Relation of PON1 and CYP1A1 genetic polymorphisms to clinical findings in a cross-sectional study of a Greek rural population professionally exposed to pesticides

Abstract

Allelic variants of CYP1A1 and PON1 have been extensively studied as susceptibility factors in toxic response, although little is known about the role of these variants as risk factors for the plethora of diseases appearing in the human population. In this study we investigated the hypothesis of correlation of CYP1A1 and PON1 enzymes with the incidence of various medical examination findings in a Greek rural population professionally exposed to a variety of pesticides. The medical history of 492 individuals, randomly selected for the total population of 42,000, was acquired by interviews and their genotype determined for the CYP1A1*2A, PON1 M/L and PON1 Q/R polymorphisms. The assessment of exposure to pesticides of the population was verified by analytical methods. Analysis of the genetic data revealed that the allele frequencies of PON1 R, M and CYP1A1*2A alleles were 0.243, 0.39 and 0.107 respectively. The CYP1A1*2A polymorphism was found to have significant association with chronic obstructive pneumonopathy (p = 0.045), peripheral circulatory problems (trend p = 0.042), arteritis (p = 0.022), allergies (trend p = 0.046), hemorrhoids (trend p = 0.026), allergic dermatitis (p = 0.0016) and miscarriages (p = 0.012). The PON1 Q/R polymorphism was found to have significant association with hypertension (p = 0.046) and chronic constipation (p = 0.028) whereas, the L/M polymorphism, with diabetes (p = 0.036), arteritis (trend p = 0.022) and hemorrhoids (trend p = 0.027). Our results demonstrated an association between the CYP1A1/PON1 polymorphisms and several medical examination findings, thus indicating the possible involvement of the human detoxification system to health effects in a rural population exposed professionally to pesticides.

Introduction

Pesticides are a diverse class of chemicals, widely used in agricultural cultivation for the protection and improvement of the quality of the products. Workers employed in the manufacture and application of pesticides are the most highly exposed group (Tsatsakis et al., 2001). While acute exposure to pesticides usually leads to intoxication with well defined and studied symptoms and only concerns a small part of the population, the interest of public health officials is focused in understanding and recording the effects of low level long term exposure to pesticides.

The cytochrome P450 and the paraoxonase gene family comprise the major enzymatic detoxification system for xenobiotics in mammals. CYP1A1 is a major hepatic and extrahepatic cytochrome P450 and it encodes the aryl hydrocarbon hydroxylase (AHH). It contributes greatly to the toxicity of chemicals, such as polycyclic aromatic hydrocarbon (PAHs), since it is responsible for the creation of the toxic DNA binding forms; catalyzing the first step of the metabolism of these substances to electrophilic compounds (Shimada et al., 1996). CYP1A1 constitutive expression is negligible, but high levels of CYP1A1 mRNA, protein and enzyme activity are detectable following induction by AhR ligands in almost every tissue studied; including lung, lymphocytes, mammary gland, and placenta (Raunio et al., 1995). Most inducers are in turn metabolized by CYP1A1. At the present time, 15 allelic variants of CYP1A1 have been characterized. The first polymorphism found (CYP1A1*2A) involved the transition of thymidine to cytosine at position 3801 (3801T>C) downstream from the polyadenylation site in the 3′ non-coding region (3′ UTR) of the CYP1A1 structural gene (Kawajiri et al., 1990). The metabolic consequences of this non-coding region substitution are not clear, although there is some evidence of increased ethoxyresorufin O-deethylation (EROD) activity in the lymphocytes of individuals after treatment with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), in the absence of increased mRNA levels (Landi et al., 1994). However, other studies detected no effects on AHH enzymatic activity or gene expression for the CYP1A1*2A allele (Crofts et al., 1994, Persson et al., 1997, Smart and Daly, 2000).

Serum paraoxonase I (PON1) belongs to a family of paraoxonases which has a total of three members located in chromosomal region 7q21.3-22.1 (Hassett et al., 1991). All paraoxonase genes share high sequence similarity (>60%) and normally accommodate functions related to lipid metabolism and homeostasis. PON1 is synthesized in the liver, enters the blood circulation and associates with high density serum lipoproteins. PON1 has been shown to have a protective role in cardiovascular disease (CVD). The exact mechanism is not known but the main hypothesis states that PON1 is capable of hydrolyzing lipid peroxides in low density lipoprotein (LDL) (Mackness et al., 1991), thereby preventing LDL uptake by macrophages during the atherogenic process, as well as downplaying its proinflamatory properties (Mertens and Holvoet, 2001). Furthermore, PON1 has been shown to protect high density lipoprotein (HDL) from oxidation thus preserving its function (Aviram et al., 1998). The hypothesis of a protective effect in CVD for PON1 is supported by observations with PON1 deficient mice, where it was demonstrated that these mice develop atherosclerosis when fed with a high fat/cholesterol diet (Shih et al., 1998). Significant interindividual variability of human serum PON activity has been described previously. This variability is attributed to the presence of polymorphisms in the PON1 gene, amongst other factors. The two most important genetic variations that have been described are the Q192R and the L55M, both involving a single amino acid substitution. The R isoform has arginine at position 192, and the Q isoform has glutamine (Adkins et al., 1993, Humbert et al., 1993). The R isoform demonstrates sixfold higher hydrolytic activity towards paraoxon than the Q isoform(Humbert et al., 1993). Furthermore, the Q/R alleles differentially affect PON1 hydrolytic activity towards a number of substrates, significantly altering its role in a number of biological processes (Davies et al., 1996). The PON1 leucine/methionine substitution at position 54 of the amino acid sequence seems to significantly affect mainly PON1 serum concentration (Garin et al., 1997).

Allelic variants of CYP1A1 and PON1 have been extensively studied as susceptibility factors in toxic response (Autrup, 2000). To the contrary, little is known about the role of these variants as risk factors for the plethora of diseases appearing in the human population. The objective of our study was to investigate the possible correlation of CYP1A1 and PON1 enzymes in the incidence of various clinicopathological findings on a Greek rural population professionally exposed to a variety of pesticides.