Genetic alterations in Ewing’s sarcoma: What do we know so far?
How to cite this article: Alhejaily AG, Yassin EM. Genetic alterations in Ewing’s sarcoma: What do we know so far? J Musculoskelet Surg Res 2022;6:117-20.
Ewing’s sarcoma (EwS) is the second most common cancer that affects bone in children and adolescents. EwS is an aggressive malignancy with a projected overall survival of 70% for the low risk localized and 30% for the metastatic tumors. EwS is genetically described by its unique translocation that fuses FET family genes with the ETS family genes encoding transcription factors. The most frequent molecular event in over 90% of EwS cases is the translocation between EWSR1–FLI1. Additional events, ingincluding TP53, mutations, and CDKN2A deletions, have been reported but in low incidences. Hitherto, new research on EwS molecular processes is needed to lead to early diagnosis, disease monitoring, treatment development, improve patients’ survival, and quality of life. In this review, we outline the presently available information on the genetics underlying EwS carcinogenesis, investigate the existing understanding of the genetics underpinning EwS, and discuss the future directions for research on EwS.
Ewing’s sarcoma (EwS) (OMIM: 612219) is a highly aggressive cancer that develops in bone and soft tissue and affects children and adolescents at a higher rate than the general population. Studies have shown that genetic alterations are the main cause of this cancer. EwS, in most instances, has a non-specific clinical course. Patients may have localized discomfort, often modest, rising somewhat at night or after a workout, but some individuals might have no pain at all. Without alarming pain, the sole indication may be the accidental development of a solid lump. Blood tests may reveal increased levels of indicators of inflammation such as alkaline phosphatase and erythrocyte sedimentation rate. Individuals affected with localized EwS have a good chance of being cured in about two-thirds of cases. Others with solitary metastases might have a long-term survival rate of around 30%, while those with more widespread malignancy that typically involves the bone marrow have a cure rate of fewer than 20% with the available therapies.
The radiologic examination is often more suggestive, with the characteristic of multiple lytic bone lesions. Computed tomography (CT) and/or magnetic resonance imaging are usually used to assess metastatic disease, disease progression, and response to therapy. However, the histologic and molecular examination of biopsy specimens or surgically removed tumor tissue is used to confirm the diagnosis. The biopsy of malignant tissues is essential for a definite diagnosis of the tumor. This may be accomplished through a fine needle or core biopsy from the primary location under the guidance of ultrasound or CT. The biopsy is taken for routine cytogenetic analysis, and under the microscope, sheets of small, rounded cells with a visible nucleus and minimal cytoplasm are usually seen on histologic inspection [Figure 1]. Yet, a definitive diagnosis relies mostly on the identification of the distinct molecular signature, which is the chromosomal translocations utilizing either in fluorescent in situ hybridization (FISH) or the more rapid quantitative polymerase chain reaction (qPCR). Even though FISH can detect EWSR1 rearrangements, in some instances, it cannot identify all partners of fusion events in EwS, but qPCR is considered to be a better method for detecting the fusion breakpoint and fine-tuning since they are more sensitive than FISH.
EwS is distinguished by its unique reciprocal translocation known as the t(11;22)(q24;q12), which fuses the EWSR1 on chr22 with the FLI1 on chr11, this event is predisposing for the carcinogenesis of this disease [Figure 2]. EWS-FLI1 is a chimeric oncogene that is involved in a variety of regulatory processes and is linked to secondary events, which results in the EwS phenotype. In addition to the primary genetic alteration, the accumulation of secondary genetic alterations involving several genes was found to have a more significant impact on the EwS clinical and biological behavior than the fusion gene alone. In this review, we outline the presently available information on the genetics underlying EwS carcinogenesis, investigate the existing understanding of the genetics underpinning EwS, and discuss the future directions for research on EwS.
GENETIC ALTERATIONS IN EWS
On the molecular level, EwS is marked by the fusion hallmark of the EWSR1 gene with one of the ETS families of transcription factors. The most frequent fusion is the EWSR1-FLI1. This fusion is the result of the chromosomal translocation entails the FLI1 gene that encodes a transcription factor associated with cell proliferation, growth, and tumorigenesis, and the EWSR1 gene that encodes a protein involved in several cellular processes such as cell signaling and transcription. Tissues are found to have the EWSR1-FLI1 chromosomal event in over 90% of EwS cases. Additional translocations involving the EWSR1 gene such as t(21;22) (q22;q12)) result in the formation of the fusion gene EWSR1-ERG, as well as fusions with additional genes such as ETV1, E1AF, and FEV in 5–10% of these cases. Additional chromosomal translocation among EWS-ERG fusion has been well documented as well but with very low incidences.
Based on a study done by Akiko et al., which indicated that EWS-ETS fusion proteins activate telomerase activity in tumors through over-activation of TERT gene expression. When the chimeric EWSR1-FLI1 gene is expressed, a protein that blocks the apoptosis pathway is produced, resulting in uncontrolled tumor development, and this protein, in turn, stimulates the transcription of the TERT gene, which favorably regulates the expression of telomerase, thus increasing the immortality of tumor cells in EwS. A strong transcriptional activation domain is gained by the EWSR1-FLI1 and the binding domain is provided by FLI in the context of this fusion protein. For EWSR1-FLI to operate as an oncogene, both of these domains must be present to back up the hypothesis that the fusion protein works as a transcription factor.
