5-Fluorouracil Sensitivity


Indications for Testing

  • Predict toxicity and responsiveness of tumor to 5-fluorouracil (5-FU) therapy

Laboratory Testing

  • 5-FU response testing
    • DPYD and TYMS mutations detected that may increase risk for 5-FU toxicity and predict responsiveness to 5-FU
      • Alternative chemotherapeutic agents, therapeutic drug monitoring, altered 5-FU doses, or increased surveillance for adverse drug reactions may be indicated

Clinical Background

5-fluorouracil (5-FU) is a fluoropyrimidine drug and the most frequently used chemotherapeutic in the treatment of colorectal cancer. Genetic mutations in the DPYD and TYMS genes predict the risk of 5-FU toxicity, responsiveness to 5-FU therapy, and clinical outcome to aid in patient care.


  • Prevalence





    Historical name: N/A

    0.7% –  French Caucasians


    Historical name: DPYD*9A

    18.5% – French Caucasians


    Historical name: DPYD*13

    0.1% – French Caucasians


    Historical name: IVS14+1G>A; DPYD*2A

    Absent – Japanese, Korean, African-American

    0.47-2.2% – Dutch, German, French, Turkish, Finnish


    Historical name: N/A

    1.0% – French Caucasians


    • 6 bp Deletion (TTAAAG)
    • Historical name: 1494 del
    DEL29.5% –  Caucasian


    (Insertion is the wild-type)



    • 28bp VNTR (2R; 3R)
    • G>C SNP in 2nd repeat  of 3R allele (3RC)


    41-48% – Caucasian, Hispanic, African-American

    19% – Singapore, Chinese

    17.5% – Japanese


    26-37% – Caucasian, Hispanic, African-American

    51% – Singapore, Chinese

    42.7% – Japanese


    15-33% – Caucasian, Hispanic, African-American

    30% – Singapore, Chinese

    39.9% – Japanese


  • Five common DPYD single nucleotide polymorphisms (SNP) are associated with reduced enzymatic function and grade III-IV 5-FU toxicity

    DPYD Mutations

    Historical Name








    IVS14+1G>A; DPYD*2A



  • Commonly evaluated TYMS polymorphisms include
    • 6 base pair deletion of the 3'-UTR (DEL) versus the wild type insertion (INS)
    • Variable number (2 or 3) of tandem repeats (2R, 3R) of the 5'-promoter enhancer region (5'-TSER)
    • G>C SNP in the second repeat of the 3R allele (3RG, 3RC)
  • Presence of the 3'-UTR deletion
    • Decreases TYMS expression
    • Increases response to 5-FU
    • Increases risk of toxicity
  • 5'-TSER genotypes 3RG/3RG, 3RG/3RC and 2R/3RG are associated with           
    • Increased TYMS expression
    • Decreased 5-FU responsiveness
    • Poor prognosis
  •  5'-TSER genotypes 2R/2R, 3RC/3RC, and 2R/3RC are associated with
    • Decreased TYMS expression
    • Increased 5-FU responsiveness
    • Increased risk of toxicity


  • Dihydropyrimidine dehydrogenase (DPYD) enzyme – encoded by DPYD gene
    • Catabolizes approximately 80% of 5-FU into an inactive form that is eliminated in the urine
    • Reduced DPYD activity can lead to the accumulation of active 5-FU metabolite, increasing the risk for 5-FU toxicity
  • Thymidylate synthase (TYMS) enzyme – encoded by TYMS gene
    • Primary target for 5-FU
      • Remaining 5-FU drug is metabolized by different enzymes into an active form that inhibits the synthesis of DNA and RNA by competitive inhibition of TYMS or by direct incorporation of cytotoxic metabolites into nucleic acids
    • TYMS gene mutations result in reduced expression of TYMS 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 ~16% of treated patients)
    • Mucositis
    • Neutropenia
    • Nausea
    • Diarrhea
    • Neurological symptoms
  • c.1905+1G>A DPYD mutation can lead to toxicity-related death

Indications for Laboratory Testing

  • Tests generally appear in the order most useful for common clinical situations
  • Click on number for test-specific information in the ARUP Laboratory Test Directory
Test Name and Number Recommended Use Limitations Follow Up
5-Fluorouracil (5-FU) Toxicity and Chemotherapeutic Response, 7 Mutations 2007228
Method: Polymerase Chain Reaction/Single Nucleotide Extensions/Fragment Analysis

Predict toxicity and responsiveness of tumor to 5-FU therapy

Clinical sensitivity/specificity – unknown

Analytical sensitivity/specificity – 99%

Only targeted mutations in the DPYD and TYMS genes are evaluated

Rare diagnostic errors may occur due to primer-site mutations

Genetic and/or non-genetic factors that are not detected by this assay may affect 5-FU drug metabolism, efficacy, and risk for toxicity

Genotyping does not replace the need for therapeutic drug and clinical monitoring

Lack of detection of the targeted DPYD and TYMS mutations does not rule out risk for 5-FU toxicity or predict degree of responsiveness to 5-FU