A new missense mutation of calcium sensing receptor with isoleucine replaced by serine at codon 857 leading to type V Bartter syndrome

Several genetic defects on thick ascending limb (TAL) of Henle loop were reported to cause Bartter syndrome (BS) characterized by metabolic alkalosis, hypokalemia, and normal or low blood pressure. Among them, defective basolateral calcium sensing receptors (CaSR) on TAL could result in type V BS that not only presents typical characteristics of BS but also hypocalcemia. Herein we report a 54 years old female patient with a novel mutation of CaSR that leads to type V BS. A sequencing of CaSR gene in peripheral blood mononuclear cells and urine stem cells both disclosed a heterozygous substitution of thymine for guanine (NM_001178065.1:c.2570T > G) in exon 7 at codon 857 resulting in substitution of isoleucine for serine (p.I857S). We performed functional tests of the mutant CaSR gene in vitro using urine stem cells to determine whether this mutation is responsible for the clinical presentations. Urine stem cells expressing abundant CaSR on flow cytometry of this patient and a normal subject were obtained for in vitro functional studies, including intracellular calcium and inositol 1,4,5-trisphosphate concentrations in response to increasing concentrations of extracellular calcium. The results show all of their responses to extracellular calcium are extremely sensitive in urine stem cells of the case as compared to those of the normal subject, indicating a prominent gain-of-function mutation. A novel mutation I857S in transmembrane domain 7 of CaSR in our patient would be added to the list of mutations leading to type V BS.