GNA11 Variants Identified in Patients with Hypercalcemia or Hypocalcemia

Familial hypocalciuric hypercalcemia type 2 (FHH2) and autosomal dominant hypocalcemia type 2 (ADH2) are due to loss- and gain-of-function mutations, respectively, of the GNA11 gene that encodes the G protein subunit Gα11, a signaling partner of the calcium-sensing receptor (CaSR). To date, four probands with FHH2-associated Gα11 mutations and eight probands with ADH2-associated Gα11 mutations have been reported. In a 10-year period, we identified 37 different germline GNA11 variants in >1200 probands referred for investigation of genetic causes for hypercalcemia or hypocalcemia, comprising 14 synonymous, 12 noncoding, and 11 nonsynonymous variants. The synonymous and noncoding variants were predicted to be benign or likely benign by in silico analysis, with 5 and 3, respectively, occurring in both hypercalcemic and hypocalcemic individuals. Nine of the nonsynonymous variants (Thr54Met, Arg60His, Arg60Leu, Gly66Ser, Arg149His, Arg181Gln, Phe220Ser, Val340Met, Phe341Leu) identified in 13 probands have been reported to be FHH2- or ADH2-causing. Of the remaining nonsynonymous variants, Ala65Thr was predicted to be benign, and Met87Val, identified in a hypercalcemic individual, was predicted to be of uncertain significance. Three-dimensional homology modeling of the Val87 variant suggested it was likely benign, and expression of Val87 variant and wild-type Met87 Gα11 in CaSR-expressing HEK293 cells revealed no differences in intracellular calcium responses to alterations in extracellular calcium concentrations, consistent with Val87 being a benign polymorphism. Two noncoding region variants, a 40bp-5'UTR deletion and a 15bp-intronic deletion, identified only in hypercalcemic individuals, were associated with decreased luciferase expression in vitro but no alterations in GNA11 mRNA or Gα11 protein levels in cells from the patient and no abnormality in splicing of the GNA11 mRNA, respectively, confirming them to be benign polymorphisms. Thus, this study identified likely disease-causing GNA11 variants in <1% of probands with hypercalcemia or hypocalcemia and highlights the occurrence of GNA11 rare variants that are benign polymorphisms. © 2023 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Hypoparathyroidism: Genetics and Diagnosis

This narrative report summarizes diagnostic criteria for hypoparathyroidism and describes the clinical presentation and underlying genetic causes of the nonsurgical forms. We conducted a comprehensive literature search from January 2000 to January 2021 and included landmark articles before 2000, presenting a comprehensive update of these topics and suggesting a research agenda to improve diagnosis and, eventually, the prognosis of the disease. Hypoparathyroidism, which is characterized by insufficient secretion of parathyroid hormone (PTH) leading to hypocalcemia, is diagnosed on biochemical grounds. Low albumin-adjusted calcium or ionized calcium with concurrent inappropriately low serum PTH concentration are the hallmarks of the disease. In this review, we discuss the characteristics and pitfalls in measuring calcium and PTH. We also undertook a systematic review addressing the utility of measuring calcium and PTH within 24 hours after total thyroidectomy to predict long-term hypoparathyroidism. A summary of the findings is presented here; results of the detailed systematic review are published separately in this issue of JBMR. Several genetic disorders can present with hypoparathyroidism, either as an isolated disease or as part of a syndrome. A positive family history and, in the case of complex diseases, characteristic comorbidities raise the clinical suspicion of a genetic disorder. In addition to these disorders' phenotypic characteristics, which include autoimmune diseases, we discuss approaches for the genetic diagnosis. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Autosomal Dominant Hypocalcemia Type 1: A Systematic Review

