There is increasing new evidence that points to hitherto under-appreciated health risks of involuntary exposure to thirdhand smoke (THS) which consists of persistent tobacco residues and their byproducts. Recent attention is being paid to the mutagenic and cancer-forming tobacco-specific nitrosamines (TSNAs) formed from the reaction of indoor surface-bound nicotine with a common pollutant, nitrous acid. NNA is a major TSNA product that was identified in THS, but is not present in freshly emitted secondhand smoke. We proved for the first time that THS at near realistic doses causes DNA damage in cultured human cells. Moreover, we identified multiple base modifications from NNA reactions in vitro, including a novel and major adduct, the exocyclic 1,N2-NNA-dG. These findings support recent concerns about health threats from THS, particularly its possible role in causing cancer. These reports and publications received extensive coverage by the mainstream media, and they were used by the California State Assembly to support a legislation targeting THS.
One major task in assessing THS exposure and human health risk is to demonstrate its potential genotoxic and cancer-forming potential. However, there is no direct evidence yet for this either from animal or human subject studies. We hypothesize that THS causes mutation and cancer, and exposure to it is a previously unknown factor in the etiology of tobacco-induced lung cancer development. We propose the following specific aims in this application, in the context of collaborating with and benefiting from the California Consortium on THS:
Aim 1 will provide important mechanistic evidence on NNA genotoxicity through identification and characterization of DNA damage in NNA-exposed cell lines and mice. We will also determine whether iso-NNAC can be generated metabolically from NNA in human liver tissues. These results will also serve as the basis for exploring specific NNA adducts and metabolites as biomarkers of THS exposure. Aim 2 will address mutagenic and cancer-forming potential of THS using a next generation toxicological assessment approach, including whole exome sequencing for mutation frequency and target genes, bioinformatics for analyzing mutation patterns to explore cancer association, cell-based transformation and invasion assays for prediction of carcinogenic potential. Aim 3 will use robust and high-throughput in vitro screening tests to assess the genotoxic effects of chemical compounds identified by the THS Consortium subproject led by Dr. Destaillats, for which toxicological information is needed in order to determine their health impact. For THS particulate matter (PM2.5), the approaches of next generation toxicological risk assessments described in Aim 2 will be employed to characterize its toxicity in comparison with other environmental particles.
We believe that these proposed studies will critically analyze and elucidate the possible health harms of THS, particularly its cancer risk, by obtaining novel toxicological data and mechanistic information. The findings on “signature” changes obtained from molecular and cellular experiments in this revision will have important implications in THS-related biomarker studies and public health risk assessment. Finally, the ensuing results should provide scientific evidence to help frame and enforce further anti-smoking policies in California.