Soft Tissue Sarcoma In Children Research Articles

Prognostic factors of cutaneous soft tissue sarcomas in children: a SEER population-based study.

This study aims to analyze the clinicopathological characteristics and survival outcomes of cutaneous soft tissue sarcomas (CSTS) in children. We selected pediatric cases of CSTS diagnosed between 2000 and 2019 from the Surveillance, Epidemiology, and End Results (SEER) database. Survival rates were calculated using Kaplan-Meier methods. We performed univariate analyses with the log-rank test and multivariate survival analyses using Cox proportional-hazards models to determine factors affecting overall survival (OS). Additionally, we constructed a predictive nomogram based on the outcomes of the Cox regression. A total of 148 pediatric patients with CSTS were reviewed. The median age at diagnosis was 13 years (range: 0-18 years). Prognostically, tumors located on the extremities showed better outcomes compared to those on the head, neck, or trunk. Among the histological types, angiosarcoma had the lowest five-year survival rate at 51.3%, which was substantially lower compared to fibrous histiocytoma and leiomyosarcoma. Cox regression analysis highlighted surgical intervention as the only significant independent prognostic factor for OS, with an increased risk of mortality observed in patients not undergoing surgery. Additionally, patients with distant-stage disease exhibited significantly lower survival rates than those with localized conditions. Pediatric CSTS represents a diverse and infrequent group of tumors, predominantly fibrous histiocytoma and leiomyosarcoma. Surgery was identified as the crucial determinant of survival, underscoring its role in effectively managing these patients.

Entinostat as a combinatorial therapeutic for rhabdomyosarcoma

Rhabdomyosarcoma (RMS) is the most common childhood soft tissue sarcoma. For the alveolar subtype (ARMS), the presence of the PAX3::FOXO1 fusion gene and/or metastases are strong predictors of poor outcome. Metastatic PAX3::FOXO1+ ARMS often responds to chemotherapies initially, only to subsequently relapse and become resistant with most patients failing to survive beyond 8 years post-diagnosis. No curative intent phase II or phase III clinical trial has been available for patients in the past 10 years (ARST0921). Thus, metastatic ARMS represents a significantly unmet clinical need. Chemotherapy resistance in ARMS has previously been attributed to PAX3::FOXO1-mediated cell cycle checkpoint adaptation, which is mediated by an HDAC3-SMARCA4-miR-27a-PAX3::FOXO1 circuit that can be disrupted by HDAC3 inhibition. In this study, we investigated the therapeutic efficacy of combining the epigenetic regulator entinostat, a Class I Histone Deacetylase (HDAC1-3) inhibitor, with RMS-specific chemotherapies in patient derived xenograft (PDX) models of RMS. We identified single agent, additive or synergistic relationships between relapse-specific chemotherapies and clinically relevant drug exposures of entinostat in three PAX3::FOXO1+ ARMS mouse models. This preclinical data provides further rationale for clinical investigation of entinostat, already known to be well tolerated in a pediatric phase I clinical trial (ADVL1513).

HDAC3 genetic and pharmacologic inhibition radiosensitizes fusion positive rhabdomyosarcoma by promoting DNA double-strand breaks

