UGT1*1 genotyping in a Canadian Inuit population

Restriction fragment length polymorphism effectively identifies exon 1 mutation of UGT1A1 gene in patients with Gilbert's Syndrome

BACKGROUND & AIMS

Gilbert's syndrome causes pharmacological variation in drug glucuronidation and unexpected toxicity from therapeutic agents. The two common genotypes of Gilbert's syndrome are a dinucleotide polymorphism (TA)7 in TATA-Box as well as the 211G>A mutation in the coding exon 1, particularly in Asians, of human UGT1A1 gene. In this study, we aimed to establish an effective method to detect the 211G>A mutation.

METHODS

The coding exon 1 sequence of human UGT1A1 gene was analysed by Vector NTI software. The 211G>A mutation in the coding exon 1 of UGT1A1 gene was determined by restriction fragment length polymorphism (RFLP) method. Serum total bilirubin level was measured as well.

RESULTS

A newly identified BsmBI site was located in the coding exon 1 of UGT1A1 gene. The 211G>A mutation in the coding exon 1 of UGT1A1 gene was determined by DNA RFLP. Furthermore, we reported our present work on genetic analysis of mutations of UGT1A1 gene, and the correlation of UGT1A1 mutations with serum total bilirubin levels in Taiwanese population. The results showed that 15 subjects carried 211G>A mutation in 23 subjects related with Gilbert's syndrome. The homozygous 211G>A mutant as well as simultaneously heterozygous mutants both in TATA-Box and 211G>A significantly increased the risk of Gilbert's syndrome similar to subjects carrying homozygous TATA-Box mutant.

CONCLUSIONS

BsmBI RFLP is an effective method to detect 211G>A mutation in the coding exon 1 of UGT1A1 gene. The common 211G>A mutation is one of the causes of Gilbert's syndrome in Taiwanese population.

Crigler-Najjar syndrome: therapeutic options and consequences of mutations in the UGT1A1 complex

Crigler-Najjar syndrome (CN), a rare inherited disorder characterized by failure of bilirubin glucuronidation, can lead to severe disability and death from kernicterus. Gilbert syndrome is a more common, benign familial unconjugated hyperbilirubinemia. The underlying problem in both conditions is impaired bilirubin conjugation and elimination due to a mutation in uridine 5'-diphosphate glucuronyltransferase. The mainstay of current management of CN is phototherapy, followed by liver transplantation. Here, we review other therapies, including hepatocyte transplantation, that have been successfully used to lessen the phenotype, although long-term engraftment of cells remains elusive. Gene therapy holds hope for the future whereby the patient's hepatocytes are transduced with the wild-type gene. Outstanding issues include safety of the gene vector and establishing immunotolerance to both vector and the new protein. The significant advances in understanding the relevance of mutations in UGT not only in glucuronidation of bilirubin, but other drugs and substances, are also reviewed.

An Interethnic Comparison of Polymorphisms of the Genes Encoding Drug-Metabolizing Enzymes and Drug Transporters: Experience in Singapore

Much of the interindividual variability in drug response is attributable to the presence of single nucleotide polymorphisms (SNPs) in genes encoding drug-metabolizing enzymes and drug transporters. In recent years, we have investigated the polymorphisms in a number of genes encoding phase I and II drug-metabolizing enzymes including CYPIA1, CYP3A4, CYP3A5, GSTM1, NAT2, UGT1A1, and TPMT and drug transporter (MDR1) in three distinct Asian populations in Singapore, namely the Chinese, Malays, and Indians. Significant differences in the frequencies of common alleles encoding these proteins have been observed among these three ethnic groups. For example, the frequency of the variant A2455G polymorphism of CYP1A1 was 28% in Chinese and 31% in Malays, but only 18% in Indians. CYP3A4*4 was detected in two of 110 Chinese subjects, but absent in Indians and Malays. Many Chinese and Malays (61-63%) were homozygous for the GSTM1*0 null genotype compared with 33% of Indians. The frequency of the UGTIA1*28 allele was highest in the Indian population (35%) compared to similar frequencies that were found in the Chinese (16%) and Malay (19%) populations. More importantly, our experience over the years has shown that the pharmacogenetics of these drug-metabolizing enzymes and MDR1 in the Asian populations are different from these in the Caucasian and African populations. For example, the CYP3A4*1B allele, which contains an A-290G substitution in the promoter region of CYP3A4, is absent in all three Asian populations of Singapore studied, but occurs in more than 54% of Africans and 5% of Caucasians. There were no difference in genotype and allelic variant frequencies in exon 12 of MDR1 between the Chinese, Malay, and Indian populations. When compared with other ethnic groups, the distribution of the wild-type C allele in exon 12 in the Malays (34.2%) and Indians (32.8%) was relatively high and similar to the Japanese (38.55%) and Caucasians (41%) but different from African-Americans (15%). The frequency of wild-type TT genotype in Asians (43.5% to 52.1%) and Japanese (61.5%) was much higher than those found in Caucasians (13.3%). All the proteins we studied represent the primary hepatic or extrahepatic enzymes, and their polymorphic expression may be implicated in disease risk and the disposition of drugs or endogenous substances. As such, dose requirements of certain drugs may not be optimal for Asian populations, and a second look at the factors responsible for this difference is necessary.

