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Familial Hypocalciuric Hypercalcaemia Types 1–3 and Neonatal Severe Primary Hyperparathyroidism

The calcium-sensing receptor (CaSR) plays a key role in maintaining calcium homeostasis. The CaSR regulates parathyroid hormone (PTH) secretion and calcium reabsorption by the kidney. Loss-of-function CASR mutations on chromosome 3q21.1 cause familial hypocalciuric hypercalcaemia type 1 (FHH1). FHH1 patients are usually asymptomatic. Biochemically, FHH1 is characterized by mild-to-moderate hypercalcaemia, inappropriately low urinary calcium excretion and a normal or mildly elevated circulating PTH level. Loss-of-function GNA11 mutations on chromosome 19p13.3 lead to FHH2. GNA11 gene encodes Gα‎-11 which mediates downstream CaSR signalling. Patients with FHH2 mutations display a mild FHH phenotype. Loss-of-function AP2S1 mutations on chromosome 19q.13.3 give rise to FHH3. AP2S1 gene encodes the adaptor-related protein complex 2 sigma (AP2σ‎) which is crucial for clathrin-mediated endocytosis, CaSR signalling, and trafficking. FHH3 may be a more severe form of FHH that may produce symptomatic hypercalcaemia, low bone mineral density, and cognitive dysfunction. Calcimimetic therapy may be considered for patients with FHH types 1–3.