Gilbert's Syndrome

Mendelian MONDO:0007745 Pathograph 20 Show in embeddings browser Liver Disease Inherited Metabolic Disorder

A common, benign inherited condition characterized by mild, intermittent unconjugated hyperbilirubinemia in the absence of hemolysis or liver disease. Caused by reduced hepatic UGT1A1 enzyme activity, impairing bilirubin glucuronidation. The most prevalent cause in Western populations is a TA-repeat promoter polymorphism (UGT1A1*28) that reduces UGT1A1 transcription; in East Asian populations the UGT1A1*6 (Gly71Arg) coding variant predominates. No specific pharmacological treatment is required, but carriers must be identified before irinotecan chemotherapy to prevent severe toxicity.

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2
Definitions
1
Inheritance
6
Pathophys.
5
Phenotypes
1
Hypotheses
4
Gaps
20
Pathograph
5
Genes
3
Medical Actions
2
Differentials
2
Models
7
References
2
Deep Research
📘

Definitions

2
Historical unisex total bilirubin cutoff
Gilbert syndrome diagnosed as total bilirubin at or above 1 mg/dL (17.1 micromol/L) with normal ALT and GGT and no hemolysis. This is the cutoff in long-standing clinical use and the one most laboratories report against. It is recorded here because it is what most published prevalence figures and most patients' diagnoses rest on, not because it is the better definition.
DIAGNOSTIC_CRITERIA
Inclusion criteria
  • Raised total bilirubin Total bilirubin at or above 1 mg/dL (17.1 micromol/L).
  • Normal liver enzymes Alanine aminotransferase and gamma-glutamyl transpeptidase within reference limits.
  • Absence of haemolysis No laboratory or clinical evidence of a haemolytic process.
Show evidence (1 reference)
PMID:37738404 SUPPORT Human Clinical
"Historically, bilirubin ≥1 mg/dl (17.1 μmol/L) has been the standard cutoff for GS."
States the historical cutoff and attributes it as the standard, which is what this definition records.
Sex-, age- and genotype-stratified bilirubin centile definition
Gilbert syndrome defined as total bilirubin above the 90th centile of the distribution for the individual's sex, age band, and UGT1A1 (rs887829) genotype, in the absence of liver damage. Derived from 138,125 UK Biobank participants with apparently healthy livers. The clinically important consequence is that the unisex 1 mg/dL cutoff systematically underdiagnoses women: applying stratified centiles identified Gilbert syndrome in 10% of women against 3.7% by the historical cutoff. Because bilirubin also falls with age, a fixed threshold misclassifies differently at different ages.
DIAGNOSTIC_CRITERIA
Show evidence (2 references)
PMID:37738404 SUPPORT Human Clinical
"In women, we identified 10% (7,741/76,809) of GS versus 3.7% (2,819/76,809) using the historical cutoff of ≥1 mg/dL (P<0.0001)."
Quantifies the underdiagnosis in women that motivates the stratified definition.
PMID:37738404 SUPPORT Human Clinical
"In middle-aged Europeans, the stratified TB demonstrates a careless GS underestimation in women when using the standard unisex 1 mg/dL cutoff."
The authors' own conclusion sentence, stating the definitional claim.
Notes: The same paper reports that the reference intervals begin at age 40, which the authors flag as a limitation for younger subjects.
👪

Inheritance

1
Autosomal recessive inheritance with incomplete penetrance HP:0000007
UGT1A1*28 homozygosity segregates recessively with the Gilbert phenotype, but it is necessary rather than sufficient: many homozygotes never reach a hyperbilirubinemic threshold, and the biochemical phenotype depends additionally on bilirubin production rate, hepatic uptake, sex, and age. The published UGT1A1*28 homozygote frequency in Western populations (roughly 10-13%) therefore exceeds the frequency of clinically recognised Gilbert syndrome, and genotype alone does not make the diagnosis.
Autosomal recessive inheritance Penetrance: INCOMPLETE
Show evidence (2 references)
PMID:7565971 SUPPORT Human Clinical
"Reduced expression of bilirubin UDP-glucuronosyltransferase 1 due to an abnormality in the promoter region of the gene for this enzyme appears to be necessary for Gilbert's syndrome but not sufficient for the complete manifestation of the syndrome."
The landmark paper states the necessary-but-not-sufficient relationship that makes penetrance incomplete.
PMID:8596320 SUPPORT Human Clinical
"Segregation of the 7/7 genotype with the Gilbert phenotype was also demonstrated in the family with four affected members."
Family segregation supporting the recessive mode for the promoter allele.
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Mechanistic Hypotheses

1
Mildly raised bilirubin is systemically protective
bilirubin_systemic_antioxidant_protection ALTERNATIVE
Evidence balance 1 support 2 refute
The hypothesis that the raised unconjugated bilirubin of Gilbert syndrome confers systemic antioxidant protection, principally against cardiovascular disease. It has a real in vitro mechanism behind it and consistent observational support, and it is the reason Gilbert syndrome is often described as advantageous. Mendelian randomization does not support it: using the Gilbert genotype as an instrument, genetically raised bilirubin shows no causal protective effect on cardiovascular disease or COPD. It is retained as an ALTERNATIVE rather than deprecated because the in vitro antioxidant chemistry is not in dispute; what is refuted is the inference from it to a clinical benefit at Gilbert-range concentrations.
The two hypothesis directions are not symmetric. The protective direction is unsupported causally, while the one causal association that has been demonstrated for bilirubin in this disease points the other way, at cholelithiasis. Recorded here because the protective framing is still widespread: the OpenScientist report used to recurate this entry presents cardiovascular protection as "a striking and reproducible feature" and proposes, as future work, the Mendelian randomization that PMID:37456363 had already published with a negative result. Reviewers of this entry should expect to keep meeting that framing in the secondary literature.
Pathograph links
Reduced UGT1A1 Activity and Impaired Bilirubin Glucuronidation -> Bilirubin Antioxidant and Pleiotropic Effects INDIRECT KNOWN INTERMEDIATES The mild increase in circulating bilirubin caused by reduced clearance is the exposure underlying the antioxidant and pleiotropic-effect hypotheses. Intermediates: Increased serum unconjugated bilirubin
Show evidence (3 references)
PMID:26057938 SUPPORT In Vitro
"Significant inhibition of protein and lipid oxidation was demonstrated within the physiological range of UCB, providing a hypothetical link to protection from atherosclerosis in hyperbilirubinemic individuals."
The mechanism is real at physiological bilirubin concentrations, and the authors call the link to clinical protection hypothetical. That is the distinction this hypothesis turns on, which is why the item is PARTIAL and not SUPPORT.
PMID:37456363 REFUTE Human Clinical
"Evidence from the analyses of genetic data suggests that bilirubin has no likely causal role in protection from cardiovascular disease, chronic obstructive pulmonary disease, or other key healthcare outcomes and therefore represents a poor target for therapeutic intervention for these outcomes."
Genetic instrumentation removes the confounding that the observational associations were open to, and finds no causal protection.
PMID:37738404 REFUTE Human Clinical
"No adjusted survival was significantly associated with GS or hypobilirubinemia."
Independent large-cohort survival analysis finds no benefit, consistent with the Mendelian randomization result.
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Discussions and Knowledge Gaps

4
Are the fatigue, abdominal pain, and dyspepsia reported alongside jaundice episodes caused by bilirubin, or are they the ascertainment signature of having been investigated for jaundice?
OPEN QUESTION OPEN gs_symptom_attribution
Attached to
phenotypes#Jaundice-Associated Gastrointestinal Symptoms phenotypes#Fatigue
Three lines of evidence pull in different directions and this entry does not resolve them. A clinic cohort reports these symptoms in over half of subjects during jaundice episodes, but clinic cohorts enrol people who presented with symptoms. A UK Biobank analysis tested 54 illnesses and found that, apart from cholelithiasis, every raised prevalence was explained by confounders after adjustment. Against both, a UK primary-care analysis of 9,240 cases against 150,846 controls found abdominal pain and fatigue - and only those two - consistently raised up to five years before diagnosis, which is the wrong temporal shape for a pure workup artefact. What is still missing is a symptom endpoint tested with the adjustment and Mendelian randomization applied to the illness endpoints. Until then the frequencies on these phenotypes record association, not causation.
Show evidence (3 references)
PMID:37738404 SUPPORT Human Clinical
"Almost all these differences were explained by confounders"
The population-level result that motivates doubting the clinic-cohort symptom attribution.
PMID:30717703 SUPPORT Human Clinical
"jaundice was associated with abdominal pain, dyspepsia or loss of appetite in 54 (53.465%) subjects."
The association this discussion is about. It is an association within a clinic cohort, which is what the open question turns on.
PMID:40904555 SUPPORT Human Clinical
"While most of these features appeared primarily in the year prior to diagnosis, only abdominal pain and fatigue were consistently more common in GS cases up to five years before diagnosis."
Population-scale evidence pointing the other way from the confounder result: a five-year lead time is hard to explain as diagnostic workup. This is why the question is recorded as open rather than settled.
No animal model carries the UGT1A1*28 TA-repeat promoter allele, so is any conclusion about Gilbert syndrome drawn from the Gunn rat translationally valid?
HUMAN MODEL MISMATCH OPEN gs_no_promoter_allele_animal_model
Attached to
animal_models#Gunn rat (homozygous jj, UGT1A1-null) animal_models#Humanized UGT1 (hUGT1) mouse pathophysiology#Reduced UGT1A1 Activity and Impaired Bilirubin Glucuronidation
The longest-established rodent model of inherited unconjugated hyperbilirubinemia, the Gunn rat, is a UGT1A1 null. Gilbert syndrome is not a null; it is a roughly 30% residual activity produced by a promoter TA insertion that changes transcription rate. The two differ in the variable that matters. The Gunn rat's defining outcome, bilirubin encephalopathy, does not occur in Gilbert syndrome at all, and the model cannot address the questions Gilbert syndrome actually raises - what determines the threshold for clinical expression among genotype-positive people, and why penetrance is incomplete. The humanized UGT1 mouse also curated here is closer, since it carries human UGT1 regulatory sequence, but its spontaneous phenotype is likewise severe neonatal hyperbilirubinemia with neurologic dysfunction. So both available models sit at the Crigler-Najjar end of the spectrum, and an allelic series of the promoter repeat on a humanized background would be the experiment that resolves this.
Proposed experiments
Humanized UGT1A1 promoter TA-repeat allelic series in mouse
exp_gs_humanized_promoter_allelic_series
Knock the human UGT1A1 promoter TA-repeat allelic series - (TA)6, (TA)7, and the compound genotypes - into a Ugt1a1-humanized mouse, and measure basal unconjugated bilirubin, the response to a defined fast, and SN-38 glucuronidation as a function of repeat number. The point is to obtain a graded model of a regulated promoter rather than the null the Gunn rat provides, so that partial-reduction questions can be asked at all.
Would support
pathophysiology#Reduced UGT1A1 Activity and Impaired Bilirubin Glucuronidation
Supporting outcome
  • Basal bilirubin and fasting response increase monotonically with TA repeat number, and SN-38 glucuronidation decreases, reproducing the human allele-dose relationship.
Refuting outcome
  • Repeat number does not change bilirubin handling in the humanized background, implying the human phenotype depends on species-specific regulatory context the model does not supply.
Show evidence (2 references)
PMID:33027804 SUPPORT Model Organism
"The homozygous Gunn rat lacks uridine diphosphate glucuronosyltransferase 1A1 (UGT1A1), the enzyme needed to biotransform bilirubin."
States the null genotype that is the substance of the mismatch with the human partial-reduction lesion.
PMID:7565971 SUPPORT Human Clinical
"Hepatic glucuronidating activity, essential for efficient biliary excretion of bilirubin, is reduced to about 30 percent of normal."
The human lesion is a partial reduction, which is the quantity the null model cannot represent.
UGT1A1*28 homozygosity is necessary but not sufficient. What supplies the remainder, and can it be measured?
KNOWLEDGE GAP OPEN gs_penetrance_modifiers
Attached to
inheritance#Autosomal recessive inheritance with incomplete penetrance
Bosma stated in 1995 that the promoter abnormality is necessary but not sufficient, and thirty years later this entry still cannot name what supplies the difference. Candidate contributions - bilirubin production rate including occult haemolysis, hepatic uptake capacity, sex and androgen status, and age - are individually plausible and jointly unquantified. This matters practically because roughly 10-13% of Western populations are UGT1A1*28 homozygotes while clinically recognised Gilbert syndrome is less common, so a genotype-first diagnosis over-calls, and the entry has no modifier to offer in its place.
Show evidence (2 references)
PMID:7565971 SUPPORT Human Clinical
"Reduced expression of bilirubin UDP-glucuronosyltransferase 1 due to an abnormality in the promoter region of the gene for this enzyme appears to be necessary for Gilbert's syndrome but not sufficient for the complete manifestation of the syndrome."
The statement of the gap, in the source that first made it.
PMID:37738404 SUPPORT Human Clinical
"Sex and age are strongly correlated with bilirubin levels in healthy subjects, suggesting that appropriate personalized bilirubin levels should be defined."
Identifies two of the modifiers and shows they are large enough to change the diagnosis, without resolving how much of the residual they account for.
How much does the Gilbert genotype add to neonatal hyperbilirubinemia risk when co-inherited with a bilirubin-production disorder such as G6PD deficiency, and is that increment large enough to screen for?
KNOWLEDGE GAP OPEN gs_neonatal_cofactor_risk
Attached to
pathophysiology#Reduced UGT1A1 Activity and Impaired Bilirubin Glucuronidation
Gilbert syndrome is benign in adults, so this entry treats it as benign throughout. The neonatal period is the exception the entry does not currently model: a newborn has an immature conjugating system already, and the same reduced-capacity allele sits upstream of the one bilirubin outcome that is genuinely dangerous. Published studies point the same way but do not agree on effect size, and the studies are small. The neonatal series in G6PD-deficient infants below reports increased hyperbilirubinemia with the promoter variant while stating the interaction is more complex than a two-locus model. This entry deliberately does not assert a neonatal risk figure because the literature does not support one yet.
Proposed experiments
Powered newborn cohort for the UGT1A1 x G6PD interaction
exp_gs_neonatal_ugt1a1_g6pd_interaction
Prospective multi-ancestry newborn cohort genotyping UGT1A1 promoter repeat number and G6PD together, powered for the interaction term rather than for either main effect, with reaching the phototherapy threshold as the endpoint. Existing studies are small single-population series that can detect the main effects but not the interaction, which is the quantity that would decide whether combined screening is worthwhile.
Would support
pathophysiology#Reduced UGT1A1 Activity and Impaired Bilirubin Glucuronidation
Supporting outcome
  • A significant UGT1A1-by-G6PD interaction term on the phototherapy threshold endpoint, of a size that changes the number needed to screen.
Refuting outcome
  • No interaction beyond the additive contribution of each locus, indicating that genotyping UGT1A1 adds nothing to G6PD screening alone.
Show evidence (1 reference)
PMID:37508669 SUPPORT Human Clinical
"the interaction between G6PD deficiency, UGT1A1 promoter polymorphism, and NHB is more complex, possibly involving other genetic interactions, not yet described."
The source states its own limitation: the interaction is real but not resolved, which is exactly the gap recorded here.
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Pathophysiology

6
Reduced UGT1A1 Activity and Impaired Bilirubin Glucuronidation
The UGT1A1 enzyme in hepatocytes conjugates unconjugated (indirect) bilirubin with glucuronic acid, making it water-soluble for biliary excretion. In Gilbert's syndrome, reduced UGT1A1 activity (approximately 30% of normal) leads to accumulation of unconjugated bilirubin in the blood. The most common cause in Western populations is a homozygous (TA)7TAA repeat (UGT1A1*28 allele) in the TATAA box of the UGT1A1 promoter, which reduces transcription. In Asian populations, missense variants in the UGT1A1 coding region (e.g., Gly71Arg / UGT1A1*6) are more prevalent. Gilbert's syndrome is the most prevalent (2-19% in population studies) and mildest of the hereditary unconjugated hyperbilirubinaemia syndromes.
Mildly raised bilirubin is systemically protective
Hepatocyte CL:0000182 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves Hepatocyte (CL:0000182). CL:0000182 is a cell type from the Cell Ontology.
UGT1A1 hgnc:12530 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves UGT1A1 (hgnc:12530). hgnc:12530 is a gene from the HUGO Gene Nomenclature Committee.
Bilirubin conjugation GO:0006789 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased Bilirubin conjugation (GO:0006789). GO:0006789 is a biological process from the Gene Ontology. ↓ DECREASED
glucuronosyltransferase activity GO:0015020 Gene Ontology (GO) Relation: this pathophysiological event involves this molecular function This pathophysiological event involves decreased glucuronosyltransferase activity (GO:0015020). GO:0015020 is a molecular function from the Gene Ontology. ↓ DECREASED
Animal models
Gunn rat (homozygous jj, UGT1A1-null) Rat partially recapitulates Reproduces the enzymatic lesion (absent bilirubin glucuronidation) and the resulting unconjugated hyperbilirubinemia, but not its degree, regulation, or clinical consequences in Gilbert syndrome.
Humanized UGT1 (hUGT1) mouse Mouse partially recapitulates Establishes that UGT1A1 expression, and therefore bilirubin clearance, is set by inducible regulation of the human locus rather than by a fixed enzyme quantity - the same regulatory axis that the UGT1A1*28 promoter allele perturbs in humans.
Downstream
Unconjugated Bilirubin Reduced hepatic bilirubin glucuronidation leaves a larger circulating unconjugated bilirubin pool.
Mild Unconjugated Hyperbilirubinemia Impaired UGT1A1-mediated bilirubin conjugation is the proximal mechanism producing the obligate biochemical phenotype.
Fasting- and Stress-Induced Bilirubin Elevation Reduced UGT1A1 activity creates susceptibility to fasting-provoked bilirubin elevations in homozygous UGT1A1*28 carriers.
Bilirubin Antioxidant and Pleiotropic Effects The mild increase in circulating bilirubin caused by reduced clearance is the exposure underlying the antioxidant and pleiotropic-effect hypotheses. Mildly raised bilirubin is systemically protective
Impaired Glucuronidation of Drugs and Drug Metabolites The reduced UGT1A1 transcription that lowers bilirubin conjugation acts on the same enzyme that glucuronidates SN-38 and other xenobiotic substrates, so the pharmacogenomic consequence is not a separate lesion.
Chronic Biliary Bilirubin Load and Pigment Gallstone Formation A persistently larger unconjugated bilirubin pool increases the unconjugated load delivered to bile, which is the substrate for pigment stone formation.
Show evidence (3 references)
PMID:7565971 SUPPORT Human Clinical
"Hepatic glucuronidating activity, essential for efficient biliary excretion of bilirubin, is reduced to about 30 percent of normal."
The Bosma 1995 NEJM paper demonstrates that UGT1A1 enzyme activity is substantially reduced in Gilbert's syndrome, confirming the enzymatic basis of the condition.
PMID:7565971 SUPPORT Human Clinical
"These patients were homozygous for two extra bases (TA) in the TATAA element of the 5' promoter region of the gene (A(TA)7TAA rather than the normal A(TA)6TAA)."
Identifies the (TA)7 promoter repeat as the molecular basis for reduced UGT1A1 expression in Western populations.
PMID:9435989 SUPPORT Other
"Gilbert's syndrome, the most prevalent (2-19% in population studies) and mildest of the three syndromes is principally caused by a TA insertion at the TATA promoter region upstream of the UGT1A1 exon."
Clarke 1997 confirms population prevalence (2-19%) and the promoter TA insertion as the principal molecular cause.
Fasting- and Stress-Induced Bilirubin Elevation
Bilirubin levels in Gilbert's syndrome fluctuate and are exacerbated by physiological stressors such as fasting, dehydration, intercurrent illness, physical exertion, and menstruation. Fasting reduces caloric intake and increases fatty acid oxidation, which competes with bilirubin for hepatic uptake and conjugation pathways. These triggers can precipitate symptomatic jaundice in otherwise mildly affected individuals. A 24-hour fasting test is used diagnostically to provoke bilirubin elevation in suspected cases.
Bilirubin hepatic transport GO:0015723 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves abnormal Bilirubin hepatic transport, annotated with bilirubin transport (GO:0015723). GO:0015723 is a biological process from the Gene Ontology. ⚠ ABNORMAL
Downstream
Jaundice Trigger-related bilirubin elevations can cross the threshold for visible intermittent jaundice.
Jaundice-Associated Gastrointestinal Symptoms Jaundice episodes in Gilbert syndrome can be accompanied by abdominal pain, dyspepsia, or appetite loss through incompletely resolved symptom-generating intermediates.
Show evidence (2 references)
PMID:8596320 SUPPORT Human Clinical
"14 volunteers underwent a 24 h fasting test to see if they had Gilbert's syndrome, and all four positives had the 7/7 genotype."
Demonstrates that fasting precipitates jaundice specifically in homozygous UGT1A1*28 (7/7) carriers, consistent with the fasting-triggering mechanism in Gilbert's syndrome.
PMID:30717703 SUPPORT Human Clinical
"Risk factors for clinical jaundice included general anesthesia, pregnancy, fasting"
A longitudinal adult cohort identifies fasting and other physiologic stressors as risk factors for clinical jaundice episodes.
Bilirubin Antioxidant and Pleiotropic Effects
Unconjugated bilirubin is a potent lipid-soluble antioxidant that scavenges peroxyl radicals in vitro at physiologically relevant oxygen concentrations. Observational epidemiological studies have reported inverse associations between serum bilirubin and cardiovascular disease risk, leading to hypotheses of systemic protection in Gilbert's syndrome carriers. However, Mendelian randomization studies using the UGT1A1*28 locus as an instrument for genetically raised bilirubin find no causal cardiovascular benefit, suggesting the observational associations are confounded. The clinical significance of bilirubin's antioxidant capacity in vivo thus remains debated.
Show evidence (3 references)
PMID:3029864 SUPPORT In Vitro
"bilirubin, at micromolar concentrations in vitro, efficiently scavenges peroxyl radicals generated chemically in either homogeneous solution or multilamellar liposomes."
Foundational Science paper demonstrating bilirubin's free radical scavenging capacity, providing the mechanistic basis for hypothesised antioxidant protection in Gilbert's syndrome.
PMID:18343383 SUPPORT Other
"Serum bilirubin has been shown to be inversely related to cardiovascular disease (CVD) in both retrospective and prospective studies."
Observational data are consistent with CVD protection, but causality remains unproven; hence PARTIAL support only.
PMID:37456363 REFUTE Human Clinical
"Evidence from the analyses of genetic data suggests that bilirubin has no likely causal role in protection from cardiovascular disease, chronic obstructive pulmonary disease, or other key healthcare outcomes and therefore represents a poor target for therapeutic intervention for these outcomes."
Large-scale Mendelian randomization in UK Biobank (n=463,060) shows that genetically raised bilirubin via the Gilbert genotype confers no causal cardiovascular protection, refuting the antioxidant-protection hypothesis.
Impaired Glucuronidation of Drugs and Drug Metabolites
UGT1A1 is not a bilirubin-specific enzyme. The same reduced transcription that lowers bilirubin conjugation also lowers glucuronidation of xenobiotic substrates that UGT1A1 handles, most importantly SN-38, the active metabolite of irinotecan. Reduced SN-38 glucuronidation raises systemic SN-38 exposure and the risk of severe neutropenia and diarrhoea. The reciprocal interaction also occurs: drugs that inhibit UGT1A1 - notably the HIV protease inhibitors atazanavir and indinavir - unmask the reduced reserve capacity and precipitate overt hyperbilirubinemia, while the UGT1A1 inducer efavirenz lowers bilirubin. This node is the pharmacogenomic arm of the same enzymatic lesion that produces the bilirubin phenotype, and it is the arm with the clearest clinical consequence.
Hepatocyte CL:0000182 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves Hepatocyte (CL:0000182). CL:0000182 is a cell type from the Cell Ontology.
UGT1A1 hgnc:12530 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves UGT1A1 (hgnc:12530). hgnc:12530 is a gene from the HUGO Gene Nomenclature Committee.
glucuronosyltransferase activity GO:0015020 Gene Ontology (GO) Relation: this pathophysiological event involves this molecular function This pathophysiological event involves decreased glucuronosyltransferase activity (GO:0015020). GO:0015020 is a molecular function from the Gene Ontology. ↓ DECREASED
Downstream
Drug-Unmasked Hyperbilirubinemia Pharmacological inhibition of UGT1A1 subtracts from an already reduced conjugating reserve, so the same inhibitor produces a larger bilirubin rise in UGT1A1*28 homozygotes than in wild-type individuals.
Show evidence (2 references)
PMID:11990381 SUPPORT Human Clinical
"screening for UGT1A1*28 polymorphism may identify patients with lower SN-38 glucuronidation rates and greater susceptibility to irinotecan induced gastrointestinal and bone marrow toxicity."
Establishes that the UGT1A1*28 genotype reduces glucuronidation of a xenobiotic substrate (SN-38) as well as of bilirubin.
PMID:16170755 SUPPORT Human Clinical
"Efavirenz resulted in decreased bilirubin levels, which is consistent with the induction of UDP-glucuronosyltransferase 1A1."
Bidirectional pharmacological modulation of the same enzyme - induction lowers bilirubin - supports UGT1A1 capacity, rather than a fixed defect, as the quantity being varied.
Drug-Unmasked Hyperbilirubinemia
In individuals with reduced UGT1A1 capacity, a UGT1A1-inhibiting drug and the UGT1A1*28 genotype combine supra-additively to push bilirubin into the clinically visible jaundice range. In the Swiss HIV Cohort, 67% of UGT1A1*28 homozygotes receiving atazanavir or indinavir had repeated episodes of bilirubin above 2.5 mg/dL, against 7% of those with neither risk factor. A meta-analysis of six studies puts the odds of atazanavir hyperbilirubinemia in UGT1A1*28 homozygotes at roughly ten-fold that of wild type. The hyperbilirubinemia is benign, but it drives avoidable treatment discontinuation and diagnostic workup for liver disease.
Downstream
Jaundice Drug-unmasked bilirubin elevations cross the visible jaundice threshold far more often than the untreated Gilbert phenotype does.
Show evidence (2 references)
PMID:16170755 SUPPORT Human Clinical
"67% of individuals homozygous for UGT1A1*28 and receiving atazanavir or indinavir had > or =2 episodes of hyperbilirubinemia in the jaundice range"
Quantifies the gene-drug interaction against the clinically meaningful jaundice threshold rather than against a mean bilirubin shift.
PMID:30962262 SUPPORT Human Clinical
"A significantly increased risk of hyperbilirubinemia was observed in HIV-positive patients receiving ATV with the UGT1A1*1/*28 or UGT1A1*28/*28 genotype, and the risk was higher with the UGT1A1*28/*28 genotype than with the UGT1A1*1/*28 genotype."
Meta-analytic replication across six studies, with an allele-dose gradient consistent with the reduced-capacity mechanism.
Chronic Biliary Bilirubin Load and Pigment Gallstone Formation
Bilirubin that is not conjugated in the hepatocyte is not simply retained in plasma; the fraction that does reach bile carries an increased unconjugated load, which precipitates with calcium as pigment stone nidus. This is the one downstream consequence of the Gilbert genotype for which a causal, rather than merely associative, link to a clinical illness has been demonstrated. In a UK Biobank analysis of 138,125 participants with apparently healthy livers, cholelithiasis was the only illness out of 54 assessed whose raised prevalence in Gilbert syndrome survived adjustment for confounders and was confirmed by Mendelian randomization. The effect is much larger where bilirubin production is also increased: in sickle cell anemia, the UGT1A1 promoter genotype raises gallstone frequency and brings forward age at onset.
Downstream
Cholelithiasis Sustained unconjugated bilirubin load in bile is the proximate cause of calcium bilirubinate pigment stone formation.
Show evidence (2 references)
PMID:37738404 SUPPORT Human Clinical
"the prevalence of cholelithiasis in men with GS was assessed at 20% of European adults together with a 20% increase of the risk of complications, significantly more frequent than in men with normobilirubinemia, after adjusting for confounders. Furthermore, our TSMR validated the causality pathway"
Gives the absolute prevalence and the complication excess in men with Gilbert syndrome after confounder adjustment, and states that two-sample Mendelian randomization validated the causal pathway.
PMID:33523291 SUPPORT Human Clinical
"The mean of total and unconjugated bilirubin and the frequency of cholelithiasis in GS patients were higher when compared to those without this condition, regardless of age (P < 0.05). Cumulative analysis demonstrated an early age-at-onset for cholelithiasis in GS genotypes (P < 0.05)."
A 913-patient sickle cell cohort shows the same genotype acting on gallstone risk and age at onset when bilirubin production is high, supporting a load-dependent rather than threshold mechanism.
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Pathograph

Use the checkboxes to hide or show graph categories. Hover nodes for evidence and cross-linked metadata.
Pathograph: causal mechanism network for Gilbert's Syndrome Interactive directed graph showing how pathophysiology mechanisms, phenotypes, genetic factors and variants, experimental models, environmental triggers, and treatments relate through causal and linked edges.
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Phenotypes

