Evidence for a gene influencing serum bilirubin on chromosome 2q telomere: a genomewide scan in the Framingham study

de Paula V, Pires T, Michelon L, Camilo C, Esteban N, Pereira A, Schalling M, Vallada H. Shorter telomere length and suicidal ideation in familial bipolar disorder

Bipolar Disorder (BD) has recently been related to a process of accelerated aging, with shortened leukocyte telomere length (LTL) in this population. It has also been observed that the suicide rate in BD patients is higher than in the general population, and more recently the telomere length variation has been described as shorter in suicide completers compared with control subjects. Objectives The aim of the present study was to investigate if there is an association between LTL and BD in families where two or more members have BD including clinical symptomatology variables, along with suicide behavior. Methods Telomere length and single copy gene ratio (T/S ratio) was measured using quantitative polymerase chain reaction in a sample of 143 relatives from 22 families, of which 60 had BD. The statistical analysis was performed with a polygenic mixed model. Results LTL was associated with suicidal ideation (p = 0.02) as that there is an interaction between suicidal ideation and course of the disorder (p = 0.02). The estimated heritability for LTL in these families was 0.68. In addition, covariates that relate to severity of disease, i.e. suicidal ideation and course of the disorder, showed an association with shorter LTL in BD patients. No difference in LTL between BD patients and healthy relatives was observed. Conclusion LTL are shorter in subjects with familial BD suggesting that stress related sub-phenotypes possibly accelerate the process of cellular aging and correlate with disease severity and suicidal ideation.

Bilirubin Links HO-1 and UGT1A1*28 Gene Polymorphisms to Predict Cardiovascular Outcome in Patients Receiving Maintenance Hemodialysis

Serum bilirubin levels, which are determined by a complex interplay of various enzymes, including heme oxygenase-1 (HO-1) and uridine diphosphate–glucuronosyl transferase (UGT1A1), may be protective against progression of cardiovascular disease (CVD) in hemodialysis patients. However, the combined effect of HO-1 and UGT1A1*28 gene polymorphisms on CVD outcomes among hemodialysis patients is still unknown. This retrospective study enrolled 1080 prevalent hemodialysis patients and the combined genetic polymorphisms of HO-1 and UGT1A1 on serum bilirubin were analyzed. Endpoints were CVD events and all-cause mortality. Mean serum bilirubin was highest in patients with S/S + S/L of the HO-1 promoter and UGT1A1 7/7 genotypes (Group 1), intermediate in those with S/S + S/L of the HO-1 promoter and UGT1A1 7/6 + 6/6 genotypes (Group 2), and lowest in the carriers with the L/L HO-1 promoter and UGT1A1 7/6 + 6/6 genotypes (Group 3) (p < 0.001). During a median follow-up of 50 months, 433 patients developed CVD. Compared with patients in Group 3, individuals among Groups 1 and 2 had significantly lower risks for CVD events (adjusted hazard ratios (aHRs) of 0.35 for Group 1 and 0.63 for Group 2), respectively. Compared with the lower bilirubin tertile, the aHRs were 0.72 for the middle tertile and 0.40 for the upper tertile for CVD events. We summarized that serum bilirubin as well as HO-1 and UGT1A1 gene polymorphisms were associated with CVD among patients receiving chronic hemodialysis.

A UGT1A1 variant is associated with serum total bilirubin levels, which are causal for hypertension in African-ancestry individuals

Serum bilirubin is associated with several clinical outcomes, including hypertension, type 2 diabetes (T2D), and drug metabolism. Here, we describe findings from our genome-wide association studies (GWAS) of serum (TBIL) using a generalized linear mixed model in West Africans (n = 1127), with adjustment for age, sex, body mass index, T2D, significant principal components of population structure, and cryptic relatedness. Genome-wide conditional analysis and CAVIARBF were used to fine map significant loci. The causal effect of TBIL on hypertension was assessed by Mendelian randomization (MR) using the GWAS findings as instrumental variables (IVs) in African Americans (n = 3,067). The SNP rs887829 (UGT1A1) was significantly associated with TBIL levels (effect allele (T) frequency = 0.49, β (SE) = 0.59 (0.04), p = 9.13 × 10^-54). Genome-wide conditional analysis and regional fine mapping pointed to rs887829 as a possible causal variant with a posterior inclusion probability of 0.99. The T allele of rs887829 is associated with lower hepatic expression of UGT1A1. Using rs887829 as an IV, two-stage least-squares MR showed a causal effect of bilirubin on hypertension (β = -0.76, 95% CI [-1.52, -0.01], p = 0.0459). Our finding confirms that UGT1A1 influences bilirubin levels. Notably, lower TBIL is causally associated with the increased risk of hypertension.

UGT1A1-related Bilirubin Encephalopathy/Kernicterus in Adults

BACKGROUND AND AIMS

Bilirubin encephalopathy/kernicterus is very rare in adults. This study is aimed to investigate the clinical manifestations and genetic features of two patients with UGT1A1-related kernicterus.

METHODS

Sanger sequencing analysis was performed to identify UGT1A1 gene mutations in the patients and their families. Bioinformatics analysis was used to predict the potential functional effects of novel missense mutations. Clinical manifestations and biochemical parameters were collected and analyzed.

RESULTS

Two patients with Crigler-Najjar syndrome type II (CNS2) developed kernicterus in adulthood. Sanger sequencing identified a compound heterozygous mutation in the UGT1A1 gene in patient 1, which was inherited from his mother (G71R) and his father (c.-3279T>G; S191F). Patient 2 carried three heterozygous mutations, namely G71R, R209W and M391K; among which, the M391K mutation has not been reported before. Multiple prediction software showed that the M391K mutation was pathogenic. Symptoms were relieved in the two patients after phenobarbital and artificial liver support treatment. Patient 1 also underwent liver transplantation.

CONCLUSIONS

Adults with CNS2 are at risk for kernicterus. Phenobarbital treatment is beneficial for maintaining bilirubin levels and preventing kernicterus.

Relationship between serum bilirubin levels, urinary biopyrrin levels, and retinopathy in patients with diabetes

Objective

Previous reports have indicated that serum bilirubin levels may be associated with diabetic retinopathy. However, the detailed mechanism is not fully understood. In this study, we evaluated the relationship between the severity of diabetic retinopathy and various factors including bilirubin levels and factors influencing bilirubin metabolism.

Methods

The study participants consisted of 94 consecutive patients with diabetes mellitus admitted to Kyushu University Hospital from April 2011 to July 2012. The patients were classified into three groups: no retinopathy (NDR), simple retinopathy (SDR), and pre-proliferative or proliferative retinopathy (PDR). The relationship between the severity of retinopathy and various factors was evaluated using univariate and logistic regression analyses. In addition, multivariate regression analysis was performed to evaluate the significant determinants for bilirubin levels.