Radiotherapy (RT) plays a critical role in the management of rhabdomyosarcoma (RMS), the prevalent soft tissue sarcoma in childhood. The high risk PAX3-FOXO1 fusion-positive subtype (FP-RMS) is often resistant to RT. We have recently demonstrated that inhibition of class-I histone deacetylases (HDACs) radiosensitizes FP-RMS both in vitro and in vivo. However, HDAC inhibitors exhibited limited success on solid tumors in human clinical trials, at least in part due to the presence of off-target effects. Hence, identifying specific HDAC isoforms that can be targeted to radiosensitize FP-RMS is imperative. We, here, found that only HDAC3 silencing, among all class-I HDACs screened by siRNA, radiosensitizes FP-RMS cells by inhibiting colony formation. Thus, we dissected the effects of HDAC3 depletion using CRISPR/Cas9-dependent HDAC3 knock-out (KO) in FP-RMS cells, which resulted in Endoplasmatic Reticulum Stress activation, ERK inactivation, PARP1- and caspase-dependent apoptosis and reduced stemness when combined with irradiation compared to single treatments. HDAC3 loss-of-function increased DNA damage in irradiated cells augmenting H2AX phosphorylation and DNA double-strand breaks (DSBs) and counteracting irradiation-dependent activation of ATM and DNA-Pkcs as well as Rad51 protein induction. Moreover, HDAC3 depletion hampers FP-RMS tumor growth in vivo and maximally inhibits the growth of irradiated tumors compared to single approaches. We, then, developed a new HDAC3 inhibitor, MC4448, which showed specific cell anti-tumor effects and mirrors the radiosensitizing effects of HDAC3 depletion in vitro synergizing with ERKs inhibition. Overall, our findings dissect the pro-survival role of HDAC3 in FP-RMS and suggest HDAC3 genetic or pharmacologic inhibition as a new promising strategy to overcome radioresistance in this tumor.

Arsenic sulfide enhances radiosensitivity in rhabdomyosarcoma via activating NFATc3-RAG1 mediated DNA double strand break (DSB)

Rhabdomyosarcoma (RMS) represents one of the most lethal soft-tissue sarcomas in children. The toxic trace element arsenic has been reported to function as a radiosensitizer in sarcomas. To investigate the role of arsenic sulfide (As4S4) in enhancing radiation sensitization in RMS, this study was conducted to elucidate its underlying mechanism in radiotherapy. The combination of As4S4 and radiotherapy showed significant inhibition in RMS cells, as demonstrated by the cell counting kit-8 (CCK-8) assay and flow cytometry. Subsequently, we demonstrated for the first time that As4S4, as well as the knockdown of NFATc3 led to double-strand break (DSB) through increased expression of RAG1. In vivo experiment confirmed that co-treatment efficiently inhibited RMS growth. Furthermore, survival analysis of a clinical cohort consisting of 59 patients revealed a correlation between NFATc3 and RAG1 expression and overall survival (OS). Cox regression analysis also confirmed the independent prognostic significance of NFATc3 and RAG1.Taken together, As4S4 enhances radiosensitivity in RMS via activating NFATc3-RAG1 mediated DSB. NFATc3 and RAG1 are potential therapeutic targets. As4S4 will hopefully serve as a prospective radio-sensitizing agent for RMS.

Children and young adults with newly diagnosed rhabdomyosarcoma metastatic to bone treated on Children's Oncology Group studies.

Rhabdomyosarcoma (RMS) is the most common soft tissue sarcoma in children. Despite bone metastases being present in 5% of patients at diagnosis, there are limited studies examining these outcomes. We sought to define the prognostic factors, clinical courses, and outcomes of children treated on Children's Oncology Group (COG) clinical trials with RMS metastatic to bone at diagnosis. We performed a retrospective analysis of patients diagnosed with bone metastatic RMS enrolled on COG RMS clinical trials (D9802, D9803, ARST0431, or ARST08P1) between 1997 and 2013. RMS metastatic to bone was identified in 154 patients at a median age of 14.9years at diagnosis. Fifty-eight percent of patients were male, 90% had metastases at additional sites, 74% had alveolar histology, and extremity was the most common primary site (31%). Eighty-six percent of patients (n=133) received radiation therapy. The 3- and 5-year event-free survival (EFS) was 15.4% and 14.5%, respectively. The 3- and 5-year overall survival (OS) was 30.4% and 18.0%, respectively. We identified alveolar histology, FOXO1 fusion presence, unfavorable primary location, higher Oberlin score, and lack of radiation as poor prognostic characteristics for both EFS and OS in univariate analysis. Lack of radiation was not significant when excluding patients with events prior to 20weeks. This study is the largest analysis of patients with bone metastatic RMS, and defines the poor overall outcomes and negative prognostic factors for these patients. They may be eligible for therapy deintensification for improved quality of life or pursuit of novel treatments/approaches, which are desperately needed.

