Abstract 5088: A synthetic genetic interaction screen in yeast identifies Plk1 as a promising therapeutic target in cancer cells that overexpress Cks1b
Authors: Reid Robert J. D. , Du Xing , DIttmar John C. , Rayannavar Vinayak , Sunjevaric Ivana , Maurer Matthew , Rothstein Rodney 
Published: May 2015

Abstract
Cancer cells often over-express specific genes as a result of translocation or gene amplification. We developed a high throughput screening protocol to mimic cancer-related over-expression in Saccharomyces cerevisiae deletion strain libraries to identify genes whose functions become essential only when the cancer-related query gene is over-expressed. The resulting synthetic dosage lethal (SDL) interactions uncover conserved genes that can be targeted to kill cancer cells but leave normal cells unaffected, providing a cancer-specific therapy. The CKS1b gene is located on chromosome 1q21, a region that is frequently amplified in breast, lung and liver cancers. Cks1b is a conserved regulatory subunit of cyclin-CDK complexes which functions during cell cycle progression. The SDL screen identified multiple pathways affecting mitotic progression that are essential when yeast CKS1 is over-expressed. One pathway controls a morphological checkpoint that leads to stabilization of the mitotic inhibitor Swe1 (Wee1 in mammals) resulting in delayed mitosis when cell polarity is disrupted. Swe1, a tyrosine kinase, inhibits the cyclin dependent kinase (CDK) complex by phosphorylation of a conserved tyrosine-19 on the yeast CDK, Cdc28. We find that the SDL interaction between CKS1 and the morphological checkpoint requires both Swe1 and the Cdc28 tyrosine-19 residue. Normal turnover of Swe1 occurs via SCF-mediated degradation after phosphorylation by the cyclin-CDK complex and the Polo-like kinase (Cdc5). Since, mutant alleles of CDC5 were identified in the SDL screen, we investigated the effect of targeting human PLK1, the mammalian Cdc5 ortholog, in breast cancers with varying expression of Cks1b. We first correlated RNAi knock down of PLK1 across 28 breast cancer cell lines [1] with Cks1b expression data taken from the Cancer Cell Line Encyclopedia [2]. Importantly, we find that growth inhibition by PLK1 knockdown correlates with increased Cks1b levels (r = -0.554, top 0.8th percentile). TCGA data also provides evidence for this SDL in breast cancer, since CKS1b over-expression and down-regulation of PLK1 expression rarely co-occur (mutual exclusion p-value 6.47x10-8). Together, these analyses support the hypothesis that PLK1 activity is essential when CKS1b levels are elevated. Finally, using shRNAs, we independently confirmed the PLK1 knockdown sensitivity in 8 breast cancer cell lines. Three of the 4 sensitive lines show increased CKS1b expression, while 3 of the 4 insensitive lines show low CKS1b expression, corroborating the correlation of CKS1b and PLK1 levels and lending support to conservation of the genetic interaction.
[1] Marcotte, et al., Cancer Discovery, 2:172 (2012)
[2] Barretina, et al., Nature, 483:603 (2012)
Citation Format: Robert J. D. Reid, Xing Du, John C. DIttmar, Vinayak Rayannavar, Ivana Sunjevaric, Matthew Maurer, Rodney Rothstein. A synthetic genetic interaction screen in yeast identifies Plk1 as a promising therapeutic target in cancer cells that overexpress Cks1b. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 5088. doi:10.1158/1538-7445.AM2014-5088