Association Between Methylation Of DNA Repair Genes And Genetic Damage Among Vinyl Chloride-exposed Workers

Vinyl chloride (CH2=CHC1, VC), a synthetic chemical with gas state at room temperature and normal atmospheric press, is colorless, flammable, explosive and slightly fragrant odorous. As an important chemical raw material, VC is mainly used to make polyvinyl chloride (PVC) in industry. The production of PVC in China began in 1957. By the end of 2013, PVC production has reached 15.2954 million tons per year, ranking the first in the world. As a major PVC production and usage country, China has a huge number of occupational population exposed to VC, raising the concern that whether VC exposure has been harmful to their health.Epidemiological studies have indicated that VC is a multi-organ and multi-system carcinogen that induces ASL as well as other tumors. It was classified as a group 1 carcinogen by IARC in 1987. Because VC is a known carcinogen, great concern has been given to occupational people exposed to it. To protect the health of workers, many countries have taken a series of measures to improve the health conditions in workplace, such as developing new techniques, strengthening the management, and reducing the exposure limitation of VC. At present, the permissible exposure limit (PEL) of VC in most developed countries is 1 ppm (2.79mg/m3), while the permissible concentration-time weighted average (PC-TWA) in China is 10mg/m3. Therefore, it is likely not sufficient to protect the workers under the VC exposure limit of China. Existing studies have shown that low doses of VC exposure can also cause genetic damages in workers, which is a prerequisite for VC carcinogenesis. However, the pathogenesis mechanism of genetic damage is not yet clear, making it necessary to perform researches to reveal mechanism of VC toxicology and assess health risk.In studies of mechanisms of genetic damages by environmental chemicals, the emphasis of epigenetic regulation has been accumulated, of which DNA methylation is the most widely studied epigenetic mechanism by far, and a major epigenetic modification form of genomic DNA in regulating genome function. Abnormal DNA methylation is closely related to tumor pathogenesis, of which the promoter CpG island methylation has become a hot topic in the field of cancer research. DNA repair is a process in living cells responsible for maintenance of structural integrity of DNA in coping with damages that arises from environmental insults and normal metabolic processes. Methylation of DNA repair genes may inhibit their activation, causing dysfunction of DNA repair capacity (DRC). Suboptimal DRC may be associated with increased risk of chromosomal damage and initiate the pathogenesis and development of tumor.In this study we estimated the cumulative exposure dose (CED) of VC workers and assessed the relationship between CED and genetic damages (CBMN). We also measured methylation of DNA repair enzyme promoter region and evaluated their association with VC-induced genetic damages to further explore the mechanism of VC carcinogenesis.Firstly, we investigated the genetic damages of VC workers below our current health standard concentration. Exposure group was comprised of 155 workers occupational ly exposed to VC for over one year.107 workers from the same factory and 74 volunteers from a college, all of whom had no history of VC or other known toxicant exposure, were recruited as internal and external controls, respectively. The Cytokinesis-block micronucleus (CBMN) assay results indicated that (1) The mean MN frequency for the total VC exposure group (7.54±3.93)%o was significantly higher than that of the control group (3.42±2.40)%o. After adjusting for potential confounding factors by multiple Poisson regression, the FR and its 95%CI was 2.35(2.13～2.61). (2) To explore the effects of different exposure doses, we additionally divided our exposure group into high-exposure, middle-exposure and low-exposure groups with a cumulative dose of> 12893.16 mg,775.84-12893.16 mg and≤775.84 mg, respectively. We found that the MN frequencies of the five groups were significantly different, of which all of the three exposure groups were higher than the external control group.Then we extracted DNA and measured the promoter methylation status of MGMT and three common genes of MMR (hMLHl, hMSH2 and hMSH3) by MSP on 138 VC-exposed workers,60 internal controls and 35 external controls. We found that the methylation rate of MGMT and hMLH1 in VC exposure group were significantly higher than that of the control group, with OR (95%CI) values 7.27(3.80-13.89) and 2.56(1.45-4.53), respectively; after adjusting for potential confounding factors, they were 8.44 (4.11-17.33) and 2.52(1.37-4.62), respectively.Using the CBMN as an indicator of chromosome damage, we analyzed the relationship between VC-induced methylation of MGMT and hMLHl and genetic damage in the 138 VC-exposed workers. The results indicated that:The MN frequency of the methylated were significantly higher than that of the unmethylated for MGMTand hMLHl (P<0.05, with FRs were 1.28(1.05-1.57) and 1.16(1.03-1.31), respectively). Multiple Poisson indicated that the adjusted FRs were 1.31(1.06-1.61) and 1.17(1.03-1.33), respectively.In conclusion, VC can still cause genetic damages and alterations of DNA repair gene methylation status for workers whose exposure level was lower than the national occupational health standard. Additionally, the hypermethylation of MGMT and hMLHl was associated with VC-induced genetic damages. Our study will improve the understanding of the mechanism of VC carcinogenesis and promote the search for early and sensitive biomarkers for workers exposed to VC.