Genetic Damage And Repair Capacity In Vinyl Chloride Monomer-exposed Workers

Vinyl chloride monomer (CH2=CHC1, VCM) is widely used gas in industry producing polyvinyl chloride. In2011, production capacity of PVC is510million ton in the whole world, with216.22million ton in China. VCM is a certain human carcinogen and it has been proved to be a multi-organ and multi-system carcinogen. The mechanism of carcinogenesis was presumed to be related to the damage of genetic material induced by electrophilic metabolites of VCM.At present, the permissible exposure limit (PEL) of VCM in developed countries is1ppm (2.79mg/m3), while the Short-term exposure limit (STEL) and Time-weighted average (TWA) in China are25mg/m3and10mg/m3respectively. VCM exposure with long term and low level has become the focus of research. It is important to develop sensitive and effective biomarkers in order to prevent the adverse health effects of VCM.This study investigated occupational health effects in VCM-exposed workers whose exposure level was lower than the national occupational health standard. Though VCM is a human carcinogen, the majority of VCM-exposed workers do not develop neoplasm, suggesting the susceptibility is modulated. The degree of DNA damage partly depends on the body’s metabolic ability, but the DNA repair capacity (DRC) also plays an essential role in maintaining the stability of genome and normal function of cell activity, which certainly has impact on the susceptibility to environmental hazards and some diseases. If DRC defects or is at relatively low level, the damaged DNA will not be repaired timely and effectively, which can lead to the genetic mutation or carcinogenesis, and increase the individual’s risk of suffering from cancer.Occupational epidemiological study and molecular biologic technology were performed to investigate the relationship between DNA damage induced by VCM and the cumulative exposure dose by both cross-sectional study and corhort study, and to provide scientific evidence to the early biomarker of health effect and the mechanism of cancer induced by VCM. Occupational epidemiological study was performed to investigate the relationship between chromosome damage induced by VCM and polymorphisms of genes related cell-cycle control related genes, in order to provide scientific evidence to screen the susceptible workers.In order to explore effect biomarkers under low level VCM exposure, this study evaluated the relationship between the cumulative exposure dose and chromosome damage in exposure groups (309individuals:male224and female85) and control groups (149individuals:male61and female88) of the company as well as health groups (25individuals:male14and female11), Cytokinesis-block micronucleus (CB-MN) assay were used to detect chromosome damage of the peripheral blood lymphocyte in all subjects. The results showed that the frequency of CB-MN in VCM-exposed group were significantly higher than the control groups FR=3.24(95%CI2.80-3.75), p<0.001. So the frequency of CB-MN of peripheral blood lymphocyte can be used as an effect biomarker under low level VCM exposure.To estimate benchmark doses (BMDs) for persistent chromosome damage induced by exposure to vinyl chloride monomer (VCM), an occupational cohort was conducted in91workers who had been occupationally exposed to VCM for at least one year participated in the study in2004and were followed up in2007. Results showed cumulative exposure dose and MN frequency had dose-response relationship. The95%lower confidence limit for BMD (BMDL10) was found to be0.99and3.38mg/m3-year based on2004data and0.63and1.27mg/m3-year using2004and2007data for males and females, respectively. The BMDL dose in this study provided possible reference for further national standard revision.The yH2AX has been widely acknowledged as a sensitive indictor to reflect the stress response of DNA damage, after DNA double-strand breaks occurred, the histone H2A family member, H2AX, serine139residue phosphorylate rapidly, forming y-H2AX. Flow cytometer (FCM) assay was performed to investigate whether VCM exposure can induce DNA damage and cell apoptosis. Exposure groups consisted of55workers of the factory, and control group consisted of46individuals of non-occupational exposure to harmful factors.The results showed that DNA damage rate and geometric mean fluorescence intensity in exposure group was significantly higher than that in the control group (p<0.05).The upper bound lymphocyte apoptosis rates of95%subjects was (4.84%) in the control group defined as the threshold to determine positive lymphocyte apoptosis. The positive rate of lymphocyte apoptosis in control,low and high exposure groups were increased with the exposure levels (p<0.05). VCM exposure can result in an increase possibility of lymphocyte apoptosis and DNA damage. DNA damage rate and apoptosis occurrence of peripheral blood lymphocyte can be used as an effect biomarker under low level VCM exposure.Next, this study focus on the DNA repair capacity of43VCM-exposed workers and17workers unexposed to VCM. We found that the3AB index shows the decreasing tendency with the increasing exposure level, which pointed the VCM exposure can lower DNA repair capacity. Besides, this study shows that DNA repair capacity is a comprehensive indicator with great potential to detect health damage induced by VCM and damage risk of susceptible workers.the frequency of MN of high DNA repair capacity group are lower than those of low DNA repair capacity group, and there was a dose-response relationship between DNA repair capacity and the frequency of MN. It indicates that DNA repair capacity might tightly associate with chromosomal damage induced by VCM. The DNA repair capacity would be worse, and the risk of chromosomal damage induced by VCM could be higher.PCR-RFLP and CRS-RFLP were used to detect cell-cycle control related genes. Using Poisson regression analysis, we analyzed the relationship between polymorphism of genes related cell-cycle control and the frequency of C B-MN.In this study,309VCM-exposed workers are the target population, and149workers who did not exposed to VCM are the control population for this study. We detected the polymorphism of five genes related cell-cycle control:p53, p21, mdm2, p14ARF Gadd45a. When the confounding factors such as age, gender, VCM exposure, drinking and smoking habits were adjusted, the MN frequency in subjects with p14ARF g.22008mutant homozygous and heterozygous is lower than their wild-type homozygous counterparts, the frequency ratio (FR) and its95%confidence interval (95%CI)were0.77(0.63～0.93)(P<0.05). However, MN frequency in subjects with p53intron3, p21Ex3+70and mdm2IVS1+309T>G mutant homozygous and heterozygous is higher than their wild-type homozygous counterparts, FR and95%CI were1.47(1.09-1.98),1.74(1.43-2.13) and1.36(1.13-1.64) respectively (P<0.05). These results suggest p14ARF g.22008variant may be protective factors while p53intron 3, p21Ex3+70and mdm2IVS1+309T>G variant may be risk factors for the chromosome damage induced by VCM.In conclusion, VCM can induce chromosome damage even when the exposure level is lower than the national occupational health standard of China; The BMDLs were lower than the current VC occupational exposure limits in China, VCM exposure can reduce DNA repair capacity; the polymorphism of genes related cell-cycle control may be associated with chromosome damage induced by VCM.