. We screened 32 patients classified as PHPIc for GNAS mutations and identified three mutations (p.E392K, p.E392X, p.L388R) in four unrelated families. These and one novel mutation associated with PHPIa (p.L388P) were introduced into a pcDNA3.1(-) expression vector encoding Gsα wild-type and expressed in a Gsα-null cell line (Gnas(E2-/E2-) ). To investigate receptor-mediated cAMP accumulation, we stimulated the endogenous expressed β(2) -adrenergic receptor, or the coexpressed PTH or TSH receptors, and measured the synthesized cAMP by RIA. The results were compared to receptor-independent cholera toxin-induced cAMP accumulation. Each of the mutants associated with PHPIc significantly reduced or completely disrupted receptor-mediated activation, but displayed normal receptor-independent activation. In contrast, PHPIa associated p.L388P disrupted both receptor-mediated activation and receptor-independent activation. We present a new subgroup of PHP that is caused by Gsα deficiency and selectively affects receptor coupling functions of Gsα.

A new heterozygous mutation (D196N) in the Gs alpha gene as a cause for pseudohypoparathyroidism type IA in a boy who had gallstones

BACKGROUND

Pseudohypoparathyroidism (PHP) is characterized by hypocalcemia and hyperphosphatemia in association with an increased secretion of parathyroid hormone (PTH) due to decreased target tissue responsiveness to PTH. Patients with PHP type Ia are not only resistant to PTH, but also to other hormones that bind to receptors coupled to stimulatory G protein (Gsalpha). PHP Ia and Albright hereditary osteodystrophy (AHO) are caused by a reduced activity of the Gsalpha protein. Heterozygous inactivating Gs alpha (GNAS) gene mutations have been identified in these patients.

METHODS

We studied a boy with PHP Ia. During follow-up the patient developed elevated liver enzyme serum levels and abdominal discomfort. Gsalpha activity was measured in erythrocyte membranes from the patient and the GNAS coding region of Gsalpha sequenced.

RESULTS

Gsalpha activity was reduced (62%) and molecular analysis revealed a new heterozygous GNAS gene mutation (D196N). Gallstones were diagnosed and cholecystectomy was performed. Biochemical analysis revealed cholesterol stones, a condition that was not reported before in PHP Ia.

CONCLUSIONS

Cholesterol gallstones may rarely be associated with PHP Ia and should be taken into account.

Pseudohypoparathyreoidismus und epigenetische Veränderungen des GNAS-Genlocus

Die Bezeichnung Pseudohypoparathyreoidismus (PHP) beschreibt eine heterogene Gruppe von Erkrankungen, die durch eine Endorganresistenz gegenüber Parathormon (PTH) gekennzeichnet sind. Sie werden durch eine Defizienz der α-Untereinheit stimulierender G-Proteine (Gsα) verursacht. Gsα ist essenziell für die Signalvermittlung durch extrazelluläre Liganden über mehr als 1000 verschiedene G-Protein-gekoppelte Rezeptoren in das Zellinnere. Durch eine gewebespezifische Prägung verursachen maternale, autosomal-dominant vererbte Mutationen in dem für Gsα kodierenden GNAS-Genlocus PTH-Resistenz und klinische Zeichen der hereditären Albright-Osteodystrophie (AHO), einschließlich Brachymetakarpie, Kleinwuchs, subkutaner Ossifikationen und mentaler Retardierung (PHP-Typ Ia). Paternal vererbte GNAS-Mutationen führen zu selektiven AHO-Zeichen ohne Parathormonresistenz (Pseudo-PHP). Der PHP-Typ Ib, bei dem eine isolierte PTH-Resistenz vorliegt, wird durch heterozygote, maternal vererbte Deletionen stromaufwärts vom oder innerhalb des GNAS-Locus hervorgerufen, die durch eine Störung des Imprintingmusters eine gewebespezifische Stilllegung der Gsα-Expression hervorrufen. Diese Patienten zeigen in der Regel keine Zeichen der AHO. In der vorliegenden Arbeit wird ein Überblick über die Rolle epigenetischer Faktoren bei der Ätiopathogenese verschiedener PHP-Formen gegeben.

