Cytochromes P450 and drug metabolism polymorphisms ... (Polymorphisms of cytochromes P450 and Drug methabolism) - Genes CYP2C9, CYP2C19 and CYP2D6

The P450 cytochrome system is a superfamily of monooxygenases present in animals, plants and prokaryotes. The human genome encodes 57 isoenzymes of this superfamily. These enzymes are involved in the synthesis and metabolism of various types of intracellular molecules. Involved in the synthesis of molecules such as steroid hormones, some lipids (cholesterol and fatty acids) and acids for fat digestion (bile acids). In addition, these enzymes metabolize foreign molecules such as drugs given. In mammals, these enzymes are found mainly in the endoplasmic reticulum and in the mitochondria of hepatic cells and small intestine, but also found in other body cells. Enzymes that are in the endoplasmic reticulum are used mainly for the metabolism of foreign molecules, for example drugs, while those found in the mitochondria involved generally in the synthesis and metabolism of endogenous substances.

Genetic polymorphisms may affect P450 genes function enzymes encoded by them, and therefore, these genetic variations may affect the degradation of drugs administered. According to the gene involved, and polymorphism, drugs can be metabolized to form small (PM: Poor metabolizers), fast (MS: Extensive metabolizers), or intermediate (IM: Intermediate metabolizers). When a polymorphism entails a poor metabolisers, the drug remains active longer, and less dosage is needed to obtain the desired effect; whereas if a normal dose is administered, will probably cause undesirable side effects. Conversely, when the polymorphism leads to a rapid degradation, a higher dose is required to obtain the same effects. That is, the duration of action of many drugs depends on the rate of inactivation by the P450 system, so that the protective action of removal of foreign substances (drugs), by citocrómicos P450 enzymes is not always beneficial.

Each cytochrome P450 gene is called with the acronym CYP, indicating that it is part of the P450 gene superfamily. Each cytochrome P450 gene subfamilies is involved in metabolizing a group of molecules. The main families involved in metabolizing molecules are: CYP1A2, CYP2B6, CYP2C8, CYP2C9, CYP2C19, CYP2D6, CYP2E1 and CYP3A4,5,7. Of these genes, the most involved in drug metabolism are CYP2C9, CYP2C19 and CYP2D6.

CYP2C9 gene, located on the long arm of chromosome 10 (10q24) belongs to CYP2C subfamily of cytochrome oxidase system mixed function P-450, so it is actively involved in xenobiotic metabolism. In fact, she involved in the metabolism of some groups of drugs very relevant for clinical use, some of which with a very narrow therapeutic range, including oral anticoagulant acenocoumarol (Sintrom) include, various nonsteroidal antiinflammatory drugs (NSAIDs) , as hypoglycemic sulfonylurea, phenytoin, warfarin, etc. CYP2C9 catalyzes most of these substrates through hydroxylation reactions.

A high percentage of the Spanish company carries genetic alterations in the gene encoding CYP2C9, which have been described so far 25 variants (although not have a significant effect on the ability to metabolize drugs). These mutations consist absence or complete gene duplication and alteration of a single nucleotide alteration can cause, reduction, absence or increase of enzyme activity. Thus, these genetic abnormalities are associated with pharmacodynamic changes, changes in the therapeutic response and adverse effects. Two examples of this are the CYP2C9 variants that predispose to bleeding with warfarin or NSAID treatments. The three major allelic variants are CYP2C9 * 1, * 2 and CYP2C9 CYP2C9 * 3. The first corresponds to an enzyme activity of 100% and not accompanied by alterations, that is, it is the variant "wild type". The second variant, CYP2C9 * 2, has a population frequency of 17% and generates a decreased activity of cytochrome because less interaction with the cofactor. It presents an efficiency of 12% compared to the unmutated variant and occurs by mutation (Arg144Cys or R144C) located in exon 3 of the CYP2C9 gene. The third variant, CYP2C9 * 3, has a frequency of 6% in the population and is due to a mutation (I357L) located in exon 7 of CYP2C9 gene. It is characterized by causing a decrease in enzyme activity -has an efficiency of 5% compared to the originally due to variations in the substrate binding site.

