An unusual patient with hypercalciuria, recurrent nephrolithiasis, hypomagnesemia and G227R mutation of Paracellin-1. An unusual patient with hypercalciuria and hypomagnesemia unresponsive to thiazide diuretics

Familial Hypomagnesemia with Hypercalciuria and Nephrocalcinosis Due to CLDN16 Gene Mutations: Novel Findings in Two Cases with Diverse Clinical Features

Biallelic loss of function mutations in the CLDN16 gene cause familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC), and chronic kidney disease. Here we report two cases of FHHNC with diverse clinical presentations and hypercalcemia in one as a novel finding. Pt#1 initially presented with urinary tract infection and failure to thrive at 5.5 months of age to another center. Bilateral nephrocalcinosis, hypercalcemia (Ca: 12.2 mg/dl), elevated parathyroid hormone (PTH) level, and hypercalciuria were detected. Persistently elevated PTH with high/normal Ca levels led to subtotal-parathyroidectomy at the age of 2.5. However, PTH levels remained elevated with progressive deterioration in renal function. At 9-year-old, she was referred to us for evaluation of hyperparathyroidism and, hypomagnesemia together with hypercalciuria, elevated PTH with normal Ca levels, and medullary nephrocalcinosis were detected. Compound heterozygosity of CLDN16 variants (c.715G>A, p.G239R; and novel c.360C>A, p.C120*) confirmed the diagnosis. Pt#2 was a 10-month-old boy, admitted with irritability and urinary crystals. Hypocalcemia, hypophosphatemia, elevated PTH and ALP, low 25(OH)D levels, and radiographic findings of rickets were detected. However, additional findings of hypercalciuria and bilateral nephrocalcinosis were inconsistent with the nutritional rickets. Low/normal serum Mg levels suggested the diagnosis of FHHNC which was confirmed genetically as a homozygous missense (c.602G > A; p.G201E) variant in CLDN16. Yet, hypocalcemia and hypomagnesemia persisted in spite of treatment. In conclusion, FHHNC may present with diverse clinical features with mild hypomagnesemia leading to secondary hyperparathyroidism with changing Ca levels from low to high. Early and accurate clinical and molecular genetic diagnosis is important for proper management.

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.

Claudins in Renal Physiology and Pathology

Claudins are integral proteins expressed at the tight junctions of epithelial and endothelial cells. In the mammalian kidney, every tubular segment express a specific set of claudins that give to that segment unique properties regarding permeability and selectivity of the paracellular pathway. So far, 3 claudins (10b, 16 and 19) have been causally traced to rare human syndromes: variants of CLDN10b cause HELIX syndrome and variants of CLDN16 or CLDN19 cause familial hypomagnesemia with hypercalciuria and nephrocalcinosis. The review summarizes our current knowledge on the physiology of mammalian tight junctions and paracellular ion transport, as well as on the role of the 3 above-mentioned claudins in health and disease. Claudin 14, although not having been causally linked to any rare renal disease, is also considered, because available evidence suggests that it may interact with claudin 16. Some single-nucleotide polymorphisms of CLDN14 are associated with urinary calcium excretion and/or kidney stones. For each claudin considered, the pattern of expression, the function and the human syndrome caused by pathogenic variants are described.

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.

Identification of the first large deletion in the CLDN16 gene in a patient with FHHNC and late-onset of chronic kidney disease: case report

Background

Familial hypomagnesemia with hypercalciuria and nephrocalcinosis is a rare autosomal recessive renal disease characterized by tubular disorders at the thick ascending limb of Henle’s loop. It is caused by mutations in the tight junction structural proteins claudin-16 or claudin-19, which are encoded by the CLDN16 and CLDN19 genes, respectively. Patients exhibit excessive wasting of calcium and magnesium, nephrocalcinosis, chronic kidney disease, and early progression to end-stage renal failure during infancy.

Case presentation

We here report the phenotype and molecular analysis of a female Brazilian patient with a novel large homozygous deletion in the CLDN16 gene. The proband, born from consanguineous parents, presented the first symptoms at age 20. Clinical examination revealed hypocalcemia, hypomagnesemia, nephrocalcinosis, mild myopia, high serum levels of uric acid and intact parathyroid hormone, and moderate chronic kidney disease (stage 3). She and her mother were subjected to CLDN16 and CLDN19 mutational analysis. In addition, the multiplex ligation-dependent probe amplification method was used to confirm a CLDN16 multi-exon deletion. Direct sequencing revealed a normal CLDN19 sequence and suggested a large deletion in the CLDN16 gene. Multiplex ligation-dependent probe amplification showed a homozygous CLDN16 multi-exon deletion (E2_E5del). The patient initiated conventional treatment for familial hypomagnesemia with hypercalciuria and nephrocalcinosis and progressed to end-stage kidney disease after five years.

Conclusions

This study provides the first report of a large homozygous deletion in the CLDN16 gene causing familial hypomagnesemia with hypercalciuria and nephrocalcinosis with late onset of the first symptoms. This description expands the phenotypic and genotypic characterization of the disease. The late-onset chronic kidney disease in the presence of a homozygous deletion in the CLDN16 gene reinforces the great variability of genotype-phenotype manifestation in patients with familial hypomagnesemia with hypercalciuria and nephrocalcinosis.

Biology of claudins

Claudins are a family of tight junction membrane proteins that regulate paracellular permeability of epithelia, likely by forming the lining of the paracellular pore. Claudins are expressed throughout the renal tubule, and mutations in two claudin genes are now known to cause familial hypercalciuric hypomagnesemia with nephrocalcinosis. In this review, we discuss recent advances in our understanding of the physiological role of various claudins in normal kidney function, and in understanding the fundamental biology of claudins, including the molecular basis for selectivity of permeation, claudin interactions in tight junction formation, and regulation of claudins by protein kinases and other intracellular signals.

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

Background

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.

Methods

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.

Results

Mutation analysis revealed 2 heterozygous mutations in the claudin 16 gene (CLDN16) 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 CLDN16 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.

Conclusion

The mutations in CLDN16 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.

Report of a family with two different hereditary diseases leading to early nephrocalcinosis

The etiologies of early onset nephrocalcinosis in consanguineous families include five major inherited recessive disorders: primary hyperoxaluria (PH), familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC), distal renal tubular acidosis (dRTA), hereditary hypophosphatemic rickets with hypercalciuria (HHRH) and antenatal Bartter syndrome. In this paper, we describe two girls from consanguineous parents with early onset nephrocalcinosis. Based on both clinical and biochemical assessment in combination with molecular genetics, we have shown that the etiology of nephrocalcinosis is different in each girl: one had FHHNC and her sister had dRTA.