Genetic diversity at the UGT1 locus is amplified by a novel 3′ alternative splicing mechanism leading to nine additional UGT1A proteins that act as regulators of glucuronidation activity

Background The gene UGT1 encodes phase 11 detoxification proteins involved in the elimination of small hydrophobic substances of both endogenous and exogenous origin. To date, nine functional UGT1 A proteins are known to be produced from a single gene composed of alternative first exons shared with four common exons. Recently, a novel exon (referred to as exon 5b) was identified in the common shared region. Results We now reveal a novel alternative splicing mechanism and demonstrate that the exon 5a and the new exon 5b are alternatively spliced, generating several variant mRNAs and up to nine previously unknown variant UGT1A proteins, referred to as isoforms 2 or i2. lsoform-specific RT-PCR analyses reveal that the alternatively spliced mRNAs are widely distributed in human tissues. Immunoreactive proteins at the predicted molecular weight of similar to 45 kDa were confirmed in microsomes of human tissues using antibodies against UGT1A1 and anti-UGT1 A7/8/9/10. Functional enzyme assays demonstrate that i2 proteins containing exon 5b are enzymatically inactive. On the other hand, co-expression experiments of i2 of UGT1 A1p, UGT1A7, UGT1A8 and UGT1A9 with their classical isoform 1 homologs results in a significant repression (15 to 79%) of UGT1A_i1-mediated drug metabolism. Conclusion The UGT1A isoforms 2 act as negative modulators of their isoform 1 homologs in microsome preparations, revealing a new regulatory mechanism of the glucuronidation pathway. Findings further provide the first direct evidence of a novel alternative splicing mechanism at the 3' end of the UGT1 locus that further increases the number of proteins derived from this single gene. Pharmacogenetics and Genomics 17:1077-1089 (c) 2007 Wolters Kluwer Health vertical bar Lippincott Williams & Wilkins.