Table II. Size of nuclei in small intestinal cells Mouse Wild type Min, diploid Min, 4n Min, >2n Min, 3n n 50 43 22 28 5 Mean nuclear diameter μm2 34.43 35.36 51.28 48.9 40.15 SD 10.4 12.99 15.66 14.8 5.63 P < 0.00005, Min diploid versus Min 4n by t test. The mitotic defects we observe in Min mice are remarkably similar to those observed in human 293 cells conditionally expressing APC1–1,450. Notably, the frequent occurrence of misoriented spindles in Min mice is consistent with an in vitro spindle-anchoring defect, and the appearance of tetraploid cells is similar to the for- mation of binucleate and multinucleate cells in vitro. Together, the collection of parallel phenotypes makes a compelling case that the Min mutation compromises microtubule dynamics in vivo, leading to spindle-anchoring defects and the formation of cells with tetraploid genotypes. Therefore, it is important to consider how changes in microtubule behavior that alter cell polarity and chromo- some ploidy affect the earliest events in cancer progression. Do the tetraploid cells we observe in Min mice contribute to tumor progression? We observe a remarkably large number of tetraploid cells in crypts: projections based on the number of crypts in the small intestine, the half-life of cells, and their divi- sion rate (Marshman et al., 2002) suggest that there are nearly 105 tetraploid cells in the small intestine, resulting from one failure in 10−2 divisions. Notably, this failure rate is similar to previously published in vitro rates (Lengauer et al., 1997). Obviously, the low number of tumors in Min mice (mean = 29 tumors/Min mouse) means that not all tetraploid cells give rise to cancer cells. The ability of tetraploid cells to successfully complete another round of mitosis is dependent on their ability to resolve the abnormal configuration of centrosomes and chro- mosomes. The large number of mitotic cells with tetrapolar spindles in Min mice suggests that another round of division is attempted (Fig. 6 D). Such a resolution may give rise to the near diploid or hypotriploid cells observed in intestinal tumors in humans and mice (Cardoso et al., 2006). Although these results may appear to conflict with previous reports of stable genomes in Min tumors, there are at least two ex- planations for the differences. The first is that we analyzed tissue earlier in cancer progression than the adenomas reported by Haigis et al. (2002). It is possible that the stabilization of near diploid karyotypes occurs in late stages of tumor formation, when high chromosome number may become a liability to the cancer cell. A second potential factor relates to how the analysis was reported: Haigis et al. (2002) report the total number of FISH signals di- vided by the total number of cells counted. Although reasonable for gross changes in tumor ploidy, such an approach will fail to identify the low frequency of aneuploid events that we observe in crypt cells or dysplasias. As proof of this point, a similar report of our data gives exactly the same numbers reported by Haigis et al. (2002; unpublished data). The implication is that a constant low frequency of changes in ploidy is ongoing in dysplastic and tumor tissues. Although this rate of ploidy change may be specific for Min mice, its impact on tumor progression remains to be deter- mined. It is interesting to note that Min tumors do not advance Figure 7. A model for a transient tetraploid intermediate in cancer progression. (A) The diagram outlines how genetic lesions or changes caused by environmental stress could, in principle, give rise to mitotic defects and tetraploid cells. Such transient tetraploid cells must resolve their extra centrosomes and large chromosome number to proceed. A strong selection occurs for cells that achieve a balance of structural and nu- merical changes in chromosomes that favor ﬁtness over invia- bility and give rise to nascent tumor cells. (B) The data presented in this study are in red, which support the corre- sponding proposed steps in the model shown in A. The num- bers of tetraploid cells and dysplastic regions are reported in this study, whereas the numbers of adenomas are the mean/ Min heterozygote reported by Moser et al. (1990). 1118 JCB • VOLUME 178 • NUMBER 7 • 2007