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Bilirubin Genetics for the Nongeneticist: Hereditary Defects of Neonatal Bilirubin Conjugation

Abbreviations: UGT, uridinediphosphoglucuronate glucuronosyltransferase • T, thymine • A, adenine • mRNA, messenger ribonucleic acid • C-N, Crigler-Najjar (syndrome) • STB, serum total bilirubin • G-6-PD, glucose-6-phosphate dehydrogenase

	INTRODUCTION

TOP INTRODUCTION STRUCTURE AND FUNCTION OF... THE UGTs MUTATIONS OF THE UGT... MUTATIONS OF THE UGT... C-N SYNDROMES HETEROZYGOSITY FOR UGT 1A1... UGT MUTATIONS AND PROLONGED... CONCLUSION REFERENCES

 
Neonatal unconjugated bilirubinemia is one of the most common conditions encountered by the practicing pediatrician. Although it is usually self-limited and benign, the condition is of importance because of the rare instances in which severe hyperbilirubinemia can lead to bilirubin encephalopathy or kernicterus. The disorders that produce unconjugated hyperbilirubinemia can be divided into those in which excessive bilirubin is being produced (hemolysis), those in which the clearance of bilirubin is inadequate,¹ and combinations of the two. To be excreted from the body, unconjugated bilirubin has to be conjugated with glucuronic acid in the hepatocyte to form water-soluble bilirubin glucuronides. This process is catalyzed by a specific hepatic enzyme isoform (1A1) belonging to the uridinediphosphoglucuronate glucuronosyltransferase (UGT) family of enzymes. In the last few years much has been learned about the functions of this enzyme group and the genetic factors that affect it. In this commentary, we will highlight some of the recent advances made in understanding the genetics encoding the enzyme UGT 1A1, and emphasize the effect of mutations of this gene on the pathogenesis of neonatal jaundice.

	STRUCTURE AND FUNCTION OF THE UGT 1A1 GENE

TOP INTRODUCTION STRUCTURE AND FUNCTION OF... THE UGTs MUTATIONS OF THE UGT... MUTATIONS OF THE UGT... C-N SYNDROMES HETEROZYGOSITY FOR UGT 1A1... UGT MUTATIONS AND PROLONGED... CONCLUSION REFERENCES

 
The UGT 1A1 gene controls bilirubin conjugation by determining the structure of the enzyme glucuronosyltransferase, which is synthesized in the hepatocyte. Like any gene, UGT 1A1 consists of coding and noncoding areas (Fig 1). Of primary importance to the subsequent discussion is the exon, that portion of the DNA molecule which carries the genetic code. Separating the exons from each other are the introns, which are noncoding sequences of base pairs, and thus are not expressed in protein synthesis. An additional noncoding area of the gene is the promoter (bottom left of Fig 1, top of Fig 2), which is an upstream regulatory region controlling gene expression. The UGT promoter contains a TATAA box, which is a DNA sequence of thymine (T) and adenine (A).

View larger version (17K): [in this window] [in a new window]   Fig 1. The human UGT1 gene locus. Upper panel, Schematic representation of the genomic structure of the UGT1 gene complex. Lower panel, Exploded view of exon 1A1 and common exons 2–5 of the gene complex that have been identified as sites for genetic mutations associated with absent or decreased UGT activity that cause deficiencies of bilirubin conjugation. From Clarke et al.4 Reproduced with the permission of Elsevier Science BV.

View larger version (22K): [in this window] [in a new window]   Fig 2. Locations of mutations leading to C-N syndrome types 1 and 2 in the coding area of the UGT1 gene. The diagram includes a promoter (noncoding) with a variant (TA)7 TATAA box in the region of exon 1A1, which is the variant encountered in whites with Gilbert syndrome. From Jansen.49 Reproduced with the permission of Springer-Verlag. CNS indicates C-N syndrome.

Existence in the same natural population group of 2 or more genetic variants of a particular trait, in such proportions that they cannot be maintained simply by mutation, is referred to as a genetic polymorphism.

	THE UGTs

TOP INTRODUCTION STRUCTURE AND FUNCTION OF... THE UGTs MUTATIONS OF THE UGT... MUTATIONS OF THE UGT... C-N SYNDROMES HETEROZYGOSITY FOR UGT 1A1... UGT MUTATIONS AND PROLONGED... CONCLUSION REFERENCES

 
The UGTs represent a superfamily of microsomal membrane-bound enzymes that catalyze the conjugation not only of bilirubin, but also steroids, bile acids, drugs, and other xenobiotics.^2–5 Based on the nucleic acid sequences of their genes, the UGTs comprise of 2 separate families, UGT1 and UGT2. The former, also designated bilirubin-UGT, includes primarily the bilirubin-conjugating isoform, although UGT1 has also been demonstrated to catalyze the glucuronidation of additional compounds, including xenobiotics (phenols, anthraquinolones, and flavones), estriols, and estradioles.⁶ The UGT2 family has the ability to glucuronidate steroids, drugs including morphine and nonsteroidal anti-inflammatories, and the bile acid hyodeoxycholic acid.⁷ Although the UGT1 family contains several isoforms, only the A1 isoform (UGT 1A1) participates in the conjugation of bilirubin. The gene encoding the UGT1 enzyme is located on chromosome 2, at 2q37⁸ and consists of 4 common exons and 13 variable exons⁹ (Fig 1). The UGT enzyme isoform 1A1 is encoded by exon 1A1 and the common exons 2–5 of the gene complex. Mutations in the 1A1 exon or its promoter may produce structural or functional deficiencies in the enzyme that may result in impaired bilirubin conjugation and as a consequence, hyperbilirubinemia. Examples of these deleterious mutations are found in Gilbert syndrome and the C-N syndromes.

