Selective resistance to parathyroid hormone caused by a novel uncoupling mutation in the carboxyl terminus of G alpha(s). A cause of pseudohypoparathyroidism type Ib

G(s) is a heterotrimeric (alpha, beta, and gamma chains) G protein that couples heptahelical plasma membrane receptors to stimulation of adenylyl cyclase. Inactivation of one GNAS1 gene allele encoding the alpha chain of G(s) (G alpha(s)) causes pseudohypoparathyroidism type Ia. Affected subjects have resistance to parathyroid hormone (PTH) and other hormones that activate adenylyl cyclase plus somatic features termed Albright hereditary osteodystrophy. By contrast, subjects with pseudohypoparathyroidism type Ib have hormone resistance that is limited to PTH and lack Albright hereditary osteodystrophy. The molecular basis for pseudohypoparathyroidism type Ib is unknown. We analyzed the GNAS1 gene for mutations using polymerase chain reaction to amplify genomic DNA from three brothers with pseudohypoparathyroidism type Ib. We identified a novel heterozygous 3-base pair deletion causing loss of isoleucine 382 in the three affected boys and their clinically unaffected mother and maternal grandfather. This mutation was absent in other family members and 15 additional unrelated subjects with pseudohypoparathyroidism type Ib. To characterize the signaling properties of the mutant G alpha(s), we used site-directed mutagenesis to introduce the isoleucine 382 deletion into a wild type G alpha(s) cDNA, transfected HEK293 cells with either wild type or mutant G alpha(s) cDNA, plus cDNAs encoding heptahelical receptors for PTH, thyrotropic hormone, or luteinizing hormone, and we measured cAMP production in response to hormone stimulation. The mutant G alpha(s) protein was unable to interact with the receptor for PTH but showed normal coupling to the other coexpressed heptahelical receptors. These results provide evidence of selective uncoupling of the mutant G alpha(s) from PTH receptors and explain PTH-specific hormone resistance in these three brothers with pseudohypoparathyroidism type Ib. The absence of PTH resistance in the mother and maternal grandfather who carry the same mutation is consistent with current models of paternal imprinting of the GNAS1 gene.     

GNAS Epigenetic Defects and Pseudohypoparathyroidism: Time for a New Classification?

Pseudohypoparathyroidism-Ia and -Ib (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), together with resistance to the action of different hormones that activate the Gs-coupled pathway. In PHP-Ib patients AHO is classically absent and hormone resistance is limited to PTH and TSH. This disorder is caused by GNAS methylation alterations with loss of imprinting at the exon A/B differentially methylated region (DMR) being the most consistent and recurrent defect. The familial form of the disease (AD-PHP-Ib) is typically associated with an isolated loss of imprinting at the exon A/B DMR due to microdeletions disrupting the upstream STX16 gene. In addition, deletions removing the entire NESP55 DMR, located within GNAS, associated with loss of all the maternal GNAS imprints have been identified in some AD-PHP-Ib kindreds. Conversely, most sporadic PHP-Ib cases have GNAS imprinting abnormalities that involve multiple DMRs, but the genetic lesion underlying these defects is unknown. Recently, methylation defects have been detected in a subset of patients with PHP-Ia and variable degrees of AHO, indicating a molecular overlap between the 2 forms. Imprinting defects do not seem to be associated with the severity of AHO neither with specific AHO signs. In conclusion, the latest findings on the molecular basis underlying these defects suggest the existence of a clinical and genetic/epigenetic overlap between PHP-Ia and PHP-Ib, and highlight the necessity of a new clinical classification of these disorders based on molecular findings.     

Epigenetic mechanisms of tumorigenesis.

In the majority of human cancers, heritable loss of gene function through cell division may be mediated as often by epigenetic as by genetic abnormalities. Epigenetic modification occurs through a process of interrelated changes in CpG island methylation and histone modifications. Candidate gene approaches of cell cycle, growth regulatory and apoptotic genes have shown epigenetic modification associated with loss of cognate proteins in sporadic pituitary tumors. A search for novel genes on the basis of their differential methylation has led to the isolation and functional characterization of a pro-apoptotic mediator--a pituitary tumor apoptosis gene ( PTAG). Although PTAG expression is significantly underexpressed in most pituitary adenomas, mechanisms in addition to methylation most likely account for its loss. The GNAS gene is imprinted in normal pituitary, and activating mutations within Gsalpha, referred to as the gsp oncogene, are almost invariably associated with the maternal expressed allele in somatotrophic adenomas. In addition, epigenetic modification, manifesting as relaxation of imprinting, leads to biallelic expression of Gsalpha irrespective of gsp status. Pituitary tumors as components of familial syndromes represent a rare entity, and the role of epigenetic modification in their evolution and outgrowth is not known. Although speculative, these studies might provide new insight since methylation-associated gene silencing is a feature of other familial tumor types.     

