A Ca(2+)-sensing receptor mutation causes hypoparathyroidism by increasing receptor sensitivity to Ca2+ and maximal signal transduction

Advances in the treatment of hypoparathyroidism with PTH 1-34

Hypoparathyroidism is a rare disorder of calcium metabolism which is treated with calcium and vitamin D analogs. Although conventional therapy effectively raises serum calcium, it bypasses the potent calcium reabsorption effects of PTH on the kidney which leads to hypercalciuria and an increased risk of nephrocalcinosis and renal insufficiency. Twenty-five years ago, we launched the first systematic investigation into synthetic human PTH 1-34 replacement therapy in both adults and children. These studies led to our current understanding of the complex nature of PTH 1-34 therapy and to the challenges we still face in our pursuit of a safe and effective physiologic replacement therapy for hypoparathyroidism. The normalization and minimal fluctuation of serum and urine calcium levels were the primary management goals. As the frequency of PTH 1-34 injections increased, the total daily dose required to normalize calcium homeostasis decreased and episodes of hypercalcemia and hypercalciuria diminished, producing a more physiologic biochemical profile. Twice-daily injections achieved simultaneous normalization of serum and urine calcium levels in many patients but the persistent elevation of bone markers and the difficulty in reducing urine calcium to normal levels in the more severe cases, suggested an alternative to PTH 1-34 injections was needed. The studies with PTH 1-34 delivered by insulin pump represent an important advance in the management of hypoparathyroidism. PTH 1-34 delivered by insulin pump normalized serum and urine calcium and markers of bone turnover. Additionally, pump delivery of PTH 1-34 produced stable magnesium values within the normal range and reduced magnesium excretion. Currently, PTH 1-34 delivery by pump is the only alternative to PTH injections that has been tested in both adults and children and proven to achieve a physiologic biochemical profile.

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.

Identification of molecular phenotypes and biased signaling induced by naturally occurring mutations of the human calcium-sensing receptor

More than 200 naturally occurring mutations have been identified in the human CaSR, which have been linked to diseases involving dysregulation of extracellular Ca(2+) homeostasis. These mutations have classically been termed "loss-" or "gain-of-function" mutations, which is an oversimplification given that amino acid changes can alter numerous molecular properties of a receptor. We thus sought to characterize the effects of 21 clinically relevant mutations, the majority located in the heptahelical domains and extracellular loop regions of the CaSR, using flow cytometry to measure cell surface receptor expression levels, and measurements of intracellular Ca(2+) mobilization and ERK1/2 phosphorylation to monitor receptor signaling. We identified distinct molecular phenotypes caused by these naturally occurring amino acid substitutions, which included combinations of loss- and gain-of-expression and changes in intrinsic signaling capacity. Importantly, we also identified biased signaling in the response of the CaSR to different mutations across the two pathways, indicating that some mutations resulted in receptor conformations that differentially altered receptor-coupling preferences. These findings have important implications for understanding the causes of diseases linked to the CaSR. A full appreciation of the molecular effects of these amino acid changes may enable the development of therapeutics that specifically target the molecular determinant of impairment in the receptor.

A Neonatal Case of Autosomal Dominant Hypoparathyroidism without Mutation of the CASR Gene

We experienced a case of familial hypoparathyroidism with an autosomal dominant pattern of transmission and performed molecular analysis of the calcium-sensing receptor (CASR) gene. The patient was a female neonate, born by cesarean section at term because of breech presentation. Her mother had been diagnosed with idiopathic hypoparathyroidism at the age of 9 yr and had been receiving vitamin D treatment since then. At birth, the patient’s serum calcium concentration was 8.4 mg/dl, but it fell to 4.0 mg/dl on the fifth day after birth. Furthermore, her serum intact PTH level was inappropriately low, while hyperphosphatemia and hypomagnesemia were found. She was diagnosed with familial hypoparathyroidism, and was immediately started on oral administration of 1α(OH)D₃ (0.1 µg/kg/day) and continuous intravenous infusion of 8.5% calcium gluconate. Additionally, trichlormethiazide was administered because of elevated urinary calcium/creatinine (Ca/Cr) ratio. Her serum calcium concentration gradually improved thereafter. In this case, autosomal dominant hypocalcemia (ADH) due to abnormality in the CASR gene was clinically suspected, but DNA sequencing analysis revealed no mutation of the CASR gene in either the patient or her mother. This result suggests that the patient’s hypoparathyroidism may have been caused by abnormality in a gene other than CASR.

