Interaction of coding region mutations and the Gilbert-type promoter abnormality of the UGT1A1 gene causes moderate degrees of unconjugated hyperbilirubinaemia and may lead to neonatal kernicterus

Rare liver diseases in Egypt: Clinical and epidemiological characterization

Illnesses that afflict a tiny number of individuals are referred to as rare diseases (RDs), sometimes called orphan diseases. The local healthcare systems are constantly under financial, psychological, and medical strain due to low incidence rates, unusual presentations, flawed diagnostic standards, and a lack of treatment alternatives for these RDs. The effective management of the once widely spread viral hepatitis B and C has altered the spectrum of liver diseases in Egypt during the last several years. The detection of uncommon disorders such as autoimmune, cholestatic, and hereditary liver diseases has also been made easier by the increasing knowledge and greater accessibility of specific laboratory testing. Finally, despite Egypt's large population, there are more uncommon liver disorders than previously thought. This review article discusses the clinical and epidemiological characteristics of a few uncommon liver disorders and the information currently accessible concerning these illnesses in Egypt.

Clinical and genetic definition of serum bilirubin levels for the diagnosis of Gilbert syndrome and hypobilirubinemia

BACKGROUND AND AIMS

Gilbert syndrome (GS) is genotypically predetermined by UGT1A1*28 homozygosity in Europeans and is phenotypically defined by hyperbilirubinemia using total bilirubin (TB) cutoff ≥1mg/dL (17 μmol/L). The prevalence of illnesses associated with GS and hypobilirubinemia has never been studied prospectively. As TB varies with UGT1A1*28 genotyping, sex, and age, we propose stratified definitions of TB reference intervals and report the prevalence of illnesses and adjusted 15 years survival.

METHODS

UK Biobank with apparently healthy liver participants (middle-aged, n=138,125) were analyzed after the exclusion of of nonhealthy individuals. The stratified TB was classified as GS when TB >90th centile; <10th centile indicated hypobilirubinemia, and between the 10th and 90th centile was normobilirubinemia. We compared the prevalence and survival rates of 54 illnesses using odds ratio (OR), logistic regression, and Cox models adjusted for confounders, and causality by Mendelian randomizations.

RESULTS

In women, we identified 10% (7,741/76,809) of GS versus 3.7% (2,819/76,809) using the historical cutoff of ≥1 mg/dL (P<0.0001). When GS and hypobilirubinemia participants were compared with normobilirubinemia, after adjustment and Mendelian randomizations, only cholelithiasis prevalence was significantly higher (OR=1.50; 95% CI [1.3-1.7], P=0.001) in men with GS compared with normobilirubinemia and in causal association with bilirubin (P=0.04). No adjusted survival was significantly associated with GS or hypobilirubinemia.

CONCLUSIONS

In middle-aged Europeans, the stratified TB demonstrates a careless GS underestimation in women when using the standard unisex 1 mg/dL cutoff. The prevalence of illnesses is different in GS and hypobilirubinemia as well as survivals before adjusting for confounding factors. With the exception of cholelithiasis in men, these differences were no more significant after adjustment and Mendelian randomization.

Management of Crigler-Najjar syndrome

Crigler-Najjar syndrome is a rare autosomal recessive inherited non-hemolytic unconjugated hyperbilirubinemia caused by UDP-glucuronosyltransferase deficiency. There are two forms of this disorder. Type 1 disease is associated with severe jaundice and neurologic impairment due to bilirubin encephalopathy that can result in permanent neurologic sequelae. Type 2 disease is associated with a lower serum bilirubin concentration and affected patients survive into adulthood without neurologic impairment. Currently, liver transplantation is the only available therapeutic method for these patients. Developing new curative approaches is a clinical need.

Novel UGT1A1 Gene Mutations in a Boy with Crigler-Najjar Syndrome Type II

In this article, we reported a patient with Crigler-Najjar syndrome type II with high-unconjugated bilirubin levels that decreased after phenobarbital treatment. The patient had two novel missense mutations in the UGT1A1 gene and a promoter variant in one allele. One mutation was c.1001T > C, that predicted leucine to proline substitution at position 334 (p.Leu334Pro). The other, c.1139A > G, predicted glutamic acid to glycine replacement at position 380 (p.Glu380Gly). In silico analysis indicated that both mutations are likely pathogenic.

