Smoking enhances asbestos-induced genotoxicity, relative involvement of chromosome 1: a study using multicolor FISH with tandem labeling

Genome-wide Gene-Asbestos Exposure Interaction Association Study Identifies a Common Susceptibility Variant on 22q13.31 Associated with Lung Cancer Risk

BACKGROUND

Occupational asbestos exposure has been found to increase lung cancer risk in epidemiologic studies.

METHODS

We conducted an asbestos exposure-gene interaction analyses among several Caucasian populations who were current or ex-smokers. The discovery phase included 833 Caucasian cases and 739 Caucasian controls, and used a genome-wide association study (GWAS) to identify single-nucleotide polymorphisms (SNP) with gene-asbestos interaction effects. The top ranked SNPs from the discovery phase were replicated within the International Lung and Cancer Consortium (ILCCO). First, in silico replication was conducted in those groups that had GWAS and asbestos exposure data, including 1,548 cases and 1,527 controls. This step was followed by de novo genotyping to replicate the results from the in silico replication, and included 1,539 cases and 1,761 controls. Multiple logistic regression was used to assess the SNP-asbestos exposure interaction effects on lung cancer risk.

RESULTS

We observed significantly increased lung cancer risk among MIRLET7BHG (MIRLET7B host gene located at 22q13.31) polymorphisms rs13053856, rs11090910, rs11703832, and rs12170325 heterozygous and homozygous variant allele(s) carriers (P < 5 × 10(-7) by likelihood ratio test; df = 1). Among the heterozygous and homozygous variant allele(s) carriers of polymorphisms rs13053856, rs11090910, rs11703832, and rs12170325, each unit increase in the natural log-transformed asbestos exposure score was associated with age-, sex-, smoking status, and center-adjusted ORs of 1.34 [95% confidence interval (CI), 1.18-1.51], 1.24 (95% CI, 1.14-1.35), 1.28 (95% CI, 1.17-1.40), and 1.26 (95% CI, 1.15-1.38), respectively, for lung cancer risk.

CONCLUSION

Our findings suggest that MIRLET7BHG polymorphisms may be important predictive markers for asbestos exposure-related lung cancer.

IMPACT

To our knowledge, our study is the first report using a systematic genome-wide analysis in combination with detailed asbestos exposure data and replication to evaluate asbestos-associated lung cancer risk.

Molecular and genetic changes in asbestos-related lung cancer

Asbestos-exposure is associated with an increased risk of lung cancer, one of the leading causes of cancer deaths worldwide. Asbestos is known to induce DNA and chromosomal damage as well as aberrations in signalling pathways, such as the MAPK and NF-kappaB cascades, crucial for cellular homeostasis. The alterations result from both indirect effects through e.g. reactive oxygen/nitrogen species and direct mechanical disturbances of cellular constituents. This review describes the current knowledge on genomic and pathway aberrations characterizing asbestos-related lung cancer. Specific asbestos-associated molecular signatures can assist the development of early biomarkers, molecular diagnosis, and molecular targeted treatments for asbestos-exposed lung cancer patients.

Identification of specific gene copy number changes in asbestos-related lung cancer

Asbestos is a well-known lung cancer-causing mineral fiber. In vitro and in vivo experiments have shown that asbestos can cause chromosomal damage and aberrations. Lung tumors, in general, have several recurrently amplified and deleted chromosomal regions. To investigate whether a distinct chromosomal aberration profile could be detected in the lung tumors of heavily asbestos-exposed patients, we analyzed the copy number profiles of 14 lung tumors from highly asbestos-exposed patients and 14 matched tumors from nonexposed patients using classic comparative genomic hybridization (CGH). A specific profile could lead to identification of the underlying genes that may act as mediators of tumor formation and progression. In addition, array CGH analyses on cDNA microarrays (13,000 clones) were carried out on 20 of the same patients. Classic CGH showed, on average, more aberrations in asbestos-exposed than in nonexposed patients, and an altered region in chromosome 2 seemed to occur more frequently in the asbestos-exposed patients. Array CGH revealed aberrations in 18 regions that were significantly associated with either of the two groups. The most significant regions were 2p21-p16.3, 5q35.3, 9q33.3-q34.11, 9q34.13-q34.3, 11p15.5, 14q11.2, and 19p13.1-p13.3 (P < 0.005). Furthermore, 11 fragile sites coincided with the 18 asbestos-associated regions (P = 0.08), which may imply preferentially caused DNA damage at these sites. Our findings are the first evidence, indicating that asbestos exposure may produce a specific DNA damage profile.

Biomarkers in risk assessment of asbestos exposure

Developments in the field of molecular epidemiology and toxicology have given valuable tools for early detection of impending disease or toxic condition. Morbidity due to respiratory distress, which may be due to environmental and occupational exposure, has drawn attention of researchers worldwide. Among the occupational exposure to respiratory distress factors, fibers and particles have been found to be main culprits in causing diseases like asbestosis, pleural plaques, mesotheliomas and bronchogenic carcinomas. An early detection of the magnitude of exposure or its' effect using molecular end points is of growing importance. The early inflammatory responses like release of the inflammatory cells collected by non-invasive methods give an indication of the unwanted exposure and susceptibility to further complications. Since free radicals like O2-, OH, OOH, NO, NOO, etc. are involved in the progression of asbestos-related diseases and lead to cytogenetic changes, an evaluation of antioxidant states reducing equivalents like GSH and ROS generation can be a good biomarker. The cytogenetic end points like chromosomal aberration, micronucleus formation and sister chromatid exchange give indication of genetic damage, hence they are used as effective biomarkers. New techniques like fluorimetric analysis of DNA unwinding, alkaline elution test, fluorescent in situ hybridization and comet assay are powerful tools for early detection of initiation of disease process and may help in planning strategies for minimizing morbidity related to asbestos fiber exposure. The present review article covers in detail possible biomarkers for risk assessment of morbidity due to fibers/particles in exposed population.

Reduction of chrysotile asbestos-induced genotoxicity in human peripheral blood lymphocytes by garlic extract

Asbestos fibers are well known environmental carcinogen, however, the underlying mechanisms of their action have still not clearly been identified. Asbestos is capable of depleting glutathione and generating reactive oxygen species (ROS), which are important mediators of damage in biological system. Asbestos-induced mutagenecity, may be mediated by the generation. It is known that a number of scavengers and antioxidants attenuate asbestos-induced ROS release. Furthermore, it is known that garlic, contains numerous sulfur compounds and glutathione precursors which act as antioxidants and also demonstrate anticarcinogenic properties. The aim of this study was to investigate whether garlic extract has any influence on asbestos-mediated genotoxicity. As an assay system, we applied the micronucleus assay, sister chromatid exchanges, and chromosomal aberrations with human peripheral blood lymphocytes, which has already been used to analyze the genotoxicity of asbestos fibers. Our results indicate that garlic extract, when administered to the lymphocytes cell culture simultaneously with chrysotile reduced the rates of micronucleus formation, sister chromatid exchanges, and chromosomal aberrations significantly. We conclude that garlic extract may be an efficient, physiologically tolerable quencher of asbestos-mediated genotoxicity.