CLDN16 Mutations Research Articles

Urinary Acidification Does Not Explain the Absence of Nephrocalcinosis in a Mouse Model of Familial Hypomagnesaemia with Hypercalciuria and Nephrocalcinosis (FHHNC).

Patients with mutations in Cldn16 suffer from familial hypomagnesaemia with hypercalciuria and nephrocalcinosis (FHHNC) which can lead to renal insufficiency. Mice lacking claudin-16 show hypomagnesemia and hypercalciuria, but no nephrocalcinosis. Calcium oxalate and calcium phosphate are the most common insoluble calcium salts that accumulate in the kidney in the case of nephrocalcinosis, however, the formation of these salts is less favored in acidic conditions. Therefore, urine acidification has been suggested to limit the formation of calcium deposits in the kidney. Assuming that urine acidification is causative for the absence of nephrocalcinosis in the claudin-16-deficient mouse model, we aimed to alkalinize the urine of these mice by the ablation of the subunit B1 of the vesicular ATPase in addition to claudin-16. In spite of an increased urinary pH in mice lacking claudin-16 and the B1 subunit, nephrocalcinosis did not develop. Thus, urinary acidification is not the only factor preventing nephrocalcinosis in claudin-16 deficient mice.

Furosemide rescues hypercalciuria in familial hypomagnesaemia with hypercalciuria and nephrocalcinosis model

Aim: Impaired calcium homeostasis limits life expectancy and quality in familial hypomagnesaemia with hypercalciuria and nephrocalcinosis (FHHNC). This rare disease is caused by loss-of-function mutations in CLDN16 or CLDN19 genes leading to impaired paracellular reabsorption of divalent cations along the cortical thick ascending limb (cTAL). The ensuing late distal nephron and collecting duct system partially compensate for the defect in cTAL by increased transcellular Ca 2+ reabsorption via the luminal transient receptor potential channel V5 (TRPV5), as well as basolateral plasma membrane calcium ATPase (PMCA) and sodium-potassium exchanger (NCX1). The loop diuretic furosemide induces compensatory activation in these distal segments due to blockade of NaCl and divalent cation reabsorption in the preceeding TAL. Although furosemide enhances urinary calcium excretion via inhibition of the aforementioned cTAL in general population, this effect is not expected in FHHNC patients with already severely impaired Ca 2+ transport in the cTAL. The present study follows the hypothesis that furosemide may alleviate hypercalciuria in this disease by activation of the distal transcellular Ca 2+ transport proteins. Methods: Cldn16-deficient mice (Cldn16-/- ) served as a FHHNC model. Wild-type (WT) and Cldn16-/- mice were treated with furosemide (7 days of 40 mg/kg bw) or vehicle. We assessed renal electrolyte handling (metabolic cages) and key divalent transport proteins. Results: Cldn16-/- mice show increased Ca 2+ excretion than WT and compensatory stimulation of Cldn2 in the proximal tubule, as well as of TRPV5, and NCX1 in the distal nephron at baseline. Furosemide reduced hypercalciuria in Cldn16-/- mice and enhanced TRPV5 and PMCA levels in Cldn16-/- but not in WT mice. Conclusions: Furosemide alleviated hypercalciuria in a mouse FHHNC model, likely via stimulation of transcellular luminal and basolateral Ca 2+ transport systems in the distal nephron and collecting duct. These results may have a clinical implication in FHHNC patients. No conflict of interests This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Furosemide rescues hypercalciuria in familial hypomagnesaemia with hypercalciuria and nephrocalcinosis model.

