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Novel TRPM6 Mutations in 21 Families with Primary Hypomagnesemia and Secondary Hypocalcemia

Karl P. Schlingmann^*, Martin C. Sassen^*, Stefanie Weber^*,, Ulla Pechmann^*, Kerstin Kusch^*, Lutz Pelken^*, Daniel Lotan, Maria Syrrou, Jeffrey J. Prebble^||, David E.C. Cole^¶, Daniel L. Metzger^#, Shamima Rahman^**, Toshihiro Tajima, San-Ging Shu, Siegfried Waldegger^*, Hannsjoerg W. Seyberth^* and Martin Konrad^*,

^* Department of Pediatrics, Philipps-University Marburg, Marburg, Germany; Pediatric Nephrology, University Children’s Hospital Heidelberg, Heidelberg, Germany; Pediatric Nephrology, The Chaim Sheba Medical Center, Tel-Hashomer, Israel; Laboratory of General Biology, Medical School, University of Ioannina, Ioannina, Greece; ^|| Department of Pediatrics, Toowoomba Base Hospital, Toowoomba, Australia; ^¶ Department of Laboratory Medicine & Pathobiology, University of Toronto, Ontario, Canada; ^# Pediatric Endocrinology, British Columbia Children’s Hospital, Vancouver, British Columbia, Canada; ^** Hospital for Children Great Ormond Street, London, United Kingdom; Department of Pediatrics, Hokkaido University Medical School, Sapporo, Japan; Department of Pediatrics, Taichung Veterans General Hospital, Taichung, Taiwan; and Pediatric Nephrology, University Children’s Hospital, Inselspital Bern, Switzerland

Address correspondence to: Dr. Karl Peter Schlingmann, University Children's Hospital Marburg, Deutschhausstrasse 12, 35037 Marburg, Germany. Phone 49-6421-286-2483; Fax: 49-6421-286-5724; E-mail schlingm{at}staff.uni-marburg.de

Received for publication November 22, 2004. Accepted for publication July 11, 2005.

Primary hypomagnesemia with secondary hypocalcemia is a rare autosomal recessive disorder characterized by profound hypomagnesemia associated with hypocalcemia. Pathophysiology is related to impaired intestinal absorption of magnesium accompanied by renal magnesium wasting as a result of a reabsorption defect in the distal convoluted tubule. Recently, mutations in the TRPM6 gene coding for TRPM6, a member of the transient receptor potential (TRP) family of cation channels, were identified as the underlying genetic defect. Here, the results of a TRPM6 mutational analysis of 21 families with 28 affected individuals are presented. In this large patient cohort, a retrospective clinical evaluation based on a standardized questionnaire was also performed. Genotype analysis revealed TRPM6 mutations in 37 of 42 expected mutant alleles. Sixteen new TRPM6 mutations were identified, including stop mutations, frame-shift mutations, splice-site mutations, and deletions of exons. Electrophysiologic analysis of mutated ion channels after heterologous expression in Xenopus oocytes proved complete loss of function of TRPM6. Clinical evaluation revealed a homogeneous clinical picture at manifestation with onset in early infancy with generalized cerebral convulsions. Initial laboratory evaluation yielded extremely low serum magnesium levels, low serum calcium levels, and inadequately low parathyroid hormone levels. Treatment usually consisted of acute intravenous magnesium supplementation leading to relief of clinical symptoms and normocalcemia, followed by lifelong oral magnesium supplementation. Serum magnesium levels remained in the subnormal range despite adequate therapy. This is best explained by a disturbed magnesium conservation in the distal convoluted tubule, which emerged in all patients upon magnesium supplementation. Delay of diagnosis resulted in permanent neurologic damage in three patients.

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