BRCA1 Regulates Gene Expression for Orderly Mitotic Progression

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Abstract
Germline mutations of the BRCA1 confer an increased risk for breast cancer, ovarian cancer, and several other tumor types, including prostate cancer. In sporadic (non-hereditary) breast and ovarian cancers, BRCA1 expression is frequently decreased or absent, implicating BRCA1 in the etiology of these cancers. To study the contribution of BRCA1 to sporadic cancers, we tested the effect of knocking down endogenous BRCA1 on the gene expression profiles of human prostate (DU-145) and breast (MCF-7) cancer cell lines. DNA microarray and independent confirmatory RNA analyses revealed that treatment with BRCA1 small interfering (si) RNA caused decreased expression of genes involved in cell cycle regulation and in DNA replication and repair. In particular, BRCA1-siRNA caused down-regulation of multiple genes implicated in the mitotic spindle checkpoint (eg., BUB1, BUB1B, HEC, STK6, and BIRC5), chromosome segregation (eg., ESPL1, NEK2, PTTG1, and multiple kinesins and kinesin-like proteins), centrosome function (eg., ASPM), cytokinesis (eg., PRC1, PLK, MPHOSPH1, and KNSL2), and the transition into and progression through mitosis (eg., B-type cyclins, CDC2, and CDC20). Consistent with these findings, cells treated with BRCA1-siRNA showed attenuation of the mitotic spindle checkpoint, demonstrated by a failure of cells to arrest in metaphase following treatment with nocodazole, an agent that activates the spindle checkpoint. On the other hand, BRCA1-siRNA did not attenuated the ionizing radiation-inducible G2 checkpoint or the G2 decatenation checkpoint. Finally, BRCA1 knockdown caused or promoted the accumulation of binucleated and multinucleated cells, suggesting a defect in the coordination of cytokinesis and karyokinesis; and BRCA1-siRNA caused centrosome amplification. These findings suggest that BRCA1 transcriptionally regulates gene expression for orderly mitotic progession.

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