BUB1B

Abstract: Faithful segregation of replicated chromosomes is essential for maintenance of genetic stability and seems to be monitored by several mitotic checkpoints. Various components of these checkpoints have been identified in mammals, but their physiological relevance is largely unknown. Here we show that mutant mice with low levels of the spindle assembly checkpoint protein BubR1 develop progressive aneuploidy along with a variety of progeroid features, including short lifespan, cachectic dwarfism, lordokyphosis, cataracts, loss of subcutaneous fat and impaired wound healing. Graded reduction of BubR1 expression in mouse embryonic fibroblasts causes increased aneuploidy and senescence. Male and female mutant mice have defects in meiotic chromosome segregation and are infertile. Natural aging of wild-type mice is marked by decreased expression of BubR1 in multiple tissues, including testis and ovary. These results suggest a role for BubR1 in regulating aging and infertility.

Abstract: Purpose: Most breast cancers have chromosomal instability that seems related to defective mitotic spindle checkpoints. Because the molecular basis of this defect is unknown, we evaluated breast cancer cell lines and tissues for possible defects involving the major mitotic checkpoint genes responsible for maintaining chromosomal stability. Experimental Design: We analyzed sequences and expression levels (RNA and protein) of eight major spindle checkpoint genes ( MAD1L1, MAD2L1, MAD2L2, BUB1, BUB1B, BUB3, CDC20 , and TTK ) in a panel of 12 breast cancer cell lines, most with established genetic instability and defective spindle damage checkpoint response. mRNA levels of these genes were also measured in primary tumor samples, and immunohistochemical staining was used to evaluate BUB1B protein levels in a panel of 270 additional cases of breast cancer. Results: No functionally significant sequence variations were found for any of the eight genes in the breast cancer cell lines with chromosomal instability. More surprisingly, the mRNA and protein levels for these checkpoint genes are significantly higher in the genetically unstable breast cancer cell lines and in high-grade primary breast cancer tissues than in the stable (and checkpoint proficient) MCF-10A and normal mammary epithelial cells, or in normal breast tissues. In fact, overexpression of the BUB1B protein is a marker that recognizes nearly 80% of breast cancers in paraffin-embedded tissues. Conclusions: Defective mitotic spindle checkpoints in breast cancer are most likely not caused by low expression or mutations of these eight checkpoint genes. High levels of these particular transcripts could represent a cellular compensation for defects in other molecular components of the mitotic spindle damage checkpoint, and increased expression of these genes might be markers of breast cancers with chromosomal instability.

Abstract: Sequence alterations of mitotic checkpoint genes, hBUB1 and hBUBR1 , were examined, and their gene transcripts were quantified using on-line, real-time quantitative reverse transcription-PCR in surgically resected human colorectal cancers and their neighboring normal tissues. Our results reveal a new hBUB1 missense mutation (Ala130Ser) but not any hBUBR1 coding sequence mutations. hBUB1 and hBUBR1 mRNA levels were reduced to <10% of the neighboring normal tissues in 3 of 103 and 3 of 109 carcinomas, respectively, and to <50% in 7 and 7 carcinomas, whereas the overall expression levels were markedly higher in cancers than in normal tissues. Carcinomas with reduced hBUB1 and/or hBUBR1 mRNA levels, as well as the colon carcinoma harboring the hBUB1 mutation, were associated with lymph node metastasis ( P < 0.005) and shorter relapse-free survival after surgery ( P = 0.006). Thus, hBUB1 and hBUBR1 may contribute to a specific driving force in tumor metastasis and progression as a result of nonmutational, as well as mutational, inactivation.

Abstract: Aneuploidy is defined as numerical abnormalities of chromosomes and is frequently (>90%) present in solid tumors. In general, tumor cells become increasingly aneuploid with tumor progression. It has been proposed that enhanced genomic instability at least contributes significantly to, if not requires, tumor progression. Two major modes for genomic instability are microsatellite instability (MIN) and chromosome instability (CIN). MIN is associated with DNA-level defects (e.g. mismatch repair defects), and CIN is associated with mitotic errors such as chromosome mis-segregation. The mitotic spindle assembly checkpoint (SAC) ensures that cells with defective mitotic spindles or defective interaction between the spindles and kinetochores do not initiate chromosomal segregation during mitosis. Thus, the SAC functions to protect the cell from chromosome mis-segregation and anueploidy during cell division. A loss of the SAC function results in gross aneuploidy, a condition from which cells with an advantage for proliferation will be selected. During the past several years, a flurry of genetic studies in mice and humans strongly support the notion that an impaired SAC causes enhanced genomic instabilities and tumor development. This review article summarizes the roles of key spindle checkpoint proteins {i.e. Mad1/Mad1L1, Mad2/Mad2L1, BubR1/Bub1B, Bub3/Bub3 [conventional protein name (yeast or human)/mouse protein name]} and the modulators (i.e. Chfr/Chfr, Rae1/Rae1, Nup98/Nup98, Cenp-E/CenpE, Apc/Apc) in genomic stability and suppression of tumor development, with a focus on information from genetically engineered mouse model systems. Further elucidation of molecular mechanisms of the SAC signaling has the potential for identifying new targets for rational anticancer drug design.