UGT1A6 polymorphism and salicylic acid glucuronidation following aspirin

OBJECTIVES

In vivo, aspirin (acetylsalicylic acid) is rapidly deacetylated to form salicylic acid, which then undergoes primary or secondary glucuronidation catalyzed by UDP-glucuronosyltransferases (UGTs). The variant UGT1A6*2 (T181A, R184S) is associated with altered enzyme function. Our objective was to compare salicylic acid glucuronidation in individuals with different UGT1A6 genotypes.

METHODS

Following orally dosing with 650 mg aspirin, saliva and urine samples were collected over a period of 24 h from healthy individuals with homozygous wild-type UGT1A6 *1/*1 (n=19) and homozygous variant UGT1A6 *2/*2 (T181A, R184S) (n=9) genotypes.

RESULTS

No statistically significant differences were observed in salivary pharmacokinetic parameters. Urinary excretion of the sum of aspirin and its metabolites (salicyluric acid, salicyluric acid phenolic glucuronide, salicyl phenolic glucuronide, salicyl acyl glucuronide, salicylic acid) during the early period of 2-4 h of collection was significantly lower in UGT1A6 *1/*1 than in UGT1A6 *2/*2 individuals. Further, UGT1A6 *1/*1 individuals excreted a lower percentage of aspirin and its metabolites in the first 12 h and a greater percentage after 12 h than UGT1A6 *2/*2 individuals.

CONCLUSIONS

The variant UGT1A6*2 or polymorphisms in other UGTs that are in linkage disequilibrium with UGT1A6*2 may confer more rapid glucuronidation of salicylic acid than the wild-type UGT1A6 *1/*1.

Glucuronidation of antiallergic drug, Tranilast: identification of human UDP-glucuronosyltransferase isoforms and effect of its phase I metabolite

Tranilast is an oral antiallergic agent widely used in Japan. Recently, in Western populations, hyperbilirubinemia induced by tranilast was suspected during clinical trials. Tranilast has been reported to be mainly metabolized to a glucuronide and a phase I metabolite, 4-demethyltranilast (N-3). In the present study, we investigated the in vitro metabolism of tranilast in human liver and jejunum microsomes and recombinant UDP-glucuronosyltransferases (UGTs). The glucuronidation of tranilast was clarified to be mainly catalyzed by UGT1A1 in human liver and intestine. The K(m) values of tranilast glucuronosyltransferase activity were 51.5, 50.6, and 38.0 microM in human liver microsomes, human jejunum microsomes, and recombinant UGT1A1, respectively. The V(max) values were 10.4, 42.9, and 19.7 pmol/min/mg protein in human liver microsomes, human jejunum microsomes, and recombinant UGT1A1, respectively. When the intrinsic clearance was calculated using the in vitro kinetic parameters, microsomal protein content, and weight of tissues, tranilast glucuronosyltransferase activity was 2.5-fold higher in liver than in intestine. Tranilast glucuronosyltransferase activity was strongly inhibited by bilirubin, a typical UGT1A1 substrate, and N-3, indicating that the phase I metabolite could affect the tranilast glucuronosyltransferase activity. In the case of N-3 formation, the K(m) and V(max) values were 37.1 microM and 27.6 pmol/min/mg protein in human liver microsomes. The bilirubin glucuronosyltransferase activity was strongly inhibited by both tranilast and N-3, suggesting that tranilast-induced hyperbilirubinemia would be responsible for the inhibition by tranilast and N-3 of the bilirubin glucuronosyltransferase activity, as would the UGT1A1 genotype.

