Impaired basolateral sorting of pro-EGF causes isolated recessive renal hypomagnesemia

Abstract

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.

New TRPM6 missense mutations linked to hypomagnesemia with secondary hypocalcemia

“…Lower but not significantly different values for Ca 2 þ were recorded: 0.26 ± 0.03 and 0.23±0.02 mM, respectively (data not shown). Next, we analyzed the EGF-mediated upregulation of TRPM6 outward currents as previously published by Groenestege et al 29 Treatment of TRPM6-expressing HEK293 cells with 10 nM EGF for 30 min at 37 1C produced a 1.5-fold increase in WT TRPM6 compared to 1.33-fold for Q 1663 R, which was not significantly different. …”

Transformation of polarized epithelial cells by apical mistrafficking of epiregulin

“…These ligands have distinct motifs that govern their basolateral sorting (6)(7)(8). Of clinical relevance is a germline mutation in the basolateral sorting motif of EGF in individuals with isolated renal hypomagnesemia (9). In this autosomal recessive disorder, loss of EGF delivery to the basolateral surface impairs EGFR activity in the distal convoluted tubules, which is required for efficient magnesium absorption.…”

Regulation of the Epithelial Mg2+ Channel TRPM6 by Estrogen and the Associated Repressor Protein of Estrogen Receptor Activity (REA)

“…Mutations in TRPM6 lead to hypomagnesemia with secondary hypocalcemia (HSH) indicating that this channel is important for the maintenance of the Mg 2ϩ balance (6 -8). Previous studies demonstrated that expression of TRPM6 is regulated by dietary Mg 2ϩ and its channel activity is strongly inhibited by the intracellular Mg 2ϩ concentration ([Mg 2ϩ ] i ) (8,9). Generally, Mg 2ϩ -free solutions are used to measure the characteristic outwardly rectifying TRPM6 currents (8,10,11).…”

“…A recent study reported that epidermal growth factor (EGF) acts as an autocrine/paracrine magnesiotropic hormone via stimulating its receptor on the basolateral membrane of DCT cells, thereby specifically increasing TRPM6 current (18). Further, Groenestege et al (9) demonstrated that the renal TRPM6 mRNA level in ovariectomized rats was significantly reduced, whereas 17␤-estradiol treatment normalized TRPM6 mRNA levels. Next to this classical transcriptional pathway, accumulating evidence suggests also rapid estrogen effects that occur within minutes via a non-transcriptional route (19 -22).…”