Mice maintain predominantly maternal Gαs expression throughout life in brown fat tissue (BAT), but not other tissues

Pseudohypoparathyroidism: complex disease variants with unfortunate names

Several human disorders are caused by genetic or epigenetic changes involving the GNAS locus on chromosome 20q13.3 that encodes the alpha-subunit of the stimulatory G protein (Gsα) and several splice variants thereof. Thus, pseudohypoparathyroidism type Ia (PHP1A) is caused by heterozygous inactivating mutations involving the maternal GNAS exons 1-13 resulting in characteristic abnormalities referred to as Albright's hereditary osteodystrophy (AHO) that are associated with resistance to several agonist ligands, particularly to parathyroid hormone (PTH), thereby leading to hypocalcemia and hyperphosphatemia. GNAS mutations involving the paternal Gsα exons also cause most of these AHO features, but without evidence for hormonal resistance, hence the term pseudopseudohypoparathyroidism (PPHP). Autosomal dominant pseudohypoparathyroidism type Ib (PHP1B) due to maternal GNAS or STX16 mutations (deletions, duplications, insertions, and inversions) is associated with epigenetic changes at one or several differentially methylated regions (DMRs) within GNAS. Unlike the inactivating Gsα mutations that cause PHP1A and PPHP, hormonal resistance is caused in all PHP1B variants by impaired Gsα expression due to loss of methylation at GNAS exon A/B, which can be associated in some familial cases with epigenetic changes at the other maternal GNAS DMRs. The genetic defect(s) responsible for sporadic PHP1B, the most frequent variant of this disorder, remain(s) unknown for the majority of patients. However, characteristic epigenetic GNAS changes can be readily detected that include a gain of methylation at the neuroendocrine secretory protein (NESP) DMR. Multiple genetic or epigenetic GNAS abnormalities can thus impair Gsα function or expression, consequently leading to inadequate cAMP-dependent signaling events downstream of various Gsα-coupled receptors.

Frequency of de novo variants and parental mosaicism in families with inactivating PTH/PTHrP signaling disorder type 2

Objective

iPPSD2 (which includes PHP1A and PPHP/POH) is a rare inherited autosomal dominant endocrine disorder caused by inactivating GNAS pathogenic variants. A high percentage of de novo cases has been suggested. In rare cases, parental mosaicism has been described, but its real frequency is unknown.

Design

A retrospective study including a series of 95 genetically confirmed iPPSD2 probands.

Methods

The frequency of de novo cases was evaluated and the distribution of the type of variants was compared according to the type of inheritance. The putative involved allele was determined by reverse transcriptase PCR (RT-PCR) or allele specific oligonucleotide RT-PCR (ASO-RT-PCR). The possibility of GNAS mosaicism was studied by next-generation sequencing (NGS) on the corresponding parental DNA.

Results

In 41 patients the variant was of de novo origin and in 24 the origin could not be established. In both cases 66.67% of variants generated a truncated or absent protein whereas the rest of the variants were missense or in-frame deletion/duplication. Parental origin was studied in 45 of those patients and determined in 35. Curiously, the percentage of de novo variants at the paternal allele was higher than when paternally inherited (31.1% vs 6.67%). NGS detected mosaicism in three independent families: one from paternal DNA (allelic ratio 10%) and two from maternal DNA (allelic ratio 10% and 2%).

Conclusion

De novo pathogenic variants are frequent in iPPSD2 (around 45%). Parental mosaicism is infrequent (8.11%) but should be analyzed with NGS, taking into account its importance in genetic counselling.

Maternal GNAS Contributes to the Extra-Large G Protein α-Subunit (XLαs) Expression in a Cell Type-Specific Manner