UGT1A1 Variants c.864+5G>T and c.996+2_996+5del of a Crigler-Najjar Patient Induce Aberrant Splicing in Minigene Assays

A large fraction of DNA variants impairs pre-mRNA splicing in human hereditary disorders. Crigler-Najjar syndrome (CNS) is characterized by a severe unconjugated hyperbilirubinemia caused by variants in the UGT1A1 gene. We previously reported one CNS-type II patient with two splice-site variants in trans (c.864+5G>T and c.996+2_996+5del). According to MaxEntScan, both disrupt their corresponding donor sites (c.864+5G>T: 6.99 → 2.28; c.996+2_996+5del: 5.96 → -11.02), so they were selected for subsequent functional tests. Given the unavailability of patient RNA, we constructed an UGT1A1 splicing-reporter minigene with exons 1-4 to characterize the underlying splicing anomaly. The variant c.996+2_996+5del generated two aberrant transcripts, Δ(E2) (exon 2 skipping/64%) and ▼(E2q135) (intron retention of 135-nt/36%), which lead to the loss of 18 conserved amino-acids and the gain of 45 new ones of a critical functional domain, respectively. The c.864+5G>T variant mainly produced the aberrant transcript Δ(E1q141) (141-nt deletion/70.4%) and the full-length isoform (29.6%). Δ(E1q141) would provoke the loss of 47 amino-acids of the N-terminal domain that encodes for substrate specificity. Thus, the three anomalous transcripts are likely to inactivate UGT1A1. Moreover, this patient is also homozygous for the promoter variant A(TA)7TAA that decreases UGT1A1 expression by 70%, so the full-length transcript produced by c.864+5G>T would be even more reduced (<9%), thus supporting the diagnosis of CNS-type II. Therefore, minigenes represent valuable tools for the functional and clinical classifications of genetic variants.

Spectrum of UGT1A1 variants in Pakistani children affected with inherited unconjugated hyperbilirubinemias

Inherited unconjugated hyperbilirubinemias are a group of disorders characterized by increased levels of serum unconjugated bilirubin and arise because of the imbalance between its production and elimination from the body. It includes Crigler-Najjar syndrome and Gilbert syndrome. Crigler-Najjar syndrome type 1 represents the extreme severe end of the spectrum with complete absence of hepatic bilirubin uridine diphosphoglucuronate glucuronosyltransferase (UGT1A1). Crigler-Najjar syndrome type 2 patients have intermediate levels of bilirubin owing to incomplete deficiency of UGT1A1, and Gilbert syndrome lies at the extreme mild end of the spectrum with only slightly raised bilirubin level. Here, we present spectrum of UGT1A1 genetic variants in 25 Pakistani children from 23 unrelated families affected with persistent unconjugated hyperbilirubinemias. The promoter region, coding exons and splice junctions of the UGT1A1 were PCR amplified and subjected to Sanger sequencing. Eleven sequence variants were identified underlying disease phenotype including a novel c.582delC variant. Overall, c.622_625dupCAGC was the most frequent variant followed by c.1021C>T found in Crigler-Najjar syndrome type 1 patients. The evaluation of promoter polymorphism A(TA)_nTAA in the affected children and their families further supported the body of evidence that the A(TA)₇TAA allele could enhance the effect of other structural variants in Crigler-Najjar syndrome patients. To our knowledge, this is the first comprehensive study on molecular genetics of persistent unconjugated hyperbilirubinemias from Pakistan. This study expands the spectrum of UGT1A1 variants and should help in improved clinical diagnosis, genetic counseling and prenatal diagnosis of the affected families.

Effect of UDP-glucuronosyltransferase 1A1 activity on risk for developing Gilbert's syndrome

Variations at the six nucleotides -3279 (T > G), -53 (A[TA]₆ TAA > A[TA]₇ TAA), 211 (G > A), 686 (C > A), 1091 (C > T), and 1456 (T > G) in the UDP-glucuronosyltransferase 1A1 (UGT1A1) gene were determined in 178 Taiwanese patients with Gilbert's syndrome and in 200 healthy adults. Every subject was classified as a genotype depending on variation status of the six nucleotides in the UGT1A1 gene. The UGT1A1 activity for each genotype was calculated and then those genotypes were divided into 10 subgroups (Q1~Q10) according to their UGT1A1 activities, by using 10% as an interval. There were 24 genotypes observed, with UGT1A1 activity ranged 9%~100% of normal. There were two and six subjects with Gilbert's syndrome and none of healthy controls carrying genotypes in the Q1 and Q2 subgroups, respectively. The odds of developing Gilbert's syndrome were significantly higher for subjects carrying genotypes in the Q3, Q4, and Q5 subgroups than for those with genotype in the Q10 subgroup (odds ratios: 240.22, 59.80, and 14.67, respectively, P < .001 for each). Among the 178 patients of Gilbert's syndrome, serum bilirubin value was inversely correlated with UGT1A1 activity (r = -.306, P < .001). The sensitivity was 72.0% and the specificity was 90.5% by using UGT1A1 activity ≦40% of normal as the cut-off point to distinguish between healthy subjects and patients of Gilbert's syndrome. Our results demonstrate that UGT1A1 activity is certainly a determinate for serum bilirubin value and UGT1A1 activity ≦40% of normal is a proper risk factor for the development of Gilbert's syndrome.

