DPYD rs56161403: Fluorouracil Toxicity and Enzyme Function
The genetic variant rs56161403 is a rare nonsense mutation located in the DPYD gene, which provides instructions for making the dihydropyrimidine dehydrogenase (DPD) enzyme. This variant leads to a truncated, non-functional enzyme that severely impedes the body's ability to break down fluoropyrimidine chemotherapy drugs like 5-fluorouracil and capecitabine. Individuals carrying this altered allele face a substantially heightened risk of life-threatening drug toxicity if treated with standard chemotherapeutic dosing.
What each genotype means
Normal DPD enzyme activity
This genotype represents the most common form of the DPYD gene, which is typically associated with normal dihydropyrimidine dehydrogenase (DPD) enzyme function. Individuals with this profile are not expected to have the specific genetic susceptibility to fluorouracil toxicity linked to the c.1156G>T variant. Please consult with your oncologist or pharmacist regarding standard dosing protocols for fluoropyrimidine-based therapies.
This is the most common genotype, found in the vast majority of the global population.
Reduced DPD enzyme activity
This genotype includes one copy of the c.1156G>T (p.Glu386Ter) variant, which is a nonsense mutation that results in a non-functional DPD enzyme. Carriers of this variant may have reduced ability to metabolize fluoropyrimidine drugs, potentially increasing the risk of severe toxicity. It is essential to discuss this result with your healthcare provider or a clinical pharmacist before starting any fluoropyrimidine-based chemotherapy to determine if dose adjustments are necessary.
This genotype is extremely rare, with the variant allele occurring at a frequency of less than 0.0005 in major population databases.
Severely reduced DPD activity
This genotype indicates the presence of two copies of the c.1156G>T variant, which is expected to result in a complete or near-complete loss of DPD enzyme function. Individuals with this profile are at a significantly higher risk for severe or life-threatening toxicity when treated with fluoropyrimidine medications. You must inform your medical team of this result immediately, as it is critical for guiding safe medication management and avoiding standard doses of these drugs.
This genotype is exceptionally rare and has not been widely documented in general population cohorts.
Understanding rs56161403 and the DPYD Gene
The single nucleotide polymorphism rs56161403 represents a specific sequence change designated as c.1156G>T within the coding sequence of the DPYD gene. This substitution converts a codon for the amino acid glutamic acid into a premature stop codon, formally annotated at the protein level as p.Glu386Ter. The premature termination of protein translation prevents the assembly of a fully functioning dihydropyrimidine dehydrogenase (DPD) enzyme. The DPYD gene encodes the rate-limiting enzyme responsible for the catabolism of endogenous pyrimidines, such as uracil and thymine. Under physiological conditions, healthy cells rely on DPD to maintain pyrimidine balance and eliminate toxic metabolic intermediates. Because DPD is also the primary enzyme clearing synthetic fluoropyrimidines, any disruptive structural variation in this protein alters drug clearance dramatically.
Pharmacogenomic Impact on Fluoropyrimidine Chemotherapy
Fluoropyrimidine medications—principally 5-fluorouracil (5-FU) and its oral prodrug capecitabine—are fundamental chemotherapeutic agents used in the treatment of gastrointestinal, breast, and head and neck malignancies. Typically, over 80% of an administered dose of 5-fluorouracil is rapidly inactivated by DPD in the liver. In carriers of complete loss-of-function variants like rs56161403, this metabolic pathway is severely impaired. Without functional enzyme activity, active drug metabolites accumulate systematically to toxic levels. This leads to profound toxic reactions, which may manifest as severe neutropenia, mucositis, intractable diarrhea, hand-foot syndrome, neurotoxicity, and potentially fatal sepsis during early treatment cycles.
Strength of Clinical Evidence and CPIC Classification
Major international pharmacogenomics consortia, including the Clinical Pharmacogenetics Implementation Consortium (CPIC) and the Dutch Pharmacogenetics Working Group (DPWG), evaluate DPYD variations to provide evidence-based dosing guidelines. CPIC classifies the c.1156G>T allele as a definitive 'no-function' allele (activity score 0). Although empirical case cohorts documenting this specific variant are small due to its low baseline prevalence—accounting for its 'limited' volume of published clinical observations—its molecular mechanism is unambiguous. Premature translation termination renders the resulting polypeptide non-functional. Based on consensus guidelines for DPYD deficiency, carriers of non-functional alleles require substantial empiric dose reductions or alternative therapeutic regimens to avoid catastrophic adverse effects.