A family with autosomal dominant hypocalcaemia with hypercalciuria (ADHH): mutational analysis, phenotypic variability and treatment challenges

Autosomal dominant hypocalcaemia with hypercalciuria (ADHH) is an intriguing syndrome, in which activating mutations of the calcium sensing receptor (CaSR) have recently been recognised. We describe a kindred with seven affected individuals across three generations, including patients affected in the first decade of life. Age at diagnosis varied from birth to 50 years. Affected members had hypocalcaemia (1.53-1.85 mmol/l), hypercalciuria, low but detectable parathyroid hormone (PTH) and hypomagnesaemia. Four of seven affected individuals were symptomatic (seizures, abdominal pains and paraesthesias), unrelated to severity of hypocalcaemia. Additional complications include nephrocalcinosis (n = 3) and basal ganglia calcification, identified by CT scanning in all five individuals. Symptomatic individuals were treated with calcium and calcitriol to reduce the risk of hypocalcaemic seizures. DNA sequence analysis, identified a mutation in exon 3, codon 129 (TGC-->TAC) of the CaSR gene of seven affected family members, resulting in loss of a conserved cysteine residue, potentially disrupting CaSR receptor dimerisation. Thus, a novel mutation was identified in this family, who demonstrate variability of ADHH phenotype and also illustrate the complexities of clinical management. Optimal management of ADHH is difficult and we recommend judicious treatment to avoid an increased risk of nephrocalcinosis.

Hydrochlorothiazide effectively reduces urinary calcium excretion in two Japanese patients with gain-of-function mutations of the calcium-sensing receptor gene

Gain-of-function mutations of the calcium-sensing receptor (CaR) gene cause autosomal dominant and/or sporadic hypocalcemia with hypercalciuria. Because treatment of the hypocalcemia with vitamin D and/or calcium in patients with such mutations results in increased hypercalciuria, nephrocalcinosis, and renal impairment, its use should be limited to alleviating the symptoms of symptomatic patients. Because thiazide diuretics have been successfully used to treat patients with hypercalciuria and hypoparathyroidism, they are theoretically useful in reducing urine calcium excretion and maintaining serum calcium levels in patients with gain-of-function mutations of the CaR gene. In this study, we report on the clinical course, molecular analysis, and effects of hydrochlorothiazide therapy in two Japanese patients with gain-of-function mutations of the CaR gene. Within a few weeks after birth, they developed generalized tonic seizures due to hypocalcemia (serum calcium values: 1.1 mmol/liter and 1.3 mmol/liter, respectively). Despite treatment with the standard dose of 1,25-dihydroxyvitamin D(3) in one patient and 1alpha-hydroxyvitamin D(3) in the other, acceptable serum calcium levels near the lower limit of normal were not established, and their urinary calcium excretion inappropriately increased. Addition of hydrochlorothiazide (1 mg/kg) reduced their urinary calcium excretion and maintained their serum calcium concentrations near the lower limit of normal, allowing the 1,25-dihydroxyvitamin D(3) and 1alpha-hydroxyvitamin D(3) doses to be reduced, and it alleviated their symptoms. A heterozygous missense mutation was identified in both patients. In one patient, the mutation was A843E in the seventh transmembrane domain of the CaR, and in the other it was L125P in the N-terminal extracellular domain. In vitro transient transfection of their mutant CaR cDNAs into HEK293 cells shifted the concentration-response curve of Ca(2+) to the left. In conclusion, two sporadic cases of hypercalciuric hypocalcemia were due to de novo gain-of-function mutations of the CaR gene. Hydrochlorothiazide with vitamin D(3) successfully reduced the patients' urinary calcium excretion and controlled their serum calcium concentrations and symptoms. Thiazide diuretics are effective in patients with gain-of function mutations of the CaR gene.

Mutations of the calcium-sensing receptor (CASR) in familial hypocalciuric hypercalcemia, neonatal severe hyperparathyroidism, and autosomal dominant hypocalcemia

The calcium-sensing receptor (CASR) is a plasma membrane G protein coupled receptor that is expressed in the parathyroid hormone (PTH) producing chief cells of the parathyroid gland and the cells lining the kidney tubule. By virtue of its ability to sense small changes in circulating calcium concentration ([Ca(2+)](o)) and to couple this information to intracellular signaling pathways that modify PTH secretion or renal cation handling, the CASR plays an essential role in maintaining mineral ion homeostasis. Inherited abnormalities of the CASR gene located on chromosome 3p13.3-21 can cause either hypercalcemia or hypocalcemia depending upon whether they are inactivating or activating, respectively. Heterozygous loss-of-function mutations give rise to familial (benign) hypocalciuric hypercalcemia (FHH) in which the lifelong hypercalcemia is asymptomatic. The homozygous condition manifests itself as neonatal severe hyperparathyroidism (NSHPT), a rare disorder characterized by extreme hypercalcemia and the bony changes of hyperparathyroidism which occur in infancy. The disorder autosomal dominant hypocalcemia (ADH) is due to gain-of-function mutations in the CASR gene. ADH may be asymptomatic or present with neonatal or childhood seizures. A common polymorphism in the intracellular tail of the CASR, Ala to Ser at position 986, has a modest effect on the serum calcium concentration in healthy individuals.