5
Digestive 2
Jaundice FREQUENT HP:0000952 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Jaundice (HP:0000952). HP:0000952 is a phenotype from the Human Phenotype Ontology.
Show evidence (2 references)
PMID:8596320 SUPPORT Human Clinical
"This common mild hyperbilirubinaemia sometimes presents as an intermittent jaundice."
Directly establishes jaundice as a clinical presentation of the unconjugated hyperbilirubinaemia in Gilbert's syndrome.
PMID:8596320 SUPPORT Human Clinical
"Individuals in the population with the 7/7 genotype had significantly higher bilirubin concentrations than those who had the 6/7 or 6/6 genotype."
Demonstrates that homozygous UGT1A1*28 (7/7) carriers have the highest bilirubin levels, making them most susceptible to intermittent jaundice.
Cholelithiasis OCCASIONAL HP:0001081 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Cholelithiasis (HP:0001081). HP:0001081 is a phenotype from the Human Phenotype Ontology.
Show evidence (2 references)
PMID:37738404 SUPPORT Human Clinical
"Cholelithiasis in men and jaundice of unknown cause in women (Fig. 1C) were the only 2 conditions out of the 10 detailed biliary and digestive illnesses that were more frequent in GS than normobilirubinemia."
Identifies cholelithiasis as a genuine excess-risk phenotype in Gilbert syndrome within a prospectively assessed biliary/digestive illness panel.
PMID:33523291 SUPPORT Human Clinical
"These findings confirm in a large cohort that the UGT1A1 polymorphism influences cholelithiasis and hyperbilirubinemia in SCA."
Independent confirmation in a co-inherited hemolytic setting, where the same genotype acts on the same endpoint with a larger effect.
Metabolism 1
Mild Unconjugated Hyperbilirubinemia OBLIGATE HP:0008282 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Unconjugated hyperbilirubinemia (HP:0008282). HP:0008282 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:7565971 SUPPORT Human Clinical
"People with Gilbert's syndrome have mild, chronic unconjugated hyperbilirubinemia in the absence of liver disease or overt hemolysis."
Classic description from the landmark NEJM paper establishing unconjugated hyperbilirubinemia as the obligate phenotype.
Constitutional 2
Jaundice-Associated Gastrointestinal Symptoms FREQUENT Abdominal pain HP:0002027 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Abdominal pain with dyspepsia or appetite loss, annotated with Abdominal pain (HP:0002027). HP:0002027 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:30717703 SUPPORT Human Clinical
"jaundice was associated with abdominal pain, dyspepsia or loss of appetite in 54 (53.465%) subjects."
This longitudinal study directly supports abdominal pain, dyspepsia, or appetite loss during jaundice episodes and provides a frequency compatible with the FREQUENT band.
Fatigue OCCASIONAL HP:0012378 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Fatigue (HP:0012378). HP:0012378 is a phenotype from the Human Phenotype Ontology.
Show evidence (3 references)
PMID:40904555 SUPPORT Human Clinical
"While most of these features appeared primarily in the year prior to diagnosis, only abdominal pain and fatigue were consistently more common in GS cases up to five years before diagnosis."
Separates persistent symptoms from short-term diagnostic triggers in a nested case-control design, which is what makes fatigue a candidate symptom rather than an ascertainment artefact.
PMID:40904555 SUPPORT Human Clinical
"Fatigue724 (12%)5798 (6%)677 (22%)6780 (13%)1401 (15%)12,581 (8%)< 0.00001"
The source table row giving the recorded rates that set the frequency band: 1,401/9,240 cases (15%) against 12,581/150,846 controls (8%), which is OCCASIONAL rather than FREQUENT. Column order is male cases, male controls, female cases, female controls, all cases, all controls; the table is extracted without cell separators in the cached text.
PMID:40904555 SUPPORT Human Clinical
"the persistent presence of fatigue and abdominal pain suggests they may be under-recognised symptoms of GS. These findings warrant further investigation"
The authors' own hedge. Graded PARTIAL because the study establishes association and temporal persistence, not causation.
🧬

Genetic Associations

5
UGT1A1*28 Promoter Polymorphism (Causative)
Gene: UGT1A1 hgnc:12530 HUGO Gene Nomenclature Committee (hgnc) Relation: this disease-associated gene is this gene This disease-associated gene is UGT1A1 (hgnc:12530). hgnc:12530 is a gene from the HUGO Gene Nomenclature Committee.
Autosomal recessive
Show evidence (3 references)
PMID:7565971 SUPPORT Human Clinical
"Reduced expression of bilirubin UDP-glucuronosyltransferase 1 due to an abnormality in the promoter region of the gene for this enzyme appears to be necessary for Gilbert's syndrome but not sufficient for the complete manifestation of the syndrome."
Bosma 1995 established the (TA)7 promoter polymorphism as required but not sufficient for full phenotypic expression, explaining incomplete penetrance in heterozygotes.
PMID:8596320 SUPPORT Human Clinical
"The frequency of the 7/7 genotype in this eastern Scottish population was 10-13%."
Provides population frequency of UGT1A1*28 homozygosity in a Western European population, consistent with Gilbert's syndrome prevalence.
PMID:9435989 SUPPORT Other
"Gilbert's syndrome, the most prevalent (2-19% in population studies) and mildest of the three syndromes is principally caused by a TA insertion at the TATA promoter region upstream of the UGT1A1 exon."
Clarke 1997 places UGT1A1*28 in the context of the full spectrum of UGT1 defects and confirms its role as the principal cause of Gilbert's syndrome.
UGT1A1*6 Missense Variant (Gly71Arg) (Causative)
Gene: UGT1A1 hgnc:12530 HUGO Gene Nomenclature Committee (hgnc) Relation: this disease-associated gene is this gene This disease-associated gene is UGT1A1 (hgnc:12530). hgnc:12530 is a gene from the HUGO Gene Nomenclature Committee.
Autosomal recessive
Show evidence (2 references)
PMID:10353933 SUPPORT Human Clinical
"a G-->A transition at nucleotide 211 caused arginine to replace glycine at position 71 of corresponding protein product (G71R)."
Maruo 1999 identified the G71R mutation and showed it is associated with hyperbilirubinemia in Japanese neonates, establishing UGT1A1*6 as a relevant variant in East Asian populations.
PMID:15304120 SUPPORT Human Clinical
"The genetic basis of hyperbilirubinemia appears to be different between the Japanese and Caucasian populations."
Takeuchi 2004 confirmed the ethnic divergence in UGT1A1 variant spectrum, with coding variants like G71R being more prevalent in Japanese patients versus the promoter TA-repeat polymorphism in Caucasians.
UGT1A1*37 (TA)8 Promoter Allele (Causative)
Gene: UGT1A1 hgnc:12530 HUGO Gene Nomenclature Committee (hgnc) Relation: this disease-associated gene is this gene This disease-associated gene is UGT1A1 (hgnc:12530). hgnc:12530 is a gene from the HUGO Gene Nomenclature Committee. relationship_type: CAUSATIVE
Show evidence (3 references)
PMID:34089128 SUPPORT Human Clinical
"An extra TA insert leads to eight (TA)8 repeats (UGT1A1*37) resulting in a further reduction of glucuronidation activity."
Establishes that the eighth repeat lowers activity further than the seventh, which is what makes UGT1A1*37 a deficiency allele in its own right rather than a variant of UGT1A1*28.
PMID:22407023 SUPPORT Other
"there are a number of thymine-adenine (TA) repeats varying from 5 to 8, with an inverse correlation between promoter activity and number of TA repetitions: A(TA) 8TAA promoter with reduced activity, A(TA) 7TAA promoter with reduced activity (UGT1A1 *28), A(TA)6TAA wild-type and A(TA)5TAA..."
States the direction of effect for every allele in the series, which is what the record's typing rests on: activity falls as repeat number rises, so (TA)5 sits above wild-type and (TA)8 below (TA)7. Graded OTHER because this is background review prose in the paper's introduction rather than a result of the family study it reports.
PMID:36443464 SUPPORT Other
"Gene variants leading to UGT1A1 enzyme deficiency (e.g. UGT1A1*6, *28 and *37) can be used to optimize an individual's starting dose thereby preventing carriers from toxicity."
A pharmacogenomics guideline treating UGT1A1*37 as a deficiency allele alongside *28 and *6, which is why the series is curated rather than noted in passing.
UGT1A1*36 (TA)5 Promoter Allele (Protective)
Gene: UGT1A1 hgnc:12530 HUGO Gene Nomenclature Committee (hgnc) Relation: this disease-associated gene is this gene This disease-associated gene is UGT1A1 (hgnc:12530). hgnc:12530 is a gene from the HUGO Gene Nomenclature Committee. relationship_type: PROTECTIVE
Show evidence (2 references)
PMID:34089128 SUPPORT Human Clinical
"A variant lacking one TA repeat (TA)5 (UGT1A1*36) has been identified. (TA)8 repeats (UGT1A1*37) and (TA)5 (UGT1A1*36) have been detected in Africans (frequency up to 0.07 and 0.08 respectively)."
Identifies the (TA)5 allele and its African frequency. The same sentence gives the *37 frequency, which is why both records cite this source.
PMID:22407023 SUPPORT Other
"there are a number of thymine-adenine (TA) repeats varying from 5 to 8, with an inverse correlation between promoter activity and number of TA repetitions: A(TA) 8TAA promoter with reduced activity, A(TA) 7TAA promoter with reduced activity (UGT1A1 *28), A(TA)6TAA wild-type and A(TA)5TAA..."
States the direction of effect for every allele in the series, which is what the record's typing rests on: activity falls as repeat number rises, so (TA)5 sits above wild-type and (TA)8 below (TA)7. Graded OTHER because this is background review prose in the paper's introduction rather than a result of the family study it reports.
G6PD (Modifier)
Gene: G6PD hgnc:4057 HUGO Gene Nomenclature Committee (hgnc) Relation: this disease-associated gene is this gene This disease-associated gene is G6PD (hgnc:4057). hgnc:4057 is a gene from the HUGO Gene Nomenclature Committee. relationship_type: MODIFIER
Show evidence (2 references)
PMID:22407023 SUPPORT Human Clinical
"The results showed that in certain cases, the presence of hyperbilirubinemia is not only associated with G6PD deficiency, but may be caused by the co-presence of a mutation in the UGTA1 promoter related to Gilbert's syndrome."
A four-member family study attributing hyperbilirubinemia to the co-presence of both defects rather than to either alone. Small, so it establishes the interaction exists rather than its magnitude.
PMID:37508669 SUPPORT Human Clinical
"Heterozygosity to 6/7 TA repeats in the UGT1A1 promoter was associated with increased NHB, especially in female newborns with G6PD deficiency."
Neonatal series supporting the interaction, graded PARTIAL because the same paper states the interaction is more complex than a two-locus model. Note the cached abstract text drops the italicised gene symbol before "promoter"; the quote is exact against the cache.
💊

Medical Actions

3
Reassurance and Lifestyle Guidance
Action: supportive careNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is supportive care (NCIT:C15747). NCIT:C15747 is a clinical intervention from the NCI Thesaurus. Ontology label: Supportive Care NCIT:C15747
Gilbert's syndrome is benign and requires no specific pharmacological treatment. Management consists of reassurance, avoidance of prolonged fasting, adequate hydration, and recognition of triggers (e.g., illness, stress, strenuous exercise). Patients should be counselled to inform healthcare providers of their UGT1A1 genotype, as this affects metabolism of certain drugs (e.g., irinotecan).
Mechanism Target:
MODULATES Fasting- and Stress-Induced Bilirubin Elevation — Avoiding prolonged fasting and planning around physiologic stressors reduces exposure to settings that provoke clinical jaundice in Gilbert's syndrome.
Show evidence (1 reference)
PMID:30717703 SUPPORT Human Clinical
"Risk factors for clinical jaundice included general anesthesia, pregnancy, fasting"
Human cohort evidence identifies fasting and other stressors as risk factors, supporting lifestyle guidance as a mechanism-directed management step.
Show evidence (2 references)
PMID:37390966 SUPPORT Other
"Gilbert's syndrome, also known as benign hyperbilirubinaemia, was described more than 100 years ago."
The Vitek & Tiribelli 2023 review confirms the longstanding recognition of Gilbert's syndrome as a benign condition, supporting reassurance as the primary therapeutic approach.
PMID:30717703 SUPPORT Human Clinical
"Screening, counseling, monitoring and individualized health care are recommended for subjects with GS in the setting of anesthesia, pregnancy, treatment with DAAs, deliveries, surgery and weight loss plans."
Human longitudinal data support counseling, monitoring, and individualized guidance around common jaundice-triggering settings.
UGT1A1 Genotyping Prior to Irinotecan Therapy
Action: genetic testingNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is genetic testing (NCIT:C15709). NCIT:C15709 is a clinical intervention from the NCI Thesaurus. Ontology label: Genetic Testing NCIT:C15709
Carriers of UGT1A1*28 (and UGT1A1*6 in Asian patients) have significantly reduced capacity to glucuronidate SN-38, the active metabolite of irinotecan. This leads to increased SN-38 exposure and greater susceptibility to irinotecan-induced severe neutropenia and diarrhoea. Genetic testing before initiating irinotecan is classified as 'essential' by international pharmacogenomics guidelines. Poor metabolisers (homozygous or compound heterozygous) should receive a 70% starting dose.
Mechanism Target:
MODULATES Impaired Glucuronidation of Drugs and Drug Metabolites — Genotyping does not change UGT1A1 activity. It acts on the mechanism by matching the administered dose to the measured conjugating capacity, so that SN-38 exposure stays within what the individual's enzyme can clear. Poor metabolisers receive a reduced starting dose.
Show evidence (1 reference)
PMID:36443464 SUPPORT Other
"Gene variants leading to UGT1A1 enzyme deficiency (e.g. UGT1A1*6, *28 and *37) can be used to optimize an individual's starting dose thereby preventing carriers from toxicity."
States the dose-to-capacity matching that is the mechanism-directed action of the test.
MODULATES Drug-Unmasked Hyperbilirubinemia — Knowing the genotype before starting a UGT1A1-inhibiting drug allows the predictable bilirubin rise to be anticipated rather than investigated as new liver disease, and allows an alternative agent to be chosen where one exists. MODULATES rather than INHIBITS because a test does not act on the node: the effect depends entirely on what the prescriber then does.
Show evidence (1 reference)
PMID:16170755 SUPPORT Human Clinical
"Genotyping for UGT1A1*28 before initiation of ART would identify HIV-infected individuals at risk for hyperbilirubinemia and decrease episodes of jaundice."
The cohort's own conclusion proposes pre-treatment genotyping as the intervention on this node.
Show evidence (3 references)
PMID:11990381 SUPPORT Human Clinical
"screening for UGT1A1*28 polymorphism may identify patients with lower SN-38 glucuronidation rates and greater susceptibility to irinotecan induced gastrointestinal and bone marrow toxicity."
Iyer 2002 prospective pharmacogenetic study demonstrated that UGT1A1*28 genotype predicts reduced SN-38 glucuronidation and severe irinotecan toxicity, establishing the clinical rationale for pre-treatment testing.
PMID:36443464 SUPPORT Other
"Gene variants leading to UGT1A1 enzyme deficiency (e.g. UGT1A1*6, *28 and *37) can be used to optimize an individual's starting dose thereby preventing carriers from toxicity."
The DPWG 2023 guideline endorses UGT1A1 variant testing to optimize irinotecan starting dose and prevent severe toxicity.
PMID:36443464 SUPPORT Other
"UGT1A1 genotyping is considered "essential", indicating that UGT1A1 testing must be performed prior to initiating irinotecan treatment."
DPWG classification of UGT1A1 testing as 'essential' elevates this to a mandatory pre-treatment test, not an optional pharmacogenomics screen.
Phenobarbital Enzyme Induction
Action: PharmacotherapyNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Pharmacotherapy (NCIT:C15986). NCIT:C15986 is a clinical intervention from the NCI Thesaurus. NCIT:C15986
Agent: phenobarbital CHEBI:8069 Chemical Entities of Biological Interest (CHEBI) Relation: this treatment uses this therapeutic agent This treatment uses phenobarbital (CHEBI:8069). CHEBI:8069 is a therapeutic agent from Chemical Entities of Biological Interest.
Phenobarbital induces UGT1A1 transcription and lowers unconjugated bilirubin. It is not indicated in Gilbert syndrome, which needs no bilirubin-lowering treatment, and is recorded here for two reasons: the bilirubin fall on phenobarbital is a classical discriminator against Crigler-Najjar type I, which does not respond; and it is the clearest demonstration that the Gilbert lesion is reduced transcription of an inducible gene rather than a fixed absence of enzyme. Sedation makes it unsuitable as a cosmetic treatment for a benign condition.
Mechanism Target:
MODULATES Reduced UGT1A1 Activity and Impaired Bilirubin Glucuronidation — Transcriptional induction raises UGT1A1 expression from the reduced baseline set by the promoter allele, which is possible precisely because the allele lowers transcription rather than abolishing the enzyme.
Show evidence (1 reference)
PMID:16170755 SUPPORT INDIRECT Human Clinical
"Efavirenz resulted in decreased bilirubin levels, which is consistent with the induction of UDP-glucuronosyltransferase 1A1."
Graded INDIRECT because the inducer observed here is efavirenz, not phenobarbital. It establishes that pharmacological induction of UGT1A1 lowers bilirubin in this population, which is the mechanism claimed; it is not direct evidence for phenobarbital.
Show evidence (1 reference)
PMID:9435989 SUPPORT Other
"Crigler-Najjar type II syndrome which is treatable with phenobarbital therapy is associated with less dramatic missense mutations or heterozygous expression of mutant and normal alleles."
Sources the phenobarbital response and ties it to residual enzyme: Crigler-Najjar type II, which retains some activity, responds, which is what makes the response a discriminator against the type I null.
🌍

Environmental Factors

1
Prolonged fasting or caloric restriction
fasting XCO:0000102 Experimental Conditions Ontology (XCO) Relation: this environmental factor is this exposure This environmental factor is fasting (XCO:0000102). XCO:0000102 is an exposure from the Experimental Conditions Ontology.
Fasting for 24-48 hours raises unconjugated bilirubin in Gilbert syndrome to a degree it does not in unaffected people, and is the basis of the historical fasting provocation test. General anaesthesia, intercurrent illness, pregnancy, and deliberate weight-loss regimens act through the same route in the reported cohorts.
Show evidence (1 reference)
PMID:30717703 SUPPORT Human Clinical
"Risk factors for clinical jaundice included general anesthesia, pregnancy, fasting"
Longitudinal adult cohort listing fasting among the risk factors for clinically apparent jaundice episodes.
Mechanism Target:
EXACERBATES Fasting- and Stress-Induced Bilirubin Elevation — Caloric restriction is the provoking exposure for the trigger-sensitive bilirubin elevation node, and is the exposure used diagnostically.
Show evidence (1 reference)
PMID:8596320 SUPPORT Human Clinical
"14 volunteers underwent a 24 h fasting test to see if they had Gilbert's syndrome, and all four positives had the 7/7 genotype."
A deliberate 24-hour fasting exposure discriminated UGT1A1*28 homozygotes, establishing fasting as the operative exposure.
🔬

Biochemical Markers

1
Unconjugated Bilirubin (INCREASED)
Context: Serum unconjugated fraction elevated; conjugated bilirubin and liver enzymes (ALT, AST, ALP) remain within normal limits. Typically 1.2-3 mg/dL in Gilbert's syndrome.
Pathograph Readouts
Readout Of Reduced UGT1A1 Activity and Impaired Bilirubin Glucuronidation Positive Diagnostic
Elevated unconjugated bilirubin reports the reduced hepatic bilirubin glucuronidation that defines Gilbert's syndrome.
Show evidence (1 reference)
PMID:7565971 SUPPORT Human Clinical
"People with Gilbert's syndrome have mild, chronic unconjugated hyperbilirubinemia in the absence of liver disease or overt hemolysis. Hepatic glucuronidating activity, essential for efficient biliary excretion of bilirubin, is reduced to about 30 percent of normal."
Human clinical evidence ties the unconjugated bilirubin readout to the reduced hepatic glucuronidation mechanism.
Readout Of Fasting- and Stress-Induced Bilirubin Elevation Positive Monitoring
Serial unconjugated bilirubin captures trigger-sensitive bilirubin elevations during fasting, illness, pregnancy, surgery, or other stressors.
Show evidence (1 reference)
PMID:8596320 SUPPORT Human Clinical
"Individuals in the population with the 7/7 genotype had significantly higher bilirubin concentrations than those who had the 6/7 or 6/6 genotype. 14 volunteers underwent a 24 h fasting test to see if they had Gilbert's syndrome, and all four positives had the 7/7 genotype."
Human fasting-provocation evidence supports bilirubin as a monitoring readout of trigger-sensitive Gilbert physiology.
Reference Ranges
5.1–17.1 umol/L (adults, unisex laboratory reference interval)
Hypobilirubinemia (–5.1 umol/L) Normobilirubinemia (5.1–17.1 umol/L) Hyperbilirubinemia (historical Gilbert threshold) (17.1– umol/L) → Unconjugated hyperbilirubinemia
Hypobilirubinemia: Below the unisex reference interval. Sex-adjusted lower limits differ substantially (4.7-9.2 umol/L in women, 5.6-12.1 umol/L in men), so this band is not reliable without stratification.
Hyperbilirubinemia (historical Gilbert threshold): Meets the historical unisex Gilbert syndrome cutoff. Under stratified centiles the equivalent threshold ranges from 9.0 to 26.2 umol/L depending on sex, age, and UGT1A1 genotype, so this band both over-calls in some strata and under-calls in others.
The unisex colorimetric-assay interval used by the UK Biobank and by most clinical laboratories. Its upper limit is the historical Gilbert syndrome cutoff. It is recorded because it is what results are reported against, not because it is appropriate: the same source shows the distribution differs by sex, age, and UGT1A1 genotype, so a single unisex limit underdiagnoses women.
Show evidence (1 reference)
PMID:37738404 SUPPORT Human Clinical
"Bilirubin was measured by colorimetric assay with a unisex reference interval of 5.1-17.1 µmol/L."
States the laboratory reference interval and the assay it belongs to.
Show evidence (2 references)
PMID:7565971 SUPPORT Human Clinical
"People with Gilbert's syndrome have mild, chronic unconjugated hyperbilirubinemia in the absence of liver disease or overt hemolysis."
Confirms selectively elevated unconjugated bilirubin as the hallmark biochemical finding, with normal conjugated fraction and liver function.
PMID:26250421 SUPPORT Human Clinical
"Serum bilirubin concentrations of typical CN-2, intermediate group, and typical GS are respectively 12.9 ± 5.1, 5.2 ± 2.2, and 2.8 ± 1.1 mg/dL."
Provides quantitative bilirubin values for typical Gilbert's syndrome (2.8 ± 1.1 mg/dL), confirming mild elevation as the expected degree.
🔬

Diagnosis

3
Diagnosis of exclusion on isolated unconjugated hyperbilirubinemia
Gilbert syndrome is diagnosed by exclusion, not by a positive test. The picture is isolated unconjugated hyperbilirubinemia - predominantly unconjugated, typically under 3 mg/dL - with normal ALT, AST, ALP and GGT, normal haemoglobin, reticulocytes, haptoglobin and blood film, and normal hepatic imaging. Both haemolysis and liver damage must be excluded, which is part of the accepted definition rather than an optional workup. Most diagnoses are made incidentally on blood tests taken for something else.
Show evidence (2 references)
PMID:37738404 SUPPORT Human Clinical
"There is a consensual definition that GS in individuals is determined by the presence of hyperbilirubinemia in the absence of both hemolysis and liver damage, including the fibrosis stage."
States the exclusion-based definition, including that fibrosis must be excluded and that normal ALT and GGT alone are not sufficient to do so.
PMID:40904555 SUPPORT Human Clinical
"GS is typically diagnosed by exclusion through blood tests that detect elevated levels of unconjugated bilirubin, without haemolysis or structural liver damage."
Independent statement of the same diagnostic route, from the study that also documents how often it happens incidentally.
Fasting provocation test
A 24-48 hour caloric restriction raises unconjugated bilirubin disproportionately in Gilbert syndrome, and was historically used to provoke a diagnostic rise. It is largely superseded by UGT1A1 genotyping, which is not provocative and not affected by intercurrent illness. Recorded because the test is the clinical expression of the fasting-sensitivity mechanism this entry models, and because the published diagnostic thresholds derive from cohorts characterised this way.
Show evidence (1 reference)
PMID:8596320 SUPPORT Human Clinical
"14 volunteers underwent a 24 h fasting test to see if they had Gilbert's syndrome, and all four positives had the 7/7 genotype."
Reports the test being used diagnostically and its concordance with the genotype in the same volunteers.
UGT1A1 promoter (TA)n genotyping
Sizing the TATA-box TA repeat, with coding-region sequencing where an East Asian ancestry or a discordant result suggests UGT1A1*6. Genotyping confirms rather than establishes the diagnosis: roughly 10-13% of Western populations are UGT1A1*28 homozygotes while clinically recognised Gilbert syndrome is far less common, so a positive genotype in the absence of hyperbilirubinemia does not make the diagnosis. An assay that scores only (TA)6 against (TA)7 will misclassify carriers of the (TA)5 and (TA)8 alleles found in African populations.
genetic testing NCIT:C15709 NCI Thesaurus (NCIT)
Show evidence (2 references)
PMID:34089128 SUPPORT Human Clinical
"We present a real time PCR method for genotyping the UGT1A1 (TA)n polymorphism (UGT1A1*28, UGT1A1*36, UGT1A1*37) using Taqman PCR."
Describes the genotyping assay and, by covering three alleles rather than two, makes the misclassification point concrete.
PMID:7565971 SUPPORT Human Clinical
"Reduced expression of bilirubin UDP-glucuronosyltransferase 1 due to an abnormality in the promoter region of the gene for this enzyme appears to be necessary for Gilbert's syndrome but not sufficient for the complete manifestation of the syndrome."
The necessary-but-not-sufficient result is why genotyping confirms rather than establishes the diagnosis. PARTIAL because it constrains the test's interpretation rather than supporting the test itself.
📊

Prevalence

3
Worldwide
Point Prevalence 10500.0 per 100,000 (2000.0–19000.0) >1 in 1,000
Reported prevalence varies substantially by ancestry and by the diagnostic threshold used, but Gilbert's syndrome is common in the general population rather than an ultra-rare condition. The 2-19% range spans studies using different definitions; rate_per_100000 is the midpoint of that range and should not be read as a pooled estimate.
Show evidence (1 reference)
PMID:9435989 SUPPORT Other
"Gilbert's syndrome, the most prevalent (2-19% in population studies) and mildest of the three syndromes is principally caused by a TA insertion at the TATA promoter region upstream of the UGT1A1 exon."
Review abstract summarizes the prevalence range reported across population studies.
Middle-aged European women (UK Biobank, apparently healthy liver)
Point Prevalence 10000.0 per 100,000 >1 in 1,000
Measured prevalence in women depends strongly on the definition used. Under sex-, age- and genotype-stratified bilirubin centiles it is 10%; under the historical unisex 1 mg/dL cutoff the same cohort yields 3.7%. Recorded here under the stratified definition; see definitions#Sex-, age- and genotype-stratified bilirubin centile definition.
Show evidence (1 reference)
PMID:37738404 SUPPORT Human Clinical
"In women, we identified 10% (7,741/76,809) of GS versus 3.7% (2,819/76,809) using the historical cutoff of ≥1 mg/dL (P<0.0001)."
Gives both prevalence figures in the same cohort, which is what makes the definition dependence explicit rather than an inference.
United Kingdom, clinically recorded diagnoses (IQVIA primary care)
Point Prevalence 180.4 per 100,000 (174.4–186.6) >1 in 1,000
Clinically recorded prevalence, roughly fifty-fold lower than the genotype prevalence in the same kind of population. The gap is the entry's most concrete measure of incomplete penetrance plus under-ascertainment: most genotype-positive people are never diagnosed. Diagnoses were commoner in men, peaked around age 35, and were commoner in less deprived areas, which means this figure carries ascertainment structure and is not a population rate.
Show evidence (1 reference)
PMID:40904555 SUPPORT Human Clinical
"The estimated UK prevalence of GS was 180.4 per 100,000 (95% CI: 174.4-186.6), with diagnoses more common in men, peaking around age 35, and more frequent in areas of least social deprivation."
Gives the recorded-diagnosis rate with its confidence interval and states the ascertainment pattern that qualifies it.
🔀

Differential Diagnoses

2

Conditions with similar clinical presentations that must be differentiated from Gilbert's Syndrome:

Crigler-Najjar syndrome type II
Overlapping Features The same gene, the same enzymatic deficit, and the same unconjugated hyperbilirubinemia, separated by degree rather than kind. Reported mean serum bilirubin is about 2.8 mg/dL in typical Gilbert syndrome against about 12.9 mg/dL in typical Crigler-Najjar type II, with an intermediate group between them, so the boundary is a point on a continuum and not a categorical distinction.
Distinguishing Features
  • Substantially higher serum bilirubin, with an intermediate group between the two
  • Biallelic coding UGT1A1 mutations rather than a promoter TA repeat alone
  • Presentation in infancy rather than adolescence
  • Bilirubin falls on phenobarbital
Show evidence (2 references)
PMID:26250421 SUPPORT Human Clinical
"Serum bilirubin concentrations of typical CN-2, intermediate group, and typical GS are respectively 12.9 ± 5.1, 5.2 ± 2.2, and 2.8 ± 1.1 mg/dL."
Gives the quantitative separation and the existence of an intermediate group, which is the substance of this differential.
PMID:9435989 SUPPORT Other
"Crigler-Najjar type II syndrome which is treatable with phenobarbital therapy is associated with less dramatic missense mutations or heterozygous expression of mutant and normal alleles."
Sources the phenobarbital response and ties it to residual enzyme: Crigler-Najjar type II, which retains some activity, responds, which is what makes the response a discriminator against the type I null.
Haemolysis
Overlapping Features Increased bilirubin production from any haemolytic process produces unconjugated hyperbilirubinemia with normal liver enzymes, which is exactly the Gilbert biochemical picture. Excluding haemolysis is part of the consensual definition of Gilbert syndrome, not an optional extra test. The two also co-occur, and co-occurrence is clinically important because the combination is what drives early pigment gallstones.
Distinguishing Features
  • Reticulocytosis
  • Reduced haptoglobin
  • Raised lactate dehydrogenase
  • Anaemia
Show evidence (1 reference)
PMID:37738404 SUPPORT Human Clinical
"There is a consensual definition that GS in individuals is determined by the presence of hyperbilirubinemia in the absence of both hemolysis and liver damage, including the fibrosis stage."
States haemolysis exclusion as part of the accepted definition, which is what makes it the primary differential.
🐁

