Aminoaciduria--its relationship to vitamin D and parathyroid hormone

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.

Renal phosphate wasting in fibrous dysplasia of bone is part of a generalized renal tubular dysfunction similar to that seen in tumor-induced osteomalacia

Fibrous dysplasia (FD) of bone is characterized by focal replacement of normal bone and marrow with abnormal bone and fibrous tissue. It arises from postzygotic activating mutations of the GNAS1 gene. Hypophosphatemia due to renal phosphate wasting has been reported in association with FD as a part of the McCune-Albright Syndrome (MAS), which is characterized by FD, skin hyperpigmentation, and precocious puberty. To date, the prevalence and mechanism of phosphate wasting has not been well studied. We evaluated 42 patients with FD/MAS. Serum and urine samples were tested for indices of mineral metabolism, amino acid handling, and markers of bone metabolism. Twenty (48%) patients had some degree of renal phosphate wasting. Nephrogenous cyclic adenosine monophosphate (cAMP) was normal in FD patients, suggesting that the underlying cause of phosphate wasting is not the presence of activating GNAS1 mutations in the kidney. In addition, there was evidence of a more generalized renal tubulopathy as represented by the presence of abnormal vitamin D metabolism, proteinuria in 36 (86%) patients, and aminoaciduria in 39 (94%) patients. Renal phosphate wasting significantly correlated with the degree of bone involvement, as assessed by serum and urine markers of bone metabolism, suggesting that a circulating factor produced by FD bone and impacting on the kidney may be the mechanism. These data show that phosphaturia as part of a generalized renal tubulopathy represents the most common extraskeletal manifestation of FD and that the observed tubulopathy is similar to that seen in tumor-induced osteomalacia (TIO).

Fanconi syndrome associated with a non-ossifying fibroma of bone

A 20-year-old man presenting with osteomalacia was found to have the Fanconi syndrome, as evidenced by hypophosphatemia with hyperphosphaturia, glycosuria in the presence of normoglycemia, and generalized aminoaciduria. After removal of a non-ossifying fibroma of the left tibia, the renal tubular abnormalities promptly resolved with subsequent healing of the osteomalacia. A humoral factor released from the tumor may have caused the disorder in proximal renal tubular cell transport.