A GNAS1 imprinting defect in pseudohypoparathyroidism type IB

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.

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.

Long noncoding RNA variations in cardiometabolic diseases

Cardiometabolic diseases are characterized as a combination of multiple risk factors for cardiovascular disease (CVD) and metabolic diseases including diabetes mellitus, dyslipidemia, hypertension and abdominal obesity. This cluster of abnormalities individually and interdependently leads to atherosclerosis and CVD morbidity and mortality. In the past decade, genome-wide association studies (GWASs) have identified a series of cardiometabolic disease-associated variants that can collectively explain a small proportion of the variability. Intriguingly, the susceptibility variants imputed from GWASs usually do not reside in the coding regions, suggesting a crucial role of the noncoding elements of the genome. In recent years, emerging evidence suggests that noncoding RNA (ncRNA) is functional for physiology and pathophysiology of human diseases. These include microRNAs and long noncoding RNAs (lncRNAs) that are now implicated in human diseases. The ncRNAs can interact with each other and with proteins, to interfere gene expressions, leading to the development of many human disorders. Although evidence suggests the functional role of lncRNAs in cardiometabolic traits, the molecular mechanisms of gene regulation underlying cardiometabolic diseases remain to be better defined. Here, we summarize the recent discoveries of lncRNA variations in the context of cardiometabolic diseases.

GNAS -Related Loss-of-Function Disorders and the Role of Imprinting

GNAS (guanine nucleotide-binding protein, α stimulating) is a complex imprinted locus coding, besides the α-stimulatory subunit of the G protein, the paternally (extra-large, antisense and A/B) and maternally (neuroendocrine secretory protein) transcripts. Heterozygous mutations in the coding sequence of GNAS produce dominant phenotypes (combination of resistances to hormones signaling through G-protein-coupled receptors, osteodystrophy and obesity) that depend on the parental origin of the mutated allele. Likewise, alterations in the methylation at promoters of GNAS transcripts, associated or not with deletions of imprinting control regions in the nearby STX16 gene or within GNAS, prompt resistance to parathormone when affecting the maternal allele. Therefore, imprinting of GNAS is the determining factor for the variability of the phenotype. Knowledge of the various phenotypes is necessary for genetic counseling as well as an appropriate therapeutic balance between regular follow-up, prevention of disease complications and iatrogeny.

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.

Causes and Consequences of Multi-Locus Imprinting Disturbances in Humans

Eight syndromes are associated with the loss of methylation at specific imprinted loci. There has been increasing evidence that these methylation defects in patients are not isolated events occurring at a given disease-associated locus but that some of these patients may have multi-locus imprinting disturbances (MLID) affecting additional imprinted regions. With the recent advances in technology, methylation profiling has revealed that imprinted loci represent only a small fraction of the methylation differences observed between the gametes. To figure out how imprinting anomalies occur at multiple imprinted domains, we have to understand the interplay between DNA methylation and histone modifications in the process of selective imprint protection during pre-implantation reprogramming, which, if disrupted, leads to these complex imprinting disorders (IDs).

Diagnosis and management of congenital hypothyroidism associated with pseudohypoparathyroidism

Hypothyroidism is a particular condition observed in pseudohypoparathyroidism (PHP), a rare disorder characterized by parathyroid (PTH) resistance leading to hypocalcemia and hyperphosphatemia associated with a GNAS (guanine nucleotide-binding protein α-subunit) mutation (PHP1A) or epimutation (PHP1B). To determine the presence of hypothyroidism at birth we conducted a retrospective study in our cohort of patients presenting with either PHP1A (n = 116) or PHP1B (n = 99). We also investigated patients presenting at birth with congenital hypothyroidism (CH) and a eutopic thyroid gland for phosphocalcium abnormalities suggesting PTH resistance and PHP. Our study reveals CH as the earliest diagnostic clue for PHP1A, but not for PHP1B. We estimated the frequency of CH at birth to be between 8 and 34% in patients presenting with PHP1A. The elevation of phosphatemia and PTH concentration precedes hypocalcemia in PHP1A. Conversely, the frequency of PHP1A in patients presenting CH is dramatically low. This may be due to the low prevalence of PHP1A which remains unknown.

CONCLUSIONS

Subclinical and overt hypothyroidism can occur in PHP1A patients at birth many years before PTH resistance becomes clinically apparent. Although such cases appear to be rare, some pediatric patients with unexplained CH are likely to benefit from measuring calcium, phosphorus, and PTH for extended periods of time.

GNAS Spectrum of Disorders

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

Pseudohypoparathyroidism type Ib in 2015

The term pseudohypoparathryoidism (PHP) refers to a group of rare genetic and epigenetic disorders characterized by resistance to the action of parathyroid hormone (PTH) that activates cAMP signaling in target cells. Together with pseudohypoparathyroidism, Albright hereditary osteodystrophy (AHO) and progressive osseous heteroplasia (POH) represent rare, related and deeply impairing disorders encompassing heterogeneous features, such as brachydactyly, ectopic ossifications, short stature, mental retardation and endocrine deficiencies due to resistance to the action of different hormones. The two main subtypes, PHP-Ia and PHP-Ib, are caused by mutations in GNAS exons 1-13 and methylation defects in the imprinted GNAS cluster respectively, while mutations in the PRKAR1A and PDE4D genes (also involved in mediating cAMP signalling) have been demonstrated in patients with acrodysostosis, a disease of bone formation with characteristics similar to AHO. The molecular overlap among these disorders indicates the need for different classification models and seriously alters our understanding of the mechanisms through which GNAS defects, together with the new recently described defects involving other components of the cAMP signalling cascade, cause AHO-related disorders.