Meanwhile, the CYP2C19 gene, located on the long arm of chromosome 10 (10q24), encoding an enzyme of clinical significance, that metabolizes a wide variety of drugs as varied as anticonvulsant activity, antiulcer, antidepressants, antifungal and antimalarial. When there are polymorphisms in the gene results in low metabolism (PM) of these drugs. They described the following alleles: CYP2C19 and CYP2C19 * * 1A 1B (wild alleles); CYP2C19 * 2 and CYP2C19 * 3 poor metabolisers (PM) (W212X: trp212ter, premature termination codon in nucleotide 636 of exon 4, and possibly other mutations such as the C-639G, to be determined by sequencing). CYP2C19 * 4 allele, with a defect in the initiation codon (AG mutation: met1val), represents an additional 3% by defective alleles (PM) of the Caucasian population. * 5A alleles CYP2C19 (R433W mutation: arg433-trp) and CYP2C19 * 5B allele also considered poor metabolizers (PM). All these alleles CYP2C19 * 2, * 3, * 4, * 5A and * 5B generate inactive enzymes, from the point of view of metabolizing drugs.

Finally, the CYP2D6 gene, located on the long arm of chromosome 22 (22q13.1), encodes an enzyme with a very prominent role in metabolizing many clinically important drugs such as antidepressants and antipsychotics, neuroleptics, some antiarrhythmics, ?-blockers and opioids. In fact, 25% of the drugs can serve as their own substrate. However, only 30-60% of patients respond adequately to treatment with antidepressants, ?-blockers, statins and antipsychotics. On the other hand, adverse drug reactions are much more common than would be desirable, representing 7% of hospital admissions -this figure increases to 30% in cases of elderly over 70 years and cause an estimate 100,000 annual deaths in the United States only. These differences in drug response is due mainly to differences in the activity of cytochrome P450 involved in drug metabolism.

Four CYP2D6 phenotypes that correspond to more than 80 allelic variants known CYP2D6 gene are distinguished. Three of them correspond to pathological phenotypes: ultrafast metabolizers, intermediate metabolizers and poor metabolizers. The remaining phenotype, called "metabolizers" is the one with the two functional alleles unchanged, no clinical consequence and response to medication is administered, in this case, ordinary. Mutations that result registered variants mentioned above are nucleotide, insertions, deletions and alterations of copy number.

The phenotype of the "poor metabolizers", present in 7-10% of the population, has its origin in autosomal recessive disorders in the gene encoding the enzyme CYP2D6. It results, therefore, the presence of two defective alleles that result in poor CYP2D6 activity. Individual who has presented the most likely adverse reaction and usually require smaller doses. In the case of the "ultrarapid metabolizers" phenotypic effects are very similar: no response to treatment and increased risk of adverse reaction, though slightly dependent gene variant that question. In this case, these effects are due to CYP2D6 activity extremely high. In fact, the genetic mechanisms that give rise to the so - called "ultrarapid metabolizers" are well other than the above. Mutations are responsible gene duplication, so that two or more copies of the gene. This variant is especially common in Asian and African individuals. In Europe, its incidence is generally lower, except in Spain and Italy, where it is around 7-10%. The "intermediate metabolizers" are due to the presence of a defective allele or two defective alleles partially. The clinical implications in these cases also depend on the variant, but generally larger doses are required to achieve minor effects.

Tests in IVAMI: IVAMI performed in detecting polymorphisms of CYP2C9, CYP2C19 gene, and CYP2D6 by the complete PCR amplification of exons of each of the genes, and subsequent sequencing.

Samples recommended: EDTA blood collected for separation of blood leukocytes, or impregnated sample card with dried blood (IVAMI may mail the card to deposit the blood sample).