Results

In univariate analysis, a significant difference was found among NDR, SDR and PDR in bilirubin levels, duration of diabetes, systolic blood pressure, and macroalbuminuria. Logistic regression analysis showed that PDR was significantly associated with bilirubin levels, duration of diabetes, and systolic blood pressure (OR 0.737, 95% CI 0.570–0.952, P = 0.012; OR 1.085, 95% CI 1.024–1.149, P = 0.006; OR 1.036, 95% CI 1.011–1.062, P = 0.005, respectively). In turn, multivariate regression analysis showed that bilirubin levels were negatively associated with high-sensitivity C-reactive protein levels and PDR, but positively correlated with urinary biopyrrin levels, oxidized metabolites of bilirubin.

Conclusion

PDR was negatively associated with bilirubin levels. This negative association may be due to a decreased production of bilirubin rather than its increased consumption considering the positive association between bilirubin and biopyrrin levels.

Serum bilirubin and chronic obstructive pulmonary disease (COPD): a systematic review

BACKGROUND

Bilirubin is a potent antioxidant and higher serum bilirubin levels have been associated with improved COPD outcomes. We performed a systematic review to evaluate the association between serum bilirubin levels and lung function (FEV1), prevalence/incidence of COPD, acute exacerbations of COPD, respiratory health status, and mortality.

METHODS

MEDLINE® and Embase were searched using Ovid® (search updated October 1st, 2019). We included studies that measured serum bilirubin levels and outcomes of interest in adults with or without underlying lung disease. We excluded studies of those with liver disease or drug-induced elevations in bilirubin. We used the Newcastle-Ottawa scale to assess individual study risk of bias (ROB) and the US Agency for Healthcare Research and Quality-Evidence Based Practice tool to assess overall strength of evidence (SOE). Two authors independently determined eligibility, performed data abstraction, assessed ROB, and determined SOE.

RESULTS

Thirteen studies (5 low risk of bias, 3 moderate and 5 high risk) were included. We found low strength of evidence for the association between higher bilirubin levels and lower risk of acute exacerbations of COPD (2 studies), mortality (3 studies), COPD diagnosis (4 studies), and lung function (FEV1) (8 studies). We found insufficient evidence on the relationship between serum bilirubin and respiratory health status/exercise capacity (1 study) and airflow obstruction (FEV1/FVC ratio) (4 studies).

CONCLUSION

Higher bilirubin levels may be associated with lower mortality and improved COPD outcomes. Randomized trials are needed to evaluate the effect of medications that raise serum bilirubin on COPD outcomes. PROSPERO registration: CRD42019145747.

From genome-wide association studies to Mendelian randomization: novel opportunities for understanding cardiovascular disease causality, pathogenesis, prevention, and treatment

The Mendelian randomization approach is an epidemiological study design incorporating genetic information into traditional epidemiological studies to infer causality of biomarkers, risk factors, or lifestyle factors on disease risk. Mendelian randomization studies often draw on novel information generated in genome-wide association studies on causal associations between genetic variants and a risk factor or lifestyle factor. Such information can then be used in a largely unconfounded study design free of reverse causation to understand if and how risk factors and lifestyle factors cause cardiovascular disease. If causation is demonstrated, an opportunity for prevention of disease is identified; importantly however, before prevention or treatment can be implemented, randomized intervention trials altering risk factor levels or improving deleterious lifestyle factors needs to document reductions in cardiovascular disease in a safe and side-effect sparse manner. Documentation of causality can also inform on potential drug targets, more likely to be successful than prior approaches often relying on animal or cell studies mainly. The present review summarizes the history and background of Mendelian randomization, the study design, assumptions for using the design, and the most common caveats, followed by a discussion on advantages and disadvantages of different types of Mendelian randomization studies using one or more samples and different levels of information on study participants. The review also provides an overview of results on many of the risk factors and lifestyle factors for cardiovascular disease examined to date using the Mendelian randomization study design.

Serum bilirubin concentration is modified by UGT1A1 haplotypes and influences risk of type-2 diabetes in the Norfolk Island genetic isolate

Background

Located in the Pacific Ocean between Australia and New Zealand, the unique population isolate of Norfolk Island has been shown to exhibit increased prevalence of metabolic disorders (type-2 diabetes, cardiovascular disease) compared to mainland Australia. We investigated this well-established genetic isolate, utilising its unique genomic structure to increase the ability to detect related genetic markers. A pedigree-based genome-wide association study of 16 routinely collected blood-based clinical traits in 382 Norfolk Island individuals was performed.

Results

A striking association peak was located at chromosome 2q37.1 for both total bilirubin and direct bilirubin, with 29 SNPs reaching statistical significance (P < 1.84 × 10⁻⁷). Strong linkage disequilibrium was observed across a 200 kb region spanning the UDP-glucuronosyltransferase family, including UGT1A1, an enzyme known to metabolise bilirubin. Given the epidemiological literature suggesting negative association between CVD-risk and serum bilirubin we further explored potential associations using stepwise multivariate regression, revealing significant association between direct bilirubin concentration and type-2 diabetes risk. In the Norfolk Island cohort increased direct bilirubin was associated with a 28 % reduction in type-2 diabetes risk (OR: 0.72, 95 % CI: 0.57-0.91, P = 0.005). When adjusted for genotypic effects the overall model was validated, with the adjusted model predicting a 30 % reduction in type-2 diabetes risk with increasing direct bilirubin concentrations (OR: 0.70, 95 % CI: 0.53-0.89, P = 0.0001).

Conclusions

In summary, a pedigree-based GWAS of blood-based clinical traits in the Norfolk Island population has identified variants within the UDPGT family directly associated with serum bilirubin levels, which is in turn implicated with reduced risk of developing type-2 diabetes within this population.

Electronic supplementary material

The online version of this article (doi:10.1186/s12863-015-0291-z) contains supplementary material, which is available to authorized users.

A Phenomic Scan of the Norfolk Island Genetic Isolate Identifies a Major Pleiotropic Effect Locus Associated with Metabolic and Renal Disorder Markers

Multiphenotype genome-wide association studies (GWAS) may reveal pleiotropic genes, which would remain undetected using single phenotype analyses. Analysis of large pedigrees offers the added advantage of more accurately assessing trait heritability, which can help prioritise genetically influenced phenotypes for GWAS analysis. In this study we performed a principal component analysis (PCA), heritability (h²) estimation and pedigree-based GWAS of 37 cardiovascular disease -related phenotypes in 330 related individuals forming a large pedigree from the Norfolk Island genetic isolate. PCA revealed 13 components explaining >75% of the total variance. Nine components yielded statistically significant h² values ranging from 0.22 to 0.54 (P<0.05). The most heritable component was loaded with 7 phenotypic measures reflecting metabolic and renal dysfunction. A GWAS of this composite phenotype revealed statistically significant associations for 3 adjacent SNPs on chromosome 1p22.2 (P<1x10^-8). These SNPs form a 42kb haplotype block and explain 11% of the genetic variance for this renal function phenotype. Replication analysis of the tagging SNP (rs1396315) in an independent US cohort supports the association (P = 0.000011). Blood transcript analysis showed 35 genes were associated with rs1396315 (P<0.05). Gene set enrichment analysis of these genes revealed the most enriched pathway was purine metabolism (P = 0.0015). Overall, our findings provide convincing evidence for a major pleiotropic effect locus on chromosome 1p22.2 influencing risk of renal dysfunction via purine metabolism pathways in the Norfolk Island population. Further studies are now warranted to interrogate the functional relevance of this locus in terms of renal pathology and cardiovascular disease risk.

