Molecular biology of bilirubin metabolism

As the genes encoding the glucuronidating enzymes are discovered, it is evident that glucuronidation is a magnificent example of how in evolution, man became adapted to his "intoxicating" environment. A superfamily of genes is necessary to dispose of the toxins and carcinogens that are encountered by inhalation and ingestion. The enzymes that glucuronidate endogenous compounds are members of this large family. For the clinician, it is important to remember that jaundice may sometimes be the result of interactions at the level of bilirubin glucuronidation. When jaundice results from inactivation of members of the UGT1 family, conjugation of certain phenols, such as the anesthetic propofol, or synthetic estrogens, such as ethinylestradiol, can also be impaired. In the case of severe bilirubin glucuronidation deficiencies, such as the Crigler Najjar syndrome type I, there are exciting prospects for a possible cure by gene therapy.

Discrimination between Crigler-Najjar type I and II by expression of mutant bilirubin uridine diphosphate-glucuronosyltransferase

Crigler-Najjar (CN) disease is classified into two subtypes, type I and II. The molecular basis for the difference between these types is not well understood. Several mutations in the bilirubin UDP-glucuronosyl-transferase (B-UGT) gene of six CN type I and two CN type II patients were identified. Recombinant cDNAs containing these mutations were expressed in COS cells. B-UGT activity was measured using HPLC and the amount of expressed protein was quantitated using a sandwich ELISA. This enabled us to determine the specific activities of the expressed enzymes. All type I patients examined had mutations in the B-UGT1 gene that lead to completely inactive enzymes. The mutations in the B-UGT1 gene of patients with CN type II only partially inactivated the enzyme. At saturating concentrations of bilirubin (75 microM) CN type II patient A had 4.4 +/- 2% residual activity and CN type II patient B had 38 +/- 2% residual activity. Kinetic constants for the glucuronidation of bilirubin were determined. The affinities for bilirubin of B-UGT1 expressed in COS cells and B-UGT from human liver microsomes were similar with Km of 5.1 +/- 0.9 microM and 7.9 +/- 5.3 microM, respectively. B-UGT1 from patient B had a tenfold decreased affinity for bilirubin, Km = 56 +/- 23 microM. At physiological concentrations of bilirubin both type II patients will have a strongly reduced conjugation capacity, whereas type I patients have no B-UGT activity. We conclude that CN type I is caused by a complete absence of functional B-UGT and that in CN type II B-UGT activity is reduced.

Genetic heterogeneity of Crigler-Najjar syndrome type I: a study of 14 cases

Crigler-Najjar syndrome type I (CN-I) is an autosomal recessive condition characterized by severe unconjugated hyperbilirubinemia caused by the lack of bilirubin-UDP-glucuronosyltransferase (B-UGT) activity in the liver. Two B-UGTs are coded for by a gene complex (UGT1) that maps to chromosome 2q37 and that also encodes two phenol-UDP-glucuronosyltransferases. Here, we report eleven mutations (including nine novel mutations) of the B-UGT1 gene in a large series of 14 unrelated CN-I children of various geographic origins: France (seven patients: A401P, Q357X, W335X, A368T, 1223insG, A291V, K426E, K437X); Portugal (two patients: G308E); Tunisia (two patients; Q357R); Turkey (one patient: S381R); italy (two siblings: S381R). Interestingly, 6/14 mutant alleles carried by unrelated probands of French ancestry bore the A401P mutation, indicating a founder effect; this effect is probably also present in Portugal, Turkey, and Tunisia. Since mutations occurred in exons 2-5 shared by all mRNAs species of the gene, a combined deficiency of B-UGT and P-UGT was observed in the liver of five patients in whom these activities were measured. The present study confirms that CN-I is genetically heterogeneous and suggests that different founder effects are involved in Western Europe, the Middle East, and North Africa.