Case report: multiple UGT1A1 gene variants in a patient with Crigler-Najjar syndrome

BACKGROUND

Inherited unconjugated hyperbilirubinemia is caused by variants in the gene UGT1A1 leading to Gilbert's syndrome and Crigler-Najjar syndrome types I and II. These syndromes are differentiated on the basis of UGT1A1 residual enzymatic activity and its affected bilirubin levels and responsiveness to phenobarbital treatment.

CASE PRESENTATION

In this report, we present a boy with Crigler-Najjar syndrome type II with high unconjugated bilirubin levels that decreased after phenobarbital treatment but increased in adolescence. Four different UGT1A1 gene variants have been identified for this patient, of which one is novel (g.11895_11898del) most likely confirming diagnose molecularly.

CONCLUSIONS

The presented case highlights the challenges encountered with the interpretation of molecular data upon identification of multiple variants in one gene that are causing different degree reducing effect on enzyme activity leading to several clinical conditions.

Unexplained cholestasis in adults and adolescents: diagnostic benefit of genetic examination

OBJECTIVES

A few adult and adolescent patients with even severe cholestatic liver disease remain unexplained after standard diagnostic work-up. We studied the value of genetic examination in such patients and developed a panel of eight genes with known cholestatic associations.

MATERIALS AND METHODS

Thirty-three patients with unexplained cholestasis despite a thorough clinical work-up were examined for sequence variations in the coding regions of the ABCB4, ABCB11, ABCC2, ABCG5, ATP8B1, JAG1, NOTCH2, and UGT1A1 genes and the promoter region of UGT1A1 by massive parallel sequencing of DNA extracted from whole blood. Hepatologists and clinical geneticists evaluated the causal potential of genetic variants.

RESULTS

In 9/33 patients (27%), we identified genetic variants as a certain causal factor and in further 9/33 (27%) variants as a possible contributing factor. In most cases, a detailed family history was necessary to establish the importance of genetic variants. Genetic causes were identified in 6/13 women (46%) with intrahepatic cholestasis during pregnancy and persisting abnormal biochemistry after delivery.

CONCLUSIONS

Our study suggests that a small number of well-known genetic variants are involved in at least 27-54% of patients with unexplained cholestasis. An expanded panel will likely explain more cases. This motivates genetic testing of these patients. Genetic testing, however, cannot stand alone but should be combined with a clinical genetic work-up in collaboration between hepatologists and clinical geneticists.

Inherited disorders of bilirubin clearance

Inherited disorders of hyperbilirubinemia may be caused by increased bilirubin production or decreased bilirubin clearance. Reduced hepatic bilirubin clearance can be due to defective (i) unconjugated bilirubin uptake and intrahepatic storage, (ii) conjugation of glucuronic acid to bilirubin (e.g., Gilbert syndrome, Crigler-Najjar syndrome, Lucey-Driscoll syndrome, breast milk jaundice), (iii) bilirubin excretion into bile (Dubin-Johnson syndrome), or (iv) conjugated bilirubin re-uptake (Rotor syndrome). In this review, the molecular mechanisms and clinical manifestations of these conditions are described, as well as current approaches to diagnosis and therapy.

Identification of Promotor and Exonic Variations, and Functional Characterization of a Splice Site Mutation in Indian Patients with Unconjugated Hyperbilirubinemia

Background

Mild unconjugated hyperbilirubinemia (UH), due to reduced activity of the enzyme uridine diphosphoglucuronate-glucuronosyltransferase family, polypeptide 1 (UGT1A1), is a common clinical condition. Most cases are caused by presence in homozygous form of an A(TA)₇TAA nucleotide sequence instead of the usual A(TA)₆TAA sequence in promoter region of the UGT1A1 gene. In some cases, other genetic variations have been identified which differ between populations. There is need for more data on such genetic variations from India.

Methods

DNA from subjects with unexplained persistent or recurrent UH was tested for the presence of TA promoter insertions. In addition, all five exons and splicing site regions of UGT1A1 gene were sequenced. Several bioinformatics tools were used to determine the biological significance of the observed genetic changes. Functional analysis was done to look for effect of a splice site mutation in UGT1A1.

Results

Of 71 subjects with UH (68 male; median age [range], 26 [16–63] years; serum bilirubin 56 [26–219] μM/L, predominantly unconjugated) studied, 65 (91.5%) subjects were homozygous for A(TA)₇TAA allele, five (7.0%) were heterozygous, and one (1.4%) lacked this change. Fifteen subjects with UH had missense exonic single nucleotide changes (14 heterozygous, 1 homozygous), including one subject with a novel nucleotide change (p.Thr205Asn). Bioinformatics tools predicted some of these variations (p.Arg108Cys, p.Ile159Thr and p.Glu463Val) to be deleterious. Functional characterization of an exonic variation (c.1084G>A) located at a splice site revealed that it results in frameshift deletion of 31 nucleotides and premature truncation of the protein.

