Genome-Wide Association Studies of Serum Magnesium, Potassium, and Sodium Concentrations Identify Six Loci Influencing Serum Magnesium Levels

Magnesium, potassium, and sodium, cations commonly measured in serum, are involved in many physiological processes including energy metabolism, nerve and muscle function, signal transduction, and fluid and blood pressure regulation. To evaluate the contribution of common genetic variation to normal physiologic variation in serum concentrations of these cations, we conducted genome-wide association studies of serum magnesium, potassium, and sodium concentrations using ∼2.5 million genotyped and imputed common single nucleotide polymorphisms (SNPs) in 15,366 participants of European descent from the international CHARGE Consortium. Study-specific results were combined using fixed-effects inverse-variance weighted meta-analysis. SNPs demonstrating genome-wide significant (p<5×10−8) or suggestive associations (p<4×10−7) were evaluated for replication in an additional 8,463 subjects of European descent. The association of common variants at six genomic regions (in or near MUC1, ATP2B1, DCDC5, TRPM6, SHROOM3, and MDS1) with serum magnesium levels was genome-wide significant when meta-analyzed with the replication dataset. All initially significant SNPs from the CHARGE Consortium showed nominal association with clinically defined hypomagnesemia, two showed association with kidney function, two with bone mineral density, and one of these also associated with fasting glucose levels. Common variants in CNNM2, a magnesium transporter studied only in model systems to date, as well as in CNNM3 and CNNM4, were also associated with magnesium concentrations in this study. We observed no associations with serum sodium or potassium levels exceeding p<4×10−7. Follow-up studies of newly implicated genomic loci may provide additional insights into the regulation and homeostasis of human serum magnesium levels

Mutations in CNNM4 Cause Jalili Syndrome, Consisting of Autosomal-Recessive Cone-Rod Dystrophy and Amelogenesis Imperfecta

The combination of recessively inherited cone-rod dystrophy (CRD) and amelogenesis imperfecta (AI) was first reported by Jalili and Smith in 1988 in a family subsequently linked to a locus on chromosome 2q11, and it has since been reported in a second small family. We have identified five further ethnically diverse families cosegregating CRD and AI. Phenotypic characterization of teeth and visual function in the published and new families reveals a consistent syndrome in all seven families, and all link or are consistent with linkage to 2q11, confirming the existence of a genetically homogenous condition that we now propose to call Jalili syndrome. Using a positional-candidate approach, we have identified mutations in the CNNM4 gene, encoding a putative metal transporter, accounting for the condition in all seven families. Nine mutations are described in all, three missense, three terminations, two large deletions, and a single base insertion. We confirmed expression of Cnnm4 in the neural retina and in ameloblasts in the developing tooth, suggesting a hitherto unknown connection between tooth biomineralization and retinal function. The identification of CNNM4 as the causative gene for Jalili syndrome, characterized by syndromic CRD with AI, has the potential to provide new insights into the roles of metal transport in visual function and biomineralization

Identification and characterization of a novel mammalian Mg2+ transporter with channel-like properties

Background: Intracellular magnesium is abundant, highly regulated and plays an important role in biochemical functions. Despite the extensive evidence for unique mammalian Mg²⁺ transporters, few proteins have been biochemically identified to date that fulfill this role. We have shown that epithelial magnesium conservation is controlled, in part, by differential gene expression leading to regulation of Mg²⁺ transport. We used this knowledge to identify a novel gene that is regulated by magnesium. Results: Oligonucleotide microarray analysis was used to identify a novel human gene that encodes a protein involved with Mg²⁺-evoked transport. We have designated this magnesium transporter (MagT1) protein. MagT1 is a novel protein with no amino acid sequence identity to other known transporters. The corresponding cDNA comprises an open reading frame of 1005 base pairs encoding a protein of 335 amino acids. It possesses five putative transmembrane (TM) regions with a cleavage site, a N- glycosylation site, and a number of phosphorylation sites. Based on Northern analysis of mouse tissues, a 2.4 kilobase transcript is present in many tissues. When expressed in Xenopus laevis oocytes, MagT1 mediates saturable Mg²⁺ uptake with a Michaelis constant of 0.23 mM. Transport of Mg²⁺ by MagT1 is rheogenic, voltage-dependent, does not display any time-dependent inactivation. Transport is very specific to Mg²⁺ as other divalent cations did not evoke currents. Large external concentrations of some cations inhibited Mg²⁺ transport (Ni²⁺, Zn²⁺, Mn²⁺) in MagT1-expressing oocytes. Ca²⁺and Fe²⁺ were without effect. Real-time reverse transcription polymerase chain reaction and Western blot analysis using a specific antibody demonstrated that MagT1 mRNA and protein is increased by about 2.1-fold and 32%, respectively, in kidney epithelial cells cultured in low magnesium media relative to normal media and in kidney cortex of mice maintained on low magnesium diets compared to those animals consuming normal diets. Accordingly, it is apparent that an increase in mRNA levels is translated into higher protein expression. Conclusion: These studies suggest that MagT1 may provide a selective and regulated pathway for Mg²⁺ transport in epithelial cells.Medicine, Department ofMedicine, Faculty ofReviewedFacult

Loading rat heart myocytes with Mg(2+) using low-[Na(+)] solutions

The objective of our study was to investigate how Mg(2+) enters mammalian cardiac cells. During this work, we found evidence for a previously undescribed route for Mg(2+) entry, and now provide a preliminary account of its properties. Changes in Mg(2+) influx into rat ventricular myocytes were deduced from changes in intracellular ionized Mg(2+) concentration ([fMg(2+)](i)) measured from the fluorescence of mag-fura-2 loaded into isolated cells. Superfusion of myocytes at 37°C with Ca(2+)-free solutions with both reduced [Na(+)] and raised [Mg(2+)] caused myocytes to load with Mg(2+). Uptake was seen with solutions containing 5 mm Mg(2+) and 95 mm Na(+), and increased linearly with increasing extracellular [Mg(2+)] or decreasing extracellular [Na(+)]. It was very sensitive to temperature (Q(10) > 9, 25–37°C), was observed even in myocytes with very low Na(+) contents, and stopped abruptly when external [Na(+)] was returned to normal. Uptake was greatly reduced by imipramine or KB-R7943 if these were added when [fMg(2+)](i) was close to the physiological level, but was unaffected if they were applied when [fMg(2+)](i) was above 2 mm. Uptake was also reduced by depolarizing the membrane potential by increasing extracellular [K(+)] or voltage clamp to 0 mV. We suggest that initial Mg(2+) uptake may involve several transporters, including reversed Na(+)–Mg(2+) antiport and, depending on the exact conditions, reversed Na(+)–Ca(2+) antiport. The ensuing rise of [fMg(2+)](i), in conjunction with reduced [Na(+)], may then activate a new Mg(2+) transporter that is highly sensitive to temperature, is insensitive to imipramine or KB-R7943, but is inactivated by depolarization