Analysis of the promoter of human bilirubin UDP-glucuronosyltransferase gene (UGT1*1) in relevance to Gilbert's syndrome

Abstract Bilirubin UDP-glucuronosyltransferase 1 (B-UGT 1 ) is an enzyme responsible for hepatic bilirubin glucuronidation and in Gilbert's syndrome, its activity is decreased to 30% of normal. About one third of the patients we analyzed with the syndrome had a homozygous TATA box mutation in UGT1*1 coding for B-UGT 1 , i.e. (TA) 7 TAA instead of (TA) 6 TAA. Since the frequency was much higher than in the normal Japanese population (≈1%), the mutation was considered to be closely associated with the etiology of the syndrome. Since these patients did not have any structural mutations in UGT1*1, we suspected a transcriptional defect of the gene. We analyzed the promoter region (up to −3190) of UGT1*1 by transient transfection assay to identify transcriptionally regulatory sequences and found two elements; one was between −1362 and −1220 and the other between −113 and −70. The proximal one (PE) consisted of two elements, E-box (−104 to −95) and an HNF-1 site (−91 to −79). The two TATA boxes were compared in promoter activity, but no substantial difference was observed. Distal element (DE) and PE were then analyzed in these patients' DNA ( n =6) by PCR and direct sequencing. The same homozygous mutation (C-1353A) was found in two of them, but the reduction in promoter activity was only 15%. These results suggest that the TATA box mutation itself is not the major cause of the syndrome and may be genetically linked to an, as yet, unidentified defect in the further upstream or in the intronic region of UGT1*1.