Analysis of Cytochrome P450 Polymorphisms

A number of common functtonally significant genetic polymorphisms in genes encoding certain cytochromes P450 have been described and characterized (for review, see refs. 1 and 2 ). Individuals may lack certain enzyme activities or have higher or lower than normal activities owing to the presence of certain variant alleles. These individuals may be at altered risk of developing adverse drug reactions or drseases associated wtth xenobiotrc exposure, including cancer. Variant alleles can be detected by the direct approach of genotyping where the individual’s DNA is directly examined usually by an assay involving use of the polymerase chain reactton (PCR), or by phenotypmg, where the individual’s pattern of metabolites produced from a probe drug is examined. This chapter describes the detection of common polymorphisms in cytochrome P450 genes using both genotyping and phenotypmg approaches. Genotyping is now generally preferred to phenotyping because it is more convenient, requiring only a single blood sample, which can be taken at any time. It can also be used on stored blood or other tissue samples. Phenotyping, however, is useful when investigating drug interactrons in viva and in situations where the molecular basis of a polymorphism is unclear. In the sections of this chapter covering genotyping, we describe general methods for PCR and analysts of PCR products that have been used to genotype for pharmacogenetic polymorphisms, together with the specific conditions used to detect a range of known polymorphisms in cytochrome P450 genes. Methods for phenotypic detection of polymorphisms in CYP2D6 and CYP2A6 are also described.