A pharmacogenetics study of the human glucuronosyltransferase UGT1A4

Background: UGT1A4 is primarily expressed in the liver, where it plays a key role in the metabolism of various drugs. Among the few UGT1A4 polymorphisms studied, some variants, including Pro²⁴Thr and Leu⁴⁸Val (designated as UGT1A4*2 and *3), have been shown to alter UGT1A4-mediated glucuronidation.

Methods: To investigate the genetic mechanisms underlying individual differences in UGT1A4 expression and activity, we sequenced the UGT1A4 gene from -4963 bp upstream of the ATG start site to 2000 bp beyond the first exon. This analysis was conducted on 184 unrelated individuals of Caucasian and African-American descent.

Results: Our study identified numerous genetic variations, including 13 intronic, 39 promoter, and 14 exonic polymorphisms, with 10 resulting in amino acid changes. Of the promoter region variants within the -5 kb region, five were located in the proximal 500 bp and aligned with putative HNF-1 and OCT-1 transcription factor binding sites. Four of these proximal (Z)-4-Hydroxytamoxifen variants, located at -163, -219, -419, and -463, showed complete linkage disequilibrium with the Leu⁴⁸Val coding variant and several upstream promoter variants. Functional assays using transient transfections of reference and variant promoter constructs (-500 to +1) in various cell lines, with and without HNF-1 and/or OCT-1 co-expression, revealed limited impact of these promoter variations.

Conclusion: Further functional studies are needed to determine the potential in vivo impact of promoter variants on UGT1A4 expression. Additionally, several coding variants (Val⁴⁸, Asp⁵⁰, Gln⁵⁶, Phe¹⁷⁶, Asn²⁵⁰, Leu²⁷⁶) significantly altered the enzyme kinetics for tamoxifen and Z-4-hydroxytamoxifen, suggesting they may influence drug metabolism in vivo.