Conclusion

Our study revealed several single nucleotide variations in UGT1A1 gene in Indian subjects with UH. Functional characterization of a splice site variation indicated that it leads to disordered splicing. These variations may explain UH in subjects who lacked homozygous A(TA)₇TAA promoter alleles.

Inherited disorders of bilirubin transport and conjugation: new insights into molecular mechanisms and consequences

Inherited disorders of bilirubin metabolism might reduce bilirubin uptake by hepatocytes, bilirubin conjugation, or secretion of bilirubin into bile. Reductions in uptake could increase levels of unconjugated or conjugated bilirubin (Rotor syndrome). Defects in bilirubin conjugation could increase levels of unconjugated bilirubin; the effects can be benign and frequent (Gilbert syndrome) or rare but severe, increasing the risk of bilirubin encephalopathy (Crigler-Najjar syndrome). Impairment of bilirubin secretion leads to accumulation of conjugated bilirubin (Dubin-Johnson syndrome). We review the genetic causes and pathophysiology of disorders of bilirubin transport and conjugation as well as clinical and therapeutic aspects. We also discuss the possible mechanisms by which hyperbilirubinemia protects against cardiovascular disease and the metabolic syndrome and the effects of specific genetic variants on drug metabolism and cancer development.

Correlation of UGT1A1 TATA-box polymorphism and jaundice in breastfed newborns-early presentation of Gilbert's syndrome

OBJECTIVE

The etiology of jaundice in otherwise healthy breastfed newborns that can present as early-onset exaggerated physiologic jaundice, or late breast milk jaundice (BMJ), is not yet entirely understood. This study tested the hypothesis that molecular marker for Gilbert's syndrome (GS), UGT1A1 TATA-box polymorphism, is associated with this disorders.

METHODS

We have investigated the UGT1A1 polymorphism frequency and its relation to severity of hyperbilirubinemia and jaundice duration among 220 exclusively breastfed term newborns; 57 of them with non-physiologic hyperbilirubinemia (NH), and 163 with BMJ, and in 187 healthy controls.

RESULTS

Significant differences in TA7/7 genotype frequency were established. The highest frequency was observed among the newborns with BMJ (42.0%), intermediate in the NH group (24.6%), while the controls had the lowest TA7/7 frequency (12.8%). Linear increase in TA7/7 frequency was observed depending on the duration of jaundice, peaking at 42.4% in newborns with the longest jaundice duration. Positive correlation between the serum bilirubin levels and the TATA-box length was established in all groups.

CONCLUSION

This study provides evidence that UGT1A1 TATA-box polymorphism is an important risk factor for developing jaundice in term breastfed newborns, presented as either early non-physiologic hyperbilirubinemia or breast milk jaundice. These results further support the original Odell's idea of neonatal jaundice as an early presentation of GS.

Advances of molecular clinical pharmacology in gastroenterology and hepatology

During developmental age, differences in pharmacodynamic reactions to several drugs may reflect polymorphisms of genes encoding drug-transporting proteins, receptors, drug targets, and gene products, whose disturbed activity sometimes plays an important role in certain diseases. Administration of drugs with a narrow therapeutic index may quite easily be associated with changes in pharmacokinetics and development of adverse drug reactions, which occasionally may cause fatalities. In such cases, polypragmasy and resulting drug interactions may enhance effects of changes in drug-metabolizing enzymes' activities. Phenotyping and genotyping of patients slowly are finding their place in some therapeutic regimens used in clinical gastroenterology and hepatology. At present, some assays to measure, for example, thiopurine S-methyltransferase activity are already commercially available. Polymorphisms of CYP450 enzymes, interleukins, and altered gene expression play an important role in some patients' various gastrointestinal tract and liver diseases. Herbal drugs also affect proinflammatory and antiinflammatory cytokine and nitric oxide balance in the body. Therapeutic use of recombined proteins, such as infliximab, natalizumab, onercept, humanized antibody to integrin α-4 β-7, or IFN-β in some large-bowel diseases increased therapeutic efficacy. IFN-α used in the patients with chronic hepatitis C improved cellular immunity in these subjects and exerted antiviral activity. Practical application of progress in pharmacogenetics, pharmacokinetics, pharmacodynamics, and use of bioproducts in novel therapeutic regimens has opened therapeutic frontiers and increased clinical safety.

Prevalence of Gilbert syndrome in parents of neonates with pathologic indirect hyperbilirubinemia

BACKGROUND AND OBJECTIVES

The cause of hyperbilirubinemia cannot be found in about 45% of cases of neonatal jaundice. Gilbert syndrome (GS) is the most common congenital disease associated with bilirubin metabolism in the liver. Since the screening value of genetic tests cannot be fully determined until accurate data on the prevalence and penetrance of the GS genotype are known, we sought to estimate whether the prevalence of GS is higher in the parents of neonates with severe unexplained indirect hyperbilirubinemia.

