A pediatric case of autosomal dominant hypocalcemia type 2

OBJECTIVES

Autosomal dominant hypocalcemia (ADH) is characterized by hypocalcemia and hyperphosphatemia secondary to hypoparathyroidism. It is classified as type 1, caused by gain-of-function mutations of the calcium-sensing receptor (CASR), and type 2, caused by activating mutations in GNA11, which is a crucial mediator of CASR signaling. What is new? We report a rare pediatric case of ADH type 2.

CASE PRESENTATION

The patient was a 15-year-old girl with short stature. Blood tests demonstrated hypocalcemia and hyperphosphatemia without elevated parathyroid hormone levels. Brain computed tomography revealed calcification in the bilateral basal ganglia. Genetic testing revealed the rare GNA11 mutation, c.1023C>G (p.Phe341Leu). The patient was diagnosed with ADH type 2. She had experienced numbness and tetany in her hands for several years, which improved with alfacalcidol therapy.

CONCLUSIONS

Our patient is the third female and first pediatric reported case of a variant mutation in the GNA11 gene (ADH type 2), c.1023C>G (p.Phe341Leu).

Case Report: Calcium sensing receptor gene gain of function mutations: a case series and report of 2 novel mutations

Autosomal dominant hypocalcemia (ADH1) is a genetic disorder characterized by low serum calcium and low or inappropriately normal levels of parathyroid hormone. The disease is caused by a heterozygous activating mutation of the calcium-sensing receptor (CaSR) gene, encoding a G-Protein-coupled cell membrane sensor of extracellular calcium concentration mainly expressed by parathyroid glands, renal tubules, and the brain. ADH1 has been linked to 113 unique germline mutations, of which nearly 96% are missense mutations. There is often a lack of a clear genotype/phenotype correlation in the reported literature. Here, we described a case series of 6 unrelated ADH1 probands, each one bearing a gain-of-function CaSR mutation, and two children of one of these cases, matching our identified mutations to the same ones previously reported in the literature, and comparing the clinical and biochemical characteristics, as well as the complication profile. As a result of these genetic and clinical comparisons, we propose that a genotype/phenotype correlation may exist because our cases showed similar presentation, characteristics, and severity, with respect to published cases with the same or similar mutations. We also contend that the severity of the presentation is highly influenced by the specific CaSR variant. These findings, however, require further evaluation and assessment with a systematic review.

Autosomal Dominant Hypocalcemia With Atypical Urine Findings Accompanied by Novel CaSR Gene Mutation and VitD Deficiency

Introduction

Autosomal dominant hypocalcemia (ADH) is caused by gain-of-function mutations of the calcium sensing receptor (CaSR). It is characterized by hypercalciuria in spite of hypocalcemia. Vitamin D deficiency increases calcium reabsorption in the distal tubules of the kidneys, resulting in hypocalciuria.

Materials and methods

A 38-year-old female proband had hypocalcemia, hypocalciuria, and vitamin D deficiency. Her father and brother also had hypocalcemia, but her mother was normocalcemic. We analyzed the CaSR gene abnormality in this family. Polymerase chain reaction (PCR) and sequence analysis were performed to explore the CaSR gene mutation. Mutagenesis, transfection, and functional analysis were performed on the discovered genetic abnormalities.

Result

PCR and sequence analysis revealed that the proband, her father, and brother had a novel heterozygous mutation of the CaSR genes that causes threonine to asparagine substitution at codon 186 (T186N). Using HEK293 cells transfected with wild-type or T186N CaSR complementary DNA, we assessed the intracellular Ca²⁺ concentration in response to changes in the extracellular Ca²⁺ concentration. The cells transfected mutant CaSR gene had higher activity than that of wild-type. Therefore, we determined our patient had ADH with a novel mutation of the CaSR gene and hypocalciuria resulting from a vitamin D deficiency. We administered vitamin D to the proband, which caused elevation of her urinary calcium level, a typical finding of ADH.

Conclusion

Vitamin D deficiency was suggested to potentially mask hypercalciuria in ADH. Hypocalcemia with vitamin D deficiency should be diagnosed with care.

Long-Term Parathyroid Hormone 1-34 Replacement Therapy in Children with Hypoparathyroidism

OBJECTIVE

To determine whether multiple daily injections of parathyroid hormone (PTH) 1-34 are safe and effective as long-term therapy for children with hypoparathyroidism.

