UGT1A1 Genotype

Irinotecan is a common chemotherapy for colon, pancreatic, and lung cancers. If you ever need it, one inherited gene can strongly shape how safe a standard dose is. That gene is UGT1A1. The long name is UDP-glucuronosyltransferase 1A1. This test reads it.

The same gene explains most cases of harmless, lifelong mild jaundice. It can put a name on a liver-panel number that keeps coming back slightly high. Your result never changes, so you can learn it once and use it years later if irinotecan or a related drug ever comes up.

What the Gene Controls

UGT1A1 is the recipe for an enzyme inside your liver cells. The enzyme sticks a sugar-based tag onto certain fat-soluble molecules so they dissolve in bile and leave your body. That tagging step is called glucuronidation.

Its most important job involves bilirubin. Bilirubin is the yellow pigment left over when old red blood cells are broken down, and in humans this enzyme is the only one that can prepare it for disposal. When the enzyme runs slow, untagged bilirubin builds up in the blood. Your lab report lists it as indirect or unconjugated bilirubin.

The enzyme clears some drugs too. The best studied is SN-38, the cell-killing form irinotecan turns into once it's inside you. If you tag and remove SN-38 slowly, more of it lingers and hits the tissues that divide fastest: your bone marrow and the lining of your gut.

How Variants Are Named

Lab reports use a star system. 1 is the typical version. The best-known slow version, 28, has a small stutter in the stretch of DNA that switches the gene on: seven 'TA' repeats where most people have six, so the cell makes less enzyme. Other slow versions change the enzyme itself. *6, a single-letter change common in people of East Asian ancestry, is one example.

You carry two copies of the gene, one from each parent. Labs sort the pair into three groups:

Sources: Maruo et al.; Sissung et al.; Ghimire et al.

Gilbert Syndrome

Two reduced-function copies often produce the Gilbert syndrome pattern. Your bilirubin drifts up and down, sometimes enough to tint the whites of your eyes, and your liver is healthy and your red cells aren't breaking down any faster than normal. It's common, it's benign, and people often find out by accident from a routine panel.

The main risk is being misread. A bilirubin that stays high can set off one liver workup after another. In a study of 773 healthy adults, people with intermediate or poor metabolizer genotypes had higher bilirubin but normal liver enzyme results. A confirmed genotype lets you stop that cycle.

Sequencing is good at settling the question. At one reference laboratory, full-gene analysis confirmed Gilbert syndrome in approximately 79% of children referred for it. It also found rare changes that a single-variant test would have missed.

What this means for you: if your panel keeps flagging only indirect bilirubin while your liver enzymes stay normal, this result can explain the pattern. It doesn't replace checking for red cell breakdown, which can produce the same lab picture.

Crigler-Najjar Syndrome

At the severe end, both copies carry changes that break the enzyme outright. In type I the enzyme has almost no activity. Bilirubin in a newborn climbs high enough to damage the brain, a complication called kernicterus, and treatment means intensive light therapy or a liver transplant. Type II leaves roughly a tenth of normal activity and a milder course that is still serious.

Gilbert syndrome and type II aren't cleanly separate. In a Japanese study of 163 people, bilirubin levels formed one continuous spread from the milder condition to the severe one. That's why the genotype, and not the bilirubin number alone, often settles the diagnosis.

Irinotecan and Chemotherapy Side Effects

For most adults, this is where the test matters most. Irinotecan has two side effects that limit how much you can be given. One is a severe drop in infection-fighting white cells. The medical name is neutropenia. The other is severe diarrhea.

People with two *28 copies have about three and a half times the risk of severe neutropenia of people with two typical copies. At high doses the gap gets wider:

Sources: Innocenti et al.; Hulshof et al.; Peeters et al.

What this means for you: with a poor metabolizer result on file, your oncologist can start lower from the first cycle, so you don't find out your genotype through a hospital stay. Guidelines differ on how much to cut. A 30% reduction is common, and some protocols use smaller reductions, so bring the report to your treatment planning visit.

Dose matters here. The gap between genotypes is clearest above about 180 mg per square meter of body surface. In a study of 105 people on low-dose weekly irinotecan, genotype made no significant difference to side effects or benefit.

The genotype cuts the other way too. A one-size-fits-all dose may be too low for people with two typical copies. In a randomized trial of 213 people with metastatic colorectal cancer, raising the dose in people with typical copies kept the cancer in check longer, without significantly more severe side effects.

Does a slow genotype make the drug work less well? Mostly, no. Meta-analyses in European-ancestry and Chinese patients found the genotype predicts side effects but not tumor response or survival. A lower starting dose protects you without giving up on the treatment.

