Familial hypomagnesaemia with hypercalciuria and nephrocalcinosis (FHHNC): Compound heterozygous mutation in the claudin 16 (CLDN16) gene

<p>Abstract</p> <p>Background</p> <p>Familial hypomagnesaemia with hypercalciuria and nephrocalcinosis (FHHNC) is an autosomal recessive disorder of renal calcium and magnesium wasting frequently complicated by progressive chronic renal failure in childhood or adolescence.</p> <p>Methods</p> <p>A 7 year old boy was investigated following the findings of marked renal insufficiency and nephrocalcinosis in his 18-month old sister. He too was found to have extensive nephrocalcinosis with increased fractional excretion of magnesium: 12.4% (<4%) and hypercalciuria: 5.7 mmol (< 2.5/24 hours). He had renal impairment, partial distal renal tubular acidosis and defective urinary concentrating ability. Therapy with thiazide diuretics and magnesium supplements failed to halt the progression of the disorder. Both children subsequently underwent renal transplantation. Both children's parents are unaffected and there is one unaffected sibling.</p> <p>Results</p> <p>Mutation analysis revealed 2 heterozygous mutations in the claudin 16 gene <it>(CLDN16</it>) in both affected siblings; one missense mutation in exon 4: C646T which results in an amino acid change Arg216Cys in the second extracellular loop of <it>CLDN16 </it>and loss of function of the protein and a donor splice site mutation which changes intron 4 consensus splice site from 'GT' to 'TT' resulting in decreased splice efficiency and the formation of a truncated protein with loss of 64 amino acids in the second extracellular loop.</p> <p>Conclusion</p> <p>The mutations in <it>CLDN16 </it>in this kindred affect the second extra-cellular loop of claudin 16. The clinical course and molecular findings suggest complete loss of function of the protein in the 2 affected cases and highlight the case for molecular diagnosis in individuals with FHHNC.</p

Common genetic variants of the ion channel transient receptor potential membrane melastatin 6 and 7 (TRPM6 and TRPM7), magnesium intake, and risk of type 2 diabetes in women

<p>Abstract</p> <p>Background</p> <p>Ion channel transient receptor potential membrane melastatin 6 and 7 (TRPM6 and TRPM7) play a central role in magnesium homeostasis, which is critical for maintaining glucose and insulin metabolism. However, it is unclear whether common genetic variation in <it>TRPM6 </it>and <it>TRPM7 </it>contributes to risk of type 2 diabetes.</p> <p>Methods</p> <p>We conducted a nested case-control study in the Women's Health Study. During a median of 10 years of follow-up, 359 incident diabetes cases were diagnosed and matched by age and ethnicity with 359 controls. We analyzed 20 haplotype-tagging single nucleotide polymorphisms (SNPs) in <it>TRPM6 </it>and 5 common SNPs in <it>TRPM7 </it>for their association with diabetes risk.</p> <p>Results</p> <p>Overall, there was no robust and significant association between any single SNP and diabetes risk. Neither was there any evidence of association between common <it>TRPM6 </it>and <it>TRPM7 </it>haplotypes and diabetes risk. Our haplotype analyses suggested a significant risk of type 2 diabetes among carriers of both the rare alleles from two non-synomous SNPs in <it>TRPM6 </it>(Val1393Ile in exon 26 [rs3750425] and Lys1584Glu in exon 27 [rs2274924]) when their magnesium intake was lower than 250 mg per day. Compared with non-carriers, women who were carriers of the haplotype 1393Ile-1584Glu had an increased risk of type 2 diabetes (OR, 4.92, 95% CI, 1.05–23.0) only when they had low magnesium intake (<250 mg/day).</p> <p>Conclusion</p> <p>Our results provide suggestive evidence that two common non-synonymous <it>TRPM6 </it>coding region variants, Ile1393Val and Lys1584Glu polymorphisms, might confer susceptibility to type 2 diabetes in women with low magnesium intake. Further replication in large-scale studies is warranted.</p

Tissue-specific expression and in vivo regulation of zebrafish orthologues of mammalian genes related to symptomatic hypomagnesemia.

