GSTP1 Genotype

If you are heading into chemotherapy, one inherited letter in your DNA may help predict whether you end up with a severe drop in infection-fighting white cells. That is where GSTP1 testing has produced its most usable human evidence. You were born with this result, and you will carry it unchanged for life.

Outside cancer treatment, the story is thinner than the internet suggests. Pooled analyses of thousands of people find this variant barely moves baseline cancer risk on its own. It matters most when it meets heavy chemical exposure, and that combination is where the result earns its place.

The Two Letters That Matter

The gene is on chromosome 11 and builds an enzyme that grabs reactive chemicals, the kind thrown off by tobacco smoke, air pollution, and many drugs. The enzyme sticks glutathione onto those chemicals so your body can clear them. The same enzyme also quiets a stress signal inside cells, which turns out to matter for how some drugs work.

Two common inherited spellings do most of the work in the research. The first, known as rs1695 or Ile105Val, swaps a single building block inside the enzyme's working pocket. The second, rs1138272 or Ala114Val, changes a nearby spot. Labs often combine the two into a pair of alleles labeled A through D.

The functional consequence is real and has been measured in people. In lung tissue removed during surgery, those with two copies of the 105 variant had roughly 30% less enzyme activity than those with two copies of the common version, 52.5 versus 74.9 on the study's activity scale. One copy lands you in the middle. Your report will show this as AA, AG, or GG, or as Ile/Ile, Ile/Val, Val/Val.

Chemotherapy Side Effects

This is the part of the evidence you can actually use, and it cuts in more than one direction. Among women on anthracycline and paclitaxel regimens, carrying the 105 variant was linked to a much higher chance of severe neutropenia. Severe neutropenia means white cell counts low enough to make infection a serious threat. In colorectal cancer treated with fluoropyrimidines and oxaliplatin, the same variant tracked with more severe vomiting and skin breakdown. In ovarian cancer treated with carboplatin and paclitaxel, GSTP1 tracked with some blood toxicity while ABCB1 tracked more clearly with nerve toxicity.

Sources: Zeng et al. 2022, Mir et al. 2009, and Lu et al. 2006. These are single cohorts, mostly small, and none of them has been turned into a dosing rule. No major pharmacogenomic guideline currently recommends GSTP1 genotyping to select or dose chemotherapy, so treat this as research-grade information that adds context rather than instructions.

The practical move is to have the result in hand before the first cycle, not after a bad one. It will not by itself change which drug you are given. It may help your oncology team plan supportive care, watch early blood counts more closely, and take early tingling or numbness more seriously.

When the Common Version Is the Riskier One

Look at the table again and something breaks the pattern. The variant raises neutropenia risk, but the common spelling raises docetaxel nerve damage risk. The oddity repeats elsewhere. Among areca nut and tobacco chewers in northeast India, it was the common Ile/Ile genotype that tracked with higher oral cancer risk. And two copies of the variant were associated with roughly 70% lower odds of asthma.

The way to make this consistent is simple. This is not a good-copy, bad-copy marker. The enzyme does two separate jobs: it clears reactive chemicals, and it damps a stress-signaling pathway inside cells. Slower clearance means some carcinogens linger, but it also means some drugs stay active longer and that internal signal stays louder. Which spelling helps you depends on what you are exposed to and what you are taking.

Cancer Risk on Its Own Is Small

The single most useful thing to know is how little this variant does by itself. A pooled analysis of roughly 5,300 people with prostate cancer and 5,600 without found no meaningful difference by genotype, and a separate review of 57 studies found no consistent overall association for this variant either. A pooled review of colorectal cancer found the variant unlikely to raise risk in any meaningful way.

Where signals do appear, they are modest and inconsistent across populations. Pooled lung cancer data found a small increase in risk, with hints that it interacts with smoking in white participants. Bladder cancer data showed a modest increase, strongest in Asian study populations. Breast cancer is unsettled: one analysis of more than 34,000 people found higher susceptibility in Asian participants, while a later re-analysis found no clear increase.

