JCB: ARTICLE APC mutations lead to cytokinetic failures in vitro and tetraploid genotypes in Min mice Christine M. Caldwell, Rebecca A. Green, and Kenneth B. Kaplan Section of Molecular and Cell Biology, University of California, Davis, Davis, CA 95616 Y G O L O I B L L E C F O L A N R U O J E H T P bility contributes to cancer progression, with its initi- revious research has proposed that genomic insta- ation linked to tetraploid cell formation (Duesberg, P., and R. Li. 2003. Cell Cycle. 2:202–210; Ganem, N.J., Z. Storchova, and D. Pellman. 2007. Curr. Opin. Genet. Dev. 17:157–162). However, there is little direct evidence linking cancer-causing mutations with such events, and it remains controversial whether genomic instability is a cause or an effect of cancer. In this study, we show that adenomatous polyposis coli (APC) mutations found in human colorectal cancers dominantly inhibit cytokinesis by preventing mitotic spindle anchoring at the anaphase Introduction To explain how genomic instability arises in cancer cells, one model posits that the formation of cells with tetraploid genomes represents a transient initiating event (see model in Fig. 7; Duesberg and Li, 2003; Ganem et al., 2007). Consistent with this model, it has recently been observed that tetraploid genomes promote an increase in numerical and structural chromosomal aberrations in p53-deficient cells (Storchova and Pellman, 2004; Fujiwara et al., 2005; Storchova et al., 2006). Although the pre- cise mechanisms that lead to the chromosomal defects in these cells are unclear, the stress of maintaining tetraploid genomes may select for viable resolutions that give rise to a variety of ge- nomic configurations. Such an increase in genomic instability would allow nascent tumor cells to sample a wide genetic space, increasing their chance of finding a genomic configuration to overcome their normal growth constraints. If tetraploid genomes play an initiating role in genomic instability in cancer, it is predicted that cancer-promoting lesions will result in the early appearance of tetraploid cells in sufficient numbers to ensure the selection of genetic winners. In this study, we provide Correspondence to Kenneth B. Kaplan: kbkaplan@ucdavis.edu R.A. Green’s present address is Ludwig Institute for Cancer Research, La Jolla, CA 92093. Abbreviations used in this paper: APC, adenomatous polyposis coli; EYFP, enhanced YFP. The online version of this article contains supplemental material. © The Rockefeller University Press $30.00 The Journal of Cell Biology, Vol. 178, No. 7, September 24, 2007 1109–1120 http://www.jcb.org/cgi/doi/10.1083/jcb.200703186 cortex and, thus, blocking initiation of the cytokinetic furrow. We ﬁnd that dividing crypt cells in the small intestines of APCMin/+ mice exhibit similar mitotic defects, including misoriented spindles and misaligned chromosomes. These defects are observed in normal crypt cells with wild-type levels of β-catenin and, importantly, are associated with tetraploid genotypes. We provide direct evidence that the dominant activity of APC mutants induces aneuploidy in vivo. Our data support a model whereby tetraploid cells represent a ﬁrst step in the onset of genomic instability and colorectal cancer. evidence that dominant mutations in the tumor suppressor gene adenomatous polyposis coli (APC), which is frequently observed in human colorectal cancer, drive tetraploid formation by caus- ing failures in cytokinesis before the earliest steps associated with colorectal cancer progression. Previous work has implicated APC in the regulation of mitotic events, including formation of the mitotic spindle and proper functioning of the spindle checkpoint (Green and Kaplan, 2003; Tighe et al., 2004; Green et al., 2005; Draviam et al., 2006). APC associates with the mitotic spindle through its inter- action with microtubules and with the microtubule plus end– binding protein EB1 (Munemitsu et al., 1994; Smith et al., 1994; Su et al., 1995; Morrison et al., 1998; Green et al., 2005). In previous work, we have shown that APC mutations similar to those found in human cancer patients act dominantly to pre- vent the interaction of APC with EB1 and, thus, inhibit micro- tubule dynamics and proper chromosome alignment (Green et al., 2005). Although microtubule dynamics are important for chromosome alignment during metaphase, mitotic spindle de- fects can also interfere with cytokinesis in tumor cells (Wheatley and Wang, 1996; Alsop and Zhang, 2003; Pihan et al., 2003; Wang et al., 2004). In part, this reflects the role of multiple sets of microtubules in regulating initiation and ingression of the cleavage furrow; peripheral overlapping microtubules at the equatorial cortex converge to initiate furrow formation, whereas JCB 1109