Genetic basis for resistance to parathyroid hormone

Pseudohypoparathyroidism (PHP) is associated with biochemical hypoparathyroidism (i.e. hypocalcemia and hyperphosphatemia) due to parathyroid hormone (PTH) resistance rather than to PTH deficiency. Patients with PHP type 1a have a generalized form of hormone resistance plus a constellation of developmental defects termed Albright hereditary osteodystrophy (AHO). Within PHP type 1a families some individuals will show AHO but have normal hormone responsiveness, a variant phenotype termed pseudo-PHP. By contrast, patients with PHP type 1b manifest only PTH resistance and lack features of AHO. These various forms of PHP are due to defects in the GNAS1 gene that lead to decreased expression or activity of the alpha-subunit of the stimulatory G protein (G(s)alpha). Tissue-specific genomic imprinting of GNAS1 accounts for the variable phenotypes of patients with GNAS1 defects.     

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.     

G protein Gs alpha (GNAS 1), the probable candidate gene for Albright hereditary osteodystrophy, is assigned to human chromosome 20q12-q13.2

Guanine nucleotide-binding proteins, also known as G proteins, mediate intracellular responses to a wide variety of extracellular stimuli. A variety of genes that specify the synthesis of the components of guanine nucleotide proteins have been identified. One of these proteins, termed Gs alpha (GNAS1), is the G protein component of the olfactory signal transduction cascade. Mutations in the GNAS1 gene leading to Gs alpha protein deficiency are known to be associated with pseudohypoparathyroidism Ia (Albright hereditary osteodystrophy) and certain pituitary tumors with acromegaly. Studies on the human--mouse somatic cell hybrids provisionally assigned this gene to chromosome 20. We have now confirmed this localization on chromosome 20 and regionally assigned the GNAS1 gene to 20q12-q13.2 by in situ hybridization.     

A Sporadic Case of Pseudohypoparathyroidism Type 1 and Idiopathic Primary Adrenal Insufficiency Associated with a Novel Mutation in the Gnas1 Gene

ObjectiveWe report an atypical association of primary adrenal insufficiency and pseudohypoparathyroidism (PHP) and a novel GNAS1 gene mutation in a Caucasian female who initially presented with adrenal crisis.MethodsA case report and literature review.ResultsA 37-year-old female presented with shock at 11 years of age, and investigations revealed primary adrenal insufficiency and pseudohypoparathyroidism (PHP). She had typical features of Albright hereditary osteodystrophy (AHO) and evidence of thyroid-stimulating hormone (TSH), growth-hormone-releasing hormone (GHRH), and gonadotrophin resistance fitting with the diagnosis of PHP type 1a/1c. She did not have a family history of any autoimmune disease or PHP. Her mother was phenotypically normal. Genomic DNA sequencing of those GNAS exons and adjacent intronic regions that encode the stimulatory guanine nucleotide-binding protein Gsαrevealed a novel heterozygous mutation in exon 11, c.857-858delCT.ConclusionThe association of primary adrenal insufficiency and PHP has not been reported in literature and may prove an area for further research. The novel mutation in this case adds to the spectrum of mutations associated with these disorders. (Endocr Pract. 2014;20:e202-e206)     

G protein defects in signal transduction

G proteins couple receptors for many hormones to effectors that regulate second messenger metabolism. Several endocrine disorders have been shown to be caused by either loss- or gain-of-function mutations in G proteins or G protein-coupled receptors. In pseudohypoparathyroidism type Ia (PHP Ia), there are generalized hormone resistance (parathyroid hormone [PTH], thyroid-stimulating hormone, gonadotropins) and associated abnormal physical features, Albright hereditary osteodystrophy. Subjects with PHP Ib are normal in appearance and show renal resistance to PTH. In McCune-Albright syndrome (MAS), subjects show autonomous endocrine hyperfunction associated with fibrous dysplasia of bone and skin hyperpigmentation. Germline loss-of-function mutations have been identified in the G(s)-alpha gene in PHP Ia, and recent evidence suggests that the G(s)-alpha gene is paternally imprinted in a tissue-specific manner. Abnormal imprinting of the G(s)-alpha gene may be the cause of PHP Ib. MAS, in contrast, is caused by gain-of-function missense mutations of the G(s)-alpha gene.     

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.     