Anlotinib destabilizes PAX3-FOXO1 to induce rhabdomyosarcoma cell death via upregulating NEK2

BackgroundRhabdomyosarcoma (RMS) is one of the most common soft tissue sarcomas in children and adolescents, in which PAX3-FOXO1 fusion gene positive patients have very poor prognosis. PAX3-FOXO1 has been identified as an independent prognostic predictor in RMS, with no currently available targeted therapeutic intervention. The novel tyrosine kinase inhibitor anlotinib exhibits a wide range of anticancer effects in multiple types of cancers; however, there have been no relevant studies regarding its application in RMS. Materials and methodsWe investigated the effects of PAX3-FOXO1 on the therapeutic efficacy of anlotinib using the CCK-8 assay, flow cytometry, invasion assay, wound healing assay, western blotting, quantitative polymerase chain reaction(qPCR), and xenograft experiments. RNA-seq and co-immunoprecipitation assays were conducted to determine the specific mechanism by which anlotinib regulates PAX3-FOXO1 expression. ResultsAnlotinib effectively inhibited RMS cell proliferation and promoted apoptosis and G2/M phase arrest while impeding tumor growth in vivo. Downregulation of PAX3-FOXO1 enhances the antitumor effects of anlotinib. Anlotinib upregulates protein kinase NEK2 and increases the degradation of PAX3-FOXO1 via the ubiquitin-proteasome pathway, leading to a reduction in PAX3-FOXO1 protein levels. ConclusionAnlotinib effectively inhibited the malignant progression of RMS and promoted degradation of the fusion protein PAX3-FOXO1. Anlotinib could be a targeted therapeutic approach to treat PAX3-FOXO1 fusion-positive RMS.

Antitumour effects of SFX-01 molecule in combination with ionizing radiation in preclinical and in vivo models of rhabdomyosarcoma

BackgroundDespite a multimodal approach including surgery, chemo- and radiotherapy, the 5-year event-free survival rate for rhabdomyosarcoma (RMS), the most common soft tissue sarcoma in childhood, remains very poor for metastatic patients, mainly due to the selection and proliferation of tumour cells driving resistance mechanisms. Personalised medicine-based protocols using new drugs or targeted therapies in combination with conventional treatments have the potential to enhance the therapeutic effects, while minimizing damage to healthy tissues in a wide range of human malignancies, with several clinical trials being started. In this study, we analysed, for the first time, the antitumour activity of SFX-01, a complex of synthetic d, l-sulforaphane stabilised in alpha-cyclodextrin (Evgen Pharma plc, UK), used as single agent and in combination with irradiation, in four preclinical models of alveolar and embryonal RMS. Indeed, SFX-01 has shown promise in preclinical studies for its ability to modulate cellular pathways involved in inflammation and oxidative stress that are essential to be controlled in cancer treatment.MethodsRH30, RH4 (alveolar RMS), RD and JR1 (embryonal RMS) cell lines as well as mouse xenograft models of RMS were used to evaluate the biological and molecular effects induced by SFX-01 treatment. Flow cytometry and the modulation of key markers analysed by q-PCR and Western blot were used to assess cell proliferation, apoptosis, autophagy and production of intracellular reactive oxygen species (ROS) in RMS cells exposed to SFX-01. The ability to migrate and invade was also investigated with specific assays. The possible synergistic effects between SFX-01 and ionising radiation (IR) was studied in both the in vitro and in vivo studies. Student’s t-test or two-way ANOVA were used to test the statistical significance of two or more comparisons, respectively.ResultsSFX-01 treatment exhibited cytostatic and cytotoxic effects, mediated by G2 cell cycle arrest, apoptosis induction and suppression of autophagy. Moreover, SFX-01 was able to inhibit the formation and the proliferation of 3D tumorspheres as monotherapy and in combination with IR. Finally, SFX-01, when orally administered as single agent, displayed a pattern of efficacy at reducing the growth of tumour masses in RMS xenograft mouse models; when combined with a radiotherapy regime, it was observed to act synergistically, resulting in a more positive outcome than would be expected by adding each exposure alone.ConclusionsIn summary, our results provide evidence for the antitumour properties of SFX-01 in preclinical models of RMS tumours, both as a standalone treatment and in combination with irradiation. These forthcoming findings are crucial for deeper investigations of SFX-01 molecular mechanisms against RMS and for setting up clinical trials in RMS patients in order to use the SFX-01/IR co-treatment as a promising therapeutic approach, particularly in the clinical management of aggressive RMS disease.