While many large genetic association studies have identified genes playing a role in complex disorders, there is still concern over the amount of missing genetic heritability. With this in mind, we have used a data reduction approach alongside pedigree-based association to obtain highly heritable components which explain 'hidden' variance of multiphenotypes within a large pedigree from the Norfolk Island genetic isolate. The most heritable of these components involved 7 traits reflecting metabolic and renal functionality, association of which locates to an intergenic region on chromosome 1p22.2. By integrating gene expression information, we identified enrichment of a purine metabolism pathway, further strengthening the metabolic nature of the observed association. Adding additional support to our approach, we show association of the tagging SNP (rs1396315) in an independent US population. The findings presented here are of particular interest as they implicate pleiotropic effect loci and newly associated biological pathways underlying cardiovascular disease risk.

Metabolic abnormalities in Williams-Beuren syndrome

BACKGROUND

Williams-Beuren syndrome (WBS, OMIM-194050) is a neurodevelopmental disorder with multisystemic manifestations caused by a 1.55-1.83 Mb deletion at 7q11.23 including 26-28 genes. Reported endocrine and metabolic abnormalities include transient hypercalcaemia of infancy, subclinical hypothyroidism in ∼ 30% of children and impaired glucose tolerance in ∼ 75% of adult individuals. The purpose of this study was to further study metabolic alterations in patients with WBS, as well as in several mouse models, to establish potential candidate genes.

METHODS

We analysed several metabolic parameters in a cohort of 154 individuals with WBS (data available from 69 to 151 cases per parameter), as well as in several mouse models with complete and partial deletions of the orthologous WBS locus, and searched for causative genes and potential modifiers.

RESULTS

Triglyceride plasma levels were significantly decreased in individuals with WBS while cholesterol levels were slightly decreased compared with controls. Hyperbilirubinemia, mostly unconjugated, was found in 18.3% of WBS cases and correlated with subclinical hypothyroidism and hypotriglyceridemia, suggesting common pathogenic mechanisms. Haploinsufficiency at MLXIPL and increased penetrance for hypomorphic alleles at the UGT1A1 gene promoter might underlie the lipid and bilirubin alterations. Other disturbances included increased protein and iron levels, as well as the known subclinical hypothyroidism and glucose intolerance.

CONCLUSIONS

Our results show that several unreported biochemical alterations, related to haploinsufficiency for specific genes at 7q11.23, are relatively common in WBS. The early diagnosis, follow-up and management of these metabolic disturbances could prevent long-term complications in this disorder.

Genetic variation underlying common hereditary hyperbilirubinaemia (Gilbert's syndrome) and respiratory health in the 1946 British birth cohort

BACKGROUND & AIMS

Bilirubin has potent antioxidant properties in vitro and raised serum levels have been associated with lower rates of respiratory disease. The enzyme uridine diphosphate glucuronosyltransferase polypeptide 1A1 (UGT1A1) is solely responsible for clearing bilirubin from the blood and homozygosity for seven thymine-adenine (TA) repeats in the TATA box regulatory element of the UGT1A1 gene underlies a mild hereditary unconjugated hyperbilirubinaemia (Gilbert's syndrome). Our aim was to investigate whether this genetic variation is associated with differences in respiratory health.

METHODS

The relationship between the promoter genotype underlying Gilbert's syndrome (UGT1A1 rs8175347 [TA]7/7) and respiratory outcomes assessed at ages 43, 53, and 60-64 were examined in 2190 members of the 1946 British birth cohort.

RESULTS

The (TA)7/7 genotype, present in 9% of the cohort, was associated with higher forced expiratory volume (FEV1) and forced vital capacity (FVC). The relationship was strongest for heavy smokers (⩾20 cigarettes per day) at age 53 with mean FEV1 409 ml higher (191 to 627; p<0.001) and mean FVC 530 ml higher (95% CI 262-798; p<0.001) for UGT1A1 (TA)7/7 Gilbert's syndrome participants than for all others, indicating a protection from the pulmonary consequences of heavy smoking. The odds of respiratory disease (chronic obstructive pulmonary disease, self-reported asthma, or prescription of respiratory drugs) were half in those with Gilbert's syndrome genotype (odds ratio 0.49 [95% CI 0.39-0.74]; p<0.001) compared to those without this genotype.

CONCLUSIONS

Genetically raised unconjugated serum bilirubin is associated with higher adult respiratory function and protection from respiratory disease.

Gilbert syndrome

Gilbert syndrome is a common autosomal dominant hereditary condition with incomplete penetrance and characterized by intermittent unconjugated hyperbilirubinemia in the absence of hepatocellular disease or hemolysis. In patients with Gilbert syndrome, uridine diphosphate-glucuronyl transferase activity is reduced to 30% of the normal, resulting in indirect hyperbilirubinemia. In its typical form, hyperbilirubinemia is first noticed as intermittent mild jaundice in adolescence. However, Gilbert syndrome in combination with other prevailing conditions such as breast feeding, G-6-PD deficiency, thalassemia, spherocytosis, or cystic fibrosis may potentiate severe hyperbilirubinemia and/or cholelithiasis. It may also reduce plasma oxidation, and it may also affect drug metabolism. Although in general the diagnosis of the syndrome is one of exclusion, molecular genetic tests can now be performed when there is a diagnostic problem. The most common genotype of Gilbert syndrome is the homozygous polymorphism A(TA)7TAA in the promoter of the gene for UDP-glucuronosyltransferase 1A1 (UGT1A1), which is a TA insertion into the promoter designated UGT1A1*28. No specific management is necessary as Gilbert syndrome is a benign condition.

CONCLUSION

Gilbert genotype should be kept in the clinician's mind, at least as a contributor factor, in cases with unexplained indirect hyperbilirubinemia.