Very small deletions within the NESP55 gene in pseudohypoparathyroidism type 1b

Pseudohypoparathyroidism (PHP) is caused by reduced expression of genes within the GNAS cluster, resulting in parathormone resistance. The cluster contains multiple imprinted transcripts, including the stimulatory G protein α subunit (Gs-α) and NESP55 transcript preferentially expressed from the maternal allele, and the paternally expressed XLas, A/B and antisense transcripts. PHP1b can be caused by loss of imprinting affecting GNAS A/B alone (associated with STX16 deletion), or the entire GNAS cluster (associated with deletions of NESP55 in a minority of cases). We performed targeted genomic next-generation sequencing (NGS) of the GNAS cluster to seek variants and indels underlying PHP1b. Seven patients were sequenced by hybridisation-based capture and fourteen more by long-range PCR and transposon-mediated insertion and sequencing. A bioinformatic pipeline was developed for variant and indel detection. In one family with two affected siblings, and in a second family with a single affected individual, we detected maternally inherited deletions of 40 and 33 bp, respectively, within the deletion previously reported in rare families with PHP1b. All three affected individuals presented with atypically severe PHP1b; interestingly, the unaffected mother in one family had the detected deletion on her maternally inherited allele. Targeted NGS can reveal sequence changes undetectable by current diagnostic methods. Identification of genetic mutations underlying epigenetic changes can facilitate accurate diagnosis and counselling, and potentially highlight genetic elements critical for normal imprint setting.

Loss of methylation at GNAS exon A/B is associated with increased intrauterine growth

CONTEXT

GNAS is one of few genetic loci that undergo allelic-specific methylation resulting in the parent-specific expression of at least four different transcripts. Due to monoallelic expression, heterozygous GNAS mutations affecting either paternally or maternally derived transcripts cause different forms of pseudohypoparathyroidism (PHP), including autosomal-dominant PHP type Ib (AD-PHP1B) associated with loss of methylation (LOM) at exon A/B alone or sporadic PHP1B (sporPHP1B) associated with broad GNAS methylation changes. Similar to effects other imprinted genes have on early development, we recently observed severe intrauterine growth retardation in newborns, later diagnosed with pseudopseudohypoparathyroidism (PPHP) because of paternal GNAS loss-of-function mutations.

OBJECTIVES

This study aimed to determine whether GNAS methylation abnormalities affect intrauterine growth.

PATIENTS AND METHODS

Birth parameters were collected of patients who later developed sporPHP1B or AD-PHP1B, and of their healthy siblings. Comparisons were made to newborns affected by PPHP or PHP1A.

RESULTS

As newborns, AD-PHP1B patients were bigger than their healthy siblings and well above the reference average; increased sizes were particularly evident if the mothers were unaffected carriers of STX16 deletions. SporPHP1B newborns were slightly above average for weight and length, but their overgrowth was less pronounced than that of AD-PHP1B newborns from unaffected mothers.

CONCLUSION

LOM at GNAS exon A/B due to maternal STX16 deletions and the resulting biallelic A/B expression are associated with enhanced fetal growth. These findings are distinctly different from those of PPHP patients with paternal GNAS exons 2-13 mutations, whose birth parameters are almost 4.5 z-scores below those of AD-PHP1B patients born to healthy mothers.

Case report of GNAS epigenetic defect revealed by a congenital hypothyroidism

Pseudohypoparathyroidism (PHP) is a group of disorders characterized by end-organ resistance to the parathyroid hormone (PTH). PHP type 1A includes multihormone resistance syndrome, Albright's hereditary osteodystrophy, and obesity and is caused by mutations in GNAS exon 1 through 13. PHP type 1B (PHP1B), caused by epigenetic changes in the GNAS locus, was initially described as an isolated resistance to PTH. Epigenetic changes in GNAS have also been reported in patients who display mild Albright's hereditary osteodystrophy or mild thyroid-stimulating hormone (TSH) resistance without mutation of GNAS. Here we report a case of PHP caused by epigenetic changes in GNAS in a patient with congenital hypothyroidism. The patient was referred for a positive newborn screening for hypothyroidism (TSH 50 mIU/L). She exhibited severe clinical features of congenital hypothyroidism. The thyroid was in place, and etiologic explorations were negative. TSH was normalized under L-thyroxin, and the symptoms disappeared, except for a macroglossia. In childhood, PHP was suspected in addition to elevated PTH, obesity, brachydactyly, and a rounded face. Sequencing, methylation analysis, and large deletion research were performed in GNAS. No genetic mutations were found. Methylation analysis revealed a broad epigenetic defect without deletion in GNAS consistent with sporadic PHP1B. The multilocus methylation analysis were negative. This finding expands the known onsets of PHP1B and emphasizes the need for a new PHP classification system. This case report has important consequences for the etiologic diagnosis of congenital hypothyroidism because it adds a new cause of the disease.

Pseudohypoparathyroidism type Ib associated with novel duplications in the GNAS locus

Context

Pseudohypoparathyroidism type 1b (PHP-Ib) is characterized by renal resistance to PTH (and, sometimes, a mild resistance to TSH) and absence of any features of Albright's hereditary osteodystrophy. Patients with PHP-Ib suffer of defects in the methylation pattern of the complex GNAS locus. PHP-Ib can be either sporadic or inherited in an autosomal dominant pattern. Whereas familial PHP-Ib is well characterized at the molecular level, the genetic cause of sporadic PHP-Ib cases remains elusive, although some molecular mechanisms have been associated with this subtype.

Objective

The aim of the study was to investigate the molecular and imprinting defects in the GNAS locus in two unrelated patients with PHP-Ib.

Design

We have analyzed the GNAS locus by direct sequencing, Methylation-Specific Multiplex Ligation-dependent Probe Amplification, microsatellites, Quantitative Multiplex PCR of Short Fluorescent fragments and array-Comparative Genomic Hybridization studies in order to characterize two unrelated families with clinical features of PHP-Ib.

