Superfund Research Program

Genotoxic Potential of Mixed Dust Exposures

Project Leader: Agnes B. Kane
Co-Investigator: Charles A. Vaslet
Grant Number: P42ES013660
Funding Period: 2005-2009

Project-Specific Links

Final Progress Reports

Year:   2008 

Inhalation of dusts and fibers has been associated with development of chronic lung disease and cancer and poses a risk to workers and residents living near Superfund sites and Brownfields.  Environmental exposures are predicted to increase due to the widespread use of asbestos-containing products and construction materials during the 1970’s.  Asbestos materials remain in place in many homes, industries, and public buildings and all urban populations have detectable levels of asbestos fibers in their lungs at autopsy. As a result of widespread occupational and residential exposure to noncommercial amphiboles associated with vermiculite mining and processing, Libby, Montana was placed on the Superfund National Priority List in 2002.

Exposure to naturally occurring or noncommercial forms of asbestos is not limited to Libby, Montana.  Naturally occurring asbestos deposits are mixtures of asbestiform fibers, acicular fragments, and prismatic crystals in combination with other minerals including quartz.  Environmental exposure to these naturally occurring asbestos deposits is a potential concern in El Dorado County, California, in the taconite iron range in Minnesota, and other sites throughout the United States.  The relative potency of these naturally occurring forms of asbestos and their physical and chemical properties related to potential toxicity and adverse human health effects are unknown.  A major goal of the Genotoxic Potential Of Mixed Dust Exposures project during the current funding period of this SBRP is to assess the physical and chemical properties of Libby amphibole asbestos that are relevant for toxicity and carcinogenicity.  Dr. Agnes Kane and her research team hypothesize that processing of vermiculite ore at high temperature alters the surface properties of the contaminating amphibole asbestos fibers that increase bioavailability of iron, similar to the effects of aging and air oxidation on iron-contaminated carbon nanotubes zero-valent iron nanoparticles. Vermiculite ore is exfoliated or expanded up to 20 times its original volume by heating to more than 1000°C for a short time to vaporize the water trapped in the vermiculite sheet silicate layers.  This process is dusty and produces a light, fluffy product used in numerous consumer products and construction materials.

These experiments were conducted in collaboration with Indrek Kuolots, Aihui Yan, and Robert Hurt (Mechanisms Of Hg Adsorption From Mixed Pollutant Streams project).  A sample of Libby amphibole asbestos (U.S. Geological Service) was heated to 1000° C in air for 15 seconds to simulate exfoliation processing of vermiculite ore.  Iron mobilized from native UICC crocidolite asbestos was compared with native and heated Libby amphibole. When compared on a mass basis, less iron was mobilized from native Libby amphibole; however, following heating to 1000°C, iron mobilization expressed per mg of iron increased almost 6-fold.  Heating Libby amphibole also significantly enhanced redox activity, especially when compared per mg of iron in each sample.

Asbestos has been shown to induce various types of DNA damage in relevant target cells in vitro, including mesothelial cells and lung epithelial cells. The genotoxic effects of asbestos fibers are believed to be due, in part, to iron-dependent generation of free radicals, since iron chelators and antioxidants protect from asbestos-induced DNA damage.  In order to compare potential genotoxicity of noncommercial and commercial asbestos fibers, a highly sensitive assay is required.  In collaboration with Anatoly Zhitkovich (Biological Dosimetry Of Hexavalent Chromium project), a sensitive genotoxicity assay was developed by genetic knockdown of a key DNA repair protein.

Repair of oxidant-induced DNA damage is mediated primarily by the base excision repair (BER) pathway. BER involves excision of oxidized bases by DNA glycosylases and subsequent generation of an abasic site, which is processed by AP endonuclease I (APE-1). The integrity of the DNA is then restored through replacement of the excised nucleotides and DNA ligation. The involvement of the BER pathway in asbestos-induced damage is suggested by studies showing induction of APE-1 by asbestos in mesothelial cells.  Additionally, a recent study of genetic polymorphisms in DNA repair genes among individuals exposed to asbestos indicated an association between polymorphisms in the X-ray complementing group 1 (XRCC1) gene and malignant mesothelioma.

Preliminary studies suggest that XRCC1-deficient human lung epithelial cells are extremely sensitive to oxidant-induced DNA damage as revealed by reduced clonogenic survival and induction of micronuclei following exposure to H2O2.  The research team will use these genetically-engineered cells to compare the ability of native and heated noncommercial and commercial amphibole asbestos fibers to induce DNA damage.  These experiments are ongoing and it is anticipated that these XRCC-1 deficient human lung epithelial cells will showing increased sensitivity to asbestos-induced DNA damage producing activation of the p53-induced DNA damage response pathway.

In summary, this genetically-engineered human cell culture model is useful for understanding the mechanisms of genotoxicity produced by commercial and noncommercial asbestos fibers, as well as newly-designed nanomaterials that have similar dimensions and chemical properties as carcinogenic asbestos fibers.