Genetic Susceptibility To Silicosis In Iron Miners

Silicosis has been one of the most serious occupational health problems in China,with high morbidity and mortality. Cumulative dust exposure is the most importantfactor in the pathogenesis of silicosis, however, there appears to be quite pronouncedindividual variation in silicosis susceptibility and severity, suggesting that geneticfactors may influence progression of the disease. Although the exact mechanismsleading to silicosis have to be elucidated, recent in vivo, in vitro, and human studieshave focused on increased expression of inflammatory and fibrogenic cytokines, suchas tumor necrosis factor-alpha(TNF-α) and interleukin(IL)-1, activation of specifictranscription factors(e.g., nuclear factor kappa B(NF-kB) and AP-1) and apoptosis inmacrophages and other cells induced by the ROS, RNS, and NO generated by thesilica particles. Few studies have investigated genetic susceptibility to silicosis andthis study was to investigate the roles of genetic polymorphisms in cytokines,transcriptional factors, and apoptosis molecules in susceptibility to silicosis and toexplore the interactions of environmental factors(occupational exposure and lifestyles) and genetic factors(genetic polymorphisms) in risk of silicosis in Chinese ironminers.Enrolled in the study were 295 retired male miners from an iron mine in AnhuiProvince, China. Cumulative total dust exposure(CTE) was assesed based on thehistorical record of total airborne dust concentration in various work locations; astandardized questionnaire was used to obtain general characteristics and health statusof all participants; health surveillance data was collected to obtain the prevalence ofpulmonary tuberculosis(PTB), and pulmonary function test was conducted in allparticipants to assess pulmonary function damage of miners. A case-control study wasconducted with 183 silicosis patients and 111 silica-exposed miners who werefrequency-matched by age, dust exposure duration, work location, and type of work.Genotype analysis was performed on genomic DNA, using a polymerase chainreaction-restrained fragment length polymorphism(PCR-RFLP) assay.The occupational exposure level of each subject was assessed by cumulative totaldust exposure(CTE). Despite the recruitment of high dust exposure miners in thecontrols, the mean CTE among silicosis patients was still significantly higher than thatin the controls(242.6±98.8 mg·a/m~3 vs 217.6±100.7 mg·a/m~3, P＜0.05). The average CTE was 229.4±102.3 mg·a/m~3 in stageⅠgroup, 262.9±91.3 mg·a/m~3 in stageⅡgroup,and 260.4±88.9 mg·a/m~3 in stageⅢgroup. In further study for genetic susceptibilityto silicosis, based on the distribution of CTE, three levels of exposure were defined:＜150 mg·a/m~3(low exposure),≥150-mg·a/m~3(moderate exposure), and≥300mg·a/m~3(high exposure).The study on health effects caused by silica exposure and silicosis indicated: (1)Clinical manifestations of silicosis included: silicosis-related symptoms, such as cheststuffiness(92.9%), chronic cough(73.9%), and phlegm production(62.5%);PTB-related symptoms, such as bloody phlegm(30.4%), persistent hypothermia(24.5%), and current body weight loss(28.8%); dyspnea. Severity of dyspneaincreased with silicosis upgrading and CTE increase, and there was an 8.85-foldincrease risk of dyspnea for silicosis cases compared with controls. Furthermore, asizeable proportion of controls had respiratory symptoms. (2) The prevalence of PTBwas 27.2% in cases and 9.9% in controls. There was a 6.09-fold increased risk of PTBfor silicosis cases compared with controls. The prevalence of PTB increased withsilicosis upgrading and CTE increase. (3) The main types of pulmonary dysfunctionwere restrictive and mixed hypoventilation. Prevalence of pulmonary dysfunctionincreased with silicosis upgrading, which was 45.5% in control, 76.4% in stageⅠgroup, 86.4% in stageⅡgroup, and 100.0% in stageⅢgroup, respectively. The sametrend was found between prevalence of pulmonary dysfunction and gruops of CTE.The study on genetic susceptibility to silicosis in iron miners indicated: (1) Thefrequencies of the variant allele for polymorphisms of TNF-α-308 G＞A, TNF-α-238G＞A, IL-1α-889 C＞T, IL-1β+3953 C＞T and IL-10-1082 A＞G were low, with varianthomozygote of 5, 1, 5, 1, and 4 cases, respectively. There was only one case carryingIL-1RA VNTR *2/*2 genotype. We detected only two silicosis patients with GCheterozygote for TGF-β1+915 G＞C polymorphism, and no variant homozygote CCwas detected. No association was observed between all the cytokine polymorphismsand silicosis, however, the TNF-α-308 G＞A variant had synergistic effect withsmoking, and smoking miners with A allele were more susceptible to silicosis thanindividuals carrying homozygote G/G(OR=2.41, 95%CI: 1.13～5.13). Diplotypeanalysis showed that risk of silicosis increased in the subjects with the 3/3 diplotypecompared with those carrying the 1/1 diplotype of IL-1 gene(OR=5.17, 95%CI:1.41～18.88, P=0.008); in the subjects with 1/3 diplotype compared with those withthe 1/1 diplotype of IL-10 gene(OR=13.91, 95%CI: 1.82～106.29, P=0.001). (2) The distribution of the NFKB1-94ins/delATTG ins/del genotype was similar between casesand controls(42.6% and 52.3%, respectively), when CTE and smoking were includedin the analysis as covariates, the risk of silicosis for individuals with the ins/delgenotype was 0.57-fold(ORadj=0.57, 95%CI: 0.32～0.998, P=0.047) compared withthose with the ins/ins genotype. Stratified by the grades of silicosis, the frequencies ofins/del+del/del genotypes in cases of stageⅡand stageⅢ, and stageⅠwere lowerthan that in the controls(68.2% vs 74.8% and 58.9% vs 74.8%, respectively); rankcorrelation analysis showed a statistically significant correlation between theNFKB1-94ins/delATTG variant and silicosis severity(spearman correlationcoefficient-0.13). (3) The distribution of the FAS-1377 A/A genotype was similarbetween cases and controls(9.3% and 18.0%, respectively), when CTE and smokingwere included in the analysis as covariates, the risk of silicosis for subjects with theA/A genotype was 0.42-fold(ORadj=0.42, 95%CI: 0.19～0.93, P=0.029) comparedwith those with the G/G genotype. Haplotype analysis showed that there was astatistically significant difference for the distribution of HAP2 haplotype[(-1377Gand -670G), P=0.007] between cases and controls. Compared with those carrying theHAP1 haplotype(-1377G and -670A), there was a 2.71-fold increased risk of silicosisfor subjects with the HAP2 haplotype(OR=2.71, 95%CI: 1.22～6.03, P=0.01).In conclusion, it suggested that comprehensive measures should be applied toprevent silicosis, including dust control at workplace, application of personalprotective equipment(PPE), health surveillance, smoking cessation propaganda andhealth education, and screening the susceptible workers(e.g. individuals withNFKB1-94ins/delATTG ins/del and FAS-1377 A/A genotypes), which would becombined to reduce incidence of silicosis and to improve life quality of workers.