Structural heterogeneity at the UDP-glucuronosyltransferase 1 locus: functional consequences of three novel missense mutations in the human UGT1A7 gene

Abstract

One of the most important mechanisms involved in host defense against xenobiotic chemicals and endogenous toxins is the glucuronidation catalysed by UDP-glucuronosyltransferase enzymes (UGT). The role of genetic factors in determining variable rates of glucuronidation is not well understood, but phenotypic evidence in support of such variation has been reported. In the present study, six single nucleotide polymorphisms were discovered in the first exon of the UGT1A7 gene, which codes for the putative substrate-binding domain, revealing a high structural heterogeneity at the UGT1 gene locus. The new UGT1A7 proteins differ in their primary structure at amino acid positions 129, 131 and 208, creating four distinct UGT1A7 allelic variants in the human population: UGT1A7*1 (N129 R131 W208), *2 (K129 K131 W208), *3 (K129 K131 R208), and *4 (N129 R131 R208). In functional studies, HEK cells stably transfected to express the four allelic UGT1A7 variants exhibited significant differences in catalytic activity towards 3-, 7-, and 9-hydroxy-benzo(a)pyrene. UGT1A7*3 exhibited a 5.8-fold lower relative Vmax compared to wild-type *1, whereas *2 and *4 had a 2.6- and 2.8-fold lower relative Vmax than *1, respectively, suggesting that these mutations confer slow glucuronidation phenotype. Kinetic characterization suggested that these differences were primarily attributable to altered Vmax. Additionally, it suggested that each amino acid substitutions can independently affect the UGT1A7 catalytic activity, and that their effects are additive. The expression pattern of UGT1A7 studied herein and its catalytic activity profile suggest a possible role of UGT1A7 in the detoxification and elimination of carcinogenic products in lung. A population study demonstrated that a considerable proportion of the population (15.3%) was found homozygous for the low activity allele containing all three missense mutations, UGT1A7*3. These findings suggest that further studies are needed to investigate the impact of the low UGT1A7 conjugator genotype on individual susceptibility to chemical-induced diseases and responses to therapeutic drugs.

Pharmacogenomics of human UDP-glucuronosyltransferase enzymes

“…Functional assays were subsequently performed for UGT1A7 variant alleles *1-*9 for a variety of substrates. 65,137 Regardless of the substrate tested, the UGT1A7 enzyme velocity was shown to be altered in the presence of polymorphisms at codons 115 and 208, regardless of the presence of the mutations affecting codons 129/131 or 139.…”

“…98 h Genotypes were stratified according to predicted activities based on allele-encoded isozyme glucuronidating activities toward benzo[>a]pyrene metabolites (Guillemette et al 65,66 ). i NS=not significant.…”

“…The genetic diversity of the UGT1A7 gene was revealed after resequencing the entire gene. 65 Four forms of the UGT1A7 gene product were initially found and more recently, five additional forms were discovered. There are now nine alleles of the UGT1A7 gene, which vary at the following positions: Gly 115 Ser, Asn 129 Lys, Arg 131 Lys, Glu 139 Asp and Trp 208 Arg (UGT1A7*1-UGT1A7*9) 65,137 (Figure 3).…”

“…65 Four forms of the UGT1A7 gene product were initially found and more recently, five additional forms were discovered. There are now nine alleles of the UGT1A7 gene, which vary at the following positions: Gly 115 Ser, Asn 129 Lys, Arg 131 Lys, Glu 139 Asp and Trp 208 Arg (UGT1A7*1-UGT1A7*9) 65,137 (Figure 3). The three nonsynonymous SNPs at codons 129/131 and 208, two of which are tightly linked (129 and 131), create the variant allele UGT1A7*3, commonly observed in the population (B17% of *3/*3).…”

“…Recent findings suggest that metabolic alterations may actually determine the degree of exposure of an individual to toxic or carcinogenic substances over a long time period and may contribute to modify one's susceptibility to diseases such as cancer. [65][66][67][68][69][70][71][72][73] This review is focused on recent work in the area of the pharmacogenomics of human UGTs. The following topics will be covered: (i) overview of human UGT enzymes; (ii) description of polymorphic human UGT enzymes; (iii) pharmacogenomic phenomena involving polymorphic UGT enzymes in drug response and (iv) pharmacogenomic phenomena involving polymorphic UGT enzymes in cancer.…”