# The Puzzle of Genetic Stability and Chromosomal Copy Number Alterations for the Therapy of Ewing Sarcoma

## TL;DR

Ewing sarcoma tumors have few gene mutations but frequent chromosomal changes, which are linked to worse outcomes and may be caused by a specific genetic fusion.

## Contribution

The paper identifies how chromosomal copy number alterations and loss of heterozygosity in Ewing sarcoma are driven by EWSR1::ETS translocations and replication stress.

## Key findings

- Ewing sarcoma tumors with fewer copy number alterations have better patient outcomes.
- EWSR1::ETS translocations cause replication stress and chromosomal instability through R-loop formation and loss of EWSR1.
- Targeting copy number alterations and loss of heterozygosity could improve Ewing sarcoma therapy.

## Abstract

Ewing sarcoma (EwS) is a highly malignant tumor of bone and soft tissue that predominantly affects children and young adults, with a high propensity for early metastasis to the lungs and bones. Large-scale sequencing and SNP array studies have concluded that, in addition to the typical EWSR1::ETS translocation, the genome of EwS is remarkably simple at the single-nucleotide level, but recurrent structural chromosome alterations are frequent. The current findings indicate that translocation-mediated EWSR1 haplo-insufficiency results in replication stress and potential loss of heterozygosity and emphasize that copy number dysregulation acts as a critical secondary driver of EwS biology, shaping tumor evolution and influencing clinical outcome. The preclinical results directed at replication stress and diminished repair capacity of EwS are promising and should significantly improve the therapy of EwS.

Studies of the genomic stability of Ewing sarcoma (EwS) have produced contradictory findings. While they are generally characterized by low mutation rates of individual genes, several cases exhibit genomic alterations that manifest as chromosomal gains and losses. Taken together, these alterations represent independent biomarkers for EwS, such as loss of heterozygosity (LOH) or an altered genome. Patients with primary EwS tumors with fewer than three copy number alterations (CNAs) have a better prognosis than those with more CNAs. The functional mechanisms underlying this chromosomal instability are not yet clear. However, there are indications that this may be directly caused by the EWSR1::ETS translocations that are characteristic of EwS. The transcriptional behavior of the chimeric transcription factor EWSR1-FLI1 leads to the formation of R-loop DNA–RNA hybrids that form when RNA binds back to DNA during transcription and increased replication stress, which may result in structural chromosomal changes. Additionally, the formation of EWSR1 fusion genes in EwS results in the loss of one or both wild-type EWSR1 alleles in sarcoma cells. As chromosome segregation has been observed under loss of wild-type EWSR1, EWSR1 loss has been proposed as a potential source of LOH. So, it is highly probable that a chromosomal translocation and the subsequent formation of the EWSR1-ETS fusion protein cause the genomic alterations in EwS. This indicates that targeted therapy should be directed against the CNA and LOH biology caused by the fusion protein.