	MUTATIONS OF THE UGT 1A1 GENE PROMOTER ASSOCIATED WITH GILBERT SYNDROME

TOP INTRODUCTION STRUCTURE AND FUNCTION OF... THE UGTs MUTATIONS OF THE UGT... MUTATIONS OF THE UGT... C-N SYNDROMES HETEROZYGOSITY FOR UGT 1A1... UGT MUTATIONS AND PROLONGED... CONCLUSION REFERENCES

 
Gilbert syndrome, which affects 6% of the adult population, is a benign, inherited condition, usually first diagnosed during adolescence. The condition is characterized by mild, indirect bilirubinemia, which typically increases with fasting, in the presence of normal serum liver enzyme values. Classically, there is no overt evidence of hemolysis.^4,10 The most common genetic polymorphism encountered in whites with Gilbert syndrome is an additional TA insertion in the TATAA box of the UGT 1A1 gene promoter. Affected individuals are homozygous for the variant promoter and have 7 repeats—(TA)₇TAA (7/7) instead of the more usual 6 repeats—(TA)₆TAA (6/6). Heterozygotes have 1 allele each of the wild-type and variant promoter (6/7).^11,12 Some individuals have been demonstrated to have a (TA)₅ or (TA)₈ promoter.¹³ Because the addition of an extra TA repeat lessens the affinity of TATAA-binding protein to the TATAA box, there is an inverse relationship between the number of TA repeats and the activity of the promoter: as the number of TA repeats increases, UGT activity decreases.^11,13 Because the gene coding area is not affected, the UGT enzyme molecule synthesized in individuals with variant UGT gene promoters will be structurally normal; however, its expression, and thus activity, will be diminished. Only a portion of individuals homozygous for the variant UGT promoter display the clinical features of Gilbert syndrome, suggesting that additional icterogenic factors are necessary for full phenotypic clinical expression. One such factor could be an independent, genetically determined defect in a hepatic transport mechanism determining bilirubin uptake.¹⁴ Recent reports have confirmed that UGT promoter polymorphism leads to higher serum total bilirubin (STB) values in both adults and neonates, compared with controls.^11,12,15,16 Evidence that the mechanism of this bilirubinemia is attributable to diminished bilirubin conjugation includes the demonstration that UGT 1A1 enzyme activity was lowest in hepatic tissue from homozygotes for the variant 7/7 UGT promoter and intermediate in 6/7 heterozygotes, compared with normal 6/6 homozygotes.¹⁷ Serum total conjugated bilirubin fractions, which reflect diminished bilirubin conjugation,¹⁸ were significantly lower in neonatal 7/7 homozygotes than 6/6 counterparts, with a trend to being lower than 6/7 heterozygotes.¹⁹

Population studies have shown that black neonates have lower serum bilirubin levels, and Asian infants higher, than their white counterparts. Therefore, Beutler et al¹³ presumed that the less active (TA)₇ mutation with resultant diminished bilirubin conjugation would be less prevalent in individuals of African extraction, while the more active (TA)₆ variation (improved conjugation) would be more frequently encountered. In fact, although UGT promoter activity consistently showed an inverse relationship to the number of (TA) repeats in each of the 3 population groups studied, contrary to expectations, the (TA)₇ variety was more common in people of African, and less common in people of Asian origin, than in whites. From this study it is apparent that although within an ethnic group there is a strong correlation between the promoter repeat number, its activity, and serum bilirubin levels, the relationship may not always hold out between ethnic groups. This apparent inconsistency may be attributable to the fact that the etiology of jaundice is multifactorial, with many genetic and environmental factors playing a part.

Although Gilbert syndrome is not associated with evidence of overt hemolysis,^4,10 a shortened red cell life span has been found in some affected individuals.^20–27 However, the subjects in these studies were preselected because of clinically apparent Gilbert syndrome, ie, mildly visible jaundice or incidentally detected elevated serum bilirubin levels. The question of whether UGT 1A1 promoter polymorphism has an effect on heme catabolism, in addition to that of diminished bilirubin conjugation, was recently definitively answered. Kaplan et al¹⁹ avoided selection bias by identifying a neonatal cohort according to UGT promoter polymorphism, rather than the presence of bilirubinemia. As expected, STB conjugates, reflective of bilirubin conjugation,¹⁸ were significantly lower in the homozygotic 7/7 group than in 6/7 heterozygote or 6/6 homozygote counterparts. What was surprising was that blood carboxyhemoglobin values corrected for inspired carbon monoxide, an accurate index of heme catabolism,²⁸ were significantly higher in the UGT 7/7 homozygotes, compared with controls of both 6/7 and 6/6 promoter genotypes. It is now clear that UGT 1A1 promoter polymorphism influences STB values by both increasing heme catabolism as well as diminishing bilirubin conjugation.

It had been suggested for many years that some infants with indirect hyperbilirubinemia could be manifesting the effects of Gilbert syndrome. However, this concept could not be studied until the association between the condition and its genetic mechanism had been established. Bancroft et al,²⁹ using a transcutaneous jaundice meter to evaluate jaundice, found that homozygosity for the variant 7/7 UGT gene promoter did not necessarily result in higher peak jaundice index levels or in hyperbilirubinemia, but did cause a significantly greater increase in the jaundice index during the first 2 days of life than in controls. Roy-Chowdhury et al¹⁵ in a study of predominantly breastfeeding Greek neonates in whom direct Coombs’ positivity, ABO blood group incompatibility, and glucose-6-phosphate dehydrogenase (G-6-PD) deficiency had been excluded, found significantly higher STB values at 96 hours of life in those homozygous for the variant 7/7 UGT promoter (10.2 mg/dL ± 1.4 mg/dL) and intermediate values in 6/7 heterozygotes (8.9 mg/dL ± 3.1 mg/dL) compared with homozygous normal 6/6 controls (7.0 mg/dL ± 3.2 mg/dL, P = .005). Laforgia et al³⁰ found a significantly higher frequency of homozygosity for the variant 7/7 promoter in neonates (hemolytic conditions excluded) with STB concentrations >13.0 mg/dL compared with controls whose STB values did not exceed that concentration (26.8% vs 12.2%, P < .05). Peak STB values did not differ according to promoter genotype.