Results

We identified two duplications in the GNAS region in two patients with PHP-Ib: one of them, comprising ∼320 kb, occurred ‘de novo’ in the patient, whereas the other one, of ∼179 kb in length, was inherited from the maternal allele. In both cases, no other known genetic cause was observed.

Conclusion

In this article, we describe the to-our-knowledge biggest duplications reported so far in the GNAS region. Both are associated to PHP-Ib, one of them occurring ‘de novo’ and the other one being maternally inherited.

Transcription driven somatic DNA methylation within the imprinted Gnas cluster

Differential marking of genes in female and male gametes by DNA methylation is essential to genomic imprinting. In female gametes transcription traversing differentially methylated regions (DMRs) is a common requirement for de novo methylation at DMRs. At the imprinted Gnas cluster oocyte specific transcription of a protein-coding transcript, Nesp, is needed for methylation of two DMRs intragenic to Nesp, namely the Nespas-Gnasxl DMR and the Exon1A DMR, thereby enabling expression of the Gnas transcript and repression of the Gnasxl transcript. On the paternal allele, Nesp is repressed, the germline DMRs are unmethylated, Gnas is repressed and Gnasxl is expressed. Using mutant mouse models, we show that on the paternal allele, ectopic transcription of Nesp traversing the intragenic Exon1A DMR (which regulates Gnas expression) results in de novo methylation of the Exon1A DMR and de-repression of Gnas just as on the maternal allele. However, unlike the maternal allele, methylation on the mutant paternal allele occurs post-fertilisation, i.e. in somatic cells. This, to our knowledge is the first example of transcript/transcription driven DNA methylation of an intragenic CpG island, in somatic tissues, suggesting that transcription driven de novo methylation is not restricted to the germline in the mouse. Additionally, Gnasxl is repressed on a paternal chromosome on which Nesp is ectopically expressed. Thus, a paternally inherited Gnas cluster showing ectopic expression of Nesp is “maternalised” in terms of Gnasxl and Gnas expression. We show that these mice have a phenotype similar to mutants with two expressed doses of Gnas and none of Gnasxl.

Renal phosphate wasting in the absence of adenylyl cyclase 6

Parathyroid hormone (PTH) and fibroblast growth factor 23 (FGF-23) enhance phosphate excretion by the proximal tubule of the kidney by retrieval of the sodium-dependent phosphate transporters (Npt2a and Npt2c) from the apical plasma membrane. PTH activates adenylyl cyclase (AC) through PTH 1 receptors and stimulates the cAMP/PKA signaling pathway. However, the precise role and isoform(s) of AC in phosphate homeostasis are not known. We report here that mice lacking AC6 (AC6(-/-)) have increased plasma PTH and FGF-23 levels compared with wild-type (WT) mice but comparable plasma phosphate concentrations. Acute activation of the calcium-sensing receptor or feeding a zero phosphate diet almost completely suppressed plasma PTH levels in both AC6(-/-) and WT mice, indicating a secondary cause for hyperparathyroidism. Pharmacologic blockade of FGF receptors resulted in a comparable increase in plasma phosphate between genotypes, whereas urinary phosphate remained significantly higher in AC6(-/-) mice. Compared with WT mice, AC6(-/-) mice had reduced renal Npt2a and Npt2c protein abundance, with approximately 80% of Npt2a residing in lysosomes. WT mice responded to exogenous PTH with redistribution of Npt2a from proximal tubule microvilli to intracellular compartments and lysosomes alongside a PTH-induced dose-response relationship for fractional phosphate excretion and urinary cAMP excretion. These responses were absent in AC6(-/-) mice. In conclusion, AC6 in the proximal tubule modulates cAMP formation, Npt2a trafficking, and urinary phosphate excretion, which are highlighted by renal phosphate wasting in AC6(-/-) mice.

Interplay between polymorphisms and methylation in the H19/IGF2 gene region may contribute to obesity in Mexican-American children

Imprinted genes often affect body size-related traits such as weight. However, the association of imprinting with obesity, especially childhood obesity, has not been well studied. Mexican-American children have a high prevalence, approaching 50%, of obesity and/or overweight. In a pilot study of 75 Mexican-American children, we analyzed the relationships among obese/overweight status, methylation status and single-nucleotide polymorphism (SNP) status at a CpG site in a differentially methylated region (DMR) of the imprinted H19/IGF2 locus. We observed a significant difference in SNP rs10732516 frequency between boys and girls among the overweight and obese children but not among the lean children. We also found that children with lower methylation of the polymorphic CpG site (CpG4) in the H19 DMR had higher birth weights than did children with higher methylation (P = 0.04). Our results suggest that CpG4 methylation status may be associated with childhood obesity in Mexican-American children in a sex-specific manner.

European guidance for the molecular diagnosis of pseudohypoparathyroidism not caused by point genetic variants at GNAS: an EQA study

Pseudohypoparathyroidism is a rare endocrine disorder that can be caused by genetic (mainly maternally inherited inactivating point mutations, although intragenic and gross deletions have rarely been reported) or epigenetic alterations at GNAS locus. Clinical and molecular characterization of this disease is not that easy because of phenotypic, biochemical and molecular overlapping features between both subtypes of the disease. The European Consortium for the study of PHP (EuroPHP) designed the present work with the intention of generating the standards of diagnostic clinical molecular (epi)genetic testing in PHP patients. With this aim, DNA samples of eight independent PHP patients carrying GNAS genetic and/or epigenetic defects (three patients with GNAS deletions, two with 20q uniparental disomy and three with a methylation defect of unknown origin) without GNAS point mutations were anonymized and sent to the five participant laboratories for their routine genetic analysis (methylation-specific (MS)-MLPA, pyrosequencing and EpiTYPER) and interpretations. All laboratories were able to detect methylation defects and, after the data analysis, the Consortium compared the results to define technical advantages and disadvantages of different techniques. To conclude, we propose as first-level investigation in PHP patients copy number and methylation analysis by MS-MLPA. Then, in patients with partial methylation defect, the result should be confirmed by single CpG bisulphite-based methods (ie pyrosequencing), whereas in case of a complete methylation defect without detectable deletion, microsatellites or SNP genotyping should be performed to exclude uniparental disomy 20.

