**5. Targeting fusion oncoproteins**

Fusion proteins of the sarcomas shown here appear to block the differentiation potential of these cells. This is achieved by hijacking transcriptional regulatory mechanisms to maintain the expression of stem cell transcriptional programs or by repressing differentiation programs. In Ewing sarcomas, the EWSR1-FLI1 protein upregulates EZH2 by binding to its promoter, thereby blocking its endothelial and neuronal differentiation capabilities [90]. Recent data show that in this process EZH2-containing PRC2 complexes interact with HDAC1, 2 and this HDAC activity mediates the immature, tumorigenic phenotype of Ewing sarcoma [91]. However, in the alveolar RMS HDAC1,2,3 also appears to serve an essential function of P3F-driven super-enhancers, as appropriate inhibitors disrupt the activity of these tumor-specific super-enhancers [92].

Transcription factors such as EWSR1-FLI1 can bind to DNA target sites on chromatin and initiate chromatin remodeling by recruiting other transcription factors and coactivator complexes. One way to achieve this chromatin remodeling is through association with BAF complexes. These multimembered complexes use ATP to move, displace, or exchange nucleosomes on chromatin. In Ewing sarcomas, BAF complexes can directly interact with the N-terminal EWSR1 protein of the fusion protein to promote and direct its tumor-specific activity at GGAA microsatellites. This binding activity is attributed to a specific prion-like domain in the N-terminal EWSR1 protein that is sufficient to drive chromatin remodeling and oncogenic gene transcription when fused to FLI1 [37]. In alveolar RMS, no direct interaction of P3F with the BAF complex has yet been shown. However, prion-like domains are suspected in a growing class of genes involved in oncogenic fusions, including FOXO1 and SS18 [93]. In synovial sarcomas, the SS18-SSX fusion also relocalizes and disrupts the BAF complex. The SS18-SSX fusion protein not only displaces wild-type SS18 binding and the tumor suppressor BAF47 from the complex [79]. Moreover, the SS18-SSX-containing BAF complexes interact with various repressive polycomb complexes in a contextdependent manner, thereby promoting the transcription of oncogenic genes [89], or alternatively, SS18-SSX and the BAF complex can localize and activate target genes via interaction with KDM2B and the PRC1.1 complex [88], as described above.

### *Drug Targeting of Chromosomal Translocations in Fusion-Positive Sarcoma DOI: http://dx.doi.org/10.5772/intechopen.106671*

Despite these preclinical and clinical data, to date, there are few examples of targeted therapies that directly target these fusion transcription factors in solid tumors. However, all of these examples do not directly target structures of these chimeric transcription factors that are considered undruggable but attempt to identify processes or proteins that are essential for the activity or stability of these fusion proteins. An example is the observed interaction of EWSR1-FLI1 with RNA helicase A: YK-4-279 interferes with the interaction of EWSR1-FLI1 with RNA helicase A and thereby efficiently impairs both the activity of the fusion protein and cell proliferation of Ewing sarcoma cells [40]. Based on these data, the derivative TK-216 is now being tested in a clinical trial in patients with relapsed or refractory Ewing sarcoma. Another example is BAF complexes in which the SS18-SSX fusion protein is present in synovial sarcomas. Recent studies have shown that targeting the BRD9 protein, which is a component of SS18-SSX-containing complexes, provides potent antitumor effects in this context [94, 95]. BRD9 and SS18-SSX bind together to regions of the synovial sarcoma genome, and small molecule-triggered targeted degradation of BRD9 prevents oncogenic transcriptional programs in cell lines and blocks tumor progression in vivo [94]. These results will form the basis for future clinical trials in patients with synovial sarcomas. Furthermore, efforts are underway to identify downstream target genes that have critical roles in mediating the oncogenic effects of fusion transcription factors. Examples of these are described below.
