Racial variability in the UDP-glucuronosyltransferase 1 (UGT1A1) promoter: a balanced polymorphism for regulation of bilirubin metabolism?

Abstract

A polymorphism in the promoter of the UDP-glucuronosyltransferase 1 (UGT1A1) gene has been shown to cause Gilbert syndrome, a benign form of unconjugated bilirubinemia. Promoters containing seven thymine adenine (ta) repeats have been found to be less active than the wild-type six repeats, and the serum bilirubin levels of persons homozygous or even heterozygous for seven repeats have been found to be higher than those with the wild-type six repeats. We have now examined the genotypes in persons of Asian, African, and Caucasian ancestry. Although within the Caucasian ethnic group there is a strong correlation between promoter repeat number and bilirubin level, between ethnic groups we found that this relationship to be inverse. Among people of African ancestry there are, in addition to those with six and seven repeats, also persons who have five or eight repeats. Using a reporter gene we show that there is an inverse relationship between the number of ta repeats and the activity of the promoter through the range of 5-8 ta repeats. An incidental finding was a polymorphism at nucleotide -106, tightly linked to the (ta)5 haplotype. Serum bilirubin levels are influenced by many factors, both genetic and environmental. We suggest that the unstable UGT1A1 polymorphism may serve to "fine-tune" the plasma bilirubin level within population groups, maintaining it at a high enough level to provide protection against oxidative damage, but at a level that is sufficiently low to prevent kernicterus in infants.

Figure 1

Autoradiograph demonstrating the number of (ta) repeats in the UGT1A1 promoter. M denotes the SequaMark molecular weight marker ranging in size from 80 to 110 bp with 1-bp incremental bands in the center. DNA samples from subjects with (ta)₇/(ta)₅, (ta)₇/(ta)₆, (ta)₇/(ta)₇, (ta)₆/(ta)₆, and (ta)₈/(ta)₇ UGT1A1 genotypes are shown. Method is described in the text.

Figure 2

The effectiveness of the UGT1A1 promoters containing the variable (ta) repeats using a luciferase reporter. Promoter activity was analyzed in two human hepatoma cell lines: HepG2 (A and C) and HuH7 (B and D). The shorter promoter constructs (C and D) were 229 bp for the (ta)₆, and the longer constructs (A and B) were an additional 32 bp in length. Promoter activity with a firefly luciferase reporter gene was compared with an internal control of Renilla luciferase with an simian virus 40 promoter. Mean values of three to six replicate wells and their standard errors are shown. The short UGT1A1 constructs in HepG2 cells (C) were assayed 24 hr after transfections instead of 48 hr because the cells had reached confluency. The “wild type” (ta)₆ is shown as a solid bar; the variants are shown as open bars. Note that the (ta)₉ promoter was made by mutagenesis. All other promoters are naturally occurring.