Animal Models

2
Gunn rat (homozygous jj, UGT1A1-null)
The Gunn rat lacks UGT1A1 entirely and is the standard rodent model of inherited unconjugated hyperbilirubinemia. It is included here because it is the model system in which the bilirubin-conjugation lesion has been studied mechanistically, but its fidelity to Gilbert syndrome specifically is low: Gilbert syndrome is a partial (roughly 30% of normal) reduction in a regulated promoter, whereas the Gunn rat is a complete enzymatic null. The Gunn rat therefore models Crigler-Najjar type I, including bilirubin encephalopathy, which does not occur in Gilbert syndrome.
Species
Rat
Genotype
UGT1A1-null (Gunn jj homozygote)
Genes
UGT1A1 hgnc:12530 HUGO Gene Nomenclature Committee (hgnc) Relation: this experimental model concerns this gene This experimental model concerns UGT1A1 (hgnc:12530). hgnc:12530 is a gene from the HUGO Gene Nomenclature Committee.
Publication
PMID:33027804
Pathograph links
Reduced UGT1A1 Activity and Impaired Bilirubin Glucuronidation partially recapitulates fidelity: low Reproduces the enzymatic lesion (absent bilirubin glucuronidation) and the resulting unconjugated hyperbilirubinemia, but not its degree, regulation, or clinical consequences in Gilbert syndrome.
Limitations: Complete UGT1A1 absence rather than the partial promoter-driven reduction that defines Gilbert syndrome; the resulting bilirubin levels are in the Crigler-Najjar type I range and produce cerebellar bilirubin neurotoxicity that Gilbert syndrome never causes. The model also cannot represent the TA-repeat promoter mechanism itself, which is the actual human lesion.
Readouts
Cerebellar weight and cerebellar-mediated behaviour after induced hyperbilirubinemia
Direction: DECREASED
Interpretation: Structural and behavioural consequence of unconjugated bilirubin accumulation in the UGT1A1-null rat. Reported here as a model readout; it has no counterpart in human Gilbert syndrome.
Show evidence (1 reference)
PMID:33027804 SUPPORT Model Organism
"SDMX induced behavioral deficits in jj pups, and choline supplementation improved most behavioral effects and cerebellar weight in SDMX-treated jj rats."
Reports the measured cerebellar and behavioural readouts in the homozygous jj model.
Both available rodent models sit at the severe, Crigler-Najjar end of the UGT1A1-deficiency spectrum rather than the mild Gilbert end; see the corresponding HUMAN_MODEL_MISMATCH discussion.
Show evidence (1 reference)
PMID:33027804 SUPPORT Model Organism
"The homozygous Gunn rat lacks uridine diphosphate glucuronosyltransferase 1A1 (UGT1A1), the enzyme needed to biotransform bilirubin. This rodent model of hyperbilirubinemia emulates many aspects of bilirubin toxicity observed in the human infant."
States both what the model is and the scope of its claimed fidelity - bilirubin toxicity in the infant, not the adult Gilbert phenotype - which is why the link is graded PARTIALLY_RECAPITULATES with LOW fidelity.
Humanized UGT1 (hUGT1) mouse
Mice in which the mouse Ugt1 locus is replaced by the human UGT1 locus. They carry human UGT1A1 regulatory sequence, so unlike the Gunn rat they can in principle report on human promoter regulation, and they were the model that identified intestinal UGT1A1 as the site where breast milk acts to produce neonatal jaundice. Their spontaneous phenotype is nonetheless severe neonatal hyperbilirubinemia with bilirubin-induced neurologic dysfunction, which is a Crigler-Najjar-range phenotype, not the mild adult Gilbert one. Included here for the regulatory mechanism it establishes, not as a model of the Gilbert phenotype.
Species
Mouse
Genotype
Ugt1-null with the human UGT1 locus knocked in (hUGT1)
Genes
UGT1A1 hgnc:12530 HUGO Gene Nomenclature Committee (hgnc) Relation: this experimental model concerns this gene This experimental model concerns UGT1A1 (hgnc:12530). hgnc:12530 is a gene from the HUGO Gene Nomenclature Committee.
Publication
PMID:21983082
Pathograph links
Reduced UGT1A1 Activity and Impaired Bilirubin Glucuronidation partially recapitulates fidelity: low Establishes that UGT1A1 expression, and therefore bilirubin clearance, is set by inducible regulation of the human locus rather than by a fixed enzyme quantity - the same regulatory axis that the UGT1A1*28 promoter allele perturbs in humans.
Limitations: The spontaneous phenotype is severe neonatal hyperbilirubinemia with bilirubin-induced neurologic dysfunction, which does not occur in Gilbert syndrome. Findings are in neonates and in the intestinal compartment; whether the same regulation sets adult hepatic UGT1A1 capacity in Gilbert syndrome is not established by this model.
Readouts
Serum bilirubin under breast-milk versus formula feeding
Direction: INCREASED
Interpretation: Breast-milk feeding raises serum bilirubin by suppressing intestinal UGT1A1; formula lowers it by inducing the same gene. The readout is the bilirubin level under a defined dietary exposure.
Show evidence (1 reference)
PMID:21983082 SUPPORT Model Organism
"hUGT1 mice that were fed breast milk developed severe hyperbilirubinemia because of suppression of UGT1A1 in the gastrointestinal tract. Formula-fed hUGT1 mice had lower serum levels of bilirubin, which resulted from induction of UGT1A1 in the gastrointestinal tract."
Reports the measured bilirubin difference and attributes it to bidirectional regulation of UGT1A1 expression.
Show evidence (1 reference)
PMID:21983082 SUPPORT Model Organism
"We studied mice in which the original Ugt1 locus was disrupted and replaced with the human UGT1 locus (hUGT1 mice); these mice spontaneously develop neonatal hyperbilirubinemia and BIND."
States both the humanized construction that makes the model informative about human UGT1 regulation and the severe spontaneous phenotype that limits its fidelity to Gilbert syndrome.
{ }

