**3. Rhabdomyosarcoma**

Rhabdomyosarcoma (RMS), the most common soft tissue sarcoma in children and adolescents, comprises a diverse group of cancers [42]. There are several subtypes: embryonal, alveolar, and pleomorphic rhabdomyosarcoma. Embryonal RMS occurs in infants and children, and as patients age, the proportion of embryonal RMS decreases. Conversely, the proportion of alveolar and pleomorphic types increases in adolescents and older patients. Currently available multimodal therapy results in an overall survival rate of approximately 65% in children and adolescents diagnosed with RMS [43]. However, cure rates have stagnated since the 1990s. Rhabdomyosarcoma is very sensitive to cytotoxic combination chemotherapy [44]. For low- or intermediate-risk RMS patients (who are mostly pediatric patients with embryonal-type tumors), a high cure rate can be expected with current standard treatment. In adolescents and elderly patients, most of whom have had alveolar or pleomorphic type RMS, the prognosis is poor [6].

Chromosomal translocations are observed in alveolar rhabdomyosarcomas in two translocation patterns: The DNA binding site of PAX, a member of the pairedbox family of transcription factors, is fused to a transactivation domain on FOXO1 (FKHR), a member of the forkhead transcription factor family [45, 46]. The t(2;13) translocation results in the fusion of the PAX3 gene with FOXO1, whereas the t(1;13) translocation fuses PAX7 with FOXO1, both of which now serve as important prognostic biomarkers for this disease (Barr et al. 1995). The O subgroup of the FOX family includes four members (FOXO1, FOXO3, FOXO4, and FOXO6). FOXO factors are considered tumor suppressors that are inactivated by the phosphatidylinositol 3-kinase (PI3K)-AKT pathway, which is regulated by several microRNAs [47]. The prevalence of the translocation with PAX3-FOXO1 is higher than that with PAX7- FOXO1 [48].

PAX3-FOXO1 is an aberrant transcription factor that disrupts gene regulatory networks that control myogenic differentiation, proliferation, cell death, and invasiveness [49, 50]. The translocation product overlaps with wild-type PAX3 function while modifying it through changes in abundance, transcriptional activity, target gene recognition, and chromatin regulation [51–56]. Patients with fusion-positive alveolar rhabdomyosarcoma (FP-ARMS) have a strikingly low somatic mutation burden and are also associated with a significantly higher rate of metastasis and lower survival compared to FP-negative RMS [56]. Metastatic FP-RMS remains essentially incurable [57].

PAX3/7:FOXO1-positive RMS (FP-RMS) is associated with alveolar histology [58]. Silencing of PAX3/7:FOXO1 (P3F) *in vitro* has been associated with decreased growth of human FP-RMS cells [59]. The effects of the fusion on tumor induction have been studied by ectopic expression and conditional activation in various cell types [60–62]. The fusion was necessary but not sufficient to induce FP+ myogenic tumors, as the fusion oncoprotein alone did not reliably induce tumor formation [60–62]. When combined with additional oncogenic hits, only those cells that expressed the fusion prior to the introduction of additional events formed tumors [62]. These observations are consistent with genomic subclonality analyses identifying PAX3/7:FOXO1 as a founding event and driver in FP-RMS [63]. Cooperating genetic events in FP-ARMS include amplification of MYCN or CDK4 or loss of CDKN2A, TP53, or ARF [17, 61].

PAX-FOXO1 fusions are thought to contribute to the phenotype and malignancy of ARMS by dysregulating PAX-specific target genes such as the epigenetic regulator JARID2, the receptor tyrosine kinases MET and FGFR4, and IGF2, Hippo and their downstream signaling pathways [64–67]. In addition, rearrangement of the PAX gene

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

is thought to lead to dysregulation and amplification of a shared receptor tyrosine kinase/RAS/PIK3CA signaling axis [17]. PAX-FOXO1 fusion is also thought to affect normal FOXO function and its regulation of TGF-β signaling [68]. Recently, PAX3- FOXO1 was shown to directly establish super-enhancers in cooperation with the master transcription factors MYOG, MYOD, and MYCN to drive a myogenic transcriptional program in ARMS [55]. Thus, as in Ewing sarcoma, both aberrant transcriptional and epigenetic regulation drive the development and maintenance of FP-ARMS.