Perhaps the most dramatic contribution of UGT (TA) promoter polymorphism to the pathogenesis of neonatal jaundice was demonstrated by Kaplan et al³¹ in Israeli G-6-PD (Mediterranean) deficient neonates. In line with the aforementioned studies, among the G-6-PD normal neonates, no significant difference in the incidence of hyperbilirubinemia, defined as STB >15 mg/dL, was demonstrated between neonates bearing the 3 different UGT 1A1 promoter genotypes. In contrast, in G-6-PD-deficient counterparts, superimposition of the variant UGT promoter resulted in a significant increase in the incidence of hyperbilirubinemia: STB levels >15 mg/dL occurred in 10% of G-6-PD-deficient infants homozygous (6/6) for the normal promoter, 35% of 6/7 heterozygotes (P < .0001), and in 50% of 7/7 homozygotes (P = .02) for the variant promoter—an allele dose-dependent response. Interestingly, neonates who were G-6-PD-deficient, but homozygous for the normal (6/6) UGT promoter, did not have a higher incidence of hyperbilirubinemia than G-6-PD-normal controls of the same UGT promoter genotype. Therefore, neither G-6-PD deficiency individually nor presence of the variant UGT1 promoter gene in its 6/7 or 7/7 forms in the absence of G-6-PD deficiency could be designated risk factors for neonatal hyperbilirubinemia. The authors concluded that the development of jaundice was dependent on an interaction between these 2 factors.

In Italian adults, homozygosity for the variant 7/7 UGT promoter was a major determinant of increased serum bilirubin levels associated with heterozygous ß-thalassemia^32,33 and G-6-PD deficiency.³² Therefore, it is surprising that in Italian G-6-PD-deficient neonates^34,35 homozygosity for the variant 7/7 promoter apparently did not increase the risk of hyperbilibinemia as it did in Israeli counterparts.³¹ This discrepancy could possibly be explained by genetic differences between the populations studied, or lack of uniformity in definitions of hyperbilirubinemia used by the researchers. However, while the Israeli study was population-based, in the Italian study, analysis was performed only among those neonates who actually developed hyperbilirubinemia. Therefore, it is also possible that these differences in study populations and statistical analyses may have resulted in an apparent lack of effect of the variant 7/7 UGT promoter among the Italian neonatal population.^36,37

In Scotland, it was found that 31% of neonates (almost all of whom were breastfeeding) with prolonged jaundice (STB levels >5.8 mg/dL >14 days of life) were homozygous for the 7/7 Gilbert syndrome promoter genotype compared with only 6% of a control group with acute jaundice (P < .05).³⁸

By itself, hereditary spherocytosis can produce hemolysis and hyperbilirubinemia in neonates but the incidence of hyperbilirubinemia is further increased when the variant UGT promoter is also present in these infants.³⁹ Several studies have found no increase in the incidence of hyperbilirubinemia in direct Coombs’ negative blood group A or B infants of group O mothers.^40,41 Similarly, Kaplan et al⁴² did not find a significantly increased incidence of hyperbilirubinemia (STB 15.0 mg/dL) when direct Coombs’ negative ABO-incompatible and -compatible neonatal cohorts were compared. However, among the subset of ABO-incompatible neonates of that same cohort who were homozygous for the variant 7/7 UGT promoter, the incidence of hyperbilirubinemia became significantly greater than in 6/6 homozygote counterparts (43% vs 0%; P = .02).

It is now evident that in the absence of additional icterogenic factors, presence of the variant 7/7 UGT gene promoter may be associated with higher STB levels, but does not necessarily lead to significant hyperbilirubinemia. However, when combined with factors that increase bilirubin production, homozygosity for 7/7 UGT promoter polymorphisms, and sometimes 6/7 heterozygosity, may potentiate the risk of neonates developing STB levels >15 mg/dL. The basic mechanism of the hyperbilirubinemia is probably a combination of increased hemolysis, however slight, and diminished bilirubin conjugation, resulting in loss of equilibrium between these 2 processes with bilirubin production outweighing bilirubin clearance.

	MUTATIONS OF THE UGT 1A1 GENE CODING AREA ASSOCIATED WITH GILBERT SYNDROME

TOP INTRODUCTION STRUCTURE AND FUNCTION OF... THE UGTs MUTATIONS OF THE UGT... MUTATIONS OF THE UGT... C-N SYNDROMES HETEROZYGOSITY FOR UGT 1A1... UGT MUTATIONS AND PROLONGED... CONCLUSION REFERENCES

 
In contrast to the promoter polymorphism seen in white populations with Gilbert syndrome, in some East Asian populations TATAA promoter variations are rare, and Gilbert syndrome appears to result from missense mutations in the coding area of the UGT 1A1 gene. The most common of these is a GA transition at nucleotide 211, which causes arginine to replace glycine at position 71, of the corresponding protein product.^43,44 Therefore, this variant is also known as G71R, and is prevalent in Japanese, Korean, and Chinese populations. Some other mutations reported in association with Gilbert syndrome in Asian populations include Pro229Gln, Tyr486Asp, Arg209Trp, and Arg367Gly.^45,46 A full list of coding area mutations encountered in >400 Taiwanese neonatal subjects has recently been published and included heterozygous coding area UGT mutations, compound heterozygous coding area mutations in combination with 6/7 (TA) promoter heterozygosity, and homozygous coding area mutations.⁴⁷