Bilirubin UDP-glucuronosyltransferase 1 is the only relevant bilirubin glucuronidating isoform in man

Crigler-Najjar syndrome type I (CN-I) is caused by an inherited absence of UDP-glucuronosyltransferase activity toward bilirubin (B-UGT), resulting in severe non-hemolytic unconjugated hyperbilirubinemia. Based on the expression of cDNAs in COS cells, two UGT isoforms in human liver, B-UGT1 and B-UGT2, have been reported to catalyze bilirubin glucuronidation. These isoforms, which are derived from a single gene, ugt1, have identical carboxyl-terminal domains that are encoded by four consecutive exons shared by both isoforms. A critical lesion in any of these common exons should inactivate both B-UGT isoforms, giving rise to CN-I. The amino-terminal domains of the B-UGT isoforms are unique, each being encoded by a different 5' exon. If both B-UGT isoforms contribute significantly to bilirubin glucuronidation, a mutation in one of these unique 5' exons should affect a single isoform, while the other isoforms should provide residual B-UGT activity. However, in two patients with CN-I, we found a mutation only in the unique exon of B-UGT1, the other exons being normal. To clarify this apparent paradox, we expressed the cDNA for each B-UGT isoform in COS cells and determined the specific B-UGT activity. These studies show that only B-UGT1 has quantitatively significant catalytic activity. Furthermore, we show that the mutation in B-UGT1 observed in each of the two CN-I patients inactivates B-UGT1. Together, the results indicate that B-UGT1 is the only physiologically relevant isoform in bilirubin glucuronidation.

A new type of defect in the gene for bilirubin uridine 5'-diphosphate-glucuronosyltransferase in a patient with Crigler-Najjar syndrome type I

Crigler-Najjar syndrome (CN) type I, which is characterized by the complete absence of bilirubin uridine 5'-diphosphate-glucuronosyltransferase (UGT) activity, is inherited as an autosomal recessive trait associated with unconjugated hyperbilirubinemia. Phenobarbital has no effect on the bilirubin concentration in the serum of patients with CN type I. Recently, cDNA for two human liver bilirubin UGT (UGT1A and UGT1D) were isolated, and their genetic organization was determined. The UGT1A (UGT1*1) and UGT1D (UGT1*4) genes each have a unique exon 1, whereas exons 2-5 are common to both genes. It has been predicted that some defect in the exons common to both genes is responsible for the absence of UGT1A and UGT1D activities in CN type I, and five cases with such a mutation have been reported. We describe here a new type of defect in the gene for bilirubin UGT in a patient with CN type I, namely, an abnormality in the exon 1 that is characteristic of the UGT1A gene. This mutation is a single nucleotide substitution, that is, C is changed to A at base position 840 in UGT1A cDNA, and this change results in a stop codon. Our patient is homozygous for the defect, and his nonconsanguineous parents and elder brother, who are clinically normal, are heterozygous for the defective allele. No mutation was detected in any exons of the UGT1D gene. Our results suggest that a homozygous nonsense or deletion mutation is detected not only in the exons common to UGT1A and UGT1D genes but also in unique exon 1 of UGT1A in CN type I.

Identification of two single base substitutions in the UGT1 gene locus which abolish bilirubin uridine diphosphate glucuronosyltransferase activity in vitro

Accumulating evidence indicates that mutations in the human UGT1 gene locus abolish hepatic bilirubin UDP-glucuronosyltransferase activity and cause the subsequent accumulation of bilirubin to toxic levels in patients with Crigler-Najjar type 1 (CN-I). Genetic and biochemical criteria are required to link CN-I with mutations in UGT1. Here we present analysis of mutations at the UGT1 locus in three individuals that were clinically diagnosed with CN-I (two related and one unrelated). Each patient carries a single base substitution that alters conserved residues in the transferase enzyme molecule, serine to phenylalanine at codon 376 and glycine to glutamic acid at codon 309. Each was homozygous for the defect as demonstrated by sequencing and RFLPs. Mutant cDNAs, constructed by site-directed mutagenesis, inserted into expression vectors, and transfected into COS-1 cells, supported the synthesis of the bilirubin transferase protein but only cells transfected with the wild-type cDNA expressed bilirubin UDP-glucuronosyltransferase activity. The data provide conclusive evidence that alterations at Gly 309 and Ser 376 are the genetic basis for CN-I in these families. These results suggest that the two codons, located in conserved regions of the molecule, are part of the active site of the bilirubin enzyme.