A genome-wide search for non-UGT1A1 markers associated with unconjugated bilirubin level reveals significant association with a polymorphic marker near a gene of the nucleoporin family

Variants in the UGT1A1 gene and its promoter are known to determine levels of unconjugated bilirubin (UCB), but do not explain all cases of unconjugated hyperbilirubinemia. To discover associations with variants in genes other than UGT1A1, we undertook a genome-wide association study. We recruited 200 participants to cover the entire range of quantitative variation in UCB level. The data set -- after data curation, including analyses for population stratification and cryptic relatedness -- comprised genotypes at 512,349 SNP loci on 182 individuals. Quantitative trait locus (QTL) association analyses were performed, after adjusting the UCB level for effects of age, gender, and genotype at the dinucleotide (TA) insertion locus in UGT1A1 that is known to significantly modulate UCB level. A significant association of a polymorphic marker (rs2328136) near the NUP153 gene (which produces a 153 kDa nucleoporin) was obtained (p = 0.002, after multiple-testing correction). The frequency of the variant allele (A) at the rs2328136 locus in our study population is 40%, higher than most global populations. NUP153, whose product is a major regulatory factor in bidirectional transport of biomolecules across nucleus to cytosol, is associated with the transport of biliverdin reductase, which is important for bilirubin conjugation.

Pharmacogenomic contribution to drug response

There has been significant progress made in individualizing cancer therapy, especially for colorectal and breast cancer. This has included objective determination of aggressiveness of therapy using molecular predictors of disease recurrence (i.e., Mammaprint, OncotypeDX), identifying altered drug activation for dose modifications (i.e., DPYD, CYP2D6, UGT1A1), or variation in drug targets or components of a pharmacodynamic pathway (TYMS, EGFR, KRAS). With patient-specific molecular characteristics increasingly guiding therapy, this review provides important and timely insights on targeted therapy. Ultimately, integration of both pharmacogenomic and clinical characteristics can provide powerful predictive tools for stratifying responders from nonresponders and identifying patients at increased risk for toxicity.

Serum bilirubin links UGT1A1*28 polymorphism and predicts long-term cardiovascular events and mortality in chronic hemodialysis patients

BACKGROUND AND OBJECTIVES

Bilirubin is a protective factor with antioxidant and anti-inflammatory properties, but its association with clinical outcomes of hemodialysis patients is unknown. Bilirubin degradation is mainly determined by the activity of hepatic bilirubin uridine diphosphate-glucuronosyltransferase (UGT1A1), which is significantly influenced by a TA-repeat polymorphism in the gene's promoter, an allele designated UGT1A1*28. The study aimed to clarify the association between serum bilirubin and UGT1A1*28 polymorphism and their respective effect on outcomes of chronic hemodialysis patients.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

The cohort study comprised 661 chronic hemodialysis patients who were prospectively followed for 12 years. The endpoints were cardiovascular events (CVEs) and all-cause mortality.

RESULTS

After adjustment for traditional and dialysis-related risk factors, individuals with bilirubin in the upper tertile had an adjusted hazard ratio of 0.32 for CVEs and 0.48 for all-cause mortality compared with those in the lower tertile. Individuals homozygous for UGT1A1*28 (genotype 7/7) had significantly higher bilirubin levels than those with 6/6 and 7/6 genotypes. In the same multivariable-adjusted model, individuals with 7/7 had approximately one tenth the risk for CVEs and one fourth the risk for all-cause mortality as compared with carriers of the 6 allele.

CONCLUSIONS

A graded, reverse association was noted between serum bilirubin and adverse outcomes among chronic hemodialysis patients. Moreover, the UGT1A1*28 polymorphism had strong effects on bilirubin levels and the 7/7 genotype might have an important effect on reducing CVEs and death.

Genetic influences on serum bilirubin in American Indians: The Strong Heart Family Study

OBJECTIVE

To identify genetic variation influencing serum bilirubin levels in American Indians, we performed genome-wide screening and association analyses in the Strong Heart Family Study. Bilirubin is an endogenous antioxidant that has demonstrated an inverse relationship with cardiovascular disease. Genetic variation within the promoter region of uridine diphosphate glucuronosyltransferase (UGT1A1) on chromosome 2q has been associated with elevated serum bilirubin levels in European populations. However, no study has investigated the UGT1A1 promoter in American Indians.

METHODS

Statistical analyses were carried out with 3,484 participants aged 14 to 93 years recruited from three geographic areas in the United States; Arizona, Oklahoma, and North and South Dakota.

RESULTS

Variance components linkage analysis detected a quantitative trait locus (QTL) for bilirubin on chromosome 2q in the combined centers (LOD = 6.61, P = 4.24 × 10⁻⁶) and in Oklahoma (LOD = 5.65, P = 4.57 24 × 10⁻⁵). Genetic association of the UGT1A1 promoter polymorphism was significant for all geographic locations. After adjustment using conditional linkage for UGT1A1 promoter variance, the linkage signal dropped to 1.10 in the combined sample and to 3.32 (P = 0.02) in Oklahoma, indicating this polymorphism is not completely responsible for the linkage signal in American Indians. We also detected suggestive linkage signals in the Dakotas on chromosome 10p12 (LOD = 2.18) and in the combined centers (LOD = 2.24) on chromosome 10q21.

CONCLUSIONS

Replication of a serum bilirubin QTL on chromosome 2q in American Indians implicates UGT1A1 but further genotyping is warranted to identify additional causative polymorphisms. Evidence also supports a potential novel locus for bilirubin on chromosome 10.

Serum bilirubin and genes controlling bilirubin concentrations as biomarkers for cardiovascular disease

BACKGROUND

Serum bilirubin has been consistently shown to be inversely related to cardiovascular disease (CVD). Recent studies showed serum bilirubin to be associated with CVD-related factors such as diabetes, metabolic syndrome, and body mass index. Although the association of serum bilirubin with CVD has been found in both retrospective and prospective studies, less information is available on the role of genes that control bilirubin concentrations and their association with CVD.

CONTENT

In this review, we provide detailed information on the identity of the major genes that control bilirubin concentrations and their association with serum bilirubin concentrations and CVD risk. We also update the results of the major studies that have been performed on the association between serum bilirubin, CVD, and CVD-related diseases such as diabetes or metabolic syndrome. Studies consistently indicate that bilirubin concentrations are inversely associated with different types of CVD and CVD-related diseases. A conditional linkage study indicates that UGT1A1 is the major gene controlling serum bilirubin concentrations, and this finding has been confirmed in recent genomewide association studies. Studies also indicate that individuals homozygous for UGT1A1*28 have a significantly lower risk of developing CVD than carriers of the wild-type alleles.

SUMMARY

Serum bilirubin has a protective effect on CVD and CVD-related diseases, and UGT1A1 is the major gene controlling serum bilirubin concentrations. Pharmacologic, nonpharmacologic, or genetic interventions that increase serum bilirubin concentrations could provide more direct evidence on the role of bilirubin in CVD prevention.

