LC-MS/MS for identifying patients with CYP24A1 mutations

BACKGROUND: Patients have been described with lossoffunction CYP24A1 (cytochrome P450, family 24, subfamily A, polypeptide 1) mutations that cause a high ratio of 25-hydroxyvitamin D to 24,25-dihydroxyvitamin D [25(OH)D/24,25(OH)₂D], increased serum 1,25-dihydroxyvitamin D, and resulting hypercalcemia, hypercalciuria and nephrolithiasis. A 25(OH)D/24,25(OH)₂Dratio that can identify patients who are candidates for confirmatory CYP24A1 genetic testing would be valuable. We validated an LC-MS/MS assay for 24,25(OH)₂D (D₃ and D₂) and determined a 25(OH)D/24,25(OH)₂D cutoff to identify candidates for confirmatory genetic testing. METHODS: After addition of isotope-labeled internal standard, serum samples were extracted by solid-phase extraction, derivatized with 4-phenyl-1,2,4,-triazoline-3,5-dione, and quantified by LC-MS/MS. We measured 25(OH)D/24,25(OH)₂D in 91 healthy patients and 34 patients with clinically suspected CYP24A1-mediated hypercalcemia. RESULTS: The limits of detection and quantification were 0.03 (0.2) and 0.1 (0.24) nmol/L, respectively, for 24,25(OH)₂D₃, and 0.1 (0.23) and 0.5 (1.16) nmol/L for 24,25(OH)₂D₂. Intra- and interassay imprecision was 4%-15% across the analytical measurement range of 0.1-25 ng/mL (0.2-60 nmol/L). No interference was observed with 25(OH)D and 1,25(OH)₂D. 25(OH)D/24,25(OH)₂D of 7-35 was observed in healthy patients, whereas in 2 patients with CYP24A1 mutations, 25(OH)D/24,25(OH)₂D was significantly increased (99-467; P < 0.001). A 25(OH)D/24,25(OH)₂D ratio ≥99 identified patients who were candidates for CYP24A1 genetic testing. CONCLUSIONS: Increased 25(OH)D/24,25(OH)₂D supports the diagnosis of reduced CYP24A1 activity due to mutations in CYP24A1. Measurement of 25(OH)D/24,25(OH)₂D should be considered a part of the clinical workup in patients with hypercalcemia of otherwise unknown etiology.