Identification of defect in the genes for bilirubin UDP-glucuronosyl-transferase in a patient with Crigler-Najjar syndrome type II

Crigler-Najjar syndrome (CN) type II is characterized by severe chronic nonhemolytic unconjugated hyperbilirubinemia due to reduced hepatic bilirubin UDP-glucuronosyl-transferase (UGT) activity. Two bilirubin UGT isozymes, UGT1A and UGT1D, have been identified. We analyzed the DNA sequence of the bilirubin UGT genes in a 5-year-old Japanese male patient with CN type II, who had consanguineous parents. Point mutations were found on exons 1 of the UGT1A and UGT1D genes. The abnormalities were single nucleotide substitutions of G by A and of T by C at base position 211 of UGT1A cDNA and at base position 395 of the UGT1D, respectively. We found another single nucleotide substitution of T by G on exon 5 common to both genes at base position 1456 of the UGT1A cDNA or 1459 of the UGT1D cDNA. These three mutations result in changes of glycine to arginine and of tyrosine to aspartic acid at amino acid positions 71 and 486 of the UGT1A protein, and of leucine to proline and of tyrosine to aspartic acid at amino acid positions 132 and 487 of the UGT1D protein, respectively. Our patient was homozygous for all defects and his parents and elder brother were heterozygous for all defective alleles. The findings suggest that the CN Type II is inherited as an autosomal recessive trait.

A phenylalanine codon deletion at the UGT1 gene complex locus of a Crigler-Najjar type I patient generates a pH-sensitive bilirubin UDP-glucuronosyltransferase

The characterization (Ritter, J. K., Chen, F., Sheen, Y. Y., Tran, H. M., Kimura, S., Yeatman, M. T., and Owens, I. S. (1992) J. Biol. Chem. 267, 3257-3261) of the single-copy UGT1 gene complex encoding both bilirubin and phenol UDP-glucuronosyltransferases (transferase) has been critical to the determination of genetic defects in Crigler-Najjar Type I patients. The complex (UGT1A-UGT1G) codes for at least two bilirubin, three bilirubin-like, and two phenol transferases. Seven different exons 1, each with an upstream promoter and each encoding the amino terminus of an isoform, are arrayed in series with four common exons (encoding seven identical carboxyl termini) in the 3'-region of the locus. Predictably, a critical mutation in a common exon inactivates the entire locus. A deleterious mutation in an exon 1, as we report here for the UGT1A gene in a Crigler-Najjar Type I patient, predictably affects the amino terminus of that single isoform. The code for the predominant bilirubin isozyme, the HUG-Br1 protein, is missing the phenylalanine codon at position 170 in exon 1 of UGT1A, abolishing a conserved diphenylalanine. We demonstrate that, at the pH (7.6) routinely used for bilirubin glucuronidation studies, both the HUG-Br1 protein and human liver microsomes have approximately one-third the activity seen at the major pH optimum of 6.4 and at low ionic strength. The altered isozyme with nearly normal activity at pH 7.6 is inactive at pH 6.4, a result consistent with the definition of a pH-sensitive mutant. The Km value for bilirubin using the wild-type protein is approximately 2.5 microM at both pH 6.4 and 7.6 and that for the mutant is 5.0 microns at pH 7.6. The structure of the wild-type enzyme compared to that of the mutant indicates that hydrophobic properties at the active center are critical for metabolizing the lipophile-like substrate. The low ion/pH requirements for bilirubin glucuronidation may signal the basis for the distribution of these isozymes to an organelle (endoplasmic reticulum) that can establish compatible conditions/compartments for each catalysis.

Human UDP-glucuronosyl transferases: chemical defence, jaundice and gene therapy

Human UDP-glucuronosyltransferases (UDPGTs) are a family of enzymes which detoxify many hundreds of compounds by their conjugation to glucuronic acid, rendering them both harmless and more water soluble, hence, excretable. The level of expression of each UDPGT isoform in the body is the result of interplay between temporal, tissue-specific and environmental regulators. This complexity contributes to the difficulty in predicting the metabolic fate of compounds. Genetic defects and polymorphisms affecting individual isoform activities have deleterious and potentially lethal effects, as exemplified by the severe hyperbilirubinaemia observed in Crigler-Najjar Syndrome. Such severe genetic defects in bilirubin glucuronidation are obvious candidates for antenatal screening and gene therapy.

