Abstract 5383: Overexpression of the vitamin D catabolizing enzyme CYP24A1 is caused by gene amplification and results in highly proliferative colorectal tumors.

Abstract Vitamin D insufficiency increases risk of colorectal cancer. Vitamin D is produced photochemically in the skin, thus, low sunlight exposure results in vitamin D insufficiency. Vitamin D is further processed in the liver to its storage form calcidiol (25-OH vitamin D3). Calcidiol can be activated by 1α-hydroxylation to the secosteroid hormone calcitriol (1α,25-OH vitamin D3). Although systemic levels of bioactive calcitriol are regulated by the kidneys, almost every tissue can synthesize and degrade calcitriol. Tissue calcitriol acts in an autocrine/paracrine manner and controls proliferation, apoptosis, and differentiation. In colorectal cancer, the calcidiol and calcitriol-degrading enzyme CYP24A1 is substantially overexpressed both on mRNA and protein level. High CYP24A1 levels markedly reduce the half-life of vitamin D metabolites, likely reducing the anti-tumorigenic effects of calcitriol in the tumor. The causes and consequences of this overexpression are not fully understood. Here, we investigated gene amplification of the CYP24A1 locus (20q13.2) as a possible cause of CYP24A1 overexpression and increased proliferation as a consequence thereof. Quantitative real time PCR assays showed that approximately 60% of colorectal tumors carry CYP24A1 gene amplification (n=127). This gene amplification correlated with increased mRNA expression (ρ=0.38, p<0.001). Aberrantly high CYP24A1 expression may reduce the anti-proliferative actions of calcitriol and lead to increased proliferation. In our colorectal cancer cohort, CYP24A1 mRNA expression correlated with expression of several proliferation markers (e.g. CDC6 ρ=0.60, p<0.001). In conclusion, our data suggest that CYP24A1 gene amplification results in increased mRNA expression in colorectal tumors. Further, high CYP24A1 expression correlates with increased proliferation, possibly caused by an inhibition of the anti-proliferative effects of calcitriol. Tumor specific inhibition of CYP24A1 may provide a future strategy to restore local vitamin D levels and its anti-tumorigenic activities. Citation Format: Julia Höbaus, Abhishek Aggarwal, Doris M. Hummel, Ursula Thiem, Irfete Fetahu, Ildiko Mesteri, Enikö Kallay. Overexpression of the vitamin D catabolizing enzyme CYP24A1 is caused by gene amplification and results in highly proliferative colorectal tumors. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 5383. doi:10.1158/1538-7445.AM2013-5383