CLUSTERED INACTIVATING MUTATIONS AND BENIGN POLYMORPHISMS OF THE CALCIUM RECEPTOR GENE IN FAMILIAL BENIGN HYPOCALCIURIC HYPERCALCEMIA SUGGEST RECEPTOR FUNCTIONAL DOMAINS

The predominant variety of familial benign hypocalciuric hypercalcemia (FBHH) is FBHH3q, which is associated with presumed inactivating muta tions of the cell surface calcium receptor (CaR) gene on chromosome 3q 13.3-q21. We sought mutations of the CaR gene in FBHH by direct sequen cing of PCR-amplified genomic DNA from 14 affected families: 8 mapped to 3913, 1 mapped to chromosome 19p, and 5 unmapped. We sequenced the entire coding region of the gene (exons 2-7) in one or two affected me mbers of each family and found six point mutations that altered one am ino acid, cosegregated with hypercalcemia, and were absent in more tha n 100 unaffected persons. Four mutations were unique (S53P, D215G, S65 7Y, and P748R), and two had been reported previously (P55L and R185Q). Of four mutant CaR proteins expressed in Xenopus oocytes, three were deficient in extracellular Ca2+-induced signaling. No CaR mutations we re found in eight families, including the one mapped to chromosome 19p . Three benign polymorphisms occurred in the COOH-terminal region of t he CaR protein in 10%, 15%, and 30% of more than 100 unaffected person s. Thus, FBHH-causing CaR mutations were clustered in the NH2-terminal extracellular and membrane-spanning regions of the receptor protein. We suggest that these are important functional domains, probably for c alcium binding and signal transduction, respectively. Finally, mutatio ns in regulatory or intronic regions of the CaR gene may also underlie many cases of FBHH.