No abstract

The physiologic function of BUBR1, a key component of the spindle checkpoint, was examined by generating BUBR1-mutant mice. BUBR1 ؊/؊ embryos failed to survive beyond day 8.5 in utero as a result of extensive apoptosis. Whereas BUBR1 ؉/؊ blastocysts grew relatively normally in vitro, BUBR1 ؊/؊ blastocysts exhibited impaired proliferation and atrophied. Adult BUBR1 ؉/؊ mice manifested splenomegaly and abnormal megakaryopoiesis. BUBR1 haploinsufficiency resulted in an increase in the number of splenic megakaryocytes, which was correlated with an increase in megakaryocytic, but a decrease in erythroid, progenitors in bone marrow cells. RNA interference-mediated down-regulation of BUBR1 also caused an increase in polyploidy formation in murine embryonic fibroblast cells and enhanced megakaryopoiesis in bone marrow progenitor cells. However, enhanced megakaryopoiesis in BUBR1 ؉/؊ mice was not correlated with a significant increase in platelets in peripheral blood, which was at least partly due to a defect in the formation of proplateletproducing megakaryocytes. Together, these results indicate that BUBR1 is essential for early embryonic development and normal

Faithful chromosome segregation is essential for the maintenance of genetic stability during cell division and it is at least partly monitored by the spindle checkpoint, a surveillance mechanism preventing the cell from prematurely entering anaphase. The adenomatous polyposis coli (Apc) gene also plays an important role in regulating genomic stability, as mutations of Apc cause aneuploidy. Here we show that whereas Apc Min/؉ mice developed many adenomatous polyps, mostly in the small intestine, by 3 mo of age; BubR1 ؉/؊ Apc Min/؉ compound mutant mice developed 10 times more colonic tumors than Apc Min/؉ mice. The colonic tumors in BubR1 ؉/؊ Apc Min/؉ mice were in higher grades than those observed in Apc Min/؉ mice. Consistently, BubR1 ؉/؊ Apc Min/؉ murine embryonic fibroblasts (MEFs) contained more ␤-catenin and proliferated at a faster rate than WT or BubR1 ؉/؊ MEFs. Moreover, BubR1 ؉/؊ Apc Min/؉ MEFs slipped through mitosis in the presence of nocodazole and exhibited a higher rate of genomic instability than that of WT or BubR1 ؉/؊ or Apc Min/؉ MEFs, accompanied by premature separation of sister chromatids. Together, our studies suggest that BubR1 and Apc functionally interact in regulating metaphase-anaphase transition, deregulation of which may play a key role in genomic instability and development and progression of colorectal cancer.polyposis ͉ genetic instability ͉ colon cancer ͉ spindle checkpoint ͉ mitosis