CNNM2, Encoding a Basolateral Protein Required for Renal Mg2+ Handling, Is Mutated in Dominant Hypomagnesemia

Familial hypomagnesemia is a rare human disorder caused by renal or intestinal magnesium (Mg2+) wasting, which may lead to symptoms of Mg2+ depletion such as tetany, seizures, and cardiac arrhythmias. Our knowledge of the physiology of Mg2+ (re)absorption, particularly the luminal uptake of Mg2+ along the nephron, has benefitted from positional cloning approaches in families with Mg2+ reabsorption disorders; however, basolateral Mg2+ transport and its regulation are still poorly understood. Here, by using a candidate screening approach, we identified CNNM2 as a gene involved in renal Mg2+ handling in patients of two unrelated families with unexplained dominant hypomagnesemia. In the kidney, CNNM2 was predominantly found along the basolateral membrane of distal tubular segments involved in Mg2+ reabsorption. The basolateral localization of endogenous and recombinant CNNM2 was confirmed in epithelial kidney cell lines. Electrophysiological analysis showed that CNNM2 mediated Mg2+-sensitive Na+ currents that were significantly diminished in mutant protein and were blocked by increased extracellular Mg2+ concentrations. Our data support the findings of a recent genome-wide association study showing the CNNM2 locus to be associated with serum Mg2+ concentrations. The mutations found in CNNM2, its observed sensitivity to extracellular Mg2+, and its basolateral localization signify a critical role for CNNM2 in epithelial Mg2+ transport.