Oncogenic addiction provides an opportunity to develop new treatment options, especially for childhood cancers. Pediatric tumors contain a lower number of oncogenic mutations compared to most adult cancers, suggesting stronger dependency on individual oncogenes, such as chimeric transcription factors that have the ability to control multiple oncogenic pathways. Taking advantage of this addiction, targeting of oncogenic transcription factors becomes a new powerful strategy for therapy of translocation positive pediatric tumors like alveolar rhabdomyosarcoma (aRMS), which is characterized by a very dismal prognosis.

As transcription factors are considered to be difficult to target, we developed an approach to screen for druggable upstream regulators like kinases. In two parallel activity reporter assays, using a kinome siRNA library and a small molecule library, we identified PLK1 to contribute to the activity of PAX3-FOXO1, the oncogenic transcription factor specific to aRMS. Mechanistically, we were able to demonstrate a direct interaction of the two proteins by co-immunoprecipitation. PLK1 silencing or inhibition increased degradation of PAX3-FOXO1, which suggests a stabilization of PAX3-FOXO1 due to phosphorylation by PLK1. Candidate phosphorylation sites were identified by in vitro kinase assays and mass spectrometric analysis and site-specific mutagenesis is currently utilized to validate the impact of these sites on fusion protein turnover. To highlight PLK1 as therapeutic target in aRMS, in vivo treatment studies using the PLK1 inhibitor BI 2536 resulted in complete tumor regression in two aRMS xenograft mouse models. Finally, tissue microarray analysis of human aRMS tumor biopsies showed PLK1 to be overexpressed and predictive of overall survival.

Hence, our preclinical studies validate PLK1 as highly relevant drug target in alveolar rhabdomyosarcoma, similar to recent findings in other pediatric tumors. Our data suggests that PLK1 inhibitors warrant further investigations to evaluate the potential to be translated into a clinical setting.

Citation Format: Verena Thalhammer, David Herrero-Martin, Regina Hecker, Dominik Laubscher, Laura Lopez-Garcia, Marco Wachtel, Peter Bode, Beat Schäfer. PLK1 regulates PAX3-FOXO1 stability and its inhibition mediates regression of alveolar rhabdomyosarcoma xenograft tumors. [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 3966. doi:10.1158/1538-7445.AM2014-3966