Although homozygosity for both the 7/7 UGT promoter polymorphism seen in whites, and the G71R polymorphisms encountered in East Asia are associated with clinically apparent Gilbert syndrome, the 2 polymorphisms appear to behave differently with regard to their ability to influence the development of neonatal hyperbilirubinemia. Maruo et al,⁴³ studying Japanese hyperbilirubinemic infants with no obvious cause for their jaundice, determined the allele frequency of a mutated UGT 1A1 (G71R) to be more than double that of controls. Both hetero- and homozygosity for the G71R mutation were associated with hyperbilirubinemia. In contrast to Western infants with the variant 7/7 UGT 1A1 promoter, the Japanese neonates with the G71R mutation did not seem to require any additional jaundice-provoking factors for their mutation to have its effect. It is possible that the high incidence of hyperbilirubinemia in Japanese newborns may be a function of the frequency of the G71R mutation in that population. Similarly, a recent study of Taiwanese neonates⁴⁷ demonstrated that homozygosity for UGT mutations, primarily G71R, in the absence of additional risk factors including G-6-PD deficiency, resulted in a significantly higher relative risk of hyperbilirubinemia (STB >15.0 mg/dL) compared with subjects carrying the wild genotype (5.2; 95% confidence interval: 1.7–16.1; P = .005). Superimposition of G-6-PD deficiency on homozygosity for UGT-coding area mutations further increased the relative risk of hyperbilirubinemia (6.4; 95% confidence interval: 3.3–12.1; P < .001), with all 11 neonates homozygous for the G71R mutation developing STB >15.0 mg/dL. Although the G-6-PD mutations encountered were primarily 1376 G to T, the interaction between G-6-PD deficiency and homozygosity for coding area UGT mutations adds support to the previous observation by Kaplan et al³¹ of an interaction between the G-6-PD Mediterranean mutation and UGT promoter (TA) polymorphisms.

	C-N SYNDROMES

TOP INTRODUCTION STRUCTURE AND FUNCTION OF... THE UGTs MUTATIONS OF THE UGT... MUTATIONS OF THE UGT... C-N SYNDROMES HETEROZYGOSITY FOR UGT 1A1... UGT MUTATIONS AND PROLONGED... CONCLUSION REFERENCES

 
Except in some inbred populations, the C-N syndromes are rare diseases characterized by very high serum levels of unconjugated bilirubin. If left untreated, the hyperbilirubinemia can lead to bilirubin encephalopathy and death.^48,49 Type 1 C-N syndrome (autosomal recessive inheritance) is characterized by almost complete absence of UGT 1A1 enzyme activity and is refractory to phenobarbital treatment. In type 2 C-N syndrome (autosomal recessive and dominant inheritance), enzyme activity is severely reduced but can be induced by phenobarbital administration. It is now recognized that phenobarbital does not act directly on the glucuronosyltransferase enzyme as previously thought, but rather functions via a phenobarbital-responsive enhancer module that stimulates the UGT 1A1 gene to induce production of the bilirubin-conjugating enzyme.⁵⁰ In type 2 C-N syndrome, phenobarbital may lower STB levels by 30% to 80%.^3,4 Increases in biliary-conjugated bilirubin fraction concentrations in response to phenobarbital therapy confirm its effect on enhancing bilirubin conjugation.⁵¹ It should be noted that the distinction between types 1 and 2 C-N syndrome is clinical, and somewhat artificial, as there is no clear dividing line between the 2. Definitive diagnosis of the C-N syndrome requires high-performance liquid chromatography analysis of bile or tissue enzyme assay from a liver biopsy, tests that are available at only a few centers.⁵² Phototherapy and subsequent hepatic transplantation have increased survival in the type 1 condition.⁴⁹

The C-N syndromes are usually caused by 1 or more mutations in any 1 of the 5 exons of the gene coding for the UGT 1A1 enzyme that result in complete to severe impairment of bilirubin conjugation (Fig 2).^3,48 Recently, 2 type 1 C-N patients were reported with splice-site mutations in the noncoding, intronic region of the UGT 1A1 gene.⁵³ Therefore, gene therapy represents the greatest hope for a cure of this condition but, although successful in the Gunn rat, has yet to be tried in the human.^54,55 Hepatocyte transplantation^56,57 or Sn-protoporphyrin therapy⁵⁸ may offer help.

	HETEROZYGOSITY FOR UGT 1A1 NONCODING AND CODING AREA MUTATIONS

TOP INTRODUCTION STRUCTURE AND FUNCTION OF... THE UGTs MUTATIONS OF THE UGT... MUTATIONS OF THE UGT... C-N SYNDROMES HETEROZYGOSITY FOR UGT 1A1... UGT MUTATIONS AND PROLONGED... CONCLUSION REFERENCES

 
Heterozygosity for a UGT 1A1 coding area mutation is not normally associated with neonatal hyperbilirubinemia. However, as the gene frequency for the variant UGT promoter is as high as 0.3 in white populations,^11,12,31 we should not be surprised to find a variant promoter gene coexisting with a coding area mutation. Indeed, Kadakol et al⁵⁹ recently reported individuals with compound heterozygosity for a combination of a variant UGT promoter and a coding region mutation of the gene. The most dramatic of these examples includes neonatal twins who also had direct Coombs’ positive ABO heterospecificity with their mother. Both developed severe neonatal hyperbilirubinemia and subsequently chronic bilirubin encephalopathy. This report reinforces the principle that, in the presence of additional icterogenic factors, otherwise benign genetic mutations of UGT may have tragic consequences. Huang et al⁴⁷ have documented patients with compound heterozygosity between (TA) promoter mutations and East Asian coding area mutations.