Perturbed calcium homeostasis limits life expectancy in familial hypomagnesaemia with hypercalciuria and nephrocalcinosis (FHHNC). This rare disease occurs by loss-of-function mutations in CLDN16 or CLDN19 genes, causing impaired paracellular reabsorption of divalent cations along the cortical thick ascending limb (cTAL). Only partial compensation takes place in the ensuing late distal convoluted tubule, connecting tubule, and collecting duct, where the luminal transient receptor potential channel V5 (TRPV5), as well as basolateral plasma membrane calcium ATPase (PMCA) and sodium-potassium exchanger (NCX1) mediate transcellular Ca2+ reabsorption. The loop diuretic furosemide induces compensatory activation in these distal segments. Normally, furosemide enhances urinary calcium excretion via inhibition of the aforementioned cTAL. As Ca2+ reabsorption in the cTAL is already severely impaired in FHHNC patients, furosemide may alleviate hypercalciuria in this disease by activation of the distal transcellular Ca2+ transport proteins. Cldn16-deficient mice (Cldn16-/- ) served as a FHHNC model. Wild-type (WT) and Cldn16-/- mice were treated with furosemide (7 days of 40 mg/kg bw) or vehicle. We assessed renal electrolyte handling (metabolic cages) and key divalent transport proteins. Cldn16-/- mice show higher Ca2+ excretion than WT and compensatory stimulation of Cldn2, TRPV5, and NCX1 at baseline. Furosemide reduced hypercalciuria in Cldn16-/- mice and enhanced TRPV5 and PMCA levels in Cldn16-/- but not in WT mice. Furosemide significantly reduces hypercalciuria, likely via upregulation of luminal and basolateral Ca2+ transport systems in the distal nephron and collecting duct in this model for FHHNC.

Familial Hypomagnesemia with Hypercalciuria, Nephrocalcinosis, and Bilateral Chorioretinal Atrophy in a Patient with Homozygous p.G75S Variant in CLDN19

Introduction Familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC) is a rare disorder caused by perturbation in renal reabsorption of magnesium and calcium. Biallelic pathogenic variants either in gene CLDN16 or CLDN19 are responsible for molecular defects. Most patients with CLDN19 variants have been associated with ocular involvements (FHHNCOI). Patient and Methods We had a pediatric patient with hypercalciuric hypomagnesemia and bilateral chorioretinal atrophy. Metabolic profiling and radiology examinations were performed, in addition to whole exome sequencing (WES) used for detection of the causative variant. Results Analysis of WES revealed a homozygous c.223G > A (p.G75S) variant in CLDN19 . MutationTaster and Combined Annotation-Dependent Depletion support its deleterious effect and SHERLOC's criteria put it in pathogenic category. This variant is previously reported in compound heterozygous state with other known pathogenic variant. As far as we know, it is the first report of this variant in homozygous state. Conclusion The variant found in our patient is pathogenic and compatible with FHHNCOI characteristics. WES is an advantageous tool in molecular diagnosis and finding genetic pathology of this disease. In line with other reports, ocular abnormalities are variable in patients with CLDN19 mutations, and chronic kidney disease and retinal damages must be considered in this group.

Heterogeneity is a common ground in familial hypomagnesemia with hypercalciuria and nephrocalcinosis caused by CLDN19 gene mutations.

Familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC) is a rare tubulopathy caused by mutations in the CLDN16 or CLDN19 genes. Patients usuallydevelop hypomagnesemia, hypercalciuria, nephrocalcinosis and renal failure early in life. Patients with CLDN19 mutations may also have ocular abnormalities. Despite clinical variability, factors associated with kidney functionimpairment, especially in patients with CLDN19 mutations, have not been addressed. Retrospective multicenter study of 30 genetically confirmed FHHNC Spanish patients. We analyzed kidney function impairmentconsidering as outcomeschronic kidney disease (CKD) stage3 and annual estimated glomerular filtration rate (eGFR) decline, to identify factors associated with thedifferent phenotypes. Of thirty patients, 27 had mutations in the CLDN19 gene (20 homozygous for the p.G20D mutation) and 3 in the CLDN16. Age at diagnosis was 1.71 (0.67-6.04) years and follow-up time was 8.34 ± 4.30years. No differences in CKD stage3-free survival based on CLDN19 mutation (p.G20D homozygous vs. other mutations) or gender were found, although females seemed to progress faster than males. Patients with more pronounced eGFR decline had higher PTH levels at diagnosis than those with stablekidney function, despite similar initial eGFR. Approximately 60% of CLDN19 patients presented ocular abnormalities. Furthermore, we confirmed high phenotypic intrafamilial variability. In a contemporary cohort of FHHNC patients with CLDN19 mutations, females seemed to progress to CKD-stage3 faster than males. Increased PTH levels at baseline may indicate amore severe renal course. There was high phenotype variability among patients with CLDN19 mutations and kidney function impairment differedeven betweensiblings.