Cosegregation of intragenic markers with a novel mutation that causes Crigler-Najjar syndrome type I: implication in carrier detection and prenatal diagnosis

Crigler-Najjar syndrome type 1 (CN-1) is a familial disorder characterized by severe unconjugated hyperbilirubinemia and jaundice and leads to kernicterus, neurological damage, and eventual death unless treated with liver transplantation. Previous reports identified mutations in the UGT1 gene complex to be the cause of the disease. The total absence of all phenol/bilirubin UGT proteins and their activities in liver homogenate of a CN-1 patient was determined by enzymological and immunochemical analysis. A novel homozygous nonsense mutation (CGA-->TGA) was identified in the patient by the combined techniques of PCR and direct sequencing. This mutation was located in exon 3 of the constant region in the gene complex which is common to all phenol and bilirubin UGTs. The segregation of the mutation in the patient's family was analyzed and confirmed the recessive nature of the disease. Newly developed intragenic polymorphic probes (UGT1* 4 and UGT-Const) were used on Southern blots of MspI-digested genomic DNA of the patient and his family. The segregation of individual alleles within the family was observed from haplotypes generated. Comparison of the segregation of haplotypes with the mutation for the patient and his family revealed the allele identified by the A1-B1-C2 haplotype to be carrying the mutation. The risk of recombination occurring is negligible, because of the intragenic nature of the probes. This study demonstrates the potential usefulness of these probes in carrier detection and prenatal/presymptomatic diagnosis.

A mutation in bilirubin uridine 5'-diphosphate-glucuronosyltransferase isoform 1 causing Crigler-Najjar syndrome type II

BACKGROUND

Inherited unconjugated hyperbilirubinemia in Crigler-Najjar type II (CN II) is caused by a strong reduction of bilirubin uridine 5'-diphosphate-glucuronosyltransferase (B-UGT) activity. Both B-UGT isoenzymes (B-UGT1 and B-UGT2) identified in humans are derived from a single gene by alternative splicing. To clarify the genetic background of CN II and the role of both B-UGT forms in the physiological clearance of bilirubin, we have studied a large kindred with two CN II patients.

METHODS

From genomic DNA all B-UGT encoding exons were amplified by polymerase chain reaction and sequenced to identify mutations causing CN II.

RESULTS

The CN II patients were found to be homozygous for a nucleotide shift in the unique region of B-UGT1, changing a arginine into a tryptophan, and also for a nucleotide shift in the unique region of B-UGT2, changing a leucine into a valine. Analysis of other family members and of 50 control subjects showed that the mutation in B-UGT1 causes CN II, whereas the mutation in B-UGT2 is a polymorphism.

CONCLUSIONS

CN II syndrome appears to be caused by a homozygous mutation in B-UGT1. This indicates that B-UGT1 is the physiological important bilirubin glucuronidating isoform.

Marked endogenous activation of the CYP1A1 and CYP1A2 genes in the congenitally jaundiced Gunn rat

The homozygous recessive jaundiced Gunn rat lacks expression of bilirubin UDP-glucuronosyltransferase and serves as a model for Crigler-Najjar syndrome type I, in which high and toxic plasma levels of bilirubin result from this genetic defect in bilirubin conjugation. Both rats and humans dispose of this heme waste product by an alternate metabolic route that involves oxidation of the compound, followed by biliary excretion of the more polar metabolites. To determine the role of cytochrome P450 in this process, hepatic levels of cytochrome P450 mRNA and protein were measured in jaundiced and nonjaundiced Gunn rats as a function of age and sex. The mRNA and protein levels of cytochrome P450(CYP) 1A1 and CYP1A2 were markedly elevated in the jaundiced rats at the age of 10 days, compared with their nonjaundiced littermates. Levels of CYP2E1 mRNA and protein did not differ between these rats, indicating that the CYP1A P450 genes were specifically induced. CYP1A1 mRNA and protein levels increased further in the jaundiced animals between 10 days and 1 month of postnatal life but remained undetectable in the nonjaundiced littermates. On the other hand, CYP1A2 mRNA and protein content increased during this time period in both jaundiced and nonjaundiced rats, but at the age of 1 month there were no major differences between the two groups. CYP1A2 mRNA and protein levels were indistinguishable in 3-month-old jaundiced and nonjaundiced Gunn rats, whereas CYP1A1 could not be detected in either group. These data suggest that young jaundiced Gunn rats cope with the degradation of toxic bilirubin by increasing hepatic levels of CYP1A1 and CYP1A2. On the other hand, normal developmental activation of CYP1A2 may provide the alternative pathway for bilirubin degradation in adult animals. This is the first demonstration of the induction of cytochrome P450 gene expression to permit the elimination of an endogenously generated neurotoxic chemical in a genetic disease in which the normal excretory mechanism is impaired.