Comprehensive and quantitative multilocus methylation analysis reveals the susceptibility of specific imprinted differentially methylated regions to aberrant methylation in Beckwith-Wiedemann syndrome with epimutations

Purpose:

Expression of imprinted genes is regulated by DNA methylation of differentially methylated regions (DMRs). Beckwith–Wiedemann syndrome is an imprinting disorder caused by epimutations of DMRs at 11p15.5. To date, multiple methylation defects have been reported in Beckwith–Wiedemann syndrome patients with epimutations; however, limited numbers of DMRs have been analyzed. The susceptibility of DMRs to aberrant methylation, alteration of gene expression due to aberrant methylation, and causative factors for multiple methylation defects remain undetermined.

Methods:

Comprehensive methylation analysis with two quantitative methods, matrix-assisted laser desorption/ionization mass spectrometry and bisulfite pyrosequencing, was conducted across 29 DMRs in 54 Beckwith–Wiedemann syndrome patients with epimutations. Allelic expressions of three genes with aberrant methylation were analyzed. All DMRs with aberrant methylation were sequenced.

Results:

Thirty-four percent of KvDMR1–loss of methylation patients and 30% of H19DMR–gain of methylation patients showed multiple methylation defects. Maternally methylated DMRs were susceptible to aberrant hypomethylation in KvDMR1–loss of methylation patients. Biallelic expression of the genes was associated with aberrant methylation. Cis-acting pathological variations were not found in any aberrantly methylated DMR.

Conclusion:

Maternally methylated DMRs may be vulnerable to DNA demethylation during the preimplantation stage, when hypomethylation of KvDMR1 occurs, and aberrant methylation of DMRs affects imprinted gene expression. Cis-acting variations of the DMRs are not involved in the multiple methylation defects.

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.

Genomic imprinting in mammals

Genomic imprinting affects a subset of genes in mammals and results in a monoallelic, parental-specific expression pattern. Most of these genes are located in clusters that are regulated through the use of insulators or long noncoding RNAs (lncRNAs). To distinguish the parental alleles, imprinted genes are epigenetically marked in gametes at imprinting control elements through the use of DNA methylation at the very least. Imprinted gene expression is subsequently conferred through lncRNAs, histone modifications, insulators, and higher-order chromatin structure. Such imprints are maintained after fertilization through these mechanisms despite extensive reprogramming of the mammalian genome. Genomic imprinting is an excellent model for understanding mammalian epigenetic regulation.

Early origins of adult disease: approaches for investigating the programmable epigenome in humans, nonhuman primates, and rodents

According to the developmental origins of health and disease hypothesis, in utero experiences reprogram an individual for immediate adaptation to gestational perturbations, with the sequelae of later-in-life risk of metabolic disease. An altered gestational milieu with resultant adult metabolic disease has been observed in instances of both in utero constraint (e.g., from famine or uteroplacental insufficiency) and overt caloric abundance (e.g., from a maternal high-fat, caloric-dense diet). The commonality of the adult metabolic phenotype begs the question of how diverse in utero experiences (i.e., reprogramming events) converge on common metabolic pathways and how the memory of these events is maintained across the lifespan. We and others have investigated the molecular mechanisms underlying fetal programming and observed that epigenetic modifications to the fetal and placental epigenome accompany these reprogramming events. Based on several lines of emerging data in human and nonhuman primates, it is now felt that modified epigenetic signature--and the histone code in particular--underlies alterations in postnatal gene expression and metabolic pathways central to accurate functioning and maintenance of health. Because of the tissue lineage specificity of many of these modifications, nonhuman primates serve as an apt model system for the capacity to recapitulate human gene expression and regulation during development. This review summarizes recent epigenetic advances using rodent and primate (both human and nonhuman) models during in utero development and contributing to adult diseases later in life.

DNMT1 and AIM1 Imprinting in human placenta revealed through a genome-wide screen for allele-specific DNA methylation

BACKGROUND

Genomic imprinting is an epigenetically regulated process wherein genes are expressed in a parent-of-origin specific manner. Many imprinted genes were initially identified in mice; some of these were subsequently shown not to be imprinted in humans. Such discrepancy reflects developmental, morphological and physiological differences between mouse and human tissues. This is particularly relevant for the placenta. Study of genomic imprinting thus needs to be carried out in a species and developmental stage-specific manner. We describe here a new strategy to study allele-specific DNA methylation in the human placenta for the discovery of novel imprinted genes.

RESULTS

Using this methodology, we confirmed 16 differentially methylated regions (DMRs) associated with known imprinted genes. We chose 28 genomic regions for further testing and identified two imprinted genes (DNMT1 and AIM1). Both genes showed maternal allele-specific methylation and paternal allele-specific transcription. Imprinted expression for AIM1 was conserved in the cynomolgus macaque placenta, but not in other macaque tissues or in the mouse.

CONCLUSIONS

Our study indicates that while there are many genomic regions with allele-specific methylation in tissues like the placenta, only a small sub-set of them are associated with allele-specific transcription, suggesting alternative functions for such genomic regions. Nonetheless, novel tissue-specific imprinted genes remain to be discovered in humans. Their identification may help us better understand embryonic and fetal development.

