GADD45α induction in the GreenScreen HC indicator assay does not occur independently of cytotoxicity

An integrative test strategy for cancer hazard identification

Assessment of genotoxic and carcinogenic potential is considered one of the basic requirements when evaluating possible human health risks associated with exposure to chemicals. Test strategies currently in place focus primarily on identifying genotoxic potential due to the strong association between the accumulation of genetic damage and cancer. Using genotoxicity assays to predict carcinogenic potential has the significant drawback that risks from non-genotoxic carcinogens remain largely undetected unless carcinogenicity studies are performed. Furthermore, test systems already developed to reduce animal use are not easily accepted and implemented by either industries or regulators. This manuscript reviews the test methods for cancer hazard identification that have been adopted by the regulatory authorities, and discusses the most promising alternative methods that have been developed to date. Based on these findings, a generally applicable tiered test strategy is proposed that can be considered capable of detecting both genotoxic as well as non-genotoxic carcinogens and will improve understanding of the underlying mode of action. Finally, strengths and weaknesses of this new integrative test strategy for cancer hazard identification are presented.

Fluorescence-based recombination assay for sensitive and specific detection of genotoxic carcinogens in human cells

In vitro genotoxicity tests are known to suffer from several shortcomings, mammalian cell-based assays, in particular, from low specificities. Following a novel concept of genotoxicity detection, we developed a fluorescence-based method in living human cells. The assay quantifies DNA recombination events triggered by DNA double-strand breaks and damage-induced replication fork stalling predicted to detect a broad spectrum of genotoxic modes of action. To maximize sensitivities, we engineered a DNA substrate encompassing a chemoresponsive element from the human genome. Using this substrate, we screened various human tumor and non-transformed cell types differing in the DNA damage response, which revealed that detection of genotoxic carcinogens was independent of the p53 status but abrogated by apoptosis. Cell types enabling robust and sensitive genotoxicity detection were selected for the generation of reporter clones with chromosomally integrated DNA recombination substrate. Reporter cell lines were scrutinized with 21 compounds, stratified into five sets according to the established categories for identification of carcinogenic compounds: genotoxic carcinogens ("true positives"), non-genotoxic carcinogens, compounds without genotoxic or carcinogenic effect ("true negatives") and non-carcinogenic compounds, which have been reported to induce chromosomal aberrations or mutations in mammalian cell-based assays ("false positives"). Our results document detection of genotoxic carcinogens in independent cell clones and at levels of cellular toxicities <60 % with a sensitivity of >85 %, specificity of ≥90 % and detection of false-positive compounds <17 %. Importantly, through testing cyclophosphamide in combination with primary hepatocyte cultures, we additionally provide proof-of-concept for the identification of carcinogens requiring metabolic activation using this novel assay system.