Genome-wide association of serum bilirubin levels in Korean population

A large-scale, genome-wide association study was performed to identify genetic variations influencing serum bilirubin levels using 8841 Korean individuals. Significant associations were observed at UGT1A1 (rs11891311, P = 4.78 x 10(-148)) and SLCO1B3 (rs2417940, P = 1.03 x 10(-17)), which are two previously identified loci. The two single-nucleotide polymorphisms (SNPs) were replicated (rs11891311, P = 3.18 x 10(-15)) or marginally significant (rs2417940, P = 8.56 x 10(-4)) in an independent cohort of 1096 individuals. In a conditional analysis adjusted for the top UGT1A1 variant (rs11891311), another variant in UGT1A1 (rs4148323, P = 1.22 x 10(-121)) remained significant; this suggests that in UGT1A1 at least two independent genetic variations influence the bilirubin levels in the Korean population. The protein coding variant rs4148323, which is monomorphic in European-derived populations, may be specifically associated with serum bilirubin levels in Asians (P = 2.56 x 10(-70)). The SLCO1B3 variant (rs2417940, P = 1.67 x 10(-18)) remained significant in a conditional analysis for the top UGT1A1 variant. Interestingly, there were significant differences in the associated variations of SLCO1B3 between Koreans and European-derived populations. While the variant rs2417940 at intron 7 of SLCO1B3 was more significantly associated in Koreans, variants rs17680137 (P = 0.584) and rs2117032 (P = 2.76 x 10(-5)), two of the top-ranked SNPs in European-derived populations, did not reach the genome-wide significance level. Also, variants in SLCO1B1 did not reach genome-wide significance in Koreans. Our result supports the idea that there are considerable ethnic differences in genetic association of bilirubin levels between Koreans and European-derived populations.

Exploring the genetic architecture of neonatal hyperbilirubinemia

The potential for genetic variation to modulate neonatal hyperbilirubinemia risk is increasingly being recognized. In particular, polymorphisms across three genes involved in bilirubin production and metabolism [glucose-6-phosphate dehydrogenase (G6PD), uridine diphosphate glucuronosyl transferase 1A1 (UGT1A1), and solute carrier organic anion transporter polypeptide 1B1 (SLCO1B1)] may interact with each other and/or environmental contributors to produce significant hyperbilirubinemia. Variant gene co-expression including compound and synergistic heterozygosity enhances hyperbilirubinemia risk, contributing to the etiologic heterogeneity and complex nature of neonatal jaundice.

Acute effect of weight loss on levels of total bilirubin in obese, cardiovascular high-risk patients: an analysis from the lead-in period of the Sibutramine Cardiovascular Outcome trial

Low levels of bilirubin are associated with an increased risk of cardiovascular adverse events. Weight reduction is known to reduce several cardiovascular risk factors, but effects on bilirubin levels have not been reported. We studied the response of weight loss therapy with sibutramine and lifestyle change on levels of total bilirubin in an overweight or obese, cardiovascular high-risk population. Data from the first 4 weeks of the lead-in period of the Sibutramine Cardiovascular Outcome study were analyzed. A total of 10 198 patients provided body weight measurements before and after 4 weeks of sibutramine treatment (10 mg daily), of whom 1059 (10.4%) gained weight, 1467 (13.7%) lost greater than 0% to 1%, 2492 (23.2%) lost greater than 1% to 2%, 2280 (21.2%) lost greater than 2% to 3%, 1498 (13.9%) lost greater than 3% to 4%, and 1402 (13.1%) lost greater than 4% of their initial weight, respectively. At screening, bilirubin concentrations were similar between weight loss groups (around 11 micromol/L, P = .7) and increased linearly as a function of weight loss. The effect was significantly more pronounced in men compared with women (P for interaction = .003). Adjusted for multiple variables, each 1% increase in weight loss was associated with 0.21-micromol/L (+/- standard error 0.027) increase in men (P < .0001) and 0.11-micromol/L (+/-0.024) increase in women (P < .0001). Short-term weight loss during administration of sibutramine in combination with diet and exercise advice is effective in increasing bilirubin levels within the reference range, with bilirubin increasing as a linear function of weight change. The effect is greater in men than in women.

Gilbert syndrome as a predisposing factor for cholelithiasis risk in the Greek adult population

We investigated the hypothesis that coinheritance of the common A(TA)(n)TAA promoter mutation at the UGT1A1 locus associated with Gilbert syndrome is a risk factor for gallstone formation in a homogeneous adult population, by conducting a case-control study that included 198 adult patients with cholelithiasis and 152 healthy controls both of Greek origin. Three genotypes were found: 7/7 (17.8% in controls and 23.3% in patients), 6/7 (33.5% in controls and 46.5% in patients), and normal homozygous 6/6 (48.7% in controls and 30.3% in patients). The Gilbert UGT1A1 genotypes 6/7 and 7/7 show significant association (odds ratio 2.225, 95% confidence interval 1.373-3.605, p=0.001, and odds ratio 2.101, 95% confidence interval 1.171-3.770, p=0.013, respectively) with cholelithiasis risk. This association supports the theory that genetic factors are responsible for a fraction of symptomatic gallstone disease; however, further studies are required in different ethnic groups to fully elucidate the involvement of Gilbert syndrome in gallstone disease.

Common variants of four bilirubin metabolism genes and their association with serum bilirubin and coronary artery disease in Chinese Han population

OBJECTIVES

Studies have revealed an inverse relationship between serum total bilirubin (TBIL) levels and coronary artery disease (CAD). This study investigated the genetic variants of four bilirubin metabolism genes--heme oxygenase-1 (HMOX1), biliverdin reductase A (BLVRA), solute carrier organic anion transporter family member 1B1 (SLCO1B1), and uridine diphosphate glycosyltransferase 1A1 (UGT1A1)--in relation to TBIL levels and CAD.

METHODS AND RESULTS

Thirty-five common single nucleotide polymorphisms (SNPs) were genotyped in 2380 unrelated Han participants who underwent angiocardiography at hospitals in Shanghai, China. Only three genetic variants--rs4399719 (UGT1A1 T-2473G), rs887829 (UGT1A1 G-364A), and rs4148323 (UGT1A1 G211A)--were associated with TBIL levels (each P<0.001). Four significant associations with CAD were detected after controlling age and the false discovery rate at 15%: the recessive effect of SNP rs887829 (UGT1A1 G-364A) [age-adjusted odds ratio (OR): 0.24; 95% confidence interval (CI): 0.10-0.60; P=0.0014] and dominant effect of rs4149013 (SLCO1B1 A-12099G) (age-adjusted OR: 0.70; 95% CI: 0.55-0.91; P=0.0069) on male CAD, and the additive effects of rs2877262 (BLVRA G+1238/in6C) (age-adjusted OR: 0.73; 95% CI: 0.59-0.89; P=0.0021) and rs2690381 (BLVRA G+2613/in6A) (age-adjusted OR: 0.70; 95% CI: 0.56-0.86; P=0.0008) on female CAD. SNPs rs2877262 and rs2690381 were both in a linkage disequilibrium block within BLVRA with r greater than 0.750. Correspondingly, this block was identified to be associated with female CAD.