Treat this as a modifier, not a verdict. If someone tells you this genotype predicts your cancer risk the way a high-penetrance gene does, the pooled human data says otherwise.

Tobacco, Areca Nut, and Chemical Exposure

Exposure is what turns a near-null marker into an informative one. Among smokers, people with two copies of the 105 variant carried roughly twice as much smoke-related DNA damage in lung tissue as those with two common copies, 15.5 versus 7.9 marks per 100 million DNA building blocks. That is the mechanism showing up in human tissue rather than in a model system, though the study was small and observational.

The same pattern appears in the stomach. In a Chinese study of about 2,400 people, the variant allele combined with H. pylori infection, smoking, and alcohol to raise gastric cancer risk, while the genotype alone did much less. If you smoke, chew tobacco or areca nut, or work around solvents and combustion products, your genotype is worth knowing. If you have none of those exposures, it tells you far less.

Asthma and the Airway

The airway is where the variant looks protective. In a case-control study of roughly 475 people, two copies of the 105 variant were associated with about 70% lower odds of bronchial asthma. Earlier work linked the same region to lower bronchial hyperresponsiveness. That means airways were less twitchy in response to irritants.

One caution on chronic obstructive pulmonary disease. A case-control study found a link to COPD, but it involved a different variant in the same gene, rs4147581, not the two spellings most panels report. Same gene does not mean same finding.

A One-Time Result

There is nothing to trend here. This is inherited DNA. The answer is the same at 30 and at 70, and repeating the test only buys you a second copy of the same report. The one reason to re-run it is technical: if the original call came from a consumer genotyping chip rather than clinical-grade sequencing, confirming it by sequencing is reasonable before anyone acts on it.

What does need tracking is whatever the result makes you watch. During chemotherapy, that means blood counts on the schedule your oncology team sets, plus early reporting of numbness or tingling. If you smoke or used to, it means staying inside lung cancer screening guidelines rather than assuming a genotype settles anything. The value of this test comes from having it on file before the decision arrives.

What to Do With Your Result

Single-gene results have repeatedly failed to classify chemotherapy response on their own. In gastric cancer, analyzing this gene in isolation was judged inadequate for separating responders from non-responders, and one lung cancer cohort found no response signal. The fix is context. Interpret GSTP1 alongside GSTM1 and GSTT1 when those results are available, since they encode related clearance enzymes and are the genes most often paired with this one in the literature. On platinum regimens, DNA-repair genes such as ERCC1 and ERCC2/XPD add the other half of the picture.

If cancer treatment is imminent, bring the report to the oncologist or clinical pharmacist managing your regimen before cycle one, because supportive-care intensity is the decision this result can realistically inform. If the result arrived inside a large sequencing panel that also flagged high-penetrance genes, that is a genetic counselor conversation, and this variant is not the reason.

Think in combinations, not single results. A slower-clearance genotype plus a long smoking history plus a family cancer pattern is a reason to get rigorous about screening eligibility and to stop smoking. A slower-clearance genotype in a non-smoker with no chemotherapy on the horizon is mostly a piece of background biology.

When Results Can Be Misleading

• Panel coverage: most assays read the two common coding changes. A clean result does not rule out every variant in the same gene, including variants studied in COPD and colorectal cancer risk.
• Ancestry: the frequency of these alleles differs sharply between populations, and associations found in one ancestry group often fail to replicate in another. A risk estimate from a study population unlike you may not transfer.
• Common variant versus VUS: clinical sequencing can report rare GSTP1 changes with unclear meaning. Do not treat a VUS like one of the two common variants.
• Inherited versus tumor-only DNA: a tumor report can contain changes that arose only in the cancer. This test is about the DNA you inherited.
• Genotype versus methylation: these are different tests. Methylation testing looks for a chemical off-switch on this gene inside tumor cells and is used in prostate cancer research. Your inherited spelling says nothing about it.
• Assay grade: a clean buccal, saliva, or blood sample should give the same inherited call, but a consumer genotyping chip and clinical sequencing can disagree at a single position. Confirm before acting.