The uridine diphosphate glucuronosyltransferase (UGT) enzymes are a superfamily of enzymes responsible for the glucuronidation of target substrates. The transfer of glucuronic acid renders xenobiotics and other endogenous compounds water soluble, allowing for their biliary or renal elimination [Article:18518849]. The UGT family is responsible for the glucuronidation of hundreds of compounds, including hormones, flavonoids and environmental mutagens [Article:18518849]. Most of the members of the UGT family are expressed in the liver, as well as in other types of tissues, such as intestinal, stomach or breast tissue. A few members are expressed only extrahepatically, such as UGT1A7, UGT1A8, UGT1A10 and UGT2A1 [Article:12893990]. Four families exist within the UGT superfamily: UGT1A, UGT2, UGT3 and UGT8 [Article:16141793]. UGT2 is further divided into two subfamilies, UGT2A and UGT2B, both of which are present on chromosome 4 [Article:12893990]. UGT2A enzymes are involved in the glucuronidation of compounds such as phenolic odorants and polycyclic aromatic hydrocarbon metabolites, though limited studies have been done on this subfamily [Article:23086198]; UGT2B proteins are mainly responsible for the metabolism of steroids [Article:11159850]. The roles of UGT3 and UGT8 family members have not been well characterized [Article:16141793]. The UGT1A family is located on chromosome 2q37, and the members of this group glucuronidate a large variety of compounds. Pharmaceutical drugs are also a common substrate of the UGT family [Article:18518849], which makes the enzymes in this group relevant to pharmacogenetic research.
The UGT1A gene locus
The UGT1A gene locus enables the transcription of nine unique enzymes, UGT1A1, and UGT1A3 through UGT1A10 [Articles:8467709, 18518849]. This occurs by exon sharing, in which one of nine unique exon 1 sequences at the 5' end of the gene is combined with the common exons 2-4 and 5a at the 3' end, forming individual UGT1A transcripts [Article:11434514] (Figure 1). Alternatively spliced isoforms of UGT1A exist, and are formed when exon 5b (seen in the common exon region) is used instead of, or in addition to, exon 5a [Article:18004212]. These alternative isoforms are known as isoforms 2 or UGT1As_i2, and are enzymatically inactive [Articles:18004212, 19997083]. Additionally, four UGT1A pseudogenes exist: UGT1A2p, 11p, 12p and 13p [Articles:11434514, 15179404]. These pseudogenes can be seen in Figure 1, along with the location of two principal UGT1A1 pharmacogenetic variants, *28 and *6, both of which are discussed in detail further on within this summary.
| Figure 1. A graphic representation of the human UGT1A locus (not drawn to scale). (A) The locus spans approximately 200 kbp and contains multiple alternative first exons, which together constitute exon 1. Each unique first exon has its own promoter site. The individual exons for each isoform are combined with the common exons 2-4 and 5a by splicing out any intervening sequence. Exons 2-4 and 5a are therefore present in every UGT1A isoform. However, alternatively spliced UGT1A isoforms do exist, and are known as isoforms 2 or UGT1As_i2; these are created when exon 5b is used instead of, or in addition to, exon 5a. An example of the formation of UGT1A4 mRNA is also shown. In (A) the promoter for UGT1A4 can be seen upstream of the gene, (B) shows the pre-mRNA formed after transcription, and (C) shows the final UGT1A4 mRNA transcript after splicing. Though splicing occurring for common exons 2-5a has not been shown in this figure, it is important to note the absence of exon 5b in (C); this alternative exon has been spliced out in order to create the classical form of UGT1A4. The figure also shows the location of two important UGT1A1 pharmacogenetic variants, *28 and *6, both of which are discussed in detail further on. Adapted from [Articles:18491077, 19794410]. |
One of the transcripts encoded by the UGT1A locus is UGT1A1, which is at the furthest 3' end of the UGT1A exon 1 region. UGT1A1 is expressed hepatically as well as within the colon, intestine and stomach [Articles:10748067, 9765507]. One of the main functions of UGT1A1 lies within the liver, where it is the sole enzyme responsible for the metabolism of bilirubin, the hydrophobic breakdown product of heme catabolism [Articles:8027054, 18518849]. In general, UGT1A enzymes have considerable overlap in substrate specificities [Article:10836148], however no other isozyme can substitute for the bilirubin glucuronidation activity of UGT1A1 [Article:18518849]. Additionally, no effective alternative pathways exist for the detoxification and elimination of bilirubin, excluding that of photoisomeration, a relatively inefficient pathway as compared to UGT1A1 glucuronidation [Article:12891498]. Patients with Crigler-Najjar Type I (CN1) disease (discussed below) act as models for this concept: they are either homozygotes or compound heterozygotes for inactive enzyme variants, and are also incapable of glucuronidating or eliminating bilirubin [Article:15712364].