Role of pharmacogenetics in irinotecan therapy

In the treatment of advanced colorectal cancer, irinotecan has become one of the most important drugs, despite its sometimes unpredictable adverse effects. To understand why some patients experience severe adverse effects (diarrhea and neutropenia), while others do not, the metabolic pathways of this drug have to be unraveled in detail. Individual variation in expression of several phase I and phase II metabolizing enzymes and ABC-transporters involved in irinotecan metabolism and excretion, at least partly explains the observed pharmacokinetic interpatient variability. Although the difference in expression-level of these proteins to a certain amount is explained by physiologic and environmental factors, the presence of specific genetic determinants also does influence their expression and function. In this review, the role of genetic polymorphisms in the main enzyme-systems (carboxylesterase, cytochrome P450 3A, and uridine diphosphate-glucuronosyltransferase) and ABC-transporters (ABCB1, ABCC2, and ABCG2) involved in irinotecan metabolism, are discussed. Since at this moment the field of pharmacogenetics and pharmacogenomics is rapidly expanding and simultaneously more rapid and cost-effective screening methods are emerging, a wealth of future data is expected to enrich our knowledge of the genetic basis of irinotecan metabolism. Eventually, this may help to truly individualize the dosing of this (and other) anti-cancer agent(s), using a personal genetic profile of the most relevant enzymes for every patient.

Genetic factors related to unconjugated hyperbilirubinemia amongst adults

Some variations in the UDP-glucuronosyltransferase 1A1 (UGT1A1) gene are involved in the development of unconjugated hyperbilirubinemia. We hypothesize that other genetic factors may also be associated with this disease. A total of 227 adults with normal routine haematology and liver function (apart from bilirubin testing for which they revealed bilirubin > or = 25.7 micromol/l and unconjugated bilirubin/total bilirubin > or = 80%), and 235 sex- and age-matched controls, were recruited. All subjects were analysed for UGT1A1, glucose-6-phosphate dehydrogenase (G6PD) and organic anion transporter polypeptide 2 (OATP2) genotypes using polymerase chain reaction-restriction fragment length polymorphism. The results indicated that G6PD deficiency, variant UGT1A1 gene and variant OATP2 gene were risk factors for hyperbilirubinemia. The odds ratios (OR) (with 95% confidence interval) were 220.83 (34.68-1406.30), 73.61 (17.01-318.63), 45.15 (11.19-182.22), 15.46 (4.35-54.99) and 6.51 (1.83-23.09), respectively, for individuals featuring the common UGT1A1/OATP2 haplotypes homozygous/heterozygous, compound heterozygous/heterozygous, compound heterozygous/wild-type, heterozygous/heterozygous and heterozygous/wild-type variations amongst subjects with normal G6PD activity. Amongst the subjects with G6PD deficiency, the OR was 159.00 (24.57-1028.94) for individuals carrying variations in both UGT1A1 and OATP2 genes. The UGT1A1/OATP2 haplotypes homozygous/wild-type, homozygous/compound heterozygous and homozygous/homozygous for G6PD normal and variant/wild-type for G6PD deficient individuals were only observed in the case group, and not in the control group. Amongst hyperbilirubinemic adults, bilirubin values tended to parallel variation status of their haplotypes. Adults featuring certain haplotypes in UGT1A1, OATP2 and G6PD genes face a high risk of developing unconjugated hyperbilirubinemia.

Polymorphisms of uridine-diphosphoglucuronosyltransferase 1A7 gene in Taiwan Chinese

AIM: Single nucleotide polymorphisms (SNPs) of uridine-diphosphoglucuro-nosyltransferase 1A7 (UGT1A7) gene are associated with the development of orolaryngeal cancer, hepatocellular carcinoma, and colorectal cancer. We performed this research to establish the techniques for determining UGT1A7 gene and basic data of this gene for Taiwan Chinese.

METHODS: We collected blood samples from 112 healthy adults and 505 subjects carrying different genotypes of UGT1A1, and determined the promoter area and the entire sequence of UGT1A7 exon 1 by polymerase chain reaction. We designed appropriate primers and restriction enzymes to detect variant UGT1A7 genotypes found in the study subjects.

RESULTS: Six SNPs at nucleotides 33, 387, 391, 392, 622, and 756 within the coding region of UGT1A7 exon 1 were found. The incidence of UGT1A7 *1/*2 (N129R131W208/ K129K131W208) was predominant (35.7%) while that of UGT1A7 *3/*3 (K129K131R208/K129K131R208) was the least (2.7%). The allele frequency of UGT1A7*3, which exists in a considerable proportion of Caucasians (0.361) and Japanese (0.255), was identified only to be 0.152 in our study subjects. A novel variation at nucleotide -57 in the upstream was found, which was associated with SNPs at nucleotides 33, 387, 391, 392, and 622 in one of the variant haplotypes. The nucleotide changes at positions 387, 391, 392 and 756 were in linkage in another variant haplotype. The allele frequency of UGT1A7*3 was 0.018, 0.158, 0.242, 0.433, and 0.920 in subjects carrying wild, A(TA)₆TAA/A(TA)₇TAA, A(TA)₇TAA/A(TA)₇TAA, 211G/211A, and 211A/211A variants of UGT1A1 gene, respectively. By using natural or mutagenesis primers, we successfully detected the variations at nucleotides -57, 33, 387, and 622 with the restriction enzymes HpyCH4 IV, Taq I, Afl II, and Rsa I, respectively.