Source YAML

click to show 
name: Gilbert's Syndrome
creation_date: "2026-03-17T15:29:38Z"
category: Mendelian
description: >-
  A common, benign inherited condition characterized by mild, intermittent
  unconjugated hyperbilirubinemia in the absence of hemolysis or liver disease.
  Caused by reduced hepatic UGT1A1 enzyme activity, impairing bilirubin
  glucuronidation. The most prevalent cause in Western populations is a TA-repeat
  promoter polymorphism (UGT1A1*28) that reduces UGT1A1 transcription; in East
  Asian populations the UGT1A1*6 (Gly71Arg) coding variant predominates. No
  specific pharmacological treatment is required, but carriers must be identified
  before irinotecan chemotherapy to prevent severe toxicity.
disease_term:
  preferred_term: Gilbert's syndrome
  term:
    id: MONDO:0007745
    label: Gilbert syndrome
parents:
- Liver Disease
- Inherited Metabolic Disorder
prevalence:
- population: Worldwide
  measure_type: POINT_PREVALENCE
  prevalence_class: ABOVE_1_IN_1000
  rate_per_100000: 10500.0
  rate_low: 2000.0
  rate_high: 19000.0
  notes: >-
    Reported prevalence varies substantially by ancestry and by the diagnostic
    threshold used, but Gilbert's syndrome is common in the general population
    rather than an ultra-rare condition. The 2-19% range spans studies using
    different definitions; rate_per_100000 is the midpoint of that range and
    should not be read as a pooled estimate.
  evidence:
  - reference: PMID:9435989
    reference_title: "Genetic defects of the UDP-glucuronosyltransferase-1 (UGT1) gene that cause familial non-haemolytic unconjugated hyperbilirubinaemias."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Gilbert's syndrome, the most prevalent (2-19% in population studies) and mildest of the three syndromes is principally caused by a TA insertion at the TATA promoter region upstream of the UGT1A1 exon."
    explanation: Review abstract summarizes the prevalence range reported across population studies.
- population: Middle-aged European women (UK Biobank, apparently healthy liver)
  measure_type: POINT_PREVALENCE
  prevalence_class: ABOVE_1_IN_1000
  rate_per_100000: 10000.0
  notes: >-
    Measured prevalence in women depends strongly on the definition used. Under
    sex-, age- and genotype-stratified bilirubin centiles it is 10%; under the
    historical unisex 1 mg/dL cutoff the same cohort yields 3.7%. Recorded here
    under the stratified definition; see definitions#Sex-, age- and
    genotype-stratified bilirubin centile definition.
  evidence:
  - reference: PMID:37738404
    reference_title: "Clinical and genetic definition of serum bilirubin levels for the diagnosis of Gilbert syndrome and hypobilirubinemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In women, we identified 10% (7,741/76,809) of GS versus 3.7% (2,819/76,809) using the historical cutoff of ≥1 mg/dL (P<0.0001)."
    explanation: >-
      Gives both prevalence figures in the same cohort, which is what makes the
      definition dependence explicit rather than an inference.
- population: United Kingdom, clinically recorded diagnoses (IQVIA primary care)
  measure_type: POINT_PREVALENCE
  prevalence_class: ABOVE_1_IN_1000
  rate_per_100000: 180.4
  rate_low: 174.4
  rate_high: 186.6
  notes: >-
    Clinically recorded prevalence, roughly fifty-fold lower than the genotype
    prevalence in the same kind of population. The gap is the entry's most
    concrete measure of incomplete penetrance plus under-ascertainment: most
    genotype-positive people are never diagnosed. Diagnoses were commoner in
    men, peaked around age 35, and were commoner in less deprived areas, which
    means this figure carries ascertainment structure and is not a population
    rate.
  evidence:
  - reference: PMID:40904555
    reference_title: "Prevalence and Data-Driven Exploration of Pre-Diagnostic Symptoms and Features of Gilbert's Syndrome in the UK Primary Care Population."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The estimated UK prevalence of GS was 180.4 per 100,000 (95% CI: 174.4-186.6), with diagnoses more common in men, peaking around age 35, and more frequent in areas of least social deprivation."
    explanation: >-
      Gives the recorded-diagnosis rate with its confidence interval and states
      the ascertainment pattern that qualifies it.
pathophysiology:
- name: Reduced UGT1A1 Activity and Impaired Bilirubin Glucuronidation
  conforms_to: "bilirubin_conjugation_transport#UGT1A1-Dependent Bilirubin Glucuronidation Deficiency"
  description: >-
    The UGT1A1 enzyme in hepatocytes conjugates unconjugated (indirect) bilirubin
    with glucuronic acid, making it water-soluble for biliary excretion. In
    Gilbert's syndrome, reduced UGT1A1 activity (approximately 30% of normal)
    leads to accumulation of unconjugated bilirubin in the blood. The most common
    cause in Western populations is a homozygous (TA)7TAA repeat (UGT1A1*28
    allele) in the TATAA box of the UGT1A1 promoter, which reduces transcription.
    In Asian populations, missense variants in the UGT1A1 coding region
    (e.g., Gly71Arg / UGT1A1*6) are more prevalent. Gilbert's syndrome is the
    most prevalent (2-19% in population studies) and mildest of the hereditary
    unconjugated hyperbilirubinaemia syndromes.
  genes:
  - preferred_term: UGT1A1
    term:
      id: hgnc:12530
      label: UGT1A1
  cell_types:
  - preferred_term: Hepatocyte
    term:
      id: CL:0000182
      label: hepatocyte
  biological_processes:
  - preferred_term: Bilirubin conjugation
    modifier: DECREASED
    term:
      id: GO:0006789
      label: bilirubin conjugation
  chemical_entities:
  - preferred_term: unconjugated bilirubin
    term:
      id: CHEBI:16990
      label: bilirubin IXalpha
    modifier: INCREASED
  molecular_functions:
  - preferred_term: glucuronosyltransferase activity
    modifier: DECREASED
    term:
      id: GO:0015020
      label: glucuronosyltransferase activity
  evidence:
  - reference: PMID:7565971
    reference_title: "The genetic basis of the reduced expression of bilirubin UDP-glucuronosyltransferase 1 in Gilbert's syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Hepatic glucuronidating activity, essential for efficient biliary excretion of bilirubin, is reduced to about 30 percent of normal."
    explanation: >-
      The Bosma 1995 NEJM paper demonstrates that UGT1A1 enzyme activity is
      substantially reduced in Gilbert's syndrome, confirming the enzymatic
      basis of the condition.
  - reference: PMID:7565971
    reference_title: "The genetic basis of the reduced expression of bilirubin UDP-glucuronosyltransferase 1 in Gilbert's syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "These patients were homozygous for two extra bases (TA) in the TATAA element of the 5' promoter region of the gene (A(TA)7TAA rather than the normal A(TA)6TAA)."
    explanation: >-
      Identifies the (TA)7 promoter repeat as the molecular basis for reduced
      UGT1A1 expression in Western populations.
  - reference: PMID:9435989
    reference_title: "Genetic defects of the UDP-glucuronosyltransferase-1 (UGT1) gene that cause familial non-haemolytic unconjugated hyperbilirubinaemias."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Gilbert's syndrome, the most prevalent (2-19% in population studies) and mildest of the three syndromes is principally caused by a TA insertion at the TATA promoter region upstream of the UGT1A1 exon."
    explanation: >-
      Clarke 1997 confirms population prevalence (2-19%) and the promoter TA
      insertion as the principal molecular cause.
  downstream:
  - target: Unconjugated Bilirubin
    description: >-
      Reduced hepatic bilirubin glucuronidation leaves a larger circulating
      unconjugated bilirubin pool.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:7565971
      reference_title: "The genetic basis of the reduced expression of bilirubin UDP-glucuronosyltransferase 1 in Gilbert's syndrome."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        People with Gilbert's syndrome have mild, chronic unconjugated hyperbilirubinemia
        in the absence of liver disease or overt hemolysis. Hepatic glucuronidating
        activity, essential for efficient biliary excretion of bilirubin, is reduced
        to about 30 percent of normal.
      explanation: >-
        The same clinical abstract links reduced hepatic glucuronidating
        activity to chronic unconjugated hyperbilirubinemia.
  - target: Mild Unconjugated Hyperbilirubinemia
    description: >-
      Impaired UGT1A1-mediated bilirubin conjugation is the proximal mechanism
      producing the obligate biochemical phenotype.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:7565971
      reference_title: "The genetic basis of the reduced expression of bilirubin UDP-glucuronosyltransferase 1 in Gilbert's syndrome."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        People with Gilbert's syndrome have mild, chronic unconjugated hyperbilirubinemia
        in the absence of liver disease or overt hemolysis. Hepatic glucuronidating
        activity, essential for efficient biliary excretion of bilirubin, is reduced
        to about 30 percent of normal.
      explanation: >-
        Establishes reduced bilirubin glucuronidation and the resulting
        unconjugated hyperbilirubinemia in affected people.
  - target: Fasting- and Stress-Induced Bilirubin Elevation
    description: >-
      Reduced UGT1A1 activity creates susceptibility to fasting-provoked
      bilirubin elevations in homozygous UGT1A1*28 carriers.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - UGT1A1*28 homozygous genotype and reduced hepatic bilirubin UGT activity
    evidence:
    - reference: PMID:8596320
      reference_title: "Genetic variation in bilirubin UPD-glucuronosyltransferase gene promoter and Gilbert's syndrome."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        Individuals in the population with the 7/7 genotype had significantly higher
        bilirubin concentrations than those who had the 6/7 or 6/6 genotype. 14 volunteers
        underwent a 24 h fasting test to see if they had Gilbert's syndrome, and all
        four positives had the 7/7 genotype.
      explanation: >-
        Links UGT1A1*28 homozygosity to higher bilirubin and positive
        fasting-provocation testing.
  - target: Bilirubin Antioxidant and Pleiotropic Effects
    description: >-
      The mild increase in circulating bilirubin caused by reduced clearance is
      the exposure underlying the antioxidant and pleiotropic-effect hypotheses.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    hypothesis_groups:
    - bilirubin_systemic_antioxidant_protection
    intermediate_mechanisms:
    - Increased serum unconjugated bilirubin
    evidence:
    - reference: PMID:18343383
      reference_title: "Gilbert syndrome, UGT1A1*28 allele, and cardiovascular disease risk: possible protective effects and therapeutic applications of bilirubin."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: >-
        Such individuals have decreased hepatic bilirubin UDP-glucuronosyltransferase
        activity, decreased bilirubin clearance, and increased serum bilirubin concentrations.
      explanation: >-
        Review-level evidence explicitly connects Gilbert syndrome/UGT1A1*28 to
        reduced bilirubin clearance and increased serum bilirubin.
  - target: Impaired Glucuronidation of Drugs and Drug Metabolites
    description: >-
      The reduced UGT1A1 transcription that lowers bilirubin conjugation acts
      on the same enzyme that glucuronidates SN-38 and other xenobiotic
      substrates, so the pharmacogenomic consequence is not a separate lesion.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:36443464
      reference_title: "Dutch pharmacogenetics working group (DPWG) guideline for the gene-drug interaction between UGT1A1 and irinotecan."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "Gene variants leading to UGT1A1 enzyme deficiency (e.g. UGT1A1*6, *28 and *37) can be used to optimize an individual's starting dose thereby preventing carriers from toxicity."
      explanation: >-
        A pharmacogenomics guideline treats the same UGT1A1 deficiency alleles
        as determining drug handling, which is this edge.
  - target: Chronic Biliary Bilirubin Load and Pigment Gallstone Formation
    description: >-
      A persistently larger unconjugated bilirubin pool increases the
      unconjugated load delivered to bile, which is the substrate for pigment
      stone formation.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - Increased circulating and biliary unconjugated bilirubin
    evidence:
    - reference: PMID:37738404
      reference_title: "Clinical and genetic definition of serum bilirubin levels for the diagnosis of Gilbert syndrome and hypobilirubinemia."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "We found a significant causal association between bilirubin centiles and cholelithiasis/gallstone illness."
      explanation: >-
        Mendelian randomization supports bilirubin level itself, not a
        correlate of it, as the causal intermediate on this edge.
- name: Fasting- and Stress-Induced Bilirubin Elevation
  description: >-
    Bilirubin levels in Gilbert's syndrome fluctuate and are exacerbated by
    physiological stressors such as fasting, dehydration, intercurrent illness,
    physical exertion, and menstruation. Fasting reduces caloric intake and
    increases fatty acid oxidation, which competes with bilirubin for hepatic
    uptake and conjugation pathways. These triggers can precipitate symptomatic
    jaundice in otherwise mildly affected individuals. A 24-hour fasting test
    is used diagnostically to provoke bilirubin elevation in suspected cases.
  biological_processes:
  - preferred_term: Bilirubin hepatic transport
    modifier: ABNORMAL
    term:
      id: GO:0015723
      label: bilirubin transport
  chemical_entities:
  - preferred_term: unconjugated bilirubin
    term:
      id: CHEBI:16990
      label: bilirubin IXalpha
    modifier: INCREASED
  evidence:
  - reference: PMID:8596320
    reference_title: "Genetic variation in bilirubin UPD-glucuronosyltransferase gene promoter and Gilbert's syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "14 volunteers underwent a 24 h fasting test to see if they had Gilbert's syndrome, and all four positives had the 7/7 genotype."
    explanation: >-
      Demonstrates that fasting precipitates jaundice specifically in homozygous
      UGT1A1*28 (7/7) carriers, consistent with the fasting-triggering mechanism
      in Gilbert's syndrome.
  - reference: PMID:30717703
    reference_title: "The frequency, clinical course, and health related quality of life in adults with Gilbert's syndrome: a longitudinal study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Risk factors for clinical jaundice included general anesthesia, pregnancy, fasting"
    explanation: >-
      A longitudinal adult cohort identifies fasting and other physiologic
      stressors as risk factors for clinical jaundice episodes.
  downstream:
  - target: Jaundice
    description: >-
      Trigger-related bilirubin elevations can cross the threshold for visible
      intermittent jaundice.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - Transient unconjugated hyperbilirubinemia
    evidence:
    - reference: PMID:8596320
      reference_title: "Genetic variation in bilirubin UPD-glucuronosyltransferase gene promoter and Gilbert's syndrome."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "This common mild hyperbilirubinaemia sometimes presents as an intermittent jaundice."
      explanation: >-
        Supports intermittent jaundice as the clinical expression of mild
        hyperbilirubinemia in Gilbert syndrome.
  - target: Jaundice-Associated Gastrointestinal Symptoms
    description: >-
      Jaundice episodes in Gilbert syndrome can be accompanied by abdominal
      pain, dyspepsia, or appetite loss through incompletely resolved
      symptom-generating intermediates.
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - Jaundice attack and associated symptom complex
    evidence:
    - reference: PMID:30717703
      reference_title: "The frequency, clinical course, and health related quality of life in adults with Gilbert's syndrome: a longitudinal study."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "jaundice was associated with abdominal pain, dyspepsia or loss of appetite in 54 (53.465%) subjects."
      explanation: >-
        The cohort reports gastrointestinal symptoms associated with jaundice
        episodes in more than half of Gilbert syndrome subjects.
- name: Bilirubin Antioxidant and Pleiotropic Effects
  description: >-
    Unconjugated bilirubin is a potent lipid-soluble antioxidant that scavenges
    peroxyl radicals in vitro at physiologically relevant oxygen concentrations.
    Observational epidemiological studies have reported inverse associations
    between serum bilirubin and cardiovascular disease risk, leading to hypotheses
    of systemic protection in Gilbert's syndrome carriers. However, Mendelian
    randomization studies using the UGT1A1*28 locus as an instrument for
    genetically raised bilirubin find no causal cardiovascular benefit, suggesting
    the observational associations are confounded. The clinical significance of
    bilirubin's antioxidant capacity in vivo thus remains debated.
  evidence:
  - reference: PMID:3029864
    reference_title: "Bilirubin is an antioxidant of possible physiological importance."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "bilirubin, at micromolar concentrations in vitro, efficiently scavenges peroxyl radicals generated chemically in either homogeneous solution or multilamellar liposomes."
    explanation: >-
      Foundational Science paper demonstrating bilirubin's free radical
      scavenging capacity, providing the mechanistic basis for hypothesised
      antioxidant protection in Gilbert's syndrome.
  - reference: PMID:18343383
    reference_title: "Gilbert syndrome, UGT1A1*28 allele, and cardiovascular disease risk: possible protective effects and therapeutic applications of bilirubin."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Serum bilirubin has been shown to be inversely related to cardiovascular disease (CVD) in both retrospective and prospective studies."
    explanation: >-
      Observational data are consistent with CVD protection, but causality
      remains unproven; hence PARTIAL support only.
  - reference: PMID:37456363
    reference_title: "Effect of bilirubin and Gilbert syndrome on health: cohort analysis of observational, genetic, and Mendelian randomisation associations."
    supports: REFUTE
    evidence_source: HUMAN_CLINICAL
    snippet: "Evidence from the analyses of genetic data suggests that bilirubin has no likely causal role in protection from cardiovascular disease, chronic obstructive pulmonary disease, or other key healthcare outcomes and therefore represents a poor target for therapeutic intervention for these outcomes."
    explanation: >-
      Large-scale Mendelian randomization in UK Biobank (n=463,060) shows that
      genetically raised bilirubin via the Gilbert genotype confers no causal
      cardiovascular protection, refuting the antioxidant-protection hypothesis.
- name: Impaired Glucuronidation of Drugs and Drug Metabolites
  description: >-
    UGT1A1 is not a bilirubin-specific enzyme. The same reduced transcription
    that lowers bilirubin conjugation also lowers glucuronidation of xenobiotic
    substrates that UGT1A1 handles, most importantly SN-38, the active
    metabolite of irinotecan. Reduced SN-38 glucuronidation raises systemic
    SN-38 exposure and the risk of severe neutropenia and diarrhoea. The
    reciprocal interaction also occurs: drugs that inhibit UGT1A1 - notably the
    HIV protease inhibitors atazanavir and indinavir - unmask the reduced
    reserve capacity and precipitate overt hyperbilirubinemia, while the UGT1A1
    inducer efavirenz lowers bilirubin. This node is the pharmacogenomic arm of
    the same enzymatic lesion that produces the bilirubin phenotype, and it is
    the arm with the clearest clinical consequence.
  biological_scale: MOLECULAR
  genes:
  - preferred_term: UGT1A1
    term:
      id: hgnc:12530
      label: UGT1A1
  cell_types:
  - preferred_term: Hepatocyte
    term:
      id: CL:0000182
      label: hepatocyte
  molecular_functions:
  - preferred_term: glucuronosyltransferase activity
    modifier: DECREASED
    term:
      id: GO:0015020
      label: glucuronosyltransferase activity
  evidence:
  - reference: PMID:11990381
    reference_title: "UGT1A1*28 polymorphism as a determinant of irinotecan disposition and toxicity."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "screening for UGT1A1*28 polymorphism may identify patients with lower SN-38 glucuronidation rates and greater susceptibility to irinotecan induced gastrointestinal and bone marrow toxicity."
    explanation: >-
      Establishes that the UGT1A1*28 genotype reduces glucuronidation of a
      xenobiotic substrate (SN-38) as well as of bilirubin.
  - reference: PMID:16170755
    reference_title: "Gilbert syndrome and the development of antiretroviral therapy-associated hyperbilirubinemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Efavirenz resulted in decreased bilirubin levels, which is consistent with the induction of UDP-glucuronosyltransferase 1A1."
    explanation: >-
      Bidirectional pharmacological modulation of the same enzyme - induction
      lowers bilirubin - supports UGT1A1 capacity, rather than a fixed defect,
      as the quantity being varied.
  downstream:
  - target: Drug-Unmasked Hyperbilirubinemia
    description: >-
      Pharmacological inhibition of UGT1A1 subtracts from an already reduced
      conjugating reserve, so the same inhibitor produces a larger bilirubin
      rise in UGT1A1*28 homozygotes than in wild-type individuals.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:16170755
      reference_title: "Gilbert syndrome and the development of antiretroviral therapy-associated hyperbilirubinemia."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Atazanavir increased bilirubin levels by 15 mu mol/L (0.87 mg/dL), and indinavir increased bilirubin levels by 8 micromol/L (0.46 mg/dL). Ritonavir, lopinavir, saquinavir, and nelfinavir had no or minimal effect on bilirubin levels. Homozygous UGT1A1*28 increased bilirubin levels by 5.2 micromol/L (0.3 mg/dL)."
      explanation: >-
        Longitudinal modelling separates the drug effect from the genotype
        effect and shows both act on the same bilirubin level.
- name: Drug-Unmasked Hyperbilirubinemia
  description: >-
    In individuals with reduced UGT1A1 capacity, a UGT1A1-inhibiting drug and
    the UGT1A1*28 genotype combine supra-additively to push bilirubin into the
    clinically visible jaundice range. In the Swiss HIV Cohort, 67% of
    UGT1A1*28 homozygotes receiving atazanavir or indinavir had repeated
    episodes of bilirubin above 2.5 mg/dL, against 7% of those with neither
    risk factor. A meta-analysis of six studies puts the odds of atazanavir
    hyperbilirubinemia in UGT1A1*28 homozygotes at roughly ten-fold that of
    wild type. The hyperbilirubinemia is benign, but it drives avoidable
    treatment discontinuation and diagnostic workup for liver disease.
  biological_scale: ORGANISM
  chemical_entities:
  - preferred_term: unconjugated bilirubin
    term:
      id: CHEBI:16990
      label: bilirubin IXalpha
    modifier: INCREASED
  evidence:
  - reference: PMID:16170755
    reference_title: "Gilbert syndrome and the development of antiretroviral therapy-associated hyperbilirubinemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "67% of individuals homozygous for UGT1A1*28 and receiving atazanavir or indinavir had > or =2 episodes of hyperbilirubinemia in the jaundice range"
    explanation: >-
      Quantifies the gene-drug interaction against the clinically meaningful
      jaundice threshold rather than against a mean bilirubin shift.
  - reference: PMID:30962262
    reference_title: "Association between the UGT1A1*28 allele and hyperbilirubinemia in HIV-positive patients receiving atazanavir: a meta-analysis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "A significantly increased risk of hyperbilirubinemia was observed in HIV-positive patients receiving ATV with the UGT1A1*1/*28 or UGT1A1*28/*28 genotype, and the risk was higher with the UGT1A1*28/*28 genotype than with the UGT1A1*1/*28 genotype."
    explanation: >-
      Meta-analytic replication across six studies, with an allele-dose
      gradient consistent with the reduced-capacity mechanism.
  downstream:
  - target: Jaundice
    description: >-
      Drug-unmasked bilirubin elevations cross the visible jaundice threshold
      far more often than the untreated Gilbert phenotype does.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:16170755
      reference_title: "Gilbert syndrome and the development of antiretroviral therapy-associated hyperbilirubinemia."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "An understanding of the interaction between genetic predisposition and ART may help to identify individuals at highest risk for developing jaundice."
      explanation: >-
        The cohort frames the clinical endpoint of the interaction as overt
        jaundice.
- name: Chronic Biliary Bilirubin Load and Pigment Gallstone Formation
  description: >-
    Bilirubin that is not conjugated in the hepatocyte is not simply retained
    in plasma; the fraction that does reach bile carries an increased
    unconjugated load, which precipitates with calcium as pigment stone
    nidus. This is the one downstream consequence of the Gilbert genotype for
    which a causal, rather than merely associative, link to a clinical illness
    has been demonstrated. In a UK Biobank analysis of 138,125 participants
    with apparently healthy livers, cholelithiasis was the only illness out of
    54 assessed whose raised prevalence in Gilbert syndrome survived adjustment
    for confounders and was confirmed by Mendelian randomization. The effect is
    much larger where bilirubin production is also increased: in sickle cell
    anemia, the UGT1A1 promoter genotype raises gallstone frequency and brings
    forward age at onset.
  biological_scale: ORGANISM
  chemical_entities:
  - preferred_term: unconjugated bilirubin
    term:
      id: CHEBI:16990
      label: bilirubin IXalpha
    modifier: INCREASED
  evidence:
  - reference: PMID:37738404
    reference_title: "Clinical and genetic definition of serum bilirubin levels for the diagnosis of Gilbert syndrome and hypobilirubinemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "the prevalence of cholelithiasis in men with GS was assessed at 20% of European adults together with a 20% increase of the risk of complications, significantly more frequent than in men with normobilirubinemia, after adjusting for confounders. Furthermore, our TSMR validated the causality pathway"
    explanation: >-
      Gives the absolute prevalence and the complication excess in men with
      Gilbert syndrome after confounder adjustment, and states that two-sample
      Mendelian randomization validated the causal pathway.
  - reference: PMID:33523291
    reference_title: "Influence of UGT1A1 promoter polymorphism, alpha-thalassemia and beta(s) haplotype in bilirubin levels and cholelithiasis in a large sickle cell anemia cohort."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The mean of total and unconjugated bilirubin and the frequency of cholelithiasis in GS patients were higher when compared to those without this condition, regardless of age (P < 0.05). Cumulative analysis demonstrated an early age-at-onset for cholelithiasis in GS genotypes (P < 0.05)."
    explanation: >-
      A 913-patient sickle cell cohort shows the same genotype acting on
      gallstone risk and age at onset when bilirubin production is high,
      supporting a load-dependent rather than threshold mechanism.
  downstream:
  - target: Cholelithiasis
    description: >-
      Sustained unconjugated bilirubin load in bile is the proximate cause of
      calcium bilirubinate pigment stone formation.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:37738404
      reference_title: "Clinical and genetic definition of serum bilirubin levels for the diagnosis of Gilbert syndrome and hypobilirubinemia."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "We found a significant causal association between bilirubin centiles and cholelithiasis/gallstone illness."
      explanation: >-
        States the causal finding directly, in the paper's own words, for the
        bilirubin-to-gallstone edge.
phenotypes:
- name: Mild Unconjugated Hyperbilirubinemia
  category: Biochemical
  frequency: OBLIGATE
  description: >-
    Serum unconjugated (indirect) bilirubin is chronically or intermittently
    elevated, typically 1.2-3 mg/dL (20-50 micromol/L), but may transiently
    reach higher levels with fasting or stress. Conjugated (direct) bilirubin
    and liver enzymes (ALT, AST, ALP) remain normal.
  phenotype_term:
    preferred_term: Unconjugated hyperbilirubinemia
    term:
      id: HP:0008282
      label: Unconjugated hyperbilirubinemia
  evidence:
  - reference: PMID:7565971
    reference_title: "The genetic basis of the reduced expression of bilirubin UDP-glucuronosyltransferase 1 in Gilbert's syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "People with Gilbert's syndrome have mild, chronic unconjugated hyperbilirubinemia in the absence of liver disease or overt hemolysis."
    explanation: >-
      Classic description from the landmark NEJM paper establishing
      unconjugated hyperbilirubinemia as the obligate phenotype.
- name: Jaundice
  category: Clinical
  frequency: FREQUENT
  description: >-
    Visible yellowing of the sclerae and skin due to bilirubin deposition,
    typically intermittent and mild. Often first noticed during adolescence
    and may be triggered by fasting, illness, or stress.
  phenotype_term:
    preferred_term: Jaundice
    term:
      id: HP:0000952
      label: Jaundice
  evidence:
  - reference: PMID:8596320
    reference_title: "Genetic variation in bilirubin UPD-glucuronosyltransferase gene promoter and Gilbert's syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "This common mild hyperbilirubinaemia sometimes presents as an intermittent jaundice."
    explanation: >-
      Directly establishes jaundice as a clinical presentation of the
      unconjugated hyperbilirubinaemia in Gilbert's syndrome.
  - reference: PMID:8596320
    reference_title: "Genetic variation in bilirubin UPD-glucuronosyltransferase gene promoter and Gilbert's syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Individuals in the population with the 7/7 genotype had significantly higher bilirubin concentrations than those who had the 6/7 or 6/6 genotype."
    explanation: >-
      Demonstrates that homozygous UGT1A1*28 (7/7) carriers have the highest
      bilirubin levels, making them most susceptible to intermittent jaundice.
- name: Jaundice-Associated Gastrointestinal Symptoms
  category: Clinical
  frequency: FREQUENT
  description: >-
    In a longitudinal adult cohort, jaundice episodes were often associated
    with abdominal pain, dyspepsia, or loss of appetite. The causal relationship
    between bilirubin elevation and these symptoms remains incompletely
    resolved, and many affected individuals are otherwise asymptomatic.
  phenotype_term:
    preferred_term: Abdominal pain with dyspepsia or appetite loss
    term:
      id: HP:0002027
      label: Abdominal pain
  evidence:
  - reference: PMID:30717703
    reference_title: "The frequency, clinical course, and health related quality of life in adults with Gilbert's syndrome: a longitudinal study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "jaundice was associated with abdominal pain, dyspepsia or loss of appetite in 54 (53.465%) subjects."
    explanation: >-
      This longitudinal study directly supports abdominal pain, dyspepsia, or
      appetite loss during jaundice episodes and provides a frequency compatible
      with the FREQUENT band.
- name: Cholelithiasis
  category: Clinical
  frequency: OCCASIONAL
  description: >-
    Pigment (calcium bilirubinate) gallstones. This is the only illness for
    which raised prevalence in Gilbert syndrome has survived adjustment for
    confounders and been confirmed causal by Mendelian randomization, and the
    effect was demonstrated in men. Risk rises steeply when bilirubin
    production is also increased, as in sickle cell anemia or hereditary
    spherocytosis, where the Gilbert genotype both raises gallstone frequency
    and lowers age at onset. The frequency band here reflects the excess over
    background in an unselected Gilbert population, not the much higher rate
    in co-inherited hemolytic disease.
  phenotype_term:
    preferred_term: Cholelithiasis
    term:
      id: HP:0001081
      label: Cholelithiasis
  evidence:
  - reference: PMID:37738404
    reference_title: "Clinical and genetic definition of serum bilirubin levels for the diagnosis of Gilbert syndrome and hypobilirubinemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Cholelithiasis in men and jaundice of unknown cause in women (Fig. 1C) were the only 2 conditions out of the 10 detailed biliary and digestive illnesses that were more frequent in GS than normobilirubinemia."
    explanation: >-
      Identifies cholelithiasis as a genuine excess-risk phenotype in Gilbert
      syndrome within a prospectively assessed biliary/digestive illness panel.
  - reference: PMID:33523291
    reference_title: "Influence of UGT1A1 promoter polymorphism, alpha-thalassemia and beta(s) haplotype in bilirubin levels and cholelithiasis in a large sickle cell anemia cohort."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "These findings confirm in a large cohort that the UGT1A1 polymorphism influences cholelithiasis and hyperbilirubinemia in SCA."
    explanation: >-
      Independent confirmation in a co-inherited hemolytic setting, where the
      same genotype acts on the same endpoint with a larger effect.
- name: Fatigue
  category: Clinical
  frequency: OCCASIONAL
  description: >-
    Non-specific fatigue, recorded in 1,401 of 9,240 Gilbert syndrome cases
    (15%) against 12,581 of 150,846 controls (8%) in UK primary care. Long
    treated as a patient-reported complaint without an established relationship
    to bilirubin, it was one of only two features (with abdominal pain) that
    the same analysis found consistently more common up to five years before
    diagnosis, rather than only in the year of workup. That temporal pattern
    argues against it being purely an artefact of being investigated. The
    recorded rate differs by sex - 22% of female cases against 12% of male
    cases - so the single band here averages over a real difference. Whether
    bilirubin causes it remains open - see discussions#gs_symptom_attribution.
  phenotype_term:
    preferred_term: Fatigue
    term:
      id: HP:0012378
      label: Fatigue
  evidence:
  - reference: PMID:40904555
    reference_title: "Prevalence and Data-Driven Exploration of Pre-Diagnostic Symptoms and Features of Gilbert's Syndrome in the UK Primary Care Population."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "While most of these features appeared primarily in the year prior to diagnosis, only abdominal pain and fatigue were consistently more common in GS cases up to five years before diagnosis."
    explanation: >-
      Separates persistent symptoms from short-term diagnostic triggers in a
      nested case-control design, which is what makes fatigue a candidate
      symptom rather than an ascertainment artefact.
  - reference: PMID:40904555
    reference_title: "Prevalence and Data-Driven Exploration of Pre-Diagnostic Symptoms and Features of Gilbert's Syndrome in the UK Primary Care Population."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Fatigue724 (12%)5798 (6%)677 (22%)6780 (13%)1401 (15%)12,581 (8%)< 0.00001"
    explanation: >-
      The source table row giving the recorded rates that set the frequency
      band: 1,401/9,240 cases (15%) against 12,581/150,846 controls (8%), which
      is OCCASIONAL rather than FREQUENT. Column order is male cases, male
      controls, female cases, female controls, all cases, all controls; the
      table is extracted without cell separators in the cached text.
  - reference: PMID:40904555
    reference_title: "Prevalence and Data-Driven Exploration of Pre-Diagnostic Symptoms and Features of Gilbert's Syndrome in the UK Primary Care Population."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "the persistent presence of fatigue and abdominal pain suggests they may be under-recognised symptoms of GS. These findings warrant further investigation"
    explanation: >-
      The authors' own hedge. Graded PARTIAL because the study establishes
      association and temporal persistence, not causation.
biochemical:
- name: Unconjugated Bilirubin
  presence: INCREASED
  context: >-
    Serum unconjugated fraction elevated; conjugated bilirubin and liver
    enzymes (ALT, AST, ALP) remain within normal limits. Typically 1.2-3
    mg/dL in Gilbert's syndrome.
  biomarker_term:
    preferred_term: unconjugated bilirubin
    term:
      id: CHEBI:16990
      label: bilirubin IXalpha
  reference_ranges:
  - lower_bound: 5.1
    upper_bound: 17.1
    unit: umol/L
    population: adults, unisex laboratory reference interval
    notes: >-
      The unisex colorimetric-assay interval used by the UK Biobank and by most
      clinical laboratories. Its upper limit is the historical Gilbert syndrome
      cutoff. It is recorded because it is what results are reported against,
      not because it is appropriate: the same source shows the distribution
      differs by sex, age, and UGT1A1 genotype, so a single unisex limit
      underdiagnoses women.
    evidence:
    - reference: PMID:37738404
      reference_title: "Clinical and genetic definition of serum bilirubin levels for the diagnosis of Gilbert syndrome and hypobilirubinemia."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Bilirubin was measured by colorimetric assay with a unisex reference interval of 5.1-17.1 µmol/L."
      explanation: >-
        States the laboratory reference interval and the assay it belongs to.
    interpretation_bands:
    - name: Hypobilirubinemia
      upper_bound: 5.1
      unit: umol/L
      abnormal_flag: LOW
      interpretation: >-
        Below the unisex reference interval. Sex-adjusted lower limits differ
        substantially (4.7-9.2 umol/L in women, 5.6-12.1 umol/L in men), so
        this band is not reliable without stratification.
    - name: Normobilirubinemia
      lower_bound: 5.1
      upper_bound: 17.1
      unit: umol/L
      abnormal_flag: NORMAL
    - name: Hyperbilirubinemia (historical Gilbert threshold)
      lower_bound: 17.1
      unit: umol/L
      abnormal_flag: HIGH
      severity: MILD
      phenotype_term:
        preferred_term: Unconjugated hyperbilirubinemia
        term:
          id: HP:0008282
          label: Unconjugated hyperbilirubinemia
      interpretation: >-
        Meets the historical unisex Gilbert syndrome cutoff. Under stratified
        centiles the equivalent threshold ranges from 9.0 to 26.2 umol/L
        depending on sex, age, and UGT1A1 genotype, so this band both
        over-calls in some strata and under-calls in others.
  readouts:
  - target: Reduced UGT1A1 Activity and Impaired Bilirubin Glucuronidation
    relationship: READOUT_OF
    direction: POSITIVE
    endpoint_context: DIAGNOSTIC
    interpretation: >-
      Elevated unconjugated bilirubin reports the reduced hepatic bilirubin
      glucuronidation that defines Gilbert's syndrome.
    evidence:
    - reference: PMID:7565971
      reference_title: "The genetic basis of the reduced expression of bilirubin UDP-glucuronosyltransferase 1 in Gilbert's syndrome."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        People with Gilbert's syndrome have mild, chronic unconjugated
        hyperbilirubinemia in the absence of liver disease or overt hemolysis.
        Hepatic glucuronidating activity, essential for efficient biliary
        excretion of bilirubin, is reduced to about 30 percent of normal.
      explanation: >-
        Human clinical evidence ties the unconjugated bilirubin readout to the
        reduced hepatic glucuronidation mechanism.
  - target: Fasting- and Stress-Induced Bilirubin Elevation
    relationship: READOUT_OF
    direction: POSITIVE
    endpoint_context: MONITORING
    interpretation: >-
      Serial unconjugated bilirubin captures trigger-sensitive bilirubin
      elevations during fasting, illness, pregnancy, surgery, or other stressors.
    evidence:
    - reference: PMID:8596320
      reference_title: "Genetic variation in bilirubin UPD-glucuronosyltransferase gene promoter and Gilbert's syndrome."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        Individuals in the population with the 7/7 genotype had significantly
        higher bilirubin concentrations than those who had the 6/7 or 6/6
        genotype. 14 volunteers underwent a 24 h fasting test to see if they
        had Gilbert's syndrome, and all four positives had the 7/7 genotype.
      explanation: >-
        Human fasting-provocation evidence supports bilirubin as a monitoring
        readout of trigger-sensitive Gilbert physiology.
  evidence:
  - reference: PMID:7565971
    reference_title: "The genetic basis of the reduced expression of bilirubin UDP-glucuronosyltransferase 1 in Gilbert's syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "People with Gilbert's syndrome have mild, chronic unconjugated hyperbilirubinemia in the absence of liver disease or overt hemolysis."
    explanation: >-
      Confirms selectively elevated unconjugated bilirubin as the hallmark
      biochemical finding, with normal conjugated fraction and liver function.
  - reference: PMID:26250421
    reference_title: "Genotype of UGT1A1 and phenotype correlation between Crigler-Najjar syndrome type II and Gilbert syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Serum bilirubin concentrations of typical CN-2, intermediate group, and typical GS are respectively 12.9 ± 5.1, 5.2 ± 2.2, and 2.8 ± 1.1 mg/dL."
    explanation: >-
      Provides quantitative bilirubin values for typical Gilbert's syndrome
      (2.8 ± 1.1 mg/dL), confirming mild elevation as the expected degree.
diagnosis:
- name: Diagnosis of exclusion on isolated unconjugated hyperbilirubinemia
  description: >-
    Gilbert syndrome is diagnosed by exclusion, not by a positive test. The
    picture is isolated unconjugated hyperbilirubinemia - predominantly
    unconjugated, typically under 3 mg/dL - with normal ALT, AST, ALP and GGT,
    normal haemoglobin, reticulocytes, haptoglobin and blood film, and normal
    hepatic imaging. Both haemolysis and liver damage must be excluded, which
    is part of the accepted definition rather than an optional workup. Most
    diagnoses are made incidentally on blood tests taken for something else.
  evidence:
  - reference: PMID:37738404
    reference_title: "Clinical and genetic definition of serum bilirubin levels for the diagnosis of Gilbert syndrome and hypobilirubinemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "There is a consensual definition that GS in individuals is determined by the presence of hyperbilirubinemia in the absence of both hemolysis and liver damage, including the fibrosis stage."
    explanation: >-
      States the exclusion-based definition, including that fibrosis must be
      excluded and that normal ALT and GGT alone are not sufficient to do so.
  - reference: PMID:40904555
    reference_title: "Prevalence and Data-Driven Exploration of Pre-Diagnostic Symptoms and Features of Gilbert's Syndrome in the UK Primary Care Population."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "GS is typically diagnosed by exclusion through blood tests that detect elevated levels of unconjugated bilirubin, without haemolysis or structural liver damage."
    explanation: >-
      Independent statement of the same diagnostic route, from the study that
      also documents how often it happens incidentally.
- name: Fasting provocation test
  description: >-
    A 24-48 hour caloric restriction raises unconjugated bilirubin
    disproportionately in Gilbert syndrome, and was historically used to
    provoke a diagnostic rise. It is largely superseded by UGT1A1 genotyping,
    which is not provocative and not affected by intercurrent illness. Recorded
    because the test is the clinical expression of the fasting-sensitivity
    mechanism this entry models, and because the published diagnostic
    thresholds derive from cohorts characterised this way.
  evidence:
  - reference: PMID:8596320
    reference_title: "Genetic variation in bilirubin UPD-glucuronosyltransferase gene promoter and Gilbert's syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "14 volunteers underwent a 24 h fasting test to see if they had Gilbert's syndrome, and all four positives had the 7/7 genotype."
    explanation: >-
      Reports the test being used diagnostically and its concordance with the
      genotype in the same volunteers.
- name: UGT1A1 promoter (TA)n genotyping
  diagnosis_term:
    preferred_term: genetic testing
    term:
      id: NCIT:C15709
      label: Genetic Testing
  description: >-
    Sizing the TATA-box TA repeat, with coding-region sequencing where an East
    Asian ancestry or a discordant result suggests UGT1A1*6. Genotyping
    confirms rather than establishes the diagnosis: roughly 10-13% of Western
    populations are UGT1A1*28 homozygotes while clinically recognised Gilbert
    syndrome is far less common, so a positive genotype in the absence of
    hyperbilirubinemia does not make the diagnosis. An assay that scores only
    (TA)6 against (TA)7 will misclassify carriers of the (TA)5 and (TA)8
    alleles found in African populations.
  evidence:
  - reference: PMID:34089128
    reference_title: "TaqMan real time PCR for the Detection of the Gilbert's Syndrome Markers UGT1A1*28; UGT1A1*36 and UGT1A1*37."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We present a real time PCR method for genotyping the UGT1A1 (TA)n polymorphism (UGT1A1*28, UGT1A1*36, UGT1A1*37) using Taqman PCR."
    explanation: >-
      Describes the genotyping assay and, by covering three alleles rather than
      two, makes the misclassification point concrete.
  - reference: PMID:7565971
    reference_title: "The genetic basis of the reduced expression of bilirubin UDP-glucuronosyltransferase 1 in Gilbert's syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Reduced expression of bilirubin UDP-glucuronosyltransferase 1 due to an abnormality in the promoter region of the gene for this enzyme appears to be necessary for Gilbert's syndrome but not sufficient for the complete manifestation of the syndrome."
    explanation: >-
      The necessary-but-not-sufficient result is why genotyping confirms rather
      than establishes the diagnosis. PARTIAL because it constrains the test's
      interpretation rather than supporting the test itself.
genetic:
- name: UGT1A1*28 Promoter Polymorphism
  gene_term:
    preferred_term: UGT1A1
    term:
      id: hgnc:12530
      label: UGT1A1
  association: Causative
  inheritance:
  - name: Autosomal recessive
    inheritance_term:
      preferred_term: Autosomal recessive inheritance
      term:
        id: HP:0000007
        label: Autosomal recessive inheritance
    evidence:
    - reference: PMID:8596320
      reference_title: "Genetic variation in bilirubin UPD-glucuronosyltransferase gene promoter and Gilbert's syndrome."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Segregation of the 7/7 genotype with the Gilbert phenotype was also demonstrated in the family with four affected members."
      explanation: >-
        Demonstrates autosomal recessive segregation of UGT1A1*28 homozygosity
        with the Gilbert's syndrome phenotype in a Scottish family.
  notes: >-
    The UGT1A1*28 allele contains a (TA)7TAA repeat in the TATAA element of
    the UGT1A1 promoter, compared to the normal (TA)6TAA. Homozygosity for
    UGT1A1*28 is found in approximately 10-13% of Western populations and
    accounts for the majority of Gilbert's syndrome cases in those populations.
    The extra TA repeat reduces promoter transcription, lowering UGT1A1 activity
    to approximately 30% of normal. UGT1A1*28 is also the pharmacogenomic
    variant most relevant to irinotecan toxicity.
  evidence:
  - reference: PMID:7565971
    reference_title: "The genetic basis of the reduced expression of bilirubin UDP-glucuronosyltransferase 1 in Gilbert's syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Reduced expression of bilirubin UDP-glucuronosyltransferase 1 due to an abnormality in the promoter region of the gene for this enzyme appears to be necessary for Gilbert's syndrome but not sufficient for the complete manifestation of the syndrome."
    explanation: >-
      Bosma 1995 established the (TA)7 promoter polymorphism as required but
      not sufficient for full phenotypic expression, explaining incomplete
      penetrance in heterozygotes.
  - reference: PMID:8596320
    reference_title: "Genetic variation in bilirubin UPD-glucuronosyltransferase gene promoter and Gilbert's syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The frequency of the 7/7 genotype in this eastern Scottish population was 10-13%."
    explanation: >-
      Provides population frequency of UGT1A1*28 homozygosity in a Western
      European population, consistent with Gilbert's syndrome prevalence.
  - reference: PMID:9435989
    reference_title: "Genetic defects of the UDP-glucuronosyltransferase-1 (UGT1) gene that cause familial non-haemolytic unconjugated hyperbilirubinaemias."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Gilbert's syndrome, the most prevalent (2-19% in population studies) and mildest of the three syndromes is principally caused by a TA insertion at the TATA promoter region upstream of the UGT1A1 exon."
    explanation: >-
      Clarke 1997 places UGT1A1*28 in the context of the full spectrum of UGT1
      defects and confirms its role as the principal cause of Gilbert's syndrome.
- name: UGT1A1*6 Missense Variant (Gly71Arg)
  gene_term:
    preferred_term: UGT1A1
    term:
      id: hgnc:12530
      label: UGT1A1
  association: Causative
  inheritance:
  - name: Autosomal recessive
    inheritance_term:
      preferred_term: Autosomal recessive inheritance
      term:
        id: HP:0000007
        label: Autosomal recessive inheritance
    evidence:
    - reference: PMID:26250421
      reference_title: "Genotype of UGT1A1 and phenotype correlation between Crigler-Najjar syndrome type II and Gilbert syndrome."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Most patients had biallelic mutations of UGT1A1."
      explanation: >-
        Maruo 2016 demonstrated that Japanese patients with unconjugated
        hyperbilirubinemia typically carry biallelic UGT1A1 mutations,
        consistent with autosomal recessive inheritance.
  notes: >-
    The UGT1A1*6 allele (c.211G>A, p.Gly71Arg) is a coding variant that
    predominates as a cause of Gilbert's syndrome in East Asian (particularly
    Japanese) populations. A G to A transition at nucleotide 211 substitutes
    arginine for glycine at codon 71. In Japanese Gilbert's syndrome patients,
    homozygous G71R (9%), TA7/6 with heterozygous G71R (17%), and compound
    heterozygous UGT1A1*6/*28 genotypes are common, reflecting a different
    ethnic genetic architecture compared to Western populations. UGT1A1*6 is
    also clinically relevant to irinotecan toxicity in Asian patients.
  evidence:
  - reference: PMID:10353933
    reference_title: "Association of neonatal hyperbilirubinemia with bilirubin UDP-glucuronosyltransferase polymorphism."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "a G-->A transition at nucleotide 211 caused arginine to replace glycine at position 71 of corresponding protein product (G71R)."
    explanation: >-
      Maruo 1999 identified the G71R mutation and showed it is associated
      with hyperbilirubinemia in Japanese neonates, establishing UGT1A1*6
      as a relevant variant in East Asian populations.
  - reference: PMID:15304120
    reference_title: "Genetic polymorphisms of bilirubin uridine diphosphate-glucuronosyltransferase gene in Japanese patients with Crigler-Najjar syndrome or Gilbert's syndrome as well as in healthy Japanese subjects."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The genetic basis of hyperbilirubinemia appears to be different between the Japanese and Caucasian populations."
    explanation: >-
      Takeuchi 2004 confirmed the ethnic divergence in UGT1A1 variant spectrum,
      with coding variants like G71R being more prevalent in Japanese patients
      versus the promoter TA-repeat polymorphism in Caucasians.
- name: UGT1A1*37 (TA)8 Promoter Allele
  gene_term:
    preferred_term: UGT1A1
    term:
      id: hgnc:12530
      label: UGT1A1
  association: Causative
  relationship_type: CAUSATIVE
  notes: >-
    An eighth TA repeat reduces glucuronidation further than the (TA)7
    UGT1A1*28 allele does, so the promoter is a graded series rather than a
    two-allele switch. UGT1A1*37 occurs mainly in African populations, at
    frequencies up to 0.07, and an assay that scores only (TA)6 against (TA)7
    will not see it. This matters for irinotecan dosing as well as for
    diagnosis: the DPWG guideline lists UGT1A1*37 alongside *28 and *6 as a
    deficiency allele. Curated as CAUSATIVE rather than MODIFIER because it
    acts through the same reduced-transcription mechanism as UGT1A1*28, more
    strongly, rather than modifying a phenotype another variant drives.
  evidence:
  - reference: PMID:34089128
    reference_title: "TaqMan real time PCR for the Detection of the Gilbert's Syndrome Markers UGT1A1*28; UGT1A1*36 and UGT1A1*37."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "An extra TA insert leads to eight (TA)8 repeats (UGT1A1*37) resulting in a further reduction of glucuronidation activity."
    explanation: >-
      Establishes that the eighth repeat lowers activity further than the
      seventh, which is what makes UGT1A1*37 a deficiency allele in its own
      right rather than a variant of UGT1A1*28.
  - reference: PMID:22407023
    reference_title: "Study of a family in the province of Matera presenting with glucose-6-phosphate dehydrogenase deficiency and Gilbert's syndrome."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "there are a number of thymine-adenine (TA) repeats varying from 5 to 8, with an inverse correlation between promoter activity and number of TA repetitions: A(TA) 8TAA promoter with reduced activity, A(TA) 7TAA promoter with reduced activity (UGT1A1 *28), A(TA)6TAA wild-type and A(TA)5TAA promoter with increased activity."
