The human renal 25-hydroxyvitamin D3-1 alpha-hydroxylase: properties studied by isotope-dilution mass spectrometry

Case report: Reversible Fanconi syndrome due to vitamin D deficiency in a patient with epilepsy harbouring a pathogenic variant in the SLC34A1 gene

We report on a 3-year and 5-month-old boy who was referred for suspected rickets, due to knee valgus deformity developed over the previous year. The child had a history of epilepsy well-controlled with phenobarbital. His psychomotor development and growth metrics were appropriate for his age. On admission, laboratory work-up revealed elevated alkaline phosphatase (1289 U/L) and parathyroid hormone (PTH) (417 pg/ml), normal corrected calcium (9,3 mg/dl) and creatinine (0,21 mg/dl), low phosphate (3,2 mg/dl), 25-hydroxy vitamin D (6 ng/ml) and 1-25 hydroxy vitamin D (13.4 pg/mL, nv 20-80) concentrations. Urinalysis indicated low tubular reabsorption of phosphate (TRP % 10,7), along with bicarbonate, uric acid and amino acid loss, consistent with renal Fanconi syndrome. Based on these results, a genetic form of renal tubular dysfunction was suspected, and thus a clinical exome sequencing was requested. In the meanwhile, the child was commenced on Joulie solution (70 mg/kg/day of phosphate), calcitriol (0.03 mcg/kg/die), and ergocalciferol (1000 IU daily). FGF-23 concentrations were found to be within the normal range, thus ruling out FGF23-dependent forms of rickets. Surprisingly, we observed a dramatic improvement in laboratory parameters within two weeks from the treatment initiation, including normalisation of phosphate and PTH concentrations and resolution of Fanconi syndrome, prompting discontinuation of phosphate supplements. Molecular analysis identified a de novo monoallelic mutation (C.1006 + 1 G>A) in the solute carrier family 34 member 1(SLC34A1) gene encoding a protein involved in actively transporting phosphate into cells via Na+ cotransport in the renal brush border membrane. However, even without phosphate substitution no further drops in serum phosphate concentrations and persistently normal proximal renal tubular function were observed. Moreover the rickets changes had almost healed six months after starting vitamin D supplementation. This case provides further evidence that vitamin D deficiency may rarely cause renal Fanconi syndrome, reversible upon vitamin D replacement. This is particularly relevant in children with risk factors for vitamin D deficiency, including use of anticonvulsants.

Interactions of vitamin D and the proximal tubule

Severe vitamin D deficiency (reduction in serum 25(OH)D concentration) in infants and children can cause features of the Fanconi syndrome, including phosphaturia, glycosuria, aminoaciduria, and renal tubular acidosis. This indicates that vitamin D and its metabolites influence proximal tubule function. Filtered 25(OH)D bound to vitamin D binding protein (DBP) is endocytosed by megalin-cubilin in the apical membrane. Intracellular 25(OH)D is metabolized to 1,25(OH)2D or calcitroic acid by 1-α-hydroxylase or 24-hydroxylase in tubule cell mitochondria. Bone-produced fibroblast growth factor 23 (FGF23) bound to Klotho in tubule cells and intracellular phosphate concentrations are regulators of 1-α-hydroxylase activity and cause proximal tubule phosphaturia. Aminoaciduria occurs when amino acid transporter synthesis is deficient, and 1,25(OH)2D along with retinoic acid up-regulate transporter synthesis by a vitamin D response element in the promoter region of the transporter gene. This review discusses evidence gained from studies in animals or cell lines, as well as from human disorders, that provide insight into vitamin D-proximal tubule interactions.

Expression of 25-hydroxyvitamin D3-1alpha-hydroxylase in the human kidney

The secosteroid hormone 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) plays a vital role in calcium metabolism, tissue differentiation, and normal bone growth. Biosynthesis of 1,25(OH)2D3 is catalyzed by the mitochondrial cytochrome P450 enzyme 25-hydroxyvitamin D3 1alpha-hydroxylase (1alpha-hydroxylase). Although activity of this enzyme has been described in several tissues, the kidneys are recognized to be the principal site of 1,25(OH)2D3 production. To date, enzyme activity studies using vitamin D-deficient animals have suggested that 1alpha-hydroxylase is expressed exclusively in proximal convoluted tubules. With the recent cloning of 1alpha-hydroxylase, specific cRNA probes and in-house polyclonal antiserum have been used to determine the distribution of 1alpha-hydroxylase along the human nephron. Immunohistochemistry and in situ hybridization studies indicated strong expression of 1alpha-hydroxylase protein and mRNA in the distal convoluted tubule, the cortical and medullary part of the collecting ducts, and the papillary epithelia. Lower expression was observed along the thick ascending limb of the loop of Henle and Bowman's capsule. Weaker and more variable expression of 1alpha-hydroxylase protein and mRNA was seen in proximal convoluted tubules, and no expression was observed in glomeruli or vascular structures. These data show for the first time the distribution of alpha1-hydroxylase expression in normal human kidney. In contrast to earlier enzyme activity studies conducted in vitamin D-deficient animals, our data indicate that the distal nephron is the predominant site of 1alpha-hydroxylase expression under conditions of vitamin D sufficiency.