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

Abstract

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.

Three Japanese patients with Crigler-Najjar syndrome type I carry an identical nonsense mutation in the gene for UDP-glucuronosyltransferase

“…Recently, cDNAs for rat and human bilirubin UGT were isolated (Sato et al, 1990;Ritter et al, 1991) and the structure of the gene for human bilirubin UGT in terms of exon-intron organization was determined (Ritter et al, 1992;Bosma et aL, 1992a). The gene is located on chromosome 2 (van Es et al, 1993) and consists of five exons (Bosma et al, 1992a(Bosma et al, , 1994.…”

Prediction of Deleterious Non-synonymous Single-Nucleotide Polymorphisms of Human Uridine Diphosphate Glucuronosyltransferase Genes

“…The confirmed variants were collected from results derived from site-directed mutagenesis studies of the enzyme using biochemical characterization (18,(39)(40)(41)(42)(43)(44)(45)(46)(47) or clinical data from family-based and association studies (48)(49)(50)(51)(52)(53)(54)(55)(56). The biochemical/ in vitro and in vivo UGT variants and the predictions for their functional impact scores by SIFT and PolyPhen are displayed in Table VI.…”

“…In the confirmed variants of UGT1A1, the associated phenotypes were CN I, CN II (15), and Gilbert's syndrome (16) which are all closely related to bilirubin levels. To date, 47 mutant UGT1A1 alleles have been identified (Table VI; 17,18,39,[42][43][44][45][46][47][48][49][50][51][53][54][55][56][57][58][59]. Many of these SNPs were predicted to have phenotypical effects by the algorithms and the correct prediction rates were 68% and 81% by SIFT and PolyPhen, respectively.…”

“…Variation in UGT1A coding sequences may lead to deficiency of UGT1A1 activity, often resulting in high level of unconjugated bilirubin which is closely related to diseases such as Crigler-Najjar syndrome (CN; 15) and Gilbert's syndrome (16). Nonsense mutations in the UGT1 gene resulting in a premature termination codon and absent UGT1A1 activity have been reported in CN patients (17,18). Furthermore, some genetic variations in the UGT1A1 promoter (*28) is associated with increased drug toxicity and hyperbilirubinemia in cancer patients receiving irinotecan-based chemotherapy (19)(20)(21)(22), and reports also show increased risk of tranilast-induced hyperbilirubinemia (23).…”

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

“…The activities of the two phenol UDPGT isoforms expressed at this locus are also presumed to be affected by the location of the mutations in the common exons for these enzymes. Indeed, N.H. has been shown to have reduced hepatic microsomal activity toward 4-nitrophenol and 4-methylumbelliferone ( 13). A partial rather than complete loss of phenol glucuronidating activities in N.H. is due to the existence of other UDPGT genes at unlinked loci that encode isoforms active towards phenolic substrates, possibly the testosterone isoform ( 19).…”