The role of radiotherapy in multimodal treatment of non-rhabdomyosarcoma soft tissue sarcomas in children: A real life report from a tertiary center.

The current treatment of pediatric non-rhabdomyosarcoma soft tissue sarcomas (NRSTS) is a multimodal risk-based approach. Today, smaller fields and lower doses of radiotherapy (RT) have become standard. In this study, it was aimed to evaluate the treatment outcomes and toxicity profile in children with NRSTS that received RT as a part of multimodal therapy. Twenty-nine patients with pediatric NRSTS treated with neoadjuvant or adjuvant RT between 1998 and 2022 were evaluated retrospectively. Kaplan-Meier method was used for survival analyses. Median follow-up was 36 months (range, 6-291 months). The median neoadjuvant and adjuvant RT doses were 50 Gy (range, 45-66 Gy) and 54 Gy (45-66 Gy), respectively. During follow-up, six (21%) patients developed a local recurrence and 10 (35%) had distant metastasis. The 5-year local control, overall survival (OS), local recurrence-free survival, and distant metastasis-free survival rate was 79%, 67%, 59%, and 61%, respectively. In multivariate analysis, a ≤5-cm tumor, gross tumor resection, Children's Oncology Group (COG) low-risk group, and absence of neoadjuvant chemotherapy were independent favorable prognostic factors for OS. Severe (≥ grade 3) late toxicity was observed in 6 (20%) patients. RT is a crucial component in the multimodal risk-based treatment approach for pediatric NRSTS. However, late toxicity rates are still high and should be improved. Patients with a ≤5-cm tumor, COG low-risk group and treated with gross tumor resection have increased survival rates.

Targeted immunotherapy and nanomedicine for rhabdomyosarcoma: The way of the future.

Rhabdomyosarcoma (RMS) is the most common soft tissue sarcoma of childhood. Histology separates two main subtypes: embryonal RMS (eRMS; 60%-70%) and alveolar RMS (aRMS; 20%-30%). The aggressive aRMS carry one of two characteristic chromosomal translocations that result in the expression of a PAX3::FOXO1 or PAX7::FOXO1 fusion transcription factor; therefore, aRMS are now classified as fusion-positive (FP) RMS. Embryonal RMS have a better prognosis and are clinically indistinguishable from fusion-negative (FN) RMS. Next to histology and molecular characteristics, RMS risk groupings are now available defining low risk tumors with excellent outcomes and advanced stage disease with poor prognosis, with an overall survival of about only 20% despite intensified multimodal treatment. Therefore, development of novel effective targeted strategies to increase survival and to decrease long-term side effects is urgently needed. Recently, immunotherapies and nanomedicine have been emerging for potent and effective tumor treatments with minimal side effects, raising hopes for effective and safe cures for RMS patients. This review aims to describe the most relevant preclinical and clinical studies in immunotherapy and targeted nanomedicine performed so far in RMS and to provide an insight in future developments.

Effect of oncolytic herpes simplex virus on fusion-positive rhabdomyosarcoma to radiotherapy.

