Hypomagnesemia with Secondary Hypocalcemia due to a Missense Mutation in the Putative Pore-forming Region of TRPM6

Vladimir Chubanov,Karl P Schlingmann,Janine Wäring,Jolanta Heinzinger,Silke Kaske,Siegfried Waldegger,Michael Mederos Y Schnitzler,Thomas Gudermann

doi:10.1074/jbc.m611117200

Abstract

Hypomagnesemia with secondary hypocalcemia is an autosomal recessive disorder caused by mutations in the TRPM6 gene. Current experimental evidence suggests that TRPM6 may function in a specific association with TRPM7 by means of heterooligomeric channel complex formation. Here, we report the identification and functional characterization of a new hypomagnesemia with secondary hypocalcemia missense mutation in TRPM6. The affected subject presented with profound hypomagnesemia and hypocalcemia caused by compound heterozygous mutation in the TRPM6 gene: 1208(-1)G > A affecting the acceptor splice site preceding exon 11, and 3050C > G resulting in the amino acid change (P1017R) in the putative pore-forming region of TRPM6. To assess the functional consequences of the P1017R mutation, TRPM6(P1017R) and wild-type TRPM6 were co-expressed with TRPM7 in Xenopus oocytes and HEK 293 cells, and currents were assessed by two-electrode voltage clamp and whole cell patch clamp measurements, respectively. Co-expression of wild-type TRPM6 and TRPM7 resulted in a significant increase in the amplitude of TRPM7-like currents. In contrast, TRPM6(P1017R) suppressed TRPM7 channel activity. In line with these observations, TRPM7, containing the corresponding mutation P1040R, displayed a dominant-negative effect upon co-expression with wild-type TRPM7. Confocal microscopy and fluorescence resonance energy transfer recordings demonstrated that the P1017R mutation neither affects assembly of TRPM6 with TRPM7, nor co-trafficking of heteromultimeric channel complexes to the cell surface. We conclude that a functional defect in the putative pore of TRPM6/7 channel complexes is sufficient to impair body magnesium homeostasis.

Highlights

Mg2ϩ plays a vital role in virtually all cellular pathways as a cofactor of enzymes, an essential structural element of proteins and nucleic acids, and a modulator of receptors and ion channels [1,2,3,4]
The mutated P1017 is located between the predicted S5–S6 helices of TRPM6, a segment with relatively low primary amino acid sequence homology to other TRP channels
The three-dimensional model of the TRPM6 S5–S6 region is well compatible with the recent structural prediction of the equivalent region of a more distantly related family member, TRPM4 [43, 44]

Summary

Introduction

Mg2ϩ plays a vital role in virtually all cellular pathways as a cofactor of enzymes, an essential structural element of proteins and nucleic acids, and a modulator of receptors and ion channels [1,2,3,4]. To assess the functional consequences of the P1017R mutation, TRPM6P1017R and wild-type TRPM6 were co-expressed with TRPM7 in Xenopus oocytes and HEK 293 cells, and currents were assessed by two-electrode voltage clamp and whole cell patch clamp measurements, respectively. In line with these observations, TRPM7, containing the corresponding mutation P1040R, displayed a dominant-negative effect upon co-expression with wild-type TRPM7.

Results

Conclusion