[Genetic defect of the hyperbilirubinemic Gunn rat, a model for Crigler-Najjar syndrome type I]

The hyperbilirubinemic Gunn rat lacks hepatic UDP-glucuronosyltransferase (UDPGT) activity toward bilirubin and has been used as an animal model for human Crigler-Najjar syndrome type I. Rat liver bilirubin UDPGT cDNA was isolated. The cDNA shared an identical 913-bp sequence (corresponding to the C-terminal 247 amino acid residues) with that for phenol UDPGT whose activity was also deficient in the Gunn rat. The bilirubin UDPGT gene was mapped at the position of 37 on mouse chromosome 1 by analyzing restriction endonuclease fragment length variations using the rat bilirubin UDPGT cDNA as a probe. The genetic defect of bilirubin UDPGT in the mutant rat was proved to be a -1 frameshift mutation. The mutation was found not only to be located in the region where the cDNA for bilirubin UDPGT shared the identical sequence with that for phenol UDPGT but also to occur in the same position in the two cDNAs from the mutant.

[Crigler-Najjar syndrome type II in a 17-year-old girl]

The Crigler-Najjar-Syndrome type II is characterised by the decreased activity of the bilirubin-UDP-glucuronyltransferase. This fact leads to an increase of the indirect bilirubin to 6-10 mg/dl. The mode of hereditary transmission is autosomal dominant with variable penetrance. The age of manifestation varies from the first year to the second decade of life. The Crigler-Najjar-Syndrome type II of a 17-year-old girl is reported. The diagnosis was made on the basis of the pattern of laboratory parameters in combination with an uneventful morphology of the liver. Under treatment with Chlofibrat the decrease of the serum total bilirubin from 10.4 mg/dl to 2 mg/dl was achieved.

Identification of a genetic alteration in the code for bilirubin UDP-glucuronosyltransferase in the UGT1 gene complex of a Crigler-Najjar type I patient

Patients with Crigler-Najjar syndrome (CN) type I inherit an autosomal recessive trait for hyperbilirubinemia, which is characterized by the total absence of bilirubin UDP-glucuronosyltransferase (transferase) activity. The recent identification of two bilirubin transferase isoforms with identical carboxyl termini (Ritter, J. K., J. M. Crawford, and I. S. Owens. 1991. J. Biol. Chem. 266:1043-1047) led to the discovery of a unique locus, UGT1, which encodes a family of UDP-glucuronosyltransferase isozymes, including the two bilirubin forms (Ritter, J. K., F. Chen, Y. Y. Sheen, H. M. Tran, S. Kimura, M. T. Yeatman, and I. S. Owens. 1992. J. Biol. Chem. 267:3257-3261). The UGT1 locus features a complex of six overlapping transcriptional units encoding transferases, each of which shares the four most 3' exons (2, 3, 4, and 5) specifying the 3' half of the transferase coding regions (condons 289-533) and the entire 3' untranslated region of each mRNA. This gene model predicts that a single critical mutation in any of these four "common" exons may inactivate the entire family of encoded transferases. In agreement with this prediction, we show here that in the first CN type I individual analyzed (patient F.B.), a 13-bp deletion has occurred in exon 2. Analysis of product generated by the polymerase chain reaction and genomic DNA demonstrated that F.B. is homozygous for the defective allele (UGT1*FB), and that the consanguineous parents are both heterozygotic at this locus. The mutation is predicted to result in the synthesis of severely truncated bilirubin transferase isozymes that are lacking a highly conserved sequence in the carboxyl-terminus and the characteristic membrane (endoplasmic reticulum)-anchoring segment of the protein molecule.