Contains fulltext : 128576.pdf (publisher's version ) (Closed access)Introduction of zebrafish as a model for human diseases with symptomatic hypomagnesemia urges to identify the regulatory transport genes involved in zebrafish Mg(2+) physiology. In humans, mutations related to hypomagnesemia are located in the genes TRPM6 and CNNM2, encoding for a Mg(2+) channel and transporter, respectively; EGF (epidermal growth factor); SLC12A3, which encodes for the Na(+)-Cl(-) co-transporter NCC; KCNA1 and KCNJ10, encoding for the K(+) channels Kv1.1 and Kir4.1, respectively; and FXYD2, which encodes for the gamma-subunit of the Na(+),K(+)-ATPase. Orthologues of these genes were found in the zebrafish genome. For cnnm2, kcna1 and kcnj10, two conserved paralogues were retrieved. Except for fxyd2, kcna1b and kcnj10 duplicates, transcripts of orthologues were detected in ionoregulatory organs such as the gills, kidney and gut. Gene expression analyses in zebrafish acclimated to a Mg(2+)-deficient (0 mM Mg(2+)) or a Mg(2+)-enriched (2 mM Mg(2+)) water showed that branchial trpm6, gut cnnm2b and renal slc12a3 responded to ambient Mg(2+). When changing the Mg(2+) composition of the diet (the main source for Mg(2+) in fish) to a Mg(2+)-deficient (0.01 % (w/w) Mg) or a Mg(2+)-enriched diet (0.7 % (w/w) Mg), mRNA expression of branchial trpm6, gut trpm6 and cnnm2 duplicates, and renal trpm6, egf, cnnm2a and slc12a3 was the highest in fish fed the Mg(2+)-deficient diet. The gene regulation patterns were in line with compensatory mechanisms to cope with Mg(2+)-deficiency or surplus. Our findings suggest that trpm6, egf, cnnm2 paralogues and slc12a3 are involved in the in vivo regulation of Mg(2+) transport in ionoregulatory organs of the zebrafish model.1 oktober 201

Regulation of magnesium reabsorption in DCT.

Contains fulltext : 81724.pdf (publisher's version ) (Closed access)The distal convoluted tubule (DCT) is the shortest segment of the nephron and consists of an early (DCT1) and late part (DCT2). Here, several transport proteins, like the thiazide-sensitive NaCl cotransporter (NCC) and the epithelial magnesium (Mg(2+)) channel (TRPM6), are exclusively expressed. This makes the DCT the major site of active transcellular Mg(2+) reabsorption determining the final excretion in the urine. Following the Mg(2+) influx via the apically localized TRPM6, intracellular Mg(2+) diffuses to the basolateral membrane where it is extruded to the blood compartment via still-unidentified Mg(2+) transporters. Recent years have witnessed multiple breakthroughs in the field of transcellular Mg(2+) reabsorption. Epidermal growth factor and estrogen were identified as magnesiotropic hormones by their effect on TRPM6 activity. Intracellularly, receptor of activated protein kinase C 1 and adenosine triphosphate were shown to inhibit TRPM6 activity through its alpha-kinase domain. Furthermore, dysregulation or malfunction of transcellular Mg(2+) reabsorption in DCT has been associated with renal Mg(2+) wasting. Mutations in TRPM6 are responsible for hypomagnesemia with secondary hypocalcemia. A defect in the gamma-subunit of the Na(+)/K(+)-adenosine triphosphatase causes isolated dominant hypomagnesemia resulting from renal Mg(2+) wasting. Moreover, in Gitelman's syndrome, mutations in NCC also result in impaired transcellular Mg(2+) reabsorption in DCT. This review highlights our recently obtained knowledge concerning the molecular regulation of transcellular Mg(2+) reabsorption

Expression of renal cystic genes in patients with HNF1B mutations

BACKGROUND/AIMS: HNF1B nephropathy is characterized by dominantly inherited renal hypodysplasia with few cysts, slow renal decline and hypomagnesemia. Mice with antenatal inactivation of HNF1B are characterized by polycystic kidneys, renal failure and a profound decrease in cystic gene (Pkhd1, Umod, Pkd2) expression. Mice with inactivation after postnatal day 10 have no renal phenotype. METHODS: Quantification of mRNA expression of HNF1B, six of its potential target genes (PKHD1, PKD1, PKD2, IFT88, TMEM27 and UMOD) and three genes involved in the Mg(2+) renal homeostasis (ATP1A1, FXYD2 and CLDN16) in the urinary sediment of 11 individuals with mutation of HNF1B and in 9 controls (non-invasive assessment of the renal transcriptome). RESULTS: As compared to controls, no difference was observed in the urinary mRNA amount of HNF1B and the renal cystic genes. A significant increase in the expression of ATP1A1, which encodes the α1-subunit of the Na(+)/K(+)-ATPase, was identified in HNF1B patients consistent with its role in Mg(2+) homeostasis. CONCLUSION: Assessment of mRNA expression in urinary sediment is a non-invasive method applicable to gain insights into the pathophysiology of inherited nephropathies in humans. HNF1B nephropathy is generally not associated with postnatal down-expression of renal cystic genes in human, a finding consistent with mouse models