Medullary nephrocalcinosis in an adult patient with idiopathic infantile hypercalcaemia and a novel CYP24A1 mutation
A 29-year-old man was admitted for evaluation of an elevated serum creatinine of 1.40 mg/dL (estimated glomerular filtration rate, eGFR, 70 mL/min/1.73 m2). He suffered from mild hypertension treated with 8 mg of candesartan. Urine analysis revealed microalbuminuria (albumin/creatinine 36 mg/g). A renal sonogram demonstrated medullary nephrocalcinosis (). The patient also suffered from mild nephrogenic diabetes insipidus with isosthenuria and 3 to 4 L of urine per day.\nMedical history revealed that the patient had suffered from IIH, manifesting in his third month of life, when he was admitted for failure to thrive. He was noted to have total serum calcium of 4.2 mmol/L, hypercalciuria and nephrocalcinosis. He had received 800 IU of vitamin D3 supplementation per day at that time. His serum 25(OH)D3 level was seen to be elevated to 123 ng/mL, and vitamin D was stopped. The child was treated with prednisolone and put on a calcium-free formula. Serum calcium normalized quickly. Further development under low calcium alimentation was normal. Total serum calcium levels remained in the upper range of normal or slightly elevated (2.4–2.6 mmol/L), but hypercalciuria (0.35–0.45 g calcium/g creatinine) persisted. Intact PTH levels measured on multiple occasions were always suppressed. The 25(OH)D3 levels were elevated during the first 4 years of life (range 110–609 ng/mL) and then returned to normal. The 1,25(OH)D3 was first determined at 13 years of age and was in the upper range of normal (65 pg/mL). The current results of calcium and vitamin D metabolism are shown in . Bone density determined by DEXA was normal (lumbar spine 1.247 g/cm2, T score 0.2; femoral neck 1.175 g/cm2, T score 0.7).\nSequence analysis of the CYP24A1 gene was performed in the patient. For this purpose, all exons including intron–exon boundaries of the CYP24A1 gene were amplified by the polymerase chain reaction (PCR) according to a previously published protocol (PMID: 22337913). PCR products were directly sequenced using an ABI 3130 DNA Analyser (Applied Biosystems).\nSequencing analysis revealed that the patient was homozygous for three common DNA polymorphisms (rs2296241, rs2762934 and rs6022987) and additionally for a yet undescribed nonsynonymous mutation in exon 4 (c.628T>C), causing tryptophan to be replaced with arginine in codon 210 (W210R) (). In silico analysis of the W210R mutation using the SIFT tool (; PMID: 19561590) and the PolyPhen-2 tool (; PMID: 20354512) consistently showed a probably damaging effect on the protein, reaching highest possible scores of the two algorithms (SIFT score: 0.00; PolyPhen-2 score: 1.000).\nBoth parents and the two siblings of the patient were genotyped for the presence of the W210R variant by sequence analysis of exon 4. All family members carried the heterozygous genotype of mutation c.628T>C (.\nTo assess the frequency of the W210R variant in the local population, 514 DNA samples of patients previously recruited for study purposes at Feldkirch Academic Teaching Hospital (PMID: 19135198) were genotyped for the mutation. Genotyping was carried out with the 5′ nuclease assay on a LightCycler® 480 Real-Time PCR System (F. Hoffmann-La Roche Ltd., Basel, Switzerland) using TaqMan® MGB probes and PCR primers obtained from the Assay-by-design™ service (Applied Biosystems, Forster City, CA, USA). Genotyping was successful in all patients. None of the patients were found to carry the W210R mutation.\nshows laboratory parameters of the patient's parents and his younger brother and sister. No major abnormalities in calcium metabolism were evident. However, the younger brother had a borderline total serum calcium level and elevated 1,25(OH)D3. A renal sonogram of the family members showed no nephrocalcinosis or renal calculi.\nSerum levels of fibroblast growth factor 23 (FGF23) were measured by enzyme-linked immunosorbent assay in all family members (C-terminal FGF23 assay, Immunotopics, San Clemente, CA). Whereas serum levels in the heterozygous family members were normal, the patient's FGF23 was considerably elevated (3- to 4-fold compared with patients with a similar eGFR as measured using the same assay) () []. Consequently, the patient also had moderate renal phosphate wasting with a reduced tubular reabsorption of phosphate and tubular maximum reabsorption of phosphate per litre of GFR ().