Familial hypocalciuric hypercalcemia (HHC) is a heritable disorder of mineral homeostasis that is transmitted as an autosomal dominant trait with a high degree of penetrance. HHC is characterized biochemically by lifelong elevation of serum calcium concentrations and is associated with inappropriately low urinary calcium excretion and a normal or mildly elevated circulating parathyroid hormone (PTH; 168450) level. Hypermagnesemia is typically present. Individuals with HHC are usually asymptomatic and the disorder is considered benign. However, chondrocalcinosis and pancreatitis occur in some adults (summary by Hannan et al., 2010). Characteristic features of familial hypocalciuric hypercalcemia include mild to moderate hypercalcemia, nonsuppressed parathyroid hormone, relative hypocalciuria while hypercalcemic (calcium/creatinine clearance ratio less than 0.01, or 24-hr urine calcium less than 6.25 mmol), almost 100% penetrance of hypercalcemia from birth, absence of complications, persistence of hypercalcemia following subtotal parathyroidectomy, and normal parathyroid size, weight, and histology at surgery. However, atypical presentations with severe hypercalcemia, hypercalciuria with or without nephrolithiasis or nephrocalcinosis, kindreds with affected members displaying either hypercalciuria or hypocalciuria, postoperative normocalcemia, and pancreatitis have all been described in FHH (Warner et al., 2004). Genetic Heterogeneity of Hypocalciuric Hypercalcemia Familial hypocalciuric hypercalcemia type II (HHC2; 145981) is caused by mutation in the GNA11 gene (139313) on chromosome 19p13, and HHC3 (600740) is caused by mutation in the AP2S1 gene (602242) on chromosome 19q13.

Any familial hypocalciuric hypercalcemia in which the cause of the disease is a mutation in the AP2S1 gene.

Familial hypocalciuric hypercalcemia type II (HHC2) is an autosomal dominant disorder characterized by lifelong elevations of serum calcium concentrations with low urinary calcium excretion and normal circulating parathyroid hormone concentrations in most patients. Patients are generally asymptomatic (summary by Nesbit et al., 2013). For a general phenotypic description and a discussion of genetic heterogeneity of hypocalciuric hypercalcemia, see HHC1 (145980).

HELIX syndrome is an autosomal recessive disorder characterized by Hypohidrosis, Electrolyte imbalance, Lacrimal gland dysfunction, Ichthyosis, and Xerostomia (summary by Hadj-Rabia et al., 2018).