STUDY DESIGN

Linear growth, bone accrual, renal function, and mineral homeostasis were studied in a long-term observational study of PTH 1-34 injection therapy in 14 children.

METHODS

Subjects were 14 children with hypoparathyroidism attributable to autoimmune polyglandular syndrome type 1 (N = 5, ages 7-12 years) or calcium receptor mutation (N = 9, ages 7-16 years). Mean daily PTH 1-34 dose was 0.75 ± 0.15 µg/kg/day. Treatment duration was 6.9 ± 3.1 years (range 1.5-10 years). Patients were evaluated semiannually at the National Institutes of Health Clinical Center.

RESULTS

Mean height velocity and lumbar spine, whole body, and femoral neck bone accretion velocities were normal throughout the study. In the first 2 years, distal one-third radius bone accrual velocity was reduced compared with normal children (P < .003). Serum alkaline phosphatase correlated with PTH 1-34 dose (P < .006) and remained normal (235.3 ± 104.8 [SD] U/L, N: 51-332 U/L). Mean serum and 24-hour urine calcium levels were 2.05 ± 0.11 mmol/L (N: 2.05-2.5 mmol/L) and 6.93 ± 1.3 mmol/24 hour (N: 1.25-7.5 mmol/24 hour), respectively-with fewer high urine calcium levels vs baseline during calcitriol and calcium treatment (P < .001). Nephrocalcinosis progressed in 5 of 12 subjects who had repeated renal imaging although renal function remained normal.

CONCLUSIONS

Twice-daily or thrice-daily subcutaneous PTH 1-34 injections provided safe and effective replacement therapy for up to 10 years in children with hypoparathyroidism because of autoimmune polyglandular syndrome type 1 or calcium receptor mutation.

Causes and pathophysiology of hypoparathyroidism

Hypoparathyroidism, a disorder characterized by hypocalcemia ensuing from inadequate parathyroid hormone secretion, is a rather rare disorder caused by multiple etiologies. When not caused by inadvertent damage or removal of the parathyroids during neck surgery, it is usually genetically determined. Epidemiological figures of this disease are still scarce and mainly limited to countries where non-anonymous databases are available and to surgical case series. Both the surgical and non-surgical forms pose diagnostic challenges. For surgical hypoparathyroidism, transient forms have to be ruled out even in the long term, in order to avoid unnecessary chronic replacement therapy with calcium and calcitriol. Regarding non-surgical hypoparathyroidism, once referred to as idiopathic, a systematic clinically and genetically-driven approach to define the precise diagnosis have to be pursued. In the case of syndromic hypoparathyroidism, patients have to be screened for associated abnormalities. Autoimmune, non-genetic hypoparathyroidism is still a diagnosis of exclusion, since no specific autoantibodies are specific for this condition.

Novel Mutation in the CASR Gene (p.Leu123Ser) in a Case of Autosomal Dominant Hypocalcemia

Autosomal dominant hypocalcemia, caused by activating mutations of the calcium-sensing receptor (CASR) gene, is characterized by hypocalcemia with an inappropriately low concentration of parathyroid hormone (PTH). In this report, we describe the identification of a novel missense mutation in the CASR gene, in a boy with autosomal dominant hypocalcemia. Polymerase chain reaction (PCR)-single strand and DNA sequencing revealed a heterozygous mutation in CASR gene that causes a leucine substitution for serine at codon 123 (p.Leu123Ser). This mutation was absent in DNA from 50 control patients. In silico studies suggest that the identified variant was likely pathogenic. Sequencing analysis in the mother suggested mosaicism for the same variant, and she was clinically and biochemically unaffected. Clinical manifestations of the index case started with seizures at 14 months of age; cognitive impairment and several neuropsychological disabilities were noted during childhood. Extrapyramidal signs and basal ganglia calcification developed later, namely, hand tremor and rigidity at the age of 7 and 18 years, respectively. Laboratory analysis revealed hypocalcemia, hyperphosphatemia, and low-serum PTH with hypomagnesemia and mild hypercalciuria. After 2 years of treatment with calcium supplements and calcitriol, some brief periods of clinical improvement were reported; as well as an absence of nephrocalcinosis.

