Abstract 2958: Quantitative and correlative analysis of variant UGT1A mRNA expression in human liver

Abstract

Abstract UDP-glucuronosyltransferases (UGTs) play an important role in the metabolism and excretion of various endogenous and xenobiotic compounds including several carcinogens and chemotherapeutic agents. UGTs are divided into two sub-families based on sequence homology. The nine UGT1A family members are encoded by a single locus on chromosome 2q37, where each family member has a unique exon 1 and shares exons 2-5. A recently identified exon 5b in the common region of the UGT1A gene cluster leads to the expression of 18 additional mRNA species from this locus. In vitro studies indicate that the proteins encoded by these variant mRNAs negatively modulate the activity of the wild type isoforms. It is hypothesized that inter-individual variation in the ratio of wild type to variant mRNA expression may affect glucuronidation capacity. The goal of this study was to quantitatively examine variant UGT1A mRNA expression in individual human liver specimens to better assess the importance of splicing variants in overall UGT1A activity. To differentiate between the 5’ (exon 1) and 3’ (exon 5) splice variants, a nested, quantitative PCR reaction was developed that allows both variants to be distinguished simultaneously. Results of the study indicate that there is a 406-, 278-, 58-, and 139-fold inter-individual variation in the expression ratio of wild-type to variant mRNA for UGTs 1A1, 1A4, 1A6, and 1A9, respectively, in the 10 human livers examined. UGT1A variant expression reached as high as 8.2-fold more than wild-type UGT1A1, 2.9-fold higher than wild-type UGT1A4, 0.13-fold lower than wild-type UGT1A6, and 0.46-fold lower than wild-type UGT1A9. Even though the expression ratio of wild-type to variant mRNA differs widely between different individuals, results indicate that the expression ratio is consistent for all UGTs examined within a single individual. The correlation between the expression ratio of UGTs 1A1 and 1A4 in different individuals is highly significant (R2 = 0.9661, p<0.0001). Similarly, the correlation between the expression ratio of UGT1A1 and UGT1A6, UGT1A1 and UGT1A9, UGT1A4 and UGT1A6, UGT1A4 and UGT1A9, and UGT1A6 and UGT1A9 are all significant (R2=0.5265, p=0.0269; R2=0.8994, p=0.0003; R2=0.5782, p=0.0174; R2=0.9610, p<0.0001; R2=0.582, p=0.0277, respectively). Together, these data suggest that UGT1A variant expression could significantly impact overall UGT1A activity and that there is a high correlation between wild-type : variant mRNA expression ratios across the entire UGT1A locus within individuals. Future studies will be conducted to determine whether a difference in the UGT1A expression ratio has an impact on glucuronidation capacity in vivo. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 2958.