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

Abstract

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.

Against all odds: blended phenotypes of three single-gene defects

“…11 Indeed, the identified variant c.2667 +1G4A has previously been reported in several patients of Turkish origin. 12 Sanger sequencing in our patients and other family members confirmed heterozygosity of both parents and co-segregation of variant and disease ( Figure 2). Hypocalcemia was not regularly observed in either of our patients, but secondary hypocalcemia in patients with TRPM6 mutations is reportedly variable.…”

“…Hypocalcemia was not regularly observed in either of our patients, but secondary hypocalcemia in patients with TRPM6 mutations is reportedly variable. 12 A stop variant in ABCG5 causes sitosterolemia Among the remaining candidate variants, we identified the c. 1336C4T/p.…”

New TRPM6 missense mutations linked to hypomagnesemia with secondary hypocalcemia

“…Except for mutation E 157 X identified in patient F38.1 that was described before in another Turkish family, 14 all other mutations are novel. Table 1 depicts the observed TRPM6 nucleotide exchanges and consequences for the TRPM6 amino-acid sequence/structure; Figure 4 shows a topological illustration of the TRPM6 protein with missense mutations indicated.…”

“…These include stop mutations, splice-site mutations, frame-shift mutations, and deletions. 10,[14][15][16][17]20,31 Furthermore, five missense mutations leading to HSH have been described so far, from which only S 141 L and P 1017 R were further studied at the molecular level. 10,19 The S 141 L mutation found in the highly conserved serine 141 resulted in impaired trafficking of TRPM6 to the membrane, 10,18 while the P 1017 R mutation located in the putative pore-forming region is the only known example of a mutation affecting TRPM6-gating properties without altering assembly or trafficking events.…”

“…10,11,[14][15][16][17] So far, there are two missense mutations described previously, S 141 L and P 1017 R, resulting in either trafficking or gating impairment of TRPM6. 18,19 Another three-point mutations leading to HSH, where the underlying molecular mechanism was not disclosed, have also been published.…”