Familial hypomagnesemia with hypercalciuria and nephrocalcinosis.

Familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC; OMIM 248250) is a rare autosomal recessive kidney disease caused by mutations in the CLDN16 or CLDN19 genes encoding the proteins claudin-16 and claudin-19, respectively. These are involved in paracellular magnesium and calcium transport in the thick ascending limb of Henle's loop and account for most of the magnesium reabsorption in the tubules. FHHNC is characterized by hypomagnesaemia, hypercalciuria, and nephrocalcinosis, and progresses to kidney failure, requiring dialysis and kidney transplantation mainly during the second to third decades of life. Patients carrying CLDN19 mutations frequently exhibit associated congenital ocular defects leading to variable visual impairment. Despite this severe clinical course, phenotype variability even among siblings has been described in this disease, suggesting unidentified epigenetic mechanisms or other genetic or environmental modifiers. Currently, there is no specific therapy for FHHNC. Supportive treatment with high fluid intake and dietary restrictions, as well as magnesium salts, thiazides, and citrate, are commonly used in an attempt to retard the progression of kidney failure. A kidney transplant remains the only curative option for kidney failure in these patients. In this review, we summarize the current knowledge about FHHNC and discuss the remaining open questions about this disorder.

Novel pathogenic variant causing familial hypomagnesemia with hypercalciuria and nephrocalcinosis

Nephrocalcinosis in children can arise due to a variety of metabolic conditions and appropriate diagnosis can lead to specific management and has implications in prognosis. Familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC) is a rare disease with varied manifestations which is caused by mutations in CLDN16 and CLDN19, and has a high rate of progression to kidney failure in the second to third decades of life. Early diagnosis aids in retardation of the progression to kidney failure. We present two unrelated patients presenting with nephrocalcinosis and deranged renal function, who had a novel pathogenic variation (c.685C > T; p.Q229X), leading to the diagnosis of FHHNC. Genetic testing is crucial in patients presenting with nephrocalcinosis for diagnosis and prognostication.

In-Depth Bioinformatic Study of the CLDN16 Gene and Protein: Prediction of Subcellular Localization to Mitochondria.

Background and Objectives: The defects in the CLDN16 gene are a cause of primary hypomagnesemia (FHHNC), which is characterized by massive renal magnesium wasting, resulting in nephrocalcinosis and renal failure. The mutations occur throughout the gene’s coding region and can impact on intracellular trafficking of the protein or its paracellular pore forming function. To gain more understanding about the mechanisms by which CLDN16 mutations can induce FHHNC, we performed an in-depth computational analysis of the CLDN16 gene and protein, focusing specifically on the prediction of the latter’s subcellular localization. Materials and Methods: The complete nucleotide or amino acid sequence of CLDN16 in FASTA format was entered and processed in 14 databases. Results: One CpG island was identified. Twenty five promoters/enhancers were predicted. The CLDN16 interactome was found to consist of 20 genes, mainly involved in kidney diseases. No signal peptide cleavage site was identified. A probability of export to mitochondria equal to 0.9740 and a cleavable mitochondrial localization signal in the N terminal of the CLDN16 protein were predicted. The secondary structure prediction was visualized. Νo phosphorylation sites were identified within the CLDN16 protein region by applying DISPHOS to the functional class of transport. The KnotProt database did not predict any knot or slipknot in the protein structure of CLDN16. Seven putative miRNA binding sites within the 3’-UTR region of CLDN16 were identified. Conclusions: This is the first study to identify mitochondria as a probable cytoplasmic compartment for CLDN16 localization, thus providing new insights into the protein’s intracellular transport. The results relative to the CLDN16 interactome underline its role in renal pathophysiology and highlight the functional dependence of CLDNs-10, 14, 16, 19. The predictions pertaining to the miRNAs, promoters/enhancers and CpG islands of the CLDN16 gene indicate a strict regulation of its expression both transcriptionally and post-transcriptionally.