Currently, 113 different UGT1A1 variants have been described throughout the gene. These variants can confer reduced or increased activities, as well as inactive or normal enzymatic phenotypes. These individual variants are described as alleles by the UGT nomenclature committee, and denoted by the * symbol followed by a number.
UGT1A1 alleles and their role in disease
Homozygotes or compound heterozygotes for inactive UGT1A1 alleles have a complete absence of bilirubin glucuronidation and removal, leading to a high serum level of unconjugated bilirubin (hyperbilirubinemia), and a serious condition known as Crigler-Najjar Type I (CN1) disease [Article:18518849]. If left untreated, CN1 is invariably fatal [Article:19830808]. The development of hyperbilirubinemia results in kernicterus, or the buildup of bilirubin within brain tissue. This causes irreversible neurological damage, leading to severe disability or death. Intensive phototherapy can keep bilirubin levels in check, but becomes less effective with age, and the only definitive treatment is liver transplantation [Article:12891498].
Crigler-Najjar Type 2 (CN2), also results from mutations within the UGT1A1 gene, but some residual enzymatic activity remains, conferring a milder phenotype [Article:19830808]. This type can be treated successfully with phenobarbital, which induces expression of UGT1A1, allowing for reduction of unconjugated bilirubin to innocuous levels [Articles:4897277, 3306242, 7989595]. Kernicterus may still develop, however, if bilirubin levels are enhanced, such as during sepsis or trauma [Article:19830808].
Gilbert's syndrome represents the least severe of the inherited unconjugated hyperbilirubinemia conditions [Article:11013440], and results from UGT1A1 glucuronidation activity that is approximately 30% of normal [Article:7565971]. Patients with Gilbert's have fluctuating bilirubin levels, which are often within the standard range [Article:7565971]. Illness, stress or fasting can precipitate a rise in bilirubin levels, leading to hyperbilirubinemia, and symptoms such as jaundice or abdominal discomfort. However, these symptoms will typically resolve themselves, and the syndrome is harmless in adults [Article:7565971]. Though the condition is benign in itself, it is an indicator of reduced UGT1A1 activity, and is therefore important to consider in the context of drug toxicity. Gilbert's syndrome can be caused by a variety of genetic changes, but within Caucasian and African American populations it is most commonly attributed to the UGT1A1*28 variant allele (rs8175347) [Article:9653159]. This allele represents seven thymine-adenine (TA) repeats within the promoter region, as opposed to six that characterizes the wild-type allele (UGT1A1*1) [Article:17898154]. These extra repeats impair proper transcription of the gene, resulting in decreased transcriptional activity of the gene by approximately 70% [Articles:7565971, 12181419]. A different allele, UGT1A1*37, has eight TA repeats at this site, and results in reduced promoter activity to levels lower than that of promoters with the UGT1A1*28 allele [Articles:10091406, 9653159]. In contrast, the allele UGT1A1*36 has only five repeats, and is associated with increased promoter activity of the gene and a reduced risk of neonatal hyperbilirubinemia, a common and typically benign condition [Articles:9653159, 10190918]. In Asian populations, the UGT1A1*6 allele is more common [Article:9784835]. This variant results from a glycine to arginine change at position 71 within the coding region (Arg71Gly; 211G>A; rs4148323) [Article:16609363]. Individuals homozygous for this allele have enzymatic activity at 32% of normal, and can present with Gilbert's syndrome [Article:9630669] as well as neonatal hyperbilirubinemia [Article:10353933].
