Identification of 24-Hydroxylase Polymorphisms and Splicing Variants That Modulate Vitamin D Oxidative Metabolism and Serum Pharmacokinetics in Patients With Colorectal Cancer on Cholecalciferol Therapy

OBJECTIVES:

• To identify CYP24 single nucleotide polymorphisms (SNPs) using peripheral blood mononuclear cell genomic DNA from patients with colorectal cancer receiving cholecalciferol supplementation.
• To evaluate the effects of these CYP24 SNPs on baseline serum vitamin D_3 metabolites (25-D_3, 24,25-D_3, and 1,25-D_3), and parathyroid hormone levels (PTH).
• To evaluate the effects of these CYP24 SNPs on serum vitamin D_3 metabolites and PTH levels during cholecalciferol treatment.
• To examine CYP24 splicing, protein expression, and enzyme activity at baseline and during cholecalciferol treatment.
• To determine the relationship, if any, between serum cholecalciferol pharmacokinetic parameters and CYP24 SNPs, splicing variants, and enzyme activity.

OUTLINE: Patients receive oral cholecalciferol 2000 IU once daily for 1 year. Patients without response to vitamin D supplementation (serum 25-D_3 level < 32 ng/mL) by 6 months will have their cholecalciferol dose increased to 4000 IU once daily.

Blood is collected at baseline and on days 14, 30, 60, 90, 180, 270, and 360. Peripheral blood mononuclear cells for CYP24 genotyping, protein expression, enzyme activity, and splicing variants are analyzed by polymerase chain reaction (PCR), western blot, high performance liquid chromatography, and reverse transcriptase PCR, respectively. Serum is analyzed for vitamin D_3 metabolite levels (by radioimmunoassay), calcium (to monitor for hypercalcemia), and parathyroid hormone assays (to measure vitamin D effect).