CONCLUSION: The results indicate that the allele frequencies of UGT1A7 gene in Taiwan Chinese are different from those in Caucasians and Japanese. Carriage of the nucleotide 211- variant UGT1A gene is highly associated with UGT1A7*3. The restriction-enzyme-digestion method for the determination of nucleotides -57 (or 33, or 622) and 387 can rapidly identify genotypes of UGT1A7 in an individual.

Risk factors for severe hyperbilirubinemia in neonates

The incidence of severe neonatal hyperbilirubinemia is higher in Asians than in whites. A case-control study was designed to investigate the effects of eight known risk factors [breast feeding, ABO incompatibility, premature birth, infection, cephalohematoma, asphyxia, glucose-6-phosphate dehydrogenase (G6PD) deficiency, and variant UDP-glucuronosyltransferase 1A1 (UGT1A1) gene] and a suspicious analog [organic anion transporter 2 (OATP 2) gene] on severe hyperbilirubinemia in Taiwanese neonates. The 72 study subjects and 100 hospital control subjects consisted of neonates with peak serum bilirubin levels > or =342 microM and <256.5 microM, respectively. The PCR-restriction fragment length polymorphism method was applied to detect the UGT1A1, OATP 2, and G6PD genes. The results of multivariate logistic regressions, adjusted for covariates, revealed odds ratios (ORs) of 4.64 [95% confidence interval (CI): 2.25-9.57; p < 0.001], 3.36 (95% CI: 1.54-7.35; p=0.002), and 3.02 (95% CI: 1.30-6.99; p=0.010) for neonates who were fed with breast milk, and carry the variant UGT1A1 gene at nucleotide 211 and the variant OATP 2 gene at nucleotide 388, respectively. The ORs, adjusted for covariates, for the other six risk factors were not statistically significant. The ORs in neonates who had one, two, and three significant risk factors were 8.46 (95% CI: 2.75-34.48; p < 0.001), 22.0 (95% CI: 5.50-88.0; p < 0.001), and 88.0 (95% CI: 12.50-642.50; p < 0.001), respectively. In conclusion, neonates who carry the 211 and 388 variants in the UGT1A1 and OATP 2 genes, respectively, as well as feed with breast milk are at high risk to develop severe hyperbilirubinemia.

Pharmacogenomics of human UDP-glucuronosyltransferase enzymes

UDP-glucuronosyltransferase (UGT) enzymes comprise a superfamily of key proteins that catalyze the glucuronidation reaction on a wide range of structurally diverse endogenous and exogenous chemicals. Glucuronidation is one of the major phase II drug-metabolizing reactions that contributes to drug biotransformation. This biochemical process is also involved in the protection against environmental toxicants, carcinogens, dietary toxins and participates in the homeostasis of numerous endogenous molecules, including bilirubin, steroid hormones and biliary acids. Over the years, significant progress was made in the field of glucuronidation, especially with regard to the identification of human UGTs, study of their tissue distribution and substrate specificities. More recently, the degree of allelic diversity has also been revealed for several human UGT genes. Some polymorphic UGTs have demonstrated a significant pharmacological impact in addition to being relevant to drug-induced adverse reactions and cancer susceptibility. This review focuses on human UGTs, the description of the nature of polymorphic variations and their functional impact. The pharmacogenomic implication of polymorphic UGTs is presented, more specifically the role of UGT polymorphisms in modifying cancer risk and their impact on individual risk to drug-induced toxicities.

Pharmacogenomics: road to anticancer therapeutics nirvana?

Interindividual differences in the toxicity and response to anticancer therapies are currently observed for essentially all available treatment regimens. Such 'unpredictable' drug responses are particularly dangerous in the context of anticancer agents that have narrow therapeutic indices. Pharmacogenomics attempts to elucidate the inherited basis of interindividual differences in drug response, with the eventual goal of minimizing such variability through the use of 'individualized' treatments. There are several emerging examples of genetic polymorphisms of drug-metabolizing enzymes, DNA repair genes and drug targets that have been shown to influence the toxicity and efficacy of anticancer treatment. This review discusses the role of genetic variants of UGT1A1, TS and EGFR to exemplify the potential impact of phramacogenomics on the field of anticancer therapeutics.