GNAS encodes the stimulatory G protein alpha-subunit (Gsα) and its large variant XLαs. Studies have suggested that XLαs is expressed exclusively paternally. Thus, XLαs deficiency is considered to be responsible for certain findings in patients with paternal GNAS mutations, such as pseudo-pseudohypoparathyroidism, and the phenotypes associated with maternal uniparental disomy of chromosome 20, which comprises GNAS. However, a study of bone marrow stromal cells (BMSC) suggested that XLαs could be biallelically expressed. Aberrant BMSC differentiation due to constitutively activating GNAS mutations affecting both Gsα and XLαs is the underlying pathology in fibrous dysplasia of bone. To investigate allelic XLαs expression, we employed next-generation sequencing and a polymorphism common to XLαs and Gsα, as well as A/B, another paternally expressed GNAS transcript. In mouse BMSCs, Gsα transcripts were 48.4 ± 0.3% paternal, while A/B was 99.8 ± 0.2% paternal. In contrast, XLαs expression varied among different samples, paternal contribution ranging from 43.0 to 99.9%. Sample-to-sample variation in paternal XLαs expression was also detected in bone (83.7-99.6%) and cerebellum (83.8 to 100%) but not in cultured calvarial osteoblasts (99.1 ± 0.1%). Osteoblastic differentiation of BMSCs shifted the paternal XLαs expression from 83.9 ± 1.5% at baseline to 97.2 ± 1.1%. In two human BMSC samples grown under osteoinductive conditions, XLαs expression was also predominantly monoallelic (91.3 or 99.6%). Thus, the maternal GNAS contributes significantly to XLαs expression in BMSCs but not osteoblasts. Altered XLαs activity may thus occur in certain cell types irrespective of the parental origin of a GNAS defect.

Molecular Definition of Pseudohypoparathyroidism Variants

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

Imprinting analysis by droplet digital PCR coupled with locked nucleic acid TaqMan probes

Imprinted genes are differentially expressed in a parent-of-origin-specific manner. Parental origin of the alleles is discriminated by intragenic DNA polymorphisms. Comparisons of parental allelic expression have been analysed by semiquantitative RT-PCR. Here, we developed a novel quantitative method for allelic expression of the imprinted gene Ube3a, which inactivation and mutations cause Angelman syndrome and predominantly expressed by the maternal allele in neuronal tissues. In this method, cDNA was amplified by droplet digital PCR (ddPCR) coupled with allele-specific locked nucleic acid (LNA) TaqMan probes, which labelled by FAM and HEX were designed to detect the SNPs in the target regions. ddPCR assay demonstrated that the sense transcript of Ube3a was equally expressed from both parental alleles in adult tissues except neuronal tissues, where Ube3a expression from the paternal allele was about 10 to 14% of total Ube3a expression in adult brain, and 20% in spinal cord. The antisense transcript of Ube3a was expressed at 60% to 70% of the sense transcript of Ube3a in adult brain. Changes in the Ube3a transcripts during postnatal brain development were also evaluated by ddPCR. The ddPCR method is far more reliable and simpler to use than semiquantitative PCR to analyse skewed or faint allelic expression of imprinted genes.

Clinical and genetic characteristics of Pseudohypoparathyroidism in the Chinese population

BACKGROUND

Pseudohypoparathyroidism (PHP) is caused by mutations and epimutations in the GNAS locus, and characterized by the possibility of resistance to multiple hormones and Albright's hereditary osteodystrophy. PHP can be classified into the forms 1A/C, sporadic 1B and familial 1B.

OBJECTIVES

To obtain an overall view of the clinical and genetic characteristics of the Chinese PHP patient population.

METHODS

From 2000 to 2016, 120 patients were recruited and studied using Sanger sequencing, methylation-specific multiple ligation-dependent probe amplification (MS-MLPA) and combined bisulfite restriction analysis (COBRA). Of these patients, 104 had positive molecular alterations indicative of certain forms of PHP and were included in data analysis. Clinical and laboratory features were compared between PHP1A/C and PHP1B patients.

RESULTS

Ten PHP1A/C, 21 familial PHP1B and 73 sporadic PHP1B patients were identified. Four novel GNAS mutations were discovered in these patients, including c.1038+1G>T, c.530+2T>C, c.880_883delCAAG and c.311_312delAAG, insT. The most common symptoms in this series were recurrent tetany (89.4%) and epilepsy (47.1%). The prevalence of weight excess increased with age for PHP1B (10%-35%) and PHP1A/C (50%-75%). Intracranial calcification had a prevalence of 94.6% and correlated with seizures (r = .227, P = .029). Cataracts occurred in 56.2% PHP patients, and there was a trend towards longer disease duration in patients with cataracts (P = .051). Statistically significant differences (P < .05) were observed when comparing certain clinical characteristics between PHP1B and PHP1A/C patients, including age of onset (10 vs 7 year), short stature (21.3% vs 70%), rounded face (60.6% vs 100%), brachydactyly (25.5% vs 100%), ectopic ossification (1.1% vs 40%) and TSH resistance (44.6% vs 90%), respectively.

CONCLUSIONS

This study is the largest single-centre series of PHP patients and summarizes the clinical and genetic features of the Chinese PHP population. While there was substantial clinical overlap between PHP1A/C and PHP1B, differences in disease progression were observed.