Cell Biology International 2000, Vol. 24, No. 9, 635–648 doi:10.1006/cbir.2000.0558, available online at http://www.idealibrary.com on RELEASE OF MITOTIC DESCENDANTS BY GIANT CELLS FROM IRRADIATED BURKITT’S LYMPHOMA CELL LINES JEKATERINA A. ERENPREISA 1 *, MARK S. CRAGG 2 , BIRGITTA FRINGES 4 , IGOR SHARAKHOV 5 and TIMOTHY M. ILLIDGE 2,3 1 Laboratory Tumor Cell Biology, Latvian University Biomedicine Centre, Riga, Latvia; 2 Tenovus Research Laboratory, Cancer Sciences Division, Southampton General Hospital, Southampton, U.K.; 3 The CRC Laboratory, Southampton General Hospital, Southampton, U.K.; 4 Institute of Pathology, Justus-Liebig-University, Giessen, Germany; 5 Institute of Biology and Biophysics, Tomsk State University, Tomsk, Russia Received 11 January 2000; accepted 3 May 2000 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.  2000 Academic Press K: lymphoma; irradiation; giant cells; mitotic survivors. INTRODUCTION A tight co-ordination of DNA replication and mitosis is thought to be required in the eukaryotic cell cycle to preserve the genetic fidelity of daughter cells and is regulated, amongst other factors, by the tumour suppressor gene p53 (Levine, 1997). Muta- tions of this gene are found in 50–60% of primary tumours and the frequency increases with tumour progression (Greenblatt et al., 1994). Uncoupling of DNA replication from mitosis leads to the formation of polyploid giant cells (Nagl, 1995). As such, the formation of giant cells occurs through what has been termed the endocycle and appears to be a characteristic feature of the genomic instability of p53 mutant or null cells (Cross et al., 1995; Olive et al., 1996; Merritt et al., 1997). Giant cell forma- tion is further enhanced by irradiation (Peled et al., 1996) and is only one of a set of responses dis- played by damage-resistant tumours after geno- toxic insult, which have been grouped under the collective term ‘mitotic’ or ‘reproductive’ death. The mechanisms which define mitotic death are currently ill-defined (Bernhard et al., 1996; Grafi, 1998). Mitotic death was reported to be associated with abrogation of the G1/S DNA damage checkpoint and delay in the G2/M checkpoint. After adapta- tion, a few rounds of aberrant mitoses ensue, frequently followed by delayed (post-mitotic) apoptosis, micronucleation, and mitotic catas- trophe, culminating in the death of the polyploid giant cells (Lock and Ross, 1990). Importantly, *Please address all correspondence to: Dr Jekaterina Erenpreisa, Latvian University Biomedicine Centre, Ratsupites str.1, Riga, LV-1067, Latvia. E-mail: cancer@latnet.lv 1065–6995/00/090635+14 $35.00/0  2000 Academic Press