	UGT MUTATIONS AND PROLONGED BREAST MILK JAUNDICE

TOP INTRODUCTION STRUCTURE AND FUNCTION OF... THE UGTs MUTATIONS OF THE UGT... MUTATIONS OF THE UGT... C-N SYNDROMES HETEROZYGOSITY FOR UGT 1A1... UGT MUTATIONS AND PROLONGED... CONCLUSION REFERENCES

 
Gilbert syndrome has also been sought as a factor in the pathophysiology of breast milk jaundice. As already noted, Mohaghan et al³⁸ were the first to demonstrate that one third of neonates, almost all of whom were breastfeeding and who had STB levels >5.8 mg/dL >14 days of life were homozygous for the 7/7 promoter genotype, compared with 7/7 homozygosity in only 6% of a control group with acute jaundice (P < .05). Maruo et al⁶⁰ found that 16 of 17 breastfed, Japanese infants with prolonged jaundice had at least 1 mutation of the UGT 1A1 gene, mostly of the G71R type. The allele frequency of this mutation in the jaundiced infants was >4-fold greater than in the general Japanese population. Presence of the variant (TA)₇ promoter allele was rare: only 1 neonate heterozygous for the 6/7 promoter mutation was encountered. Thus, mutations of the UGT 1A1 gene-coding area appear to be an important factor in the pathogenesis of prolonged breastfeeding jaundice in Japanese infants.

Prolonged jaundice is well-described in infants with pyloric stenosis, but the pathogenesis has never been established. In a recent study, 3 formula-fed infants with pyloric stenosis developed prolonged indirect hyperbilirubinemia. Two were homozygous (7/7) and 1 heterozygous (6/7) for the variant UGT promoter. (TA) promoter polymorphism was not detected in any of 10 infants with pyloric stenosis who did not become jaundiced.⁶¹

	CONCLUSION

TOP INTRODUCTION STRUCTURE AND FUNCTION OF... THE UGTs MUTATIONS OF THE UGT... MUTATIONS OF THE UGT... C-N SYNDROMES HETEROZYGOSITY FOR UGT 1A1... UGT MUTATIONS AND PROLONGED... CONCLUSION REFERENCES

 
Identification of a molecular marker for Gilbert syndrome and the ability to identify mutations of the gene encoding the UGT1A1 enzyme have clarified several, hitherto poorly understood, aspects of neonatal hyperbilirubinemia and may well elucidate other aspects in the future. It is possible, for example, that these mutations may play a role in the occasional breastfed infant that develops extreme hyperbilirubinemia (STB levels >30 mg/dL). Of particular importance is the concept of equilibrium between bilirubin production and conjugation in maintaining concentrations of serum bilirubin within the physiologic range, and imbalance between these processes in the mechanism of severe hyperbilirubinemia. Small changes in either the rate of bilirubin production or of bilirubin conjugation may be sufficient to upset the balance and contribute to increasing serum bilirubin concentrations.^62,63

Guidelines from the American Academy of Pediatrics^64,65 have emphasized the need to identify neonates with hemolysis, a major risk factor for developing kernicterus. Awareness of bilirubin conjugation genetics and modern-day ability to positively identify genetic mutations of bilirubin conjugation may be a useful adjunct to the American Academy of Pediatrics practice parameter in identifying neonates at risk for developing severe hyperbilirubinemia. However, not every jaundiced neonate requires full genetic consultation and laboratory analysis. Many infants at risk for hyperbilirubinemia may be identified by spending a few minutes with parents and asking about ethnic background, risk factors, previous infants with hyperbilirubinemia, and whether they (the parents) were jaundiced as infants or in later life. Physical examination and a few simple laboratory tests including blood group determination, direct Coombs’ testing, G-6-PD screening where appropriate, and end-tidal carbon monoxide testing where available⁶⁶ may well contribute to singling out of infants who do not have these commonly occurring hemolytic risk factors, but whose genetic endowment nevertheless places the infant at risk for significant hyperbilirubinemia. In selected cases, such as severe, unexplained familial hyperbilirubinemia, it may be appropriate to refer a family for genetic counseling and to obtain genotype analysis of the UGT 1A1 gene, a test that is now available in some clinical laboratories. Prudent evaluation may facilitate prediction of severe jaundice and enable prevention of bilirubin encephalopathy in future siblings.

Michael Kaplan, MB, ChB

Department of Neonatology
Shaare Zedek Medical Center
Faculty of Medicine of the Hebrew University
Jerusalem 91031, Israel

Cathy Hammerman, MD

Department of Neonatology
Shaare Zedek Medical Center
Jerusalem 91031, Israel
Faculty of Health Sciences
Ben Gurion University of the Negev
Beer Sheva 84105, Israel

M. Jeffrey Maisels, MB, BCh

Department of Pediatrics
William Beaumont Hospital
Royal Oak, MI 48073-6769

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	ACKNOWLEDGMENTS

 
We thank Antony F. McDonagh, PhD, CChem, MRSC, for helpful suggestions and review of the manuscript.

FOOTNOTES

Received for publication Sep 4, 2001; Accepted Sep 26, 2002.