Pseudohypoparathyroidism and GNAS epigenetic defects: clinical evaluation of albright hereditary osteodystrophy and molecular analysis in 40 patients

CONTEXT

The two main subtypes of pseudohypoparathyroidism (PHP), PHP-Ia and -Ib, are caused by mutations in GNAS exons 1-13 and methylation defects in the imprinted GNAS cluster, respectively. PHP-Ia patients show Albright hereditary osteodystrophy (AHO) and resistance toward PTH and additional hormones, whereas PHP-Ib patients do not have AHO, and hormone resistance appears to be limited to PTH and TSH. Recently, methylation defects have been detected in few patients with PHP and mild AHO, indicating a molecular overlap between the two forms.

OBJECTIVES

The aim of the study was to screen patients with clinically diagnosed PHP-Ia for methylation defects and to investigate the presence of correlations between the molecular findings and AHO severity.

PATIENTS AND METHODS

We investigated differential methylation of GNAS regions and STX16 microdeletions in genomic DNA from 40 patients with sporadic AHO and multihormone resistance, with no mutations in Gsalpha-coding GNAS exons.

RESULTS

Molecular analysis showed GNAS cluster imprinting defects in 24 of the 40 patients analyzed. No STX16 deletion was detected. The presence of imprinting defects was not associated with the severity of AHO or with specific AHO signs.

CONCLUSIONS

We report the largest series of the literature of patients with clinical AHO and multihormone resistance and no mutation in the Gsalpha gene. Our findings of frequent GNAS imprinting defects further confirm the existence of an overlap between molecular and clinical features of PHP-Ia and PHP-Ib and highlight the necessity of a new clinical classification of the disease that takes into account the recent knowledge on the molecular basis underlying these defects.

[Epigenetics and pseudohypoparathyroidism]

Parental imprinting and the type of the genetic alteration play a determinant role in the phenotype expression of GNAS locus associated to pseudohypoparathyroidism (PHP). This imprint is tissue-specific, mainly localized in the kidney and the thyroid. Only the maternal allele is expressed at this level. An alteration in the coding sequence of the gene leads to an haplo-insufficiency and a dysmorphic phenotype (Albright's syndrome). If the alteration is on the maternal allele, there is a hormonal resistance to the PTH at the kidney level and to the TSH at the thyroid level. The phenotype is known as a PHP1a. If the alteration is on the paternal allele, there are few clinical signs with no hormonal resistance and the phenotype is known as pseudo-pseudo-hypoparathyroidism (PPHP). Methylation anomalies of GNAS locus, in particular of exon 1A, are responsible for a lack of expression of Gαs at kidney and thyroid levels only. If these anomalies concern the maternal allele (the only one expressed) with a paternal pattern, there is no haplo-insufficiency and no dysmorphic syndrome. The hormonal resistance is yet again limited to PTH and TSH. The phenotype is known as PHP1b. In the familial forms, these methylation anomalies are associated with a deletion of the syntaxine 16 gene in the maternal allele. This gene contains probably the imprinting center of the locus.

Identification of a novel GNAS mutation for pseudohypoparathyroidism in a Chinese family

Pseudohypoparathyroidism (PHP) is a heterogeneous group of diseases characterized by hormone resistance to receptors that stimulate adenylate cyclase. PHP-Ia patients show specific Gs-alpha protein deficiency, PTH/TSH/gonadotropin resistance, and a phenotype characterized by Albright hereditary osteodystrophy (AHO). Many heterozygous mutations in the GNAS gene encoding the Gs protein have been identified in PHP-Ia. We describe two boys with hypocalcemia and elevated serum levels of PTH in a Chinese family. The 13 exons of the GNAS gene were amplified using 15 pairs of GNAS-specific primers and analyzed by direct sequencing. We found a novel frame shift mutation in exon 11 of the GNAS gene identified in both of the two boys and their mother. This report provides another example of a Gs-alpha mutation leading to PHP.

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.