Genetic variation of human UDP-glucuronosyltransferase: implications in disease and drug glucuronidation

The uridine diphosphate (UDP)-glucuronosyltransferases (UGTs) are key enzymes in human detoxication of xeno- and endobiotics. Potentially toxic endogenous compounds such as bilirubin, or exogenous compounds such as drugs, pesticides, and carcinogens, are generally transformed into water-soluble glucuronides for excretion in bile and urine. The UGTs are encoded by a multigene family in humans. A relatively small number of human enzymes catalyze the glucuronidation of thousands of compounds. Genetic variations and single nucleotide polymorphisms (SNPs) within the UGT genes are remarkably common, and lead to genetic polymorphisms. The multiplicity of transferases, some exhibiting overlapping substrate specificity, may provide functional compensation for genetic deficit in some cases. Genetic variation may cause different phenotypes by affecting expression levels or activities of individual UGTs. This inter-individual variation in UGTs has resulted in functional deficit affecting endogenous metabolism and leading to jaundice and other diseases. Disruption of the normal metabolic physiology, by the reduction of bile acid excretion or steroid glucuronidation, may lead to cholestasis and organ dysfunction. Deficient glucuronidation of drugs and xenobiotics have an important pharmacological impact, which may lead to drug-induced adverse reactions, and even cancer. Additional novel polymorphisms in this gene family are yet to be revealed and studied, but will have a profound effect on the development of new drugs and therapies.

The global distribution of length polymorphisms of the promoters of the glucuronosyltransferase 1 gene (UGT1A1): hematologic and evolutionary implications

The promoter region of the UDP glucuronosyltransferase 1 gene (UGT1A1) contains a run of thymine-adenine (TA) repeats, usually six (TA)(6). As well as its relationship to Gilbert's syndrome, homozygosity for the extended sequence, (TA)(7) (TA)(7), has been found to be an important risk factor for hyperbilirubinemia and gallstones in patients with hemoglobin E-beta-thalassemia and other intermediate forms of beta thalassemia. To assess the importance of this polymorphism in these common disorders a wide-scale population study of the relative frequency of the size alleles of the UGT1A1 promoter has been carried out. Homozygosity for the (TA)(7) allele occurs in 10-25% of the populations of Africa and the Indian subcontinent, with a variable frequency in Europe. It occurs at a much lower frequency in Southeast Asia, Melanesia, and the Pacific Islands, ranging from 0 to 5%. African populations show a much greater diversity of length alleles than other populations. These findings define those populations with a high frequency of hemoglobin E-beta-thalassemia and related disorders that are at increased risk for hyperbilirubinemia and gall bladder disease and provide evolutionary insights into how these polymorphisms have arisen and are so unequally distributed among human populations.

Pharmacogenetics in cancer treatment

Interindividual variability in the efficacy and toxicity of drug therapy is associated with polymorphisms in genes encoding drug-metabolizing enzymes, transporters, or drug targets. Pharmacogenetics aims to identify individuals predisposed to high risk of toxicity from conventional doses of cancer chemotherapeutic agents. We review the role of genetic polymorphisms in UGT1A1 and TPMT, as well as mutations in DPD, in influencing drug disposition and toxicity. Recent studies show that pharmacogenetic determinants may also influence treatment outcomes. We discuss the clinical significance of polymorphisms in TS, MTHFR, and FCGR3A, as well as the polymorphic DNA repair genes XPD and XRCC1, in influencing response to chemotherapy and survival outcomes. Finally, the potential implications of transporter pharmacogenetics in influencing drug bioavailability are addressed.

Relationship between bilirubin UDP-glucuronosyl transferase 1A1 gene and neonatal hyperbilirubinemia

The variation rate within the coding region of UDP-glucuronosyl transferase 1A1 (UGT1A1) gene in Taiwan Chinese was found to be 29.3%. This study sought to determine whether that high variation rate of UGT1A1 gene is a risk factor for neonatal hyperbilirubinemia. The study subjects consisted of 123 newborn infants suffering from unconjugated hyperbilirubinemia who had no known risk factors for hyperbilirubinemia and 218 healthy control neonates. The promoter area, exons 1 to 4, coding region of exon 5, and the flanking intronic regions in UGT1A1 gene were determined by the PCR in all subjects. Wild UGT1A1 gene, variation in the promoter, variation at nucleotide 211, variation at nucleotide 1091, and compound heterozygous variation of UGT1A1 gene were found. The percentage of neonates with wild UGT1A1 gene and the percentage of neonates with variation at nucleotide 211 were significantly different between the study subjects and controls. The percentages with bilirubin >or=342 micro M (20.0 mg/dL) and with persistent hyperbilirubinemia in the subjects carrying homozygous variation at nucleotide 211 (Gly71Arg) were significantly higher than the neonates carrying wild type or other genotypes. In conclusion, this study has demonstrated that variation at nucleotide 211 of the UGT1A1 gene is a risk factor for the development of neonatal hyperbilirubinemia. Pediatricians should closely follow hyperbilirubinemic newborn infants who carry homozygous 211 G to A variation in UGT1A1 gene.

