CYP2D6 Genotyping by a Multiplex Primer Extension Reaction

Great interindividual variability in drug response leads to variation in drug safety and efficacy. Although this can be the result of environmental and physiologic factors as well as drug–drug interactions, in many cases the response is inherited, arising from a polymorphism in genes encoding drug transporters, drug receptors, and especially, drug-metabolizing enzymes. The polymorphic cytochrome P450 2D6 (CYP2D6; OMIM 124030) is one of the most widely studied drug-metabolizing enzymes, being responsible for the metabolism of many commonly used drugs belonging to classes such as antidepressants, neuroleptics, beta-blockers, and antiarrhythmics (1). The CYP2D6 gene spans a 4.2-kb region located on chromosome 22q13.1 and is part of the CYP2D cluster together with CYP2D8P and CYP2D7 pseudogenes (2). At present, more than 50 different major polymorphic CYP2D6 alleles are known (3). These include variants produced by major rearrangements (whole-gene deletion or duplication), point mutations, and single or multiple base deletions or insertions. The phenotypic consequences of this variation are considerable: the CYP2D6 enzyme activity ranges from complete deficiency, possibly giving rise to profound toxicity of medication in the case of poor-metabolizing individuals, to ultrarapid metabolism, which can lead to therapeutic failure at recommended drug dosages. Individuals with a normal or slightly below normal rate of metabolism related to the CYP2D6 enzyme are usually defined as extensive or intermediate metabolizers, respectively.