Compound heterozygous mutations in the GNAS gene of a boy with morbid obesity, thyroid-stimulating hormone resistance, pseudohypoparathyroidism, and a prothrombotic state

CONTEXT

Pseudohypoparathyroidism type Ia and pseudopseudohypoparathyroidism are characterized by Albright's hereditary osteodystrophy (AHO), respectively, with and without hormone resistance. Both clinical conditions result from decreased expression or function of the alpha-subunit of the stimulatory G protein (Gsalpha) of adenylyl cyclase due to heterozygous inactivating mutations in GNAS. Homozygous GNAS defects have not been described.

OBJECTIVE

A genetic and functional GNAS study was undertaken in a boy with morbid obesity (body mass index Z-score of 5 at the age of 3 yr, with a body fat fraction of 40%, which is more than twice normal), TSH resistance, pseudohypoparathyroidism, and a prothrombotic state.

RESULTS

The boy was found to be a first case with a compound heterozygous GNAS defect: a de novo R231C mutation on the paternal allele and on the other allele a maternally inherited unique combination of three C to T nucleotide substitutions in exon 7 (I185I), intron 7 (IVS7 + 31), and exon 13 (N371N) leading to aberrant splicing of GNAS. Platelets of this boy displayed a pronounced Gsalpha hypofunction and were spontaneously hyperreactive resulting in a prothrombotic state due to extremely low cAMP levels.

CONCLUSION

This report expands the human GNAS genotype-phenotype spectrum to include compound heterozygosity and a prothrombotic state.

Physiological Dysfunctions Associated with Mutations of the Imprinted Gnas Locus

The ubiquitous Gαs-subunit of the trimeric, stimulatory G-protein plays a central role in receptor-mediated signal transduction, coupling receptor activation with the production of cAMP. The Gαs-encoding locus Gnas is now known to consist of a complex arrangement of several protein-coding and noncoding transcripts. We provide an overview of its genomic organization, its regulation by genomic imprinting, and a summary of the physiological roles of the alternative protein variants Gαs and XLαs as determined from deficient mouse models.

The GNAS locus and pseudohypoparathyroidism

Pseudohypoparathyroidism (PHP) is a disorder of end-organ resistance primarily affecting the actions of parathyroid hormone (PTH). Genetic defects associated with different forms of PHP involve the alpha-subunit of the stimulatory G protein (Gsalpha), a signaling protein essential for the actions of PTH and many other hormones. Heterozygous inactivating mutations within Gsalpha-encoding GNAS exons are found in patients with PHP-Ia, who also show resistance to other hormones and a constellation ofphysical features called Albright's hereditary osteodystrophy (AHO). Patients who exhibit AHO features without evidence for hormone resistance, who are said to have pseudopseudohypoparathyroidism (PPHP), also carry heterozygous inactivating Gsalpha mutations. Maternal inheritance of such a mutation leads to PHP-Ia, i.e., AHO plus hormone resistance, while paternal inheritance of the same mutation leads to PPHP, i.e., AHO only. This imprinted mode of inheritance for hormone resistance can be explained by the predominantly maternal expression of Gsalpha in certain tissues, including renal proximal tubules. Patients with PHP-Ib lack coding Gsalpha mutations but display epigenetic defects of the GNAS locus, with the most consistent defect being a loss of imprinting at the exon A/B differentially methylated region (DMR). This epigenetic defect presumably silences, in cis, Gsalpha expression in tissues where this protein is derived from the maternal allele only, leading to a marked reduction of Gsa levels. The familial form of PHP-Ib (AD-PHP-Ib) is typically associated with an isolated loss of imprinting at the exon A/B DMR. A unique 3-kb microdeletion that disrupts the neighboring STX16 1ocus has been identified in this disorder and appears to be the cause of the loss of imprinting. In addition, deletions removing the entire NESP55 DMR, located within GNAS, have been identified in some AD-PHP-Ib kindreds in whom affected individuals show loss of all the maternal GNAS imprints. Mutations identified in different forms of PHP-Ib thus point to different cis-acting elements that are apparently required for the proper imprinting of the GNAS locus. Most sporadic PHP-Ib cases also have imprinting abnormalities of GNAS that involve multiple DMRs, but the genetic lesion(s) responsible for these imprinting abnormalities remain to be discovered.

