An acceptor splice site mutation in the calcium‐sensing receptor (CASR) gene in familial hypocalciuric hypercalcemia and neonatal severe hyperparathyroidism

We studied family members of a large kindred expressing both familial hypocalciuric hypercalcemia (FHH) and neonatal severe hyperparathyroidism (NSHPT) and found, by PCR amplification of the extracellular calcium‐sensing receptor ( CASR ) gene exons and flanking intronic sequences, that FHH individuals were heterozygous for a g to t substitution in the last nucleotide of intron 2 (IVS2‐1G>T). Defects in messenger RNA splicing were investigated by illegitimate transcription of the CASR gene in lymphoblastoid cells from an FHH affected individual, as well as by transfection of a CASR minigene harboring this mutation into HEK293 cells. The mutation resulted predominantly in exon III skipping causing a shift in exon IV reading frame and introduction of a premature stop codon leading to a predicted truncated protein of 153 amino acids. Interestingly, it was noted that exon III splicing is not 100% efficient in parathyroid, thyroid, and kidney; an exon III‐deleted transcript is produced approximately 15% of the time. This is the first description of a splice site mutation in the CASR gene and provides an explanation of the clinical phenotype of the patients. Hum Mutat 18:411–421, 2001. © 2001 Wiley‐Liss, Inc.