Primary hypomagnesemia constitutes a rare heterogeneous group of disorders characterized by renal or intestinal magnesium (Mg 2+ ) wasting resulting in generally shared symptoms of Mg 2+ depletion, such as tetany and generalized convulsions, and often including associated disturbances in calcium excretion. However, most of the genes involved in the physiology of Mg 2+ handling are unknown. Through the discovery of a mutation in the EGF gene in isolated autosomal recessive renal hypomagnesemia, we have, for what we believe is the first time, identified a magnesiotropic hormone crucial for total body Mg 2+ balance. The mutation leads to impaired basolateral sorting of pro-EGF. As a consequence, the renal EGFR is inadequately stimulated, resulting in insufficient activation of the epithelial Mg 2+ channel TRPM6 (transient receptor potential cation channel, subfamily M, member 6) and thereby Mg 2+ loss. Furthermore, we show that colorectal cancer patients treated with cetuximab, an antagonist of the EGFR, develop hypomagnesemia, emphasizing the significance of EGF in maintaining Mg 2+ balance. IntroductionMagnesium (Mg 2+ ) is established as a central electrolyte in a large number of cellular metabolic reactions, including DNA synthesis, neurotransmission, and hormone receptor binding. It is a component of GTPase and a cofactor for Na,K-ATPase, adenylate cyclase, phosphoinositide kinases, and phosphofructokinase (1). Mg 2+ is also important for the regulation of parathyroid hormone release (2, 3). Accordingly, Mg 2+ deficiency (plasma Mg 2+ concentrations below 0.70 mM) has an effect on multiple body functions. Symptoms of Mg 2+ deficiency are mostly related to muscle dysfunctioning, such as tetany, prolonged QT interval, and cardiac arrhythmias (4). Children with hypomagnesemia often present with tetany and/or convulsions. Hypomagnesemia is a problem frequently observed in more than 10% of hospitalized patients and occurrences can be as high as 65% in intensive care patients (5). A long-term complication seen in many adult patients with chronic hypomagnesemia is chondrocalcinosis, which can lead to impairment of joint function (4). Mg 2+ deficiency can be secondary to systemic diseases (for instance, diabetes mellitus and Crohn disease) or to the use of osmotic agents, diuretics, and drugs such as cyclosporin and cisplatin (6). In addition, primary Mg 2+ deficiency is observed in several monogenetic disorders. Failure of early diagnosis or noncompliance with treatment can be fatal or result in permanent neurological damage.

The kidney is the principal organ responsible for the regulation of the body Mg 2؉ balance.

Recent identification of a mutation in the EGF gene that causes isolated recessive hypomagnesemia led to the finding that EGF increases the activity of the epithelial magnesium (Mg 2ϩ ) channel transient receptor potential M6 (TRPM6). To investigate the molecular mechanism mediating this effect, we performed whole-cell patch-clamp recordings of TRPM6 expressed in human embryonic kidney 293 (HEK293) cells. Stimulation of the EGF receptor increased current through TRPM6 but not TRPM7. The carboxy-terminal ␣-kinase domain of TRPM6 did not participate in the EGF receptor-mediated increase in channel activity. This activation relied on both the Src family of tyrosine kinases and the downstream effector Rac1. Activation of Rac1 increased the mobility of TRPM6, assessed by fluorescence recovery after photobleaching, and a constitutively active mutant of Rac1 mimicked the stimulatory effect of EGF on TRPM6 mobility and activity. Ultimately, TRPM6 activation resulted from increased cell surface abundance. In contrast, dominant negative Rac1 decreased TRPM6 mobility, abrogated current development, and prevented the EGF-mediated increase in channel activity. In summary, EGF-mediated stimulation of TRPM6 occurs via signaling through Src kinases and Rac1, thereby redistributing endomembrane TRPM6 to the plasma membrane. These results describe a regulatory mechanism for transepithelial Mg 2ϩ transport and consequently whole-body Mg 2ϩ homeostasis.

The body maintains Mg(2+) homeostasis by renal and intestinal (re)absorption. However, the molecular mechanisms that mediate transepithelial Mg(2+) transport are largely unknown. Transient receptor potential melastatin 6 (TRPM6) was recently identified and shown to function in active epithelial Mg(2+) transport in intestine and kidney. To define the relationship between Mg(2+) status and TRPM6 expression, we used two models of hypomagnesemia: 1) C57BL/6J mice fed a mildly or severely Mg(2+)-deficient diet, and 2) mice selected for either low (MgL) or high (MgH) erythrocyte and plasma Mg(2+) status. In addition, the mice were subjected to a severely Mg(2+)-deficient diet. Our results show that C57BL/6J mice fed a severely Mg(2+)-deficient diet developed hypomagnesemia and hypomagnesuria and showed increased TRPM6 expression in kidney and intestine. When fed a Mg(2+)-adequate diet, MgL mice presented hypomagnesemia and hypermagnesuria, and lower kidney and intestinal TRPM6 expression, compared with MgH mice. A severely Mg(2+)-deficient diet led to hypomagnesemia and hypomagnesuria in both strains. Furthermore, this diet induced kidney TRPM6 expression in MgL mice, but not in MgH mice. In conclusion, as shown in C57BL/6J mice, dietary Mg(2+)-restriction results in increased Mg(2+) (re)absorption, which is correlated with increased TRPM6 expression. In MgL and MgH mice, the inherited Mg(2+) status is linked to different TRPM6 expression. The MgL and MgH mice respond differently to a low-Mg(2+) diet with regard to TRPM6 expression in the kidney, consistent with genetic factors contributing to the regulation of cellular Mg(2+) levels. Further studies of these mice strains could improve our understanding of the genetics of Mg(2+) homeostasis.