Reprint requests to (M.K.) Department of Neonatology, Shaare Zedek Medical Center, Box 3235, Jerusalem 91031, Israel. E-mail: kaplan{at}cc.huji.ac.il

	REFERENCES

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1. Dennery PA, Seidman DS, Stevenson DK. Neonatal hyperbilirubinemia. N Engl J Med.2001; 344 :581 –590[Free Full Text]
2. Iyanagi T, Emi Y, Ikushiro S-I. Biochemical and molecular aspects of genetic disorders of bilirubin metabolism. Biochim Biophys Acta.1998; 1407 :173 –184[Medline]
3. Kadakol A, Ghosh SS, Sappal BS, Sharma G, Chowdhury JR, Chowdhury NR. Genetic lesions of bilirubin uridine-diphosphoglucuronate glucuronosyltransferase (UGT1A1) causing Crigler-Najjar and Gilbert syndromes: correlation of genotype to phenotype. Hum Mutat.2000; 16 :297 –306[CrossRef][ISI][Medline]
4. Clarke DJ, Moghrabi N, Monaghan G, et al. Genetic defects of the UDP-glucuronosyltransferase-1 (UGT1) gene that cause familial non-haemolytic unconjugated hyperbilirubinaemias. Clin Chim Acta.1997; 266 :63 –74[CrossRef][ISI][Medline]
5. Sampietro M, Iolascon A. Molecular pathology of Crigler-Najjar type I and II and Gilbert’s syndromes. Haematologica.1999; 84 :150 –157[Abstract/Free Full Text]
6. Senafi SB, Clarke DJ, Burchell B. Investigation of the substrate specificity of a cloned expressed human bilirubin UDP-glucuronosyltransferase: UDP-sugar specificity and involvement in steroid and xenobiotic glucuronidation. Biochem J.1994; 303 :233 –240
7. Mackenzie PI, Miners JO, McKinnon RA. Polymorphisms in UDP glucuronosyltransferase genes: functional consequences and clinical relevance. Clin Chem Lab Med.2000; 38 :889 –892[CrossRef][ISI][Medline]
8. Van Es HH, Bout A, Liu J, et al. Assignment of the human UDP glucuronosyltransferase gene (UGT1A1) to chromosome region 2q37. Cytogenet Cell Genet.1993; 63 :114 –116[ISI][Medline]
9. Gong Q-K, Cho JW, Huang T, et al. Thirteen UDP glucuronosyltransferase genes are encoded at the human UGT1 gene complex locus. Pharmacogenetics.2001; 11 :357 –368[CrossRef][ISI][Medline]
10. Burchell B, Hume R. Molecular genetic basis of Gilbert’s syndrome. J Gastroenterol Hepatol.1999; 14 :960 –966[CrossRef][ISI][Medline]
11. Bosma PJ, Chowdhury JR, Bakker C, et al. The gene basis of the reduced expression of bilirubin UDP-glucuronosyltransferase 1 in Gilbert’s Syndrome. N Eng J Med.1995; 333 :1171 –1175[Abstract/Free Full Text]
12. Monaghan G, Ryan M, Seddon R, Hume R, Burchell B. Genetic variation in bilirubin UDP-glucuronosyltransferase gene promoter and Gilbert’s syndrome. Lancet.1996; 347 :578 –581[CrossRef][ISI][Medline]
13. Beutler E, Gelbart T, Demina A. Racial variability in the UDP-glucuronosyltransferase 1 (UGT1A1) promoter: a balanced polymorphism for regulation of bilirubin metabolism. Proc Natl Acad Sci U S A.1998; 95 :8170 –8174[Abstract/Free Full Text]
14. Persico M, Persico E, Bakker CTM, et al. Hepatic uptake of organic anions affects the plasma bilirubin level in subjects with Gilbert’s syndrome mutations in UGT1A1. Hepatology.2001; 33 :627 –632[CrossRef][ISI][Medline]
15. Roy-Chowdhury N, Deocharan B, Bejjanki HR, et al. Presence of the genetic marker for Gilbert syndrome is associated with increased level and duration of neonatal jaundice. Acta Paediatr.2002; 91 :100 –101[ISI][Medline]
16. Kaplan M, Hammerman C, Renbaum P, Levy-Lahad E, Vreman HJ, Stevenson DK. Differing pathogenesis of perinatal bilirubinemia in glucose-6-phosphate dehydrogenase-deficient versus normal neonates. Pediatr Res.2001; 50 :532 –537[ISI][Medline]
17. Raijmakers MTM, Jansen PLM, Steegers EAP, Peters WHM. Association of human liver bilirubin UDP-glucuronosyltransferase activity with a polymorphism in the promoter region of the UGT1A1 gene. J Hepatol.2000; 33 :348 –351[CrossRef][ISI][Medline]
18. Muraca M, Fevery J, Blanckaert N. Relationships between serum bilirubins and production and conjugation of bilirubin: studies in Gilbert’s Syndrome, Crigler-Najjar disease, hemolytic disorders and rat models. Gastroenterology.1987; 92 :309 –317[ISI][Medline]
19. Kaplan M, Hammerman C, Rubaltelli FF, et al. Hemolysis and bilirubin conjugation in association with UDP-glucuronosyltransferase 1A1 promoter polymorphism. Hepatology.2002; 35 :905 –911[CrossRef][ISI][Medline]
20. Foulk WT, Butt HR, Owen CR, Whitcomb FF, Mason HL. Constitutional hepatic dysfunction (Gilbert’s Disease): its natural history and related syndromes. Medicine.1959; 28 :25 –46[CrossRef]
21. Arias I. Chronic unconjugated hyperbilirubinemia without overt signs of hemolysis in adolescents and adults. J Clin Invest.1962; 41 :2233 –2245
