Background and objectives: Elevated serum vitamin D with hypercalciuria can result in nephrocalcinosis and nephrolithiasis. This study evaluated the cause of excess 1,25-dihydroxycholecalciferol (1α,25(OH)2D3) in the development of those disorders in two individuals.
Design, setting, participants, & measurements: Two patients with elevated vitamin D levels and nephrocalcinosis or nephrolithiasis were investigated at the National Institutes of Health (NIH) Clinical Center and the NIH Undiagnosed Diseases Program, by measuring calcium, phosphate, and vitamin D metabolites, and by performing CYP24A1 mutation analysis.
Results: Both patients exhibited hypercalciuria, hypercalcemia, low parathyroid hormone, elevated vitamin D (1α,25(OH)2D3), normal 25-OHD3, decreased 24,25(OH)2D, and undetectable activity of 1,25(OH)2D-24-hydroxylase (CYP24A1), the enzyme that inactivates 1α,25(OH)2D3. Both patients had bi-allelic mutations in CYP24A1 leading to loss of function of this enzyme. On the basis of dbSNP data, the frequency of predicted deleterious bi-allelic CYP24A1 variants in the general population is estimated to be as high as 4%-20%.
Conclusions: The results of this study show that 1,25(OH)2D-24-hydroxylase deficiency due to bi-allelic mutations in CYP24A1 causes elevated serum vitamin D, hypercalciuria, nephrocalcinosis, and renal stones.