Autosomal dominant hypocalcemia type 1 (ADH1) is a rare form of hypoparathyroidism due to activating variants of the calcium-sensing receptor gene (CASR). Inherited or de novo activating variants of the CASR alter the set point for extracellular calcium, resulting in inadequate parathyroid hormone (PTH) secretion and inappropriate renal calcium excretion leading to hypocalcemia and hypercalciuria. Conventional therapy includes calcium and activated vitamin D, which can worsen hypercalciuria, resulting in renal complications. A systematic literature review, using published reports from 1994 to 2021, was conducted to catalog CASR variants, to define the ADH1 clinical spectrum, and to determine the effect of treatment on patients with ADH1. There were 113 unique CASR variants reported, with a general lack of genotype/phenotype correlation. Clinical data were available in 191 patients; 27% lacked symptoms, 32% had mild/moderate symptoms, and 41% had severe symptoms. Seizures, the most frequent clinical presentation, occurred in 39% of patients. In patients with blood and urine chemistries available at the time of diagnosis (n = 91), hypocalcemia (99%), hyperphosphatemia (59%), low PTH levels (57%), and hypercalciuria (34%) were observed. Blood calcium levels were significantly lower in patients with severe symptoms compared with asymptomatic patients (6.8 ± 0.7 versus 7.6 ± 0.7 mg/dL [mean ± SD]; p < 0.0001), and the age of presentation was significantly lower in severely symptomatic patients (9.1 ± 15.0 versus 19.3 ± 19.4 years; p < 0.01). Assessments for complications including nephrocalcinosis, nephrolithiasis, renal impairment, and brain calcifications in 57 patients on conventional therapy showed that 75% had at least one complication. Hypercalciuria was associated with nephrocalcinosis, nephrolithiasis, renal impairment, or brain calcifications (odds ratio [OR] = 9.3; 95% confidence interval [CI] 2.4-37.2; p < 0.01). In 27 patients with urine calcium measures before and after starting conventional therapy, the incidence of hypercalciuria increased by 91% (p < 0.05) after therapy initiation. ADH1 is a condition often associated with severe symptomatology at presentation with an increase in the risk of renal complications after initiation of conventional therapy. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Treatment of Autosomal Dominant Hypocalcemia Type 1 With the Calcilytic NPSP795 (SHP635)

Autosomal dominant hypocalcemia type 1 (ADH1) is a rare form of hypoparathyroidism caused by heterozygous, gain-of-function mutations of the calcium-sensing receptor gene (CAR). Individuals are hypocalcemic with inappropriately low parathyroid hormone (PTH) secretion and relative hypercalciuria. Calcilytics are negative allosteric modulators of the extracellular calcium receptor (CaR) and therefore may have therapeutic benefits in ADH1. Five adults with ADH1 due to four distinct CAR mutations received escalating doses of the calcilytic compound NPSP795 (SHP635) on 3 consecutive days. Pharmacokinetics, pharmacodynamics, efficacy, and safety were assessed. Parallel in vitro testing with subject CaR mutations assessed the effects of NPSP795 on cytoplasmic calcium concentrations (Ca²⁺_i ), and ERK and p38^MAPK phosphorylation. These effects were correlated with clinical responses to administration of NPSP795. NPSP795 increased plasma PTH levels in a concentration-dependent manner up to 129% above baseline (p = 0.013) at the highest exposure levels. Fractional excretion of calcium (FECa) trended down but not significantly so. Blood ionized calcium levels remained stable during NPSP795 infusion despite fasting, no calcitriol supplementation, and little calcium supplementation. NPSP795 was generally safe and well-tolerated. There was significant variability in response clinically across genotypes. In vitro, all mutant CaRs were half-maximally activated (EC₅₀ ) at lower concentrations of extracellular calcium (Ca²⁺_o ) compared to wild-type (WT) CaR; NPSP795 exposure increased the EC₅₀ for all CaR activity readouts. However, the in vitro responses to NPSP795 did not correlate with any clinical parameters. NPSP795 increased plasma PTH levels in subjects with ADH1 in a dose-dependent manner, and thus, serves as proof-of-concept that calcilytics could be an effective treatment for ADH1. Albeit all mutations appear to be activating at the CaR, in vitro observations were not predictive of the in vivo phenotype or the response to calcilytics, suggesting that other parameters impact the response to the drug. © 2019 American Society for Bone and Mineral Research.

