Despite progress in the treatment of ES, the survival rate of ES remains poor13. Most patients diagnosed with ES have a chromosomal translocation14. Some researchers have reported that chromosomal translocation can facilitate widespread epigenetic alterations15. Therefore, it is essential to elucidate the underlying epigenetic changes in ES.
For hypomethylated-upregulated genes, GO enrichment analysis demonstrated that the biological processes were associated with intracellular signal transduction, proteolysis, and positive regulation of apoptotic processes. This is reasonable because previous studies have demonstrated that both altered signal transduction cascades and evasion of apoptosis are hallmarks of cancers, including ES16, 17. PARP proteolysis has been found in the process of apoptosis in Ewing’s sarcoma18. KEGG pathway enrichment analysis showed several significantly enriched pathways including arginine and proline metabolism, the PPAR signaling pathway, and pertussis. The PPAR signaling pathway has been reported to be involved in a variety of cancers, but its specific function is unclear19. Some researchers believe that PPAR pathway activation could promote the initiation or development of tumors, while others hold the opposite opinion20. Arginine and proline are two important nonessential amino acids, and their synthesis could promote tumor growth21. Besides, pertussis toxin can influence the stimulation and inhibition of adenylate cyclase, which plays an important role in Ewing’s sarcoma activities22.
Besides, the PPI network illustrated the functional connections between gene expression and gene methylation in ES. Through Cytoscape, the top 5 hub genes were presented: C3, TF, PNPLA2, VWA1, and FUCA2. The C3 gene encodes complement C3, which is a central protein associated with most complement activation pathways, and the C3 gene is a direct target of PPAR receptor α, which could function in certain functional pathways of ES metabolism 23, 24. The TF gene encodes transferrin, an iron-binding transport protein that is usually overexpressed on the surface of tumor cells, including ES cells 25, 26, some researchers found that transferrin and its receptor were also closely associated with tumor cell growth and proliferation27. The PNPLA2 gene encodes a protein named adipose triglyceride lipase, which is expressed in human tumors and related to tumor aggressiveness 28. The VWA1 gene encodes von Willebrand factor A-domain-related protein (WARP), the function of which is still unclear29. However, there is translational upregulation of VWA1 in hepatocellular carcinoma (HCC), and cell proliferation can be inhibited after knockdown of VWA130. The expression of the FUCA2 can be upregulated by interferon-gamma (IFN-γ), which is closely associated with the initiation and development of tumours31. However, little is known about its function in Ewing sarcoma.
For hypermethylated-downregulated genes, GO enrichment analysis demonstrated that the biological processes as cell division, apoptotic process, transcription from the RNA polymerase II promoter, and DNA replication. Molecular function enrichment included protein binding, DNA binding, poly (A) RNA binding and structural constituent of ribosome. KEGG analysis showed that the enriched pathways were pyrimidine metabolism, cell cycle, purine metabolism, and ubiquitin-mediated proteolysis. These results implied the important role of DNA replication and transcription dysregulation in the development of ES, which might be regulated by DNA methylation32.
After the construction of the PPI network, the top 5 hub genes were identified through Cytoscape: CDC20, TCEB1, CUL2, ASB6, and KLHL13. CDC20 is important in the regulation of cell cycle, and its expression is upregulated in a variety of malignant tumors 33, 34. Hypoxia-inducible factor-1α (HIF-1α) was found to directly bind to the ES-FLI-1 promoter and could be closely related to the invasiveness of Ewing sarcoma, while TCEB1 could increase HIF stabilization 35, 36. Thus, TCEB1 targeted drug might be helpful for ES treatment. The CUL2 gene is believed to be a candidate tumor suppressor gene, and the protein it encodes is a member of the cullin family, which is participated in ubiquitination and degradation of certain cellular proteins 37, 38. The ASB6 gene encodes a kind of ankyrin repeat protein that contains a C-terminal suppressor of cytokines signaling (SOCS) box motif, and the SOCS functions in ubiquitination and proteasomal degradation of certain proteins39. The Kelch domain-containing protein encoded by KLHL13 was found to be associated with cytoskeletal functions40. The role of ASB6 and KLHL13 in the development of ES requires further exploration.
CMap database is commonly used for drug development. In this study, based on CMap analysis, we determined 6 small-molecule compounds that might be potential treatment options for ES. Iloprost has been shown of the highest significance among our results. As a prostaglandin analog, Iloprost has antithrombotic, vasodilatation, and cytoprotection effects41. In some malignant tumors, increased platelets were frequently observed, and researchers speculated that those platelets played a critical role in tumor invasion and metastasis42. Some preclinical studies demonstrated Iloprost could reduce adhesion of tumor cells to platelets, inhibit tumor angiogenesis, and hematogenous spread43, and iloprost significantly inhibited migration and invasion, but not cell growth, in a dose-dependent manner in human ovarian cancer cells by downregulating MMP-2 expression via the IP-mediated PKA pathway. Thus, iloprost might be a potential tumor inhibitor for clinical use. Benzbromarone was also reported with an anti-tumor effect. It might influence the motility of vascular endothelial cells, and subsequently inhibit tumor angiogenesis and cancer metastasis 44. Fendiline was FDA approved. Recent studies showed fendiline could influence multiple tumor phenotypes, including proliferation, migration, invasion, and independent growth45.
In this study, several limitations should be acknowledged. First, we only analyzed two datasets, and a larger sample size is required to verify our results. Also, cell and animal experiments are demanded to further examine the effects of the filtered hub genes and associated pathways on ES. Analysis of clinical parameters and prognosis is also lacking due to the limitations of data. Therefore, further experiments are indispensable to verify the specific function of methylation and hub genes in ES.