Different mutations within or upstream of the GNAS locus cause distinct forms of pseudohypoparathyroidism

Osteocytes control myeloid cell proliferation and differentiation through Gsα-dependent and -independent mechanisms

Osteocytes, the bone cells embedded in the mineralized matrix, control bone modeling, and remodeling through direct contact with adjacent cells and via paracrine and endocrine factors that affect cells in the bone marrow microenvironment or distant organs. Osteocytes express numerous G protein-coupled receptors (GPCRs) and thus mice lacking the stimulatory subunit of G-protein (Gsα) in osteocytes (Dmp1-Gsα^KO mice) have abnormal myelopoiesis, osteopenia, and reduced adipose tissue. We previously reported that the severe osteopenia and the changes in adipose tissue present in these mice were mediated by increased sclerostin, which suppress osteoblast functions and promote browning of white adipocytes. Inversely, the myeloproliferation was driven by granulocyte colony-stimulating factor (G-CSF) and administration of neutralizing antibodies against G-CSF only partially restored the myeloproliferation, suggesting that additional osteocyte-derived factors might be involved. We hypothesized that osteocytes secrete Gsα-dependent factor(s) which regulate the myeloid cells proliferation. To identify osteocyte-secreted proteins, we used the osteocytic cell line Ocy454 expressing or lacking Gsα expression (Ocy454-Gsα^cont and Ocy454-Gsα^KO ) to delineate the osteocyte "secretome" and its regulation by Gsα. Here we reported that factors secreted by osteocytes increased the number of myeloid colonies and promoted macrophage proliferation. The proliferation of myeloid cells was further promoted by osteocytes lacking Gsα expression. Myeloid cells can differentiate into bone-resorbing osteoclasts, therefore, we hypothesized that osteocyte-secreted factors might also regulate osteoclastogenesis in a Gsα-dependent manner. Conditioned medium (CM) from Ocy454 (both Gsα^cont and Gsα^KO ) significanlty increased the proliferation of bone marrow mononuclear cells (BMNC) and, at the same time, inhibited their differentiation into mature osteoclasts via a Gsα-dependent mechanism. Proteomics analysis of CM from Ocy454 Gsα^cont and Gsα^KO cells identified neuropilin-1 (Nrp-1) and granulin (Grn) as osteocytic-secreted proteins upregulated in Ocy454-Gsα^KO cells compared to Ocy454-Gsα^cont , whereas semaphorin3A was significantly suppressed. Treatment of Ocy454-Gsα^cont cells with recombinant proteins or knockdown of Nrp-1 and Grn in Ocy454-Gsα^KO cells partially rescued the inhibition of osteoclasts, demonstrating that osteocytes control osteoclasts differentiation through Nrp-1 and Grn which are regulated by Gsα signaling.

The Prevalence of GNAS Deficiency-Related Diseases in a Large Cohort of Patients Characterized by the EuroPHP Network

CONTEXT

The term pseudohypoparathyroidism (PHP) was coined to describe the clinical condition resulting from end-organ resistance to parathormone (rPTH), caused by genetic and/or epigenetic alterations within or upstream of GNAS. Although knowledge about PHP is growing, there are few data on the prevalence of underlying molecular defects.

OBJECTIVE

The purpose of our study was to ascertain the relative prevalence of PHP-associated molecular defects.

DESIGN

With a specially designed questionnaire, we collected data from all patients (n = 407) clinically and molecularly characterized to date by expert referral centers in France, Italy, and Spain.

RESULTS

Isolated rPTH (126/407, 31%) was caused only by epigenetic defects, 70% of patients showing loss of imprinting affecting all four GNAS differentially methylated regions and 30% loss of methylation restricted to the GNAS A/B:TSS-DMR. Multihormone resistance with no Albright's hereditary osteodystrophy (AHO) signs (61/407, 15%) was essentially due to epigenetic defects, although 10% of patients had point mutations. In patients with rPTH and AHO (40/407, 10%), the rate of point mutations was higher (28%) and methylation defects lower (about 70%). In patients with multihormone resistance and AHO (155/407, 38%), all types of molecular defects appeared with different frequencies. Finally, isolated AHO (18/407, 4%) and progressive osseous heteroplasia (7/407, 2%) were exclusively caused by point mutations.