10063 Background: Rhabdomyosarcoma (RMS) is the most common soft tissue sarcoma in children. Despite multimodal interventions including surgery, chemotherapy, and radiotherapy, nearly a third of patients with localized disease will experience tumor recurrence. Patients with tumors harboring an oncogenic FOXO1 fusion are at greater risk of relapse and are most in need of novel therapies. Oncolytic viruses are being explored as treatments for pediatric tumors, and data suggest that oncolytic viruses may enhance responses to chemoradiotherapy. M002 is a genetically engineered oncolytic herpes simplex virus (oHSV) that exhibits aneurovirulence in normal cells. M002 selectively replicates in malignant cells, eliminating these cells through cellular burst during the lytic cycle. In the present study, we explored the efficacy of the M002 virus alone and in combination with low-dose ionizing radiotherapy in a human patient-derived xenograft (PDX) model of fusion-positive RMS (FP-RMS). Methods: A human FP-RMS PDX was established and maintained by injection into the quadriceps femoris muscle of athymic nude mice under Institutional Animal Care and Use Committee guidelines (IACUC-09363). Tumors were dissociated and cells were treated with increasing multiplicity of infection (MOI) of either the oHSV, R3659 or its genetically engineered variant, M002. A colorimetric assay evaluated the cytotoxicity of oHSV treatments on the RMS PDX cells after 24, 48, and 72 hours of infection. In separate experiments, cells treated with vehicle (control) or infected with M002 were exposed to irradiation (2 Gy), and viability was determined at 24, 48, 72, and 96 hours. Results: Treatment with M002 or its parent construct, R3659, significantly reduced RMS PDX cell viability in a dose-dependent manner at all time points. M002 showed slightly higher cytotoxicity than R3659 as measured by lethal dose 50% (LD50, 37±1.25 vs 44±0.95 PFU/cell, M002 vs R3659, at 24 hours, p=0.02). When sub-lethal doses of M002 were combined with low-dose irradiation, RMS PDX cell viability was significantly decreased compared to cells treated with either M002 or irradiation alone (26% decrease at 10 PFU/cell). Conclusions: Our preliminary data highlight that M002 exhibits cytotoxicity in a PDX model of FP-RMS. Treatment with M002 further increased the cytotoxicity of ionizing radiation compared to individual treatments, suggesting that oHSV sensitized the cells to radiotherapy. Future studies in vivo may provide more pre-clinical evidence for the radio-sensitizing and therapeutic potential of M002 in FP-RMS.

The deubiquitinase USP7 and E3 ligase TRIM21 regulate vasculogenic mimicry and malignant progression of RMS by balancing SNAI2 homeostasis

BackgroundRhabdomyosarcoma (RMS) is a rare malignancy and the most common soft tissue sarcoma in children. Vasculogenic mimicry (VM) is a novel tumor microcirculation model different from traditional tumor angiogenesis, which does not rely on endothelial cells to provide sufficient blood supply for tumor growth. In recent years, VM has been confirmed to be closely associated with tumor progression. However, the ability of RMS to form VM has not yet been reported.MethodsImmunohistochemistry, RT-qPCR and western blot were used to test the expression level of SNAI2 and its clinical significance. The biological function in regulating vasculogenic mimicry and malignant progression of SNAI2 was examined both in vitro and in vivo. Mass spectrometry, co-immunohistochemistry, immunofluorescence staining, and ubiquitin assays were performed to explore the regulatory mechanism of SNAI2.ResultsOur study indicated that SNAI2 was abnormally expressed in patients with RMS and RMS cell lines and promoted the proliferation and metastasis of RMS. Through cell tubule formation experiments, nude mice Matrigel plug experiments, and immunohistochemistry (IHC), we confirmed that RMS can form VM and that SNAI2 promotes the formation of VM. Due to SNAI2 is a transcription factor that is not easily drugged, we used Co-IP combined with mass spectrometry to screen for the SNAI2-binding protein USP7 and TRIM21. USP7 depletion inhibited RMS VM formation, proliferation and metastasis by promoting SNAI2 degradation. We further demonstrated that TRIM21 is expressed at low levels in human RMS tissues and inhibits VM in RMS cells. TRIM21 promotes SNAI2 protein degradation through ubiquitination in the RMS. The deubiquitinase USP7 and E3 ligase TRIM21 function in an antagonistic rather than competitive mode and play a key role in controlling the stability of SNAI2 to determine the VM formation and progression of RMS.ConclusionOur findings reveal a previously unknown mechanism by which USP7 and TRIM21 balance the level of SNAI2 ubiquitination, determining RMS vasculogenic mimicry, proliferation, and migration. This new mechanism may provide new targeted therapies to inhibit the development of RMS by restoring TRIM21 expression or inhibiting USP7 expression in RMS patients with high SNAI2 protein levels.