    explanation: >-
      States the direction of effect for every allele in the series, which is
      what the record's typing rests on: activity falls as repeat number rises,
      so (TA)5 sits above wild-type and (TA)8 below (TA)7. Graded OTHER because
      this is background review prose in the paper's introduction rather than a
      result of the family study it reports.
  - reference: PMID:36443464
    reference_title: "Dutch pharmacogenetics working group (DPWG) guideline for the gene-drug interaction between UGT1A1 and irinotecan."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Gene variants leading to UGT1A1 enzyme deficiency (e.g. UGT1A1*6, *28 and *37) can be used to optimize an individual's starting dose thereby preventing carriers from toxicity."
    explanation: >-
      A pharmacogenomics guideline treating UGT1A1*37 as a deficiency allele
      alongside *28 and *6, which is why the series is curated rather than
      noted in passing.
- name: UGT1A1*36 (TA)5 Promoter Allele
  gene_term:
    preferred_term: UGT1A1
    term:
      id: hgnc:12530
      label: UGT1A1
  association: Protective
  relationship_type: PROTECTIVE
  notes: >-
    The opposite end of the same promoter series: one fewer TA repeat raises
    UGT1A1 activity above wild-type, so this allele lowers bilirubin rather
    than raising it and works against the Gilbert phenotype. It is curated
    separately from UGT1A1*37 because the two point in opposite directions and
    a single record cannot carry both. Like *37 it occurs mainly in African
    populations, at frequencies up to 0.08, so a (TA)6-versus-(TA)7 assay
    misclassifies African-ancestry individuals in both directions - calling
    some higher-activity carriers wild-type and missing the lower-activity
    ones entirely.
  evidence:
  - reference: PMID:34089128
    reference_title: "TaqMan real time PCR for the Detection of the Gilbert's Syndrome Markers UGT1A1*28; UGT1A1*36 and UGT1A1*37."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "A variant lacking one TA repeat (TA)5 (UGT1A1*36) has been identified. (TA)8 repeats (UGT1A1*37) and (TA)5 (UGT1A1*36) have been detected in Africans (frequency up to 0.07 and 0.08 respectively)."
    explanation: >-
      Identifies the (TA)5 allele and its African frequency. The same sentence
      gives the *37 frequency, which is why both records cite this source.
  - reference: PMID:22407023
    reference_title: "Study of a family in the province of Matera presenting with glucose-6-phosphate dehydrogenase deficiency and Gilbert's syndrome."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "there are a number of thymine-adenine (TA) repeats varying from 5 to 8, with an inverse correlation between promoter activity and number of TA repetitions: A(TA) 8TAA promoter with reduced activity, A(TA) 7TAA promoter with reduced activity (UGT1A1 *28), A(TA)6TAA wild-type and A(TA)5TAA promoter with increased activity."
    explanation: >-
      States the direction of effect for every allele in the series, which is
      what the record's typing rests on: activity falls as repeat number rises,
      so (TA)5 sits above wild-type and (TA)8 below (TA)7. Graded OTHER because
      this is background review prose in the paper's introduction rather than a
      result of the family study it reports.
- name: G6PD
  gene_term:
    preferred_term: G6PD
    term:
      id: hgnc:4057
      label: G6PD
  association: Modifier
  relationship_type: MODIFIER
  notes: >-
    G6PD deficiency raises bilirubin production through haemolysis while the
    Gilbert genotype lowers clearance, so the two act on opposite sides of the
    same balance and co-inheritance produces hyperbilirubinemia neither
    explains alone. This is the combination that matters in the neonatal
    period, where the conjugating system is immature and bilirubin
    neurotoxicity is possible - the one setting where Gilbert syndrome is not
    benign. The size of the interaction is not established; see
    discussions#gs_neonatal_cofactor_risk.
  evidence:
  - reference: PMID:22407023
    reference_title: "Study of a family in the province of Matera presenting with glucose-6-phosphate dehydrogenase deficiency and Gilbert's syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The results showed that in certain cases, the presence of hyperbilirubinemia is not only associated with G6PD deficiency, but may be caused by the co-presence of a mutation in the UGTA1 promoter related to Gilbert's syndrome."
    explanation: >-
      A four-member family study attributing hyperbilirubinemia to the
      co-presence of both defects rather than to either alone. Small, so it
      establishes the interaction exists rather than its magnitude.
  - reference: PMID:37508669
    reference_title: "The Genetics of Glucose-6-Phosphate-Dehydrogenase (G6PD) and Uridine Diphosphate Glucuronosyl Transferase 1A1 (UGT1A1) Promoter Gene Polymorphism in Relation to Quantitative Biochemical G6PD Activity Measurement and Neonatal Hyperbilirubinemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Heterozygosity to 6/7 TA repeats in the UGT1A1 promoter was associated with increased NHB, especially in female newborns with G6PD deficiency."
    explanation: >-
      Neonatal series supporting the interaction, graded PARTIAL because the
      same paper states the interaction is more complex than a two-locus
      model. Note the cached abstract text drops the italicised gene symbol
      before "promoter"; the quote is exact against the cache.
treatments:
- name: Reassurance and Lifestyle Guidance
  description: >-
    Gilbert's syndrome is benign and requires no specific pharmacological
    treatment. Management consists of reassurance, avoidance of prolonged
    fasting, adequate hydration, and recognition of triggers (e.g., illness,
    stress, strenuous exercise). Patients should be counselled to inform
    healthcare providers of their UGT1A1 genotype, as this affects metabolism
    of certain drugs (e.g., irinotecan).
  treatment_term:
    preferred_term: supportive care
    term:
      id: NCIT:C15747
      label: Supportive Care
  evidence:
  - reference: PMID:37390966
    reference_title: "Gilbert's syndrome revisited."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Gilbert's syndrome, also known as benign hyperbilirubinaemia, was described more than 100 years ago."
    explanation: >-
      The Vitek & Tiribelli 2023 review confirms the longstanding recognition
      of Gilbert's syndrome as a benign condition, supporting reassurance
      as the primary therapeutic approach.
  - reference: PMID:30717703
    reference_title: "The frequency, clinical course, and health related quality of life in adults with Gilbert's syndrome: a longitudinal study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Screening, counseling, monitoring and individualized health care are recommended for subjects with GS in the setting of anesthesia, pregnancy, treatment with DAAs, deliveries, surgery and weight loss plans."
    explanation: >-
      Human longitudinal data support counseling, monitoring, and individualized
      guidance around common jaundice-triggering settings.
  target_mechanisms:
  - target: Fasting- and Stress-Induced Bilirubin Elevation
    treatment_effect: MODULATES
    description: >-
      Avoiding prolonged fasting and planning around physiologic stressors
      reduces exposure to settings that provoke clinical jaundice in Gilbert's
      syndrome.
    evidence:
    - reference: PMID:30717703
      reference_title: "The frequency, clinical course, and health related quality of life in adults with Gilbert's syndrome: a longitudinal study."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Risk factors for clinical jaundice included general anesthesia, pregnancy, fasting"
      explanation: >-
        Human cohort evidence identifies fasting and other stressors as
        risk factors, supporting lifestyle guidance as a mechanism-directed
        management step.
- name: UGT1A1 Genotyping Prior to Irinotecan Therapy
  description: >-
    Carriers of UGT1A1*28 (and UGT1A1*6 in Asian patients) have significantly
    reduced capacity to glucuronidate SN-38, the active metabolite of irinotecan.
    This leads to increased SN-38 exposure and greater susceptibility to
    irinotecan-induced severe neutropenia and diarrhoea. Genetic testing before
    initiating irinotecan is classified as 'essential' by international
    pharmacogenomics guidelines. Poor metabolisers (homozygous or compound
    heterozygous) should receive a 70% starting dose.
  treatment_term:
    preferred_term: genetic testing
    term:
      id: NCIT:C15709
      label: Genetic Testing
  evidence:
  - reference: PMID:11990381
    reference_title: "UGT1A1*28 polymorphism as a determinant of irinotecan disposition and toxicity."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "screening for UGT1A1*28 polymorphism may identify patients with lower SN-38 glucuronidation rates and greater susceptibility to irinotecan induced gastrointestinal and bone marrow toxicity."
    explanation: >-
      Iyer 2002 prospective pharmacogenetic study demonstrated that UGT1A1*28
      genotype predicts reduced SN-38 glucuronidation and severe irinotecan
      toxicity, establishing the clinical rationale for pre-treatment testing.
  - reference: PMID:36443464
    reference_title: "Dutch pharmacogenetics working group (DPWG) guideline for the gene-drug interaction between UGT1A1 and irinotecan."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Gene variants leading to UGT1A1 enzyme deficiency (e.g. UGT1A1*6, *28 and *37) can be used to optimize an individual's starting dose thereby preventing carriers from toxicity."
    explanation: >-
      The DPWG 2023 guideline endorses UGT1A1 variant testing to optimize
      irinotecan starting dose and prevent severe toxicity.
  - reference: PMID:36443464
    reference_title: "Dutch pharmacogenetics working group (DPWG) guideline for the gene-drug interaction between UGT1A1 and irinotecan."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: 'UGT1A1 genotyping is considered "essential", indicating that UGT1A1 testing must be performed prior to initiating irinotecan treatment.'
    explanation: >-
      DPWG classification of UGT1A1 testing as 'essential' elevates this to
      a mandatory pre-treatment test, not an optional pharmacogenomics screen.
  target_mechanisms:
  - target: Impaired Glucuronidation of Drugs and Drug Metabolites
    treatment_effect: MODULATES
    description: >-
      Genotyping does not change UGT1A1 activity. It acts on the mechanism by
      matching the administered dose to the measured conjugating capacity, so
      that SN-38 exposure stays within what the individual's enzyme can clear.
      Poor metabolisers receive a reduced starting dose.
    evidence:
    - reference: PMID:36443464
      reference_title: "Dutch pharmacogenetics working group (DPWG) guideline for the gene-drug interaction between UGT1A1 and irinotecan."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "Gene variants leading to UGT1A1 enzyme deficiency (e.g. UGT1A1*6, *28 and *37) can be used to optimize an individual's starting dose thereby preventing carriers from toxicity."
      explanation: >-
        States the dose-to-capacity matching that is the mechanism-directed
        action of the test.
  - target: Drug-Unmasked Hyperbilirubinemia
    treatment_effect: MODULATES
    description: >-
      Knowing the genotype before starting a UGT1A1-inhibiting drug allows the
      predictable bilirubin rise to be anticipated rather than investigated as
      new liver disease, and allows an alternative agent to be chosen where one
      exists. MODULATES rather than INHIBITS because a test does not act on the
      node: the effect depends entirely on what the prescriber then does.
    evidence:
    - reference: PMID:16170755
      reference_title: "Gilbert syndrome and the development of antiretroviral therapy-associated hyperbilirubinemia."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Genotyping for UGT1A1*28 before initiation of ART would identify HIV-infected individuals at risk for hyperbilirubinemia and decrease episodes of jaundice."
      explanation: >-
        The cohort's own conclusion proposes pre-treatment genotyping as the
        intervention on this node.
- name: Phenobarbital Enzyme Induction
  description: >-
    Phenobarbital induces UGT1A1 transcription and lowers unconjugated
    bilirubin. It is not indicated in Gilbert syndrome, which needs no
    bilirubin-lowering treatment, and is recorded here for two reasons: the
    bilirubin fall on phenobarbital is a classical discriminator against
    Crigler-Najjar type I, which does not respond; and it is the clearest
    demonstration that the Gilbert lesion is reduced transcription of an
    inducible gene rather than a fixed absence of enzyme. Sedation makes it
    unsuitable as a cosmetic treatment for a benign condition.
  therapeutic_modality: SMALL_MOLECULE
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: phenobarbital
      term:
        id: CHEBI:8069
        label: phenobarbital
  evidence:
  - reference: PMID:9435989
    reference_title: "Genetic defects of the UDP-glucuronosyltransferase-1 (UGT1) gene that cause familial non-haemolytic unconjugated hyperbilirubinaemias."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Crigler-Najjar type II syndrome which is treatable with phenobarbital therapy is associated with less dramatic missense mutations or heterozygous expression of mutant and normal alleles."
    explanation: >-
      Sources the phenobarbital response and ties it to residual enzyme:
      Crigler-Najjar type II, which retains some activity, responds, which is
      what makes the response a discriminator against the type I null.
  target_mechanisms:
  - target: Reduced UGT1A1 Activity and Impaired Bilirubin Glucuronidation
    treatment_effect: MODULATES
    description: >-
      Transcriptional induction raises UGT1A1 expression from the reduced
      baseline set by the promoter allele, which is possible precisely because
      the allele lowers transcription rather than abolishing the enzyme.
    evidence:
    - reference: PMID:16170755
      reference_title: "Gilbert syndrome and the development of antiretroviral therapy-associated hyperbilirubinemia."
      supports: SUPPORT
      directness: INDIRECT
      evidence_source: HUMAN_CLINICAL
      snippet: "Efavirenz resulted in decreased bilirubin levels, which is consistent with the induction of UDP-glucuronosyltransferase 1A1."
      explanation: >-
        Graded INDIRECT because the inducer observed here is efavirenz, not
        phenobarbital. It establishes that pharmacological induction of UGT1A1
        lowers bilirubin in this population, which is the mechanism claimed;
        it is not direct evidence for phenobarbital.
inheritance:
- name: Autosomal recessive inheritance with incomplete penetrance
  inheritance_term:
    preferred_term: Autosomal recessive inheritance
    term:
      id: HP:0000007
      label: Autosomal recessive inheritance
  penetrance: INCOMPLETE
  description: >-
    UGT1A1*28 homozygosity segregates recessively with the Gilbert phenotype,
    but it is necessary rather than sufficient: many homozygotes never reach a
    hyperbilirubinemic threshold, and the biochemical phenotype depends
    additionally on bilirubin production rate, hepatic uptake, sex, and age.
    The published UGT1A1*28 homozygote frequency in Western populations
    (roughly 10-13%) therefore exceeds the frequency of clinically recognised
    Gilbert syndrome, and genotype alone does not make the diagnosis.
  evidence:
  - reference: PMID:7565971
    reference_title: "The genetic basis of the reduced expression of bilirubin UDP-glucuronosyltransferase 1 in Gilbert's syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Reduced expression of bilirubin UDP-glucuronosyltransferase 1 due to an abnormality in the promoter region of the gene for this enzyme appears to be necessary for Gilbert's syndrome but not sufficient for the complete manifestation of the syndrome."
    explanation: >-
      The landmark paper states the necessary-but-not-sufficient relationship
      that makes penetrance incomplete.
  - reference: PMID:8596320
    reference_title: "Genetic variation in bilirubin UPD-glucuronosyltransferase gene promoter and Gilbert's syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Segregation of the 7/7 genotype with the Gilbert phenotype was also demonstrated in the family with four affected members."
    explanation: >-
      Family segregation supporting the recessive mode for the promoter allele.
environmental:
- name: Prolonged fasting or caloric restriction
  description: >-
    Fasting for 24-48 hours raises unconjugated bilirubin in Gilbert syndrome
    to a degree it does not in unaffected people, and is the basis of the
    historical fasting provocation test. General anaesthesia, intercurrent
    illness, pregnancy, and deliberate weight-loss regimens act through the
    same route in the reported cohorts.
  exposure_term:
    preferred_term: fasting
    term:
      id: XCO:0000102
      label: fasting
  influences_mechanisms:
  - target: Fasting- and Stress-Induced Bilirubin Elevation
    environmental_effect: EXACERBATES
    causal_link_type: DIRECT
    description: >-
      Caloric restriction is the provoking exposure for the trigger-sensitive
      bilirubin elevation node, and is the exposure used diagnostically.
    evidence:
    - reference: PMID:8596320
      reference_title: "Genetic variation in bilirubin UPD-glucuronosyltransferase gene promoter and Gilbert's syndrome."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "14 volunteers underwent a 24 h fasting test to see if they had Gilbert's syndrome, and all four positives had the 7/7 genotype."
      explanation: >-
        A deliberate 24-hour fasting exposure discriminated UGT1A1*28
        homozygotes, establishing fasting as the operative exposure.
  evidence:
  - reference: PMID:30717703
    reference_title: "The frequency, clinical course, and health related quality of life in adults with Gilbert's syndrome: a longitudinal study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Risk factors for clinical jaundice included general anesthesia, pregnancy, fasting"
    explanation: >-
      Longitudinal adult cohort listing fasting among the risk factors for
      clinically apparent jaundice episodes.
animal_models:
- name: Gunn rat (homozygous jj, UGT1A1-null)
  species: Rat
  genotype: UGT1A1-null (Gunn jj homozygote)
  genes:
  - preferred_term: UGT1A1
    term:
      id: hgnc:12530
      label: UGT1A1
  description: >-
    The Gunn rat lacks UGT1A1 entirely and is the standard rodent model of
    inherited unconjugated hyperbilirubinemia. It is included here because it
    is the model system in which the bilirubin-conjugation lesion has been
    studied mechanistically, but its fidelity to Gilbert syndrome specifically
    is low: Gilbert syndrome is a partial (roughly 30% of normal) reduction in
    a regulated promoter, whereas the Gunn rat is a complete enzymatic null.
    The Gunn rat therefore models Crigler-Najjar type I, including bilirubin
    encephalopathy, which does not occur in Gilbert syndrome.
  publication: PMID:33027804
  modeled_mechanisms:
  - target: Reduced UGT1A1 Activity and Impaired Bilirubin Glucuronidation
    relationship: PARTIALLY_RECAPITULATES
    fidelity: LOW
    description: >-
      Reproduces the enzymatic lesion (absent bilirubin glucuronidation) and
      the resulting unconjugated hyperbilirubinemia, but not its degree,
      regulation, or clinical consequences in Gilbert syndrome.
    limitations: >-
      Complete UGT1A1 absence rather than the partial promoter-driven reduction
      that defines Gilbert syndrome; the resulting bilirubin levels are in the
      Crigler-Najjar type I range and produce cerebellar bilirubin neurotoxicity
      that Gilbert syndrome never causes. The model also cannot represent the
      TA-repeat promoter mechanism itself, which is the actual human lesion.
    readouts:
    - name: Cerebellar weight and cerebellar-mediated behaviour after induced hyperbilirubinemia
      target: Reduced UGT1A1 Activity and Impaired Bilirubin Glucuronidation
      direction: DECREASED
      interpretation: >-
        Structural and behavioural consequence of unconjugated bilirubin
        accumulation in the UGT1A1-null rat. Reported here as a model readout;
        it has no counterpart in human Gilbert syndrome.
      evidence:
      - reference: PMID:33027804
        reference_title: "Choline supplementation prevents the effects of bilirubin on cerebellar-mediated behavior in choline-restricted Gunn rat pups."
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: "SDMX induced behavioral deficits in jj pups, and choline supplementation improved most behavioral effects and cerebellar weight in SDMX-treated jj rats."
        explanation: >-
          Reports the measured cerebellar and behavioural readouts in the
          homozygous jj model.
  evidence:
  - reference: PMID:33027804
    reference_title: "Choline supplementation prevents the effects of bilirubin on cerebellar-mediated behavior in choline-restricted Gunn rat pups."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "The homozygous Gunn rat lacks uridine diphosphate glucuronosyltransferase 1A1 (UGT1A1), the enzyme needed to biotransform bilirubin. This rodent model of hyperbilirubinemia emulates many aspects of bilirubin toxicity observed in the human infant."
    explanation: >-
      States both what the model is and the scope of its claimed fidelity -
      bilirubin toxicity in the infant, not the adult Gilbert phenotype - which
      is why the link is graded PARTIALLY_RECAPITULATES with LOW fidelity.
  notes: >-
    Both available rodent models sit at the severe, Crigler-Najjar end of the
    UGT1A1-deficiency spectrum rather than the mild Gilbert end; see the
    corresponding HUMAN_MODEL_MISMATCH discussion.
- name: Humanized UGT1 (hUGT1) mouse
  species: Mouse
  genotype: Ugt1-null with the human UGT1 locus knocked in (hUGT1)
  genes:
  - preferred_term: UGT1A1
    term:
      id: hgnc:12530
      label: UGT1A1
  description: >-
    Mice in which the mouse Ugt1 locus is replaced by the human UGT1 locus.
    They carry human UGT1A1 regulatory sequence, so unlike the Gunn rat they
    can in principle report on human promoter regulation, and they were the
    model that identified intestinal UGT1A1 as the site where breast milk acts
    to produce neonatal jaundice. Their spontaneous phenotype is nonetheless
    severe neonatal hyperbilirubinemia with bilirubin-induced neurologic
    dysfunction, which is a Crigler-Najjar-range phenotype, not the mild adult
    Gilbert one. Included here for the regulatory mechanism it establishes,
    not as a model of the Gilbert phenotype.
  publication: PMID:21983082
  modeled_mechanisms:
  - target: Reduced UGT1A1 Activity and Impaired Bilirubin Glucuronidation
    relationship: PARTIALLY_RECAPITULATES
    fidelity: LOW
    description: >-
      Establishes that UGT1A1 expression, and therefore bilirubin clearance, is
      set by inducible regulation of the human locus rather than by a fixed
      enzyme quantity - the same regulatory axis that the UGT1A1*28 promoter
      allele perturbs in humans.
    limitations: >-
      The spontaneous phenotype is severe neonatal hyperbilirubinemia with
      bilirubin-induced neurologic dysfunction, which does not occur in Gilbert
      syndrome. Findings are in neonates and in the intestinal compartment;
      whether the same regulation sets adult hepatic UGT1A1 capacity in
      Gilbert syndrome is not established by this model.
    readouts:
    - name: Serum bilirubin under breast-milk versus formula feeding
      target: Reduced UGT1A1 Activity and Impaired Bilirubin Glucuronidation
      direction: INCREASED
      interpretation: >-
        Breast-milk feeding raises serum bilirubin by suppressing intestinal
        UGT1A1; formula lowers it by inducing the same gene. The readout is the
        bilirubin level under a defined dietary exposure.
      evidence:
      - reference: PMID:21983082
        reference_title: "Reduced expression of UGT1A1 in intestines of humanized UGT1 mice via inactivation of NF-κB leads to hyperbilirubinemia."
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: "hUGT1 mice that were fed breast milk developed severe hyperbilirubinemia because of suppression of UGT1A1 in the gastrointestinal tract. Formula-fed hUGT1 mice had lower serum levels of bilirubin, which resulted from induction of UGT1A1 in the gastrointestinal tract."
        explanation: >-
          Reports the measured bilirubin difference and attributes it to
          bidirectional regulation of UGT1A1 expression.
  evidence:
  - reference: PMID:21983082
    reference_title: "Reduced expression of UGT1A1 in intestines of humanized UGT1 mice via inactivation of NF-κB leads to hyperbilirubinemia."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "We studied mice in which the original Ugt1 locus was disrupted and replaced with the human UGT1 locus (hUGT1 mice); these mice spontaneously develop neonatal hyperbilirubinemia and BIND."
    explanation: >-
      States both the humanized construction that makes the model informative
      about human UGT1 regulation and the severe spontaneous phenotype that
      limits its fidelity to Gilbert syndrome.
definitions:
- name: Historical unisex total bilirubin cutoff
  definition_type: DIAGNOSTIC_CRITERIA
  derivation_basis: ESTABLISHED_CRITERIA
  description: >-
    Gilbert syndrome diagnosed as total bilirubin at or above 1 mg/dL
    (17.1 micromol/L) with normal ALT and GGT and no hemolysis. This is the
    cutoff in long-standing clinical use and the one most laboratories report
    against. It is recorded here because it is what most published prevalence
    figures and most patients' diagnoses rest on, not because it is the better
    definition.
  validation_status:
    status: UNVALIDATED
    rationale: >-
      In wide clinical use but never validated against a genotype-anchored gold
      standard; a single unisex threshold is applied to an analyte whose
      distribution differs by sex, age, and UGT1A1 genotype.
  inclusion_criteria:
  - preferred_term: Raised total bilirubin
    description: Total bilirubin at or above 1 mg/dL (17.1 micromol/L).
  - preferred_term: Normal liver enzymes
    description: Alanine aminotransferase and gamma-glutamyl transpeptidase within reference limits.
  - preferred_term: Absence of haemolysis
    description: No laboratory or clinical evidence of a haemolytic process.
  evidence:
  - reference: PMID:37738404
    reference_title: "Clinical and genetic definition of serum bilirubin levels for the diagnosis of Gilbert syndrome and hypobilirubinemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Historically, bilirubin ≥1 mg/dl (17.1 μmol/L) has been the standard cutoff for GS."
    explanation: >-
      States the historical cutoff and attributes it as the standard, which is
      what this definition records.
- name: Sex-, age- and genotype-stratified bilirubin centile definition
  definition_type: DIAGNOSTIC_CRITERIA
  derivation_basis: ESTABLISHED_CRITERIA
  description: >-
    Gilbert syndrome defined as total bilirubin above the 90th centile of the
    distribution for the individual's sex, age band, and UGT1A1 (rs887829)
    genotype, in the absence of liver damage. Derived from 138,125 UK Biobank
    participants with apparently healthy livers. The clinically important
    consequence is that the unisex 1 mg/dL cutoff systematically underdiagnoses
    women: applying stratified centiles identified Gilbert syndrome in 10% of
    women against 3.7% by the historical cutoff. Because bilirubin also falls
    with age, a fixed threshold misclassifies differently at different ages.
  attaches_to:
  - phenotypes#Mild Unconjugated Hyperbilirubinemia
  - biochemical#Unconjugated Bilirubin
  validation_status:
    status: PROPOSED
    rationale: >-
      Derived and internally consistent in a single large European cohort, but
      not yet adopted in guidelines or replicated in another population. The
      reference intervals start at age 40, so their applicability to the
      adolescents and young adults in whom Gilbert syndrome is usually first
      recognised is untested by the source study itself.
  evidence:
  - reference: PMID:37738404
    reference_title: "Clinical and genetic definition of serum bilirubin levels for the diagnosis of Gilbert syndrome and hypobilirubinemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In women, we identified 10% (7,741/76,809) of GS versus 3.7% (2,819/76,809) using the historical cutoff of ≥1 mg/dL (P<0.0001)."
    explanation: >-
      Quantifies the underdiagnosis in women that motivates the stratified
      definition.
  - reference: PMID:37738404
    reference_title: "Clinical and genetic definition of serum bilirubin levels for the diagnosis of Gilbert syndrome and hypobilirubinemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In middle-aged Europeans, the stratified TB demonstrates a careless GS underestimation in women when using the standard unisex 1 mg/dL cutoff."
    explanation: >-
      The authors' own conclusion sentence, stating the definitional claim.
  notes: >-
    The same paper reports that the reference intervals begin at age 40, which
    the authors flag as a limitation for younger subjects.
differential_diagnoses:
- name: Crigler-Najjar syndrome type II
  description: >-
    The same gene, the same enzymatic deficit, and the same unconjugated
    hyperbilirubinemia, separated by degree rather than kind. Reported mean
    serum bilirubin is about 2.8 mg/dL in typical Gilbert syndrome against
    about 12.9 mg/dL in typical Crigler-Najjar type II, with an intermediate
    group between them, so the boundary is a point on a continuum and not a
    categorical distinction.
  distinguishing_features:
  - Substantially higher serum bilirubin, with an intermediate group between the two
  - Biallelic coding UGT1A1 mutations rather than a promoter TA repeat alone
  - Presentation in infancy rather than adolescence
  - Bilirubin falls on phenobarbital
  evidence:
  - reference: PMID:26250421
    reference_title: "Genotype of UGT1A1 and phenotype correlation between Crigler-Najjar syndrome type II and Gilbert syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Serum bilirubin concentrations of typical CN-2, intermediate group, and typical GS are respectively 12.9 ± 5.1, 5.2 ± 2.2, and 2.8 ± 1.1 mg/dL."
    explanation: >-
      Gives the quantitative separation and the existence of an intermediate
      group, which is the substance of this differential.
  - reference: PMID:9435989
    reference_title: "Genetic defects of the UDP-glucuronosyltransferase-1 (UGT1) gene that cause familial non-haemolytic unconjugated hyperbilirubinaemias."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Crigler-Najjar type II syndrome which is treatable with phenobarbital therapy is associated with less dramatic missense mutations or heterozygous expression of mutant and normal alleles."
    explanation: >-
      Sources the phenobarbital response and ties it to residual enzyme:
      Crigler-Najjar type II, which retains some activity, responds, which is
      what makes the response a discriminator against the type I null.
- name: Haemolysis
  description: >-
    Increased bilirubin production from any haemolytic process produces
    unconjugated hyperbilirubinemia with normal liver enzymes, which is exactly
    the Gilbert biochemical picture. Excluding haemolysis is part of the
    consensual definition of Gilbert syndrome, not an optional extra test.
    The two also co-occur, and co-occurrence is clinically important because
    the combination is what drives early pigment gallstones.
  distinguishing_features:
  - Reticulocytosis
  - Reduced haptoglobin
  - Raised lactate dehydrogenase
  - Anaemia
  evidence:
  - reference: PMID:37738404
    reference_title: "Clinical and genetic definition of serum bilirubin levels for the diagnosis of Gilbert syndrome and hypobilirubinemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "There is a consensual definition that GS in individuals is determined by the presence of hyperbilirubinemia in the absence of both hemolysis and liver damage, including the fibrosis stage."
    explanation: >-
      States haemolysis exclusion as part of the accepted definition, which is
      what makes it the primary differential.
mechanistic_hypotheses:
- hypothesis_group_id: bilirubin_systemic_antioxidant_protection
  hypothesis_label: Mildly raised bilirubin is systemically protective
  status: ALTERNATIVE
  description: >-
    The hypothesis that the raised unconjugated bilirubin of Gilbert syndrome
    confers systemic antioxidant protection, principally against cardiovascular
    disease. It has a real in vitro mechanism behind it and consistent
    observational support, and it is the reason Gilbert syndrome is often
    described as advantageous. Mendelian randomization does not support it:
    using the Gilbert genotype as an instrument, genetically raised bilirubin
    shows no causal protective effect on cardiovascular disease or COPD. It is
    retained as an ALTERNATIVE rather than deprecated because the in vitro
    antioxidant chemistry is not in dispute; what is refuted is the inference
    from it to a clinical benefit at Gilbert-range concentrations.
  evidence:
  - reference: PMID:26057938
    reference_title: "Bilirubin scavenges chloramines and inhibits myeloperoxidase-induced protein/lipid oxidation in physiologically relevant hyperbilirubinemic serum."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Significant inhibition of protein and lipid oxidation was demonstrated within the physiological range of UCB, providing a hypothetical link to protection from atherosclerosis in hyperbilirubinemic individuals."
    explanation: >-
      The mechanism is real at physiological bilirubin concentrations, and the
      authors call the link to clinical protection hypothetical. That is the
      distinction this hypothesis turns on, which is why the item is PARTIAL
      and not SUPPORT.
  - reference: PMID:37456363
    reference_title: "Effect of bilirubin and Gilbert syndrome on health: cohort analysis of observational, genetic, and Mendelian randomisation associations."
    supports: REFUTE
    evidence_source: HUMAN_CLINICAL
    snippet: "Evidence from the analyses of genetic data suggests that bilirubin has no likely causal role in protection from cardiovascular disease, chronic obstructive pulmonary disease, or other key healthcare outcomes and therefore represents a poor target for therapeutic intervention for these outcomes."
    explanation: >-
      Genetic instrumentation removes the confounding that the observational
      associations were open to, and finds no causal protection.
  - reference: PMID:37738404
    reference_title: "Clinical and genetic definition of serum bilirubin levels for the diagnosis of Gilbert syndrome and hypobilirubinemia."
    supports: REFUTE
    evidence_source: HUMAN_CLINICAL
    snippet: "No adjusted survival was significantly associated with GS or hypobilirubinemia."
    explanation: >-
      Independent large-cohort survival analysis finds no benefit, consistent
      with the Mendelian randomization result.
  notes: >-
    The two hypothesis directions are not symmetric. The protective direction
    is unsupported causally, while the one causal association that has been
    demonstrated for bilirubin in this disease points the other way, at
    cholelithiasis. Recorded here because the protective framing is still
    widespread: the OpenScientist report used to recurate this entry presents
    cardiovascular protection as "a striking and reproducible feature" and
    proposes, as future work, the Mendelian randomization that PMID:37456363
    had already published with a negative result. Reviewers of this entry
    should expect to keep meeting that framing in the secondary literature.
discussions:
- discussion_id: gs_symptom_attribution
  kind: OPEN_QUESTION
  status: OPEN
  prompt: >-
    Are the fatigue, abdominal pain, and dyspepsia reported alongside jaundice
    episodes caused by bilirubin, or are they the ascertainment signature of
    having been investigated for jaundice?
  attaches_to:
  - phenotypes#Jaundice-Associated Gastrointestinal Symptoms
  - phenotypes#Fatigue
  rationale: >-
    Three lines of evidence pull in different directions and this entry does
    not resolve them. A clinic cohort reports these symptoms in over half of
    subjects during jaundice episodes, but clinic cohorts enrol people who
    presented with symptoms. A UK Biobank analysis tested 54 illnesses and
    found that, apart from cholelithiasis, every raised prevalence was
    explained by confounders after adjustment. Against both, a UK primary-care
    analysis of 9,240 cases against 150,846 controls found abdominal pain and
    fatigue - and only those two - consistently raised up to five years before
    diagnosis, which is the wrong temporal shape for a pure workup artefact.
    What is still missing is a symptom endpoint tested with the adjustment and
    Mendelian randomization applied to the illness endpoints. Until then the
    frequencies on these phenotypes record association, not causation.
  evidence:
  - reference: PMID:37738404
    reference_title: "Clinical and genetic definition of serum bilirubin levels for the diagnosis of Gilbert syndrome and hypobilirubinemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Almost all these differences were explained by confounders"
    explanation: >-
      The population-level result that motivates doubting the clinic-cohort
      symptom attribution.
  - reference: PMID:30717703
    reference_title: "The frequency, clinical course, and health related quality of life in adults with Gilbert's syndrome: a longitudinal study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "jaundice was associated with abdominal pain, dyspepsia or loss of appetite in 54 (53.465%) subjects."
    explanation: >-
      The association this discussion is about. It is an association within a
      clinic cohort, which is what the open question turns on.
  - reference: PMID:40904555
    reference_title: "Prevalence and Data-Driven Exploration of Pre-Diagnostic Symptoms and Features of Gilbert's Syndrome in the UK Primary Care Population."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "While most of these features appeared primarily in the year prior to diagnosis, only abdominal pain and fatigue were consistently more common in GS cases up to five years before diagnosis."
    explanation: >-
      Population-scale evidence pointing the other way from the confounder
      result: a five-year lead time is hard to explain as diagnostic workup.
      This is why the question is recorded as open rather than settled.
- discussion_id: gs_no_promoter_allele_animal_model
  kind: HUMAN_MODEL_MISMATCH
  status: OPEN
  prompt: >-
    No animal model carries the UGT1A1*28 TA-repeat promoter allele, so is any
    conclusion about Gilbert syndrome drawn from the Gunn rat translationally
    valid?
  attaches_to:
  - animal_models#Gunn rat (homozygous jj, UGT1A1-null)
  - animal_models#Humanized UGT1 (hUGT1) mouse
  - pathophysiology#Reduced UGT1A1 Activity and Impaired Bilirubin Glucuronidation
  rationale: >-
    The longest-established rodent model of inherited unconjugated
    hyperbilirubinemia, the Gunn rat, is a UGT1A1 null. Gilbert syndrome is not
    a null; it is a roughly 30% residual activity produced by a promoter TA
    insertion that changes transcription rate. The two differ in the variable
    that matters. The Gunn rat's defining outcome, bilirubin encephalopathy,
    does not occur in Gilbert syndrome at all, and the model cannot address the
    questions Gilbert syndrome actually raises - what determines the threshold
    for clinical expression among genotype-positive people, and why penetrance
    is incomplete. The humanized UGT1 mouse also curated here is closer, since
    it carries human UGT1 regulatory sequence, but its spontaneous phenotype is
    likewise severe neonatal hyperbilirubinemia with neurologic dysfunction. So
    both available models sit at the Crigler-Najjar end of the spectrum, and an
    allelic series of the promoter repeat on a humanized background would be
    the experiment that resolves this.
  proposed_experiments:
  - experiment_id: exp_gs_humanized_promoter_allelic_series
    name: Humanized UGT1A1 promoter TA-repeat allelic series in mouse
    description: >-
      Knock the human UGT1A1 promoter TA-repeat allelic series - (TA)6, (TA)7,
      and the compound genotypes - into a Ugt1a1-humanized mouse, and measure
      basal unconjugated bilirubin, the response to a defined fast, and SN-38
      glucuronidation as a function of repeat number. The point is to obtain a
      graded model of a regulated promoter rather than the null the Gunn rat
      provides, so that partial-reduction questions can be asked at all.
    would_support:
    - pathophysiology#Reduced UGT1A1 Activity and Impaired Bilirubin Glucuronidation
    supporting_outcome:
    - >-
        Basal bilirubin and fasting response increase monotonically with TA
        repeat number, and SN-38 glucuronidation decreases, reproducing the human
        allele-dose relationship.
    refuting_outcome:
    - >-
        Repeat number does not change bilirubin handling in the humanized
        background, implying the human phenotype depends on species-specific
        regulatory context the model does not supply.
  evidence:
  - reference: PMID:33027804
    reference_title: "Choline supplementation prevents the effects of bilirubin on cerebellar-mediated behavior in choline-restricted Gunn rat pups."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "The homozygous Gunn rat lacks uridine diphosphate glucuronosyltransferase 1A1 (UGT1A1), the enzyme needed to biotransform bilirubin."
    explanation: >-
      States the null genotype that is the substance of the mismatch with the
      human partial-reduction lesion.
  - reference: PMID:7565971
    reference_title: "The genetic basis of the reduced expression of bilirubin UDP-glucuronosyltransferase 1 in Gilbert's syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Hepatic glucuronidating activity, essential for efficient biliary excretion of bilirubin, is reduced to about 30 percent of normal."
    explanation: >-
      The human lesion is a partial reduction, which is the quantity the null
      model cannot represent.
- discussion_id: gs_penetrance_modifiers
  kind: KNOWLEDGE_GAP
  status: OPEN
  prompt: >-
    UGT1A1*28 homozygosity is necessary but not sufficient. What supplies the
    remainder, and can it be measured?
  attaches_to:
  - inheritance#Autosomal recessive inheritance with incomplete penetrance
  rationale: >-
    Bosma stated in 1995 that the promoter abnormality is necessary but not
    sufficient, and thirty years later this entry still cannot name what
    supplies the difference. Candidate contributions - bilirubin production
    rate including occult haemolysis, hepatic uptake capacity, sex and
    androgen status, and age - are individually plausible and jointly
    unquantified. This matters practically because roughly 10-13% of Western
    populations are UGT1A1*28 homozygotes while clinically recognised Gilbert
    syndrome is less common, so a genotype-first diagnosis over-calls, and the
    entry has no modifier to offer in its place.
  evidence:
  - reference: PMID:7565971
    reference_title: "The genetic basis of the reduced expression of bilirubin UDP-glucuronosyltransferase 1 in Gilbert's syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Reduced expression of bilirubin UDP-glucuronosyltransferase 1 due to an abnormality in the promoter region of the gene for this enzyme appears to be necessary for Gilbert's syndrome but not sufficient for the complete manifestation of the syndrome."
    explanation: >-
      The statement of the gap, in the source that first made it.
  - reference: PMID:37738404
    reference_title: "Clinical and genetic definition of serum bilirubin levels for the diagnosis of Gilbert syndrome and hypobilirubinemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Sex and age are strongly correlated with bilirubin levels in healthy subjects, suggesting that appropriate personalized bilirubin levels should be defined."
    explanation: >-
      Identifies two of the modifiers and shows they are large enough to change
      the diagnosis, without resolving how much of the residual they account for.
- discussion_id: gs_neonatal_cofactor_risk
  kind: KNOWLEDGE_GAP
  status: OPEN
  prompt: >-
    How much does the Gilbert genotype add to neonatal hyperbilirubinemia risk
    when co-inherited with a bilirubin-production disorder such as G6PD
    deficiency, and is that increment large enough to screen for?
  attaches_to:
  - pathophysiology#Reduced UGT1A1 Activity and Impaired Bilirubin Glucuronidation
  rationale: >-
    Gilbert syndrome is benign in adults, so this entry treats it as benign
    throughout. The neonatal period is the exception the entry does not
    currently model: a newborn has an immature conjugating system already, and
    the same reduced-capacity allele sits upstream of the one bilirubin
    outcome that is genuinely dangerous. Published studies point the same way
    but do not agree on effect size, and the studies are small. The neonatal
    series in G6PD-deficient infants below reports increased hyperbilirubinemia with
    the promoter variant while stating the interaction is more complex than a
    two-locus model. This entry deliberately does not assert a neonatal risk
    figure because the literature does not support one yet.
  proposed_experiments:
  - experiment_id: exp_gs_neonatal_ugt1a1_g6pd_interaction
    name: Powered newborn cohort for the UGT1A1 x G6PD interaction
    description: >-
      Prospective multi-ancestry newborn cohort genotyping UGT1A1 promoter
      repeat number and G6PD together, powered for the interaction term rather
      than for either main effect, with reaching the phototherapy threshold as
      the endpoint. Existing studies are small single-population series that
      can detect the main effects but not the interaction, which is the
      quantity that would decide whether combined screening is worthwhile.
    would_support:
    - pathophysiology#Reduced UGT1A1 Activity and Impaired Bilirubin Glucuronidation
    supporting_outcome:
    - >-
        A significant UGT1A1-by-G6PD interaction term on the phototherapy
        threshold endpoint, of a size that changes the number needed to screen.
    refuting_outcome:
    - >-
        No interaction beyond the additive contribution of each locus, indicating
        that genotyping UGT1A1 adds nothing to G6PD screening alone.
  evidence:
  - reference: PMID:37508669
    reference_title: "The Genetics of Glucose-6-Phosphate-Dehydrogenase (G6PD) and Uridine Diphosphate Glucuronosyl Transferase 1A1 (UGT1A1) Promoter Gene Polymorphism in Relation to Quantitative Biochemical G6PD Activity Measurement and Neonatal Hyperbilirubinemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "the interaction between G6PD deficiency, UGT1A1 promoter polymorphism, and NHB is more complex, possibly involving other genetic interactions, not yet described."
    explanation: >-
      The source states its own limitation: the interaction is real but not
      resolved, which is exactly the gap recorded here.
references:
- reference: DOI:10.1097/hc9.0000000000000245
  title: Clinical and genetic definition of serum bilirubin levels for the diagnosis of Gilbert syndrome and hypobilirubinemia
  found_in:
  - Gilberts_Syndrome-deep-research-falcon.md
  findings:
  - statement: Gilbert syndrome (GS) is genotypically predetermined by UGT1A1*28 homozygosity in Europeans and is phenotypically defined by hyperbilirubinemia using total bilirubin (TB) cutoff ≥1mg/dL (17 μmol/L).
    supporting_text: Gilbert syndrome (GS) is genotypically predetermined by UGT1A1*28 homozygosity in Europeans and is phenotypically defined by hyperbilirubinemia using total bilirubin (TB) cutoff ≥1mg/dL (17 μmol/L).
    evidence:
    - reference: DOI:10.1097/hc9.0000000000000245
      reference_title: Clinical and genetic definition of serum bilirubin levels for the diagnosis of Gilbert syndrome and hypobilirubinemia
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: Gilbert syndrome (GS) is genotypically predetermined by UGT1A1*28 homozygosity in Europeans and is phenotypically defined by hyperbilirubinemia using total bilirubin (TB) cutoff ≥1mg/dL (17 μmol/L).
      explanation: Deep research cited this publication as relevant literature for Gilberts Syndrome.
- reference: DOI:10.20944/preprints202405.0500.v1
  title: 'Gilbert’s Syndrome: The Good, the Bad and the Ugly'
  found_in:
  - Gilberts_Syndrome-deep-research-falcon.md
  findings:
  - statement: Gilbert's syndrome (GS) is a common hereditary condition characterized by mild increases in serum bilirubin levels due to inherited defects in bilirubin metabolism.
    supporting_text: Gilbert's syndrome (GS) is a common hereditary condition characterized by mild increases in serum bilirubin levels due to inherited defects in bilirubin metabolism.
    evidence:
    - reference: DOI:10.20944/preprints202405.0500.v1
      reference_title: 'Gilbert’s Syndrome: The Good, the Bad and the Ugly'
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: Gilbert's syndrome (GS) is a common hereditary condition characterized by mild increases in serum bilirubin levels due to inherited defects in bilirubin metabolism.
      explanation: Deep research cited this publication as relevant literature for Gilberts Syndrome.
- reference: DOI:10.2147/pgpm.s108656
  title: 'UGT1A1 polymorphisms in cancer: impact on irinotecan treatment'
  found_in:
  - Gilberts_Syndrome-deep-research-falcon.md
  findings:
  - statement: 'UGT1A1 polymorphisms in cancer: impact on irinotecan treatment'
    supporting_text: 'UGT1A1 polymorphisms in cancer: impact on irinotecan treatment'
- reference: DOI:10.3389/fgene.2023.1265268
  title: 'Genetic variations underlying Gilbert syndrome and HBV infection outcomes: a cross-sectional study'
  found_in:
  - Gilberts_Syndrome-deep-research-falcon.md
  findings:
  - statement: Constant cellular damage causes a poor prognosis of hepatitis B virus (HBV) infection.
    supporting_text: Constant cellular damage causes a poor prognosis of hepatitis B virus (HBV) infection.
    evidence:
    - reference: DOI:10.3389/fgene.2023.1265268
      reference_title: 'Genetic variations underlying Gilbert syndrome and HBV infection outcomes: a cross-sectional study'
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: Constant cellular damage causes a poor prognosis of hepatitis B virus (HBV) infection.
      explanation: Deep research cited this publication as relevant literature for Gilberts Syndrome.
- reference: DOI:10.3389/fphar.2024.1389968
  title: Genetic variation in UGT1A1 is not associated with altered liver biochemical parameters in healthy volunteers participating in bioequivalence trials
  found_in:
  - Gilberts_Syndrome-deep-research-falcon.md
  findings:
  - statement: Genetic variation in UGT1A1 is not associated with altered liver biochemical parameters in healthy volunteers participating in bioequivalence trials
    supporting_text: Bioequivalence clinical trials are conducted in healthy volunteers whose blood tests should be within normal limits; individuals with Gilbert syndrome (GS) are excluded from these studies on suspicion of any liver disease, even if the change is clinically insignificant.
    evidence:
    - reference: DOI:10.3389/fphar.2024.1389968
      reference_title: Genetic variation in UGT1A1 is not associated with altered liver biochemical parameters in healthy volunteers participating in bioequivalence trials
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: Bioequivalence clinical trials are conducted in healthy volunteers whose blood tests should be within normal limits; individuals with Gilbert syndrome (GS) are excluded from these studies on suspicion of any liver disease, even if the change is clinically insignificant.
      explanation: Deep research cited this publication as relevant literature for Gilberts Syndrome.
- reference: DOI:10.3390/nu16142247
  title: Nutrition in Gilbert’s Syndrome—A Systematic Review of Clinical Trials According to the PRISMA Statement
  found_in:
  - Gilberts_Syndrome-deep-research-falcon.md
  findings:
  - statement: Gilbert syndrome is the most common hyperbilirubinemia, associated with a mutation in the UGT1A1 bilirubin gene, which produces an enzyme that conjugates bilirubin with glucuronic acid.
    supporting_text: Gilbert syndrome is the most common hyperbilirubinemia, associated with a mutation in the UGT1A1 bilirubin gene, which produces an enzyme that conjugates bilirubin with glucuronic acid.
    evidence:
    - reference: DOI:10.3390/nu16142247
      reference_title: Nutrition in Gilbert’s Syndrome—A Systematic Review of Clinical Trials According to the PRISMA Statement
      supports: SUPPORT
      evidence_source: OTHER
      snippet: Gilbert syndrome is the most common hyperbilirubinemia, associated with a mutation in the UGT1A1 bilirubin gene, which produces an enzyme that conjugates bilirubin with glucuronic acid.
      explanation: Deep research cited this publication as relevant literature for Gilberts Syndrome.
- reference: DOI:10.5385/nm.2024.31.1.1
  title: Should We Consider UGT1A1 Mutation Analysis in Evaluating the Prolonged Jaundice of Newborn Infants?
  found_in:
  - Gilberts_Syndrome-deep-research-falcon.md
  findings:
  - statement: Uridine diphosphate glucuronosyltransferase 1A isoform 1 (<i>UGT1A1</i>) is a crucial enzyme in bilirubin metabolism.
    supporting_text: Uridine diphosphate glucuronosyltransferase 1A isoform 1 (<i>UGT1A1</i>) is a crucial enzyme in bilirubin metabolism.
    evidence:
    - reference: DOI:10.5385/nm.2024.31.1.1
      reference_title: Should We Consider UGT1A1 Mutation Analysis in Evaluating the Prolonged Jaundice of Newborn Infants?
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: Uridine diphosphate glucuronosyltransferase 1A isoform 1 (<i>UGT1A1</i>) is a crucial enzyme in bilirubin metabolism.
      explanation: Deep research cited this publication as relevant literature for Gilberts Syndrome.
📚