Mechanisms of inherited deficiencies of multiple UDP-glucuronosyltransferase isoforms in two patients with Crigler-Najjar syndrome, type I

Crigler-Najjar syndrome, type I (CN-I) is a potentially lethal disorder characterized by severe unconjugated hyperbilirubinemia resulting from a recessively inherited deficiency of hepatic UDP-glucuronosyl-transferase (UGT) activity toward bilirubin (B-UGT). Two forms of B-UGT exist in human liver. mRNAs for these two forms and that for another isoform with activity toward simple phenols (P-UGT) have unique 5' regions, but their 3' regions are identical. The three mRNA species are derived from a single locus; the unique 5' regions are encoded by single unique exons and the identical 3' regions consist of four consecutive exons that are shared by all three isoforms. In this paper, we determined genetic lesions in two CN-I patients with deficiency of hepatic B-UGT and P-UGT activities. In one patient, there was a C----T substitution in exon 4 (common region) predicting the substitution of a serine residue with a phenylalanine residue; this mutation was present in the identical region of B-UGT and P-UGT mRNAs. In the other patient, a C----T substitution in exon 2 (common region) of the B-UGT/P-UGT locus resulted in a premature stop codon. This exon (132 nt) was absent in heptic B-UGT and P-UGT mRNAs of this patient due to exon skipping during pre-mRNA processing. Sequence abnormality of three distinct mRNA species explains the abnormality of multiple UGT isoforms in these patients. Presence of identical abnormalities in the common regions of the three mRNAs is consistent with the finding that the common 3' regions of the two B-UGT mRNAs and the P-UGT mRNA are encoded by four shared exons.

The novel bilirubin/phenol UDP-glucuronosyltransferase UGT1 gene locus: implications for multiple nonhemolytic familial hyperbilirubinemia phenotypes

At least three types of congenital nonhemolytic unconjugated hyperbilirubinemias, including the rare Crigler-Najjar (CN) diseases (Types I or II) and Gilbert's syndrome (affecting 6% of the population) are associated with either absent or reduced hepatic UDP-glucuronosyltransferase (transferase) activity towards the potentially toxic endogenous acceptor, bilirubin. Here, we review the biochemical studies associated with these deficiencies. Accumulated evidence from studies with an animal model of CN Type I syndrome, the Gunn strain of hyperbilirubinemic rats, suggested that multiple isozymes are absent. These confounding observations have been clarified by a flurry of reports which have revealed the molecular basis for the complex disease phenotype in the Gunn rat and by the isolation and description of a novel human gene complex, UGT1, which encodes multiple and independently-regulated transferase isozymes that contain identical carboxyl terminal regions (246 amino acids). Finally, we discuss the implications of the gene organization and genetic defects determined for four different CN Type I individuals as a basis for a model which explains the inheritance pattern and genotypes of other familial unconjugated hyperbilirubinemias.

Sequence of exons and the flanking regions of human bilirubin-UDP-glucuronosyltransferase gene complex and identification of a genetic mutation in a patient with Crigler-Najjar syndrome, type I

Crigler-Najjar syndrome, type I is a heterogeneous disorder that may result from mutations of various regions of the bilirubin-UDP-glucuronosyltransferase gene complex that encodes two bilirubin-UDP-glucuronosyltransferase isoforms and a phenol-UDP-glucuronosyltransferase isoform in the human liver. The two bilirubin-UDP-glucuronosyltransferase messenger RNAs and the phenol-UDP-glucuronosyltransferase messenger RNA have identical 3' regions derived from four consecutive exons. The 5' region of each messenger RNA is unique and is derived from distinct single exons. By screening a human genomic library with probes corresponding to various regions of the messenger RNAs, we have isolated five cosmid clones containing overlapping segments of this large gene complex that spans at least 84 kb of the human genome. To facilitate the amplification of each exon by polymerase chain reaction and their adjacent splice junctions, we have delineated the intron-exon boundaries of the four common region exons and the two single exons that encode the unique regions of the two bilirubin-UDP-glucuronosyltransferase isoforms and have described sequences of the regions flanking each exon. All exons encoding the two bilirubin-UDP-glucuronosyltransferase isoforms and their splice junctions were amplified from the DNA of two control subjects and a Crigler-Najjar syndrome, type I patient. The DNA from the Crigler-Najjar syndrome, type I patient revealed a point mutation in exon 3 (a common region exon) resulting in a stop codon. RNA blot showed that the two bilirubin-UDP-glucuronosyltransferase messenger RNAs in the liver of the Crigler-Najjar syndrome, type I patient were of normal length but were reduced in concentration.(ABSTRACT TRUNCATED AT 250 WORDS)

