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UGT1A1
UDP glucuronosyltransferase family 1 member A1
UGT1A1 encodes the only enzyme capable of converting toxic bilirubin into a form the body can safely eliminate, with variants affecting bilirubin metabolism and drug processing. The UGT1A1 gene provides instructions for making bilirubin-UGT, an enzyme exclusively responsible for processing bilirubin - a breakdown product of red blood cells.
UGT1A1 is located on the long (q) arm of chromosome 2, at band 2q37.1. Arm ratio per GRCh38 - banding schematic.
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Overview
UGT1A1 belongs to the UDP-glucuronosyltransferase gene family, which encodes enzymes that attach glucuronic acid molecules to various substances to facilitate their elimination from the body. The protein produced by UGT1A1 performs the critical task of glucuronidating bilirubin, transforming it from an insoluble, toxic compound into a water-soluble form that can be excreted in bile.
This gene is particularly important in newborns, who produce substantial amounts of bilirubin as foetal haemoglobin is replaced. Variants in UGT1A1 are associated with a spectrum of disorders where unconjugated bilirubin accumulates in the blood, from the common and generally benign Gilbert syndrome to the rare and severe Crigler-Najjar syndromes. The gene also plays a role in pharmacogenomics, as UGT1A1 variants can affect how individuals metabolise certain medications.
What the gene does
The bilirubin-UGT enzyme catalyses a conjugation reaction in which glucuronic acid is transferred from UDP-glucuronic acid to unconjugated bilirubin. This chemical modification converts lipophilic (fat-soluble) bilirubin into a hydrophilic (water-soluble) conjugate that can dissolve in bile and be eliminated through the digestive system.
The enzyme operates primarily within hepatocytes - the main functional cells of the liver - where bilirubin processing occurs as part of normal metabolism. Bilirubin is produced when red blood cells break down, and must be processed before excretion to prevent toxic accumulation in tissues. Once conjugated, bilirubin becomes soluble in bile, a liver-produced fluid, and leaves the body with solid waste.
Beyond bilirubin, UGT1A1 demonstrates activity towards certain pharmaceutical compounds and other endogenous substrates, contributing to phase II metabolism - the body's system for modifying substances to make them easier to eliminate. This broader metabolic role means that genetic variation in UGT1A1 can influence individual responses to medications, particularly drugs that undergo glucuronidation as part of their clearance pathway.
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Chromosome location
UGT1A1 is located on the long arm of chromosome 2 at position 2q37.1, near the end of the chromosome. This chromosomal region contains a cluster of related UDP-glucuronosyltransferase genes that likely arose through duplication events during evolution. The gene's position within this cluster influences its regulation and expression patterns.
Protein structure
The UGT1A1 protein comprises 533 amino acids and localises to the endoplasmic reticulum membrane within liver cells, where it can access substrates and cofactors required for the glucuronidation reaction. Like other members of the UGT superfamily, the protein is thought to contain regions responsible for substrate recognition and catalytic activity.
Key variants
At least 85 variants in UGT1A1 have been documented in association with Crigler-Najjar syndrome. The severity of clinical effects depends on the degree to which variants reduce enzyme activity. Complete loss of function in both gene copies causes Crigler-Najjar syndrome type I, whilst variants that permit residual enzyme function result in the milder type II form. Changes in UGT1A1 also cause Gilbert syndrome, which occurs worldwide with some mutations seen more frequently in particular populations.
Sample of pathogenic variants
1 pathogenic / likely-pathogenic variants from ClinVar, ranked by review status (expert-panel-reviewed first). This is a sample; recurrent founder variants in a specific population may not appear here - see the full ClinVar listing via the link above.
| Variant (HGVS) | Protein change | Classification | Evidence | Associated condition |
|---|---|---|---|---|
UGT1A1, 1-BP DEL, 1223A | - | Pathogenic | - | Crigler-Najjar syndrome, type II |
Evidence stars indicate ClinVar review status. Individual variant interpretation should always be performed by a qualified clinical laboratory - many variants remain classified as Variants of Uncertain Significance (VUS) pending more research.
