A novel claudin-16 mutation, severe bone disease, and nephrocalcinosis

Abstract

In Feb, 2013, a 26-year-old Pakistani male student came to the emergency department with two self-terminating tonic-clonic seizures. He reported a 3 month history of lethargy, weight loss, polyuria, and polydipsia. He had no notable past medical history and did not take any regular medication. His parents, fi rst cousins, were well. He had seven siblings: three brothers died in infancy and one sister died in her 30s from an undetermined kidney problem. No further clinical information was available because his family lived in Pakistan. Physical examination was normal. There were no ocular abnormalities. He had a severe metabolic acidosis with a pH of 6·58 (normal range 7·38–7·42), an almost undetect able bicarbonate level of 0·8 mmol/L (22–26 mmol/L), and renal failure with a creatinine of 1611 μmol/L (80–115 μmol/L). He had severe hypocalcaemia, 1·65 mmol/L (2·2–2·6 mmol/L) and hypomagnesaemia, 0·65 mmol/L (0·7–1·0 mmol/L). Para thyroid hormone (PTH) was 50 pmol/L (1·6–7 pmol/L) and alkaline phosphate was 215 mmol/L (30–130 mmol/L). His urinary pH was 6·3 and dipstick urinalysis was unremarkable. He was admitted to the intensive care unit for haemofi ltration. CT identifi ed two small kidneys with striking bilateral medullary nephrocalcinosis (appendix), which was con sistent with end stage renal failure. Plain radiography showed bilateral neck of femur fractures, presumably sustained during hypo calcaemic seizures. He required bilateral hip replace ments. His bone was so severely diseased that his right hip prosthesis evulsed through the shaft of his femur post-operatively (appendix), neces sitating salvage surgery. Metabolic acidosis resolved with haemo dialysis. However, hypo magnesaemia and hypo calc aemia were refractory to treatment with calcium supple ments, and fi nally responded to calcitriol and magnesium replacement. He remained seizure-free on maintenance haemodialysis and returned to Pakistan on July 30, 2013. Medullary nephrocalcinosis with normal renal function can result from dysregulated calcium metabolism or tubular or anatomical disease, and is most commonly caused by primary hyper para thy roidism, distal renal tubular acidosis (dRTA), or medul lary sponge kidney. Nephrocalcinosis with a severe metabolic acidosis and an alkaline urine (pH>5·3) is diagnostic for dRTA. However, end stage renal failure is highly unusual in primary dRTA. Nephrocalcinosis associated with end stage renal failure is usually seen with three genetic diseases: primary hyperoxaluria, Dent’s disease, and familial hypomagnes aemia with hypercalciuria and nephrocalcinosis (auto somal reces sive due to mutations in the claudin-16 or claudin-19 genes). The patient’s family history, hypocalcaemia, and hypo magnesaemia suggested familial hypomagnesaemia with hypercalciuria and nephro calcinosis. He had no ocular abnormalities (nystagmus, colobomata, myopia) associated with claudin-19 muta tions, so we suspected a claudin-16 mutation. Using Sanger gene sequencing, we identifi ed a novel homo zygous splice site mutation in intron 1 of the claudin-16 gene (c.324 + 3_324 + 4insT), which was predicted to disrupt gene splicing with con sequent aberrant protein production. Claudin-16 and 19 are tight junction proteins implicated in the paracellular absorption of calcium and magnesium from the thick ascending limb of the loop of Henle. Familial hypo magnesaemia with hypercalciuria and nephrocalcinosis causes urinary cal cium and mag nesium wasting and subsequent nephro calcinosis. Secon dary dRTA (present in our patient) and nephrogenic diabetes insipidus can occur secondary to nephrocalcinosis, which also causes renal failure. Chronic hypocalcaemia and metabolic acidosis can cause severe bone demineralisation, pre disposing to pathological frac tures. Supportive treat ment includes calcium and mag nesium replacement and a thiazide diuretic to reduce cal ciuria. When possible, screening of family members is warranted. Magnesium concentrations should be checked when treating any refractory hypocalcaemia; hypomagnesaemia impairs the PTH response to lowered calcium, which can exacerbate persistent hypocalcaemia. Management should include magnesium, calcium, and vitamin D replacement.