Mouse imprinting defect mutations that model Angelman syndrome

Prader-Willi syndrome (PWS) and Angelman syndrome (AS) are neurobehavioral disorders resulting from deficiency of imprinted gene expression from paternal or maternal chromosome 15q11-15q13, respectively. In humans, expression of the imprinted genes is under control of a bipartite cis-acting imprinting center (IC). Families with deletions causing PWS imprinting defects localize the PWS-IC to 4.3 kb overlapping with SNRPN exon 1. Families with deletions causing AS imprinting defects localize the AS-IC to 880 bp 35 kb upstream of the PWS-IC. We report two mouse mutations resulting in defects similar to that seen in AS patients with deletion of the AS-IC. An insertion/duplication mutation 13 kb upstream of Snrpn exon 1 resulted in lack of methylation at the maternal Snrpn promoter, activation of maternally repressed genes, and decreased expression of paternally repressed genes. The acquisition of a paternal epigenotype on the maternal chromosome in the mutant mice was demonstrated by the ability to rescue the lethality and growth retardation in a mouse model of a PWS imprinting defect. A second mutation, an 80-kb deletion extending upstream of the first mutation, caused a similar imprinting defect with variable penetrance. These results suggest that there is a mouse functional equivalent to the human AS-IC.     

Novel mutations of CDKN1C in Japanese patients with Beckwith-Wiedemann syndrome

Beckwith-Wiedemann syndrome (BWS) is an imprinting-related human disease that is characterized by macrosomia, macroglossia, abdominal wall defects, and variable minor features. BWS is caused by several genetic/epigenetic alterations, such as loss of methylation at KvDMR1, gain of methylation at H19-DMR, paternal uniparental disomy of chromosome 11, CDKN1C mutations, and structural abnormalities of chromosome 11. CDKN1C is an imprinted gene with maternal preferential expression, encoding for a cyclin-dependent kinase (CDK) inhibitor. Mutations in CDKN1C are found in 40 % of familial BWS cases with dominant maternal transmission and in ~5 % of sporadic cases. In this study, we searched for CDKN1C mutations in 37 BWS cases that had no evidence for other alterations. We found five mutations—four novel and one known—from a total of six patients. Four were maternally inherited and one was a de novo mutation. Two frame-shift mutations and one nonsense mutation abolished the QT domain, containing a PCNA-binding domain and a nuclear localization signal. Two missense mutations occurred in the CDK inhibitory domain, diminishing its inhibitory function. The above-mentioned mutations were predicted by in silico analysis to lead to loss of function; therefore, we strongly suspect that such anomalies are causative in the etiology of BWS.     

Screening for Parathyroid Hormone Resistance In Patients With Nonphenotypically Evident Pseudohypoparathyroidism

ObjectiveHypocalcemia and hyperphosphatemia in the setting of elevated parathyroid hormone (PTH) and normal vitamin D metabolites, raises the possibility of PTH resistance. The idiopathic and inherited forms of PTH resistance are referred to as pseudohypoparathyroidism. Nonphenotypically evident pseudohypoparathyroidism can go undiagnosed for decades. We have designed a new test to diagnose PTH resistance and confirmed its clinical utility in the diagnosis of pseudohypoparathyroidism.MethodsOur test consists of a subcutaneous injection of commercially available recombinant PTH and concomi tant measurement of cyclic adenosine monophosphate in urine. We implemented the test in 2 patients with recalcitrant hypocalcemia and a healthy control subject.ResultsOur test unequivocally demonstrated PTH resistance in both patients. One of the patients had phenotypically evident pseudohypoparathyroidism type-1a hence, PTH resistance was suspected. The other patient with nonphenotypically evident disease, also showed PTH resistance and was later demonstrated to have pseudohypoparathyroidism type-1b at the genomic level and confirmed to be of familial type.ConclusionOur results show for the first time the implementation of a simple new diagnostic tool designed to check for PTH resistance. This new test has already proven to be useful in few occasions at our institution. Larger pop ulations, however, should be tested before implementation of such a test is considered a standard of care. (Endocr Pract. 2012;18:864-869)     

Albright's hereditary osteodystrophy associated with cerebellar pilocytic astrocytoma: coincidence or genetic relationship?

Albright's hereditary osteodystrophy (AHO) is a rare inherited disease characterized by skeletal abnormalities, short stature, and, in some cases, resistance to parathyroid hormone, resulting in pseudohypoparathyroidism (PHP). Heterozygous inactivating mutations of the GNAS1 gene are responsible for reduced activity of the alpha subunit of the Gs protein (G(Salpha)), a protein that mediates hormone signal transduction across cell membranes. G(salpha) is also known to have oncogenic potentials, leading to the development of human pituitary tumors and Leydig cell tumors. Here, we report the 1st case, a 3.5-year-old girl, with classic AHO phenotype and PHP type 1A associated with a cerebellar pilocytic astrocytoma. Coincidence or genetic relationships of both diseases are discussed according to molecular findings and current literature.