Identification and Functional Characterization of a Novel Mutation in the Calcium-Sensing Receptor Gene in Familial Hypocalciuric Hypercalcemia: Modulation of Clinical Severity by Vitamin D Status

Abstract

Familial hypocalciuric hypercalcemia (FHH) is a benign condition associated with heterogeneous inactivating mutations in the calcium-sensing receptor (CASR) gene. The objective of the study was to identify and characterize a CASR mutation in a moderately hypercalcemic, hyperparathyroid individual and his family and assess the influence of vitamin D status on the clinical expression of the defect. We studied a kindred with FHH, in which the proband (a 34-yr-old male) was initially diagnosed with primary hyperparathyroidism due to frankly elevated serum PTH levels. CASR gene mutation analysis was performed on genomic DNA of the proband and family members. The mutant CASR was functionally characterized by transient transfection studies in kidney cells in vitro. A novel heterozygous mutation (F180C, TTC>TGC) in exon 4 of the CASR gene was identified. Although the mutant receptor was expressed normally at the cell surface, it was unresponsive with respect to intracellular signaling (MAPK activation) to increases in extracellular calcium concentrations. The baby daughter of the proband presented with neonatal hyperparathyroidism with markedly elevated PTH. Vitamin D supplementation of both the proband and the baby resulted in reduction of serum PTH levels to the normal range. The serum calcium level remained at a constant and moderately elevated level. The identification of a novel CASR gene mutation established the basis of the hypercalcemia in the kindred. Concomitant vitamin D deficiency modulates the severity of the presentation of FHH.