Drug glucuronidation in clinical psychopharmacology

Glucuronidation is a phase II metabolic process and one of the most common pathways in the formation of hydrophilic drug metabolites. At least 33 families of uridine diphosphate-glucuronosyltransferases have been identified in vitro, and specific nomenclature similar to that used to classify the cytochrome (CYP) P450 system has been established. The UGT1 and UGT2 subfamilies represent the most important of these enzymes in human drug metabolism. Factors affecting glucuronidation include the following: cigarette smoking, obesity, age, and gender. In addition, several drugs have been found in vitro to be substrates, inhibitors, or inducers of UGT enzymes. Induction or inhibition of both UGT and CYP isoforms may occur simultaneously. Some important drug interactions involving glucuronidation have been documented and others can be postulated. This review summarizes the relevant literature pertaining to drug glucuronidation and its implications for clinical psychopharmacology.

Pharmacogenetics: a tool for individualizing antineoplastic therapy

This article reviews the clinical relevance of pharmacogenetics in cancer chemotherapy, with emphasis on drugs for which genetic differences in enzyme metabolism have been demonstrated to affect patient outcome. About 10% of children with leukaemia are intolerant to mercaptopurine (6-mercaptopurine) because of genetic defects in mercaptopurine inactivation by thiopurine S-methyltransferase. However, mercaptopurine dose intensity, a critical factor for outcome in patients deficient in thiopurine S-methyltransferase, can be maintained by means of thiopurine S-methyltransferase phenotyping or genotyping. Patients with reduced fluorouracil (5-fluorouracil) catabolism are more likely to be exposed to severe toxicity. The measurement of dihydropyrimidine dehydrogenase activity in patients cannot be considered fully predictive, and the role of dihydropyrimidine dehydrogenase gene variants in this syndrome has yet to be clarified. With regard to irinotecan, patients with Gilbert's syndrome phenotype have reduced inactivation of the active topoisomerase I inhibitor 7-ethyl-10-hydroxycamptothecin (SN-38) caused by a mutation in the UDP-glucuronosyltransferase 1A1 gene promoter. This subset of patients is more likely to be exposed to irinotecan toxicity and could be identified by genotyping for gene promoter variants. Finally, the experience with amonafide represents a model for dose individualization approaches that use simple phenotypic probes.

Genetic polymorphism of UDP-glucuronosyltransferase 2B7 (UGT2B7) at amino acid 268: ethnic diversity of alleles and potential clinical significance

UGT2B7 catalyses the glucuronidation of a diverse range of drugs, environmental chemicals and endogenous compounds. Hence, coding region polymorphisms of UGT2B7 are potentially of pharmacological, toxicological and physiological significance. Two variant UGT2B7 cDNAs encoding enzymes with either His or Tyr at residue 268 have been isolated. The variants, referred to as UGT2B7*1 and UGT2B7*2, respectively, arise from a C to T transversion at nucleotide 802 of the UGT2B7 coding region. Analysis of genomic DNA from 91 unrelated Caucasians and 84 unrelated Japanese demonstrated the presence of the variant alleles encoding UGT2B7*1 and UGT2B7*2 in both populations. However, while there was an approximately equal distribution of subjects homozygous for each allele in the Caucasian population, subjects homozygous for the UGT2B7*1 allele were over 10-fold more prevalent than UGT2B7*2 homozygotes in Japanese. The frequencies of the UGT2B7*1 and UGT2B7*2 alleles were 0.511 and 0.489, respectively, in Caucasians, and 0.732 and 0.268, respectively, in Japanese. The 95% confidence intervals for the two alleles did not overlap between Caucasians and Japanese. Rates of microsomal androsterone, menthol and morphine (3-position) glucuronidation were determined for genotyped livers from Caucasian donors. Statistically significant inter-genotypic differences were not apparent for any of the three substrates. Although the UGT2B7 polymorphism characterized here is probably not associated with altered enzyme activity, the results highlight the need to consider ethnic variability in assessing the consequences of UGT polymorphisms.