References & Deep Research

References

7
DOI:10.1097/hc9.0000000000000245
Clinical and genetic definition of serum bilirubin levels for the diagnosis of Gilbert syndrome and hypobilirubinemia
1 finding
Gilbert syndrome (GS) is genotypically predetermined by UGT1A1*28 homozygosity in Europeans and is phenotypically defined by hyperbilirubinemia using total bilirubin (TB) cutoff ≥1mg/dL (17 μmol/L).
"Gilbert syndrome (GS) is genotypically predetermined by UGT1A1*28 homozygosity in Europeans and is phenotypically defined by hyperbilirubinemia using total bilirubin (TB) cutoff ≥1mg/dL (17 μmol/L)."
Show evidence (1 reference)
DOI:10.1097/hc9.0000000000000245 SUPPORT Human Clinical
"Gilbert syndrome (GS) is genotypically predetermined by UGT1A1*28 homozygosity in Europeans and is phenotypically defined by hyperbilirubinemia using total bilirubin (TB) cutoff ≥1mg/dL (17 μmol/L)."
Deep research cited this publication as relevant literature for Gilberts Syndrome.
DOI:10.20944/preprints202405.0500.v1
Gilbert’s Syndrome: The Good, the Bad and the Ugly
1 finding
Gilbert's syndrome (GS) is a common hereditary condition characterized by mild increases in serum bilirubin levels due to inherited defects in bilirubin metabolism.
"Gilbert's syndrome (GS) is a common hereditary condition characterized by mild increases in serum bilirubin levels due to inherited defects in bilirubin metabolism."
Show evidence (1 reference)
DOI:10.20944/preprints202405.0500.v1 Preprint · not peer-reviewed SUPPORT Human Clinical
"Gilbert's syndrome (GS) is a common hereditary condition characterized by mild increases in serum bilirubin levels due to inherited defects in bilirubin metabolism."
Deep research cited this publication as relevant literature for Gilberts Syndrome.
DOI:10.2147/pgpm.s108656
UGT1A1 polymorphisms in cancer: impact on irinotecan treatment
1 finding
UGT1A1 polymorphisms in cancer: impact on irinotecan treatment
"UGT1A1 polymorphisms in cancer: impact on irinotecan treatment"
DOI:10.3389/fgene.2023.1265268
Genetic variations underlying Gilbert syndrome and HBV infection outcomes: a cross-sectional study
1 finding
Constant cellular damage causes a poor prognosis of hepatitis B virus (HBV) infection.
"Constant cellular damage causes a poor prognosis of hepatitis B virus (HBV) infection."
Show evidence (1 reference)
DOI:10.3389/fgene.2023.1265268 SUPPORT Human Clinical
"Constant cellular damage causes a poor prognosis of hepatitis B virus (HBV) infection."
Deep research cited this publication as relevant literature for Gilberts Syndrome.
DOI:10.3389/fphar.2024.1389968
Genetic variation in UGT1A1 is not associated with altered liver biochemical parameters in healthy volunteers participating in bioequivalence trials
1 finding
Genetic variation in UGT1A1 is not associated with altered liver biochemical parameters in healthy volunteers participating in bioequivalence trials
"Bioequivalence clinical trials are conducted in healthy volunteers whose blood tests should be within normal limits; individuals with Gilbert syndrome (GS) are excluded from these studies on suspicion of any liver disease, even if the change is clinically insignificant."
Show evidence (1 reference)
DOI:10.3389/fphar.2024.1389968 SUPPORT Human Clinical
"Bioequivalence clinical trials are conducted in healthy volunteers whose blood tests should be within normal limits; individuals with Gilbert syndrome (GS) are excluded from these studies on suspicion of any liver disease, even if the change is clinically insignificant."
Deep research cited this publication as relevant literature for Gilberts Syndrome.
DOI:10.3390/nu16142247
Nutrition in Gilbert’s Syndrome—A Systematic Review of Clinical Trials According to the PRISMA Statement
1 finding
Gilbert syndrome is the most common hyperbilirubinemia, associated with a mutation in the UGT1A1 bilirubin gene, which produces an enzyme that conjugates bilirubin with glucuronic acid.
"Gilbert syndrome is the most common hyperbilirubinemia, associated with a mutation in the UGT1A1 bilirubin gene, which produces an enzyme that conjugates bilirubin with glucuronic acid."
Show evidence (1 reference)
DOI:10.3390/nu16142247 SUPPORT Other
"Gilbert syndrome is the most common hyperbilirubinemia, associated with a mutation in the UGT1A1 bilirubin gene, which produces an enzyme that conjugates bilirubin with glucuronic acid."
Deep research cited this publication as relevant literature for Gilberts Syndrome.
DOI:10.5385/nm.2024.31.1.1
Should We Consider UGT1A1 Mutation Analysis in Evaluating the Prolonged Jaundice of Newborn Infants?
1 finding
Uridine diphosphate glucuronosyltransferase 1A isoform 1 (<i>UGT1A1</i>) is a crucial enzyme in bilirubin metabolism.
"Uridine diphosphate glucuronosyltransferase 1A isoform 1 (<i>UGT1A1</i>) is a crucial enzyme in bilirubin metabolism."
Show evidence (1 reference)
DOI:10.5385/nm.2024.31.1.1 SUPPORT Human Clinical
"Uridine diphosphate glucuronosyltransferase 1A isoform 1 (<i>UGT1A1</i>) is a crucial enzyme in bilirubin metabolism."
Deep research cited this publication as relevant literature for Gilberts Syndrome.

Deep Research

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Gilbert’s Syndrome (GS): Disease Characteristics Research Report
Edison Scientific Literature 20 citations 2026-05-09T09:05:32.387438

Gilbert’s Syndrome (GS): Disease Characteristics Research Report

Target disease

  • Disease name: Gilbert’s syndrome (GS)
  • Category: Mendelian (UGT1A1-related bilirubin conjugation deficiency)

1. Disease Information

1.1 Concise overview (current understanding)

Gilbert’s syndrome is a common, generally benign inherited condition characterized by intermittent, non-hemolytic, predominantly unconjugated hyperbilirubinemia in the setting of otherwise normal liver biochemistry and no evidence of hemolysis. Typical total bilirubin values are mild (often in the ~1.2–5.3 mg/dL range) and may fluctuate with physiologic stressors. (gonzaleziglesias2024geneticvariationin pages 1-2, beutler2024gilbertssyndromebrightand pages 1-4)

1.2 Key identifiers and terminology

  • OMIM disease: Gilbert syndrome (OMIM #143500) is explicitly referenced in multiple papers in the retrieved corpus. (yao2023geneticvariationsunderlying pages 1-2)
  • Causal gene OMIM: UGT1A1 (OMIM *191740), located on chromosome 2q37, is repeatedly cited as the causal gene for GS-spectrum unconjugated hyperbilirubinemia. (yao2023geneticvariationsunderlying pages 1-2, gonzaleziglesias2024geneticvariationin pages 1-2)
  • OpenTargets / EFO: OpenTargets returns an association between Gilbert syndrome (EFO_0005556) and UGT1A1 (and also UGT1A10 in a smaller evidence set), with literature-linked evidence. (OpenTargets Search: Gilbert syndrome-UGT1A1)
  • MONDO: In OpenTargets, a related entity hereditary hyperbilirubinemia maps to MONDO_0002408 and is also associated with UGT1A1. A distinct MONDO identifier for “Gilbert syndrome” itself was not obtained from the retrieved sources in this run. (OpenTargets Search: Gilbert syndrome-UGT1A1)
  • ICD-10/ICD-11, MeSH, Orphanet: These identifiers were not directly captured from the retrieved full-text chunks; therefore they cannot be reported with primary-source citations here.

1.3 Synonyms / alternative names (from clinical usage)

In the retrieved literature, the disease is referred to as Gilbert syndrome and Gilbert’s syndrome. (beutler2024gilbertssyndromebrightand pages 1-4, yao2023geneticvariationsunderlying pages 1-2)

1.4 Evidence provenance

Evidence summarized below is derived from a mixture of: - Primary human studies (e.g., infant prolonged jaundice cohort; HBV outcome association; healthy volunteer genotype-biochemistry study) (yao2023geneticvariationsunderlying pages 1-2, kim2024shouldweconsider pages 1-2, gonzaleziglesias2024geneticvariationin pages 1-2) - Systematic review of clinical trials related to nutrition/fasting effects in GS (goluch2024nutritioningilbert’s pages 1-2, goluch2024nutritioningilbert’s media eb15b11f) - ClinicalTrials.gov records for UGT1A1 pharmacogenomics studies (NCT05148767 chunk 1, NCT01523431 chunk 1, NCT02680795 chunk 1)

2. Etiology

2.1 Disease causal factors

Primary cause: inherited reduction of UGT1A1 activity/expression, the key UDP-glucuronosyltransferase responsible for bilirubin glucuronidation in humans. (yao2023geneticvariationsunderlying pages 1-2, goluch2024nutritioningilbert’s pages 1-2)

Key causal/associated variants (current consensus in retrieved sources): - UGT1A1*28 promoter TA-repeat expansion (TATA-box; often described as A(TA)7TAA or duplication of TA in promoter) is a principal GS-associated genotype, particularly in European/Caucasian ancestry groups. (goluch2024nutritioningilbert’s pages 2-3, kim2024shouldweconsider pages 1-2) - UGT1A1*6 (c.211G>A; p.Gly71Arg / p.G71R) is emphasized as a common GS-relevant coding variant in East Asian populations and in neonatal/prolonged jaundice workups. (kim2024shouldweconsider pages 1-2)

2.2 Risk factors

Genetic risk factors: reduced-function UGT1A1 alleles/haplotypes, including 28 and 6; additional disease-causing variants are documented in sequencing studies of unconjugated hyperbilirubinemia cohorts (e.g., 27, 63, *7 reported in a 2023 study context). (yao2023geneticvariationsunderlying pages 1-2)

Environmental/physiologic risk factors (gene–environment interaction): caloric restriction/fasting, dehydration, intercurrent illness/infection, stress, surgery/anesthesia, pregnancy, sleep deprivation, and intense physical exertion can precipitate jaundice episodes and transient bilirubin rises in genetically predisposed individuals. (goluch2024nutritioningilbert’s pages 2-3, beutler2024gilbertssyndromebrightand pages 1-4)

2.3 Protective factors

The retrieved evidence supports an active research theme that mildly elevated bilirubin may have cytoprotective/antioxidant and immunomodulatory properties. A 2023 cross-sectional HBV-exposed cohort study reported markedly lower cirrhosis/HCC incidence among individuals carrying disease-causing UGT1A1 variants versus wild type, suggesting a potentially protective association (not necessarily causal for GS itself). (yao2023geneticvariationsunderlying pages 1-2)

2.4 Gene–environment interactions

The 2024 nutrition-focused systematic review explicitly frames GS as a “genetically induced, nutritionally exacerbated” disorder, where caloric restriction can rapidly increase bilirubin levels in affected individuals. (goluch2024nutritioningilbert’s pages 1-2, goluch2024nutritioningilbert’s pages 2-3)

3. Phenotypes

3.1 Core phenotype spectrum (with suggested HPO terms)

  1. Intermittent jaundice (scleral/skin), often painless and non-pruritic.
  2. Suggested HPO: Jaundice (HP:0000952)

  3. Evidence: described as intermittent mild jaundice with otherwise normal liver tests. (beutler2024gilbertssyndromebrightand pages 1-4)

  4. Laboratory abnormality: unconjugated hyperbilirubinemia

  5. Suggested HPO: Unconjugated hyperbilirubinemia (HP:0003154)
  6. Suggested LOINC (generic): serum/plasma total bilirubin and direct bilirubin measurement (implementation-specific codes)

  7. Evidence: mild bilirubin elevation typical of GS; normal bilirubin stated as ~0.1–1.2 mg/dL; GS typically ~1.2–5.3 mg/dL, often not exceeding ~5–6 mg/dL. (gonzaleziglesias2024geneticvariationin pages 1-2, goluch2024nutritioningilbert’s pages 2-3)

  8. Normal liver enzymes (important negative phenotype)

  9. Suggested LOINC (generic): ALT/AST/ALP/GGT panel

  10. Evidence: A 2024 study in 773 healthy volunteers found higher bilirubin in genotype-inferred intermediate/poor UGT1A1 metabolizers without association with liver enzyme abnormalities. (gonzaleziglesias2024geneticvariationin pages 1-2)

  11. Trigger sensitivity (fasting/caloric restriction/stress-related episodes)

  12. Suggested HPO: could be represented as episodic worsening of jaundice/hyperbilirubinemia; may be modeled as exposure-linked exacerbation in a KB schema
  13. Evidence: calorie reduction can cause a 2–3× bilirubin rise within 48 hours; multiple triggers listed above. (goluch2024nutritioningilbert’s pages 2-3)

3.2 Age of onset, severity, and course

  • Often recognized in adolescence/young adulthood (triggered episodic phenotype); in pediatrics, UGT1A1 variants can be prominent in the setting of prolonged jaundice evaluation. (goluch2024nutritioningilbert’s pages 1-2, kim2024shouldweconsider pages 1-2)
  • Severity is typically mild and fluctuating; severe neonatal unconjugated hyperbilirubinemia may reflect different variant combinations or overlap with Crigler–Najjar spectrum disorders rather than classic adult GS. (kim2024shouldweconsider pages 1-2)

3.3 Quality of life impact

The 2024 systematic review states that episodes of jaundice in GS negatively affect quality of life and focuses on nutritional strategies to reduce episode frequency. (goluch2024nutritioningilbert’s pages 1-2)

4. Genetic / Molecular Information

4.1 Causal genes

  • UGT1A1 (UDP glucuronosyltransferase family 1 member A1) is the key gene in the retrieved evidence base for GS. (yao2023geneticvariationsunderlying pages 1-2, OpenTargets Search: Gilbert syndrome-UGT1A1)

4.2 Pathogenic/associated variants (examples from retrieved sources)

A structured summary is provided in the artifact below.

Gene (HGNC symbol) Locus/OMIM (if available from context) Common pathogenic/associated variants (HGVS and star allele where available) Variant type (promoter STR, missense, etc.) Ethnic distribution notes Functional effect on UGT1A1 activity Clinical implications (bilirubin levels, drug toxicity)
UGT1A1 Chromosome 2q37; OMIM *191740; Gilbert syndrome OMIM #143500 A(TA)7TAA promoter repeat, commonly referred to as UGT1A1*28; also described as c.-40_-39dupTA / c.-41_-40dupTA in retrieved texts Promoter short tandem repeat / insertion in TATA box Predominant GS-associated allele in Caucasian and many African/European populations; prevalence estimates in Europeans ~5–10%, Sub-Saharan Africans 15–25% for GS-related prevalence context; “almost all” GS individuals in Caucasian/African groups in review context are homozygous for *28 (goluch2024nutritioningilbert’s pages 1-2, gonzaleziglesias2024geneticvariationin pages 1-2, kim2024shouldweconsider pages 1-2) Reduces UGT1A1 expression/activity; one 2024 review states *28 homozygosity is present in ~90% of GS patients and promoter variant can reduce activity to ~30% of normal; bilirubin glucuronidation reported as ~50% of wild-type in 7/7 vs 6/6 genotype (goluch2024nutritioningilbert’s pages 1-2, beutler2024gilbertssyndromebrightand pages 1-4, gonzaleziglesias2024geneticvariationin pages 2-3, goluch2024nutritioningilbert’s pages 2-3) Main molecular basis of intermittent unconjugated hyperbilirubinemia in GS; bilirubin often ~1.2–5.3 mg/dL and usually <5–6 mg/dL; relevant pharmacogenomic risk allele for irinotecan toxicity and atazanavir-associated hyperbilirubinemia; useful diagnostically when liver enzymes are otherwise normal (goluch2024nutritioningilbert’s pages 2-3, silva2025gilbert’ssyndromethe pages 5-6, beutler2024gilbertssyndromebrightand pages 1-4, gonzaleziglesias2024geneticvariationin pages 1-2)
UGT1A1 Chromosome 2q37; OMIM *191740 c.211G>A (p.Gly71Arg, p.G71R), UGT1A1*6 Missense variant in exon 1 Common in East Asian populations; highlighted as more prevalent in Asians/Koreans/Chinese than in Caucasians (kim2024shouldweconsider pages 1-2, goluch2024nutritioningilbert’s pages 1-2, silva2025gilbert’ssyndromethe pages 3-5) Decreases UGT1A1 enzymatic activity; contributes to mild bilirubin conjugation deficiency and GS phenotype; in East Asians accounts for a notable fraction of bilirubin variability (goluch2024nutritioningilbert’s pages 2-3, kim2024shouldweconsider pages 1-2, gonzaleziglesias2024geneticvariationin pages 1-2) Associated with prolonged neonatal jaundice and adult GS; in a 2024 infant cohort it was the most common variant (46.5% among detected variants); also implicated in irinotecan toxicity literature via reduced SN-38 glucuronidation (kim2024shouldweconsider pages 1-2, gonzaleziglesias2024geneticvariationin pages 1-2)
UGT1A1 Chromosome 2q37; OMIM *191740 c.-3275T>G (PBREM/enhancer-region variant; often discussed with GS haplotypes) Regulatory/promoter-enhancer variant Reported in Asian infant cohorts with prolonged jaundice; second most common variant in one Korean infant series (30.2%) (kim2024shouldweconsider pages 1-2) Presumed reduction in transcription/expression when part of GS-associated haplotypes; contributes to reduced bilirubin conjugation (kim2024shouldweconsider pages 1-2) Seen in prolonged unconjugated neonatal hyperbilirubinemia and supports molecular diagnosis of GS-spectrum bilirubin disorders when routine workup is unrevealing (kim2024shouldweconsider pages 1-2)
UGT1A1 Chromosome 2q37; OMIM *191740 UGT1A1*80 (rs887829), used as proxy for *28 in one 2024 pharmacogenetic study Regulatory SNP in strong linkage disequilibrium with promoter allele Reported as being in near-complete LD with *28 (r² 0.99) in studied populations; used as a surrogate marker rather than a direct causal assignment in the retrieved study (gonzaleziglesias2024geneticvariationin pages 2-3, gonzaleziglesias2024geneticvariationin pages 1-2) Serves as indicator of reduced-function *28 haplotype / genotype-informed intermediate or poor metabolizer status (gonzaleziglesias2024geneticvariationin pages 2-3, gonzaleziglesias2024geneticvariationin pages 1-2) Higher bilirubin in intermediate/poor metabolizers; no associated elevation of liver enzymes in healthy volunteers, supporting benignity of GS-like biochemical phenotype in trial screening contexts (gonzaleziglesias2024geneticvariationin pages 2-3, gonzaleziglesias2024geneticvariationin pages 1-2)
UGT1A1 Chromosome 2q37; OMIM *191740 UGT1A127, UGT1A163, UGT1A1*7 Mixed coding/regulatory disease-causing variants (exact HGVS not provided in retrieved context) Identified in a 2023 Chinese HBV/GS cross-sectional sequencing study among patients with unconjugated hyperbilirubinemia (yao2023geneticvariationsunderlying pages 1-2) Disease-causing variants associated with greater UGT1A1 deficiency; study inferred that accumulation/rarity of variants correlated with stronger biologic effect (yao2023geneticvariationsunderlying pages 1-2) In HBV-exposed individuals, carriers had lower LC/HCC incidence (13.14% vs 78.95%) and some achieved HBsAg clearance only in the variant group; these findings concern comorbidity/outcomes rather than routine GS diagnosis (yao2023geneticvariationsunderlying pages 1-2)
UGT1A1 Chromosome 2q37; OMIM *191740 c.1091C>T (p.Pro364Leu, P364L) Missense variant Reported in Chinese neonates with severe prolonged unconjugated hyperbilirubinemia; may blur GS vs Crigler-Najjar type II boundaries (kim2024shouldweconsider pages 1-2) Residual activity reported in retrieved context as ~35.6% of wild-type enzyme activity (kim2024shouldweconsider pages 1-2) Can present with bilirubin >15 mg/dL in neonates and respond to phenobarbital, later normalizing; illustrates that some UGT1A1 variants produce phenotypes overlapping GS and CN-II rather than classic mild adult GS alone (kim2024shouldweconsider pages 1-2)
UGT1A1 Chromosome 2q37; OMIM *191740 UGT1A1*36 [A(TA)6TAA], UGT1A1*37 [A(TA)9TAA] Promoter STR alleles Mentioned as alternative promoter-repeat alleles in pharmacogenetic context rather than classic GS-causing variants; 36 aligns with normal-function haplotypes, 37 with reduced-function haplotypes (gonzaleziglesias2024geneticvariationin pages 2-3) 36 generally reflects more normal promoter configuration; 37 associates with reduced activity via linkage with low-function haplotypes (gonzaleziglesias2024geneticvariationin pages 2-3) Important for genotype interpretation in pharmacogenomics and bilirubin phenotyping; less central than 28/6 for classic GS diagnosis in retrieved evidence (gonzaleziglesias2024geneticvariationin pages 2-3)

Table: This table summarizes the main UGT1A1 variants linked to Gilbert syndrome and related unconjugated hyperbilirubinemia phenotypes, including population patterns, functional effects, and pharmacogenomic relevance. It is useful for mapping disease genetics to clinical interpretation and drug-response implications.

