Atypical presentation of familial hypomagnesemia with hypercalciuria and nephrocalcinosis in a patient with a new claudin-16 gene mutation

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.

Magnesium: The recent research and developments

Magnesium is the fourth most abundant mineral in the human body, which facilitates more than 300 enzymatic reactions. Magnesium is essential for nucleic material and protein synthesis, neuromuscular conduction, cardiac contractility, energy metabolism, and immune system function. Gastrointestinal system and kidneys closely regulate magnesium absorption and elimination to maintain adequate storage of magnesium. Magnesium deficiency has been linked to many diseases and poor health outcomes. Magnesium has also been proven to be an effective therapeutic agent in many diseases, such as bronchial asthma, cardiac arrhythmia, and pre-eclampsia.

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.

Nephrocalcinosis: A Review of Monogenic Causes and Insights They Provide into This Heterogeneous Condition

The abnormal deposition of calcium within renal parenchyma, termed nephrocalcinosis, frequently occurs as a result of impaired renal calcium handling. It is closely associated with renal stone formation (nephrolithiasis) as elevated urinary calcium levels (hypercalciuria) are a key common pathological feature underlying these clinical presentations. Although monogenic causes of nephrocalcinosis and nephrolithiasis are rare, they account for a significant disease burden with many patients developing chronic or end-stage renal disease. Identifying underlying genetic mutations in hereditary cases of nephrocalcinosis has provided valuable insights into renal tubulopathies that include hypercalciuria within their varied phenotypes. Genotypes affecting other enzyme pathways, including vitamin D metabolism and hepatic glyoxylate metabolism, are also associated with nephrocalcinosis. As the availability of genetic testing becomes widespread, we cannot be imprecise in our approach to nephrocalcinosis. Monogenic causes of nephrocalcinosis account for a broad range of phenotypes. In cases such as Dent disease, supportive therapies are limited, and early renal replacement therapies are necessitated. In cases such as renal tubular acidosis, a good renal prognosis can be expected providing effective treatment is implemented. It is imperative we adopt a precision-medicine approach to ensure patients and their families receive prompt diagnosis, effective, tailored treatment and accurate prognostic information.