Exposure Assessment And Genetic Damage In Vinyl Chloride Monomer (VCM) Exposed Workers

Vinyl chloride monomer (CH2=CHC1, VCM) is widely used in industry,95% of vinyl chloride was polymerized to polyvinyl chloride (PVC). China is one of the important PVC production countries, and its annual production accounts for about 10% of the global production. So the health of VCM exposed workers should be paid more attention.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 genetic material damage induced by electrophilic metabolites of VCM. At present, the permissible exposure limit (PEL)of VCM in developed countries is 1 ppm (2.79mg/m3), while the STEL and TWA in China are 20mg/m3 and 10mg/m3 respectively. This study analyzed occupational health effects in VCM-exposed workers whose exposure level was lower than the national occupational health standard. Since VCM is a human carcinogen, the majority of VCM-exposed workers do not develop neoplasm, suggesting the susceptibility is modulated, at least in part, by polymorphisms in genes encoding metabolic enzymes, DNA repair proteins. Many studies have been done to investigate the effect of genetic polymorphisms of genes involved in VCM metabolism. But only a few studies paid attention to the DNA repair genes, and to our knowledge, there was limited reports concerning the interaction effect of genetic polymorphisms and exposure dose.In order to explore effect biomarkers under low level VCM exposure, this study evaluated the relationship between the cumulative exposure dose and VCM-induced liver lesion, chromosome damage in exposure groups (335 individuals:male 272 and female 63) and control groups (165 indivuals:male 119 and female 46) of this company as well as health groups (41indivuals:male 20 and female 21). Occupational epidemiological study was performed to investigate the relationship between chromosome damage induced by VCM and polymorphisms of DNA damage repair genes, in order to provide scientific evidence to screen the susceptible workers.Using ALT as an indicator of liver function and liver ultrasonography to detect liver morphological changes, we found VCM exposure was not associated with ALT level. Although liver morphological changes had statistical difference between the exposure and control groups (P<0.05), this result lacked of specificity.Concentration of 8-OHdG, which is a well-known biomarker of oxidative damage in serum, was also detected in parts of workers (143 indivuals:male 129 and female 14). No difference was observed among different cumulative exposure individuals (P>0.05), even there was a statistical difference in gender (P<0.05). This substance in serum was not available to be a sensitive biomarker for VCM exposed in this study.Cytokinesis-block micronucleus (CB-MN) assay were used to detect chromosome damage and DNA damage of the peripheral blood lymphocyte in VCM-exposed workers. The results showed that the frequency of CB-MN in VCM-exposed group were significantly higher than the control groups FR=2.31 (95%CI 1.81-3.01), P<0.01. So the frequency of CB-MN of peripheral blood lymphocyte can be used as an effect biomarker under low level VCM exposure.Moreover, the dose-response relationship between cumulative exposure dose and CBMN frequency in all individuals was analyzed with benchmark dose (BMD) methods. Results showed cumulative exposure dose and MN frequency had dose-response relationship, also the benchmark dose low (BMDL) for male and female was 6.37g and 5.29g respectively. Female had more susceptibility for CBMN damage than male workers. The BMDL dose in this study provided possible reference for further national standard.Next, PCR-RFLP and CRS-RFLP were used to detect polymorphisms of DNA repair genes related to VCM-induced DNA damage. Using Poisson regression analysis, we analyzed the relationship between polymorphism of DNA repair genes and the frequency of CB-MN. 