The role of the Bub1 gene in chromosomal instability and cancer progression

Abstract

Colorectal cancer (CRC) is an important cause of mortality and morbidity in the world. Some CRC tumors show chromosomal instability (CIN) and others show microsatellite instability. The cause for microsatellite instability is considered to be the result of mutations in DNA mismatch repair genes but the genetic basis for CIN is not well understood. We hypothesized that CIN might result from mutations in genes involved in the mitotic checkpoint.;Bub1 is a gene involved in mitotic checkpoint. Mutations in Bub1 have been identified in CRC tumors. To examine the role of Bub1 in cancer and chromosomal instability, we generated mice that carry a null mutation in the Bub1 gene.;Our data shows that Bub1 is necessary for embryogenesis because homozygous Bub1 mutant mice do not survive beyond 3.5 days of development. The generation of Bub1 double targeted ES cells demonstrated that inactivation of the gene does not lead to cell lethality.;Chromosome segregation was examined in Bub1+/- and wild-type (WT) cell lines. At different passages the Bub1+/- line showed a higher percentage of aneuploid cells. To confirm this observation FISH analysis was performed in blood cells of WT and Bub1+/- mice using probes for chromosomes 9 and 17. Modest chromosomal instability was observed in the Bub1+/- samples.;Bub1+/- mice are fertile and susceptible to develop tumors late in their lives. Bub1+/- mice were bred to Apc1638N and Msh2 mutant mice. There was no difference in the incidence or multiplicity of tumors suggesting that genomic instability provided by Bub1 heterozygosity is insufficient to significantly affect the biology of the tumors in these double mutants. The tumors were analyzed by microarray-based Comparative Genomic Hybridization (aCGH) and no difference was observed in the genomic changes of the tumors in the presence or absence of the Bub1+/- mutation. The results showed clear evidence of the role of Msh2 in the regulation of homeologous recombination.;Taken together, these results suggest that the Bub1 gene is essential for normal survival. Bub1 heterozygosity leads to a mild chromosomal instability phenotype. This degree of chromosomal instability does not significantly affect the phenotype of Apc 1638N and Msh2-/- mutant mice.