No abstract

Uridine‐diphosphoglucuronate glucuronosyltransferases (UGTs) are a family of enzymes that conjugate various endogenous and exogenous compounds with glucuronic acid and facilitate their excretion in the bile. Bilirubin‐UGT1 (UGT1A1) is the only isoform that significantly contributes to the conjugation of bilirubin. Lesions in the gene encoding bilirubin‐UGT1, lead to complete or partial inactivation of the enzyme causing the rare autosomal recessively inherited conditions, Crigler‐Najjar syndrome type‐1 (CN‐1) and type 2 (CN‐2), respectively. Inactivation of the enzyme leads to accumulation of unconjugated bilirubin in the serum. Severe hyperbilirubinemia seen in CN‐1 can cause bilirubin encephalopathy (kernicterus). Kernicterus can be fatal or may leave behind permanent neurological sequelae. Here, we have compiled more than 50 genetic lesions of UGT1A1 that cause CN‐1 (including 9 novel mutations) or CN‐2 (including 3 novel mutations) and have presented a correlation of structure to function of UGT1A1. In contrast to Crigler‐Najjar syndromes, Gilbert syndrome is a common inherited condition characterized by mild hyperbilirubinemia. An insertional mutation of the TATAA element upstream to UGT1A1 results in a reduced level of expression of the gene. Homozygosity for the variant promoter is required for Gilbert syndrome, but not sufficient for manifestation of hyperbilirubinemia, which is partly dependent on the rate of bilirubin production. Several structural mutations of UGT1A1, for example, a G71R substitution, have been reported to cause mild reduction of UGT activity toward bilirubin, resulting in mild hyperbilirubinemia, consistent with Gilbert syndrome. When the normal allele of a heterozygote carrier for a Crigler‐Najjar type structural mutation contains a Gilbert type promoter, intermediate levels of hyperbilirubinemia, consistent with the diagnosis of CN‐2, may be observed. Hum Mutat 16:297–306, 2000. © 2000 Wiley‐Liss, Inc.

This article is available online at http://dmd.aspetjournals.org ABSTRACT:This article is an updated report of a symposium held at the June 2000 annual meeting of the American Society for Pharmacology and Experimental Therapeutics in Boston. The symposium was sponsored by the ASPET Divisions for Drug Metabolism and Molecular Pharmacology. The report covers research from the authors' laboratories on the structure and regulation of UDP-glucuronosyltransferase (UGT) genes, glucuronidation of xenobiotics and endobiotics, the toxicological relevance of UGTs, the role of UGT polymorphisms in cancer susceptibility, and gene therapy for UGT deficiencies.

Familial hypercholesterolemia (FH) is an inherited disorder in humans that is caused by a deficiency of low density lipoprotein receptors (LDLRs). An animal model for FH, the Watanabe Heritable Hyperlipidemic rabbit, was used to develop an approach for liver-directed gene therapy based on transplantation of autologous hepatocytes that were genetically corrected ex vivo with recombinant retroviruses. Animals transplanted with LDLR-transduced autologous hepatocytes demonstrated a 30 to 50 percent decrease in total serum cholesterol that persisted for the duration of the experiment (122 days). Recombinant-derived LDLR RNA was harvested from tissues with no diminution for up to 6.5 months after transplantation.

Crigler-Najjar (CN) 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 ,uM and 7.9±5.3 ,uM, respectively. B-UGT1 from patient B had a tenfold decreased affinity for bilirubin, Km = 56±23 ,LM.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. (J. Clin. Invest. 1994Invest. .94:2385Invest. -2391

