Familial hypocalciuric hypercalcemia is a genetically heterogeneous disorder with three variants: types 1, 2, and 3. Type 1 is due to loss-of-function mutations of the calcium-sensing receptor, a guanine nucleotide–binding protein (G-protein)–coupled receptor that signals through the G-protein subunit α₁₁ (Gα₁₁). Type 3 is associated with adaptor-related protein complex 2, sigma 1 subunit (AP2S1) mutations, which result in altered calcium-sensing receptor endocytosis. We hypothesized that type 2 is due to mutations effecting Gα₁₁ loss of function, since Gα₁₁ is involved in calcium-sensing receptor signaling, and its gene (GNA11) and the type 2 locus are colocalized on chromosome 19p13.3. We also postulated that mutations effecting Gα₁₁ gain of function, like the mutations effecting calcium-sensing receptor gain of function that cause autosomal dominant hypocalcemia type 1, may lead to hypocalcemia. We performed GNA11 mutational analysis in a kindred with familial hypocalciuric hypercalcemia type 2 and in nine unrelated patients with familial hypocalciuric hypercalcemia who did not have mutations in the gene encoding the calcium-sensing receptor (CASR) or AP2S1. We also performed this analysis in eight unrelated patients with hypocalcemia who did not have CASR mutations. In addition, we studied the effects of GNA11 mutations on Gα₁₁ protein structure and calcium-sensing receptor signaling in human embryonic kidney 293 (HEK293) cells. The kindred with familial hypocalciuric hypercalcemia type 2 had an in-frame deletion of a conserved Gα₁₁ isoleucine (Ile200del), and one of the nine unrelated patients with familial hypocalciuric hypercalcemia had a missense GNA11 mutation (Leu135Gln). Missense GNA11 mutations (Arg181Gln and Phe341Leu) were detected in two unrelated patients with hypocalcemia; they were therefore identified as having autosomal dominant hypocalcemia type 2. All four GNA11 mutations predicted disrupted protein structures, and assessment on the basis of in vitro expression showed that familial hypocalciuric hypercalcemia type 2–associated mutations decreased the sensitivity of cells expressing calcium-sensing receptors to changes in extracellular calcium concentrations, whereas autosomal dominant hypocalcemia type 2–associated mutations increased cell sensitivity. Gα₁₁ mutants with loss of function cause familial hypocalciuric hypercalcemia type 2, and Gα₁₁ mutants with gain of function cause a clinical disorder designated as autosomal dominant hypocalcemia type 2. (Funded by the United Kingdom Medical Research Council and others.)

Rajesh Thakker and colleagues show that missense mutations affecting codon 15 of AP2S1 cause familial hypocalciuric hypercalcemia type 3, a disorder of calcium homeostasis. AP2S1 encodes a protein involved in clathrin-mediated endocytosis, and the mutations probably cause disease by disrupting internalization of the calcium-sensing receptor CaSR. Adaptor protein-2 (AP2), a central component of clathrin-coated vesicles (CCVs), is pivotal in clathrin-mediated endocytosis, which internalizes plasma membrane constituents such as G protein–coupled receptors (GPCRs)1,2,3. AP2, a heterotetramer of α, β, μ and σ subunits, links clathrin to vesicle membranes and binds to tyrosine- and dileucine-based motifs of membrane-associated cargo proteins1,4. Here we show that missense mutations of AP2 σ subunit (AP2S1) affecting Arg15, which forms key contacts with dileucine-based motifs of CCV cargo proteins4, result in familial hypocalciuric hypercalcemia type 3 (FHH3), an extracellular calcium homeostasis disorder affecting the parathyroids, kidneys and bone5,6,7. We found AP2S1 mutations in >20% of cases of FHH without mutations in calcium-sensing GPCR (CASR)8,9,10,11,12, which cause FHH1. AP2S1 mutations decreased the sensitivity of CaSR-expressing cells to extracellular calcium and reduced CaSR endocytosis, probably through loss of interaction with a C-terminal CaSR dileucine-based motif, whose disruption also decreased intracellular signaling. Thus, our results identify a new role for AP2 in extracellular calcium homeostasis.

In this narrative review, we present data gathered over four decades (1980–2020) on the epidemiology, pathophysiology and genetics of primary hyperparathyroidism (PHPT). PHPT is typically a disease of postmenopausal women, but its prevalence and incidence vary globally and depend on a number of factors, the most important being the availability to measure serum calcium and parathyroid hormone levels for screening. In the Western world, the change in presentation to asymptomatic PHPT is likely to occur, over time also, in Eastern regions. The selection of the population to be screened will, of course, affect the epidemiological data (ie, general practice as opposed to tertiary center). Parathyroid hormone has a pivotal role in regulating calcium homeostasis; small changes in extracellular Ca++ concentrations are detected by parathyroid cells, which express calcium-sensing receptors (CaSRs). Clonally dysregulated overgrowth of one or more parathyroid glands together with reduced expression of CaSRs is the most important pathophysiologic basis of PHPT. The spectrum of skeletal disease reflects different degrees of dysregulated bone remodeling. Intestinal calcium hyperabsorption together with increased bone resorption lead to increased filtered load of calcium that, in addition to other metabolic factors, predispose to the appearance of calcium-containing kidney stones. A genetic basis of PHPT can be identified in about 10% of all cases. These may occur as a part of multiple endocrine neoplasia syndromes (MEN1–MEN4), or the hyperparathyroidism jaw-tumor syndrome, or it may be caused by nonsyndromic isolated endocrinopathy, such as familial isolated PHPT and neonatal severe hyperparathyroidism. DNA testing may have value in: confirming the clinical diagnosis in a proband; eg, by distinguishing PHPT from familial hypocalciuric hypercalcemia (FHH). Mutation-specific carrier testing can be performed on a proband's relatives and identify where the proband is a mutation carrier, ruling out phenocopies that may confound the diagnosis; and potentially prevention via prenatal/preimplantation 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). Abstract The prevalence and incidence of PHPT vary around the world and depend on a number of factors, the most important being availability of serum calcium (and parathyroid hormone) measurement for screening, which in the Western world has shifted its presentation to an asymptomatic disorder. Clonally dysregulated overgrowth of one or more parathyroid glands together with reduced expression of CaSRs is the most important pathophysiologic basis of PHPT, which may also occur as a part of inherited syndromes or as a nonsyndromic isolated endocrinopathy in 10% of cases. Genetic testing has roles in establishing the clinical diagnosis, although its value requires assessments using GRADE(176) methodology, which is recommended for future consideration.