A missense mutation in the seventh transmembrane domain constitutively activates the human Ca2+ receptor

A missense mutation, A843E, in the seventh transmembrane domain of the human Ca2+ receptor, identified in a subject with autosomal dominant hypocalcemia, was found to cause a constitutive activation while at the same time lowering the maximal response of the receptor to Ca2+. A truncated human Ca2+ receptor lacking the majority of the N-terminal extracellular domain failed to respond to Ca2+ despite an excellent cell surface expression. The A843E mutant version of this truncated receptor showed constitutive activation. These results identify A843 as a critical residue for maintaining the inactive conformation of the human Ca2+ receptor. Substitution of glutamate, but not lysine or valine, for alanine 843 leads to activation of the human Ca2+ receptor in a manner that no longer depends upon Ca2+ binding to the extracellular domain.

A randomized, cross-over trial of once-daily versus twice-daily parathyroid hormone 1-34 in treatment of hypoparathyroidism

Once-daily sc injection of PTH 1-34 can normalize mean serum and urine calcium levels in patients with hypoparathyroidism; however, once-daily PTH has diminishing effects on serum calcium after 12 h, such that serum calcium levels fall below the normal range in some patients. Once-daily PTH also causes a marked increase in bone turnover, with persistent increases in markers of bone formation and resorption. To test the hypothesis that a twice-daily PTH regimen can produce more physiological control than a once-daily regimen, we performed a randomized cross-over trial, lasting 28 weeks, in 17 adult subjects with hypoparathyroidism. Each 14-week study arm was divided into a 2-week inpatient dose-adjustment phase and a 12-week outpatient phase. The PTH dose (given sc once daily at 0900 h or twice daily with one dose at 0900 h and the other at 2100 h) was adjusted to maintain both serum and urine calcium within, or close to, the normal range. During the second half of the day (12-24 h), twice-daily PTH increased serum calcium and magnesium levels more effectively than once-daily PTH. In patients with calcium receptor mutations (CaR), once-daily PTH normalized urine calcium, provided that serum calcium was maintained at levels below normal range. However, twice-daily PTH treatment produced higher mean serum calcium in patients with CaR with no significant rise in urine calcium excretion, and with no significant differences in either serum or urine calcium levels between CaR and patients with acquired or idiopathic hypoparathyroidism. Thus, treatment with twice-daily PTH is the better regimen for patients with CaR to overcome their tendency to hypercalciuria while producing near-normal levels of serum calcium. The total daily PTH dose was markedly reduced with the twice-daily regimen (twice daily 46+/-52 vs. once daily 97+/-60 microg/day, P < 0.001). We conclude that a twice-daily PTH regimen provides effective treatment of hypoparathyroidism and reduces the variation in serum calcium levels at a lower total daily PTH dose.

Activating mutations of the Ca2+-sensing receptor

The Ca2+-sensing receptor (CaR) is a member of the seven-transmembrane domain, G-protein-coupled receptor superfamily. It is expressed in parathyroid, kidney, and other tissues. In parathyroid, activation of the CaR by extracellular Ca2+ negatively regulates the secretion of parathyroid hormone. In the the thick ascending limb of Henle's loop, receptor activation decreases renal reabsorption of Ca2+. Heterozygous inactivating mutations of the CaR cause familial benign hypocalciuric hypercalcemia while homozygous inactivating mutations cause neonatal severe hyperparathyroidism. Conversely, activating mutations of the CaR cause autosomal dominant and sporadic hypoparathyroidism. Affected individuals have hypocalcemia which ranges from mild and asymptomatic to life-threatening. They also show a greater tendency to hypercalciuria than do other patients with hypoparathyroidism. Most, but not all, of the reported activating mutations occur in the amino-terminal, extracellular domain of the receptor. When expressed in cultured cells, mutant receptors can show both increased receptor sensitivity to Ca2+ and increased maximal signal transduction capacity.