4.3 Functional consequences

Reduced-function variants decrease bilirubin glucuronidation capacity; one review/source describes a promoter-repeat variant reducing activity to ~30% of normal, and genotype-activity relationships are reported (e.g., ~50% activity for 7/7 vs 6/6; ~80% for 6/7 vs 6/6). (goluch2024nutritioningilbert’s pages 2-3, gonzaleziglesias2024geneticvariationin pages 2-3)

4.4 Modifier genes / epigenetics / chromosomal abnormalities

No specific modifier genes or epigenetic mechanisms were captured in the retrieved GS-focused evidence snippets in this run.

5. Environmental Information

5.1 Lifestyle and nutritional factors

A 2024 PRISMA-based systematic review of GS-related clinical trials (1963–2023) emphasizes that caloric restriction and dietary composition can meaningfully alter bilirubin levels in GS and suggests practical strategies to reduce episodes (avoid excessive calorie restriction; consider dietary fats and bioactive compounds in certain plant families). (goluch2024nutritioningilbert’s pages 1-2, goluch2024nutritioningilbert’s media eb15b11f)

Visual evidence (systematic review evidence base): PRISMA flow diagram and trial-summary tables are available from the review. - PRISMA diagram and trial tables: (goluch2024nutritioningilbert’s media eb15b11f)

6. Mechanism / Pathophysiology

6.1 Causal chain (from trigger to phenotype)

  1. Heme/hemoglobin catabolism produces bilirubin.
  2. Unconjugated bilirubin must be taken up by hepatocytes and conjugated with glucuronic acid to become bile-soluble.
  3. UGT1A1 is the key enzyme mediating bilirubin glucuronidation; reduced UGT1A1 activity/expression yields impaired conjugation and accumulation of unconjugated bilirubin.
  4. Physiologic stressors (fasting/calorie restriction, illness, dehydration, exertion, etc.) transiently increase bilirubin or reduce clearance, triggering episodic jaundice in genetically predisposed individuals.

This mechanistic framing and the “bottleneck” concept at hepatic conjugation are explicitly described in the 2024 systematic review background. (goluch2024nutritioningilbert’s pages 1-2, goluch2024nutritioningilbert’s pages 2-3)

6.2 Molecular pathways and ontology suggestions

  • Suggested GO (process/function level): bilirubin metabolic process; glucuronosyltransferase activity; bilirubin glucuronosyltransferase activity (GO term IDs not retrieved in evidence chunks). (goluch2024nutritioningilbert’s pages 1-2)
  • Primary organ/cell type: liver and hepatocytes.
  • Suggested UBERON: liver (UBERON:0002107)
  • Suggested CL: hepatocyte (CL:0000182)
  • Evidence: UGT1A1 described as key hepatic conjugation enzyme. (yao2023geneticvariationsunderlying pages 1-2, gonzaleziglesias2024geneticvariationin pages 1-2)

6.3 Pharmacogenomics mechanism (drug metabolism)

UGT1A1 also glucuronidates drugs and xenobiotics; thus, reduced UGT1A1 function can increase exposure/toxicity of some UGT1A1 substrates (notably irinotecan’s active metabolite SN-38 in oncology pharmacogenomics literature). (takano2017ugt1a1polymorphismsin pages 1-2)

7. Anatomical Structures Affected

7.1 Organ/tissue/cell

  • Primary: liver (bilirubin conjugation) and hepatocytes. (gonzaleziglesias2024geneticvariationin pages 1-2)
  • Secondary/clinical manifestation site: skin/sclera (visible jaundice) as a downstream manifestation of hyperbilirubinemia. (beutler2024gilbertssyndromebrightand pages 1-4)

8. Temporal Development

  • Onset pattern: typically chronic lifelong predisposition with episodic hyperbilirubinemia; episodes can be triggered by environmental/physiologic stressors. (beutler2024gilbertssyndromebrightand pages 1-4, goluch2024nutritioningilbert’s pages 2-3)
  • Course: fluctuating; bilirubin may normalize after trigger resolution/diet normalization. (goluch2024nutritioningilbert’s pages 2-3)

9. Inheritance and Population

9.1 Inheritance

The retrieved sources treat GS as a genetically determined disorder due to UGT1A1 variants; a specific Mendelian inheritance label (e.g., autosomal recessive vs complex haplotype) was not consistently provided in the excerpts captured in this run.

9.2 Epidemiology (recently summarized)

Reported prevalence varies substantially by ancestry and region: - One 2024 primary-source background section summarizes ethnic prevalence patterns: Sub-Saharan African ~15–25%, Europeans ~5–10%, East Asian ~0–5%. (gonzaleziglesias2024geneticvariationin pages 1-2) - The 2024 systematic review summarizes wider ranges: ~3–16% overall, ~2% in East Asians, and up to ~20% in India/South Asia/Middle East. (goluch2024nutritioningilbert’s pages 1-2)

Sex differences are repeatedly noted, with male predominance in diagnosis; the nutrition systematic review reports male:female diagnosis ratios and example prevalence estimates by sex in secondary summaries. (goluch2024nutritioningilbert’s pages 1-2)

10. Diagnostics

10.1 Clinical tests and laboratory findings

Typical diagnostic pattern is repeated demonstration of isolated, predominantly unconjugated hyperbilirubinemia with normal liver enzymes and absence of hemolysis or structural liver disease. (goluch2024nutritioningilbert’s pages 2-3, gonzaleziglesias2024geneticvariationin pages 1-2)

Quantitative diagnostic context captured: - Normal bilirubin: ~0.1–1.2 mg/dL; GS often ~1.2–5.3 mg/dL. (gonzaleziglesias2024geneticvariationin pages 1-2) - GS-compatible ranges cited in one review: unconjugated hyperbilirubinemia usually ≤4–5 mg/dL with normal liver tests. (beutler2024gilbertssyndromebrightand pages 1-4)

10.2 Genetic testing (real-world implementation)

UGT1A1 genotyping/sequencing is used in selected clinical scenarios: - In a Korean infant cohort with prolonged jaundice (>21 days), 30/33 tested infants (90.9%) had UGT1A1 variants; the most frequent was c.211G>A (46.5%), followed by c.-3275T>G (30.2%). (kim2024shouldweconsider pages 1-2)

Direct abstract quote (Kim 2024): “Thirty-three infants agreed to UGT1A1 mutation analysis, and 30 (90.9%) were positive for UGT1A1 mutations.” (kim2024shouldweconsider pages 1-2)

10.3 Differential diagnosis

Reviews emphasize diagnosis by exclusion, including excluding hemolysis and other hereditary jaundice disorders such as Crigler–Najjar syndrome, Rotor syndrome, and Dubin–Johnson syndrome. (beutler2024gilbertssyndromebrightand pages 1-4)

11. Outcome / Prognosis

Overall, GS is described as benign with no evidence of liver injury and typically no disease-specific treatment requirement. (beutler2024gilbertssyndromebrightand pages 1-4, gonzaleziglesias2024geneticvariationin pages 1-2)

A key clinically relevant outcome is drug-related toxicity risk for certain UGT1A1 substrates/inhibitors (see Treatment/Applications). (takano2017ugt1a1polymorphismsin pages 1-2)

12. Treatment

12.1 Standard management

No disease-specific pharmacotherapy is typically required; management centers on reassurance, avoidance of triggers (e.g., prolonged fasting/caloric restriction), and appropriate diagnostic workup to exclude other causes. (goluch2024nutritioningilbert’s pages 1-2, beutler2024gilbertssyndromebrightand pages 1-4)

MAXO suggestions (knowledge-base oriented): - Patient education / counseling - Dietary modification / avoidance of prolonged fasting - Genetic testing (selected contexts)

(ontology suggestions summarized in artifact below).

Item (phenotype/diagnostic test/mechanism) Suggested ontology term(s) Brief description Evidence notes
Intermittent jaundice HPO: Jaundice (HP:0000952); HPO: Intermittent jaundice (suggested specific child term if used locally) Episodic mild scleral/skin icterus, usually benign and fluctuating Typical GS presentation is intermittent jaundice with otherwise normal liver tests; bilirubin usually ~1.2–5.3 mg/dL and often ≤4–5 mg/dL in adults (gonzaleziglesias2024geneticvariationin pages 1-2, beutler2024gilbertssyndromebrightand pages 1-4)
Unconjugated hyperbilirubinemia HPO: Hyperbilirubinemia (HP:0002904); HPO: Unconjugated hyperbilirubinemia (HP:0003154); LOINC: Total bilirubin in Serum/Plasma; Direct bilirubin in Serum/Plasma Core laboratory abnormality with predominantly indirect bilirubin elevation GS is defined by mild unconjugated hyperbilirubinemia; diagnostic ranges cited include bilirubin <5–6 mg/dL with direct bilirubin <0.7 mg/dL and no hemolysis/liver disease (goluch2024nutritioningilbert’s pages 2-3)
Normal liver enzymes HPO: Abnormality of liver physiology (negated/normal finding in local schema); LOINC: ALT, AST, ALP, GGT panels Absence of hepatocellular injury markers helps distinguish GS from liver disease 2024 healthy-volunteer study found higher bilirubin in UGT1A1 IM/PM phenotypes but no association with liver enzyme abnormalities (gonzaleziglesias2024geneticvariationin pages 1-2)
Triggered bilirubin rises with fasting/stress HPO: Abnormality of metabolism/homeostasis (context-dependent); MAXO: Dietary modification / avoidance of fasting (suggested); LOINC: serial bilirubin measurement Hyperbilirubinemia worsens during fasting, illness, dehydration, exertion, surgery, stress Caloric restriction can increase bilirubin 2–3-fold within 48 h; triggers include fasting >12 h, dehydration, infection, intense exercise, surgery, pregnancy, stress, alcohol, sleep deprivation (goluch2024nutritioningilbert’s pages 2-3, beutler2024gilbertssyndromebrightand pages 1-4)
Gallstones / cholelithiasis risk HPO: Cholelithiasis (HP:0001081) Recognized complication/comorbidity, especially with coexisting hemolysis UK Biobank analysis found cholelithiasis significantly higher in men with GS, OR 1.50 (95% CI 1.3–1.7); review sources also note gallstone risk (goluch2024nutritioningilbert’s pages 2-3, beutler2024gilbertssyndromebrightand pages 1-4)
UGT1A1 deficiency / reduced bilirubin glucuronidation GO: bilirubin glucuronosyltransferase activity; GO: glucuronosyltransferase activity; GO: bilirubin metabolic process Molecular defect is reduced UGT1A1-mediated conjugation of bilirubin Common variants include promoter TA repeat 28 and coding variant 6; promoter variant may reduce activity to ~30% of normal, with 7/7 genotype ~50% bilirubin glucuronidation relative to 6/6 (gonzaleziglesias2024geneticvariationin pages 2-3, goluch2024nutritioningilbert’s pages 2-3, yao2023geneticvariationsunderlying pages 1-2)
Hepatocyte involvement UBERON: liver (UBERON:0002107); CL: hepatocyte (CL:0000182); GO Cellular Component: endoplasmic reticulum membrane Primary affected cell type and organ for bilirubin conjugation UGT1A1 is the key bilirubin-conjugating enzyme in liver; GS reflects reduced hepatic glucuronidation rather than structural liver disease (yao2023geneticvariationsunderlying pages 1-2, gonzaleziglesias2024geneticvariationin pages 1-2)
Genetic confirmation / UGT1A1 testing LOINC: Molecular genetics tests for UGT1A1 (local implementation-specific); MAXO: Genetic testing; HPO: Abnormality of bilirubin metabolism Sequencing/genotyping can support diagnosis, especially atypical or prolonged cases Common tested variants include UGT1A128, 6, c.-3275T>G, c.211G>A; in prolonged infant jaundice, 30/33 tested infants had UGT1A1 variants (kim2024shouldweconsider pages 1-2)
Differential diagnosis exclusion MAXO: Diagnostic laboratory testing; HPO terms for hemolysis/liver disease used as exclusions Diagnosis is typically by exclusion of hemolysis and other hereditary/acquired jaundice disorders Reviews emphasize ruling out hemolysis, Crigler–Najjar syndrome, Rotor syndrome, Dubin–Johnson syndrome, and other liver disease before labeling GS (beutler2024gilbertssyndromebrightand pages 1-4, silva2025gilbert’ssyndromethe pages 3-5)
Supportive management / reassurance MAXO: Patient education; MAXO: Counseling; MAXO: Dietary modification Usually no disease-specific pharmacotherapy required; management focuses on education and trigger avoidance GS is generally benign, no hepatic toxicity demonstrated in 2024 volunteer data; avoiding excessive calorie restriction may reduce jaundice episodes (goluch2024nutritioningilbert’s pages 1-2, gonzaleziglesias2024geneticvariationin pages 1-2)

Table: This table maps the core clinical features, diagnostics, and mechanisms of Gilbert syndrome to practical ontology suggestions (HPO, LOINC, GO, UBERON, CL, MAXO). It is useful for structuring disease knowledge-base entries and annotating phenotypes, pathways, and clinical workflows with recent evidence.

12.2 Pharmacogenomics / precision medicine applications (real-world)

UGT1A1 genotyping is widely implemented in oncology and other drug-safety contexts; many trials and clinical programs use UGT1A1 (28/6) status to mitigate toxicity.

ClinicalTrials.gov examples (UGT1A1-guided dosing/PK; not GS-specific interventions): - NCT05148767 (posted 2022; recruiting; Phase 4): UGT1A1-guided irinotecan dosing in neoadjuvant chemoradiotherapy for locally advanced rectal cancer; requires UGT1A1 6 and 28 testing and assigns irinotecan dose by genotype. (NCT05148767 chunk 1) - NCT01523431 (posted 2012; completed; Phase 2/3): genotype-guided irinotecan dosing in metastatic colorectal cancer; homozygotes randomized to standard vs 50% reduced irinotecan dose. (NCT01523431 chunk 1) - NCT02680795 (posted 2016; completed; Phase 1): belinostat PK/safety stratified by UGT1A1*28 genotype. (NCT02680795 chunk 1)

12.3 Experimental / advanced therapeutics

For severe UGT1A1 deficiency disorders (Crigler–Najjar type I), gene replacement is under study (not GS): - NCT06641154 (posted 2024; recruiting; Phase 1/2): AAV8 vector carrying normal human UGT1A1 for Crigler–Najjar type I. (NCT06641154 chunk 1)

13. Prevention

Primary prevention of GS is not applicable (genetic predisposition). Practical prevention of symptomatic episodes is framed as trigger avoidance, particularly avoiding excessive caloric restriction/fasting and dehydration. (goluch2024nutritioningilbert’s pages 2-3)

14. Other Species / Natural Disease

No naturally occurring GS-equivalent disease in non-human species was captured in the retrieved GS-specific snippets; however, Ugt1a1 deficiency models exist (see Model Organisms).

15. Model Organisms

Model systems in the retrieved literature largely address UGT1A1 deficiency and neonatal hyperbilirubinemia (often more severe than classic GS), and also include humanized UGT1A1*28 mouse models used for drug-metabolism work.

Examples retrieved (not fully extracted as evidence snippets in this run but available as papers in the corpus): - Humanized UGT1 mouse models expressing human UGT1 locus and/or UGT1A1*28 allele for studying bilirubin metabolism and drug clearance. (silva2025gilbert’ssyndromethe pages 8-9) - Ugt1 locus knockout mice modeling severe unconjugated hyperbilirubinemia (Crigler–Najjar type I-like). (silva2025gilbert’ssyndromethe pages 8-9)

Recent developments and latest research (2023–2024 prioritized)

The table below summarizes key recent (2023–2024) evidence sources captured in this run.

Year First author Type (primary study/review/systematic review) Population/setting Key findings (quantitative where possible) PMID/DOI/URL Notes
2024 Goluch Systematic review of clinical trials Clinical studies in people with Gilbert syndrome, literature 1963–2023; 19 studies included GS described as the most common benign hyperbilirubinemia due to reduced UGT1A1 activity; prevalence reported as 3–16% overall, ~2% in East Asians, up to ~20% in India/South Asia/Middle East; ~90% of GS patients carry homozygous A(TA)7TAA (*28); fasting/caloric restriction can raise bilirubin 2–3× within 48 h; avoiding excessive calorie restriction and consuming certain fats/bioactive plant compounds may reduce jaundice episodes; episodes negatively affect quality of life (goluch2024nutritioningilbert’s pages 2-3, goluch2024nutritioningilbert’s pages 1-2, goluch2024nutritioningilbert’s media eb15b11f) DOI: 10.3390/nu16142247; https://doi.org/10.3390/nu16142247 Diet/nutrition; triggers; QoL; ethnicity-specific genetics
2024 González-Iglesias Primary study 773 healthy volunteers in 29 bioequivalence trials, Spain Bilirubin higher in UGT1A1 intermediate and poor metabolizers vs normal metabolizers; decreased uric acid in poor metabolizers; no association between UGT1A1 phenotype and liver enzyme levels; supports inclusion of likely GS participants in bioequivalence studies; paper cites prevalence by ancestry: Sub-Saharan African 15–25%, Europeans 5–10%, East Asian 0–5% (gonzaleziglesias2024geneticvariationin pages 2-3, gonzaleziglesias2024geneticvariationin pages 1-2) DOI: 10.3389/fphar.2024.1389968; https://doi.org/10.3389/fphar.2024.1389968 Pharmacogenomics; real-world trial eligibility; benign liver biochemistry
2024 Kim Primary study 74 Korean infants with prolonged jaundice >21 days; 33 underwent UGT1A1 testing 30/33 tested infants (90.9%) had UGT1A1 mutations; common variants were c.211G>A (46.5%) and c.-3275T>G (30.2%); breastfeeding was the only significant factor associated with mutation-positive cases (P=0.027); supports utility of UGT1A1 testing in prolonged unexplained neonatal jaundice (kim2024shouldweconsider pages 1-2) DOI: 10.5385/nm.2024.31.1.1; https://doi.org/10.5385/nm.2024.31.1.1 Neonatal/prolonged jaundice; East Asian variant spectrum
2024 Beutler Review Literature review GS summarized as the most common inherited jaundice affecting ~5–10% of people, more common in men; common promoter variant reduces UGT activity to ~30% of normal; typical unconjugated bilirubin usually up to ~4–5 mg/dL with normal liver tests; triggers include stress, alcohol, dehydration, heavy exercise, surgery, sleep deprivation, starvation; notes gallstones/cholelithiasis and hemolytic anemia associations (beutler2024gilbertssyndromebrightand pages 1-4) 2024 review; journal/PMID not available in context Clinical overview; triggers; comorbidity; protective vs adverse aspects
2023 Poynard Primary study UK Biobank, apparently healthy middle-aged Europeans with liver data (n=138,125) GS phenotypically defined using stratified total bilirubin centiles; in women, stratified approach identified 10% GS (7,741/76,809) vs 3.7% using the historical ≥1 mg/dL cutoff; after adjustment/Mendelian randomization, only cholelithiasis remained significantly higher in men with GS (OR 1.50, 95% CI 1.3–1.7; P=0.001); no adjusted survival difference over 15 years (poynard2023clinicalandgenetic pages 1-1) DOI: 10.1097/HC9.0000000000000245; https://doi.org/10.1097/HC9.0000000000000245 Population definition; sex-specific diagnostic thresholds; gallstones risk
2023 Yao Primary study Single-ethnic Chinese cross-sectional study; 2,792 screened for unconjugated hyperbilirubinemia, 175 with confirmed HBV exposure analyzed Five disease-causing UGT1A1 variants detected (28, 6, 27, 63, *7); cirrhosis/HCC incidence lower in UGT1A1-variant hosts than wild type (13.14% vs 78.95%, P<0.0001); HBsAg clearance in non-cirrhotic patients seen only in variant group (12.32% vs 0%); suggests mildly elevated bilirubin may be protective in HBV outcomes (yao2023geneticvariationsunderlying pages 1-2) DOI: 10.3389/fgene.2023.1265268; https://doi.org/10.3389/fgene.2023.1265268 Comorbidity/prognosis; potential protective biology of mild hyperbilirubinemia

Table: This table summarizes key 2023–2024 evidence sources for Gilbert syndrome, prioritizing recent primary studies and systematic reviews. It highlights epidemiology, genetics, triggers, diagnostics, pharmacogenomics, and comorbidity findings useful for a disease knowledge base.

Important note: Some identifier-level items (e.g., ICD/MeSH/Orphanet codes) and some mechanistic details (e.g., detailed transcriptional regulation of the TA-repeat promoter) were not directly available in the retrieved full-text snippets; this report avoids asserting those details without primary-source capture.

References

  1. (gonzaleziglesias2024geneticvariationin pages 1-2): Eva González-Iglesias, Dolores Ochoa, Manuel Román, Paula Soria-Chacartegui, Samuel Martín-Vilchez, Marcos Navares-Gómez, Alejandro De Miguel, Pablo Zubiaur, Andrea Rodríguez-Lopez, Francisco Abad-Santos, and Jesús Novalbos. Genetic variation in ugt1a1 is not associated with altered liver biochemical parameters in healthy volunteers participating in bioequivalence trials. Frontiers in Pharmacology, May 2024. URL: https://doi.org/10.3389/fphar.2024.1389968, doi:10.3389/fphar.2024.1389968. This article has 3 citations.

  2. (beutler2024gilbertssyndromebrightand pages 1-4): K Beutler and J Lewandowski. Gilbert's syndrome-bright and dark sides of the disease-literature review. Unknown journal, 2024.

  3. (yao2023geneticvariationsunderlying pages 1-2): Bilian Yao, Qi Xu, Xinxin Zhang, and Yue Han. Genetic variations underlying gilbert syndrome and hbv infection outcomes: a cross-sectional study. Frontiers in Genetics, Nov 2023. URL: https://doi.org/10.3389/fgene.2023.1265268, doi:10.3389/fgene.2023.1265268. This article has 3 citations and is from a peer-reviewed journal.

  4. (OpenTargets Search: Gilbert syndrome-UGT1A1): Open Targets Query (Gilbert syndrome-UGT1A1, 4 results). Buniello, A. et al. (2025). Open Targets Platform: facilitating therapeutic hypotheses building in drug discovery. Nucleic Acids Research.

  5. (kim2024shouldweconsider pages 1-2): Young Don Kim. Should we consider ugt1a1 mutation analysis in evaluating the prolonged jaundice of newborn infants? Neonatal Medicine, 31:1-8, Feb 2024. URL: https://doi.org/10.5385/nm.2024.31.1.1, doi:10.5385/nm.2024.31.1.1. This article has 0 citations.

  6. (goluch2024nutritioningilbert’s pages 1-2): Zuzanna Goluch, Aldona Wierzbicka-Rucińska, and Ewelina Książek. Nutrition in gilbert’s syndrome—a systematic review of clinical trials according to the prisma statement. Nutrients, 16:2247, Jul 2024. URL: https://doi.org/10.3390/nu16142247, doi:10.3390/nu16142247. This article has 2 citations.

  7. (goluch2024nutritioningilbert’s media eb15b11f): Zuzanna Goluch, Aldona Wierzbicka-Rucińska, and Ewelina Książek. Nutrition in gilbert’s syndrome—a systematic review of clinical trials according to the prisma statement. Nutrients, 16:2247, Jul 2024. URL: https://doi.org/10.3390/nu16142247, doi:10.3390/nu16142247. This article has 2 citations.

  8. (NCT05148767 chunk 1): Ji Zhu, MD. UGT1A1-Based Irinotecan Therapy for Locally Advanced Rectal Cancer. Zhejiang Cancer Hospital. 2022. ClinicalTrials.gov Identifier: NCT05148767

  9. (NCT01523431 chunk 1): Xu jianming. Individual Dosage Selection of Irinotecan (CPT-11) Based on UGT1A1 Genotype in Metastatic Colorectal Cancer Patients. The Affiliated Hospital of the Chinese Academy of Military Medical Sciences. 2012. ClinicalTrials.gov Identifier: NCT01523431

  10. (NCT02680795 chunk 1): Establish the PK of Belinostat in Patients With Wild-type, Heterozygous, and Homozygous UGT1A1*28 Genotypes. Acrotech Biopharma Inc.. 2016. ClinicalTrials.gov Identifier: NCT02680795

  11. (goluch2024nutritioningilbert’s pages 2-3): Zuzanna Goluch, Aldona Wierzbicka-Rucińska, and Ewelina Książek. Nutrition in gilbert’s syndrome—a systematic review of clinical trials according to the prisma statement. Nutrients, 16:2247, Jul 2024. URL: https://doi.org/10.3390/nu16142247, doi:10.3390/nu16142247. This article has 2 citations.

  12. (gonzaleziglesias2024geneticvariationin pages 2-3): Eva González-Iglesias, Dolores Ochoa, Manuel Román, Paula Soria-Chacartegui, Samuel Martín-Vilchez, Marcos Navares-Gómez, Alejandro De Miguel, Pablo Zubiaur, Andrea Rodríguez-Lopez, Francisco Abad-Santos, and Jesús Novalbos. Genetic variation in ugt1a1 is not associated with altered liver biochemical parameters in healthy volunteers participating in bioequivalence trials. Frontiers in Pharmacology, May 2024. URL: https://doi.org/10.3389/fphar.2024.1389968, doi:10.3389/fphar.2024.1389968. This article has 3 citations.

  13. (silva2025gilbert’ssyndromethe pages 5-6): Arjuna P De Silva, Nilushi Nuwanshika, Madunil A Niriella, and Janaka H De Silva. Gilbert’s syndrome: the good, the bad and the ugly. Unknown journal, May 2025. URL: https://doi.org/10.20944/preprints202405.0500.v1, doi:10.20944/preprints202405.0500.v1.

  14. (silva2025gilbert’ssyndromethe pages 3-5): Arjuna P De Silva, Nilushi Nuwanshika, Madunil A Niriella, and Janaka H De Silva. Gilbert’s syndrome: the good, the bad and the ugly. Unknown journal, May 2025. URL: https://doi.org/10.20944/preprints202405.0500.v1, doi:10.20944/preprints202405.0500.v1.

  15. (takano2017ugt1a1polymorphismsin pages 1-2): Masashi Takano and Toru Sugiyama. Ugt1a1 polymorphisms in cancer: impact on irinotecan treatment. Pharmacogenomics and Personalized Medicine, 10:61-68, Feb 2017. URL: https://doi.org/10.2147/pgpm.s108656, doi:10.2147/pgpm.s108656. This article has 141 citations and is from a peer-reviewed journal.

  16. (NCT06641154 chunk 1): Gene Therapy for Crigler Najjar Syndrome Type I (AlphaCN). Federal State Budget Institution Research Center for Obstetrics, Gynecology and Perinatology Ministry of Healthcare. 2024. ClinicalTrials.gov Identifier: NCT06641154

  17. (silva2025gilbert’ssyndromethe pages 8-9): Arjuna P De Silva, Nilushi Nuwanshika, Madunil A Niriella, and Janaka H De Silva. Gilbert’s syndrome: the good, the bad and the ugly. Unknown journal, May 2025. URL: https://doi.org/10.20944/preprints202405.0500.v1, doi:10.20944/preprints202405.0500.v1.

  18. (poynard2023clinicalandgenetic pages 1-1): Thierry Poynard, Olivier Deckmyn, Valentina Peta, Mehdi Sakka, Pascal Lebray, Joseph Moussalli, Raluca Pais, Chantal Housset, Vlad Ratziu, Eric Pasmant, and Dominique Thabut. Clinical and genetic definition of serum bilirubin levels for the diagnosis of gilbert syndrome and hypobilirubinemia. Hepatology Communications, Sep 2023. URL: https://doi.org/10.1097/hc9.0000000000000245, doi:10.1097/hc9.0000000000000245. This article has 8 citations and is from a peer-reviewed journal.

OpenScientist ▸
Gilbert's Syndrome — Comprehensive Disease Characteristics Report
openscientist-autonomous 29 citations 2026-08-28T11:26:57.678705

Gilbert's Syndrome — Comprehensive Disease Characteristics Report

Category: Mendelian (inborn error of bilirubin metabolism) Evidence basis: Primary literature (PMID-anchored), 10 confirmed findings across 5 investigation iterations, 42 papers reviewed.

Summary

Gilbert's Syndrome (GS) is a common, benign, inherited disorder of bilirubin metabolism characterized by mild, chronic, fluctuating unconjugated (indirect) hyperbilirubinemia in the absence of liver disease or overt hemolysis. It affects an estimated 3–10% of the general population and is caused by reduced transcriptional/enzymatic activity of UDP-glucuronosyltransferase 1A1 (UGT1A1), the hepatic enzyme responsible for conjugating bilirubin with glucuronic acid for biliary excretion. The canonical molecular lesion in European and African populations is homozygosity for a 2-bp thymine-adenine (TA) insertion in the UGT1A1 TATA-box promoter — the A(TA)7TAA allele, UGT1A1*28 (rs8175347) — which lowers UGT1A1 expression to roughly 30% of normal. In East Asians, the coding variant UGT1A1*6 (c.211G>A, p.Gly71Arg) is a major cause. The jaundice phenotype is inherited in an autosomal-recessive manner with incomplete, age- and sex-dependent penetrance and variable expressivity.

Clinically, GS is remarkable for what it does not do: it causes no liver damage, no reduction in life expectancy, and requires no disease-directed treatment. Jaundice is typically mild (total bilirubin usually <3 mg/dL, >70–80% unconjugated), intermittent, and provoked by fasting, dehydration, intercurrent illness, physical exertion, stress, or menstruation. Onset of biochemically detectable hyperbilirubinemia is usually around puberty/early adulthood, and the condition is more frequently diagnosed in men, peaking around age 35. Beyond jaundice, the most consistent associated symptoms are non-specific fatigue and abdominal pain.

The syndrome carries two areas of genuine clinical importance. First, mildly elevated unconjugated bilirubin is a potent circulating antioxidant that appears to confer protection against cardiovascular disease and all-cause mortality through antioxidant, anti-inflammatory, anti-thrombotic, and lipid-lowering effects. Second, GS is a clinically actionable pharmacogenomic trait: because UGT1A1 detoxifies several drugs, UGT1A1*28 homozygosity predisposes to severe toxicity from the chemotherapeutic irinotecan (via impaired clearance of its active metabolite SN-38) and to benign hyperbilirubinemia from the HIV protease inhibitor atazanavir. GS also acts as a genetic modifier that additively worsens neonatal hyperbilirubinemia when co-inherited with G6PD deficiency and other hemolytic or bilirubin-transport conditions, raising kernicterus risk in newborns.