Proton Therapy in Non-Rhabdomyosarcoma Soft Tissue Sarcomas of Children and Adolescents.

This paper provides insights into the use of Proton Beam Therapy (PBT) in pediatric patients with non-rhabdomyosarcoma soft tissue sarcomas (NRSTS). NRSTS are a heterogeneous group of rare and aggressive mesenchymal extraskeletal tumors, presenting complex and challenging clinical management scenarios. The overall survival rate for patients with NRSTS is around 70%, but the outcome is strictly related to the presence of various variables, such as the histological subtype, grade of malignancy and tumor stage at diagnosis. Multimodal therapy is typically considered the preferred treatment for high-grade NRSTS. Radiotherapy plays a key role in the treatment of children and adolescents with NRSTS. However, the potential for radiation-induced side effects partially limits its use. Therefore, PBT represents a very suitable therapeutic option for these patients. The unique depth-dose characteristics of protons can be leveraged to minimize doses to healthy tissue significantly, potentially allowing for increased tumor doses and enhanced preservation of surrounding tissues. These benefits suggest that PBT may improve local control while reducing toxicity and improving quality of life. While clear evidence of therapeutic superiority of PBT over other modern photon techniques in NRSTS is still lacking-partly due to the limited data available-PBT can be an excellent treatment option for young patients with these tumors. A dedicated international comprehensive collaborative approach is essential to better define its role within the multidisciplinary management of NRSTS. Shared guidelines for PBT indications-based on the patient's age, estimated outcome, and tumor location-and centralization in high-level referral centers are needed to optimize the use of resources, since access to PBT remains a challenge due to the limited number of available proton therapy facilities.

FOXO3 polymorphisms influence the risk and prognosis of rhabdomyosarcoma in children.

Rhabdomyosarcoma(RMS) is the most common soft tissue sarcoma in children and single nucleotide polymorphisms(SNPs) in certain genes influence risk of RMS. Although FOXO3 had been reported in multiple cancers including RMS, the role of FOXO3 polymorphisms in RMS remains unclear. In this case-control study, we evaluated the association of FOXO3 SNPs with RMS risk and prognosis in children. Four FOXO3 SNPs(rs17069665 A>G, rs4946936 T>C, rs4945816 C>T and rs9400241 C>A) were genotyped in 110 RMS cases and 359 controls. The associations between FOXO3 polymorphisms and RMS risk were determined by odds ratios(ORs) with 95% confidence intervals(CIs). The associations of rs17069665 and rs4946936 with overall survival in RMS children were estimated using the Kaplan-Meier method and log-rank test. Functional analysis in silico was performed to estimate the probability that rs17069665 and rs4946936 might influence the regulation of FOXO3. We found that rs17069665 (GG vs. AA+AG, adjusted OR=2.96; 95%CI [1.10-3.32]; P=0.010) and rs4946936 (TC+CC vs. TT, adjusted OR=0.48; 95%CI [0.25-0.90]; P=0.023) were related to the increased and decreased RMS risk, respectively. Besides, rs17069665(P<0.001) and rs4946936(P<0.001) were associated with decreased and increased overall survival in RMS patients, respectively. Functional analysis showed that rs17069665 and rs4946936 might influence the transcription and expression of FOXO3 via altering the bindings to MYC, CTCF, and/or RELA. This study revealed that FOXO3 polymorphisms influence the RMS susceptibility and prognosis in children, and might altered the expression of FOXO3. FOXO3 polymorphism was suggested as a biomarker for RMS susceptibility and prognosis.