DESIGN AND SETTING

Case-control study of parents of neonates with severe unexplained indirect hyperbilirubinemia admitted to a neonatal ward.

METHODS

We used the rifampin test (checked bilirubin before and 4 hours after administration of 600 mg rifampin) for diagnosis of GS in parents of 115 neonates with severe unexplained indirect hyperbilirubinemia. We compared the prevalence of GS in these parents with that of a control group of 115 couples referred for premarital counseling.

RESULTS

The 115 neonates were aged 5.2 (1.6) days (mean, standard deviation), all were breast-fed, and males constituted 56.5%. Mean total serum bilirubin (TSB) level was 20.96 (5.48) mg/dL. 14.8% were glucose 6 phosphate dehydrogenase (G6PD) deficiency was present in 14.8%, and 10.4% had A, B or O blood group (ABO) incompatibilities with their mothers. There was no difference in the prevalence of GS between parents of the group with hyperbilirubinemia (22.2%) and the control group (19.13%) (P=.42). Mean TSB in neonates with parents who had GS was more (about 3 mg/dL) than in neonates with normal parents (P=.004). Fathers had GS twice as often as the mothers among the parents of neonates with hyperbilirubinemia (P=.003), among the control group (P=.009) and among neonates (P=.014).

CONCLUSION

This study showed that GS cannot cause severe indirect hyperbilirubinemia by itself, but it may have a summative effect on rising bilirubin when combined with other factors, for example, G6PD. Our results showed that in GS, males are affected about twice as much as the females.

Developmental pharmacogenomics

Interindividual variability in the disposition and action associated with similar doses of a given medication is an inherent characteristic of both adult and pediatric populations. Genotype-phenotype relationships in infants and children must take into account the role that ontogeny plays in producing variability in both pharmacokinetics and pharmacodynamics. This review explores pharmacogenomics in the context of ontogeny and relates these to the expression of drug-metabolizing enzymes and transporters and the consequent effect on the exposure-response relationship in the early years of life.

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.

Risk factors for severe hyperbilirubinemia among infants with borderline bilirubin levels: a nested case-control study

OBJECTIVE

To estimate the effect of phototherapy and other predictors on the risk of total serum bilirubin (TSB) >or= 25 mg/dL in infants with a TSB of 17 to 22.9 mg/dL at age >or= 48 hours.

STUDY DESIGN

From a cohort of 285295 infants >or= 34 weeks gestation and >or= 2000 g born between 1995 and 2004 in northern California Kaiser hospitals, we identified 17986 with a TSB of 17 to 22.9 mg/dL at age >or= 48 hours. All infants exhibiting a TSB >or= 25 mg/dL were selected as cases for the study. Four randomly selected controls were matched to each case based on the difference between their qualifying TSB and the American Academy of Pediatrics' phototherapy threshold.

RESULTS

A total of 62 cases were identified (0.4%). Six of these (10%) received inpatient phototherapy within 8 hours, along with 101 controls (41%) (adjusted odds ratio [AOR] 0.15; 95% confidence interval [CI] 0.06 to 0.40). Cases more often had lower gestational age (AOR 3.24; 95% CI 1.24 to 8.47 for 38 to 39 weeks and AOR = 3.70; 95% CI 0.61 to 22.4 for 34 to 37 weeks compared with >or= 40-week infants), bruising, (AOR 2.52; 95% CI 1.16 to 5.50), exclusive breast-feeding (AOR 2.09; 95% CI 1.05 to 4.03), and TSB increase of >or= 6 mg/dL/day (AOR 2.39; 95% CI 1.18 to 4.85).

CONCLUSIONS

Phototherapy was 85% effective in preventing TSB >or= 25 mg/dL. The strongest predictors of TSB >or= 25 mg/dL were gestational age, bruising, family history, and rapid rise in TSB.

Gilbert syndrome

Az ismert familiáris, nem konjugált hyperbilirubinaemiák közé tartozó Gilbert-szindróma az átlagpopuláció 7–10%-át érintő benignus kórkép. Tünetei rendszerint aspecifikusak, egyedül az esetlegesen előforduló sárgaság, továbbá az enyhén emelkedett nem konjugált bilirubin szintje utal a rendellenességre. Más laborértékek és a májbiopszia általában nem mutat eltérést a normálistól. A betegség hátterében a legtöbb esetben az UDP-glükuronil-transzferáz gén részleges működészavara áll. A gén azt az enzimet kódolja, amely a bilirubin glükuronsavval való konjugációját segíti. Noha a Gilbert-szindróma diagnosztizálása korábban hagyományos laboratóriumi módszerekkel és családi anamnézis felvételével történt, napjainkban lehetőség van az UDP-glükuronil-transzferáz gén genetikai variánsainak meghatározására. A gén promoterrégiójában, homozigóta formában lévő (TA)-inszerció – (TA)7/(TA)7– a betegek 80–100%-ában jelen van, és az aktív UDP-glükuronil-transzferáz mennyiségének csökkenéséhez vezet. A kódolórégióban található missense mutációk szerepe még nem teljesen tisztázott, de a (TA)7promotervariánssal való együttes előfordulásuk magyarázatul szolgálhat az emelkedett bilirubinszintre és a sárgaságra, valamint a Gilbert-szindróma családi halmozódására.

