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Interpretive Handbook

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Test 89401 :
Cytochrome P450 1A2 Genotype

Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test

Primary metabolism of many drugs is performed by cytochrome P450 (CYP), a group of oxidative/dealkylating enzymes localized in the microsomes of many tissues including the intestines and liver. One of these CYP enzymes, CYP1A2, is wholly or partially responsible for the hydroxylation or dealkylation of many commonly prescribed drugs (see above). The current clinical application of this test is focused on the impact of allelic variation on antidepressant and antipsychotic metabolism.

 

CYP1A2-mediated drug metabolism is highly variable. CYP1A2*1A is the wildtype or normal allele. Some individuals have altered CYP1A2 gene sequences that result in synthesis defective enzyme. These individuals metabolize CYP1A2 substrates poorly. Changes in the promoter impacting gene induction of the CYP1A2 gene has been observed, which results in either an increase or decrease of overall metabolic activity. Dosing of drugs that are metabolized through CYP1A2 may require adjustment based on the individual patient's genotype. Patients who are poor metabolizers may require lower than usual doses to achieve optimal response. Patients who are ultrarapid metabolizers may benefit from increased doses. Patients with either ultrarapid or poor metabolism also may benefit by conversion to other comparable drugs that are not primarily metabolized by CYP1A2.

 

A number of specific polymorphisms have been found in the CYP1A2 gene that results in enzymatic deficiencies. The frequency of these polymorphisms varies within the major ethnic groups.

 

All of the identified polymorphisms associated with CYP1A2 are autosomal recessive. Consequently, only individuals who are homozygous or compound heterozygous for these polymorphisms are poor metabolizers. Individuals who are heterozygous, with 1 normal gene and 1 defective polymorphic gene, will have metabolism intermediate between the extensive (normal) and poor metabolizers.

 

The following information outlines the relationship between the polymorphisms detected in this assay and the effect on the activity of the enzyme produced by that allele:

 

Nucleotide Change


Effect on Enzyme Metabolism*

-3860G->A

Lower inducibility in Asians but increased inducibility in Northern Europeans

-2467T->del T

Increased inducibility

-729C->T

Decreased activity and decreased inducibility

-163C->A

Increased inducibility

125C->G

Greatly reduced activity

558C->A

Greatly reduced activity

2385G->A

Decreased activity

2473G->A

Greatly reduced activity

2499A->T

Decreased activity

3497G->A

Decreased activity

3533G->A

No activity

5090C->T

Greatly reduced activity

5166G->A

Decreased activity

 

*Effect of a specific polymorphism on the activity of the CYP1A2 enzyme can only be estimated since the literature does not provide precise data.

 

A complicating factor in correlating CYP1A2 genotype with phenotype is that some drugs or their metabolites are inhibitors of CYP1A2 catalytic activity. These drugs may reduce CYP1A2 catalytic activity. Consequently, an individual may require a dosing decrease greater than predicted based upon genotype alone. Another complicating factor is that the CYP1A2 gene is inducible by several drugs and environmental agents (eg, cigarette smoke) and the degree of inducibility is under genetic control. It is important to interpret the results of testing in the context of other coadministered drugs and environmental factors.

 

Cytochrome P450 Patient Education Brochure (Supply T526) is available upon request.

Useful For Suggests clinical disorders or settings where the test may be helpful

Identifying patients who are poor, intermediate, extensive, or ultrarapid metabolizers of drugs metabolized by CYP1A2

 

Adjusting dosages for drugs that are metabolized by CYP1A2

Interpretation Provides information to assist in interpretation of the test results

An interpretive report will be provided that includes assay information, genotype, and an interpretation indicating whether results are consistent with a poor, intermediate, extensive, or ultra-rapid metabolizer phenotype. The report will list drugs known to affect metabolism by CYP1A2. Direct polymorphism analysis for -3860G->A, -2467T->del T, -729C->T, -163C->A, 125C->G, 558C->A, 2385G->A, 2473G->A, 2499A->T, 3497G->A, 3533G->A, 5090C->T, or 5166G->A is performed following PCR amplification. Direct DNA testing will not detect all the known mutations that result in decreased or inactive CYP1A2 alleles. This assay does not test for some known polymorphisms because those polymorphisms have not been associated with alterations in enzymatic activity. Rare or undescribed variants may not have been found during validation but will be sequence verified upon detection. See http:///www.cypalleles.ki.se/cyp1a2.html for a full description of CYP1A2 alleles. Absence of a detectable gene mutation or polymorphism does not rule out the possibility that a patient has a metabolizer status other than predicted above. The frequency of polymorphisms causing poor metabolism has not been fully characterized in various ethnic groups. Patients with an ultrarapid, extensive, or intermediate genotype may have CYP1A2 enzyme activity inhibited or induced by a variety of substances, medications, or their metabolites. The following is a listing of substances known to affect CYP1A2 activity as the date of this report.

