Polyploidy Rewires The Spindle Assembly Checkpoint. Benjamin M Stormo¹, Ruth Montague², Sarah Paramore², Don Fox^1,2. 1) Department of Cell Biology, Duke University, Durham, NC; 2) Department of Cancer Biology and Pharmacology, Duke University, Durham, NC.

   Many types of human cancers are known to be polyploid, however whether polyploidy is a result or a cause of genome instability is not known. However, recent studies of mitotic polyploid cells in both Drosophila and mice have shown these cells are particularly susceptible to errors in separating their chromosomes during mitosis. Studying developmentally programmed polyploid cell divisions that occur in Drosophila rectal papillae, we found polyploid cells show significantly levels of unaligned chromosomes and chromosomal bridging. We hypothesized that these mitotic errors are caused by an aberrant Spindle Assembly Checkpoint (SAC). The SAC is a complex of proteins that binds to unattached kinetochores during metaphase and inhibits the Anaphase Promoting Complex (APC). We suspected the SAC might not function in polyploid cells, leading to genomic instability. By treating developing papillae with microtubule poisons, we find the SAC remains intact, but appears less robust, in polyploid cells. Further, we find the SAC regulator Mad2 fails to localize to papillar kinetochores, and that loss of Mad2 has no effect on recruitment of unaligned chromosomes. However, we do detect a polyploid-specific role for Mad2 that is independent of the kinetochore. Through live cell imaging, we find mad2 null animals have a high rate of chromosome bridging, specifically in polyploid cells, suggesting cytoplasmic Mad2 regulate anaphase timing without localizing to the kinetochore. These results suggest study of papillar cells may help to resolve the long-standing controversy regarding Mad2 function outside of the kinetochore. Taken together, our work suggests a novel SAC configuration in polyploid cells that is less robust, providing a link between polyploidy and genomic instability.