A dominant interfering Bub1 mutant is insufficient to induce or alter thymic tumorigenesis in vivo, even in a sensitized genetic background.

Aneuploidy is a common feature of human tumors, often correlating with poor prognosis. The mitotic spindle checkpoint is thought to play a major role in aneuploidy suppression. To investigate the role of the spindle checkpoint in tumor suppression in vivo, we developed transgenic mice in which thymocytes express a dominant interfering fragment of Bub1, a kinase regulator of the spindle checkpoint. We report that, despite high-level expression of dominant-negative Bub1 (Bub1DN), a protein known to inhibit spindle checkpoint activity in cultured cells, thymocytes show no evidence of spindle checkpoint impairment. Transgenic animals also failed to show an increased predisposition to spontaneous tumors. Moreover, the Bub1DN transgene failed to alter the timing or characteristics of thymic lymphoma development in p53 heterozygous or homozygous null backgrounds, indicating that the lack of tumorigenesis is not due to suppression by p53-dependent checkpoints. These results indicate that overexpression of a Bub1 N-terminal fragment is insufficient to impair the spindle checkpoint in vivo or to drive tumorigenesis in the highly susceptible murine thymocyte system, either alone or in combination with G(1) checkpoint disruption.