Key Findings

Finding 1 — Molecular cause: reduced UGT1A1 activity, most often via the UGT1A1*28 promoter TA-repeat polymorphism

Gilbert syndrome is an inherited unconjugated hyperbilirubinemia caused by reduced transcriptional activity of UGT1A1 (UDP-glucuronosyltransferase 1A1), the gene located on chromosome 2q37. The canonical cause is homozygosity for a 2-bp (TA) insertion in the TATA-box promoter, producing A(TA)7TAA (UGT1A1*28, rs8175347) instead of the wild-type A(TA)6TAA; the extra repeat lowers transcription and enzyme activity to approximately 30% of normal. A defining review states that "Gilbert syndrome (GS) is characterized by unconjugated hyperbilirubinemia without liver disease or overt hemolysis and it is found in 3-10% of the general population. Inherited hyperbilirubinaemia is attributable to a reduced UGT1A1 activity" PMID: 28338110.

The TA-repeat locus forms an allelic series with graded functional consequences: an additional TA insert yields eight repeats — (TA)8 = UGT1A1*37 — causing further reduction of glucuronidation activity, whereas a variant lacking one repeat — (TA)5 = UGT1A1*36 — increases activity; both variants have been detected in Africans at frequencies up to 0.07 and 0.08 respectively PMID: 34089128. In East Asians, the coding variant UGT1A1*6 (c.211G>A, p.Gly71Arg) is a common cause, and the enhancer variant UGT1A1*60 (c.-3279T>G) frequently co-segregates; in one patient cohort "83.02% presented missense mutations at UGT1A1*60 (c.-3279T > G), 54.72% had heterozygous or homozygous insertions in the TATA box in the promoter, 52.38% had the UGT1A1*6 variant (c.211G > A, G71R)" PMID: 40038193.

Importantly, the genotype–phenotype relationship is incomplete: some A(TA)7TAA homozygotes have entirely normal bilirubin. As one study notes, "individuals with normal bilirubin levels and no clinical symptoms of Gilbert's syndrome may also present this in a homozygous condition" PMID: 25887876.

Ontology terms: Gene HGNC:12530 (UGT1A1); OMIM #143500 (Gilbert syndrome); CHEBI:16990 (bilirubin); GO:0015020 (glucuronosyltransferase activity); MONDO:0008738 (Gilbert syndrome). Suggested MeSH: "Gilbert Disease".

Finding 2 — Cardiovascular protection via antioxidant, anti-inflammatory, anti-thrombotic, and lipid-lowering effects

A striking and reproducible feature of GS is that the mildly elevated unconjugated bilirubin (UCB) is protective against atherosclerosis and cardiovascular disease. As stated directly, "Individuals with mildly elevated bilirubin concentrations (i.e., Gilbert syndrome; GS) are protected from atherosclerosis, cardiovascular disease, and related mortality" PMID: 26057938. Three mechanistic pillars support this:

  1. Increased antioxidant capacity. UCB scavenges hypochlorous acid (HOCl) and chloramines and inhibits myeloperoxidase-induced protein/lipid oxidation (measured as protein carbonyls and malondialdehyde) at physiologically relevant concentrations, validated in Gunn rat and human GS serum PMID: 26057938.

  2. Reduced platelet activation/thrombogenesis. In a matched GS vs control study (n=14/group), "A statistically significant decrease in the expression of P-selectin (P = 0.030) on activated platelets was observed in GS subjects. Collagen and AA-induced platelet aggregation were significantly (P = 0.018; P = 0.032 for respective agonists) reduced in GS versus control group. Elevated UCB (P = 0.001) and high density lipoprotein (P = 0.033) in addition to reduced low density lipoprotein (P = 0.024) and high sensitive C-reactive protein (P = 0.043) were also observed in GS" PMID: 28300459.

  3. Improved lipid and inflammatory profile. In a larger study (n=59/group), "GS subjects had significantly (P<0.05) improved lipid profile with reduced total cholesterol, LDL-C (LDL-cholesterol), TAG, low- and pro-atherogenic LDL subfractions (LDL-1+LDL-2), Apo-B, Apo-B/Apo-A1 ratio and lower IL-6 (interleukin 6) and SAA (serum amyloid A) concentrations" — effects that were especially pronounced in older GS subjects PMID: 23566065.

Ontology terms: GO:0016209 (antioxidant activity); GO:0030193 (regulation of blood coagulation); HP:0003330 (abnormality of bilirubin metabolism); CHEBI:16990 (bilirubin).

Finding 3 — UGT1A1*28 is a clinically actionable pharmacogenomic determinant of irinotecan (SN-38) toxicity

Irinotecan's active metabolite SN-38 is detoxified by hepatic UGT1A1 glucuronidation: "Its anticancer activity results from its bioactivation into SN-38 metabolite, which is cleared through glucuronidation by the hepatic enzyme uridine diphosphate-glucuronosyltransferase 1A1 (UGT1A1). In the general population, there is wide inter-subject variability in UGT1A1 enzyme activity related to UGT1A1 gene polymorphisms" PMID: 24977443. The UGT1A1*28 promoter polymorphism reduces SN-38 clearance, increasing exposure and the risk of severe neutropenia and diarrhea, particularly at doses above 180 mg/m². A pre-treatment blood genotype test can prevent this: "for doses higher than 180 mg/m(2), hematologic and digestive irinotecan-induced toxicities could be prevented in daily clinical practice by generalizing the use of a simple pharmacogenetic test before starting treatment" PMID: 24977443. The FDA irinotecan label and CPIC/pharmacogenetic guidelines recognize UGT1A1*28. Atazanavir (an HIV protease inhibitor) similarly causes benign hyperbilirubinemia more frequently in *28 carriers.

Ontology terms: NCIT:C1249 (Irinotecan); NCIT:C1876 (Atazanavir); pharmacogenomic biomarker (PharmGKB/CPIC "very important pharmacogene" UGT1A1).

Finding 4 — Epidemiology and clinical phenotype

GS is found in ~3–10% of the general population PMID: 28338110. A large UK primary-care EHR study (IQVIA, >11 million patients; 9,240 GS cases vs 150,846 controls) established the clinically-recorded prevalence and phenotype: "The estimated UK prevalence of GS was 180.4 per 100,000 (95% CI: 174.4-186.6), with diagnoses more common in men, peaking around age 35, and more frequent in areas of least social deprivation. Among 9,240 GS cases and 150,846 controls, machine learning identified key diagnostic themes including jaundice, abnormal liver function tests, abdominal pain, fatigue, bowel changes, and sleep disturbances. While most of these features appeared primarily in the year prior to diagnosis, only abdominal pain and fatigue were consistently more common in GS cases up to [5 years pre-diagnosis]" PMID: 40904555.

Note the difference between genotype prevalence (3–10%) and clinically diagnosed prevalence (~180/100,000) — most genotypic individuals never come to clinical attention, reflecting incomplete penetrance and the benign nature of the condition. Jaundice is typically mild and episodic, triggered by fasting, illness, dehydration, exertion, stress, or menstruation; total bilirubin is usually <3 mg/dL and predominantly unconjugated, with normal liver enzymes and no hemolysis.

Ontology terms: HP:0000952 (Jaundice); HP:0012378 (Fatigue); HP:0002027 (Abdominal pain); HP:0002904 (Hyperbilirubinemia); HP:0003073 (Unconjugated hyperbilirubinemia).

Finding 5 — Bilirubin pathway and gene–environment mechanism

The upstream pathway (KEGG hsa00860 porphyrin metabolism; Reactome heme degradation) is: heme → biliverdin-IXα → bilirubin-IXα → UGT1A1 glucuronidation → biliary excretion. "Heme oxygenase (HO) metabolizes heme into ferrous iron, carbon monoxide (CO), and biliverdin-IXα (BV), the latter being reduced into bilirubin-IXα (BR) by the biliverdin reductase-A (BVR)" PMID: 40002374. In GS, reduced UGT1A1 causes unconjugated bilirubin to accumulate.

Several gene–environment interactions modulate bilirubin: - Fasting/caloric restriction raises UCB in GS (the basis of the classic fasting provocation test). - Diet in neonates: in humanized UGT1 (hUGT1) mice, "hUGT1 mice that were fed breast milk developed severe hyperbilirubinemia because of suppression of UGT1A1 in the gastrointestinal tract. Formula-fed hUGT1 mice had lower serum levels of bilirubin, which resulted from induction of UGT1A1 in the gastrointestinal tract" — the mechanism of breast-milk jaundice, mediated by intestinal IKKα/β and NF-κB PMID: 21983082. - Xenobiotics: oral inorganic arsenic induces intestinal UGT1A1 via a Keap1-Nrf2/PXR pathway — "oral administration of iAs to neonatal hUGT1 mice that display severe neonatal hyperbilirubinemia leads to induction of intestinal UGT1A1 and a reduction in total serum bilirubin values" PMID: 36720308. - Metabolic signaling: bilirubin binds the nuclear receptor PPARα — "Bilirubin is an antioxidant with fat-burning actions by binding to the PPARα nuclear receptor transcription factor, improving insulin sensitivity, reducing inflammation, and reversing metabolic dysfunction" PMID: 39873298.

Ontology terms: GO:0006788 (heme oxidation); GO:0042167 (heme catabolic process); GO:0006789 (bilirubin conjugation); CHEBI:16990 (bilirubin); CHEBI:17033 (biliverdin); GO:0005789 (endoplasmic reticulum membrane — site of conjugation).

Finding 6 — Model organisms: the Gunn rat and the humanized UGT1 (hUGT1) mouse

Two principal models recapitulate UGT1A1 deficiency:

  1. Gunn rat (Rattus norvegicus, NCBI:txid10116): homozygous "jj" animals carry a spontaneous frameshift in Ugt1a1 causing complete loss of bilirubin glucuronidation and lifelong unconjugated hyperbilirubinemia — "The Gunn rat is a molecular and metabolic model of Crigler-Najjar syndrome type 1, which is characterized by lifelong unconjugated hyperbilirubinemia due to the lack of ... (UGT1A1)-mediated bilirubin glucuronidation" PMID: 27830550. Acute encephalopathy is inducible with sulfadimethoxine, which "displaces bilirubin from albumin and thus increases free bilirubin" PMID: 31578041, modeling kernicterus. The model is used to test hepatocyte transplantation, iPSC-derived hepatic stem-cell therapy ("bilirubinemia was significantly decreased (around 30% decrease, P < .05) and remained stable throughout the 6-month study" PMID: 31342573), and gene therapy.

  2. Humanized UGT1 (hUGT1) mouse: "We studied mice in which the original Ugt1 locus was disrupted and replaced with the human UGT1 locus (hUGT1 mice); these mice spontaneously develop neonatal hyperbilirubinemia and BIND [bilirubin-induced neurologic dysfunction]" PMID: 21983082. This model reproduces breast-milk jaundice and xenobiotic (PXR/CAR/Nrf2) regulation of intestinal UGT1A1.

Note these models represent the severe (Crigler-Najjar type 1) end of the UGT1A1 deficiency spectrum, not the mild GS phenotype; no dedicated animal model recapitulates GS itself.

Ontology terms: NCBI:txid10116 (Rattus norvegicus); NCBI:txid10090 (Mus musculus); orthologous rat gene Ugt1a1.

Finding 7 — UGT1A1 as a genetic modifier that worsens neonatal hyperbilirubinemia

GS co-inherited with hemolytic or transport conditions produces additive (cumulative) hyperbilirubinemia and elevated kernicterus risk. In a northern-Guangdong study of infants with unexplained jaundice, "These cases involved six diseases: Gilbert syndrome in 7 cases (12.5%), sodium taurocholate co-transporting polypeptide (NTCP) deficiency in 8 cases (14.2%), glucose-6-phosphate dehydrogenase (G6PD) deficiency in 4 cases (7.1%), a combination of Gilbert syndrome and G6PD deficiency in 5 cases (8.9%)" PMID: 42051946. More broadly, "Genetic variants may play an important role in an increased risk of neonatal hyperbilirubinemia, and severe jaundice in neonates may be related to a cumulative effect of genetic variants" PMID: 36051115.

The interaction with G6PD deficiency is clinically documented: "the presence of hyperbilirubinemia is not only associated with G6PD deficiency, but may be caused by the co-presence of a mutation in the UGTA1 promoter related to Gilbert's syndrome. As being affected by these two conditions predisposes to adverse effects towards certain drug treatments, it is advisable to study the UGTA1 gene before prescribing drugs" PMID: 22407023. Other co-acting bilirubin genes include OATP2/SLCO1B1, HMOX1, and BLVRA; one study found ~82% of hyperbilirubinemia cases carried ≥4 variants vs 37% of controls (P<0.0001) PMID: 27943244. Note: one G6PD study found no significant GS–hyperbilirubinemia association in its cohort PMID: 24783083, so the modifier effect is real but not universal across all populations/studies.

Ontology terms: Gene HGNC:4118 (G6PD); HGNC:10959 (SLCO1B1); HP:0001937 (Neonatal unconjugated hyperbilirubinemia).

Finding 8 — Diagnosis, differential diagnosis, and excellent prognosis

GS belongs to the congenital non-hemolytic hyperbilirubinemias (CNH), which are "characterized by an abnormal serum bilirubin level without other abnormalities in routine liver functional tests. Liver histology on light microscopy is normal" PMID: 16146029. Diagnosis is largely clinical/biochemical: - Isolated mildly elevated unconjugated (indirect) bilirubin (typically <3 mg/dL / <51 µmol/L, >70–80% unconjugated) - Normal ALT, AST, ALP, GGT - Normal hemoglobin, reticulocytes, haptoglobin, and blood smear (excluding hemolysis) - Normal hepatic imaging

Provocation testing supports diagnosis: 24–48h fasting or IV nicotinic acid raises UCB, and phenobarbital lowers it. The underlying biochemical defect is "a deficiency in hepatic bilirubin UDP-glucuronosyltransferase activity (B-GTA)" PMID: 98393, with an increased proportion of bilirubin monoglucuronide in bile; some cases also show a hepatic uptake (BSP kinetics) component. Molecular confirmation = UGT1A1 promoter (TA)n genotyping ± coding sequencing.

Differential diagnosis: Crigler-Najjar types I/II (severe unconjugated hyperbilirubinemia, kernicterus risk), Dubin-Johnson and Rotor syndromes (conjugated hyperbilirubinemia), hemolytic anemias, and hepatobiliary disease — "it should be differentiated from Crigler-Najjar syndrome and Dubin-Johnson syndrome" PMID: 30669779.

Prognosis is excellent: "Because CNH in adults are benign disorders and the prognosis is excellent, patients do not require any specific therapy" PMID: 16146029, and GS "prognosis is good in absence of special treatment" PMID: 30669779. There is no reduction in life expectancy; the course is lifelong and stable/episodic.

Finding 9 — Inheritance and population genetics

The GS jaundice phenotype is inherited autosomal-recessively, requiring homozygosity for the low-activity UGT1A1*28 (TA)7 allele (or compound states); it shows incomplete, age/sex-dependent penetrance and variable expressivity. The (TA)7 (rs8175347) risk allele frequency varies widely by ancestry, and "the low-activity (risk) alleles ((TA)(7) and (TA)(8)) are very frequent in Africans" PMID: 21309756. Reported allele frequencies include ~25.7% in Saudis ("The most common allele for (TA) repeats was the wild type (TA)6 with a frequency of 74.3% followed by the mutant (TA)7 (i.e., UGT1A1*28) with a frequency of 25.7%" PMID: 24049537) and ~39.8% in South Indians PMID: 22318545.

Bilirubin rises with allele dose: "Total bilirubin concentration in homozygous carriers of the -3279G and (TA)7 allele were significantly higher than those in heterozygous carriers or homozygous carriers of wild-type alleles" PMID: 17060921. In East Asians, the coding variant G71R (UGT1A1*6) dominates: among 63 Japanese GS patients, "Homozygous TA insertion in the TATA box (TA7) of the promoter region (TA7/7; 33%), homozygous G71R (9%), and combination of TA7/6 and heterozygous G71R (17%) were the most frequent findings" PMID: 15304120; Crigler-Najjar type II Japanese patients were homozygous for Y486D.

Population (TA)7 / *28 allele frequency Dominant variant
Equatorial Africans Highest; (TA)7 often most common allele (TA)7, (TA)8
South Indian ~39.8% (TA)7
Saudi/Arab ~25.7% (TA)7
European ~30–40% (allele) (TA)7 (*28)
East Asian Lower (TA)7; G71R common UGT1A1*6 (G71R)

Finding 10 — Management, prevention, and genetic counseling

GS requires no disease-directed treatment; management is reassurance about its benign nature to prevent unnecessary work-up PMID: 16146029. Where cosmetically or diagnostically desired, hepatic enzyme inducers (phenobarbital, and experimentally rifampicin) upregulate UGT1A1 and lower unconjugated bilirubin. This contrasts sharply with severe UGT1A1 deficiency: "Crigler-Najjar syndrome is the severe inherited form of unconjugated hyperbilirubinaemia due to mutations in the UGT1A1 gene, which can cause kernicterus early in life and can be even lethal when left untreated" — requiring phototherapy and liver transplantation, and motivating gene therapy PMID: 25315738.

Prevention centers on gene–environment triggers (avoid prolonged fasting/dehydration) and, most importantly, secondary prevention of drug toxicity: pre-treatment UGT1A1 (*28/*6) genotyping before irinotecan (dose reduction if homozygous) and awareness of benign hyperbilirubinemia with atazanavir/indinavir PMID: 24977443. In severe UGT1A1 deficiency, curative/experimental strategies under study in the Gunn rat and hUGT1 models include hepatocyte transplantation, hiPSC-derived hepatic stem-cell therapy (~30% durable bilirubin reduction over 6 months, P<0.05 PMID: 31342573), and AAV gene therapy — relevant to Crigler-Najjar, not needed for GS. Genetic counseling conveys: autosomal-recessive risk, benign prognosis, and high carrier frequency; no prenatal or newborn screening is indicated for GS itself, although UGT1A1 genotyping aids differential diagnosis and pharmacogenomics.

Ontology terms: NCIT:C739 (Phenobarbital); NCIT:C29524 (Genetic Counseling); NCIT:C15277 (Phototherapy — for CN, not GS).

Mechanistic Model / Interpretation

Gilbert's Syndrome is best understood as a partial loss-of-function of a single detoxifying enzyme whose principal substrate — bilirubin — happens to be a beneficial antioxidant. The causal chain is:

   GENETIC LESION                  ENZYME DEFECT              BIOCHEMICAL              CLINICAL
 ┌───────────────────┐        ┌───────────────────┐     ┌──────────────────┐    ┌──────────────────┐
 │ UGT1A1*28          │        │ ↓ UGT1A1           │     │ ↑ Unconjugated    │    │ Mild episodic     │
 │ A(TA)7TAA promoter │ ─────► │ transcription      │───► │ bilirubin (UCB)   │──► │ jaundice          │
 │ (or *6 G71R coding)│        │ → ~30% activity    │     │ in blood          │    │ (fasting/stress)  │
 └───────────────────┘        └───────────────────┘     └────────┬─────────┘    └──────────────────┘
recessive                    hepatocyte ER              │
incomplete penetrance                                   │
                                                ├──► ANTIOXIDANT ─► ↓ CVD, ↓ mortality
                                                │    (scavenges HOCl, ↓ platelet
                                                │     activation, ↓ LDL, PPARα)
                                                │
                                                └──► PHARMACOGENOMIC ─► ↑ SN-38 toxicity
                                                     (irinotecan), atazanavir jaundice

Upstream vs downstream. The upstream trigger is the germline UGT1A1 regulatory/coding variant. The proximal downstream consequence is reduced hepatic conjugation of bilirubin (a physiological byproduct of heme catabolism by heme oxygenase and biliverdin reductase A). The distal consequences bifurcate into a beneficial arm (elevated circulating antioxidant bilirubin conferring cardiovascular/metabolic protection) and a liability arm (impaired glucuronidation of xenobiotic drug substrates such as SN-38 and atazanavir, and additive contribution to neonatal hyperbilirubinemia).

Cell types and tissues. The primary cell is the hepatocyte (CL:0000182), where UGT1A1 resides in the endoplasmic reticulum membrane (GO:0005789). The intestinal epithelium is a key secondary site whose UGT1A1 is dynamically regulated by diet and xenobiotics (relevant to neonatal jaundice). The primary organ is the liver (UBERON:0002107); in severe UGT1A1 deficiency the vulnerable secondary organ is the brain (UBERON:0000955), specifically basal ganglia and cerebellum (kernicterus) — a risk essentially absent in GS but relevant to Crigler-Najjar and to GS as a neonatal modifier.

Gene–environment integration. GS is a paradigm of gene–environment interaction: an otherwise silent genotype becomes phenotypically manifest under fasting, dehydration, illness, or stress, and its severity is tunable by dietary (breast milk vs formula) and xenobiotic (arsenic, enzyme inducers) modulation of intestinal UGT1A1 through NF-κB, PXR/CAR, and Nrf2 signaling.

Evidence Base

PMID Topic Role in report
28338110 UGT1A1*28 in Romanian GS cohort Defines GS, prevalence (3–10%), reduced UGT1A1 activity
34089128 TaqMan detection of *28/*36/*37 TA-repeat allelic series and African frequencies
40038193 PAP-PCR for UGT1A1 polymorphisms Co-occurring *60/*28/*6 variants
25887876 Missense + A(TA)7TAA causes GS Most common genotype; incomplete penetrance
26057938 Bilirubin scavenges chloramines Antioxidant/CVD protection mechanism
28300459 UCB & platelet activation in GS Anti-thrombotic/lipid effects
23566065 Lipid/inflammation protection in GS Improved lipid & inflammatory profile
24977443 UGT1A1 genotyping & irinotecan Pharmacogenomic actionability
40904555 UK primary-care prevalence & symptoms EHR prevalence, demographics, symptoms
40002374 HO/BVR system Upstream heme→bilirubin pathway
21983082 Intestinal UGT1A1 & NF-κB in hUGT1 mice Breast-milk jaundice; hUGT1 model
36720308 Arsenic induces UGT1A1 via Nrf2/PXR Xenobiotic gene–environment interaction
39873298 HO/BVR/bilirubin & insulin resistance Bilirubin–PPARα metabolic link
27830550 Gunn rat as CN type 1 model Gunn rat characterization
31578041 Gunn rat preterm hyperbilirubinemia Sulfadimethoxine kernicterus model
31342573 iPSC hepatic stem cells in Gunn rats Cell-therapy efficacy (~30% reduction)
42051946 Genetic infant jaundice, Guangdong GS + G6PD as jaundice contributors
36051115 Severe neonatal hyperbilirubinemia cases Cumulative genetic-variant model
22407023 G6PD + GS family study UGT1A1 augments G6PD hyperbilirubinemia
24783083 GS & neonatal icterus in G6PD Null association (nuance)
27943244 Bilirubin gene variants Multi-gene additive effect (≥4 variants)
16146029 Congenital nonhemolytic hyperbilirubinemias Diagnostic hallmark; excellent prognosis
30669779 Inherited metabolic liver disease in adults Prognosis; key differentials
98393 Classification of hereditary hyperbilirubinemias B-GTA enzymatic defect
25315738 Gene replacement for hepatic jaundice Contrast with severe CN; gene therapy
21309756 UGT1A alleles in African populations High African risk-allele frequency
24049537 UGT1A1 polymorphisms in Saudis (TA)7 frequency ~25.7%
22318545 UGT1A1 in South Indians (TA)7 frequency ~39.8%
17060921 Intra-ethnic UGT1A1 in Chinese Allele-dose effect on bilirubin
15304120 UGT1A1 in Japanese CN/GS Asian G71R variant spectrum

Evidence source types: Human clinical/genetic (majority — case-control genotyping, EHR cohorts, matched physiological studies); model organism (Gunn rat, hUGT1 mouse); mechanistic/in-vitro (bilirubin antioxidant chemistry, platelet assays). Findings are strongly convergent across independent populations and study designs.

Section-by-Section Coverage of the Research Template

1. Disease Information. Benign inherited unconjugated hyperbilirubinemia. Identifiers: OMIM #143500; MONDO:0008738; ICD-10 E80.4; ICD-11 5C58.03; MeSH D005878 "Gilbert Disease"; Orphanet ORPHA:552. Synonyms: Gilbert-Meulengracht syndrome, constitutional hepatic dysfunction, familial non-hemolytic non-obstructive jaundice, unconjugated benign bilirubinemia. Information derives from both aggregated disease-level resources (OMIM/Orphanet) and individual-patient EHR (UK IQVIA study [PMID: 40904555]).

2. Etiology. Primary cause is genetic (UGT1A1 hypofunction). Genetic risk: homozygous UGT1A1*28/(TA)7, *37/(TA)8, *6/G71R, *60 enhancer. Environmental triggers: fasting, dehydration, illness, stress, exertion, menstruation. Protective genetic factor: (TA)5/*36 (increased activity). Protective environmental factors: adequate caloric intake. Gene–environment: fasting, diet, and xenobiotics modulate intestinal/hepatic UGT1A1 (Findings 5, 7).

3. Phenotypes. Jaundice (HP:0000952), unconjugated hyperbilirubinemia (HP:0003073), fatigue (HP:0012378), abdominal pain (HP:0002027). Onset: puberty/early adulthood. Severity: mild. Progression: episodic/fluctuating. QoL impact: minimal; non-specific fatigue may cause anxiety before diagnosis. (Finding 4)

4. Genetic/Molecular. Causal gene UGT1A1 (HGNC:12530, chr 2q37). Variant types: promoter TA-repeat (regulatory), missense (G71R, Y486D), enhancer SNP (-3279T>G). Germline. Functional consequence: partial loss of function. Modifier genes: G6PD, SLCO1B1/OATP2, HMOX1, BLVRA. (Findings 1, 7, 9)

5. Environmental. Fasting/dehydration, drug/xenobiotic exposure (arsenic induces UGT1A1). No infectious etiology, though intercurrent infection triggers jaundice. (Finding 5)

6. Mechanism. KEGG hsa00860; Reactome heme degradation; ER glucuronidation. Antioxidant, anti-thrombotic, PPARα metabolic effects. GO:0006789 (bilirubin conjugation), GO:0042167 (heme catabolism). (Findings 2, 5)

7. Anatomy. Primary organ: liver (UBERON:0002107); hepatocyte (CL:0000182); ER (GO:0005789). Secondary: intestinal epithelium; brain (only in severe spectrum). Digestive/hepatobiliary system; bilateral/systemic (skin, sclerae). (Findings 5, 6)

8. Temporal. Onset puberty/early adulthood; chronic lifelong; episodic/fluctuating course; self-limited flares resolving with removal of trigger. No progression to fibrosis. (Findings 4, 8)

9. Inheritance/Population. Autosomal recessive; incomplete age/sex-dependent penetrance; variable expressivity; high carrier frequency; marked ethnic variation (African > European/Indian > East Asian for (TA)7; G71R in Asians). Male-predominant clinical diagnosis. (Findings 4, 9)

10. Diagnostics. Isolated unconjugated hyperbilirubinemia with normal LFTs, no hemolysis, normal imaging/histology; fasting and nicotinic-acid provocation; phenobarbital suppression; UGT1A1 (TA)n genotyping. Differentials: Crigler-Najjar I/II, Dubin-Johnson, Rotor, hemolysis. (Finding 8)

11. Prognosis. Excellent; normal life expectancy; no specific therapy; potentially reduced CVD mortality. (Findings 2, 8)

12. Treatment. None required; reassurance. Phenobarbital (NCIT:C739) if desired. Pharmacogenomic dose adjustment for irinotecan. (Findings 3, 10)

13. Prevention. Primary: avoid fasting/dehydration triggers. Secondary: pre-treatment UGT1A1 genotyping (drug toxicity). Genetic counseling. No population screening for GS itself. (Finding 10)

14. Other Species / Natural Disease. Gunn rat (Rattus norvegicus, NCBI:txid10116) — naturally occurring Ugt1a1 frameshift; ortholog rat Ugt1a1. Represents severe (CN-1) end of spectrum. (Finding 6)

15. Model Organisms. Gunn rat (spontaneous mutant); humanized UGT1 (hUGT1) transgenic mouse; iPSC-derived hepatic stem cells; hepatocyte transplantation systems. These recapitulate severe UGT1A1 deficiency (kernicterus, BIND) rather than mild GS. Resources: MGI, RGD. (Finding 6)

Limitations and Knowledge Gaps

  1. No dedicated GS model. Existing animal models (Gunn rat, hUGT1 mouse) reproduce the severe Crigler-Najjar end of the UGT1A1 spectrum, not the mild GS phenotype. Mechanistic inferences about GS from these models must account for the difference between partial (~30%) and complete enzyme loss.

  2. Incomplete penetrance is unexplained. Many A(TA)7TAA homozygotes remain anicteric [PMID: 25887876], indicating additional modifiers (erythropoietic rate, sex hormones, other bilirubin-pathway genes) that are incompletely characterized.

  3. Causality of CVD protection. The cardiovascular/mortality benefit is supported by observational and mechanistic data [PMID: 26057938, 28300459, 23566065], but no randomized evidence establishes causality; residual confounding cannot be excluded, and bilirubin-raising therapeutics remain investigational.

  4. Modifier effect is not universal. While GS additively worsens neonatal hyperbilirubinemia in most cohorts, at least one G6PD study found no significant association [PMID: 24783083], suggesting population- and context-dependence.

  5. Quality-of-life data are thin. The contribution of GS itself (vs coincidental illness) to reported fatigue/abdominal pain is not rigorously separated; no validated GS-specific QoL instrument exists.

  6. This report is literature-derived, not based on primary molecular datasets; allele-frequency figures are drawn from individual cohort studies and may not represent gnomAD-scale reference estimates.

Proposed Follow-up Experiments / Actions

  1. Population-scale allele frequencies. Query gnomAD/1000 Genomes directly for rs8175347 and c.211G>A (G71R) frequencies across superpopulations to replace cohort-based estimates with reference-database values.

  2. Penetrance modeling. Analyze a biobank (e.g., UK Biobank) linking UGT1A1 genotype to serum bilirubin and covariates (sex, hemoglobin, fasting status) to quantify penetrance and identify modifiers.

  3. CVD causality. Perform Mendelian randomization using UGT1A1*28 as an instrument for lifelong bilirubin exposure vs cardiovascular endpoints to test causality of the protective association.

  4. Pharmacogenomic implementation audit. Evaluate real-world uptake and outcomes of pre-irinotecan UGT1A1 genotyping (CPIC-guided dosing) to quantify toxicity reduction.

  5. Neonatal risk stratification. Prospectively test a combined UGT1A1 + G6PD + SLCO1B1 panel for predicting severe neonatal hyperbilirubinemia and kernicterus risk.

  6. Bilirubin therapeutics. Advance nanoparticle/biosynthetic bilirubin delivery toward controlled trials for metabolic/cardiovascular indications, leveraging GS as a "natural experiment."

Report compiled from 10 confirmed findings and 42 reviewed papers across 5 investigation iterations. Evidence is predominantly human clinical/genetic, supported by model-organism and in-vitro mechanistic studies.

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