Sticky anaphase aberrations after G2-phase arrest of gamma-irradiated human skin fibroblasts: TP53 independence of formation and TP53 dependence of consequences.

We have studied the impact of TP53 status on the extent and nature of chromosome damage seen in human skin fibroblasts after gamma irradiation beyond the G1-phase checkpoint but prior to the G2-phase checkpoint. Mitotic cells were examined in the absence and presence of treatment with nocodazole and the yield of aberrations was scored as a function of time postirradiation. The results revealed substantially greater damage in the absence of nocodazole, indicating that damage was being masked in its presence. While metaphase aberrations were seen exclusively in the presence of nocodazole, anaphase aberrations were seen principally in its absence. Furthermore, these were mostly of an unseparated, or "sticky", type that showed separation of the chromatids in the centromeric region, indicating normal degradation of cohesin, with retention of adhesion further out on the chromatid arms. Using postirradiation BrdU labeling and the absence of nocodazole, we were able to identify mitotic figures up to the third postirradiation mitosis. Analysis of the data revealed that in cells wild-type for TP53 the aberrant anaphases were lost after the first postirradiation mitosis, although they were still found in gradually decreasing amounts into the second and third postirradiation mitoses in E6-expressing cells. The data indicate that the formation of these sticky anaphases is independent of TP53 status, an observation that is consistent with the TP53 independence of transient G2-phase arrest. However, the consequences of the formation of these lesions appear to be very different. In the case of cells wild-type for TP53 this is chronic G1-phase arrest, while in E6 cells it is anaphase catastrophe.