Abstract 4457: Chromatin remodeling as a potential new strategy for fusion positive rhabdomyosarcoma therapy
Authors: Marques Joana G. , Gryder Berkley , Boehm Maria , Wachtel Marco , Song Young , Chou Hsien-Chao , Patidar Rajesh , Liao Hongling , Khan Javed , Schaefer Beat W. 
Published: July 2016

Abstract
Fusion-positive rhabdomyosarcoma (FP-RMS) is a pediatric tumor driven by an oncogenic fusion protein, PAX3-FOXO1, which acts as a transcription factor. Conventional chemotherapy is effective for low risk patients who have a 5-year overall survival greater than 65%, while high risk patients, including those with metastatic disease, have less than 40% survival. Consequently, we hypothesize that targeting the fusion protein or its collaborators in transcription regulation will provide novel therapies for this aggressive subtype of RMS. To identify new druggable PAX3-FOXO1 interactors, we performed a combined proteomic and genetic screen which led to the discovery of the NuRD complex (Nucleosome Remodelling and Deacetylase) as a major PAX3-FOXO1 co-regulator. The NuRD complex is unique among the chromatin remodelling complexes due to its dual enzymatic activity. It can act by histone deacetylation through HDAC1/2 (histone deacetylases) or influence nucleosome positioning through CHD4 (chromodomain-DNA-binding protein 4). Intriguingly, it has been associated with both activating and repressive activities in gene expression and its role in cancer development is not fully understood yet. We found that in FP-RMS, silencing of CHD4 affected the expression of approximately 50% of PAX3-FOXO1 regulated target genes. These were mainly genes which are usually upregulated, suggesting an activating role for NuRD. Consistent with CHD4 activation activity, ChIP-seq experiments demonstrated that CHD4 and HDAC2 co-localize with the fusion protein in cis-regulatory sites of a subset of its target genes. Interestingly, gene expression analysis showed that both CHD4 and HDAC2 are highly expressed in tumor tissue and myoblasts when compared to normal skeletal muscle, inferring a potential role of the NuRD complex in maintaining the undifferentiated phenotype observed in FP-RMS. Importantly, CHD4 silencing had no effect on myoblasts proliferation whereas a profound growth reduction was seen in FP-RMS cell lines, suggesting a unique tumour dependency on this chromatin remodeler. In addition, depletion of CHD4 caused a complete regression of xenograft tumours in mice.In summary, we have identified the NuRD complex as an essential positive co-regulator of PAX3-FOXO1 transcriptional activity. Our data propose a critival role of one of the NuRD core component CHD4 in FP-RMS cell viability, making CHD4 an attractive new target for therapy. To our knowledge, CHD4 is the first chromatin remodeler identified to associate with PAX3-FOXO1 transcriptional activity, thus highlighting the relevance of epigenetic regulation in FP-RMS tumour development. Collectively, our findings suggest CHD4 as a potential novel therapeutic target in this childhood malignancy.Ongoing work is currently underway to identify first-in-class small molecules to inhibit CHD4 protein.
Citation Format: Joana G. Marques, Berkley Gryder, Maria Boehm, Marco Wachtel, Young Song, Hsien-Chao Chou, Rajesh Patidar, Hongling Liao, Javed Khan, Beat W. Schaefer. Chromatin remodeling as a potential new strategy for fusion positive rhabdomyosarcoma therapy. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4457.