22. Pitcher CS, Williams R. Reduced red cell survival in jaundice and its relation to abnormal glutathione metabolism. Clin Sci.1963; 24 :239 –252[ISI][Medline]
23. Powell LW, Billing BH, Williams HS. An assessment of red cell survival in idiopathic unconjugated hyperbilirubinaemia (Gilbert’s syndrome) by the use of radioactive isopropylfluorophosphate and chromium. Australas Ann Med.1967; 16 :221 –225[ISI][Medline]
24. Powell LW, Hemingway E, Billing BH, Sherlock S. Idiopathic unconjugated hyperbilirubinemia (Gilbert’s syndrome): a study of 42 families. N Engl J Med.1967; 227 :1108 –1112
25. Berk PD, Blaschke TF. Detection of Gilbert’s syndrome in patients with hemolysis: a method using radioactive chromium. Ann Intern Med.1972; 77 :527 –531
26. Bensinger TA, Maisels MJ, Carlson DE, Conrad ME. Effect of low caloric diet on endogenous carbon monoxide production: normal adults and Gilbert’s syndrome. Proc Soc Exp Biol Med.1973; 144 :417 –419[Medline]
27. Okolicsany L, Ghidini O, Orlando R, et al. An evaluation of bilirubin kinetics with respect to the diagnosis of Gilbert’s syndrome. Clin Sci Mol Med.1978; 54 :539 –547[ISI][Medline]
28. Vreman HJ, Mahoney JJ, Stevenson DK. Carbon monoxide and carboxyhemoglobin. Adv Pediatr.1995; 42 :303 –334[Medline]
29. Bancroft JD, Kreamer B, Gourley GR. Gilbert syndrome accelerates the development of neonatal jaundice. J Pediatr.1998; 132 :656 –660[CrossRef][ISI][Medline]
30. Laforgia N, Faienza MF, Rinaldi A, D’Amato G, Rinaldi G, Iolascon A. Neonatal hyperbilirubinemia and Gilbert’s syndrome. J Perinat Med.2002; 30 :166 –169[CrossRef][ISI][Medline]
31. Kaplan M, Renbaum P, Levi-Lahad E, Hammerman C, Lahad A, Beutler E. Gilbert syndrome and glucose-6-phosphate dehydrogenase deficiency: a dose-dependent genetic interaction crucial to neonatal hyperbilirubinemia. Proc Natl Acad Sci U S A.1997; 94 :12128 –12132[Abstract/Free Full Text]
32. Sampietro M, Lupica L, Perrero L, et al. The expression of uridine diphosphate glucuronosyltransferase gene is a major determinant of bilirubin level in heterozygous ß-thalassaemia and in glucose-6-phosphate dehydrogenase deficiency. Br J Haematol.1997; 99 :437 –439[CrossRef][ISI][Medline]
33. Galanello R, Perseu L, Melil MA, et al. Hyperbilirubinemia in heterozygous ß-thalassaemia is related to co-inherited Gilbert’s syndrome. Br J Haematol.1997; 99 :433 –436.[CrossRef][ISI][Medline]
34. Iolascon A, Faienza MF, Perrotta S, Meloni GF, Ruggiu G, del Giudice EM. Gilbert’s syndrome and jaundice in glucose-6-phosphate dehydrogenase deficient neonates. Haematologica.1999; 84 :99 –102[Abstract/Free Full Text]
35. Galanello R, Cipollina MD, Carboni G, et al. Hyperbilirubinemia, glucose-6-phosphate dehydrogenase deficiency and Gilbert’s syndrome. Eur J Pediatr.1999; 158 :914 –916[CrossRef][ISI][Medline]
36. Kaplan M, Hammerman C, Beutler E. Hyperbilirubinemia, glucose-6-phosphate dehydrogenase deficiency and Gilbert syndrome. Eur J Pediatr.2001; 160 :195[CrossRef][ISI][Medline]
37. Kaplan M, Hammerman C. Gilbert’s syndrome and jaundice in glucose-6-phosphate dehydrogenase deficient neonates. Haematologica.2000; 85(e-letters) :01
38. Monaghan G, McLellan A, McGeehan A, et al. Gilbert’s syndrome is a contributory factor in prolonged unconjugated hyperbilirubinemia of the newborn. J Pediatr.1999; 134 :441 –446[CrossRef][ISI][Medline]
39. Iolascon A, Faienza MF, Moretti A, Perotta S, Miraglia del Giudice E. UGT1 promoter polymorphism accounts for increased neonatal appearance of hereditary spherocytosis. Blood.1998; 91 :1093[Free Full Text]
40. Ozolek JA, Watchko JF, Mimouni F. Prevalence and lack of clinical significance of blood group incompatibility in mothers with blood type A or B. J Pediatr.1994; 125 :87 –91[CrossRef][ISI][Medline]
41. Leistikow EA, Collin MF, Savastano GD, de Sierra TM, Leistikow BN. Wasted health care dollars: routine cord blood type and Coombs’ testing. Arch Pediatr Adolesc Med.1995; 149 :1147 –1151[Abstract]
42. Kaplan M, Hammerman C, Renbaum P, Klein G, Levy-Lahad E. Gilbert’s syndrome and hyperbilirubinaemia in ABO-incompatible neonates. Lancet.2000; 356 :652 –653[CrossRef][ISI][Medline]
43. Maruo Y, Nishizawa K, Sato H, Doida Y, Shimada M. Association of neonatal hyperbilirubinemia with bilirubin UDP-glucuronosyltransferase polymorphism. Pediatrics.1999; 103 :1224 –1227[Abstract/Free Full Text]
44. Akaba K, Kimura T, Sasaki A, et al. Neonatal hyperbilirubinemia and a common mutation of the bilirubin uridine diphosphate-glucuronosyltransferase gene in Japanese. J Hum Genet.1999; 44 :22 –25[CrossRef][ISI][Medline]
