Pseudohypoparathyroidism type I-b with neurological involvement is associated with a homozygous PTH1R mutation

G protein-coupled receptor (GPCR) gene variants and human genetic disease

Genetic variations in the genes encoding G protein-coupled receptors (GPCRs) can disrupt receptor structure and function, which can result in human genetic diseases. Disease-causing mutations have been reported in at least 55 GPCRs for more than 66 monogenic diseases in humans. The spectrum of pathogenic and likely pathogenic variants includes loss of function variants that decrease receptor signaling on one extreme and gain of function that may result in biased signaling or constitutive activity, originally modeled on prototypical rhodopsin GPCR variants identified in retinitis pigmentosa, on the other. GPCR variants disrupt ligand binding, G protein coupling, accessory protein function, receptor desensitization and receptor recycling. Next generation sequencing has made it possible to identify variants of uncertain significance (VUS). We discuss variants in receptors known to result in disease and in silico strategies for disambiguation of VUS such as sorting intolerant from tolerant and polymorphism phenotyping. Modeling of variants has contributed to drug development and precision medicine, including drugs that target the melanocortin receptor in obesity and interventions that reverse loss of gonadotropin-releasing hormone receptor from the cell surface in idiopathic hypogonadotropic hypogonadism. Activating and inactivating variants of the calcium sensing receptor (CaSR) gene that are pathogenic in familial hypocalciuric hypercalcemia and autosomal dominant hypocalcemia have enabled the development of calcimimetics and calcilytics. Next generation sequencing has continued to identify variants in GPCR genes, including orphan receptors, that contribute to human phenotypes and may have therapeutic potential. Variants of the CaSR gene, some encoding an arginine-rich region that promotes receptor phosphorylation and intracellular retention, have been linked to an idiopathic epilepsy syndrome. Agnostic strategies have identified variants of the pyroglutamylated RF amide peptide receptor gene in intellectual disability and G protein-coupled receptor 39 identified in psoriatic arthropathy. Coding variants of the G protein-coupled receptor L1 (GPR37L1) orphan receptor gene have been identified in a rare familial progressive myoclonus epilepsy. The study of the role of GPCR variants in monogenic, Mendelian phenotypes has provided the basis of modeling the significance of more common variants of pharmacogenetic significance.

Identification of a novel heterozygous PTH1R variant in a Chinese family with incomplete penetrance

BACKGROUND

Mutations in PTH1R are associated with Jansen-type metaphyseal chondrodysplasia (JMC), Blomstrand osteochondrodysplasia (BOCD), Eiken syndrome, enchondroma, and primary failure of tooth eruption (PFE). Inheritance of the PTH1R gene can be either autosomal dominant or autosomal recessive, indicating the complexity of the gene. Our objective was to identify the phenotypic differences in members of a family with a novel PTH1R mutation.

METHODS

The proband was a 13-year, 6-month-old girl presenting with short stature, abnormal tooth eruption, skeletal dysplasia, and midface hypoplasia. The brother and father of the proband presented with short stature and abnormal tooth eruption. High-throughput sequencing was performed on the proband, and the variant was confirmed in the proband and other family members by Sanger sequencing. Amino acid sequence alignment was performed using ClustalX software. Three-dimensional structures were analyzed and displayed using the I-TASSER website and PyMOL software.

RESULTS

High-throughput genome sequencing and Sanger sequencing validation showed that the proband, her father, and her brother all carried the PTH1R (NM_000316) c.1393G>A (p.E465K) mutation. The c.1393G>A (p.E465K) mutation was novel, as it has not been reported in the literature database. According to the American College of Medical Genetics and Genomics (ACMG) guidelines, the p.E465K variant was considered to have uncertain significance. Biological information analysis demonstrated that this identified variant was highly conserved and highly likely pathogenic.

CONCLUSIONS

We identified a novel heterozygous mutation in the PTH1R gene leading to clinical manifestations with incomplete penetrance that expands the spectrum of known PTH1R mutations.

Venous thrombosis in a pseudohypoparathyroidism patient with a novel GNAS frameshift mutation and complete resolution of vascular calcifications with acetazolamide treatment

Introduction Pseudohypoparathyroidism type IA (PHP1A) is characterized by end-organ resistance to multiple hormones and Albright's hereditary osteodystrophy (AHO). PHP1A is caused by inactivating mutations of the GNAS gene encoding the α-subunit of the stimulatory G protein (Gsα). In line with the underlying genetic defect, impaired inhibition of platelet aggregation has been demonstrated in some patients. However, no PHP1A case with thrombotic events has been described. Also, PHP1A cases typically have subcutaneous ossifications, but soft tissue calcifications are another common finding. Treatment options for those and other non-hormonal features of PHP1A are limited. Case Presentation A female patient presented with short stature, fatigue, and exercise-induced carpopedal spasms at age 117/12 years. Diagnosis of PHP1A was made based on hypocalcemia, hyperphosphatemia, elevated serum PTH, and AHO features, including short stature and brachydactyly. A novel frameshift variant was detected in the last exon of GNAS (c.1065_1068delGCGT, p.R356Tfs*47), showing complete loss of baseline and receptor-stimulated activity in transfected cells. The patient developed venous thrombosis and vascular and subcutaneous calcifications on both forearms after venous puncture on the right and extravasation of calcium gluconate during treatment on the left. The thrombosis and calcifications completely resolved following treatment with low molecular weight heparin and acetazolamide for 5 and 8 months, respectively. Conclusions This case represents the first PHP1A patient displaying thrombosis and the first successful use of acetazolamide for PHP1A-associated soft tissue calcifications, thus providing new insights into the treatment of non-endocrinological features in this disease.