Newer Cancer Drugs and Other Medicines

Sacituzumab govitecan, an antibody-guided breast cancer drug, carries SN-38 as its payload. A meta-analysis found that people with two *28 copies had more severe side effects, but testing still isn't a standard step before treatment. Trastuzumab deruxtecan carries a different payload, and in 292 people with advanced breast cancer, genotype didn't predict its side effects.

Outside cancer, your genotype helps predict bilirubin rises on atazanavir, an HIV drug, and it guides dosing of belinostat, a lymphoma drug. For nilotinib, pazopanib, and raltegravir, the evidence isn't strong enough yet to change doses by genotype.

When a Slow Enzyme Meets Another Condition

By itself, a slow genotype is mild. It becomes a burden when something else is making extra bilirubin. In inherited red cell disorders like hereditary spherocytosis, red cells are fragile and break down early. Two slow copies on top of that deepen the jaundice, and an Indian study of 113 patients found gallstones in 30% of those with the combination.

Newborns show the same interaction. In studies of Chinese newborns, variants in both the gene's switch and its coding region shifted bilirubin levels and the risk of newborn jaundice. A national reference laboratory found that two *28 copies combined with red cell membrane or enzyme defects made unconjugated bilirubin markedly worse.

Ancestry Decides Which Variants Matter

28 is the main slow version in people of European and African ancestry. 6 is common in East Asian ancestry, and a test that only looks for 28 can't see it. People of African ancestry also carry 36 and 37, with five and eight TA repeats, and 37 cuts enzyme output even more than *28.

In South Asia, other promoter patterns matter. In a Nepali cohort of 75 people with features of Gilbert syndrome, the 60-linked G variant made up 81.3% of the gene copies seen. 60 alone isn't treated as a slow allele in major dosing tables, but this finding shows why a *28-only result can be incomplete when your ancestry makes other variants more likely.

When the Result Can Mislead

A genotype is fixed, but a lab's call can still be wrong or incomplete. Here are the gaps worth knowing:

• The repeat stretch: the TA stutter is hard to read. Short-read sequencing can get its length wrong because of copying errors in the lab, and a study comparing eight methods recommended fluorescent PCR or fragment analysis to confirm a repeat call.
• Coverage limits: an assay only finds the variants it reads. Some commercial arrays don't test for the eight-repeat *37 at all, so a normal call from them can be wrong for people of African ancestry.
• Ancestry: a negative result means less when the panel was built around variants common in a different ancestry group. 6, 37, and rare local variants can matter even when *28 is absent.
• Unfamiliar variants: full sequencing sometimes finds a change nobody has described before. The lab reports it as a variant of uncertain significance because its effect on the enzyme is unknown.
• Tumor versus inherited DNA: a tumor-only sequencing report can mix cancer changes with inherited DNA, and it may not call this gene at all. Use a clinical germline result for dosing.
• Consumer DNA reports: an app-based report may check one stand-in marker, such as 80. In Spanish cohorts 80 nearly tracked *28, but that's still not a clinical reading of the whole gene.

A One-Time Result

Your genotype was set at conception. Unless the lab flags an uncertain call in the repeat stretch, you never need to repeat this test. The value comes from using the result for decades, every time a new drug or a stray lab number comes up.

What you do keep tracking is the bilirubin your genotype shapes. If you carry slow copies, include total and indirect bilirubin when you run routine yearly labs so you know your own normal. Then a rise above your usual level gets read in context instead of chased as liver disease. If you also have a red cell disorder, add hemoglobin and a reticulocyte count, since that combination is what drives gallstones.

What to Do With Your Result

• Two typical copies: file the report. It tells a future oncologist that standard irinotecan dosing, and in some regimens higher dosing, may be reasonable for you.
• One slow copy: keep the report and bring it up before any irinotecan or SN-38 drug, since some studies of combination chemotherapy found more severe side effects even in single carriers.
• Two slow copies: have it added to your medical record as a drug flag and share it before chemotherapy or atazanavir. Once liver enzymes and red cell tests come back normal, stop repeat workups set off only by indirect bilirubin.
• Rare or unclear variant with very high bilirubin: this is the pattern that calls for a hepatologist or clinical geneticist, and for confirming the variant with a second lab method.

Two combinations should change what you do next. High indirect bilirubin with a normal genotype means the enzyme isn't the explanation, so check for red cell breakdown with hemoglobin and a reticulocyte count. A slow genotype together with signs of red cell breakdown means both are adding bilirubin, and a hematology evaluation is worth getting.