Genetic and epigenetic states of the GNAS complex in pseudohypoparathyroidism type Ib using methylation-specific multiplex ligation-dependent probe amplification assay

CONTEXT

Pseudohypoparathyroidism type Ib (PHP-Ib) is a rare disorder resulting from genetic and epigenetic aberrations in the GNAS complex. PHP-Ib, usually defined by renal resistance to parathyroid hormone, is due to a maternal loss of GNAS exon A/B methylation and leads to decreased expression of the stimulatory G protein α (Gsα) in specific tissues.

OBJECTIVE

To clarify the usefulness of methylation-specific multiplex ligation-dependent probe amplification (MS-MLPA), we evaluated genetic and epigenetic changes of the GNAS locus in Japanese PHP-Ib patients.

DESIGN

Retrospective case series.

PATIENTS

We studied 13 subjects with PHP-Ib (three families with eight affected members and one unaffected member and four sporadic cases).

MEASUREMENTS

The methylation status of GNAS differentially methylated regions (DMRs) was evaluated using MS-MLPA. The main outcome measure was the presence of deletion mutations in the GNAS locus and STX16, which were assessed using MLPA.

RESULTS

In all familial PHP-Ib cases, a ~3 kb deletion of STX16 and demethylation of the A/B domain were identified. In contrast, no deletion was detected throughout the entire GNAS locus region in the sporadic cases. Broad methylation abnormalities were observed in the GNAS DMRs.

CONCLUSIONS

MS-MLPA allows for precise and rapid analysis of the methylation status in GNAS DMRs as well as the detection of microdeletion mutations in PHP-Ib. Results confirm the previous findings in this disorder and demonstrate that this method is valuable for the genetic evaluation and visualizing the methylation status. The MS-MLPA assay is a useful tool that may facilitate making the molecular diagnosis of PHP-Ib.

Pseudohypoparathyroidism: a rare but important cause of hypocalcaemia

We present a 46-year-old Caucasian lady with symptomatic hypocalcaemia. Investigations revealed markedly raised parathyroid hormone (PTH) levels with vitamin D deficiency. A number of conditions causing secondary hyperparathyroidism were ruled out from her medical history and initial investigations. The main differential diagnoses were vitamin D deficiency and PTH resistance (pseudohypoparathyroidism, PHP). With high-normal serum phosphate and normal alkaline phosphatase, and a lack of symptoms associated with osteomalacia, vitamin D deficiency alone was unlikely to be the cause of hypocalcaemia. Given a normal physical appearance, genetic testing was arranged and confirmed the diagnosis of PHP type Ib. She is currently taking activated vitamin D to maintain calcium homeostasis. PTH resistance is the hallmark of PHP, a rare complex genetic disorder, which can be easily missed resulting in potentially serious consequences.

The GNAS complex locus and human diseases associated with loss-of-function mutations or epimutations within this imprinted gene

GNAS is a complex imprinted locus leading to several different gene products that show exclusive monoallelic expression. GNAS also encodes the α-subunit of the stimulatory G protein (Gsα), a ubiquitously expressed signaling protein that is essential for the actions of many hormones and other endogenous molecules. Gsα is expressed biallelically in most tissues but its expression is silenced from the paternal allele in a small number of tissues. The tissue-specific paternal silencing of Gsα results in different parent-of-origin-specific phenotypes in patients who carry inactivating GNAS mutations. In this paper, we review the GNAS complex locus and discuss how disruption of Gsα expression and the expression of other GNAS products shape the phenotypes of human disorders caused by mutations in this gene.

Cellular functions of genetically imprinted genes in human and mouse as annotated in the gene ontology

By analyzing the cellular functions of genetically imprinted genes as annotated in the Gene Ontology for human and mouse, we found that imprinted genes are often involved in developmental, transport and regulatory processes. In the human, paternally expressed genes are enriched in GO terms related to the development of organs and of anatomical structures. In the mouse, maternally expressed genes regulate cation transport as well as G-protein signaling processes. Furthermore, we investigated if imprinted genes are regulated by common transcription factors. We identified 25 TF families that showed an enrichment of binding sites in the set of imprinted genes in human and 40 TF families in mouse. In general, maternally and paternally expressed genes are not regulated by different transcription factors. The genes Nnat, Klf14, Blcap, Gnas and Ube3a contribute most to the enrichment of TF families. In the mouse, genes that are maternally expressed in placenta are enriched for AP1 binding sites. In the human, we found that these genes possessed binding sites for both, AP1 and SP1.

Non-Coding RNAs at the Gnas and Snrpn-Ube3a Imprinted Gene Loci and Their Involvement in Hereditary Disorders

Non-coding RNAs (ncRNAs) have long been recognized at imprinted gene loci and provided early paradigms to investigate their functions and molecular mechanisms of action. The characteristic feature of imprinted genes, their monoallelic, parental-origin-dependent expression, is achieved through complex epigenetic regulation, which is modulated by ncRNAs. This minireview focuses on two imprinted gene clusters, in which changes in ncRNA expression contribute to human disorders. At the GNAS locus loss of NESP RNA can cause autosomal dominant Pseudohypoparathyroidism type 1b (AD-PHP-Ib), while at the SNRPN-UBE3A locus a long ncRNA and processed snoRNAs play a role in Angelman-Syndrome (AS) and Prader–Willi-Syndrome (PWS). The ncRNAs silence overlapping protein-coding transcripts in sense or anti-sense orientation through changes in histone modifications as well as DNA methylation at CpG-rich sequence motifs. Their epigenetic modulatory functions are required in early development in the pre-implantation embryo or already in the parental germ cells. However, it remains unclear whether the sequence homology-carrying ncRNA itself is required, or whether the process of its transcription through other promoters causes the silencing effect.