Interestingly, in EwS cases where EWSR1-FLI1 does not exist, alternative events, mostly translocations, are present that fuse EWSR1 to another member of the ETS family, such as the ERG, ETV1, ETV4, and FEV with very low incidences. To ascertain further potentially EwS causing mutations, Mitchell et al., at the Center for Cancer Research (CCR) in the National Cancer Institute launched a large-scale study protocol on a cohort of (2000 cases) for children and young adults, enrolled in CCR clinical trials to sequence their tumors DNA to ascertain mutations that underlie EwS carcinogenesis. They identified additional three loci at 6p25.1, 20p11.22, and 20p11.23, with low frequency associated with increased EwS risk. This study finding support the inherited genetic to EwS risk and proposes the prominence of the interaction among germline variation and the acquired EWSR1-FLI1 translocations to the etiologic EwS.
Additional genetic alterations
Despite the prominence of EWSR1-FLI1 fusion for the initiation of EwS, this fusion alone found not to be enough to transform human cells in vitro, suggesting that the additional secondary alteration or related parallel pathways might be crucial for EwS pathogenesis a high rate of likely pathogenic mutations, accounting for 13.1% of the population.
EwS has been the subject of various studies, all of which have shown that the tumors have a relatively silent genome, with a scarcity of mutations in pathways that would be susceptible to therapy with innovative targeted treatments. These articles discovered recurrent genomic abnormalities in several genes, including studies that showed EWSR1-FLI1 fusion stimulates tumor cells’ production of a protein known as pappalysin-1 (PAPPA), which is responsible for breaking down proteins known as insulin-like growth factor binding proteins (IGFBPs). This breakdown results in the production of the hormone-IGF-1, which, in turn, promotes the proliferation of cancer cells. Through utilizing cell and animal models, researchers were able to effectively show that inactivating PAPPA is a feasible therapeutic option for EwS.
Further mutations in the tumor suppressor genes such as CDKN2A and TP53 have been identified as a secondary genetic alteration in EwS in multiple studies. By utilizing a combination of whole-genome sequencing and targeted sequencing approaches, Hemelin et al. found a homozygous deletion affecting CDKN2A in 13.8% and 50% and mutations of TP53 in 6.2% and 71.9% in both EwS’ clinical tumors and cell lines, respectively.[18,19] Another study by Brohl et al., the same group examined the sequencing data from the genomes of 175 Ewing patients, discovered a high rate of pathogenic or likely pathogenic mutations in 13.1% of patients. Some of these patients possess genetic mutations in DNA repair genes. However, their analysis in both studies showed that EwS has a very low mutational burden in general (0.15 mutations/Mb).
EWS-FLI1 tends to activate genes involved in cellular differentiation, proliferation, and survival, such as IGF1, NKX2, TOPK, SOX2, and EZH2. EWS-FLI1 also tends to deactivate genes involved in apoptosis and cell cycle, including the IGFBP3, CDKN1C, CDKN1A, and TGFB2.[18,19] Furthermore, because the EWSR1–FLI1 fusion is constitutively active, it is found to work as an activator of the MYC gene, which is a proto-oncogene that is essential for embryonic development but that relies on extra external and internal stimuli to become activated its ability to function. Although the MYC gene has decreased activity in differentiated cells, its inappropriate activation results in gene amplification, which results in the expression of proteins that are engaged in the control of the cell cycle, differentiation, and genomic instability, which might promote the tumor initiation.
EwS is a rare and highly fatal disease that has a poor prognosis when metastatic or recurrent. EwS also serves as a good model for studying carcinogenesis because it has several features that are both helpful and significant. For example, almost all cases (>90%) of EwS have the t(11;22) (q24;q12) chromosomal translocation, which encodes the EWSR1-FLI oncoprotein. However, apart from the t(11;22) translocation, many instances have generally straightforward karyotypes with no other discernible anomalies. It also seems that underlying genetic predisposition to EwS, if it exists, must be a very uncommon occurrence. Much has been revealed about the biology of EwS over the past three decades, including the key role of oncogenic EWSR1-FLI1 fusions that have an extent and importance of the carcinogenesis process in this disease. This information opens the door to targeted therapies, including blocking IGF signaling and, more recently, interfering with key EWSR1-FLI1 protein-protein interactions conserved pathways in cancer which is likely to suggest additional ways to attack EwS therapeutically.
Genomic analysis of EwS is critical to understanding the genetic and molecular alterations associated with this malignancy. NGS-based genetic analysis has the potential to enhance the prognosis, as it can be used to reach a precise and early diagnosis.
AGA drafted the manuscript and did the preparation. EMY revised the article and helped in the literature review. All authors have critically reviewed and approved the final draft and are responsible for the manuscript’s content and similarity index.
The authors confirm that this review had been prepared following COPE roles and regulations. Given the nature of the review, the IRB review was not required.
DECLARATION OF PATIENT CONSENT
Patient’s consent not required as there are no patients in this study.
FINANCIAL SUPPORT AND SPONSORSHIP
This study did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
CONFLICTS OF INTEREST
There are no conflicts of interest.