Familial hypocalciuric hypercalcemia in an infant: diagnosis and management quandaries.

Familial hypocalciuric hypercalcemia (FHH) is typically a benign condition characterized by elevated serum calcium, low urinary calcium excretion, and non-suppressed circulating levels of parathyroid hormone (PTH), usually requiring no intervention. FHH is inherited in an autosomal-dominant manner. Three subtypes are described, representing variants in genes with critical roles in extracellular calcium-sensing. FHH1, due to heterozygous inactivating variants in the calcium-sensing receptor gene (CASR), accounts for the majority of cases. FHH2, due to variants in GNA11, encoding the α-subunit of the downstream signaling protein, G11, is the rarest form of FHH. FHH3, resulting from variants in AP2S1, may present with a more pronounced phenotype than FHH1 or FHH2. We describe herein a newborn girl presenting with in utero femoral fractures, hypercalcemia, hypophosphatemia, and elevated circulating PTH. She was diagnosed with mild hyperparathyroidism and provided supplemental phosphate upon hospital discharge. However, serum calcium and PTH remained elevated at 5 mo of age. The combination of low-calcium formula and cinacalcet improved the biochemical profile. No pathogenic variants in the coding region of CASR were identified; subsequent whole exome sequencing revealed a G- > T transition at c.44 (p.R15L) in AP2S1. Family studies identified this variant in the father and an affected brother. The mother was unexpectedly found to be hypocalcemic and was diagnosed with idiopathic hypoparathyroidism. This case demonstrates successful treatment of FHH3 using a low-calcium formula to limit dietary calcium availability and cinacalcet to modify PTH levels.