Spindle misorientation in tumors from APC(min/+) mice.

The adenomatous polyposis coli (APC) tumor suppressor gene is mutated in the majority of colon cancers, and its mutation may initiate cancer by multiple mechanisms. Recently, abnormal mitotic spindle orientation was shown in normal-appearing tissues from mice heterozygous for APC mutation. To determine the effect of APC mutation on spindle orientation in tumors, and to better understand its mechanism, we measured mitotic spindle orientation in intestinal tumors from APC mutant mice, with three-dimensionally reconstructed confocal stacks of microtubule immunofluorescence images. We found spindle angles were increased in crypts heterozygous for the APC(min) mutation, and further increased in tumors. Astral microtubules of these spindles were clearly evident, suggesting astral microtubule loss is not a major mechanism of spindle misorientation in intestinal cells lacking wild-type APC. beta-Catenin staining was markedly abnormal in crypts and tumors from the mutant mice, suggesting a possible role in spindle orientation. Spindle angles in colon tumors with wild-type APC were equivalent to those in wild-type mice, showing that spindle misorientation is specific to APC mutation and not a general feature of tumors. We suggest spindle misorientation may contribute to the loss of normal tissue organization during tumor formation and could offer new insights into early carcinogenic events.