Gilbert’s syndrome: High frequency of the (TA)7 TAA allele in India and its interaction with a novel CAT insertion in promoter of the gene for bilirubin UDP-glucuronosyltransferase 1 gene

AIM: To identify the variants in UDP-glucuronosyltransfe-rase 1 (UGT1A1) gene in Gilbert’s syndrome (GS) and to estimate the association between homozygosity for TA insertion and GS in India, as well as the frequency of TA insertion and its impact among normal controls in India.

METHODS: Ninety-five GS cases and 95 normal controls were selected. Liver function and other tests were done. The promoter and all 5 exons of UGT1A1 gene were resequenced. Functional assessment of a novel trinucleotide insertion was done by in silico analysis and by estimating UGT1A1 promoter activity carried out by luciferase reporter assay of appropriate constructs in Hep G2 cell line.

RESULTS: Among the GS patients, 80% were homozygous for the TA insertion, which was several-fold higher than reports from other ethnic groups. The mean UCB level was elevated among individuals with only one copy of this insertion, which was not significantly different from those with two copies. Many new DNA variants in UGT1A1 gene were discovered, including a trinucleotide (CAT) insertion in the promoter found in a subset (10%) of GS patients, but not among normal controls. In-silico analysis showed marked changes in the DNA-folding of the promoter and functional analysis showed a 20-fold reduction in transcription efficiency of UGT1A1 gene resulting from this insertion, thereby significantly elevating the UCB level.

CONCLUSION: The genetic epidemiology of GS is variable across ethnic groups and the epistatic interactions among UGT1A1 promoter variants modulate bilirubin glucuronidation.

Role of cysteine residues in the function of human UDP glucuronosyltransferase isoform 1A1 (UGT1A1)

Bilirubin glucuronidation, catalysed by UGT1A1 [UGT (UDP glucuronosyltransferase) isoform 1A1, EC 2.4.1.17], is critical for biliary elimination of bilirubin. UGT1A1 deficiency causes CN-1 (Crigler-Najjar syndrome type 1), which is characterized by potentially lethal unconjugated hyperbilirubinaemia. Nucleotide sequence analysis of UGT1A1 in two CN-1 patients revealed that patient A was homozygous for a nt 530 G-->A (where nt 530 G-->A means guanine to adenine transition at nucleotide 530) mutation, predicting a C177Y substitution, and patient B had a nt 466 T-->C mutation on one allele and a nt 1070 A-->G mutation on the other, predicting a C156R and a Q357R substitution respectively. All 11 cysteine residues of mature human UGT1A1 are highly conserved in other human UGT isoforms and in rat, mouse and Rhesus monkey UGT1A1, suggesting their functional importance. Expression of mutagenized UGT1A1 plasmids showed that substitution of any of the seven cysteine residues located within the endoplasmic reticulum cisternae (including those mutated in patients A and B) abolished UGT1A1 activity or markedly increased its apparent K(m) for bilirubin. Substitution of the three cysteine residues within the C-terminal cytosolic tail had minimal effect on basal UGT1A1 activity, but prevented UGT1A1 activation by UDP-GlcNAc. N-Ethylmaleimide did not inhibit UGT1A1 activity in native microsomes, but prevented UGT1A1 activation by UDP-GlcNAc and inhibited the activity in digitonin-permeabilized microsomes. Dithiothreitol did not affect UGT1A1 activity in human liver microsomes. Together, the results suggested that free thiol groups, but not disulphide bonding, of seven cysteine residues within the intracisternal region of human UGT1A1 are important for its catalytic activity, while cysteine residues in the cytosolic domain may be involved in its physiological activation by UDP-GlcNAc.

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.

Vigintiphobia revisited

In this review the historical tenets and evidence-based clinical research in support of a bilirubin exchange threshold of >20 mg/dL for the healthy term neonate are revisited. In addition, a hypothesis is ventured that recent cases of kernicterus are related in part to changes in population factors coupled with genetic predispositions that have unmasked an unappreciated potential for marked neonatal hyperbilirubinemia.

