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

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, 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.

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.

Rescue of tight junctional localization of a claudin-16 mutant D97S by antimalarial medicine primaquine in Madin-Darby canine kidney cells

Magnesium ion (Mg²⁺) is paracellularly reabsorbed through claudin-16 (CLDN16) in the thick ascending limb (TAL) of Henle's loop in the kidney. Genetic disorders of CLDN16 cause mislocalization of CLDN16, resulting in hypomagnesemia. There is no effective treatment for hypomagnesemia except for magnesium administration. Here, we searched for a novel drug to restore tight junctional localization of a CLDN16 mutant. A D97S mutant, which has a mutation in the first extracellular loop (ECL) of CLDN16, was mainly colocalized with endosome marker, whereas wild-type (WT) CLDN16 was colocalized with ZO-1, an adaptor protein of tight junctions. The protein stability of the D97S mutant was lower than that of WT. The expression level of the D97S mutant was increased by lactacystin, a proteasomal inhibitor. Endocytosis inhibitors increased the tight junctional localization of the D97S mutant. We found that primaquine, an antimalarial agent, increased the protein stability and cell surface localization of the D97S mutant, but the localization of other mutants, which have mutations in the cytosolic domain or second ECL, was not affected. Transepithelial Mg²⁺ flux was increased by primaquine in D97S mutant-expressing cells. The expression of chaperon proteins, proteasome activity, and lactate dehydrogenase release were decreased by primaquine, and the proportion of viable cells increased. In contrast, these effects were not observed in WT CLDN16-expressing cells. These results suggested that primaquine increases the tight junctional localization of the D97S mutant, resulting in a reduction in ER stress and cellular injury. Primaquine may become an effective treatment drug for selected patients with mutant CLDN16.

Familial hypomagnesaemia, Hypercalciuria and Nephrocalcinosis associated with a novel mutation of the highly conserved leucine residue 116 of Claudin 16 in a Chinese patient with a delayed diagnosis: a case report

Background

Sixty mutations of claudin 16 coding gene have been reported in familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC) patients. Recent investigations revealed that a highly conserved glycine-leucine-tryptophan (¹¹⁵G-L-W¹¹⁷) motif in the first extracellular segment (ESC1) of claudin 16 might be essential for stabilization of the appropriately folded ECS1 structure and conservation of normal claudin 16 function. However, neither missense nor nonsense mutation has ever been described in this motif. Our study aimed at identifying mutations in a Chinese patient with FHHNC and exploring the association between genotype and phenotype.

Case presentation

A 33-year-old female presented with 4 years history of recurrent acute pyelonephritis without other notable past medical history. Her healthy parents, who aged 56 and 53 respectively, were second cousins, and her only sibling died from renal failure without definite cause at age 25. Renal ultrasound imaging demonstrated atrophic kidneys and bilateral nephrocalcinosis. The laboratory workup revealed impaired renal function (Stage CKD IV), hypocalcemia and mild hypomagnesemia, accompanied with marked renal loss of magnesium and hypercalciuria. During the follow-up, treatment with calcitriol and calcium but not with magnesium was difficult to achieve normal serum calcium levels, whereas her serum magnesium concentration fluctuated within normal ranges. In the end, the patient unavoidably reached ESRD at 36 years old. The clinical features and family history suggested the diagnosis of FHHNC. To make a definite diagnosis, we use whole-exome sequencing to identify the disease-causing mutations and Sanger sequencing to confirm the mutation co-segregation in the family. As a result, a novel homozygous mutation (c.346C > G, p.Leu116Val) in ¹¹⁵G-L-W¹¹⁷ motif of claudin 16 was identified. Her parents, grandmother and one of her cousins carried heterozygous p.Leu116Val, whereas 200 unrelated controls did not carry this mutation.

Conclusions

We described a delayed diagnosis patient with FHHNC in the Chinese population and identified a novel missense mutation in the highly conserved ¹¹⁵G-L-W¹¹⁷ motif of claudin 16 for the first time. According to the reported data and the information deduced from 3D modeling, we speculate that this mutation probably reserve partial residual function which might be related to the slight phenotype of the patient.

Electronic supplementary material

The online version of this article (10.1186/s12882-018-0979-1) contains supplementary material, which is available to authorized users.