Neuromuscular symptoms in a patient with familial pseudohypoparathyroidism type Ib diagnosed by methylation-specific multiplex ligation-dependent probe amplification

Pseudohypoparathyroidism type Ib (PHP-Ib) is a rare genetic disorder characterized by hypocalcemia and hyperphosphatemia due to imprinting defects in the maternally derived GNAS allele. Patients with PHP-Ib are usually identified by tetany, convulsions, and/or muscle cramps, whereas a substantial fraction of patients remain asymptomatic and are identified by familial studies. Although previous studies on patients with primary hypoparathyroidism have indicated that hypocalcemia can be associated with various neuromuscular abnormalities, such clinical features have been rarely described in patients with PHP-Ib. Here, we report a 12-year-old male patient with familial PHP-Ib and unique neuromuscular symptoms. The patient presented with general fatigue, steppage gait, and myalgia. Physical examinations revealed muscular weakness and atrophies in the lower legs, a shortening of the bilateral Achilles' tendons and absence of deep tendon reflexes. Laboratory tests showed hypocalcemia, hyperphosphatemia, elevated serum intact PTH level, and impaired responses of urinary phosphate and cyclic AMP in an Ellsworth-Howard test, in addition to an elevated serum creatine kinase level. Clinical features of the patient were significantly improved after 1 month of treatment with alfacalcidol and calcium. Methylation-specific multiplex ligation-dependent probe amplification (MS-MLPA) and subsequent PCR analyses identified a methylation defect at exon A/B of GNAS and a microdeletion involving exons 4-6 of the GNAS neighboring gene STX16 in the patient and in his asymptomatic brother. The results suggest that various neuromuscular features probably associated with hypocalcemia can be the first symptoms of PHP-Ib, and that MS-MLPA serves as a powerful tool for screening of GNAS abnormalities in patients with atypical manifestations.

Development and treatment of tertiary hyperparathyroidism in patients with pseudohypoparathyroidism type 1B

CONTEXT

Pseudohypoparathyroidism type 1B (PHP1B) patients have PTH resistance at the renal proximal tubule and develop hypocalcemia and secondary hyperparathyroidism. Hyperparathyroid bone disease also develops in some patients. PHP1B patients are at theoretical risk of developing tertiary hyperparathyroidism.

SETTING

Patients were studied in a clinical research center.

PATIENTS

Five female PHP1B patients presented with hypercalcemia and elevated PTH.

INTERVENTION

Patients either underwent parathyroidectomy (n = 4) or received cinacalcet (n = 1).

MAIN OUTCOME MEASURES

Serum calcium and PTH were serially measured before and after intervention.

RESULTS

Five PHP1B patients developed concomitantly elevated serum calcium and PTH levels (range, 235-864 ng/liter) requiring termination of calcium and vitamin D therapy (time after diagnosis, 21-42 yr; median, 34 yr), consistent with tertiary hyperparathyroidism. Four patients underwent parathyroidectomy with removal of one (n = 2) or two (n = 2) enlarged parathyroid glands. Calcium and vitamin D therapy was reinstituted postoperatively, and at 93-month median follow-up, PTH levels ranged between 56 and 182 (normal, <87) ng/liter. One patient was treated with cinacalcet, resulting in resolution of hypercalcemia.

CONCLUSIONS

PHP1B patients are at risk of developing tertiary hyperparathyroidism and/or hyperparathyroid bone disease and should therefore be treated with sufficient doses of calcium and vitamin D to achieve serum calcium and PTH levels within or as close to the normal range as possible. Surgery is the treatment of choice in this setting. Cinacalcet may be a useful alternative in those who do not undergo surgery.

Methylation defect in imprinted genes detected in patients with an Albright's hereditary osteodystrophy like phenotype and platelet Gs hypofunction

BACKGROUND

Pseudohypoparathyroidism (PHP) indicates a group of heterogeneous disorders whose common feature is represented by impaired signaling of hormones that activate Gsalpha, encoded by the imprinted GNAS gene. PHP-Ib patients have isolated Parathormone (PTH) resistance and GNAS epigenetic defects while PHP-Ia cases present with hormone resistance and characteristic features jointly termed as Albright's Hereditary Osteodystrophy (AHO) due to maternally inherited GNAS mutations or similar epigenetic defects as found for PHP-Ib. Pseudopseudohypoparathyroidism (PPHP) patients with an AHO phenotype and no hormone resistance and progressive osseous heteroplasia (POH) cases have inactivating paternally inherited GNAS mutations.

METHODOLOGY/PRINCIPAL FINDINGS

We here describe 17 subjects with an AHO-like phenotype that could be compatible with having PPHP but none of them carried Gsalpha mutations. Functional platelet studies however showed an obvious Gs hypofunction in the 13 patients that were available for testing. Methylation for the three differentially methylated GNAS regions was quantified via the Sequenom EpiTYPER. Patients showed significant hypermethylation of the XL amplicon compared to controls (36 ± 3 vs. 29 ± 3%; p<0.001); a pattern that is reversed to XL hypomethylation found in PHPIb. Interestingly, XL hypermethylation was associated with reduced XLalphaS protein levels in the patients' platelets. Methylation for NESP and ExonA/B was significantly different for some but not all patients, though most patients have site-specific CpG methylation abnormalities in these amplicons. Since some AHO features are present in other imprinting disorders, the methylation of IGF2, H19, SNURF and GRB10 was quantified. Surprisingly, significant IGF2 hypermethylation (20 ± 10 vs. 14 ± 7%; p<0.05) and SNURF hypomethylation (23 ± 6 vs. 32 6%; p<0.001) was found in patients vs. controls, while H19 and GRB10 methylation was normal.