Recombinant adenoviruses (Ads) are highly efficient at transferring foreign genes to the liver in vivo; however, the duration of gene expression is limited by the host antiviral immune response, which prevents expression upon readministration of the virus. To test whether overexpression of the immunomodulatory products of the early Ad genome region 3 (E3) could prevent the antiviral immune response and prolong expression of foreign genes delivered by Ad vectors, we injected a recombinant Ad (Ad-E3-hBUGT), containing both E3 and the human bilirubin-uridine-diphosphoglucuronate-glucuronosyltransferase (BUGT) genes, into BUGT-deficient hyperbilirubinemic Gunn rats. Control Gunn rats received Ad-hBUGT, which expresses human BUGT alone. An initial injection of either virus resulted in hepatic expression of human BUGT as evidenced by excretion of bilirubin glucuronides in bile and a reduction of mean serum bilirubin levels from 7.0 mg͞dl to 1.9-2.7 mg͞dl within 7 days. In Ad-E3-hBUGT-injected rats, serum bilirubin levels increased to 4.5 mg͞dl by 84 days after infection, but a second administration of the virus on that day resulted in a hypobilirubinemic response similar to that seen with the first injection. In contrast, rats receiving Ad-hBUGT had serum bilirubin levels of 7 mg͞dl on day 84 after infection, but showed no reduction of serum bilirubin by reinjection of the virus on that day. In the rats injected with Ad-E3-hBUGT, but not in the ones injected with Ad-hBUGT, there was a marked inhibition of the antiviral antibody and Ad-specific cytotoxic T lymphocyte responses. This is the first demonstration that insertion of E3 genes in recombinant Ads facilitates readministration of a functional vector for long-term correction of an inherited metabolic disorder.

Crigler-Najjar syndrome type I is characterized by unconjugated hyperbilirubinemia resulting from an autosomal recessive inherited deficiency of hepatic UDPglucuronosyltransferase (UGT) 1A1 activity. The enzyme is essential for glucuronidation and biliary excretion of bilirubin, and its absence can be fatal. The Gunn rat is an excellent animal model of this disease, exhibiting a single guanosine (G) base deletion within the UGT1A1 gene. The defect results in a frameshift and a premature stop codon, absence of enzyme activity, and hyperbilirubinemia. Here, we show permanent correction of the UGT1A1 genetic defect in Gunn rat liver with site-specific replacement of the absent G residue at nucleotide 1206 by using an RNA͞DNA oligonucleotide designed to promote endogenous repair of genomic DNA. The chimeric oligonucleotide was either complexed with polyethylenimine or encapsulated in anionic liposomes, administered i.v., and targeted to the hepatocyte via the asialoglycoprotein receptor. G insertion was determined by PCR amplification, colony lift hybridizations, restriction endonuclease digestion, and DNA sequencing, and confirmed by genomic Southern blot analysis. DNA repair was specific, efficient, stable throughout the 6-month observation period, and associated with reduction of serum bilirubin levels. Our results indicate that correction of the UGT1A1 genetic lesion in the Gunn rat restores enzyme expression and bilirubin conjugating activity, with consequent improvement in the metabolic abnormality.UDP-glucuronosyltransferases (UGTs) are a family of membrane-bound enzymes that catalyze the conjugation of numerous xenobiotics and endogenous substrates with glucuronic acid. Of the known isoforms, only UGT1A1 is physiologically relevant in bilirubin glucuronidation and biliary excretion of this potentially toxic metabolite (1, 2). Crigler-Najjar (CN) syndrome is the inherited deficiency of hepatic UGT1A1 activity and is characterized by elevated serum levels of unconjugated bilirubin (3). Of the two types of CN syndrome, type I is more severe and is characterized by a nearly complete absence of UGT1A1 activity, whereas incomplete deficiency of the enzyme is associated with the less severe type II form (4, 5).The homozygous Gunn rat, a mutant strain of Wistar rat, is an accurate animal model for CN syndrome type I. Its liver lacks UGT1A1 activity because of the deletion of a single guanosine (G) base in UGT1A1 that results in a frameshift and a premature stop codon (6, 7). Recombinant adenoviral vectors have been used in vivo to correct the hyperbilirubinemia in the Gunn rat with persistent expression of the human bilirubin UGT1A1 enzyme for as long as 2 months (8, 9). Significant progress also has been made in overcoming the immunogenicity of the adenoviral-based vectors (9-11), but their use requires repeated treatments and immunomodulation of the host to maintain therapeutic levels of UGT1A1.A novel approach, based on mechanisms of DNA repair (12), was reported to correct single nucleotide mutations in ...

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)