UGT1A1 alleles have also been associated with the development of various cancers. Along with bilirubin and pharmaceuticals, UGT1A1 enzymes have been seen to glucuronidate benzo(α)pyrene-trans-7,8-dihydrodiol, a precursor to the potent carcinogen
benzo(α)pyrene-7,8-dihydrodiol-9,10-epoxide, which is found in charbroiled food, coal tar, and cigarette smoke [Article:11929814]. They have also been noted to glucuronidate estradiol [Article:12386134], as well as 2-amino-1-methyl-6-phenylimidazo(4,5-b)pyridine (PhIP), another carcinogen present in cooked meat [Article:15986396]. UGT1A1 therefore exhibits a protective effect against benzo(α)pyrene and PhIP-mediated carcinogenicity, and modulates levels of estradiol within the body [Articles:11929814, 12386134, 15986396]. The *28 allele has been shown to increase the risk of developing colorectal and breast cancer across multiple studies in Chinese and Caucasian populations 15111762, 10706110, 22559977. The *6 allele was seen to increase the risk of colorectal cancer in one study in a Chinese cohort [Article:15929176]. Since these alleles result in reduced UGT1A1 activity, any associations seen are potentially due to increased exposure to carcinogens and estradiol [Article:18518849]; increased levels of the latter are associated with the development of breast cancer [Article:21813404]. However, several studies have shown no associations between the *28 allele and risk of breast cancer [Articles:17949292, 11401924], and one showed a decreased risk of breast cancer in *28 carriers [Article:15318931].
There is also evidence suggesting that the UGT1A1*28 allele may offer protection from cardiovascular disease (CVD). Bilirubin is a known antioxidant, and is thought to be capable of preventing plaque formation leading to atherosclerosis [Article:20693308]. Multiple studies have found a link between low bilirubin concentrations and an increased risk of CVD [Article:20562445], though this association may be affected by confounders such as obesity or cholesterol levels [Articles:20562445, 22416852]. Since the *28 allele impairs transcription of the UGT1A1 gene, it is associated with significantly increased bilirubin concentrations, and therefore could be an important biomarker for predicting those at decreased risk of CVD [Article:21411679]. Additionally, testing for associations between the *28 allele and CVD allows for a Mendelian randomization approach, which helps avoid confounding or reverse causation, limitations present in the studies linking bilirubin levels with CVD [Articles:22416852, 22805420]. A 2006 study utilizing the Framingham Heart Study population followed 1780 individuals for 24 years, and found that those with the *28/*28 genotype had one third the risk for cardiovascular disease as compared to those with the *1/*28 or *1/*1 genotypes [Article:17000907]. However, additional studies and meta-analyses [Articles:20562445, 12816916], including one with over 67,000 participants [Article:22805420], have failed to find a link between the *28 allele or bilirubin levels and risk for cardiovascular disease or myocardial infarction. (Note: PMID 22805420 conducted analyses using the rs6742078 variant, shown to be in strong linkage disequilbrium with rs8175347).
UGT1A1 allele frequencies
UGT1A1*28 occurs with a frequency of 0.26 - 0.31 in Caucasians, and 0.42 - 0.56 in African Americans, and only 0.09 - 0.16 in Asian populations [Articles:10591539, 9653159]. UGT1A1*6 has allele frequencies in Japanese, Korean and Chinese populations of 0.13, 0.23 and 0.23, respectively [Article:9784835]. Both UGT1A1*36 and UGT1A1*37 occur almost exclusively in populations of African origin, with estimated allele frequencies of 0.03 - 0.10 and 0.02 - 0.07, respectively [Articles:9653159, 10591539, 15388579].
Both the *28 and *6 alleles have been well studied in regard to pharmaceutical toxicities. In particular, both alleles have shown associations with the development of irinotecan toxicities [Articles:20562211, 17728214, 19390945]. Irinotecan is a topoisomerase I inhibitor, and is primarily used to treat colorectal cancer, though it is also used to treat solid tumors within other organs [Article:19852077]. As a pro-drug, irinotecan is converted by carboxylesterases to SN-38, a metabolite which has 100-fold higher anti-tumor activity than its parent compound [Article:1651156]. UGT1A1 is the predominant isoform responsible for the glucuronidation of this toxic metabolite, enabling its eventual excretion. However, in vitro studies show that UGT1A7 and UGT1A9 are also involved in SN-38 glucuronidation [Article:12181437]. Irinotecan has a very narrow therapeutic range, and treatment can lead to a variety of side effects, mainly neutropenia and diarrhea, which can be severe enough to reduce dosage or discontinue the drug [Article:18349289]. Indeed, approximately 7% patients who undergo irinotecan treatment and present with severe neutropenia and fever will die from these complications [Article:18466101]. Several studies have also shown that genotyping for the *28 allele before irinotecan treatment for colorectal cancer is cost-effective [Articles:18466101, 19517472], suggesting that testing for this allele may have a place in clinical practice.
