Familial hypomagnesemic hypercalciuric nephrocalcinosis (FHHNC) (OMIM: 248250) is characterized by hypomagnesemia, hypercalciuria and nephrocalcinosis. FHHNC inevitably progresses to end-stage renal disease in decades. Mutations in CLDN-16 and CLDN-19 genes are associated with disrupted magnesium handling in the thick ascending limp of Henle`s loop. Patients with mutations in these genes share similar clinical features, and those with CLDN-19 gene mutations have ocular findings in addition. A 2-month-old boy, was admitted to our clinic with the complaints of upper respiratory tract infection. He was the first-born child of consanguineous parents. Laboratory findings revealed hypocalcemia and hypomagnesemia. Bilateral medullary nephrocalcinosis was detected on abdominal ultrasound. His ophthalmologic examination was unremarkable. With hypomagnesemia, hypercalciuria and nephrocalcinosis, the patient was considered to have FHHNC. Oral magnessium supplementation was initiated. Four years of follow-up has been completed uneventfully. When 6-days-old the brother of the case above was admitted with seizure. The patient was resistant to calcium and anticonvulsant drugs and the seizure activity could only be controlled after magnesium infusion. Biochemistry profile revealed hypocalcemia and hypomagnesemia. Urinary calcium extraction was 11 mg/kg/day. Medullary nephrocalcinosis was reported on renal ultrasound. His eye examination, echocardiography, transfontanel ultrasound and electroencephalography were normal. Due to the triad of hypomagnesemia, hypercalciuria and nephrocalcinosis, and the medical history of his elder brother, he was diagnosed with FHHNC. After correction of the electrolyte abnormalities, he was discharged from hospital and is currently being followed-up without any problem. In this manuscript, we shared our experience about a novel homozygous mutation (W99C) in CLDN-16 gene causing FHHNC in a couple of Turkish siblings.

Keywords: children, hypercalciuria, hypomagnesemia, nephrocalcinosis