Cognitive impairment is prevalent in pseudohypoparathyroidism type Ia, but not in pseudopseudohypoparathyroidism: possible cerebral imprinting of Gsalpha

OBJECTIVE

Pseudohypoparathyroidism type Ia (PHP-Ia) is a hereditary disorder characterized by resistance to multiple hormones that work via cAMP such as PTH and TSH, accompanied by typical skeletal features including short stature and brachydactyly, termed Albright hereditary osteodystrophy (AHO). In affected kindreds, some members may have AHO but not hormone resistance; they are termed as pseudopseudohypoparathyroidism (PPHP). The molecular basis for the disorder is heterozygous inactivating mutation of the Gsalpha gene. In affected families, subjects with both PHP-Ia and PPHP have the same Gsalpha mutations. The skeletal features common to PPHP and PHP-Ia are presumably caused by tissue-specific Gsalpha haploinsufficiency. Other features that distinguish between PPHP and PHP-Ia, such as the multihormone resistance, are presumably caused by tissue-specific paternal imprinting of Gsalpha. This suggests that major differences in phenotype between PHP-Ia and PPHP point to specific tissues with Gsalpha imprinting. One such major difference may be cognitive function in PHP-Ia and PPHP.

DESIGN

Description of a large family with PHP-Ia and PPHP.

PATIENTS

Eleven affected subjects with PHP-Ia or PPHP in one family.

MEASUREMENTS

Cognitive impairment (CI) was defined by a history of developmental delay, learning disability and the Wechsler intelligence scale.

RESULTS

CI occurred only in the five PHP-Ia but not in the six PPHP subjects. Hypothyroidism which occurred in all PHP-Ia subjects was apparently not the cause of CI as it was mild, and was treated promptly. Analysis of additional Israeli cases, and the published cases from the literature, all with documented Gsalpha mutations, revealed that CI is prevalent in PHP-Ia [60 of 77 subjects (79%)] but not in PPHP [3 of 30 subjects (10%)] (P < 1 x 10(-6)).

CONCLUSION

We suggest that Gsalpha is imprinted in the brain.

Patients with mutations in Gsalpha have reduced activation of a downstream target in epithelial tissues due to haploinsufficiency

CONTEXT

Patients with Albright hereditary osteodystrophy (AHO) have defects in stimulatory G protein signaling due to loss of function mutations in GNAS. The mechanism by which these mutations lead to the AHO phenotype has been difficult to establish due to the inaccessibility of the affected tissues.

OBJECTIVE

The objective of the study was to gain insight into the downstream consequences of abnormal stimulatory G protein signaling in human epithelial tissues.

PATIENTS AND DESIGN

We assessed transcription of GNAS and Gsalpha-stimulated activation of the cystic fibrosis transmembrane conductance regulator (CFTR) in AHO patients, compared with normal controls and patients with cystic fibrosis.

MAIN OUTCOME MEASURES

Relative expression of Gsalpha transcripts from each parental GNAS allele and cAMP measurements from nasal epithelial cells were compared among normal controls and AHO patients. In vivo measurements of CFTR function, pulmonary function, and pancreatic function were assessed in AHO patients.

RESULTS

GNAS was expressed equally from each allele in normals and two of five AHO patients. cAMP generation was significantly reduced in nasal respiratory epithelial cells from AHO patients, compared with normal controls (0.4 vs. 0.6, P = 0.0008). Activation of CFTR in vivo in nasal (P = 0.0065) and sweat gland epithelia (P = 0.01) of AHO patients was significantly reduced from normal. In three patients, the reduction in activity was comparable with patients with cystic fibrosis due to mutations in CFTR. Yet no AHO patients had pulmonary or pancreatic disease consistent with cystic fibrosis.

CONCLUSIONS

In humans, haploinsufficiency of GNAS causes a significant reduction in the activation of the downstream target, CFTR, in vivo.