CONCLUSION

With European data, we have established the prevalence of various genetic and epigenetic lesions in PHP-affected patients. Using these findings, we will develop objective criteria to guide cost-effective strategies for genetic testing and explore the implications for management and prognosis.

Identification of a novel mutation in a patient with pseudohypoparathyroidism type Ia

Pseudohypoparathyroidism type Ia (PHP Ia) is a disorder characterized by multiform hormonal resistance including parathyroid hormone (PTH) resistance and Albright hereditary osteodystrophy (AHO). It is caused by heterozygous inactivating mutations within the Gs alpha-encoding GNAS exons. A 9-year-old boy presented with clinical and laboratory abnormalities including hypocalcemia, hyperphosphatemia, PTH resistance, multihormone resistance and AHO (round face, short stature, obesity, brachydactyly and osteoma cutis) which were typical of PHP Ia. He had a history of repeated convulsive episodes that started from the age of 2 months. A cranial computed tomography scan showed bilateral calcifications in the basal ganglia and his intelligence quotient testing indicated mild mental retardation. Family history revealed that the patient's maternal relatives, including his grandmother and 2 of his mother's siblings, had features suggestive of AHO. Sequencing of the GNAS gene of the patient identified a heterozygous nonsense mutation within exon 11 (c.637 C>T). The C>T transversion results in an amino acid substitution from Gln to stop codon at codon 213 (p.Gln213^*). To our knowledge, this is a novel mutation in GNAS.

Autosomal dominant pseudohypoparathyroidism type Ib: a novel inherited deletion ablating STX16 causes loss of imprinting at the A/B DMR

CONTEXT

Pseudohypoparathyroidism type Ib (PHP-Ib) is a rare imprinting disorder characterized by end-organ resistance to PTH and, frequently, to thyroid-stimulating hormone. PHP-Ib familial form, with an autosomal dominant pattern of transmission (autosomal dominant pseudohypoparathyroidism type Ib [AD-PHP-Ib]), is typically characterized by an isolated loss of methylation at the guanine nucleotide-binding protein α-stimulating activity polypeptide 1 A/B differentially methylated region (DMR), secondary to genetic deletions disrupting the upstream imprinting control region in the syntaxin-16 (STX16) locus. However, deletions described up to now failed to account some cases of patients with a methylation defect limited to the A/B DMR; thus, it is expected the existence of other still unknown rearrangements, undetectable with conventional molecular diagnostic methods.

OBJECTIVE

We investigated a PHP-Ib patient with a methylation defect limited to the A/B DMR and no known STX16 deletions to find the underlying primary genetic defect.

PATIENT AND METHODS

A PHP-Ib patient (hypocalcaemia, hyperphosphataemia, raised serum PTH levels, no vitamin D deficiency) and his unaffected relatives were investigated by methylation specific-multiplex ligand-dependent probe amplification to search for novel pathogenetic defects affecting the guanine nucleotide-binding protein α-stimulating activity polypeptide 1 and STX16 loci.

RESULTS

We report the clinical, biochemical, and molecular analysis of an AD-PHP-Ib patient with a novel STX16 deletion overlapping with previously identified STX16 deletions but that, unlike these genetic defects associated with AD-PHP-Ib, goes unnoticed with commonly used first-level diagnostic techniques.

CONCLUSIONS

Our work highlights the importance of performing accurate investigations in PHP-Ib patients with methylation defects to allow precise genetic counseling because, in case of deletions, the segregation ratio is about 50% and the disease phenotype is transmitted in an autosomal dominant fashion via the mother.

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.

Pseudohypoparathyroidism as a rare cause of bilateral slipped capital femoral epiphysis

Slipped capital femoral epiphysis (SCFE) is a disorder of adolescent age. Presentation of SCFE earlier than the expected age range should prompt the clinician to consider the presence of an underlying endocrinopathy. Early recognition and aggressive management of the predisposing endocrine disorder is crucial to prevent treatment failure and associated morbidity. We report the clinical presentation and treatment of an 8-year-old girl with bilateral slipped capital femoral epiphysis. The unusual age, persistent hypocalcemia, and associated distal femoral physeal deformities prompted further evaluations, which led to the diagnosis of pseudohypoparathyroidism type 1b. PHP type 1b is an extremely rare cause of SCFE and only a few cases have been reported. A delay in diagnosis in such case is not uncommon.