Paternal isodisomy for chromosome 2 as the cause of Crigler–Najjar type I syndrome

Crigler-Najjar syndrome type I (CN-I) is a rare and severe autosomal recessive metabolic disease due to a total deficiency of bilirubin uridine diphosphate glucuronosyltransferase located on chromosome 2. We report on a child with CN-I due to a phenylalanine residue deletion inherited only from the father carrying this deletion at the heterozygous state. Cytogenetic analyses showed no deletion of the chromosomal 2q37 region. Microsatellite analysis of the child and his parents was consistent with paternal isodisomy for chromosome 2 in the child. This report demonstrates that uniparental disomy may be at the origin of very rare diseases transmitted as autosomal recessive traits and emphasizes the need for parental DNA analysis in such cases.

Molecular basis of bilirubin-induced neurotoxicity

Unconjugated bilirubin (UCB), at slightly elevated unbound concentrations, is toxic to astrocytes and neurons, damaging mitochondria (causing impaired energy metabolism and apoptosis) and plasma membranes (causing oxidative damage and disrupting transport of neurotransmitters). Accumulation of UCB in the CSF and CNS is limited by its active export, probably mediated by MRP1/Mrp1 present in choroid plexus epithelia, capillary endothelia, astrocytes and neurons. Upregulation of MRP1/Mrp1 protein levels by UCB might represent an important adaptive mechanism that protects the CNS from UCB toxicity. These concepts could explain the varied susceptibility of newborns to bilirubin neurotoxicity and the occurrence of neurological damage at plasma UCB concentrations well below therapeutic guidelines, and are relevant to the increasing prevalence of bilirubin encephalopathy in newborns.

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.

New concepts in bilirubin encephalopathy

Revised concepts of bilirubin encephalopathy have been revealed by studies of bilirubin toxicity in cultured CNS cells and in congenitally jaundiced Gunn rats. Bilirubin neurotoxicity is related to the unbound (free) fraction of unconjugated bilirubin (Bf), of which the dominant species at physiological pH is the protonated diacid, which can passively diffuse across cell membranes. As the binding affinity of plasma albumin for bilirubin decreases strikingly as albumin concentration increases, previously reported Bf values were underestimated. Newer diagnostic tests can detect reversible neurotoxicity before permanent damage occurs from precipitation of bilirubin (kernicterus). Early toxicity can occur at Bf only modestly above aqueous saturation and affects astrocytes and neurons, causing mitochondrial damage, resulting in impaired energy metabolism and apoptosis, plus cell-membrane perturbation, which causes enzyme leakage and hampers transport of neurotransmitters. The concentrations of unbound bilirubin in the cerebro-spinal fluid and CNS cells are probably limited mainly by active export of bilirubin back into plasma, mediated by ABC transporters present in the brain capillary endothelium and choroid plexus epithelium. Intracellular bilirubin levels may be diminished also by oxidation, conjugation and binding to cytosolic proteins. These new concepts may explain the varied susceptibility of neonates to develop encephalopathy at any given plasma bilirubin level and the selective distribution of CNS lesions in bilirubin encephalopathy. They also can suggest better strategies for predicting, preventing and treating this syndrome.

Bilirubin toxicity in the developing nervous system

Bilirubin toxicity remains a significant problem despite recent advances in the care of jaundiced (hyperbilirubinemic) neonates. A recent surge in reported cases of classical kernicterus, due in part to earlier hospital discharge and relaxation of treatment criteria for hyperbilirubinemia, and new reports of hyperbilirubinemia-induced auditory dysfunction using evoked potential based infant testing and hearing screening, underscore the need to better understand how hyperbilirubinemia causes brain damage in some infants, especially because the damage is preventable. Recent progress in understanding bilirubin binding and neurotoxicity resulting from unbound or "free" unconjugated bilirubin, how bilirubin affects the central nervous system in vivo and in vitro, and the use of new clinical tools in neonates, for example magnetic resonance imaging revealing bilateral lesions in globus pallidus and subthalamus, and abnormal brainstem auditory evoked potentials with normal inner ear function, may lead to improved detection and prevention of neurologic dysfunction and damage from bilirubin. Finally, the concern is raised that partial or isolated neurologic sequelae, for example auditory neuropathy and other central auditory processing disorders, may result from excessive amount and duration of exposure to free, unconjugated bilirubin at different stages of neurodevelopment.