Conditional overexpression of the wild-type Gs alpha as the gsp oncogene initiates chronic extracellularly regulated kinase 1/2 activation and hormone hypersecretion in pituitary cell lines

In pituitary cells, activation of the cAMP pathway by specific G protein-coupled receptors controls differentiative functions and proliferation. Constitutively active forms of the alpha subunit of the heterotrimeric G(s) protein resulting from mutations at codon 201 or 227 (gsp oncogene) were first identified in 30-40% of human GH-secreting pituitary adenomas. This rate of occurrence suggests that the gsp oncogene is not responsible for initiating the majority of these tumors. Moreover, there is a large overlap between the clinical phenotypes observed in patients with tumors bearing the gsp oncogene and those devoid of this oncogene. To explore the role of G(s)alpha in GH-secreting adenomas, we obtained somatolactotroph GH4C1 cell lines by performing doxycycline-dependent conditional overexpression of the wild-type G(s)alpha protein and expression of the gsp oncogene. Although the resulting adenylyl cyclase and cAMP levels were 10-fold lower in the wild-type G(s)alpha-overexpressing cell line, a sustained MAPK ERK1/2 activation was observed in both cell lines. Overexpression of the wild-type G(s)alpha protein as the gsp oncogene initiated chronic activation of endogenous prolactin synthesis and release, as well as chronic activation of ERK1/2-sensitive human prolactin and GH promoters.

Epigenetic defects of GNAS in patients with pseudohypoparathyroidism and mild features of Albright's hereditary osteodystrophy

CONTEXT

Several endocrine disorders that share resistance to PTH are grouped under the term pseudohypoparathyroidism (PHP). PHP type I, associated with blunted PTH-induced nephrogenous cAMP formation and phosphate excretion, is subdivided according to the presence or absence of additional endocrine abnormalities, Albright's hereditary osteodystrophy (AHO), and reduced Gsalpha activity caused by GNAS mutations.

OBJECTIVE

We sought to identify the molecular defect in four unrelated patients who were thought to have PHP-Ia because of PTH and TSH resistance and mild AHO features.

METHODS

Gsalpha activity and mutation analysis, and assessment of GNAS haplotype, methylation, and gene expression were performed for probands and family members.

RESULTS

Two patients showed modest decreases in erythrocyte Gsalpha activity. Instead of Gsalpha point mutations, however, all four patients showed methylation defects of the GNAS locus, a feature previously described only for PHP-Ib. Furthermore, one patient with an isolated loss of GNAS exon A/B methylation had the 3-kb STX16 deletion frequently identified in PHP-Ib patients. In all but one of the remaining patients, haplotype analysis excluded large deletions or uniparental disomy as the cause of the observed methylation changes.

CONCLUSIONS

Our investigations indicate that an overlap may exist between molecular and clinical features of PHP-Ia and PHP-Ib. No current mechanisms can explain the AHO-like features of our patients, some of which may not be linked to GNAS. Therefore, patients with hormone resistance and AHO-like features in whom coding Gsalpha mutations have been excluded should be evaluated for epigenetic alterations within GNAS.

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.

Progressive osseous heteroplasia-like heterotopic ossification in a male infant with pseudohypoparathyroidism type Ia: a case report

Pseudohypoparathyroidism (PHP) Ia is a rare condition associated with multiple hormone resistance and the Albright Hereditary Osteodystrophy (AHO) phenotype. Progressive osseous heteroplasia (POH) is characterized by progressive ossifications of dermal, skeletal muscle and deep connective tissue during childhood. Both PHP Ia and POH are caused by heterozygous inactivating mutations in the GNAS gene. Maternal inheritance of a GNAS mutation leads to an AHO phenotype with hormonal resistance (PHP Ia), whereas paternal inheritance leads to an AHO phenotype without the hormonal resistance (pseudopseudo-hypoparathyroidism). Pure POH (no other AHO features) is also caused by a paternal inheritance of GNAS mutations. Mutations that cause PHP Ia when maternally inherited can cause POH when paternally inherited. We present an unusual case of a boy with clinical features of both POH and PHP Ia, and a GNAS inactivating mutation.

