Coupling cytotoxicity biomarkers with DNA damage assessment in TK6 human lymphoblast cells

There is considerable discussion within the scientific community as to the appropriate measures of cytotoxicity to use when deciding on the maximum concentration of a substance to test in vitro for its ability to induce DNA damage using the Comet assay. Conventional cytotoxicity assessment methods, such as trypan blue dye exclusion or relative cell number (cell counts) may not be the most biologically relevant measurement for cytotoxicity in this assay. Thus, we evaluated for decreased levels of adenosine triphosphate (ATP) and activation of Caspase-3/7 as well as relative cell number and trypan blue exclusion in order to understand the correlation among test compound concentration, cytotoxicity and genotoxicity outcomes in the Comet assay. We tested two non-genotoxic and non-cytotoxic compounds (d-glucose and ethanol), two non-genotoxic but cytotoxic compounds (2,4-dichlorophenol and tunicamycin) and four genotoxic and cytotoxic compounds (methyl methanesulfonate, ethyl methanesulfonate, etoposide and 4-nitroquinoline-N-oxide) in TK6 human lymphoblast cells. Our data show that measuring ATP and Caspase-3/7 levels provides more rapid and perhaps more biologically relevant measures of cytotoxicity compared with trypan blue dye exclusion and relative cell number. Furthermore, incorporating these two assays into the Comet assay also provided insight on the cytotoxic mode of action of the chemicals tested. By extrapolation, such assays may also be useful in other in vitro genotoxicity assays.