 

Drugs and substances known to increase (induce) CYP1A2 activity include: Broccoli, brussel sprouts, char-grilled meat, insulin, methylcholanthrene, modafinil, nafcillin, beta-naphthoflavone, omeprazole, and tobacco

Coadministration will increase the rate of metabolism of CYP1A2 metabolized drugs and may change the effectiveness of the drug.

 

Drugs and substances known to decrease CYP1A2 activity include: Amiodarone, cimetidine, ciprofloxacin, fluoroquinolones, fluvoxamine, furafylline, interferon, methoxsalen, and mibefradil

Coadministration will decrease the rate of metabolism of CYP1A2 metabolized drugs, increasing the possibility of toxicity.

 

Drugs and substances that undergo metabolism by CYP1A2 include: Acetaminophen, amitriptyline, caffeine, clomipramine, clozapine, cyclobenzaprine, estradiol, fluvoxamine, haloperidol, imipramine, mexiletine, naproxen, olanzapine, ondansetron, phenacetin, propranolol, riluzole, ropivacaine, tacrine, theophylline, tizanidine, verapamil, (R)warfarin, zileuton, and zolmitriptan

Coadministration may decrease the rate of elimination of other drugs metabolized by CYP1A2.

 

Drug-drug interactions and drug/metabolite inhibition or induction must be considered when dealing with heterozygous individuals. Drug/metabolite inhibition occurs with the drugs noted above resulting in inhibition of residual functional CYP1A2 catalytic activity. Drug/metabolite induction occurs with the drugs noted above resulting in variable increase in CYP1A2 enzyme function. This inducibility is under genetic control and this is further varies per ethnicity. Each report will include a list of commonly prescribed drugs that are known to alter CYP1A2 activity. This list includes only those drugs for which established, peer-reviewed literature substantiates the effect. The list provided may not be all-inclusive.

 

CYP1A2 activity also is dependent upon hepatic and renal function status, as well as age. Patients also may develop toxicity if hepatic or renal function is decreased. Drug metabolism also is known to decrease with age. It is important to interpret the results of testing and dose adjustments in the context of hepatic and renal function and age.

Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances

Patients who have received a heterologous blood transfusion within the preceding 6 weeks, or who have received an allogeneic blood or marrow transplant, can have inaccurate genetic test results due to presence of donor DNA. 

 

CYP1A2 genetic test results in patients who have undergone liver transplantation may not accurately reflect the patient's CYP1A2 status.

 

Direct DNA testing will not detect all the known mutations that result in decreased or inactive CYP1A2. Absence of a detectable gene mutation or polymorphism does not rule out the possibility that a patient has an intermediate or poor metabolizer phenotype.

 

This test does not detect polymorphisms other than those listed. Other polymorphisms in the primer binding regions can affect the testing, and ultimately, the genotyping assessments made.

 

Patients with an extensive or intermediate metabolizer genotype may have CYP1A2 enzyme activity inhibited by a variety of medications or their metabolites as described previously. Treatment with drugs that are inhibitors of CYP1A2, or produce inhibitors through metabolism, may generate a poor metabolizer phenotype in an individual who has an ultrarapid, extensive, or intermediate metabolizer genotype.

 

The drug application that we currently support for testing and interpretation is for the treatment of psychiatric disorders.

Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.

An interpretive report will be provided.

Clinical References Provides recommendations for further in-depth reading of a clinical nature

1. Shirley KL, Hon YY, Penzak SR, et al: Correlation of cytochrome P450 (CYP) 1A2 activity using caffeine phenotyping and olanzapine disposition in healthy volunteers. Neuropsychopharmacology 2003;28(5):961-966

2. Shimoda K, Someya T, Morita S, et al: Lack of impact of CYP1A2 genetic polymorphism (C/A polymorphism at position 734 in intron 1 and G/A polymorphism at position -2964 in the 5'-flanking region of CYP1A2) on the plasma concentration of haloperidol in smoking male Japanese with schizophrenia. Prog Neuropsychopharmacol Biol Psychiatry 2002;26(2):261-265

3. Obase Y, Shimoda T, Kawano T, et al: Polymorphisms in the CYP1A2 gene and theophylline metabolism in patients with asthma. Clin Pharmacol Ther 2003;73(5):468-474

4. Cornelis MC, El-Sohemy A, Kabagambe EK, et al: CYP1A2 genotype, and risk of myocardial infarction. JAMA 2006 Aug 16;296(7):764-765


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