Genome damage triggers non-canonical cell death during Drosophila polyploid mitosis. Heidi Bretscher, Don Fox. Duke University, Durham, NC.

   Maintaining a stable genome prevents damaged DNA, altered cellular function, and ultimately diseases such as cancer. Genome instability is monitored by a checkpoint regulated by the tumor suppressor p53, which prevents cells with damaged genomes from progressing into mitosis, where such damage can contribute to chromosome number imbalance (aneuploidy). This p53-dependent checkpoint is inhibited in murine trophoblast cells, several larval Drosophila tissues as well as certain cancerous cells, allowing cell cycling despite DNA damage. All of these cell types undergo endoreplication. During endoreplication cells alternate between G and S phases thus increasing in ploidy but not cell number. In contrast to endoreplicating cells in cancer, most programmed endocycling cells are terminally differentiated and do not re-enter the mitotic cell cycle, preventing study of connections between polyploidy, DNA damage, and aneuploidy. We previously established a model to address mechanisms by which polyploidy promotes genome instability during mitosis. In the Drosophila rectum, we found endoreplicated cells can re-enter mitosis as polyploid cells, but that such divisions are error-prone. Given this new connection between endoreplication and genome instability, we next examined the status of the p53 checkpoint in endoreplicating and mitotic rectal cells. Like other endoreplicating cells, we find Drosophila rectal cells tolerate significant DNA damage. p53 over-expression does not sensitize rectal cells to cell death, indicating this canonical death pathway is silenced in rectal cells. However, when rectal cells with severely damaged genomes enter mitosis, they undergo cell death. Unlike in diploid cells, we find such death is caspase- and p53- independent. However, this death is dependent on re-entry into the mitotic cell cycle. Our data suggest genome damage in naturally occurring polyploid cells can trigger Mitotic Catastrophe (MC), a poorly understood cell death mechanism. Lack of this mechanism could thus contribute to expansion of cancerous polyploid cells.