Genetic Susceptibility Of Chromosomal Damage Induced By 1, 3-butadiene-exposed Workers

Master candidate:Mr. TAN, Hong-shan Supervisor:Prof. XIA, Zhao-lin1,3-butadiene (BD) is a monomer chemical widely used in the production of a wide range of polymers, including the production of rubber and thermoplastic resins. It is a colorless flammable gas at room temperature. BD is also commonly found in automobile exhaust and cigarette smoke at much lower concentration compared to industrial emissions. It is estimated that the total amount of productive capacity for BD is approximately 12 million tons worldwide in 2006, in China there are nearly 1.61 million tons. It is necessary to pay more attention to the health of BD-exposed workers in our country.As a confirmed human carcinogen, BD would increase the risk of human lymphatic and hematopoietic cancers. However, BD itself is not a carcinogen; its genotoxicity is attributed to its metabolites, which can be conjugated with DNA, RNA and other macromolecular protein to induce cancer. At present, the permissible exposure limit (PEL) of BD in developed countries is 1 ppm (2.2 mg/m3), while the permissible concentration-time weighted average (PC-TWA) in China is 5 mg/m3. So it is necessary to analyze occupational health effects in BD-exposed workers whose exposure level was two-fold than the occupational health standard in developed countries. Under the same working environment and BD-exposed level, only some BD-exposed workers had got genetic damage, which indicated that there were genetic susceptible differences among the workers. There are many reasons inducing gredients genetic damage, including age, gender, health and nutritional status, occupational and personal exposures (e.g., alcohol drinking and cigarette smoking); moreover polymorphism of metabolic and DNA repair genes. Presently, only a few studies paid attention to polymorphisms of metabolic enzyme genes on BD-induced genetic damage, especially little to DNA repair genes. Studies on relationship between genetic damage in BD-exposed workers and susceptible genotypes of metabolic and/or DNA repair genes would be useful for revealing mechanism of toxicology and evaluation to health risk.This study used the questionnaire, physical examination, cytokinesis-blocked micronucleus (CBMN) assay and sister chromatid exchange (SCE) assay to evaluate the occupational health and genetic damage of 164 BD-exposed workers with 41 healthy controls. Through calculating cumulative exposure dose to evaluate the relationship between the cumulative exposure dose and genetic damage. Moreover, the genotypes of metabolic and/or DNA repair genes were used to find biomarkers of susceptibility of genetic damage induced by BD-exposure.The result of CBMN assay showed that BD-exposed workers got higher MN frequency than controls (P<0.01). Within the exposed workers, Poisson regression analyses demonstrated that the workers with high BD exposure had a significantly increased MN frequency, compared with those from low-exposure group (P＜0.05). However, no significant difference in MN frequency was found in term of gender, age, smoking or alcohol drinking status. Through the sister-chromatid exchange assay, the high exposure workers were found to have higher SCE frequency than low exposure group, but the difference was not significant. Similar response was found in age, sex, smoking and alcohol status. Our research suggests that the frequency of CB-MN of peripheral blood lymphocyte can be used as an effect biomarker under low level BD exposure.Next, PCR and PCR-RFLP were used to detect polymorphisms of twelve polymorphisms of seven genes. Using Poisson regression analysis, we analyzed the relationship of polymorphism of the metabolic and DNA repair genes between the frequency of MN and SCE. The PHASE 2.0.2 software was used to obtain maximum-likelihood estimates of the haplotype frequencies. In this study, we detected the polymorphism of three metabolic genes:GSTT1, GSTM1, CYP2E1, mEH 113 and mEH 139; four DNA repair genes which participate in the pathways of BER: XRCC1 194, XRCC1 280, XRCC1 399, XRCC1-77, APE1 148, MGMT 84, and ADPRT 762.Using backward stepwise selection method, Multivariate Poisson regression analysis about MN frequency was performed. Ultimately, gender, cumulative BD exposure, GSTM1 (+), CYP2E1 and mEH were enrolled in the model. When adjusted the confounding factors, MN frequency has a relationship with gender, that female workers got higher MN frequency than male (FR=1.72,95%CI:1.20-2.41, P<0.05); higher BD cumulative dose that had an increased MN frequency compared to the lower dose group (FR=1.31,95%CI:1.06-1.62, P＜0.05); genotypes that individuals carrying GSTM1 (+), CYP2E1 c1c2/c2c2, mEH intermediate (I) group and XRCC1 399 AG/AA polymorphisms had a significantly higher MN frequency than those carrying GSTM1 (-), CYP2E1 c1c1, mEH high (H) group and XRCC1 399 GG, respectively. FRs were 1.23 (95%CI:1.01-1.51),1.43 (95%CI:1.15-1.77),1.53 (95%CI:1.04-2.29) and 1.50 (95%CI:1.02-2.24), respectively. Our founding consider that gender, cumulative exposure, and GSTM1, CYP2E1, mEH, and XRCC1 399 genotypes are the major factors that modulate MN induction in BD-exposed workers. Diplotype analysis of XRCC1 194,280,399 and-77 demonstrated that the MN frequency in subjects with TCAG/TTGG diplotype was significantly higher than that in subjects with CCGG/CCGG (P<0.05), and TCAG/TTGG diplotype is potentially one of the risk factors for chromosomal damage in BD-exposed workers. However, our study did not found the significant difference between SCE and those genotypes.In conclusion, BD can induce genetic damage of workers whose exposure level was lower than the national occupational health standard. The MN frequency can be a sensitive index of early damage among BD exposed workers. We should pay attention to the genotoxic effect from the aspects such as hereditary factors, lifestyles and BD exposure. The polymorphisms of metabolic genes and DNA repair genes are associated with chromosomal damage induced by BD exposure, which could help us to understand the carcinogenesis mechanisms of BD and find the biomarkers of susceptible individuals.