Restoration of radiation damage examined by the analysis of HeLa cell clones.

Abstract

This paper reports a small, but significant, increase in the survival of irradiated HeLa cells after treatment with DNA precursors. Clone-size analysis is used to indicate that this may be regarded as a true restoration of radiation damage and not merely a nutritive phenomenon. Single cell survival was estimated by the technique of Puck and Marcus (1956) and involved the usual comparison of the numbers of clones which have attained a minimum size within a given period. This criterion provides evidence of the reproductive integrity of single cells, but it conceals a further criterion of survival, namely the ability of a cell to proliferate at the normal rate. Analysis of HeLa cell clones has shown (Nias, Gilbert, Lajtha and Lange 1965) that a broad spectrum of clone sizes may be found after irradiation, as described for Chinese hamster cells by Sinclair (1964). Such broad clone-size distributions indicate the range of proliferation rates induced by radiation. These rates extend from the maximum control rate down to zero. Those within the control range can now be defined as ' survivors' in the full sense: cells able to proliferate at the control rate. Smaller clones may well attain the 50-cell level, given time, but these exhibit radiation damage, albeit non-lethal damage. If radiation damage is restored by treatment with DNA precursors, such restoration may involve two effects: (a) an increase in the number of cells with reproductive integrity; (b) an increase in the proliferation rate of non-lethally damaged cells. The second effect might be considered to be a growth-promoting phenomenon as distinct from the first which requires an absolute increase in the number of viable cells. The usual clone counting procedure cannot distinguish the two effects, however, since an increase in the proliferation rate or non-lethally damaged cells may produce more 'over-50-cell' clones just as much as would an increase in the absolute number of viable cells. Clone-size analysis allows these two effects to be examined. A difference in clone-size distribution implies either a growth-promoting or a toxic effect (e.g. figure 1). If the clone-size distributions are similar, on the other hand, any increase in the number of 'over-50-cells' clones implies an increase in the number of viable cells. This would represent a true restoration of radiation damage to the reproductive integrity of the cells.