Expression of delayed toxicity and lethal mutations in the progeny of human cells surviving exposure to radiation and other environmental mutagens

Purpose: Delayed expression of lethal mutations in the progeny of cells whi ch survived a toxic insult was first shown for ionizing radiation and is on e of the signs of induced genomic instability. The effect appears to be rel ated to DNA strand breakage or repair but not to the physical break itself. To investigate this and the relationship of lethal mutations or delayed de ath to other instability endpoints, cultures of immortal but non-transforme d human keratinocytes were exposed to a range of environmental mutagens or cytotoxic compounds with different DNA damaging properties. Methods: Delayed expression of damage was assessed by scoring a number of e ndpoints in the progeny of cells which survived exposure and underwent at l east 15 population doublings. Endpoints included delayed apoptosis, cloning efficiency of cells in 'healthy' colonies and expression of the apoptosis regulatory proteins bcl-2 and BAX. Results: The results clearly linked expression of delayed lethal mutations with substances that induced DNA strand breaks. All these substances are kn own also to induce oxidative stress. The occurrence of delayed damage requi red a threshold level of toxicity in the initially exposed population, whic h was remarkably similar for all the effective substances except cadmium. A lkylating agents or microtubule poisons that do not permit repair of DNA da mage did not cause any delayed death. Conclusion: It is concluded that delayed cell death may be caused by widesp read radical damage to DNA which is either signalled, thereby inducing an a poptotic response, or (mis-)repaired yielding a weak or unstable genome. It is likely that the process may be an important factor in determining the l ong-term response of populations to 'sublethal' levels of environmental mut agens whose mechanism of action includes DNA strand breakage and repair.