A naturally occurring isoform inhibits parathyroid hormone receptor trafficking and signaling

Parathyroid hormone (PTH) regulates calcium homeostasis and bone remodeling through its cognitive receptor (PTHR). We describe here a PTHR isoform harboring an in-frame 42-bp deletion of exon 14 (Δe14-PTHR) that encodes transmembrane domain 7. Δe14-PTHR was detected in human kidney and buccal epithelial cells. We characterized its topology, cellular localization, and signaling, as well as its interactions with PTHR. The C-terminus of the Δe14-PTHR is extracellular, and cell surface expression is strikingly reduced compared with the PTHR. Δe14-PTHR displayed impaired trafficking and accumulated in endoplasmic reticulum. Signaling and activation of cAMP and ERK by Δe14-PTHR was decreased significantly compared with PTHR. Δe14-PTHR acts as a functional dominant-negative by suppressing the action of PTHR. Cells cotransfected with both receptors exhibit markedly reduced PTHR cell membrane expression, colocalization with Δe14-PTHR in endoplasmic reticulum, and diminished cAMP activation and ERK phosphorylation in response to challenge with PTH. Δe14-PTHR forms heterodimers with PTHR, which may account for cytoplasmic retention of PTHR in the presence of Δe14-PTHR. Analysis of the PTHR heteronuclear RNA suggests that base-pair complementarity in introns surrounding exon 14 causes exon skipping and accounts for generation of the Δe14-PTHR isoform. Thus Δe14-PTHR is a poorly functional receptor that acts as a dominant-negative of PTHR trafficking and signaling and may contribute to PTH resistance.

Imprinting status of Galpha(s), NESP55, and XLalphas in cell cultures derived from human embryonic germ cells: GNAS imprinting in human embryonic germ cells

GNAS is a complex gene that through use of alternative first exons encodes signaling proteins Galpha(s) and XLalphas plus neurosecretory protein NESP55. Tissue-specific expression of these proteins is regulated through reciprocal genomic imprinting in fully differentiated and developed tissue. Mutations in GNAS account for several human disorders, including McCune-Albright syndrome and Albright hereditary osteodystrophy, and further knowledge of GNAS imprinting may provide insights into variable phenotypes of these disorders. We therefore analyzed expression of Galpha(s), NESP55, and XLalphas prior to tissue differentiation in cell cultures derived from human primordia germ cells. We found that the expression of Galpha(s) was biallelic (maternal allele: 52.6%+/- 2.5%; paternal allele: 47.2%+/- 2.5%; p= 0.07), whereas NESP55 was expressed preferentially from the maternal allele (maternal allele: 81.9%+/- 10%; paternal allele: 18.1%+/- 10%; p= 0.002) and XLalphas was preferentially expressed from the paternal allele (maternal allele: 2.7%+/- 0.3%; paternal allele: 97.3%+/- 0.3%; p= 0.007). These results demonstrate that imprinting of NESP55 occurs very early in development, although complete imprinting appears to take place later than 5-11 weeks postfertilization, and that imprinting of XLalphas occurs very early postfertilization. By contrast, imprinting of Galpha(s) most likely occurs after 11 weeks postfertilization and after tissue differentiation.

Spinal-cord compression related to pseudohypoparathyroidism

We report a 24-year-old male with pseudohypoparathyroidism and a 6-month history of sensory disturbance in both legs which was associated with difficulty in walking. His physical signs included a short stature, a thick neck, short fourth metacarpals and metatarsals, a spastic paraparesis and sphincteric disturbance. His serum electrolytes included low serum calcium and high serum phosphorus levels. CT reconstruction showed compression of the spinal cord in association with ossified ligamentum flavum at the C2-7 and T9-10 levels. These findings were confirmed by MRI scans.

