Proteins Required for Centrosome Clustering in Cancer Cells

Identified in an RNA interference screen, proteins that prevent spindle multipolarity in human cancer cells may prove to be promising drug targets. Releasing Tension Could Help Tame Tumor Cells Tumor cells do some things very well—proliferate, for example—but do other things very badly. Instead of orderly, bipolar cytoskeletal machinery that neatly pulls the chromosomes apart during cell division, tumor cells are often multipolar, a state caused by extra copies of the microtubule organizer, the centrosome. These extra organizing centers form multipolar spindle apparatuses, the scaffolds on which cell division takes place, thus producing abnormal division and cell death. Some cells successfully correct this defect by bundling the extra spindles along with the primary ones, and these cells go on to divide properly and proliferate, becoming thriving tumors. Leber and colleagues have taken advantage of this organizational weakness of tumor cells and have identified the proteins these cells use to bundle their spindles next to the primary ones. These proteins are prime targets for new chemotherapeutic drugs, which would prevent corrective bundling, sentencing the cells to abnormal cell division and death. The authors used a small interfering RNA screen to eliminate the expression of 21,000 genes, one by one, in a squamous cell carcinoma cell line. By finding which cells had more than the usual two spindle poles during mitosis—indicative of extra centrosomes—and then verifying these in a second screen, the authors identified 82 genes that encode proteins that the cancer cells use to inhibit damaging multiple centrosomes. Among these proteins are many involved in attaching chromosomes to the cytoskeletal scaffold: those in the chromosomal passenger complex, a regulator of chromosome-microtubule interactions and spindle tension at the point where the spindle attaches; members of the Ndc80 complex, a contact point for chromosome-microtubule attachment; and CENPT, part of a DNA binding complex at the attachment point. Another particularly interesting protein picked up by the screen is CEP164, which is found at the ends of centriole appendages, which are microtubule anchors at the centrosome. By looking for commonalities among these proteins, the authors hypothesized that spindle tension might be a key characteristic required for effective corrective spindle bundling. They tested this idea by measuring the distance between the chromosomes, confirming a central role for tension in this homeostatic mechanism, by which cancer cells compensate for extra centrosomes. Therefore, interfering with the function of any of the proteins found in the screen could, in principle, eliminate this tension and prevent bundling, leading to tumor cell death. These potential drug targets that the cell uses to bypass normal protective mechanisms against cancer are new weapons that can be added to the arsenal of anticancer drug developers. Current cancer chemotherapies are limited by the lack of tumor-specific targets, which would allow for selective eradication of malignant cells without affecting healthy tissues. In contrast to normal cells, most tumor cells contain multiple centrosomes, which tend to cause the formation of multipolar mitotic spindles, chromosome segregation defects, and cell death. Nevertheless, many cancer cells divide successfully because they can cluster multiple centrosomes into two spindle poles. Inhibition of this centrosomal clustering, with consequent induction of multipolar spindles and subsequent cell death, would specifically target cancer cells and overcome one limitation of current cancer treatments. We have performed a genome-wide RNA interference screen to identify proteins involved in the prevention of spindle multipolarity in human cancer cells with supernumerary centrosomes. The chromosomal passenger complex, Ndc80 microtubule-kinetochore attachment complex, sister chromatid cohesion, and microtubule formation via the augmin complex were identified as necessary for centrosomal clustering. We show that spindle tension is required to cluster multiple centrosomes into a bipolar spindle array in tumor cells with extra centrosomes. These findings may explain the specificity of drugs that interfere with spindle tension for cancer cells and provide entry points for the development of therapeutics.