Exonic CLDN16 mutations associated with familial hypomagnesemia with hypercalciuria and nephrocalcinosis can induce deleterious mRNA alterations

BackgroundFamilial hypomagnesaemia with hypercalciuria and nephrocalcinosis type 1 is an autosomal recessive disease characterized by excessive renal magnesium and calcium excretion, bilateral nephrocalcinosis, and progressive chronic renal failure. This rare disease is caused by mutations in CLDN16 that encodes claudin-16, a tight-junction protein involved in paracellular reabsorption of magnesium and calcium in the renal tubule. Most of these variants are located in exons and have been classified as missense mutations. The functional consequences of some of these claudin-16 mutant proteins have been analysed after heterologous expression showing indeed a significant loss of function compared to the wild-type claudin-16. We hypothesize that a number of CLDN16 exonic mutations can be responsible for the disease phenotype by disrupting the pre-mRNA splicing process.MethodsWe selected 12 previously described presumed CLDN16 missense mutations and analysed their potential effect on pre-mRNA splicing using a minigene assay.ResultsOur results indicate that five of these mutations induce significant splicing alterations. Mutations c.453G > T and c.446G > T seem to inactivate exonic splicing enhancers and promote the use of an internal cryptic acceptor splice site resulting in inclusion of a truncated exon 3 in the mature mRNA. Mutation c.571G > A affects an exonic splicing enhancer resulting in partial skipping of exon 3. Mutations c.593G > C and c.593G > A disturb the acceptor splice site of intron 3 and cause complete exon 4 skipping.ConclusionsTo our knowledge, this is the first report of CLDN16 exonic mutations producing alterations in splicing. We suggest that in the absence of patients RNA samples, splicing functional assays with minigenes could be valuable for evaluating the effect of exonic CLDN16 mutations on pre-mRNA splicing.

Characterization of two novel mutations in the claudin-16 and claudin-19 genes that cause familial hypomagnesemia with hypercalciuria and nephrocalcinosis

Familial hypomagnesemia with hypercalciuria and nephrocalcinosis is an autosomal-recessive renal tubular disorder characterized by excessive urinary losses of magnesium and calcium, bilateral nephrocalcinosis and progressive chronic renal failure in childhood or adolescence. The disease is caused by mutations in the tight-junction proteins claudin-16 and claudin-19 that are encoded by the CLDN16 and CLDN19 genes, respectively. Patients with CLDN19 mutations also are affected with severe ocular abnormalities. The aim of our study was to identify and characterize the molecular defects causing this disease in a Georgian girl and two Spanish siblings. Clinical and biochemical parameters were studied. The CLDN16 and CLDN19 genes were analyzed by DNA sequencing. The functional consequences of the identified mutations on pre-mRNA splicing were investigated using a minigene assay. Sequence analysis revealed that the patient from Georgia was homozygous for a novel mutation, c.602G > A; p.(G201E), in exon 4 of the CLDN16 gene. The two Spanish siblings were homozygous for a new CLDN19 mutation, c.388G > T; p.(G130C), located in exon 2, and both parents were heterozygous carriers of the mutation. Bioinformatics analysis predicted that the amino acid substitutions generated by these mutations were pathogenic. Functional studies showed that mutation c.388G > T also results in partial skipping of CLDN19 exon 2, which would imply significant alterations in the claudin-19 protein structure. Conversely, CLDN16 mutation c.602G > A had no effect on pre-mRNA splicing. Our study expands the genotypic classification of this rare disease and provides the first report of a CLDN19 mutation affecting splicing.