Recessive versus imprinted disorder: consanguinity can impede establishing the diagnosis of autosomal dominant pseudohypoparathyroidism type Ib

Hypocalcemia and hyperphosphatemia with low/normal parathyroid hormone (PTH) levels can be observed in hypoparathyroidism (HP), a disorder that may follow an autosomal dominant (AD) or autosomal recessive (AR) mode of inheritance. Similar biochemical changes are also observed in pseudohypoparathyroidism (PHP) type Ia and Ib, but affected patients usually show elevated PTH levels indicative of hormonal resistance. Features of Albright's hereditary osteodystrophy (AHO) are typically not observed in patients affected by familial forms of PHP-Ib, which are most frequently caused by maternally inherited, heterozygous microdeletions within STX16 and are associated with isolated loss of methylation at GNAS exon A/B. We established the molecular defect in two children of consanguineous Turkish parents, who presented with hypocalcemia, hyperphosphatemia, and low 25-OH vitamin D levels, but initially normal or only mildly elevated PTH levels, i.e. findings that do not readily exclude HP. After normalizing serum magnesium levels, hypocalcemia and hyperphosphatemia persisted, and PTH levels increased, suggesting PTH resistance rather than PTH deficiency. Because of the absence of AHO and parental consanguinity, an AR form of PHP-Ib appeared plausible, which had previously been suggested for sporadic cases. However, loss of GNAS methylation was restricted to exon A/B, which led to the identification of the 3-kb STX16 microdeletion. The same mutation was also detected in the healthy mother, who did not show any GNAS methylation abnormality, indicating that her deletion resides on the paternal allele. Our findings emphasize the importance of considering a parentally imprinted, AD disorder even if consanguinity suggests an AR mode of inheritance.

Preimplantation genetic diagnosis for severe albright hereditary osteodystrophy

CONTEXT

Preimplantation genetic diagnosis (PGD) enables the selection of embryos without mutations for implantation and has not been described to our knowledge for mutations in GNAS. Phocomelia in a patient with Albright hereditary osteodystrophy (AHO) has also not been previously described.

OBJECTIVE

The aim of this study was to identify a GNAS mutation in a patient with a severe form of AHO and pseudohypoparathyroidism type 1a with phocomelia and to perform PGD on embryos derived by in vitro fertilization to deliver an unaffected infant.

DESIGN

A proband and his family are described clinically, the GNAS gene was sequenced to identify a novel mutation in the proband, and PGD was performed on embryos.

SETTING

The setting was in a tertiary-care hospital.

PATIENTS

The patients were from a single family in which the proband has a severe form of AHO.

INTERVENTIONS

Interventions were PGD and in vitro fertilization.

MAIN OUTCOME MEASURES

The main outcome measures were the clinical phenotypes and GNAS gene sequences of the proband, embryos, and family members.

RESULTS

After PGD, three genotypically normal embryos were transferred back to the mother. Pregnancy ensued, and a healthy male infant was delivered at 36.5 wk gestation. The GNAS genes in the baby were confirmed as wild-type, and the infant is free of any signs of AHO.

CONCLUSIONS

We describe herein a proband with AHO and severe skeletal deformities (including phocomelia) related to a novel GNAS mutation and the delivery of a male infant with homozygous normal GNAS genotype after PGD.

Genetics of pseudohypoparathyroidism types Ia and Ic

Pseudohypoparathyroidism (PHP) types Ia and Ic result from heterozygous inactivating mutations of Gs alpha, the alpha-subunit of the heterotrimeric stimulatory G-protein, Gs. Both are characterized by a combination of Albright's hereditary osteodystrophy and, when the mutation is maternally inherited, end-organ resistance to multiple hormones. Due to complex tissue-specific imprinting of Gs alpha, paternally-derived mutations do not usually lead to hormone resistance. More than 100 mutations have been characterized in patients with PHP-Ia and one mutation in type Ic. These are scattered throughout the gene, with one significant mutational hotspot in exon 7. Identification of mutations in a clinical service setting is important for accurate genetic counselling and clinical management of affected families. However, only 70-80% of mutations are identified by direct sequencing of coding exons and splice junctions. Screening for whole exon deletions and intronic or regulatory mutations in mutation-negative families is therefore now an important priority to establish the full mutational spectrum in these conditions.