CONCLUSION

Our study provides genetic evidences for the difference in the impact of these four bilirubin metabolism genes on TBIL levels and CAD.

Genome-wide association meta-analysis for total serum bilirubin levels

Variation in serum bilirubin is associated with altered cardiovascular disease risk and drug metabolism. We aimed to identify genetic contributors to variability in serum bilirubin levels by combining results from three genome-wide association studies (Framingham heart study, n = 3424; Rotterdam study, n = 3847; Age, Gene, Environment and Susceptibility-Reykjavik, n = 2193). Meta-analysis showed strong replication for a genetic influence on serum bilirubin levels of the UGT1A1 locus (P < 5 x 10(-324)) and a 12p12.2 locus. The peak signal in the 12p12.2 region was a non-synonymous SNP in SLCO1B1 (rs4149056, P = 6.7 x 10(-13)), which gives rise to a valine to alanine amino acid change leading to reduced activity for a hepatic transporter with known affinity for bilirubin. There were also suggestive associations with several other loci. The top variants in UGT1A1 and SLCO1B1 explain approximately 18.0 and approximately 1.0% of the variation in total serum bilirubin levels, respectively. In a conditional analysis adjusted for individual genotypes for the top UGT1A1 variant, the top SLCO1B1 variant remained highly significant (P = 7.3 x 10(-13)), but no other variants achieved genome-wide significance. In one of the largest genetic studies of bilirubin to date (n = 9464), we confirm the substantial genetic influence of UGT1A1 variants, consistent with past linkage and association studies, and additionally provide strong evidence of a role for allelic variation in SLCO1B1. Given the involvement of bilirubin in a number of physiological and disease processes, and the roles for UGT1A1 and SLCO1B1 in drug metabolism, these genetic findings have potential clinical importance. In analyses for association with gallbladder disease or gallstones, top bilirubin SNPs in UGT1A1 and SLCO1B1 were not associated.

Conditional linkage and genome-wide association studies identify UGT1A1 as a major gene for anti-atherogenic serum bilirubin levels--the Framingham Heart Study

OBJECTIVE AND METHODS

Low bilirubin levels are significantly associated with cardiovascular diseases (CVD). In previous genome-wide linkage studies we identified a major locus on chromosome 2q harboring the candidate gene UDP-glucuronosyltransferase (UGT1A1). The activity of this enzyme is significantly influenced by a TA-repeat polymorphism in the promoter of the gene. In a prospective study individuals with genotype (TA)7/(TA)7 had significantly higher bilirubin levels and approximately one-third the risk of CVD as carriers of the wildtype (TA)6 allele. In the present study we performed a conditional linkage study to investigate whether this polymorphism explains the observed linkage peak and extended our analysis by a genome-wide association study on bilirubin levels in 1345 individuals.

RESULTS

After adjustment for the bilirubin variance explained by this polymorphism, the LOD score on chromosome 2q dropped from 3.8 to 0.4, demonstrating that this polymorphism explains the previous linkage result. For the genome-wide association study, the closest marker to UGT1A1 was in the top ranking SNPs. The association became even stronger when we considered the TA-repeat polymorphism in the analysis (p=2.68 x 10(-53)). Five other SNPs in other regions reached genome-wide significance without obvious connection to bilirubin metabolism.

CONCLUSIONS

Our studies suggest that UGT1A1 may be the major gene with strong effects on bilirubin levels and the TA-repeat polymorphism might be the key polymorphism within the gene controlling bilirubin levels. Since this polymorphism has a high frequency and a substantial impact on the development of CVD, the gene might be an important drug target.

Serum bilirubin levels, UGT1A1 polymorphisms and risk for coronary artery disease

Low levels of the antioxidative serum bilirubin are associated with vascular aging and an increased risk for coronary artery disease (CAD). UGT1A1 is the major gene influencing bilirubin concentrations. Therefore, we investigated an association of bilirubin levels and two polymorphisms in the promoter of UGT1A1 (-53(TA-repeat) polymorphism and T-3279G) in 477 patients with premature, familial CAD and 619 age- and sex-matched controls. Bilirubin concentrations were significantly lower in cases than in controls (0.62+/-0.36 vs. 0.76+/-0.41 mg/dl for men, p=1.2 x 10(-10); and 0.42+/-0.29 vs. 0.55+/-0.23 mg/dl, p=1.9 x 10(-9) for women). Both polymorphisms showed a strong association with bilirubin levels with higher levels for homozygote carriers of the minor allele. These associations were most pronounced in male controls and patients (p=5.9 x 10(-26) and p=3.4 x 10(-16), respectively, for the -53(TA-repeat) polymorphism). Logistic regression analysis revealed low bilirubin levels but not the UGT1A1 polymorphisms to be significantly associated with CAD: OR (95% CI) 0.90 (0.86-0.94), p=2.6 x 10(-6) for men and 0.77 (0.68-0.87), p=3.2 x 10(-5) for women, respectively for each 0.1mg/dl increase of bilirubin. These results indicate that it is rather decreased bilirubin levels in general than the changes in the genetic variation of this gene that increase the risk for CAD.

Computational and experimental studies on the catalytic mechanism of biliverdin-IXbeta reductase

BVR-B (biliverdin-IXbeta reductase) also known as FR (flavin reductase) is a promiscuous enzyme catalysing the pyridine-nucleotide-dependent reduction of a variety of flavins, biliverdins, PQQ (pyrroloquinoline quinone) and ferric ion. Mechanistically it is a good model for BVR-A (biliverdin-IXalpha reductase), a potential pharmacological target for neonatal jaundice and also a potential target for adjunct therapy to maintain protective levels of biliverdin-IXalpha during organ transplantation. In a commentary on the structure of BVR-B it was noted that one outstanding issue remained: whether the mechanism was a concerted hydride transfer followed by protonation of a pyrrolic anion or protonation of the pyrrole followed by hydride transfer. In the present study we have attempted to address this question using QM/MM (quantum mechanics/molecular mechanics) calculations. QM/MM potential energy surfaces show that the lowest energy pathway proceeds with a positively charged pyrrole intermediate via two transition states. These initial calculations were performed with His(153) as the source of the proton. However site-directed mutagenesis studies with both the H153A and the H153N mutant reveal that His(153) is not required for catalytic activity. We have repeated the calculation with a solvent hydroxonium donor and obtain a similar energy landscape indicating that protonation of the pyrrole is the most likely first step followed by hydride transfer and that the required proton may come from bulk solvent. The implications of the present study for the design of inhibitors of BVR-A are discussed.

Association between the UGT1A1 TA-repeat polymorphism and bilirubin concentration in patients with intermittent claudication: results from the CAVASIC study

BACKGROUND

Bilirubin has antioxidative and cytoprotective properties. Low plasma concentrations of bilirubin are reportedly associated with the development of coronary and cerebrovascular disease, and bilirubin concentrations are strongly correlated with the enzyme activity of the hepatic uridine diphosphate glucuronosyltransferase (UGT1A1). The activity of UGT1A1 is influenced by a TA-repeat polymorphism in the promoter of the UGT1A1 gene (UDP glucuronosyltransferase 1 family, polypeptide A1). In a case-control study, we investigated the association between the UGT1A1 polymorphism, bilirubin concentration, and intermittent claudication.