Establishment of urinary exosome-like vesicles isolation protocol for FHHNC patients and evaluation of different exosomal RNA extraction methods

BackgroundMolecular and cellular pathophysiological events occurring in the majority of rare kidney diseases remain to be elucidated. Familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC) is a rare autosomal recessive disorder caused by mutations in either CLDN16 or CLDN19 genes. This disease is characterized by massive urinary wasting of magnesium and calcium, osmosis deregulation and polyuria. Patients with p.G20D homozygous mutation in CLDN19 gene exhibit different progression to kidney failure suggesting that beyond the pathogenic mutation itself, other molecular events are favoring disease progression. Due to the fact that biopsy is not clinically indicated in these patients, urinary exosome-like vesicles (uEVs) can be envisioned as a valuable non-invasive source of information of events occurring in the kidney. Exosome research has increased notably to identify novel disease biomarkers but there is no consensus standardized protocols for uEVs isolation in patients with polyuria. For this reason, this work was aimed to evaluate and refine different uEVs isolation methods based on differential centrifugation, the gold standard method.ResultsCharacterization by NTA, cryo-TEM and immunoblotting techniques identified the most appropriate protocol to obtain the highest yield and purest uEVs enriched fraction possible from urine control samples and FHHNC patients. Moreover, we tested five different RNA extraction methods and evaluated the miRNA expression pattern by qRT-PCR.ConclusionsIn summary, we have standardized the conditions to proceed with the identification of differentially expressed miRNAs in uEVs of FHHNC patients, or other renal diseases characterized by polyuria.

Deletion of claudin-10 rescues claudin-16–deficient mice from hypomagnesemia and hypercalciuria

The tight junction proteins claudin-10 and -16 are crucial for the paracellular reabsorption of cations along the thick ascending limb of Henle's loop in the kidney. In patients, mutations in CLDN16 cause familial hypomagnesemia with hypercalciuria and nephrocalcinosis, while mutations in CLDN10 impair kidney function. Mice lacking claudin-16 display magnesium and calcium wasting, whereas absence of claudin-10 results in hypermagnesemia and interstitial nephrocalcinosis. In order to study the functional interdependence of claudin-10 and -16 we generated double-deficient mice. These mice had normal serum magnesium and urinary excretion of magnesium and calcium and showed polyuria and sodium retention at the expense of increased renal potassium excretion, but no nephrocalcinosis. Isolated thick ascending limb tubules of double mutants displayed a complete loss of paracellular cation selectivity and functionality. Mice lacking both claudin-10 and -16 in the thick ascending limb recruited downstream compensatory mechanisms and showed hypertrophic distal convoluted tubules with changes in gene expression and phosphorylation of ion transporters inthis segment, presumably triggered by the mild decrease in serum potassium. Thus, severe individual phenotypes in claudin-10 and claudin-16 knockout mice are corrected by the additional deletion of the other claudin.

Familial hypomagnesaemia with hypercalciuria and nephrocalcinosis induced by compound heterozygous mutation of CLDN16: a case report and literature review

