Abstract
BackgroundClonogenicity gives important information about the cellular reproductive potential following ionizing irradiation, but an abortive colony that fails to continue to grow remains poorly characterized. It was recently reported that the fraction of abortive colonies increases with increasing dose. Thus, we set out to investigate the production kinetics of abortive colonies using a model of branching processes.Methodology/Principal FindingsWe firstly plotted the experimentally determined colony size distribution of abortive colonies in irradiated normal human fibroblasts, and found the linear relationship on the log-linear or log-log plot. By applying the simple model of branching processes to the linear relationship, we found the persistent reproductive cell death (RCD) over several generations following irradiation. To verify the estimated probability of RCD, abortive colony size distribution (≤15 cells) and the surviving fraction were simulated by the Monte Carlo computational approach for colony expansion. Parameters estimated from the log-log fit demonstrated the good performance in both simulations than those from the log-linear fit. Radiation-induced RCD, i.e. excess probability, lasted over 16 generations and mainly consisted of two components in the early (<3 generations) and late phases. Intriguingly, the survival curve was sensitive to the excess probability over 5 generations, whereas abortive colony size distribution was robust against it. These results suggest that, whereas short-term RCD is critical to the abortive colony size distribution, long-lasting RCD is important for the dose response of the surviving fraction.Conclusions/SignificanceOur present model provides a single framework for understanding the behavior of primary cell colonies in culture following irradiation.
Highlights
Clonogenicity, i.e., the ability of a cell to form a colony gives important information about the cellular reproductive potential that changes in response to various internal and external stimuli
Despite the difference in the patterns of the probability and ionizing radiation (IR)-induced excess probability of reproductive cell death (RCD) between them, these results suggest that IR-induced excess probability lasts over several generations
With the experimental observation and modeling approach based on the general and simple principles, we have here demonstrated that the size distribution of abortive colonies with $2 cells observed in the experiments did follow a linear relationship on the log-linear plot (Figure 2), but an optimal method for the colony-formation kinetics of the abortive colonies was a log-log fit in the colony size distribution, because it reflected the stochastic outcome of the branching processes
Summary
Clonogenicity, i.e., the ability of a cell to form a colony gives important information about the cellular reproductive potential that changes in response to various internal and external stimuli. Given mounting evidence for non-targeted effects of ionizing radiation (IR) such as genomic instability and bystander effects [5,6,7], the analysis of abortive colonies would be important and should make the colony formation assay more informative. In this regard, we previously reported that, in IR-exposed normal human fibroblasts, the fraction of abortive colonies increases with increasing dose in contrast to the case for clonogenic colonies [8], suggesting a systematic rule in the production of abortive colonies. We set out to investigate the production kinetics of abortive colonies using a model of branching processes
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