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 eight DNA repair genes:XRCC1,APE1,XPC,XPG,XPA,ERCC1,XPF,XPD, which play important role in the pathways of base excision repair (BER) and nucleus excision repair (NER).When the confounding factors such as age, gender, VCM exposure, drinking and smoking habits were adjusted, the MN frequency in subjects with XRCC1 T-77C, XRCC1Arg194Trp, XRCC1Arg280His, XRCC1Arg399Gln mutant homozygous and heterozygous is higher than their homozygous counterparts (P<0.05). XRCC1-77CT and CC, FR=1.29 (95%CI 1.14-1.47) P<0.01; XRCC1 194 Arg/Trp and Trp/Trp, FR=1.17 (95%CI 1.04-1.32) P<0.01; XRCC1 280 Arg/His and His/His, FR=1.28 (95%CI 1.13-1.44) P<0.01; XRCC1 399 Arg/Gln and XRCC1 399 Gln/Gln, FR=1.23(95%CI 1.09-1.38) P<0.01.Similar result was showed in the interaction effect of these mutant genotype and exposure dose. Take XRCC1 T-77C for example, compared with XRCC1-77TT genotyope carriers in low exposure dose groups, The MN frequency of XRCC1-77TC or CC genotyope carriers in low exposure dose groups, XRCC1-77TT genotyope carriers in high exposure dose groups, XRCC1-77TC or CC genotyope carriers in high exposure dose groups, were increased. FR=1.16,95%CI 0.94-1.43; FR=1.31, 95%CI 1.13-1.52 P<0.01; FR=1.72,95%CI 1.45-2.02 P<0.01, respectively.To further elucidate the relevance of XRCC1 variants with MN frequency, linkage disequilibrium (LD) among the four XRCC1 polymorphisms (XRCC1-77 C/T, Arg194Trp, Arg280His, and Arg399Gln) was analyzed and haplotypes were reconstructed. The D'value of the four loci of XRCC1 were 0.716(XRCC1-77 with 194), 0.776(XRCC1-77 with 280), 0.752(XRCC1-77 with 399),0.833 (XRCC1 194 with 280),0.849 (XRCC1 194 with 399), and 0.899(XRCC1 280 with 399). Haplotype analysis demonstrated the MN frequency in subjects with CAAA/CAAA, TAAA/TAAA, CGAA/CAAA was significantly higher than that in subjects with TGGG. FR=1.25, 95%CI 1.05-1.48 P<0.05; FR=1.31,95%CI 1.04-1.65 P<0.05; FR=1.56,95%CI 1.07-2.19 P<0.05, respectively.The MN frequency in subjects with XPC PAT and XPF T2063A mutant homozygous and heterozygous is lower than their wild-type homozygous counterparts, the frequency ratio (FR) and its 95% confidence interval (95% CI) were 0.82 (0.69-0.98),0.80 (0.67-0.95) respectively(P<0.05). However, MN frequency in subjects with XPA A23G and XPC Lys939Gln mutant homozygous and heterozygous is higher than their wild-type homozygous counterparts, FR and 95%CI were 1.20 (1.03-1.40) and 1.31 (1.05-1.62) respectively (P<0.05). These results suggest XPC PAT and XPF T2063A variant may be protective factors while XPA A23G and XPC Lys939Gln variant may be risk factors for the chromosome damage induced by VCM.The interaction effect of these mutant genotype and exposure dose showed XPA A23G, XPC Ala499Val,XPC Lys939Gln, XPD Metl99Ile, XPD Lys751Gln, XPGExon15G-C, ERCC13'-UTR C8092A mutant homozygous and heterozygous in low exposure groups and high exposure groups had higher MN frequency than their wild-type homozygous in low exposure groups (P<0.05). While, XPCPAT, XPF 5'-UTRT2063A mutant homozygous and heterozygous in low exposure groups and high exposure groups had lower MN frequency than their wild-type homozygous (P<0.05).Linkage disequilibrium (LD) analysis among the four XPC polymorphisms (XPC PAT+/-, Ala499Val, Lys939Gln) showed strongly LD between these sites. Haplotype analysis demonstrated the MN frequency in subjects with PAT+CA/PAT-CA, PAT-AC/PAT-TC, and PAT+TC/PAT+TA was significantly higher, FR=2.73,95%CI 1.95-3.72 P<0.01; FR=1.20,95%CI 1.03-1.40 P<0.05; FR=1.48,95%CI 1.19-1.82 P<0.01, while PAT-TC/PAT+TC lower than that in subjects with PAT-CA/PAT-CA, FR=0.33,95%CI 0.15-0.62 P<0.01.No association of Linkage disequilibrium occurred in XPD(XPD Met199Ile, Asp312Asn, Lys751Gln). Haplotype analysis of showed the MN frequency in subjects with CCC/CCC, ATA/CTA was significantly higher than that in subjects with CCA/CCA, FR=1.34,95%CI 1.10-1.63 P<0.01; FR=1.55,95%CI 1.06-2.18 P<0.05.In conclusion, VCM can induce chromosome damage even when the exposure level is lower than the national occupational health standard of China; the polymorphism of DNA damage repair genes may be associated with chromosome damage induced by VCM.