METHODS

We included 255 consecutive male patients presenting with intermittent claudication in the investigation and matched the patients by age and diabetes mellitus with 255 control individuals.

RESULTS

Plasma bilirubin concentrations were significantly lower in patients than in controls [mean (SD), 12.5 (5.3) micromol/L vs 15.4 (7.9) micromol/L; P < 0.001]. We found a clear association between the number of TA repeats and plasma bilirubin concentration. Considering the 6/6 TA-repeat genotype as the wild type, we observed a slight increase in bilirubin concentration individuals with the heterozygous 6/7 genotype and pronounced increases for those with the homozygous 7/7 genotype. This association occurred in both controls and patients; however, patients and controls were not significantly different with respect to UGT1A1 TA-repeat genotype frequencies.

CONCLUSIONS

Our study of a well-phenotyped group of patients with intermittent claudication and control individuals revealed a clear association between low bilirubin concentrations and peripheral arterial disease but no association between the UGT1A1 polymorphism and the disease.

Genome-wide association with select biomarker traits in the Framingham Heart Study

BACKGROUND

Systemic biomarkers provide insights into disease pathogenesis, diagnosis, and risk stratification. Many systemic biomarker concentrations are heritable phenotypes. Genome-wide association studies (GWAS) provide mechanisms to investigate the genetic contributions to biomarker variability unconstrained by current knowledge of physiological relations.

METHODS

We examined the association of Affymetrix 100K GeneChip single nucleotide polymorphisms (SNPs) to 22 systemic biomarker concentrations in 4 biological domains: inflammation/oxidative stress; natriuretic peptides; liver function; and vitamins. Related members of the Framingham Offspring cohort (n = 1012; mean age 59 +/- 10 years, 51% women) had both phenotype and genotype data (minimum-maximum per phenotype n = 507-1008). We used Generalized Estimating Equations (GEE), Family Based Association Tests (FBAT) and variance components linkage to relate SNPs to multivariable-adjusted biomarker residuals. Autosomal SNPs (n = 70,987) meeting the following criteria were studied: minor allele frequency > or = 10%, call rate > or = 80% and Hardy-Weinberg equilibrium p > or = 0.001.

RESULTS

With GEE, 58 SNPs had p < 10(-6): the top SNPs were rs2494250 (p = 1.00*10(-14)) and rs4128725 (p = 3.68*10(-12)) for monocyte chemoattractant protein-1 (MCP1), and rs2794520 (p = 2.83*10(-8)) and rs2808629 (p = 3.19*10(-8)) for C-reactive protein (CRP) averaged from 3 examinations (over about 20 years). With FBAT, 11 SNPs had p < 10(-6): the top SNPs were the same for MCP1 (rs4128725, p = 3.28*10(-8), and rs2494250, p = 3.55*10(-8)), and also included B-type natriuretic peptide (rs437021, p = 1.01*10(-6)) and Vitamin K percent undercarboxylated osteocalcin (rs2052028, p = 1.07*10(-6)). The peak LOD (logarithm of the odds) scores were for MCP1 (4.38, chromosome 1) and CRP (3.28, chromosome 1; previously described) concentrations; of note the 1.5 support interval included the MCP1 and CRP SNPs reported above (GEE model). Previous candidate SNP associations with circulating CRP concentrations were replicated at p < 0.05; the SNPs rs2794520 and rs2808629 are in linkage disequilibrium with previously reported SNPs. GEE, FBAT and linkage results are posted at http://www.ncbi.nlm.nih.gov/projects/gap/cgi-bin/study.cgi?id=phs000007 webcite.

CONCLUSION

The Framingham GWAS represents a resource to describe potentially novel genetic influences on systemic biomarker variability. The newly described associations will need to be replicated in other studies.

UDP-Glucuronosyltransferase 1A1 Gene Polymorphisms and Total Bilirubin Levels in an Ethnically Diverse Cohort of Women

The objective of this study was to investigate variations in UGT1A1 polymorphisms and haplotypes among African-American and Caucasian women and to assess whether variants other than UGT1A1*28 are associated with total serum bilirubin levels. The (TA)(n) repeats and 14 single nucleotide polymorphisms (SNPs) in the UGT1A1 gene were genotyped in 335 African Americans and 181 Caucasians. Total serum bilirubin levels were available in a subset of 125 women. Allele frequencies of all SNPs and (TA)(n) repeats were significantly different between African Americans and Caucasians. In Caucasians, three common haplotypes accounted for 71.8% of chromosomes, whereas five common haplotypes accounted for only 46.6% of chromosomes in African Americans. Mean total serum bilirubin levels were significantly lower (p = 0.005) in African Americans (0.36 mg/dl) than in Caucasians (0.44 mg/dl). The (TA)(n) repeats explained a significant amount of variation in total bilirubin levels (R(2) = 0.27, p < 0.0001), whereas other SNPs were less correlative. Thus, significant variations in UGT1A1 haplotype structure exist between African Americans and Caucasians in this relatively large cohort of women. The correlation of UGT1A1 with total bilirubin levels was mainly due to (TA)(n) repeats in Caucasians but a clear correlation was not observed in African Americans because of the high diversity of haplotypes and the small sample size. These data have implications for the design of epidemiologic studies of cancer susceptibility and pharmacogenetic studies for adverse drug reactions in populations of African ancestry.

Inheritance of hyperbilirubinemia: evidence for a major autosomal recessive gene

BACKGROUND AND AIM

To clarify the precise mode of inheritance of Gilbert syndrome, an unconjugated familial hyperbilirubinemia, where impaired bilirubin conjugation is caused by reduced UGT1A1 activity determined by a defective function of the A(TA)6TAA promoter region of the UGT1A1 gene.

SUBJECTS AND METHODS

Serum bilirubin levels were measured in a large, homogeneous resident population from North-Eastern Italy, consisting of 1.639 males (age 44.5+/-13.9, range 18-89 years), and 1.420 females (age 45.1+/-15.0, range 18-85). In 112 nuclear families from hyperbilirubinemic probands living in the same area a complex segregation analysis was then performed. In both samples we carefully excluded potentially confounding factors of bilirubin levels (alcohol abuse, excessive cigarette smoking, drug consumption, overt haemolysis and liver disease).

RESULTS

Mean serum bilirubin concentrations are higher in males than in females, showing fluctuations through the different age periods in males. Complex segregation results demonstrate that unconjugated hyperbilirubinemia exhibits a precise mode of inheritance in which a major recessive gene with a frequency of 0.45 is responsible for higher serum bilirubin values.