Investigation of the molecular basis of the genetic deficiency of UDP-glucuronosyltransferase in Crigler-Najjar syndrome

Liver biopsy samples were obtained from eight Crigler-Najjar patients. Bilirubin UDPGT activity, assayed by a microassay with HPLC analysis, was not detectable in type I livers, and low levels (9-26% of controls) of monoglucuronide conjugates only were observed in type II livers. 1-Naphthol UDPGT activity was normal in most patients, where membrane integrity was maintained by correct sample procurement and preparation. Our data on type II livers suggest that a defect in UDPGA transport is an unlikely cause of the hyperbilirubinaemia, but reduced affinity for UDPGA was observed in one sample. Analysis of four patient liver samples by immunoblot analysis revealed the heterogeneous nature of this inherited disease within the patient population, and one sample where 1-naphthol UDPGT activity was considerably reduced appeared to correlate with the non-detection of a phenol UDPGT protein. Progress towards a molecular genetic diagnosis of Crigler-Najjar syndromes is discussed.

Concurrence of Robinow syndrome and Crigler-Najar syndrome in two offspring of first cousins

Robinow syndrome and Crigler-Najar syndrome were encountered in 2 Saudi offspring of first cousins. Both died at age 4 months. The parents lost 2 previous children at age 2 months with progressive jaundice but without fetal facial characteristics.

Crigler-Najjar type II disease inheritance: a family study

The inheritance of Crigler-Najjar type II disease is still contested. Autosomal dominant transmission with incomplete penetrance and autosomal recessive transmission have been proposed. We had the opportunity to study the hepatic activity of bilirubin uridinediphosphate glucuronyltransferase in parents whose first child had been affected by Crigler-Najjar type II disease. The demonstration of reduced activity of glucuronidation in the liver of both parents suggests autosomal recessive inheritance. The second infant of this couple was affected by the same disease and was treated with success by phenobarbital.

Familial unconjugated hyperbilirubinemia syndromes

Our understanding of the biochemical defects underlying the hepatic forms of congenital, unconjugated hyperbilirubinemias has been greatly enhanced over the past decade. This is mostly due to the availability of pure, labeled bilirubin, the appropriate kinetic analyses, and a better understanding of the mechanisms underlying bilirubin conjugation. Although it is quite obvious that the defect underlying Gilbert's and Crigler-Najjar syndromes is deficient glucuronidation, the molecular explanation may eventually be found in altered composition of the microsomal lipids rather than in a protein defect of glucuronyl transferase. The recognition that Gilbert's syndrome is a quite heterogeneous entity will allow a better understanding of the mode of inheritance of this disorder; its relationship to Crigler-Najjar type II disease also awaits further definition. It is hoped that definition of the molecular defect in Crigler-Najjar type I will lead to better therapeutic modalities, but this remains to be seen.

Prolonged survival in three brothers with severe type 2 Crigler-Najjar syndrome. Ultrastructural and metabolic studies

Three brothers with severe type 2 Crigler-Najjar syndrome for over 50 years have been studied. Although the plasma unconjugated bilirubin (UCB) concentrations were in excess of 19 mg per 100 ml, no abnormal neurological signs were evident. Prolonged exposure to severe unconjugated hyperbilirubinemia does not therefore necessarily increase morbidity. Electron microscopy of liver tissue obtained from 2 patients before phenobarbital therapy showed hypertrophy and hyperplasia of smooth endoplasmic reticulum with unusual prominence of the Golgi apparatus and focal modification of the cell surface membranes. These changes may reflect the reactive state of the hepatocyte to high levels of unconjugated bilirubin. Phenobarbital therapy resulted in a marked reduction in UCB concentration and was accompanied by further hypertrophy of the smooth endoplasmic reticulum and minor changes in bile canaliculi. Dietary restriction to 400 cal daily for 3 days produced a dramatic increase in UCB. The addition of 2400 cal by the intravenous administration of 50% dextrose did not reduce the elevated UCB. In contrast, 2400 cal fed as a normal diet rapidly returned the UCB to basal levels. While on phenobarbital therapy, a similar response to caloric withdrawal and parenteral feeding was observed. These findings indicate that the hyperbilirubinemia of fasting does not depend on caloric deficiency per se, and suggest that either the route of caloric administration or the type of nutrient may influence the level of unconjugated hyperbilirubinemia.