45. Hsieh S-Y, Wu Y-H, Lin D-Y, Chu C-M, Wu M, Liaw Y-F. Correlation of mutational analysis to clinical features in Taiwanese patients with Gilbert’s syndrome. Am J Gastroenterol.2001; 96 :1188 –1192[CrossRef][ISI][Medline]
46. Maruo Y, Sato H, Yamano T, Doika Y, Shimada M. Gilbert syndrome caused by a homozygous missnse mutation (Tyr486Asp) of bilirubin UDP-glucuronosyltransferase gene. J Pediatr.1998; 132 :1045 –1047[CrossRef][ISI][Medline]
47. Huang C-S, Chang P-F, Huang M-J, Chen E-S, Chen W-C. Glucose-6-phosphate dehydrogenase deficiency, the UDP-glucuronosyltransferase 1A1 gene, and neonatal hyperbilirubinemia. Gastroenterol.2002; 123 :127 –133[CrossRef][ISI][Medline]
48. Jansen PLM. Genetic diseases of bilirubin metabolism: the inherited unconjugated bilirubinemias. J Hepatol.1996; 25 :398 –404[CrossRef][ISI][Medline]
49. Jansen PLM. Diagnosis and management of Crigler-Najjar syndrome. Eur J Pediatr.1999; 158(suppl 2) :S89 –S94
50. Sugatani J, Kojima H, Ueda A, et al. The phenobarbital response enhancer module in the human bilirubin UDP-glucuronosyltransferase UGT1A1 gene and regulation by the nuclear receptor CAR. Hepatology.2001; 33 :1232 –1238[CrossRef][ISI][Medline]
51. Rubaltelli FF, Novello A, Zancan L, Vilei MT, Muraca M. Serum and bile bilirubin pigments in the differential diagnosis of Crigler-Najjar disease. Pediatrics.1994; 94 :553 –555[Abstract/Free Full Text]
52. Lee WS, McKiernan PJ, Beath SV, et al. Bile bilirubin pigment analysis in disorders of bilirubin metabolism in early infancy. Arch Dis Child.2001; 85 :38 –42[Abstract/Free Full Text]
53. Gantla S, Bakker CTM, Deocharan B, et al. Splice-site mutations: a novel genetic mechanism of Crigler-Najjar syndrome type 1. Am J Hum Genet.1998; 62 :585 –592[CrossRef][ISI][Medline]
54. Li Q, Murphree SS, Willer SS, Bolli R, French BA. Gene therapy with bilirubin-UDP-glucuronosyltransferase in the Gunn rat model of Crigler-Najjar syndrome type 1. Hum Gene Ther.1998; 9 :497 –505[ISI][Medline]
55. Kren BT, Parashar B, Bandyopadhyay P, Chowdhury NR, Chowdhury JR, Steer CJ. Correction of the UDP-glucuronosyltransferase gene defect in the Gunn rat model of Crigler-Najjar syndrome type I with a chimeric oligonucleotide. Proc Natl Acad Sci U S A.1999; 96 :10349 –10354[Abstract/Free Full Text]
56. Fox IJ, Chowdhury J, Kaufman SS, et al. Hepatocyte transplantation for the treatment of Crigler-Najjar syndrome Type 1. N Eng J Med.1998; 338 :1422 –1426[Free Full Text]
57. Chowdhury JR, Chowdhury NR, Strom SC, Kaufman SS, Horshen S, Fox IJ. Human hepatocyte transplantation: gene therapy and more? Pediatrics.1988; 102 :647 –648
58. Rubaltelli FF, Guerrini P, Reddie E, Jori G. Tin-protoporphyrin in the management of children with Crigler-Najjar disease. Pediatrics.1989; 84 :728 –731[Abstract/Free Full Text]
59. Kadakol A, Sappal BS, Ghosh SS, et al. Interaction of coding area mutations and the Gilbert-type promoter abnormality of the UGT1A1 gene causes moderate degrees of unconjugated hyperbilirubinemia and may lead to neonatal kernicterus. J Med Genet.2001; 38 :244 –249[Free Full Text]
60. Maruo Y, Nishizawa K, Sato H, Sawa H, Shimada M. Prolonged unconjugated hyperbilirubinemia associated with breast milk and mutations of the bilirubin uridine diphosphate glucuronosyltransferase gene. Pediatrics.2000; 106(5) . Available at: http://www.pediatrics.org/cgi/content/full/106/5/e59
61. Trioche P, Chalas J, Francoual J, et al. Jaundice with hypertrophic pyloric stenosis as an early manifestation of Gilbert syndrome. Arch Dis Child.1999; 81 :301 –303[Abstract/Free Full Text]
62. Stevenson DK, Vreman HJ, Benaron DA. Evaluation of neonatal jaundice: monitoring the transition in bilirubin metabolism. J Perinatol.1996; 16 :562 –567
63. Kaplan M, Muraca M, Hammerman C, et al. Imbalance between production and conjugation of bilirubin: a fundamental concept in the mechanism of neonatal jaundice. Pediatrics.2002; 110(4) . Available at: http://www.pediatrics.org/cgi/content/full/110/4/e47
64. American Academy of Pediatrics, Provisional Committee for Quality Improvement and Subcommittee on Hyperbilirubinemia. Practice parameter: management of hyperbilirubinemia in the healthy term newborn. Pediatrics.1994; 94 :558 –562[Abstract/Free Full Text]
65. American Academy of Pediatrics, AAP Subcommittee on Neonatal Hyperbilirubinemia. Neonatal jaundice and kernicterus. Pediatrics.2001; 108 :763 –765[Abstract/Free Full Text]
66. Stevenson DK, Fanaroff AA, Maisels MJ, et al. Prediction of hyperbilirubinemia in near-term and term infants. Pediatrics.2001; 108 :31 –39[Abstract/Free Full Text]

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