Physiopathology of Phosphate Disorders

Intracellular phosphate is critical for cellular processes such as signaling, nucleic acid synthesis, and membrane function. Extracellular phosphate (Pi) is an important component of the skeleton. Normal levels of serum phosphate are maintained by the coordinated actions of 1,25-dihydroxyvitamin D3, parathyroid hormone and fibroblast growth factor-23, which intersect in the proximal tubule to control the reabsorption of phosphate via the sodium-phosphate cotransporters Npt2a and Npt2c. Furthermore, 1,25-dihydroxyvitamin D3 participates in the regulation of dietary phosphate absorption in the small intestine. Clinical manifestations associated with abnormal serum phosphate levels are common and occur as a result of genetic or acquired conditions affecting phosphate homeostasis. For example, chronic hypophosphatemia leads to osteomalacia in adults and rickets in children. Acute severe hypophosphatemia can affect multiple organs leading to rhabdomyolysis, respiratory dysfunction, and hemolysis. Patients with impaired kidney function, such as those with advanced CKD, have high prevalence of hyperphosphatemia, with approximately two-thirds of patients on chronic hemodialysis in the United States having serum phosphate levels above the recommended goal of 5.5 mg/dL, a cutoff associated with excess risk of cardiovascular complications. Furthermore, patients with advanced kidney disease and hyperphosphatemia (>6.5 mg/dL) have almost one-third excess risk of death than those with phosphate levels between 2.4 and 6.5 mg/dL. Given the complex mechanisms that regulate phosphate levels, the interventions to treat the various diseases associated with hypophosphatemia or hyperphosphatemia rely on the understanding of the underlying pathobiological mechanisms governing each patient condition.

Primary failure of tooth eruption: Etiology and management

Primary failure of eruption (PFE) is a rare disorder defined as incomplete tooth eruption despite the presence of a clear eruption pathway. PFE is known to be caused by rare variants in the parathyroid hormone 1 receptor gene (PTH1R). Although several PTH1R variants have been reported, the etiology of PFE remains unclear. However, important studies that help elucidate the pathology of PFE have recently been published. The purpose of this review is to summarize current treatment options, clinical symptoms or phenotypes for diagnosis, genetic information including solid evidence in mouse disease models and disease-specific induced pluripotent stem cells, thus approaching the etiology of PFE from the perspective of the latest research.

Functional Properties of Two Distinct PTH1R Mutants Associated With Either Skeletal Defects or Pseudohypoparathyroidism

Consistent with a vital role of parathyroid hormone (PTH) receptor type 1 (PTH1R) in skeletal development, homozygous loss-of-function PTH1R mutations in humans results in neonatal lethality (Blomstrand chondrodysplasia), whereas such heterozygous mutations cause a primary failure of tooth eruption (PFE). Despite a key role of PTH1R in calcium and phosphate homeostasis, blood mineral ion levels are not altered in such cases of PFE. Recently, two nonlethal homozygous PTH1R mutations were identified in two unrelated families in which affected members exhibit either dental and skeletal abnormalities (PTH1R-V204E) or hypocalcemia and hyperphosphatemia (PTH1R-R186H). Arg186 and Val204 map to the first transmembrane helix of the PTH1R, and thus to a critical region of this class B G protein-coupled receptor. We used cell-based assays and PTH and PTH-related protein (PTHrP) ligand analogs to assess the impact of the R186H and V204E mutations on PTH1R function in vitro. In transiently transfected HEK293 cells, PTH1R-R186H mediated cyclic adenosine monophosphate (cAMP) responses to PTH(1-34) and PTHrP(1-36) that were of comparable potency to those observed on wild-type PTH1R (PTH1R-WT) (half maximal effective concentrations [EC50s] = 0.4nM to 1.2nM), whereas the response-maxima were significantly reduced for the PTH1R-V204E mutant (maximum effect [Emax] = 81%-77% of PTH1R-WT, p ≤ 0.004). Antibody binding to an extracellular hemagglutinin (HA) tag was comparable for PTH1R-R186H and PTH1R-WT, but was significantly reduced for PTH1R-V204E (maximum binding level [Bmax] = 44% ± 11% of PTH1R-WT, p = 0.002). The potency of cAMP signaling induced by a PTH(1-11) analog was reduced by ninefold and threefold, respectively, for PTH1R-R186H and PTH1R-V204E, relative to PTH1R-WT, and a PTH(1-15) radioligand analog that bound adequately to PTH1R-WT exhibited little or no specific binding to either mutant receptor. The data support a general decrease in PTH1R surface expression and/or function as a mechanism for PFE and a selective impairment in PTH ligand affinity as a potential PTH1R-mutation-based mechanism for pseudohypoparathyroidism. © 2022 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Case Report: Inactivating PTH/PTHrP Signaling Disorder Type 1 Presenting With PTH Resistance