Variations of the bilirubin uridine-diphosphoglucuronosyl transferase 1A1 gene in healthy Taiwanese

The activity of uridine-diphosphoglucuronosyl transferase 1 (UGT1) may influence the concentration of serum bilirubin. Because UGT1 is too labile to be measured with classical biochemical methods, we analysed the whole UGT1A1 gene in 290 healthy Taiwanese adults by using the polymerase chain reaction method, and investigated the relationship between UGT1A1 genotypes and serum bilirubin levels. The results showed that slightly more than 50% of the subjects had one or more variant sites in UGT1A1 gene. The most common variant was A(TA)6TAA/A(TA)7TAA (6/7) in the promoter area, followed by heterozygous variation within the coding region, compound heterozygous and homozygous variations. Among the four variant sites within the coding region, 211 G to A was the predominate one, 1091 C to T was a novel variation, and 686 C to A was associated with 6/7. Subjects with 6/7 or heterozygous variation within the coding region or compound heterozygous (plus one homozygous) variation had significantly higher bilirubin levels than those with wild UGT1A1 gene. When the 290 subjects were stratified into six groups according to their serum bilirubin concentrations, the bilirubin levels were correlated well to the frequencies of variant UGT1A1 gene. Our results show that there is a strong association between UGT1A1 gene and bilirubin levels in healthy Taiwanese adults. The occurrence of A(TA)7TAA allele was relatively rare and the variation rate within the coding region was much higher in Taiwanese compared to that in Caucasians.

Drug-mediated toxicity caused by genetic deficiency of UDP-glucuronosyltransferases

Human gene families encoding UDP-Glucuronosyltransferases (UGTs) have been identified and partially characterised. This family of enzymes catalysed the glucuronidation of drugs, xenobiotics and endobiotics. Genetic mutations and polymorphisms have been identified in several UGT genes and examples should be anticipated in all UGT genes. A common genetic defect in the TATA box promoter of the UGT1A1 gene is associated with Gilbert's Syndrome (GS) causing mild hyperbilirubinaemia. Recently, adverse effects of anticancer agents have been observed in Gilbert's patients due to reduced drug or bilirubin glucuronidation.

Molecular genetic basis of Gilbert's syndrome

Gilbert's syndrome, an hereditary, chronic, mild, unconjugated hyperbilirubinaemia resulting from impaired hepatic bilirubin clearance and otherwise normal liver function, is arguably the most common syndrome known in humans. Recent molecular genetic studies have determined that the clinical phenotype can be described by a dinucleotide polymorphism in the TATA box promoter of the bilirubin uridine diphosphate-glucuronosyltransferase (UGT-1A1) gene, most frequently (TA)7TAA, affecting up to 36% of Africans, but only 3% of Asians. However, a second common heterozygous mutation in the coding exon 1 of the UGT-1A1 gene (G71R) can also cause the Gilbert's phenotype in Japanese and Asians. The clinical phenotype may not be apparent as frequently as the determined genotype, due to environmental factors such as alcohol-induced hepatic bilirubin glucuronidation, reducing serum bilirubin levels and causing a latent condition. Gilbert's disease is a contributory factor of prolonged neonatal jaundice in breast-fed infants and may precipitate jaundice when coinherited with other disorders of haem metabolism. The genetic variation described as Gilbert's syndrome may lead to pharmacological variation in drug glucuronidation and unexpected toxicity from therapeutic agents.

Gilbert's syndrome is a contributory factor in prolonged unconjugated hyperbilirubinemia of the newborn

OBJECTIVE

Prolonged neonatal jaundice, beyond day 14 of life, is very common and of concern to the clinician. The aim of this study was to investigate whether a genetic mutation in the bilirubin UGT1A1 gene, which has been associated with Gilbert's syndrome in adults, is a contributory factor in prolonged neonatal jaundice.

STUDY DESIGN

Blood was collected from 85 term newborns with unexplained hyperbilirubinemia, and DNA was prepared. The neonates were divided into 6 groups depending on whether they were breast-fed or bottle-fed and whether they had acute, prolonged, or very prolonged jaundice. UGT1A1 TATA promoter genotyping (DNA test for Gilbert's syndrome) was performed on all samples, and analysis of the entire UGT1A1 coding sequence was performed in a representative sample (11 of 26) of very prolonged cases.

RESULTS

In addition to the known common UGT1A1 TATA alleles (TA6 and TA7), a novel TATA allele (TA5) in a neonate with very prolonged jaundice was identified. Statistical analysis of the TATA genotype distributions within the group of breast-fed neonates revealed significant differences among the acute, prolonged, and very prolonged subgroups (.05 > P >.01): the incidence of familial hyperbilirubinemia genotypes (7/7 and 5/7) is 5 times greater in very prolonged cases (31%) relative to acute cases (6%). Neonates with prolonged jaundice from family pedigrees were observed to demonstrate the Gilbert's phenotype as children or young adults.

