5-fluorouracil (5-FU) is a fluoropyrimidine drug used in the treatment of colorectal cancer and other solid tumors. Pharmacogenetic variations in genes such as DPYD and TYMS, may contribute to the risk of toxicity and altered therapeutic benefits.
Epidemiology
- Prevalence
- Heterozygosity and homozygosity for DPYD alleles with impaired function occur in 3-5% and 0.1% of the general population, respectively
- Heterozygosity for 1494 del TTAAAG (6-bp deletion) in the 3’ untranslated region of the TYMS gene occurs in ~50% of Caucasians; 7% are homozygous
- This deletion genotype is in strong linkage disequilibrium with the 5’ tandem repeat genotype known commonly as 3R/3R
Genetics
- At least 17 DPYD mutations have been identified in patients with severe 5-FU toxicity
- Five common mutations are associated with reduced enzymatic function and grade III-IV toxicity
- *2A (IVS+14G>A) – most common and clinically significant variant
- *9A (c.85T>C)
- c.1590T>C
- *13 (c.1679T>G)
- c.2846A>T
- Polymorphisms in the TYMS gene encoding thymidylate synthase (TS) include the 6-bp deletion in the 3’ untranslated region, which contributes to decreased gene expression and reduced mRNA stability
- Mutation is associated with increased sensitivity to 5-FU
Pathophysiology
- Dihydropyrimidine dehydrogenase (DPD) enzyme – encoded by DPYD gene
- Responsible for the degradation and inactivation of >80% of 5-FU
- Initial and rate-limiting enzyme in the pathways that catabolize pyrimidines
- Reduced DPD activity can lead to the accumulation of active 5-FU metabolite, fluorodeoxyuridine monophosphate (5-FdUMP), which leads to 5-FU sensitivity
- DPYD mutations are associated with decreased DPD activity, leading to production of proportionately higher amounts of 5-FdUMP and an increased risk for dose-related 5-FU sensitivity
- TS – encoded by TYMS gene
- Primary target for 5-FU
- Catalyses methylation of deoxyuridine monophosphate (dUMP) to deoxythymidine monophosphate (dTMP), which is essential for DNA replication
- 5-FdUMP – active metabolite of 5-FU prevents DNA synthesis by forming stable complexes with TS, with folate as a cofactor, blocking the production of dTMP in cancer cells
- TYMS gene mutations result in reduced expression of TS and may be associated with higher clinical responsiveness to 5-FU therapy and increased risk of toxicity
Clinical Presentation
- Grade III-IV toxicity (occurs in up to 15% of treated patients)
- Mucositis
- Neutropenia
- Nausea
- Diarrhea
- Neurological symptoms
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