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.

Novel Variant in CLDN16: A Further Step in the Diagnosis of Familial Hypomagnesemia with Hypercalciuria and Nephrocalcinosis—A Case Report

Familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC) is a rare tubulopathy characterized by renal loss of calcium and magnesium leading to progressive renal failure. The disorder is caused by variants to the tight junction proteins claudin-16 and -19. While rare, this disorder causes a significant burden to patients based on its clinical manifestations of various electrolyte abnormalities, nephrocalcinosis, and early progression to renal failure. In this report we describe the diagnosis of a novel variant of CLDN16 which clinically presented with severe hypomagnesemia, hypocalcemia, nephrocalcinosis, and renal failure.

Differential localization patterns of Claudin 10, 16 and 19 in human, mouse, and rat renal tubular epithelia

Functional properties of the paracellular pathway depend critically on the set of claudins expressed at the tight junction. Two syndromes are causally linked to loss-of-function mutations of claudins: HELIX syndrome caused by genetic variations in the CLDN10 gene, and Familial Hypomagnesemia with Hypercalciuria and Nephrocalcinosis caused by genetic variations in the CLDN16 or the CLDN19 gene. All three genes are expressed in the kidney, particularly in the thick ascending limb (TAL). However, localization of these claudins in humans and rodents remains to be delineated in detail. We studied the segmental and subcellular expression of CLDN10, 16 and 19 in both paraffin-embedded and frozen kidney sections from adult human, mouse and rat, using immunohistochemistry and immunofluorescence, respectively. Here CLDN10 was present in a subset of medullary and cortical TAL cells, localizing to basolateral domains and tight junction in human and rodent kidney. A weak expression was detected at the tight junction of proximal tubular cells. CLDN16 was primarily expressed in a subset of TAL cells in cortex and outer stripe of outer medulla, restricted to basolateral domains and tight junctional structures in both human and rodent kidney. CLDN19 predominantly colocalized with CLDN16 in tight junctions and basolateral domains of TAL but was also found in basolateral and junctional domains in more distal sites. CLDN10 expression at tight junction almost never overlapped with that of CLND16 and CLDN19, consistent with distinct junctional pathways with different permeation profiles in both human and rodent kidney.

A novel TRPM6 variant (c.3179T>A) causing familial hypomagnesemia with secondary hypocalcemia

Summary Familial hypomagnesemia with secondary hypocalcemia (FHSH) is a rare autosomal recessive disorder (OMIM# 602014) characterized by profound hypomagnesemia associated with hypocalcemia. It is caused by mutations in the gene encoding transient receptor potential cation channel member 6 (TRPM6). It usually presents with neurological symptoms in the first months of life. We report a case of a neonate presenting with recurrent seizures and severe hypomagnesemia. The genetic testing revealed a novel variant in the TRPM6 gene. The patient has been treated with high-dose magnesium supplementation, remaining asymptomatic and without neurological sequelae until adulthood. Early diagnosis and treatment are important to prevent irreversible neurological damage. Learning points: Loss-of-function mutations of TRPM6 are associated with FHSH. FHSH should be considered in any child with refractory hypocalcemic seizures, especially in cases with serum magnesium levels as low as 0.2 mM. Normocalcemia and relief of clinical symptoms can be assured by administration of high doses of magnesium. Untreated, the disorder may be fatal or may result in irreversible neurological damage.