Analysis of Bub1 Deficiency in Mice

Accurate chromosome segregation is essential for living organisms. Errors in chromosome segregation can lead to the gain or loss of whole chromosomes, a condition known as aneuploidy, which is present in many cancers. One mechanism employed by the cell to monitor chromosome segregation is the spindle checkpoint, a complex signaling pathway which ensures chromosomes are properly aligned on the metaphase plate before the cell proceeds into anaphase. When lagging chromosomes are present, the spindle checkpoint delays the metaphase-to-anaphase transition, which enables the lagging chromosomes to properly orient before the cell proceeds into anaphase. Microtubule attachment to the kinetochore and tension across the kinetochore are both monitored by the spindle checkpoint, and a single unattached kinetochore is sufficient for spindle checkpoint activation. The delay in mitosis is due to the inactivation of the anaphase promoting complex (APC), a ubiquitin ligase required for sister chromatid separation.

Bubl is a spindle checkpoint protein kinase that localizes to kinetochores lacking attachment to microtubules and is necessary for the localization of other checkpoint proteins, but its role is not clearly understood. We have used Bub1 knockout mice to study the effects of Bub1 loss and its role in tumorigenesis. Our data suggest Bub1 homozygous null mutations are lethal ~6.5 d.p.c., and Bub1 heterozygous females exhibit reduced reproductive capacity. Our data also suggest Bub1 heterozygous mouse embryonic fibroblasts (MEFs), to a limited extent, exit mitosis prematurely in the presence of the spindle damaging agent nocodazole. In addition, Bub1 heterozygous cells do not differ in their growth kinetics from wildtype cells, nor do they differ with respect to colony forming assays. Our Bub1 heterozygous mice do not show an increased incidence of tumor formation compared to our wildtype mice, even at 1 ½ years of age. These data suggest other insults, in addition to the loss of one copy of Bub1, are needed to drive the tumorigenesis process.