Prenatal diagnosis of Crigler-Najjar syndrome type I by single-strand conformation polymorphism (SSCP)

Crigler-Najjar syndrome type I (CN-I) is a rare and severe inherited disorder of bilirubin metabolism, caused by the total deficiency of bilirubin-UDP-glucuronosyltransferase (UGT) activity. Enzymatic diagnosis cannot be performed in chorionic villi or amniocytes as UGT is not active in these tissues. The cloning of the UGT1 gene and the identification of disease-causing mutations have led to the possibility of performing DNA-based diagnosis. Here we report DNA-based prenatal diagnosis of CN-I in two Tunisian families in whom CN-I patients were diagnosed. As we had previously shown that CN-I was, in Tunisia, associated with homozygosity for the Q357R mutation within the UGT1 gene, we were able to detect this mutation in both families and to show that it was easily recognized by single-strand conformation polymorphism (SSCP) analysis. In both cases, SSCP analysis of fetal DNA showed that the fetus was heterozygous for the Q357R mutation. In one family, the pregnancy was carried to term and a healthy baby was born, whereas, in the other family, the pregnancy is still continuing. Thus the prenatal diagnosis of CN-I is possible, provided disease-causing mutations have been identified. SSCP analysis of DNA prepared either from amniocytes or from chorionic villi is a simple, reliable and fast method for prenatal diagnosis.

Understanding neonatal hyperbilirubinaemia in the era of genomics

The genomics revolution offers novel approaches to scientific investigation. Application of genomics technologies including microarray gene chips will provide a more complete picture of biological phenomena and help define the genetic contribution to disease by monitoring changes in expression across thousands of genes in physiological and clinical contexts. We briefly summarize identified genetic components that contribute to the genesis of neonatal hyperbilirubinaemia with a focus on inborn errors of hepatic bilirubin conjugation and discuss the potential use of microarray gene expression profiling technology to enhance our understanding of the pathogenesis of hyperbilirubinaemic neuronal cell injury. Expanded study using the tools of genomics will shed insights into the genetics of newborn jaundice and the pathogenesis of hyperbilirubinaemic encephalopathy.

A novel intronic mutation results in the use of a cryptic splice acceptor site within the coding region of UGT1A1, causing Crigler-Najjar syndrome type 1

Crigler-Najjar syndrome type 1 (CN-1) is characterized by severe unconjugated hyperbilirubinemia due to an inherited deficiency of hepatic bilirubin uridinediphosphoglucuronate glucuronosyltransferase (UGT1A1), inherited as an autosomal recessive characteristic. CN-1 is potentially lethal because of the risk of bilirubin encephalopathy (kernicterus). Genetic lesions of the coding region of the UGT1A1 gene are known to cause CN-1. Here, we report a CN-1 patient who has a novel G > A mutation at the splice acceptor site in intron 4 (IVS4-1 G > A) on one allele, and a T > A substitution followed by a 13-nt deletion in exon 2 (877T > A 878-890del) of the other allele. As the UGT1A1 gene is expressed specifically in the liver, structural analysis of the expressed UGT1A1 mRNA requires liver biopsy. To use a noninvasive approach to determine the effect of the splice site mutation on splicing of the RNA transcript, we amplified the relevant region of the genomic DNA by long-range polymerase chain reaction (PCR). The amplicon was cloned in an expression plasmid and transfected into COS-7 cells. The expressed mRNA was amplified by reverse-transcription-primed PCR. Nucleotide sequence determination of the amplicon showed that the splice acceptor site mutation caused splicing of the 3'-end of exon 4 to a cryptic splice site within exon 5. This resulted in deletion of the first 7 nucleotides of exon 5, causing a frameshift and premature truncation of UGT1A1, with consequent inactivation of the enzyme.

Homodimerization of human bilirubin-uridine-diphosphoglucuronate glucuronosyltransferase-1 (UGT1A1) and its functional implications

Genetic lesions of bilirubin-uridine-diphosphoglucuronate glucuronosyltransferase-1 (UGT1A1) completely or partially abolish hepatic bilirubin glucuronidation, causing Crigler-Najjar syndrome type 1 or 2, respectively. Clinical observations indicate that some mutant forms of human UGT1A1 (hUGT1A1) may be dominant-negative, suggesting their interaction with the wild-type enzyme. To evaluate intermolecular interaction of hUGT1A1, Gunn rat fibroblasts were stably transduced with hUGT1A1 cDNA. Gel permeation chromatography of solubilized microsomes suggested dimerization of hUGT1A1 in solution. Nearest-neighbor cross-linking analysis indicated that, within microsomal membranes, hUGT1A1 dimerized more efficiently at pH 7.4 than at pH 9. Two-hybrid analysis in yeast and mammalian systems demonstrated positive interaction of hUGT1A1 with itself, but not with another UGT isoform, human UGT1A6, which differs only in the N-terminal domain. Dimerization was abolished by deletion of the membrane-embedded helix from the N-terminal domain of hUGT1A1, but not by substitution of several individual amino acid residues or partial deletion of the C-terminal domain. A C127Y substitution abolished UGT1A1 activity, but not its dimerization. Coexpression of mutagenized and wild-type hUGT1A1 in COS-7 cells showed that the mutant form markedly suppressed the catalytic activity of wild-type hUGT1A1. Homodimerization of hUGT1A1 may explain the dominant-negative effect of some mutant forms of the enzyme.