Activating Calcium-Sensing Receptor Mutations: Prospects for Future Treatment with Calcilytics

Activating mutations of the G protein-coupled receptor, calcium-sensing receptor (CaSR), cause autosomal dominant hypocalcemia and Bartter syndrome type 5. These mutations lower the set-point for extracellular calcium sensing, thereby causing decreased parathyroid hormone secretion and disturbed renal calcium handling with hypercalciuria. Available therapies increase serum calcium levels but raise the risk of complications in affected patients. Symptom relief and the prevention of adverse outcome is currently very difficult to achieve. Calcilytics act as CaSR antagonists that attenuate its activity, thereby correcting the molecular defect of activating CaSR proteins in vitro and elevating serum calcium in mice and humans in vivo, and have emerged as the most promising therapeutics for the treatment of these rare and difficult to treat diseases.

Allosteric Modulation of the Calcium-sensing Receptor Rectifies Signaling Abnormalities Associated with G-protein α-11 Mutations Causing Hypercalcemic and Hypocalcemic Disorders

Germline loss- and gain-of-function mutations of G-protein α-11 (Gα₁₁), which couples the calcium-sensing receptor (CaSR) to intracellular calcium (Ca²⁺_i) signaling, lead to familial hypocalciuric hypercalcemia type 2 (FHH2) and autosomal dominant hypocalcemia type 2 (ADH2), respectively, whereas somatic Gα₁₁ mutations mediate uveal melanoma development by constitutively up-regulating MAPK signaling. Cinacalcet and NPS-2143 are allosteric CaSR activators and inactivators, respectively, that ameliorate signaling disturbances associated with CaSR mutations, but their potential to modulate abnormalities of the downstream Gα₁₁ protein is unknown. This study investigated whether cinacalcet and NPS-2143 may rectify Ca²⁺_i alterations associated with FHH2- and ADH2-causing Gα₁₁ mutations, and evaluated the influence of germline gain-of-function Gα₁₁ mutations on MAPK signaling by measuring ERK phosphorylation, and assessed the effect of NPS-2143 on a uveal melanoma Gα₁₁ mutant. WT and mutant Gα₁₁ proteins causing FHH2, ADH2 or uveal melanoma were transfected in CaSR-expressing HEK293 cells, and Ca²⁺_i and ERK phosphorylation responses measured by flow-cytometry and Alphascreen immunoassay following exposure to extracellular Ca²⁺ (Ca²⁺_o) and allosteric modulators. Cinacalcet and NPS-2143 rectified the Ca²⁺_i responses of FHH2- and ADH2-associated Gα₁₁ loss- and gain-of-function mutations, respectively. ADH2-causing Gα₁₁ mutations were demonstrated not to be constitutively activating and induced ERK phosphorylation following Ca²⁺_o stimulation only. The increased ERK phosphorylation associated with ADH2 and uveal melanoma mutants was rectified by NPS-2143. These findings demonstrate that CaSR-targeted compounds can rectify signaling disturbances caused by germline and somatic Gα₁₁ mutations, which respectively lead to calcium disorders and tumorigenesis; and that ADH2-causing Gα₁₁ mutations induce non-constitutive alterations in MAPK signaling.

Calcium-sensing receptor (CaSR) mutations and disorders of calcium, electrolyte and water metabolism

The extracellular calcium-sensing receptor (CaSR) is a family C G-protein-coupled receptor (GPCR) that is expressed at multiple sites, including the parathyroids and kidneys. The human CASR gene, located on chromosome 3q21.1, encodes a 1078 amino acid protein. More than 230 different disease-causing mutations of the CaSR have been reported. Loss-of-function mutations lead to three hypercalcemic disorders, which are familial hypocalciuric hypercalcemia (FHH), neonatal severe hyperparathyroidism and primary hyperparathyroidism. Gain-of-function mutations, on the other hand, result in the hypocalcemic disorders of autosomal dominant hypocalcemia and Bartter syndrome type V. Moreover, autoantibodies directed against the extracellular domain of the CaSR have been found to be associated with FHH in some patients, and also in some patients with hypoparathyroidism that may be part of autoimmune polyglandular syndrome type 1. Studies of disease-causing CASR mutations have provided insights into structure-function relationships and highlighted intra-molecular domains that are critical for ligand binding, intracellular signaling, and receptor trafficking.