Population Prevalence and Rarity
Unlike more commonly evaluated DPYD variants such as DPYD*2A (rs3918290) or c.2846A>T (rs67376798), rs56161403 is extraordinarily rare in the general population. According to global sequencing resources such as the Genome Aggregation Database (gnomAD), the minor allele frequency (MAF) of rs56161403 is well below 0.0005 across worldwide populations. Because of its scarcity, standard targeted genotyping panels used in routine pre-treatment screening may not interrogate rs56161403 unless comprehensive gene sequencing is utilized. Homozygous carriers for this specific stop-gain allele are virtually unseen in unselected population databases, though individuals may theoretically present as compound heterozygotes carrying this mutation alongside another partially or completely inactive DPYD allele.
Clinical Considerations and Patient Next Steps
Knowledge of one's rs56161403 genotype holds significant clinical relevance exclusively in the context of fluoropyrimidine therapy. This genetic variant does not cause symptoms during ordinary daily life, nor does it elevate individual cancer risk. However, if chemotherapy with 5-fluorouracil or capecitabine is ever considered, knowing your status empowers oncologists and pharmacotherapy specialists to tailor dosing. You should never alter, stop, or reject prescribed oncology treatments based solely on direct-to-consumer reports. Instead, discuss your results with your oncologist, medical geneticist, or a clinical pharmacist, who can perform confirmatory testing, calculate your cumulative DPYD gene activity score, and adapt therapy safely.
How common is this variant?
The rs56161403 variant is an extremely rare nonsense alteration with an overall minor allele frequency (MAF) below 0.0005 in large population cohorts like gnomAD.
Frequently asked questions
Does having the rs56161403 variant cause symptoms in daily life?
No. Under routine life circumstances, carrying this genetic variant does not cause illness, disability, or known outward symptoms. Its clinical significance emerges almost exclusively when an individual is exposed to fluoropyrimidine chemotherapy drugs, such as 5-fluorouracil or capecitabine.
Can I still receive cancer treatment if I test positive for this variant?
Yes, cancer care can proceed safely, but the choice and dosing of medications must be carefully adjusted. Clinical guidelines recommend either substantially reducing the initial dosage of fluoropyrimidines with therapeutic monitoring or choosing an alternative chemotherapy regimen that does not depend on the DPD enzyme.
Why is rs56161403 not included on all pharmacogenomic tests?
Many commercial and hospital panels only screen for the three or four most common DPYD variants in European populations to maximize cost-efficiency. Because rs56161403 has a population frequency under 0.05%, it is generally detected only through broader multigene panels or full-gene sequencing.
What is the function of the DPD enzyme?
Dihydropyrimidine dehydrogenase (DPD) is an essential enzyme produced primarily in the liver that breaks down uracil and thymine, which are natural building blocks of RNA and DNA. Because fluoropyrimidine chemotherapies mimic these natural pyrimidines, the DPD enzyme also serves as the body's primary mechanism for clearing and neutralizing these drugs.
What steps should I take if my test report shows I carry rs56161403?
If your genetic report indicates the presence of this variant, provide the documentation to your healthcare provider, oncologist, or clinical pharmacist before initiating any oncology treatments. A medical professional can verify the finding using clinical-grade methods and calculate your overall DPYD activity score to ensure proper treatment safety.
Sources & further reading
Educational information only, last refreshed 9/8/2026. Not medical advice — these associations describe population statistics, not individual predictions.
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This intronic variant is a marker for reduced DPYD enzyme activity, increasing the risk of severe toxicity from fluoropyrimidine-based chemotherapies.
This variant is linked to decreased dihydropyrimidine dehydrogenase activity, increasing the risk of severe toxicity during fluoropyrimidine chemotherapy.
Defines the DPYD*7 variant (p.Leu311Phe) associated with impaired pyrimidine metabolism and potential fluoropyrimidine toxicity.
Constitutes the DPYD*13 variant (p.Ile560Thr), associated with significantly reduced DPYD enzyme activity and increased risk of severe fluoropyrimidine toxicity.
Also designated as DPYD c.557A>G (p.Tyr186Cys), this variant significantly decreases dihydropyrimidine dehydrogenase activity and predisposes patients to severe fluoropyrimidine toxicity.
Causes the c.2846A>T (p.Asp949Val) missense change in DPYD, associated with reduced enzyme activity and increased risk of fluoropyrimidine-associated toxicity.