CONCLUSIONS

A genetic predisposition to develop prolonged neonatal hyperbilirubinemia in breast-fed infants is associated with TATA box polymorphism of the UGT1A1 gene and will be recognized as Gilbert's syndrome in adulthood.

Biochemical and molecular aspects of genetic disorders of bilirubin metabolism

Bilirubin, the oxidative product of heme in mammals, is excreted into the bile after its esterification with glucuronic acid to polar mono- and diconjugated derivatives. The accumulation of unconjugated and conjugated bilirubin in the serum is caused by several types of hereditary disorder. The Crigler-Najjar syndrome is caused by a defect in the gene which encodes bilirubin UDP-glucuronosyltransferase (UGT), whereas the Dubin-Johnson syndrome is characterized by a defect in the gene which encodes the canalicular bilirubin conjugate export pump of hepatocytes. Animal models such as the unconjugated hyperbilirubinemic Gunn rat, the conjugated hyperbilirubinemic GY/TR-, and the Eisai hyperbilirubinemic rat, have contributed to the understanding of the molecular basis of hyperbilirubinemia in humans. Elucidation of both the structure of the UGT1 gene complex, and the Mrp2 (cMoat) gene which encodes the canalicular conjugate export pump, has led to a greater understanding of the genetic basis of hyperbilirubinemia.

Pharmacogenetics and cancer chemotherapy

Cancer chemotherapy is limited by significant inter-individual variations in responses and toxicities. Such variations are often due to genetic alterations in drug metabolising enzymes (pharmacokinetic polymorphisms) or receptor expression (pharmacodynamic polymorphisms). Pharmacogenetic screening prior to anticancer drug administration may lead to identification of specific populations predisposed to drug toxicity or poor drug responses. The role of polymorphisms in specific enzymes, such as thiopurine S-methyltransferases (TPMT), dihydropyrimidine dehydrogenase (DPD), aldehyde dehydrogenases (ALDH), glutathione S-transferases (GST), uridine diphosphate glucuronosyl-transferases (UGTs) and cytochrome P450 (CYP 450) enzymes in cancer therapy are discussed in this review.

Genetic predisposition to the metabolism of irinotecan (CPT-11). Role of uridine diphosphate glucuronosyltransferase isoform 1A1 in the glucuronidation of its active metabolite (SN-38) in human liver microsomes

Irinotecan (CPT-11) is a promising antitumor agent, recently approved for use in patients with metastatic colorectal cancer. Its active metabolite, SN-38, is glucuronidated by hepatic uridine diphosphate glucuronosyltransferases (UGTs). The major dose-limiting toxicity of irinotecan therapy is diarrhea, which is believed to be secondary to the biliary excretion of SN-38, the extent of which is determined by SN-38 glucuronidation. The purpose of this study was to identify the specific isoform of UGT involved in SN-38 glucuronidation. In vitro glucuronidation of SN-38 was screened in hepatic microsomes from normal rats (n = 4), normal humans (n = 25), Gunn rats (n = 3), and patients (n = 4) with Crigler-Najjar type I (CN-I) syndrome. A wide intersubject variability in in vitro SN-38 glucuronide formation rates was found in humans. Gunn rats and CN-I patients lacked SN-38 glucuronidating activity, indicating the role of UGT1 isoform in SN-38 glucuronidation. A significant correlation was observed between SN-38 and bilirubin glucuronidation (r = 0.89; P = 0.001), whereas there was a poor relationship between para-nitrophenol and SN-38 glucuronidation (r = 0.08; P = 0.703). Intact SN-38 glucuronidation was observed only in HK293 cells transfected with the UGT1A1 isozyme. These results demonstrate that UGT1A1 is the isoform responsible for SN-38 glucuronidation. These findings indicate a genetic predisposition to the metabolism of irinotecan, suggesting that patients with low UGT1A1 activity, such as those with Gilbert's syndrome, may be at an increased risk for irinotecan toxicity.

Genetic defects of the UDP-glucuronosyltransferase-1 (UGT1) gene that cause familial non-haemolytic unconjugated hyperbilirubinaemias

Congenital familial non-haemolytic hyperbilirubinaemias are potentially lethal syndromes caused by genetic lesions that reduce or abolish hepatic bilirubin UDP-glucuronosyltransferase activity. Here we describe genetic defects that occur in the UGT1 gene complex that cause three non-haemolytic unconjugated hyperbilirubinaemia syndromes. The most severe syndrome, termed Crigler-Najjar syndrome type I, is mainly associated with mutations in exons 2 to 5 that affect all UGT1 enzymes and many of the mutations result in termination codons and frameshifts. 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. 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. Current methods used for the diagnosis and treatment of these diseases are discussed.