Objective To investigate the clinical features and disease-causing mutations of familial hypomagnesaemia with hypercalciuria and nephrocalcinosis. Methods In February 2016, a 24 year old female patient with left kidney stone and nephrocalcinosis in bilateral kidneys was admitted to our hospital. One month prior to this admission, she had been treated by PCNL to remove the most part of left kidney stone in other hospital. After admission, She was found hypomagnesaemia (serum magnesium 0.65 mmol/L) and hypercalciuria (24h urine calcium 364.0 mg) but with normal renal function (serum creatinine 101.5μmol/L). And the remained part of left kidney stone was removed by flexible ureteroscope. As she was considered probably with an autosomal recessive FHHNC, an analysis of CLDN16 and CLDN19 gene mutations was performed using her and her parents' peripheral white blood cells. Results Mutation analysis revealed this patient had two heterozygous mutations in the CLDN16. One is an one-base deletion mutation in the 123th codon in exon 2: 368delA. The other is a missense mutation in the 139th codon in exon 2: 416C→T which resulted in an amino acid change Ala139Val. Her parents respectively had one of each heterozygous mutation. In the six months follow-up, an oral administration with hydrochlorothiazide, potassium citrate, and calcium magesium supplements significantly reduced her hypomagnesaemia (serum magnesium 1.0 mmol/L) and hypercalciuria (24-h urine calcium 156.0 mg), and no stone recurrence and aggravation of nephrocalcinosis and renal dysfunction occurred. Conclusions We diagnosed a patient with FHHNC who had a novel compound heterozygous mutation of CLDN16. This rare disease should be suspected if there are three constant clinical features of hypomagnesaemia, hypercalciuria and nephrocalcinosis, and verified with CLDN16 and CLDN19 gene test. Currently the option for treatment of FHHNC is symptomatic treatment until severe deterioration of renal function. The hydrochlorothiazide, potassium citrate, and calcium magesium supplements may have considerable effects on hypomagnesaemia and hypercalciuria. Key words: Hypomagnesaemia; Hypercalciuria; Nephrocalcinosis; CLDN16 gene; Familial hypomagnesaemia with hypercalciuria and nephrocalcinosis

Amelogenesis imperfecta in familial hypomagnesaemia and hypercalciuria with nephrocalcinosis caused by CLDN19 gene mutations

BackgroundAmelogenesis imperfecta (AI) is a group of genetic diseases characterised by tooth enamel defects. AI was recently described in patients with familial hypercalciuria and hypomagnesaemia with nephrocalcinosis (FHHNC) caused by...

A novel mutation in CLDN16 results in rare familial hypomagnesaemia with hypercalciuria and nephrocalcinosis in a Chinese family

BackgroundFamilial hypomagnesaemia with hypercalciuria and nephrocalcinosis (FHHNC) is a rare autosomal recessively inherited disease characterized by excessive wasting of renal tubular magnesium and calcium. FHHNC is associated with various mutations in CLDN16 and CLDN19. CasesTwo children from a consanguineous family of Chinese Han origin demonstrated manifestations of rickets, polyuria, polydipsia, hematuria and failure to thrive. Hypomagnesaemia (0.49–0.50mmol/L), hypercalciuria or a trend to hypercalciuria (24hour urine calcium: 3.8–5.1mg/kg/day), and secondary hyperparathyroidism (serum PTH level: 94.7–200pg/mL) were revealed upon laboratory examination. Using targeted next-generation sequencing and subsequent confirmation by Sanger sequencing, a novel homozygous mutation was identified in the CLDN16 gene of both FHHNC patients. This specific mutation, a 16bp deletion followed by a 23bp insertion in exon 3, led to the generation of a premature termination codon. The parents and an unaffected sister were all heterozygous carriers of this mutation. ConclusionsWe detected a novel mutation in CLDN16 for the first time. The clinical and genetic findings from this study will help to expand the understanding of this rare disease, FHHNC.

Hypomagnesaemia due to lead poisoning in the context of a heterozygous CLDN-16 mutation

Hypomagnesaemia due to lead poisoning in the context of a heterozygous CLDN-16 mutation

Claudin-16 Deficiency Impairs Tight Junction Function in Ameloblasts, Leading to Abnormal Enamel Formation.