PTH resistance is characterized by elevated parathyroid hormone (PTH) levels, hypocalcemia, hyperphosphatemia and it is classically associated with GNAS locus genetic or epigenetic defects. Inactivating PTH/PTHrP signaling disorders (iPPSD) define overlapping phenotypes based on their molecular etiology. iPPSD1 is associated with PTH1R variants and variable phenotypes including ossification anomalies and primary failure of tooth eruption but no endocrine disorder. Here we report on a 10-month-old child born from consanguineous parents, who presented with mild neurodevelopmental delay, seizures, enlarged fontanelles, round face, and bilateral clinodactyly. Hand x-rays showed diffuse delayed bone age, osteopenia, short metacarpal bones and cone-shaped distal phalanges. A diagnosis of PTH resistance was made on the basis of severe hypocalcemia, hyperphosphatemia, elevated PTH and normal vitamin D levels on blood sample. The patient was treated with calcium carbonate and alfacalcidol leading to rapid bio-clinical improvement. Follow-up revealed multiple agenesis of primary teeth and delayed teeth eruption, as well as Arnold-Chiari type 1 malformation requiring a ventriculoperitoneal shunt placement. GNAS gene analysis showed no pathogenic variation, but a likely pathogenic homozygous substitution c.723C>G p.(Asp241Glu) in PTH1R gene was found by trio-based whole exome sequencing. We studied the deleterious impact of the variant on the protein conformation with bioinformatics tools. In conclusion, our study reports for the first time PTH resistance in a child with a biallelic PTH1R mutation, extending thereby the clinical spectrum of iPPSD1 phenotypes.

A novel long-range deletion spanning STX16 and NPEPL1 causing imprinting defects of the GNAS locus discovered in a patient with autosomal-dominant pseudohypoparathyroidism type 1B

BACKGROUND

Pseudohypoparathyroidism (PHP) is a rare disorder characterized by hypocalcemia, hyperphosphatemia, and resistance to parathyroid hormone (PTH). According to different GNAS mutations, PHP is divided into several subtypes, among which autosomal-dominant PHP1B (AD-PHP1B) is caused by STX16 deletion and epigenetic alteration of GNAS. Although the deletion of STX16 exons 2-6 is commonly observed, other mutations involving STX16 can also result in AD-PHP1B.

MATERIALS AND METHODS

The clinical information of a 38-year-old male PHP patient was collected. The genomic DNA from peripheral blood cells was extracted for genetic analysis of GNAS and upstream STX16 by methylation-specific multiplex ligation-dependent probe amplification (MS-MLPA) and whole-exome sequencing (WES). Sanger sequencing was performed to verify the break point of the novel long-range deletion.

RESULTS

The patient's medical history of tetany and seizure as well as laboratory examination showing hypocalcemia and elevated PTH levels indicated the diagnosis of PHP. The results of MS-MLPA showed loss of methylation of GNAS A/B:TSS-DMR and half-reduced copy number of STX16 exon 1-9, which revealed the subtype of AD-PHP1B. Furthermore, the WES study displayed a 87.5 kb missing upstream of GNAS. A 87.5 kb deletion spanning STX16 and NPEPL1 together with an insertion of 28 bp of unknown origin was verified by PCR along with Sanger sequencing.

CONCLUSIONS

A novel deletion of 87.5 kb spanning STX16 and NPEPL1 was discovered in an AD-PHP1B patient, which provides new information on molecular defects leading to AD-PHP1B.

Pseudohypoparathyroidism type 1B - a rare cause of tetany: case report

Pseudohypoparathyroidism (PHP) is a rare group of disorders characterised by end-organ resistance to the parathyroid hormone (PTH). A 16-year-old boy presented with a 2-year history of involuntary dystonic movements involving mainly the left hand, initially after writing and later during physical exercise. Serum calcium was 1.37 mmol/L (2.20-2.69), phosphate 2.1 mmol/L (0.8-1.45) and PTH 302 ng/L (12-88). CT scan of the head demonstrated multiple subcortical and diffuse basal ganglia calcifications. Genetic analysis confirmed a methylation defect in the GNAS cluster on chromosome 20q13.32 which established the diagnosis. Treatment with calcitriol and calcium carbonate led to complete remission of symptoms. Causes of hypocalcaemia should be considered in evaluating patients with movement disorders. The diagnosis of PHP-1B is challenging but the overall prognosis is excellent.

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.