CONCLUSION/SIGNIFICANCE

In conclusion, this is the first report of methylation defects including GNAS in patients with an AHO-like phenotype without endocrinological abnormalities. Additional studies are still needed to correlate the methylation defect with the clinical phenotype.

Detection of hypomethylation syndrome among patients with epigenetic alterations at the GNAS locus

CONTEXT

Genomic imprinting is the modification of the genome so that genes from only one (rather than two) of the parental alleles are expressed. The mechanism underlying imprinting is epigenetic, occurring via changes in DNA methylation and histone modifications rather than through alterations in the DNA sequence. To date, nine different imprinting disorders have been clinically and genetically identified and a considerable research effort has been focused on determining the cause of the corresponding methylation defects.

OBJECTIVE

Our objective was to identify multilocus imprinting defects and characterize any mutations in trans-acting genes in patients with pseudohypoparathyroidism (PHP) caused by epigenetic alterations at GNAS locus.

DESIGN

We have investigated multilocus imprinting defects in 22 PHP patients with aberrant methylation at the GNAS locus not due to previously described deletions or to paternal uniparental disomy (UPD) of chromosome 20.

RESULTS

We found that, in contrast to what has been described in growth disorders, multilocus hypomethylation is an uncommon event in PHP patients. We were also unable to identify any genetic alteration causative of the epigenetic defects in the currently known methylation regulatory genes.

CONCLUSION

Our work suggests that a trans-acting gene regulating the establishment or maintenance of imprinting at GNAS locus, if it exists, should be specific to PHP cases caused by epigenetic defects at GNAS.

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

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

Genomic imprinting of the type 3 thyroid hormone deiodinase gene: regulation and developmental implications

BACKGROUND

In recent years, findings in a number of animal and human models have ignited renewed interest in the type 3 deiodinase (D3), the main enzyme responsible for the inactivation of thyroid hormones. The induction of D3 in models of illness and injury has raised critical questions about the physiological significance of reduced thyroid hormone availability in those states. Phenotypes in transgenic mice lacking this enzyme also point to important developmental roles for D3. A critical determinant of D3 expression is genomic imprinting, an epigenetic phenomenon that regulates a small number of dosage-critical genes in the mammalian genome. The D3 gene (Dio3) is imprinted and preferentially expressed from one of the alleles in most tissues.

SCOPE OF REVIEW

In the context of the physiological significance of D3 and the characteristics and purported origins of genomic imprinting, we review the current knowledge about the epigenetic mechanisms specifying gene dosage in the Dio3 locus.

MAJOR CONCLUSIONS

Altered Dio3 dosage is detrimental to development, suggesting that the level of thyroid hormone action needs to be exquisitely tailored in a timely fashion to the requirements of particular tissues. An appropriate Dio3 dosage is the result of the coordinated action of certain genomic elements and epigenetic marks in the Dlk1-Dio3 domain.

GENERAL SIGNIFICANCE

The imprinting of Dio3 prompts intriguing questions about why the level of thyroid hormone signaling should be regulated in this rare epigenetic manner, and to what extent altered Dio3 expression due to aberrant imprinting may be implicated in human conditions. This article is part of a Special Issue entitled Thyroid hormone signalling.

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

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

Imprinted genes and hypothalamic function

Genomic imprinting is an important and enigmatic form of gene regulation in mammals in which one copy of a gene is silenced in a manner determined by its parental history. Imprinted genes range from those with constitutive monoallelic silencing to those, typically more remote from imprinting control regions, that display developmentally regulated, tissue-specific or partial monoallelic expression. This diversity may make these genes, and the processes they control, more or less sensitive to factors that modify or disrupt epigenetic marks. Imprinted genes have important functions in development and physiology, including major endocrine/neuroendocrine axes. Owing to is central role in coordinating growth, metabolism and reproduction, as well as evidence from genetic and knockout studies, the hypothalamus may be a focus for imprinted gene action. Are there unifying principles that explain why a gene should be imprinted? Conflict between parental genomes over limiting maternal resources, but also co-adaptation between mothers and offspring, have been invoked to explain the evolution of imprinting. Recent reports suggest there may be many more genes imprinted in the hypothalamus than hitherto expected, and it will be important for these new candidates to be validated and to determine whether they conform to current notions of how imprinting is regulated. In fully evaluating the role of imprinted genes in the hypothalamus, much work needs to be done to identify the specific neuronal populations in which particular genes are expressed, establish whether there are pathways in common and whether imprinted genes are involved in long-term programming of hypothalamic functions.

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.

Establishment of diagnosis by bisulfite-treated methylation-specific PCR method and analysis of clinical characteristics of pseudohypoparathyroidism type 1b

Pathogenesis of pseudohypoparathyroidism type 1b (PHP-1b) is related to the loss of methylation at the GNAS exon A/B region, which is combined with epigenetic defects at other differentially methylated GNAS regions in most sporadic cases. In this study, we established a method for evaluating the methylation status of a CpG island in exon A/B using a methylation-specific polymerase chain reaction (MSPCR). We designed two primer pairs specific for the methylated and unmethylated alleles and evaluated the methylation status of GNAS exon A/B in samples from PHP-1b patients and normal controls. We examined 20 Japanese patients and 20 normal controls, and one primer set was found to be appropriate for diagnosis. Furthermore, we evaluated the clinical data of patients. Weight and height of patients were not significantly different from the normal population. Short stature (adult height ≤ 2SD) was observed in one patient and short metacarpals in two. Obesity was observed in six patients, and no patient showed ectopic subcutaneous calcification. Seven patients showed subclinical hypothyroidism because of resistance to thyroid stimulating hormone, but no patient had an abnormally low free thyroxine level, and none received oral thyroid hormone replacement. For diagnosis of PHP-1b, only clinical data is not sufficient because a few PHP-1b patients show clinical features similar to PHP-1a, and hence, molecular biology techniques are required for correct diagnosis. We concluded that MSPCR is applicable for rapid molecular diagnosis of PHP-1b.

