Prevalence and clinical impact of the uncommon DPYD c.257C>T (Pro86Leu) variant in a multiethnic cohort receiving fluoropyrimidine therapy.

Pre-emptive genotyping and genotype-guided dose adjustments are generally recommended for seven hypofunctional polymorphisms in the DPYD gene that are associated with fluoropyrimidine toxicity. However, targeted genotyping may miss uncommon and population-specific hypofunctional variants. Therefore, in 2021, we replaced conventional targeted genotyping with comprehensive DPYD exome sequencing. The objective of the current study was to identify uncommon hypofunctional variants and assess their association with fluoropyrimidine toxicity. In a cohort of 762 consecutive patients pre-emptively genotyped between 2021 and 2025, among the seven established risk variants, we identified *2A and HapB3 at their expected minor allele frequencies (MAF), but no carriers of any of the five other established risk variants. Interestingly, four patients were heterozygous for the rare missense c.257C>T variant (Pro86Leu; rs568132506), previously associated with fluoropyrimidine toxicity. The MAF (2.62 × 10-3) was similar to that previously described in Middle Eastern populations but 26-fold higher than in the gnomAD global population. Three of the carriers (75%) developed grade 3-4 fluoropyrimidine toxicity after one or two cycles, corresponding to a relative risk of 14.5 (95% confidence interval: 5.0-30.4, P = 0.0009) compared with a control cohort without hypofunctional DPYD variants. Our findings add to the growing body of evidence indicating that DPYD c.257C>T may be associated with severe fluoropyrimidine toxicity. Future studies should examine the clinical utility of including uncommon but potentially actionable variants, such as c.257C>T, into pre-emptive genotyping algorithms. This may be particularly important in multiethnic populations in which these variants can be more prevalent than those variants for which pre-emptive genotyping is generally recommended.