Besides irinotecan, UGT1A1 is also responsible for the glucuronidation of drugs such as raloxifene [Article:12019197] and etoposide [Article:12969965], and some associations have been reported between the *28 allele and pharmacokinetic and pharmacodynamic parameters for these drugs [Articles:19371317, 12969965]. Additionally, the development of hyperbilirubinemia during treatment with inhibitors of UGT1A1, such as atazanavir and tranilast, has also been linked to the presence of the *28 allele [Articles:14647407, 17058217].
It has been suggested that including variants from other UGT1A isoforms may lead to stronger associations with drug side effects and pharmacokinetic measures. UGT1A7 exhibits a five-fold higher specific activity for the SN-38 metabolite than UGT1A1 [Articles:10381366, 12181437], and the inclusion of UGT1A7 alleles into association studies with irinotecan toxicity have shown persuasive results: the combination of UGT1A1*28 with UGT1A7*2 and UGT1A7 -57T/G alleles was superior for prediction of neutropenia and dose reduction, as compared to the UGT1A7 or UGT1A1*28 alleles alone. Indeed, UGT1A1*28 allele by itself showed no association with neutropenia or dose reduction in this particular study [Article:18349289]. The UGT1A7 alleles analyzed were associated with a reduction in either glucuronidation activity or transcription activity, providing a mechanistic explanation for the increased risk of toxicity seen [Article:18349289]. A later study by Cecchin et al. found that in multivariate analyses, UGT1A7*3 was the only significant predictor of hematologic toxicity in the first cycle of treatment with FOLFIRI (fluorouracil, leucovorin and irinotecan); UGT1A1*28 was not a predictor of toxicity [Article:19364970]. Another study by Lévesque et al. in patients taking FOLFIRI found in multivariate analyses that UGT1A7*4 (rs11692021) and UGT1A6*5 (rs2070959) were both significant predictors of neutropenia, while UGT1A1*28 was not [Article:23386248]. UGT1A7*4 is associated with a reduction in glucuronidation activity [Articles:12181437, 11037804] which may explain its association with increased risk for neutropenia. UGT1A6 has been shown to glucuronidate SN-38 [Articles:12181437, 10381366], though no information is currently available on how the *5 allele may affect the enzyme. The study also found a dosage effect when considering multiple alleles: assessing UGT1A7*4 and UGT1A6*5 together as a haplotype gave an odds ratio of 2.18 for the development of neutropenia; including the UGT1A9 -688A/C variant allele in the haplotype increased the odds ratio to 5.28. This result suggests that considering multiple UGT1A variants may improve risk prediction for neutropenia [Article:23386248]. In patients taking atazanavir, Lankish et al. found that the combination of the UGT1A1*28, UGT1A7 -57G and UGT1A7*2, and UGT1A3 -66C variants was associated with increased risk of jaundice and hyperbilirubinemia [Article:17058217]. Approximately 20% of atazanavir-treated subjects were homozygous for this haplotype, compared to 40% of atazanavir-treated subjects who presented with grade 3 or 4 hyperbilirubinemia a statistically significant difference. In subjects with exclusively grade 4 hyperbilirubinemia, 100% were homozygous for the haplotype [Article:17058217]. However, it remains uncertain how variants in UGT1A isoforms that are not directly involved in bilirubin metabolism lead to a propensity for atazanavir-induced hyperbiliriubinemia [Article:17058217]. The alleles present in this study were not in linkage disequilibrium (LD) [Article:17058217], but variants within the UGT1A gene cluster often do show high levels of linkage. This suggests the need for more haplotype-based studies, which can determine interactions among UGT1A variants, and potentially provide better predictions of drug toxicities [Article:19364970].