Bone Matrix Mineralization in Patients With Gain-of-Function Calcium-Sensing Receptor Mutations Is Distinctly Different From that in Postsurgical Hypoparathyroidism

The role of the calcium-sensing receptor (CaSR) as a regulator of parathyroid hormone secretion is well established, but its function in bone is less well defined. In an effort to elucidate the CaSR's skeletal role, bone tissue and material characteristics from patients with autosomal dominant hypocalcemia (ADH), a genetic form of primary hypoparathyroidism caused by CASR gain-of-function mutations, were compared to patients with postsurgical hypoparathyroidism (PSH). Bone structure and formation/resorption indices and mineralization density distribution (BMDD), were examined in transiliac biopsy samples from PSH (n = 13) and ADH (n = 6) patients by histomorphometry and quantitative backscatter electron imaging, respectively. Bone mineral density (BMD by DXA) and biochemical characteristics were measured at the time of the biopsy. Because both study groups comprised children and adults, all measured biopsy parameters and BMD outcomes were converted to Z-scores for comparison. Histomorphometric indices were normal and not different between ADH and PSH, with the exception of mineral apposition rate Z-score, which was higher in the ADH group. Similarly, average BMD Z-scores were normal and not different between ADH and PSH. Significant differences were observed for the BMDD: average Z-scores of mean and typical degree of mineralization (CaMean, CaPeak, respectively) were lower (p = 0.02 and p = 0.03, respectively), whereas the heterogeneity of mineralization (CaWidth) and percentage of lower mineralized areas (CaLow) were increased in ADH versus PSH (p = 0.01 and p = 0.002, respectively). The BMDD outcomes point toward a direct, PTH-independent role of the CaSR in the regulation of bone mineralization. © 2018 American Society for Bone and Mineral Research.

Prevention of Injury-Induced Osteoarthritis in Rodent Temporomandibular Joint by Targeting Chondrocyte CaSR

Traumatic joint injuries produce osteoarthritic cartilage manifesting accelerated chondrocyte terminal differentiation and matrix degradation via unknown cellular and molecular mechanisms. Here we report the ability of biomechanical stress to increase expression of the calcium-sensing receptor (CaSR), a pivotal driver of chondrocyte terminal differentiation, in cultured chondrogenic cells subjected to fluid flow shear stress (FFSS) and in chondrocytes of rodent temporomandibular joint (TMJ) cartilage subjected to unilateral anterior cross-bite (UAC). In cultured ATDC5 cells or TMJ chondrocytes, FFSS induced Ca²⁺ loading and CaSR localization in endoplasmic reticulum (ER), casually accelerating cell differentiation that could be abrogated by emptying ER Ca²⁺ stores or CaSR knockdown. Likewise, acute chondrocyte-specific Casr knockout (KO) prevented the UAC-induced acceleration of chondrocyte terminal differentiation and matrix degradation in TMJ cartilage in mice. More importantly, local injections of CaSR antagonist, NPS2143, replicated the effects of Casr KO in preventing the development of osteoarthritic phenotypes in TMJ cartilage of the UAC-treated rats. Our study revealed a novel pathological action of CaSR in development of osteoarthritic cartilage due to aberrant mechanical stimuli and supports a therapeutic potential of calcilytics in preventing osteoarthritis in temporomandibular joints by targeting the CaSR. © 2018 American Society for Bone and Mineral Research.

Polyclonality of Parathyroid Tumors in Neonatal Severe Hyperparathyroidism

Neonatal severe hyperparathyroidism (NSHPT) is a rare disorder characterized by major hypercalcemia, elevated parathyroid hormone levels, and marked enlargement of multiple parathyroid glands, usually associated with germline mutations in the calcium receptor gene CASR. However, little is known about the outgrowth of parathyroid tumors in NSHPT, including whether they represent monoclonal or polyclonal expansions. We sought to examine the clonality of parathyroid tissues resected from a patient with NSHPT and biallelic CASR mutations. DNA from two distinct parathyroid tumors resected from a girl with NSHPT, plus polyclonal/monoclonal control samples, were subjected to analyses of clonality by two independent methods, X-chromosome inactivation analysis at the androgen receptor locus (HUMARA) and BAC array comparative genomic hybridization (CGH). Both parathyroid tumor samples revealed polyclonal patterns by X-inactivation analysis, with polyclonal and monoclonal controls yielding the expected patterns. Similarly, by BAC array CGH, neither parathyroid sample contained monoclonal copy number changes and both appeared identical to the patient-matched polyclonal controls. Our observations provide direct experimental evidence that the markedly enlarged parathyroid tumors in the setting of NSHPT constitute polyclonal, generalized hyperplastic growths rather than monoclonal neoplasms.