Kinetics of CHO A L mutant expression after treatment with gamma radiation, EMS, and asbestos

The flow cytometry mutation assay (FCMA) uses hybrid CHO A(L) cells to measure mutations of the cd59 gene located on human chromosome 11 by the absence of fluorochrome-conjugated antibody binding to the CD59 surface antigen. Mutant expression peaks between 6 and 12 days, then decreases to a stable plateau, instead of a constant mutant fraction obtained by clonogenic assays. To evaluate this variable mutant expression time, cells were treated with radiation, EMS or asbestos and cell proliferation and survival were measured at times leading up to peak mutant expression. Potential doubling time (T(pot)) values increased by at least 75% for each agent by 3 h after treatment but returned to control levels after only 3 days. Survival returned to 90% of control within a week, close to the peak expression day for all three agents. The survival of CD59(-) cells sorted on the peak day of expression was roughly half that of CD59(+) cells. Cloned EMS-treated CD59(-) cells had a doubling time of 16.7 vs. 14.1 h for CD59(+) cells. Triple mutants (CD59(-)/CD44(-)/CD90(-)) were preferentially lost from the population over time, while the proportion of CD59(-)/CD90(-) mutants increased. In conclusion, the peak day of mutant expression occurs only when cells recover from the toxic effects of the mutagen. A fraction of cells originally quantified as mutants are lost over time due to lethal deletions and slower growth.