CONCLUSIONS

This major recessive gene accounts only for a part of the serum bilirubin concentration, thus implying additional, environmental factors for the clinical appearance of GS.

Association between the UGT1A1*28 allele, bilirubin levels, and coronary heart disease in the Framingham Heart Study

BACKGROUND

Bilirubin is an antioxidant that suppresses lipid oxidation and retards atherosclerosis formation. An inverse association between serum bilirubin and coronary heart disease has been reported. Linkage studies have identified a major locus at the chromosome 2q telomere that affects bilirubin concentrations. A candidate gene in the linkage region encodes hepatic bilirubin uridine diphosphate-glucuronosyltransferase (UGT1A1). The insertion of a TA in the TATAA box of the gene, an allele designated UGT1A1*28, decreases gene transcription. Individuals homozygous for UGT1A1*28 (genotype 7/7) have increased serum bilirubin levels compared with carriers of the 6 allele. To date, no significant association between UGT1A1*28 and cardiovascular disease (CVD) events has been reported. We performed an association study in the Framingham Heart Study population to investigate whether UGT1A1*28 is associated with the risk of CVD events.

METHODS AND RESULTS

The study population included 1780 unrelated individuals from the Offspring cohort (49% males, mean age 36 years at entry) who had been followed up for 24 years. Individuals with genotype 7/7 had significantly higher bilirubin levels (mean+/-SD 1.14+/-0.44 mg/dL) than those with genotypes 6/6 and 6/7 (mean+/-SD 0.69+/-0.27 mg/dL, P<0.01). Using the Cox proportional hazards model, we found significant associations between the UGT1A1*28 allele and decreased risk of CVD. Individuals with genotype 7/7 (population frequency of 11%) had approximately one third the risk for CVD and coronary heart disease as carriers of the 6 allele, which resulted in a hazard ratio (95% confidence interval) of 0.36 (0.18 to 0.74) and 0.30 (0.12 to 0.74), respectively.

CONCLUSIONS

Homozygote UGT1A1*28 allele carriers with higher serum bilirubin concentrations exhibit a strong association with lower risk of CVD.

Comparative genomics analysis of human sequence variation in the UGT1A gene cluster

Common polymorphisms within the human UGT1A gene locus are associated with irinotecan and tranilast toxicity. To uncover additional functional variation across this gene cluster, cross-species sequence comparisons were performed. Evolutionarily conserved segments (a total of 47.1 kb) were re-sequenced in 24 African-American, 24 European-American, and 24 Asian individuals, and 381 segregating sites (including 123 singletons) were identified. Highly conserved coding sites were less likely to be polymorphic than diverged sites (P<0.0001) but this pattern was not observed at non-coding sites (P=0.1025). Among coding variants, the distribution of those computationally predicted to affect function was skewed toward low frequencies. Some alleles occurred at similar frequencies in each population; others had wide disparities. Although strong linkage disequilibrium was detected among the hepatically expressed genes, the degree of linkage disequilibrium varied among populations. These results suggest that rare functional gene variants and inter-population variability must be considered in the interpretation of association studies between UGT1A and drug metabolism/toxicity phenotypes.

Genetic polymorphisms in uridine diphospho-glucuronosyltransferase 1A1 (UGT1A1) and risk of breast cancer

Uridine diphospho-glucuronosyltransferase 1A1 (UGT1A1) is involved in catalyzing estrogen, the hormone that plays a central role in the etiology of breast cancer. A common polymorphism [A(TA)6TAA (allele *1) to A(TA)7TAA change (allele *28)] in the TATA-box of the promoter region of the UGT1A1 gene has been reported to be associated with a reduced transcription of this gene. We investigated the association of this polymorphism with the risk of breast cancer among 1047 breast cancer cases and 1083 community controls in the Shanghai Breast Cancer Study, a population-based case-control study. Approximately same proportion of cases (12.5%) and controls (13.0%) carried the variant allele *28 in the Chinese population (p = 0.32). When stratified by age, carrying the *28 allele was associated with an increased risk of breast cancer among women aged less than 40 years (odds ratio [OR] = 1.7; 95% CI = 1.0-2.7) but not among women 40 years old and over (OR = 0.8; 0.7-1.1). Only a few women were homozygous for the *28 allele, precluding a detailed gene-dose association analysis. Additional analyses showed that, the elevated risk associated with the UGT1A1 *28 allele among young women was primarily seen in women who had a later menarche, short menstrual years, absence of family history of breast cancer, low waist-to-hip ratio, or low body-mass index. These results suggested that the *28 allele in the UGT1A1 gene may be associated with an increased risk for breast cancer among Chinese women under age 40. No significant associations were observed with *28 allele and breast cancer risk by estrogen receptor/progesterone receptor status.

Genetic variants in the UDP-glucuronosyltransferase 1A1 gene predict the risk of severe neutropenia of irinotecan

PURPOSE

Severe toxicity is commonly observed in cancer patients receiving irinotecan. UDP-glucuronosyltransferase 1A1 (UGT1A1) catalyzes the glucuronidation of the active metabolite SN-38. This study prospectively evaluated the association between the prevalence of severe toxicity and UGT1A1 genetic variation.

PATIENTS AND METHODS

Sixty-six cancer patients with advanced disease refractory to other treatments received irinotecan 350 mg/m(2) every 3 weeks. Toxicity and pharmacokinetic data were measured during cycle 1. UGT1A1 variants (-3279G>T, -3156G>A, promoter TA indel, 211G>A, 686C>A) were genotyped.

RESULTS

The prevalence of grade 4 neutropenia was 9.5%. Grade 4 neutropenia was much more common in patients with the TA indel 7/7 genotype (3 of 6 patients; 50%) compared with 6/7 (3 of 24 patients; 12.5%) and 6/6 (0 of 29 patients; 0%) (P =.001). The TA indel genotype was significantly associated with the absolute neutrophil count nadir (7/7 < 6/7 < 6/6, P =.02). The relative risk of grade 4 neutropenia was 9.3 (95% CI, 2.4 to 36.4) for the 7/7 patients versus the rest of the patients. Pretreatment total bilirubin levels (mean +/- standard deviation) were significantly higher in patients with grade 4 neutropenia (0.83 +/- 0.08 mg/dL) compared to those without grade 4 neutropenia (0.47 +/- 0.03 mg/dL; P <.001). The -3156G>A variant seemed to distinguish different phenotypes of total bilirubin within the TA indel genotypes. The -3156 genotype and the SN-38 area under the concentration versus time curve were significant predictors of ln(absolute neutrophil count nadir; r(2) = 0.51).

CONCLUSION

UGT1A1 genotype and total bilirubin levels are strongly associated with severe neutropenia, and could be used to identify cancer patients predisposed to the severe toxicity of irinotecan. The hypothesis that the -3156G>A variant is a better predictor of UGT1A1 status than the previously reported TA indel requires further testing.