GENETICS IN ENDOCRINOLOGY: Gain and loss of function mutations of the calcium-sensing receptor and associated proteins: current treatment concepts.

The calcium-sensing receptor (CASR) is the main calcium sensor in the maintenance of calcium metabolism. Mutations of the CASR, the G protein alpha 11 (GNA11) and the adaptor-related protein complex 2 sigma 1 subunit (AP2S1) genes can shift the set point for calcium sensing causing hyper- or hypo-calcemic disorders. Therapeutic concepts for these rare diseases range from general therapies of hyper- and hypo-calcemic conditions to more pathophysiology oriented approaches such as parathyroid hormone (PTH) substitution and allosteric CASR modulators. Cinacalcet is a calcimimetic that enhances receptor function and has gained approval for the treatment of hyperparathyroidism. Calcilytics in turn attenuate CASR activity and are currently under investigation for the treatment of various diseases. We conducted a literature search for reports about treatment of patients harboring inactivating or activating CASR, GNA11 or AP2S1 mutants and about in vitro effects of allosteric CASR modulators on mutated CASR. The therapeutic concepts for patients with familial hypocalciuric hypercalcemia (FHH), neonatal hyperparathyroidism (NHPT), neonatal severe hyperparathyroidism (NSHPT) and autosomal dominant hypocalcemia (ADH) are reviewed. FHH is usually benign, but symptomatic patients benefit from cinacalcet. In NSHPT patients pamidronate effectively lowers serum calcium, but most patients require parathyroidectomy. In some patients cinacalcet can obviate the need for surgery, particularly in heterozygous NHPT. Symptomatic ADH patients respond to vitamin D and calcium supplementation but this may increase calciuria and renal complications. PTH treatment can reduce relative hypercalciuria. None of the currently available therapies for ADH, however, prevent tissue calcifications and complications, which may become possible with calcilytics that correct the underlying pathophysiologic defect.

Novel activating mutations of the calcium-sensing receptor: the calcilytic NPS-2143 mitigates excessive signal transduction of mutant receptors.

CONTEXT AND OBJECTIVE: Activating mutations in the calcium-sensing receptor (CaSR) gene cause autosomal dominant hypocalcemia (ADH). The aims of the present study were the functional characterization of novel mutations of the CaSR found in patients, the comparison of in vitro receptor function with clinical parameters, and the effect of the allosteric calcilytic NPS-2143 on the signaling of mutant receptors as a potential new treatment for ADH patients. METHODS: Wild-type and mutant CaSR (T151R, P221L, E767Q, G830S, and A844T) were expressed in human embryonic kidney cells (HEK 293T). Receptor signaling was studied by measuring intracellular free calcium in response to different concentrations of extracellular calcium ([Ca(2+)](o)) in the presence or absence of NPS-2143. RESULTS: All ADH patients had lowered serum calcium ranging from 1.7 to 2.0 mm and inadequate intact PTH and urinary calcium excretion. In vitro testing of CaSR mutations from these patients revealed exaggerated [Ca(2+)](o)-induced cytosolic Ca(2+) responses with EC(50) values for [Ca(2+)](o) ranging from 1.56 to 3.15 mM, which was lower than for the wild-type receptor (4.27 mM). The calcilytic NPS-2143 diminished the responsiveness to [Ca(2+)](o) in the CaSR mutants T151R, E767Q, G830S, and A844T. The mutant P221L, however, was only responsive when coexpressed with the wild-type CaSR. CONCLUSION: Calcilytics might offer medical treatment for patients with autosomal dominant hypocalcemia caused by calcilytic-sensitive CaSR mutants.

Sympathoadrenal counterregulation in patients with hypothalamic craniopharyngioma.

In humans, the role of hypothalamic centers for activation of counterregulatory release of catecholamines and glucagon during hypoglycemia is unclear. To address this question, we investigated the counterregulatory response to acute insulin-induced hypoglycemia of glucagon, epinephrine, and norepinephrine in eight patients who had undergone transcranial surgery for a craniopharyngioma extending to the hypothalamic region. We compared the patients' responses with those of four patients suffering from hypopituitarism and of six healthy subjects. After the i.v. injection of 0.1 U of human insulin per kg of body weight in the patients or 0.15 U in healthy subjects, the plasma glucose concentrations decreased to similar minimum levels within 30 min in all three groups. All subjects recovered spontaneously from hypoglycemia within 2 h. In five of eight craniopharyngioma patients, only a small counterregulatory rise in plasma epinephrine (< or =2-fold) and norepinephrine could be observed (P < 0.05 for epinephrine and P = 0.22 for norepinephrine vs. healthy controls). During hypoglycemia, virtually no adrenergic symptoms (tremor, heart pounding, and anxiety) were reported by these five patients, and changes in the heart rate were diminished. In three craniopharyngioma patients, the counterregulatory increase in catecholamines was unimpaired, adrenergic symptoms were reported and a rise in heart rate was observed during hypoglycemia. In all craniopharyngioma patients, the counterregulatory glucagon response to hypoglycemia was preserved and orthostasis increased both catecholamines and the heart rate similar to in the patients with hypopituitarism as well as in the healthy controls. Our results demonstrate selective impairment of counterregulatory sympathoadrenal activation in patients who had undergone surgery for a craniopharyngioma extending to the hypothalamic region. This strongly suggests the involvement of hypothalamic centers in hypoglycemia-induced activation of the sympathoadrenal axis in humans. It remains unclear as to whether hypoglycemia-induced glucagon secretion is also controlled by the hypothalamus. However, a common hypothalamic center controlling both counterregulatory catecholamine and glucagon release is unlikely, and sympathoadrenal activation is not required for hypoglycemia-induced glucagon secretion in humans.