CYP2D6

Cytochrome P450 2D6 (CYP2D6), a member of the cytochrome P450 mixed-function oxidase system, is one of the most important enzymes involved in the metabolism of xenobiotics in the body. Whilst CYP2D6 is involved in the oxidation of a wide range of substrates of all the CYPs, there is considerable variability in its expression in the liver. The gene is located near two cytochrome P450 pseudogenes on chromosome 22q13.1. Alternatively spliced transcript variants encoding different isoforms have been found for this gene.

Genotype/phenotype variability
CYP2D6 shows the largest phenotypical variability amongst the CYPs, largely due to genetic polymorphism. The genotype accounts for normal, reduced and non-existent CYP2D6 function in subjects.

The CYP2D6 function in any particular subject may be described as one of the following:


 * poor metaboliser - these subjects have little or no CYP2D6 function
 * intermediate metabolizers - these subjects metabolize drugs at a rate somewhere between the poor and extensive metabolizers
 * extensive metaboliser - these subjects have normal CYP2D6 function
 * ultrarapid metaboliser - these subjects have multiple copies of the CYP2D6 gene expressed, and therefore greater-than-normal CYP2D6 function

A patient's CYP2D6 phenotype is often clinically determined via the administration of debrisoquine (a selective CYP2D6 substrate) and subsequent plasma concentration assay of the debrisoquine metabolite (4-hydroxydebrisoquine). More recently, a "DNA microarray" has been developed, known as the AmpliChip, which allows the automated determination of a patient's CYP2D6 (or CYP2C19) genotype.

Genetic basis of variability
The genetic basis for extensive and poor metaboliser variability is the CYP2D6 allele, located on chromosome 22. Subjects who possess certain allelic variants will show normal, decreased or no CYP2D6 function depending on the allele.

Ethnic factors in variability
Ethnicity is a factor in the occurrence of CYP2D6 variability. The prevalence of CYP2D6 poor metabolizers is approximately 6-10% amongst white populations, but is lower in most other ethnic groups such as Asians (2%). In blacks, the frequency of poor metabolizers is greater than for whites. The occurrence of CYP2D6 ultrarapid metabolisers appears to be greater amongst Middle Eastern and North African populations.

This variability is accounted for by the differences in the prevalence of various CYP2D6 alleles amongst the populations - approximately 10% of whites appear to have the non-functional CYP2D6*4 allele while approximately 50% of Asians possess the CYP2D6*10 allele, which should produce decreased CYP2D6 function; however this still appears to be within the normal range and are still grouped as extensive metabolisers.