Associated conditions
Pathogenic variants in UGT1A1 cause three main inherited conditions affecting bilirubin processing. Crigler-Najjar syndrome type I represents the most severe form, with complete absence of enzyme activity leading to dangerously high bilirubin levels and risk of neurological damage. Crigler-Najjar syndrome type II involves partial enzyme deficiency and typically presents with less severe hyperbilirubinaemia. Gilbert syndrome is a common, benign condition characterised by intermittent mild jaundice, often triggered by fasting, illness, or stress. Crigler-Najjar syndromes occur when both copies of the UGT1A1 gene in each cell are altered, following autosomal recessive inheritance.
Inheritance pattern
Conditions caused by pathogenic UGT1A1 variants typically follow autosomal recessive inheritance.
When both parents are carriers, each child has a 25% chance of being affected, 50% of being a carrier, and 25% of being unaffected.
UK clinical status
UGT1A1 appears on several NHS Genomic Medicine Service gene panels with green (high-evidence) classification. These include the Cholestasis panel, Neonatal Cholestasis panel, Likely Inborn Error of Metabolism panel, and Undiagnosed Metabolic Disorders panel. The gene's inclusion reflects its established role in inherited disorders affecting bilirubin metabolism and liver function. Clinical testing through NHS pathways is available for individuals presenting with unexplained jaundice or hyperbilirubinaemia, particularly in neonatal or paediatric contexts.
Sources: NHS GMS PanelApp · Genomics England PanelApp · NHS National Genomic Test Directory
Frequently asked questions
What is Gilbert syndrome and how does it relate to UGT1A1?
Gilbert syndrome is a common, generally harmless condition caused by reduced UGT1A1 enzyme activity, typically resulting from a promoter variant that decreases gene expression. Affected individuals experience intermittent mild jaundice, particularly during periods of fasting, illness, or physical stress, but the condition does not usually require treatment and does not affect life expectancy.
Can UGT1A1 variants affect medication response?
Yes, certain UGT1A1 variants can influence how the body processes medications that undergo glucuronidation. This is particularly relevant for some chemotherapy agents, where reduced enzyme activity may increase the risk of side effects. Pharmacogenomic testing can help guide medication dosing in these situations.
How do Crigler-Najjar syndromes differ from Gilbert syndrome?
Crigler-Najjar syndromes are much rarer and more severe than Gilbert syndrome. Type I involves complete absence of UGT1A1 enzyme activity and causes dangerously high bilirubin levels requiring intensive treatment, whilst type II retains some enzyme function and presents with moderate hyperbilirubinaemia. Gilbert syndrome, by contrast, causes only mild, intermittent elevation of bilirubin without serious health consequences.
References
- Long J, Zhang S, Fang X. Association of neonatal hyperbilirubinemia with uridine diphosphate-glucuronosyltransferase 1A1 gene polymorphisms: meta-analysis. Pediatrics international : official journal of the Japan Pediatric Society. 2011. PMID: 21342357
- Owens IS, Basu NK, Banerjee R. UDP-glucuronosyltransferases: gene structures of UGT1 and UGT2 families. Methods in enzymology. 2005. PMID: 16399340
- Gong QH, Cho JW, Huang T. Thirteen UDPglucuronosyltransferase genes are encoded at the human UGT1 gene complex locus. Pharmacogenetics. 2001. PMID: 11434514
- Maruo Y, Nishizawa K, Sato H. Prolonged unconjugated hyperbilirubinemia associated with breast milk and mutations of the bilirubin uridine diphosphate- glucuronosyltransferase gene. Pediatrics. 2000. PMID: 11061796
- King CD, Rios GR, Green MD. UDP-glucuronosyltransferases. Current drug metabolism. 2000. PMID: 11465080