Ewing sarcoma is an aggressive bone cancer affecting children and young adults. The main molecular hallmark of Ewing sarcoma are chromosomal translocations that produce chimeric oncogenic transcription factors, the most frequent of which is the aberrant transcription factor EWSR1–FLI1. Because this is the principal oncogenic driver of Ewing sarcoma, its inactivation should be the best therapeutic strategy to block tumor growth. In this study, we genetically inactivated EWSR1–FLI1 using CRISPR-Cas9 technology in order to cause permanent gene inactivation. We found that gene editing at the exon 9 of FLI1 was able to block cell proliferation drastically and induce senescence massively in the well-studied Ewing sarcoma cell line A673. In comparison with an extensively used cellular model of EWSR1–FLI1 knockdown (A673/TR/shEF), genetic inactivation was more effective, particularly in its capability to block cell proliferation. In summary, genetic inactivation of EWSR1–FLI1 in A673 Ewing sarcoma cells blocks cell proliferation and induces a senescence phenotype that could be exploited therapeutically. Although efficient and specific in vivo CRISPR-Cas9 editing still presents many challenges today, our data suggest that complete inactivation of EWSR1–FLI1 at the cell level should be considered a therapeutic approach to develop in the future.

The chimeric EWSR1::FLI1 transcription factor is the main oncogenic event in Ewing sarcoma. Recently, it has been proposed that EWSR1::FLI1 levels can fluctuate in Ewing sarcoma cells, giving rise to two cell populations. EWSR1::FLI1^low cells present a migratory and invasive phenotype, while EWSR1::FLI1^high cells are more proliferative. In this work, we described how the CD44 standard isoform (CD44s), a transmembrane protein involved in cell adhesion and migration, is overexpressed in the EWSR1::FLI1^low phenotype. The functional characterization of CD44s (proliferation, clonogenicity, migration, and invasion ability) was performed in three doxycycline-inducible Ewing sarcoma cell models (A673, MHH-ES1, and CADO-ES1). As a result, CD44s expression reduced cell proliferation in all the cell lines tested without affecting clonogenicity. Additionally, CD44s increased cell migration in A673 and MHH-ES1, without effects in CADO-ES1. As hyaluronan is the main ligand of CD44s, its effect on migration ability was also assessed, showing that high molecular weight hyaluronic acid (HMW-HA) blocked cell migration while low molecular weight hyaluronic acid (LMW-HA) increased it. Invasion ability was correlated with CD44 expression in A673 and MHH-ES1 cell lines. CD44s, upregulated upon EWSR1::FLI1 knockdown, regulates cell migration and invasion in Ewing sarcoma cells.

Ewing sarcoma is a rare pediatric tumor characterized by chromosomal translocations that give rise to aberrant chimeric transcription factors (e.g., EWSR1-FLI1). EWSR1-FLI1 promotes a specific cellular transcriptional program. Therefore, the study of EWSR1-FLI1 target genes is important to identify critical pathways involved in Ewing sarcoma tumorigenesis. In this work, we focused on the transcription factors regulated by EWSR1-FLI1 in Ewing sarcoma. Transcriptomic analysis of the Ewing sarcoma cell line A673 indicated that one of the genes more strongly upregulated by EWSR1-FLI1 was FEZF1 (FEZ family zinc finger protein 1), a transcriptional repressor involved in neural cell identity. The functional characterization of FEZF1 was performed in three Ewing sarcoma cell lines (A673, SK-N-MC, SK-ES-1) through an shRNA-directed silencing approach. FEZF1 knockdown inhibited clonogenicity and cell proliferation. Finally, the analysis of the FEZF1-dependent expression profile in A673 cells showed several neural genes regulated by FEZF1 and concomitantly regulated by EWSR1-FLI1. In summary, FEZF1 is transcriptionally regulated by EWSR1-FLI1 in Ewing sarcoma cells and is involved in the regulation of neural-specific genes, which could explain the neural-like phenotype observed in several Ewing sarcoma tumors and cell lines.

We have recently demonstrated that genetic inactivation of EWSR1 : : FLI1 by CRISPR/Cas9, successfully blocks cell proliferation in a cell model of Ewing sarcoma. However, CRISPR/Cas9-mediated gene editing can exhibit off-target effects, and thus, precise regulation of Cas9 expression in target cells is essential to develop gene-editing strategies to inactivate EWSR1 : : FLI1 in Ewing sarcoma cells. In this study, we demonstrate that Cas9 can be specifically expressed in Ewing sarcoma cells when located downstream a promoter consisting of GGAA repeats and a consensus TATA box (GGAAprom). Under these conditions, Cas9 is selectively expressed in Ewing sarcoma cells that express EWSR1 : : FLI1 oncoproteins, but not in cells expressing wild-type FLI1. Consequently, Ewing sarcoma cells infected with GGAAprom>Cas9 and a specific gRNA designed to inactivate EWSR1 : : FLI1, showed successful EWSR1 : : FLI1 inactivation and the subsequent blockade of cell proliferation. Notably, GGAAprom>Cas9 can be efficiently delivered to Ewing sarcoma cells via adenoviral vectors both in vitro and in vivo, highlighting the potential of this approach for Ewing sarcoma treatment. Our results demonstrate that the CRISPR/Cas9 machinery is safe and specific for Ewing sarcoma cells when driven under a GGAAprom, paving the way for the development of cancer gene therapies based on the selective expression of genes with therapeutic potential.

Pediatric cancer care in Spain lacked a national personalized medicine program. The SEHOP-PENCIL initiative, established by the SEHOP, was designed to address this gap. A national survey identified the genomic sequencing needs of hospitals treating pediatric patients with cancer. Results showed improved access to targeted next-generation sequencing panels but revealed persistent variability in implementation, limited availability of advanced sequencing (whole-exome sequencing, whole-genome sequencing, RNA sequencing, and DNA methylation profiling), and gaps in cancer predisposition clinics and molecular tumor boards (MTBs). SEHOP-PENCIL is organized as a network of 10 specialized genomic centers serving 45 hospitals through a centralized inclusion system. The program standardizes patient eligibility, genomic workflows, and result interpretation, supported by a national MTB. This framework enables informed clinical decision making, ensures access to molecular diagnosis and innovative therapies, and supports systematic data collection to advance pediatric cancer research and personalized care. SEHOP-PENCIL represents a pioneering national model for integrating precision oncology into pediatric cancer care in Spain. By fostering collaboration, standardizing genomic practices, and promoting equitable access, it aims to reduce disparities and improve outcomes, offering a scalable example for other decentralized health care systems.