Polymorphisms in UDP glucuronosyltransferase genes: functional consequences and clinical relevance.

As glucuronidation is a major process for the metabolism and removal of lipophilic chemicals, polymorphic variations in genes encoding the enzymes involved in this process, the UDP glucuronosyltransferases (UGT), may have a significant impact on our capacity to detoxify and eliminate drugs and toxins. Although 24 human UGT genes have been identified to date, only polymorphisms in five UGTs, viz. UGT1A1, UGT1A6, UGT2B4, UGT2B7 and UGT2B15 have been described. Polymorphisms in UGT1A1, the major bilirubin-glucuronidating form, often result in a decreased capacity to glucuronidate bilirubin, such as observed in Gilbert Syndrome and some forms of perinatal jaundice. The frequencies of individual UGT1A1 polymorphisms show extensive variability across ethnic groups. Two variants of UGT1A6 and UGT2B4 and one variant of UGT2B7 and UGT2B15 have been identified. However, the clinical significance of these variants is unclear. More UGT polymorphisms will undoubtedly be discovered when the human genome is sequenced. However, unless the UGT in question is responsible for the exclusive metabolism of a particular drug or chemical (e.g. UGT1A1 and bilirubin) or is the predominant or only UGT present in the cell, it is unlikely that these polymorphisms will be of major clinical significance.