Identification of a Novel Homozygous Missense Mutation in the CLDN16 Gene to Decipher the Ambiguous Clinical Presentation Associated with Autosomal Dominant Hypocalcaemia and Familial Hypomagnesemia with Hypercalciuria and Nephrocalcinosis in an Indian Family

Familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHNNC) is a rare autosomal recessive renal tubulopathy disorder characterized by excessive urinary loss of calcium and magnesium, polyuria, polydipsia, bilateral nephrocalcinosis, progressive chronic kidney disease, and renal failure. Also, sometimes amelogenesis imperfecta and severe ocular abnormalities are involved. The CLDN-16 and CLDN-19 genes encode the tight junction proteins claudin-16 and claudin-19, respectively, in the thick ascending loop of Henle in the kidney, epithelial cells of the retina, dental enamel, etc. Loss of function of the CLDN-16 and/or CLDN-19 genes leads to FHHNC. We present a case of FHHNC type 1, which was first confused with autosomal dominant hypocalcaemia (ADH) due to the presence of a very low serum parathyroid hormone (PTH) concentration and other similar clinical features before the genetic investigations. After the exome sequencing, FHHNC type 1 was confirmed by uncovering a novel homozygous missense mutation in the CLDN-16 gene (Exon 2, c.374 T > C) which causes, altered protein structure with F55S. Associated clinical, biochemical, and imaging findings also corroborate final diagnosis. Our findings expand the spectrum of the CLDN-16 mutation, which will further help in the genetic diagnosis and management of FHNNC.

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

BACKGROUND

Familial 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.

METHODS

We selected 12 previously described presumed CLDN16 missense mutations and analysed their potential effect on pre-mRNA splicing using a minigene assay.

RESULTS

Our 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.

CONCLUSIONS

To 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.

Calcium-sensing receptor: evidence and hypothesis for its role in nephrolithiasis

Calcium-sensing receptor (CaSR) is a plasma-membrane G protein-coupled receptor activated by extracellular calcium and expressed in kidney tubular cells. It inhibits calcium reabsorption in the ascending limb and distal convoluted tubule when stimulated by the increase of serum calcium levels; therefore, these tubular segments are enabled by CaSR to play a substantial role in the regulation of serum calcium levels. In addition, CaSR increases water and proton excretion in the collecting duct and promotes phosphate reabsorption and citrate excretion in the proximal tubule. These CaSR activities form a network in which they are integrated to protect the kidney against the negative effects of high calcium concentrations and calcium precipitates in urine. Therefore, the CaSR gene has been considered as a candidate to explain calcium nephrolithiasis. Epidemiological studies observed that calcium nephrolithiasis was associated with polymorphisms of the CaSR gene regulatory region, rs6776158, located within the promoter-1, rs1501899 located in the intron 1, and rs7652589 in the 5'-untranslated region. These polymorphisms were found to reduce the transcriptional activity of promoter-1. Activating rs1042636 polymorphism located in exon 7 was associated with calcium nephrolithiasis and hypercalciuria. Genetic polymorphisms decreasing CaSR expression could predispose individuals to stones because they may impair CaSR protective effects against precipitation of calcium phosphate and oxalate. Activating polymorphisms rs1042636 could predispose to calcium stones by increasing calcium excretion. These findings suggest that CaSR may play a complex role in lithogenesis through different pathways having different relevance under different clinical conditions.