CASE PRESENTATION

The patient was referred at 1 month of age with a "knot on his leg". Plain radiographs revealed subcutaneous ossifications. PE at age 4 months included: length and weight >95%, a round face, short 4th metacarpals, and extensive subcutaneous ossifications of the lower limbs, buttocks, and back. Studies at age 4 months included an elevated TSH 12.4 mIU/l, free T4 0.86 ng/dl (0.8-2.3), PTH 61 pg/ml (10-65), calcium 9. 8 mg/dl (9.0-11.0), and phosphorus 6. 4 mg/dl (3.8-6.5). By age 16 months, the PTH was elevated at 126 pg/ml. Biopsies of the skin lesions demonstrated osteoma cutis consistent with POH. GNAS analysis revealed a heterozygous deletion in exon 7. The mutation was not detected in either parent.

DISCUSSION

POH and PHP Ia are rare genetic disorders caused by loss of function mutations of the GNAS gene. POH and PHP Ia do not commonly occur in the same individual as they are associated with paternal versus maternal inheritance (imprinting) of an affected GNAS gene. Our patient has evidence of both severe POH and PHP Ia, apparently due to a de novo mutation in GNAS.

Mutations in the Gs alpha gene causing hormone resistance

G-protein-coupled receptors (GPCRs) and G proteins mediate the effects of a number of hormones of relevance to endocrinology. Genes encoding these molecules may be targets of loss- or gain-of-function mutations, resulting in endocrine disorders. The only mutational change of G proteins so far unequivocally associated with endocrine disorders occurs in the Gsalpha gene (GNAS1, guanine nucleotide binding protein alpha stimulating activity polypeptide 1), which activates cyclic AMP (cAMP)-dependent pathways. Heterozygous loss-of-function mutations of GNAS1 in the active maternal allele cause resistance to hormones acting through Gsalpha-coupled GPCRs, whereas somatic gain-of-function mutations cause proliferation of endocrine cells recognizing cAMP as mitogen. This review will focus on inactivating mutations leading to hormone resistance syndromes, i.e., pseudohypoparathyroidism types Ia and Ib.

Resistance to growth hormone releasing hormone and gonadotropins in Albright's hereditary osteodystrophy

Heterozygous inactivating mutations in the Gs alpha gene cause Albright's hereditary osteo-dystrophy (AHO). Consistent with the observation that only maternally inherited mutations lead to resistance to hormone action (pseudohypoparathyroidism type Ia [PHP-Ia), recent studies have provided evidence for a predominant maternal origin of Gs alpha transcripts in endocrine organs, such as thyroid, gonad and pituitary. Accordingly, patients with PHP-Ia display variable degrees of resistance to parathyroid hormone (PTH), thyroid stimulating hormone (TSH), gonadotropins and growth hormone (GH) releasing hormone (GHRH). Although the incidence and the clinical and biochemical characteristics of PTH and TSH resistance have been widely investigated and described, the cause and significance of the reproductive dysfunction in AHO is still poorly understood. The clinical finding of alterations of GH secretion in these patients was described for the first time only 2 years ago. The present report briefly reviews the literature focusing on the actual knowledge about these last two subjects.

Determination of Gs alpha protein activity in Albright's hereditary osteodystrophy

Albright's hereditary osteodystrophy (AHO) is a heterogeneous clinical entity in part associated with pseudohypoparathyroidism (PHP) and other endocrinopathies. It may be caused by diminished Gs alpha protein activity. Heterozygous mutations in the underlying GNAS gene on chromosome 20 have been described. One hundred and six patients with suspected AHO, were investigated, of whom 93 showed a laboratory profile of PHP with low or normal calcium and elevated parathormone with normal vitamin D metabolites, and 13 had no endocrine abnormalities. Gs alpha activity was determined in isolated erythrocyte membranes. Molecular genetic analysis of GNAS exons 2-13 was initiated. Significantly reduced Gs alpha activity was found in 91 patients. In 53 patients with reduced Gs alpha activity, a mutation within GNAS was demonstrated. The mutation detection rate was much lower in AHO patients without endocrinopathies than in those who had PHP. In addition, three of the 15 patients with AHO features but normal Gs alpha activity had genetic variations of GNAS. We conclude that determination of Gs alpha activity can be used as a diagnostic screening procedure in patients with suspected AHO. However, the mutation detection rate in GNAS is highly variable. The genetic heterogeneity of AHO needs further investigation.