Genetic, epigenetic and environmental factors of importance for CYP1A2 catalyzed drug metabolism

Abstract

The heme containing cytochrome P450 (CYP450) enzymes participate in the phase I metabolism of drugs and other xenobiotics as well as endogenous substrates. CYP1A2 is one of the major CYP450 enzymes and account for about 13% of the total CYP450 content in the liver and is responsible for the metabolism of several clinically used drugs, e.g clozapine, olanzapine and theophylline. Even though the CYP1A2 gene is relatively conserved it shows a wide interindividual and ethnic differences in enzyme activity.

The aims of this PhD thesis were to investigate the importance of genetic and environmental factors in the regulation of CYP1A2 gene expression and enzyme activity. We further wanted to elucidate the role of CYP1A2, multi drug resistant (MDR1), and uridine diphosphate glucuronosyltransferase 1A4 (UGT1A4) genetic variants for the metabolism of olanzapine in schizophrenic patients.

Comparison of CYP1A2 enzyme activity in healthy Swedes with Koreans using the paraxanthine (17X)/ caffeine (137X) ratio, indicated that Swedes had a 1.54- fold higher 17X/137 ratio, and hence higher enzyme activity compared to Koreans (p<0.0001). In both Swedes and Koreans, smokers had significantly higher CYP1A2 activity than non-smokersand oral contraceptive (OC) users had significantly lower CYP1A2 activity. Haplotype CYP1A2*1F was found to be associated with higher enzyme inducibility in Swedish smokers. There were no significant gender differences in enzyme activity in neither Swedes nor Koreans. None of the investigated CYP1A2 haplotypes are critical for variation in enzyme activity, except for CYP1A2*1F.

The effect of daily coffee consumption on CYP1A2 activity was studied in healthy Swedes and Serbs. Daily consumption of at least three cups of coffee significantly increased CYP1A2 enzyme activity among non-smokers and non-OC users in both populations. Swedes had significantly higher enzyme activity than Serbs when controlling for the effects of smoking and heavy coffee consumption. About 48% of the human livers displayed significant allele-specific expression (ASE) with no significant correlation to CYP1A2 mRNA expression. The mean DNA methylation of a CpG site in exon 2 was inversely correlated with CYP1A2 mRNA expression (p=0.018). The results imply that ASE and DNA methylation are important contributing factors in the regulation of CYP1A2 gene expression.

Smokers had significantly lower plasma olanzapine concentration to-daily dose ratio (C/D) than non-smokers (p<0.003) and men had significantly lower C/D than females (p<0.005). Investigating for the effect of one factor while controlling for other factors by using regression analysis, we found that male gender, 1 hour increase in time between last dose and sampling, smoking, CYP1A2*1F haplotype and UGT1A4 142T>G allele, each predicting a significant decrease in dose normalised plasma concentration of olanzapine of 29%, 2%, 19%, 10% and 24%, respectively, thus explaining only 30% of the interindividual variation in plasma C/D. The CYP1A2 gene expression is regulated by genetic, epigenetic and environmental factors contributing to the observed wide interindividual variation in CYP1A2 enzyme activity.