Abstract LB404: ATR inhibitors synergize with DHFR inhibitors in rhabdomyosarcoma cells by disrupting DNA damage checkpoint

Abstract Rhabdomyosarcoma (RMS) is the most common soft tissue sarcoma in children and adolescents. RMS tumors are classified into four subtypes: embryonal, alveolar, spindle cell/sclerosing, and pleomorphic. Treatment options for RMS include a combination of surgery, chemotherapy, and radiation therapy and regimens for patients with relapsed and metastatic RMS remain limited. However, a recent study has reported that pharmacological inhibitors targeting ataxia telangiectasia and Rad3-related protein (ATR) are currently undergoing phase I/II trials and may be utilized for the treatment of alveolar RMS. Interestingly, we identified that the gene signature of replication stress positively correlated with dihydrofolate reductase (DHFR) expression in the cancer cell line database (CellMinerCDB, discover.nci.nih.gov/cellminercdb; Pearson correlation (r)=0.66). DHFR is an important therapeutic target for cancer and autoimmune diseases, and its inhibitors are routinely used and clinically approved. DHFR inhibitors are known to exhibit their anticancer effects by inhibiting nucleotide synthesis and DNA replication. In this study, we aimed to explore a potential synergistic interaction using a combination treatment of ATR inhibitors (elimusertib and ceralasertib) and DHFR inhibitors (pralatrexate, methotrexate, and raltitrexated) in multiple RMS cell lines. We used a drug matrix reduction format and found that combined treatment of ATR inhibitors and DHFR inhibitors induced a synergistically decreased RMS cell viability, showing higher synergy in fusion-positive RMS cells than in fusion-negative RMS cells. Mechanistically, ATR inhibitors abrogated DNA damage response checkpoint CHK1 activation caused by DHFR inhibitor, leading to increased γH2AX and decreased DNA replication activity. Accordingly, the accumulation of DNA damage in combination with ATR inhibitor and DHFR inhibitor triggered higher cell cycle arrest and apoptotic cell death than single agent treatment of DHFR inhibitor. Our findings warrant further research into identifying genomic regions where the DNA damage is enriched in combination treatment by using END-sequencing and synthesis-associated with repair sequencing (SAR-seq). In conclusion, this study provides promising rationale for the combination treatment of ATR and DHFR inhibitors to treat RMS patients, including those with advanced, relapsed, and metastatic states. Citation Format: Ukhyun Jo, George K Annor, Jack F. Shern. ATR inhibitors synergize with DHFR inhibitors in rhabdomyosarcoma cells by disrupting DNA damage checkpoint [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(7_Suppl):Abstract nr LB404.

Characterization of the malignant cells and microenvironment of infantile fibrosarcoma via single-cell RNA sequencing.

Infantile fibrosarcoma (IFS) is the most prevalent soft tissue sarcoma in children under 1 year old and is known for its rapid growth. The tumor lacks specific immunohistochemical tumor marker and a general view of tumor microenvironment (TME). Its primary therapeutic intervention places patients at a risk of disability or mutilation. This study aimed to elucidate the universal transcriptional characteristics of IFS and explore novel targets for diagnosis and therapy using single-cell RNA sequencing (scRNA-seq). Fresh tissue samples of IFS for scRNA-seq were collected from four patients before other treatments were administered. We conducted cell clustering, inferring copy number variation from scRNA-seq (InferCNV) analysis, gene differential expression analysis, cell function evaluation, Pearson correlation analysis, and cell-cell and ligand-receptor interaction analysis to investigate the distinct ecosystem of IFS. According to the single-cell resolution data, we depicted the cell atlas of IFS, which comprised 14 cell populations. Through comparison with normal cells, the malignant cells were distinguished, and potential novel markers (POSTN, IGFBP2 and CTHRC1) were identified. We also found four various functional malignant cell subtypes, three of which exhibited cancer stem cells (CSCs) phenotypes, and investigated the interplay between these subtypes and nonmalignant cells in the TME of IFS. Endothelial cells and macrophages were found to dominate the cell-cell communication landscape within the microenvironment, promoting tumorigenesis via multiple receptor-ligand interactions. This study provides a comprehensive characterization of the tumor transcriptome and TME of IFS at the cellular level, offering valuable insights for clinically significant advancements in the immunohistochemical diagnosis and treatment of IFS.