Claudin-16 protein (CLDN16) is a component of tight junctions (TJ) with a restrictive distribution so far demonstrated mainly in the kidney. Here, we demonstrate the expression of CLDN16 also in the tooth germ and show that claudin-16 gene (CLDN16) mutations result in amelogenesis imperfecta (AI) in the 5 studied patients with familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC). To investigate the role of CLDN16 in tooth formation, we studied a murine model of FHHNC and showed that CLDN16 deficiency led to altered secretory ameloblast TJ structure, lowering of extracellular pH in the forming enamel matrix, and abnormal enamel matrix protein processing, resulting in an enamel phenotype closely resembling human AI. This study unravels an association of FHHNC owing to CLDN16 mutations with AI, which is directly related to the loss of function of CLDN16 during amelogenesis. Overall, this study indicates for the first time the importance of a TJ protein in tooth formation and underlines the need to establish a specific dental follow-up for these patients.

Familial hypomagnesaemia with hypercalciuria and nephrocalcinosis: clinical and molecular characteristics.

Familial hypomagnesaemia with hypercalciuria and nephrocalcinosis (FHHNC) is an autosomal-recessive renal tubular disorder characterized by excessive urinary losses of magnesium and calcium, bilateral nephrocalcinosis and progressive chronic renal failure. Presentation with FHHNC symptoms generally occurs early in childhood or before adolescence. At present, the only therapeutic option is supportive and consists of oral magnesium supplementation and thiazide diuretics. However, neither treatment seems to have a significant effect on the levels of serum magnesium or urine calcium or on the decline of renal function. In end-stage renal disease patients, renal transplantation is the only effective approach. This rare disease is caused by mutations in the CLDN16 or CLDN19 genes. Patients with mutations in CLDN19 also present severe ocular abnormalities such as myopia, nystagmus and macular colobamata. CLDN16 and CLDN19 encode the tight-junction proteins claudin-16 and claudin-19, respectively, which are expressed in the thick ascending limb of Henle's loop and form an essential complex for the paracellular reabsorption of magnesium and calcium. Claudin-19 is also expressed in retinal epithelium and peripheral neurons. Research studies using mouse and cell models have generated significant advances on the understanding of the pathophysiology of FHHNC. A recent finding has established that another member of the claudin family, claudin-14, plays a key regulatory role in paracellular cation reabsorption by inhibiting the claudin-16–claudin-19 complex. Furthermore, several studies on the molecular and cellular consequences of disease-causing CLDN16 and CLDN19 mutations have provided critical information for the development of potential therapeutic strategies.

Hypomagnesaemia due to lead poisoning in the context of a heterozygous CLDN-16 mutation

Hypomagnesaemia due to lead poisoning in the context of a heterozygous CLDN-16 mutation

First Report of a Novel Missense CLDN19 Mutations Causing Familial Hypomagnesemia with Hypercalciuria and Nephrocalcinosis in a Chinese Family

Familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC) is an autosomal recessive disorder caused by mutations in the CLDN16 or CLDN19 genes, encoding claudin-16 and claudin-19 in the thick ascending limb of Henle's loop. In patients with claudin-19 mutations, severe ocular involvement (macular coloboma, pigmentary retinitis, nystagmus, or visual loss) has been described. In this report, we presented a 12-year-old girl with rickets, polyuria, and polydipsia. She was the daughter of consanguineous parents, and she had a history of recurred hypocalcemic and hypomagnesemic tetany. On physical examination, bilateral horizontal nystagmus and severe myopia were detected. Laboratory examination revealed hypomagnesemia, hypocalcemia, hypercalciuria, nephrocalcinosis, and renal stone. A clinical diagnosis of FHHNC caused possibly by claudin-19 mutation was decided with the ocular findings. DNA analysis revealed a novel homozygous missense mutation c.241C>T in the CLDN19 gene. In conclusion, in a patient with hypomagnesemia, hypercalciuria, nephrocalcinosis, and ocular findings, a diagnosis of FHHNC caused by claudin-19 mutation should be considered. This is the first study of FHHNC in Chinese population. Our findings of the novel mutation c.241C>T in exon 2 add to the list of more than 16 mutations of CLDN19 gene reported.