Mutations in AP2S1 cause familial hypocalciuric hypercalcemia type 3

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 proteins 1 , 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 proteins 4 , result in familial hypocalciuric hypercalcemia type 3 (FHH3), an extracellular calcium homeostasis disorder affecting the parathyroids, kidneys and bone 5 , 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.