Tumor Suppressor function of the deubiquitinating enzyme BAP1 and its substrate gamma-tubulin In regulation of cell cycle and genome stability

In normal cells, the cell cycle is controlled by a complex series of signalling pathways by which a cell grows, replicates its DNA and divides. The critical task of the cell cycle is to ensure that DNA is faithfully replicated and that identical chromosomal copies are distributed equally to two daughter cells during mitosis with the help of the mitotic spindle. Thus, mitotic spindle formation is vital for proper chromosome congression, alignment and segregation during cell division. Dysregulation of any of the components involved in this process may cause mis-segregation of the chromosomes and chromosomal instability, which are commonly observed in cancer. There are a number of surveillance mechanisms to ensure errors are corrected, and if not, the cells commit suicide (apoptosis). In cancer, this tightly regulated process malfunctions as a result of genetic mutations, resulting in uncontrolled cell proliferation. g-Tubulin is a member of the tubulin family that is required for interphase αβ-tubulin nucleation, spindle formation and centrosomal duplication. The cell cycle is a closely controlled process, involving multiple components with regulation on several levels. The findings from our studies suggest that at the G1/S-phase transition, when retinoblastoma protein (RB) releases E2 promoter binding factors (E2Fs), g-tubulin associates with and captures E2F to ensure a transient transcription of genes necessary for S-phase entry. In addition, we found that the g-tubulin deubiquitination enzyme, BRCA1 Associated Protein 1 (BAP1), was downregulated in metastatic adenocarcinoma breast cell lines compared to non-cancerous human breast epithelial cells. Reduced expression of BAP1 in breast cancer cell lines was associated with mitotic abnormalities. Rescue experiments involving expression of BAP1 reduced ubiquitination of g-tubulin and prevented mitotic defects. Furthermore, we found that the tumour suppressor function of BAP1 in neuroblastoma is mediated through the arrest of cells in S phase, and further, it promoted cell death. In conclusion, our studies define important functions of g-tubulin and BAP1 in the regulation of the cell cycle and proper segregation of the chromosomes