Abstract 3893: Quantitative contribution of rs75017182 to dihydropyrimidine dehydrogenase mRNA splicing and enzyme activity

5-Fluorouracil (5-FU) has remained a widely prescribed anti-cancer drug for decades. However, 5-FU–based treatments show inter-individual variability, with up to 34% of 5-FU–treated patients developing severe (grade 3+) adverse toxicity. Dihydropyrimidine dehydrogenase (DPD; DPYD gene) is the rate-limiting enzyme of 5-FU catabolism and converts 80–85% of 5-FU into its inactive metabolites. Three deleterious genetic variations in DPYD are well-established predictors of severe 5-FU toxicity (i.e., *2A, p. I560S, and p. D949V); however, these variants only explain 30–35% of adverse cases. Recently, an intronic variant (rs75017182) has been suggested to contribute to 5-FU–related toxicity by promoting the alternative splicing of DPYD. However, clinical studies are conflicting as to the degree to which rs75017182 affects DPD enzyme function, and the true contribution of the rs75017182 variant towards DPD function and 5-FU toxicity remains unclear and inconclusive. In the present study, we directly examined the effect of the intronic SNP rs75017182 on DPYD mRNA splicing, DPYD expression, and DPD enzyme function. We genotyped 3950 healthy Caucasian volunteers from the Mayo Clinic Biobank to identify the rs75017182, *2A, p.I560S, and p.D949V variants. From the initial cohort, 204 volunteers who were rs75017182 heterozygous carriers were recruited as our study population. Alternative splicing and DPD activity in rs75017182 carriers were measured. The findings were confirmed using a novel mini-gene reporter system in vitro. A moderate, but significant, reduction (30%) was detected in canonically spliced DPYD expression in rs75017182 carriers compared to non-carriers. A correlative reduction (35%) in DPD enzyme activity was observed in those carriers, which was similar to that of D949V carriers (31%). The results demonstrated an association of this deep intronic variant with decreased DPD activity and suggested that rs75017182 may be a predictor of 5-FU toxicity similar to D949V. Citation Format: Qian Nie, Shikshya Shrestha, Erin E. Tapper, Colbren S. Trogstad-Isaacson, Kelly J. Bouchonville, Steven M. Offer, Robert B. Diasio. Quantitative contribution of rs75017182 to dihydropyrimidine dehydrogenase mRNA splicing and enzyme activity [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3893.