Polyploid giant cells provide a survival mechanism for p53 mutant cells after DNA damage.

The relationships between delayed apoptosis, polyploid 'giant' cells and reproductive survivors were studied in p53-mutated lymphoma cells after DNA damage. Following severe genotoxic insult with irradiation or chemotherapy, cells arrest at the G(2)-M cell cycle check-point for up to 5 days before undergoing a few rounds of aberrant mitoses. The cells then enter endoreduplication cycles resulting in the formation of polyploid giant cells. Subsequently the majority of the giant cells die, providing the main source of delayed apoptosis; however, a small proportion survives. Kinetic analyses show a reciprocal relationship between the polyploid cells and the diploid stem line, with the stem line suppressed during polyploid cell formation and restituted after giant cell disintegration. The restituted cell-line behaves with identical kinetics to the parent line, once re-irradiated. When giant cells are isolated and followed in labelling experiments, the clonogenic survivors appear to arise from these cells. These findings imply that an exchange exists between the endocyclic (polyploid) and mitotic (diploid or tetraploid) populations during the restitution period and that giant cells are not always reproductively dead as previously supposed. We propose that the formation of giant cells and their subsequent complex breakdown and subnuclear reorganization may represent an important response of p53-mutated tumours to DNA damaging agents and provide tumours with a mechanism of repair and resistance to such treatments.

Release of mitotic descendants by giant cells from irradiated Burkitt's lymphoma cell line..

Polyploid giant cells are produced as part of the response of p53 mutant Burkitt's lymphoma cell lines to high doses of irradiation. Polyploid giant cells arise by endo-reduplication in the first week after a single 10 Gray dose of irradiation. Within the giant cells a sub-nuclear structure is apparent and within this, sub-nuclear autonomy is evident, as displayed by independent nuclear structure and DNA replication in different parts of the nucleus. The majority of these cells soon die as apoptotic polykaryons. However, approximately 10-20% of giant cells remain viable into the second week after irradiation and begin vigorous extrusion of large degraded chromatin masses. During the second week, the giant cells begin to reconstruct their nuclei into polyploid 'bouquets', where chromosome double-loops are formed. Subsequently, the bouquets return to an interphase state and separate into several secondary nuclei. The individual sub-nuclei then resume DNA synthesis with mitotic divisions and sequester cytoplasmic territories around themselves, giving rise to the secondary cells, which continue mitotic propagation. This process of giant cell formation, reorganization and breakdown appears to provide an additional mechanism for repairing double-strand DNA breaks within tumour cells.