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

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

The GNAS Locus: Quintessential Complex Gene Encoding Gsalpha, XLalphas, and other Imprinted Transcripts

The currently estimated number of genes in the human genome is much smaller than previously predicted. As an explanation for this disparity, most individual genes have multiple transcriptional units that represent a variety of biologically important gene products. GNAS exemplifies a gene of such complexity. One of its products is the alpha-subunit of the stimulatory heterotrimeric G protein (Gsalpha), a ubiquitous signaling protein essential for numerous different cellular responses. Loss-of-function and gain-of-function mutations within Gsalpha-coding GNAS exons are found in various human disorders, including Albright's hereditary osteodystrophy, pseudohypoparathyroidism, fibrous dysplasia of bone, and some tumors of different origin. While Gsalpha expression in most tissues is biallelic, paternal Gsalpha expression is silenced in a small number of tissues, playing an important role in the development of phenotypes associated with GNAS mutations. Additional products derived exclusively from the paternal GNAS allele include XLalphas, a protein partially identical to Gsalpha, and two non-coding RNA molecules, the A/B transcript and the antisense transcript. The maternal GNAS allele leads to NESP55, a chromogranin-like neuroendocrine secretory protein. In vivo animal models have demonstrated the importance of each of the exclusively imprinted GNAS products in normal mammalian physiology. However, although one or more of these products are also disrupted by most naturally occurring GNAS mutations, their roles in disease pathogenesis remain unknown. To further our understanding of the significance of this gene in physiology and pathophysiology, it will be important to elucidate the cellular roles and the mechanisms regulating the expression of each GNAS product.

Paternal uniparental isodisomy of the entire chromosome 20 as a molecular cause of pseudohypoparathyroidism type Ib (PHP-Ib)

Pseudohypoparathyoridism type Ib (PHP-Ib) typically defines the presence of end-organ resistance to parathyroid hormone in the absence of Albright's hereditary osteodystrophy. Patients affected by this disorder present with imprinting defects in the complex GNAS locus. Microdeletions within STX16 or GNAS have been identified in familial cases with PHP-Ib, but the molecular cause of the GNAS imprinting defects in sporadic PHP-Ib cases remains poorly defined. We now report a case with sporadic PHP-Ib for whom a SNPlex analysis revealed loss of the maternal GNAS allele. Further analysis of the entire genome with a 100K SNP chip identified a paternal uniparental isodisomy affecting the entire chromosome 20 without evidence for another chromosomal abnormality. Our findings explain the observed GNAS methylation changes and the patient's hormone resistance, and furthermore suggest that chromosome 20 harbors, besides GNAS, no additional imprinted region that contributes to the clinical and laboratory phenotype.

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

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

Diseases associated with genomic imprinting

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

A new approach to imprinting mutation detection in GNAS by Sequenom EpiTYPER system

BACKGROUND

Pseudohypoparathyroidism type Ib (PHPIb) results from abnormal imprinting of GNAS. Familial and sporadic forms of PHPIb have distinct GNAS imprinting patterns: familial PHPIb patients have an exon A/B-only imprinting defect and an intragenic STX16 deletion, whereas sporadic PHPIb cases have abnormal imprinting of the three differentially methylated regions (DMRs) in GNAS without the STX16 deletion. Overall GNAS methylation defects have recently been detected in some PHPIa patients.

METHODS

This study describes the first quantitative methylation analysis of multiple CpG sites for three different GNAS DMRs using the Sequenom EpiTYPER in 35 controls, 12 PHPIb patients, 2 PHPIa patients and 2 patients without parathormone (PTH) resistance but having only hypocalcemia and hyperphosphatemia.

RESULTS

All patients have GNAS methylation defects typically with NESP hypermethylation versus XL and exon A/B hypomethylation while the imprinting of SNURF/SNRPN was normal. PHPIa patients showed an abnormal methylation in the three DMRs of GNAS. For the first time, a marked abnormal GNAS methylation was also found in 2 patients without PTH resistance but having hypocalcemia and hyperphosphatemia.

CONCLUSIONS

The Sequenom EpiTYPER proves to be very sensitive in detecting DNA methylation changes. Our analysis also suggests that GNAS imprinting defects might be more frequent and diverse than previously thought.

New mechanisms involved in paternal 20q disomy associated with pseudohypoparathyroidism

PURPOSE

Type I pseudohypoparathyroidism (PHP-I) can be subclassified into Ia and Ib, depending on the presence or absence of Albright's hereditary osteodystrophy's phenotype, diminished α-subunit of the stimulatory G protein (G(s)α) activity and multihormonal resistance. Whereas PHP-Ia is mainly associated with heterozygous inactivating mutations in G(s)α-coding exons of GNAS, PHP-Ib is caused by imprinting defects of GNAS. To date, just one patient with PHP and complete paternal uniparental disomy (UPD) has been described. We sought to identify the underlining molecular defect in twenty patients with parathyroid hormone resistance, hypocalcemia and hyperphosphatemia, and abnormal methylation pattern at GNAS locus.

METHODS

Microsatellite typing and comparative genome hybridization were performed for proband and parents.

RESULTS

We describe four patients with partial paternal UPD of chromosome 20 involving pat20qUPD in one case, from 20q13.13-qter in two cases, and pat20p heterodisomy plus interstitial 20q isodisomy in one patient.

CONCLUSIONS

These observations demonstrate that mitotic recombination of chromosome 20 can also give rise to UPD and PHP, a situation similar to other imprinting disorders, such as Beckwith-Wiedemann syndrome or neonatal diabetes.

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.

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.