Pediatric tumors harbor very low numbers of somatic mutations and therefore offer few targets to improve therapeutic management with targeted drugs. In particular, outcomes remain dismal for patients with metastatic alveolar rhabdomyosarcoma (aRMS), where the chimeric transcription factor PAX3/7-FOXO1 has been implicated but problematic to target.

A number of drugs developed against cancer-specific molecular targets have been shown to offer survival benefits alone or in combination with standard treatments, especially for those cases where tumor pathogenesis is dominated by a single molecular abnormality. One example for such a tumor type is alveolar rhabdomyosarcoma (aRMS) which is characterized by a specific translocation creating the oncogenic PAX3/FKHR transcription factor, believed to be the molecular basis of the disease.Recently, we were able to demonstrate that the small molecule inhibitor PKC412 (midostaurin) exhibits strong antitumor activity against aRMS by reducing the transcriptional activity of PAX3/FKHR.Here, we screened for combination strategies that are superior to PKC412-only treatment and found that the combination of PKC412 with histone deacetylase (HDAC) inhibitors like valproic acid (VPA) synergistically induced apoptosis resulting in suppressed aRMS tumor growth in vivo.We provide evidence that the antitumor effect upon combination treatment is achieved by VPAinduced reactivation of p21 which is downregulated in aRMS cells via destabilization of the transcriptional regulator EGR1 by PAX3/FKHR. Our study highlights a possible mechanism behind the increased efficacy and indicates that different arms of PAX3/FKHR oncogenicity can be exploited therapeutically by the specific combination of drugs to increase their therapeutic potential.

<div>Abstract<p>Pediatric tumors harbor very low numbers of somatic mutations and therefore offer few targets to improve therapeutic management with targeted drugs. In particular, outcomes remain dismal for patients with metastatic alveolar rhabdomyosarcoma (aRMS), where the chimeric transcription factor PAX3/7-FOXO1 has been implicated but problematic to target. In this report, we addressed this challenge by developing a two-armed screen for druggable upstream regulatory kinases in the PAX3/7-FOXO1 pathway. Screening libraries of kinome siRNA and small molecules, we defined PLK1 as an upstream-acting regulator. Mechanistically, PLK1 interacted with and phosphorylated PAX3-FOXO1 at the novel site S503, leading to protein stabilization. Notably, PLK1 inhibition led to elevated ubiquitination and rapid proteasomal degradation of the PAX3-FOXO1 chimeric oncoprotein. On this basis, we embarked on a preclinical validation of PLK1 as a target in a xenograft mouse model of aRMS, where the PLK1 inhibitor BI 2536 reduced PAX3-FOXO1–mediated gene expression and elicited tumor regression. Clinically, analysis of human aRMS tumor biopsies documented high PLK1 expression to offer prognostic significance for both event-free survival and overall survival. Taken together, these preclinical studies validate the PLK1–PAX3-FOXO1 axis as a rational target to treat aRMS. <i>Cancer Res; 75(1); 98–110. ©2014 AACR</i>.</p></div>

<p>Supplementary Table S3. PAX3-FOXO1 phospho-peptides identified by mass spectrometry.</p>

<p>Supplementary Table S1. PAX3-FOXO1 regulating kinases identified by siRNA screen.</p>

<p>Supplementary Table S2. Small-molecule library.</p>

<p>Supplementary Materials and Methods. Description of additional methods and procedures used in the study.</p>

<p>Supplementary Figures S1-S7. Establishment of screening approach (S1); BI 2536 causes apoptosis and cell cycle arrest (S2); PLK1 phosphorylates PAX3-FOXO1 in vitro (S3); PLK4 is overexpressed in tumor biopsies (S4); PLK1 expression does not correlate with MIB1 expression in tumor biopsies (S5); PLK4 silencing reduces PAX3-FOXO1 activity (S6); PLK1 inhibition causes NMYC degradation (S7).</p>