Abstract 711: HDAC3 sustains resistance to hypofractionated radiotherapy in fusion positve rhabdomyosarcoma cells

Abstract Rhabdomyosarcoma (RMS) is the most common soft tissue sarcoma of childhood. The fusion-positive (FP)-RMS variant expressing chimeric oncoproteins such as PAX3-FOXO1 and PAX7-FOXO1 shows a dismal prognosis with 5-year survival of less than 30% compared to non-metastatic fusion-negative (FN)-RMS variant. In the last years, a lot of interest has focused on new targets identification to improve the radiotherapy (RT) efficacy. HDAC inhibitors (HDACi) radio-sensitize different cancer cell types including RMS. Recently, we reported that MS-275, a Class I and IV HDACi, in combination with RT affected cell survival, reduced colony formation ability, increased DNA damage repair inhibition and reactive oxygen species formation in FP-RMS cells. However, despite promising preclinical studies, HDAC inhibitors (HDACi) achieved only modest success in human clinical trials for solid tumors, frequently showing several toxicities, probably because of the limited specificity of many HDACi. Thus, a major effort is being directed toward identification of HDACi which are selective for HDAC isoforms often uniquely implicated in the radioresistance of specific cancers. In order to identify the class I HDAC responsible of radioresistance in RMS we knocked-down HDAC1, HDAC2, HDAC3, and HDAC8 expression in combination with RT. Interestingly, we observed an increased radiosensitivity only in HDAC3-depleted FP-RMS cells. Thus, we focused on HDAC3 to understand the mechanisms by which it promotes radioresistance in RMS. We observed that HDAC3 is overexpressed in RMS patients and cell lines compared to the normal counterpart. Furthermore, RMS cells are strongly dependent by HDAC3 expression while slight dependency has been observed with other class I HDACs in DepMap portal. We demonstrated that HDAC3 depletion by CRISPR in combination with RT in FP-RMS cells increases apoptosis, reduces colony formation ability, cancer stem cells population and anchorage independent growth and reduces cell growth in vivo and in vitro. Accordingly, HDAC3 depleted cells in combination with RT reduces levels and activation of key player of FP-RMS biology such as MYCN, ERK and AKT. Moreover, HDAC3 KD increases radiotherapy-induced DNA double strand break and impairs DNA repair mechanisms reducing levels and activation of both homologous recombination (HR) and non-homologous end joining (NHEJ) factors such as ATM, RAD51 and DNA-PKcs. Thus, we developed a new potent and highly specific HDAC3 inhibitor (HDAC3i). The new HDAC3i is highly specific in targeting FP-RMS cell growth in vitro while no effects have been observed in normal cells such as myoblasts and lung fibroblast. Moreover, the drug treatment in combination with radiotherapy phenotypically and molecularly recapitulated what observed in HDAC3 depleted cells.The study has been founded by Italian Association for Cancer Research (AIRC) to FM. Citation Format: Matteo Cassandri, Antonella Porrazzo, Silvia Pomella, Simona Camero, Francesca A. Aiello, Lucrezia D'Archivio, Clemens Zwergel, Beatrice Noce, Miriam Tomaciello, Francesca Vulcano, Luisa Milazzo, Francesca Pedini, Michele Signore, Alessandro Fanzani, Cinzia Marchese, Giuseppe Minniti, Sergio Valente, Antonello Mai, Francesca Megiorni, Rossella Rota, Francesco Marampon. HDAC3 sustains resistance to hypofractionated radiotherapy in fusion positve rhabdomyosarcoma cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 711.

Abstract 1097: CD73 contributes to the pathogenesis of fusion-negative rhabdomyosarcoma through the purinergic signaling pathway

Abstract Rhabdomyosarcoma (RMS) is the most common type of soft tissue sarcoma in children and adolescents. Fusion-Negative RMS (FN-RMS) accounts for more than 80% of all RMS cases. The long-term event-free survival rate for patients with high grade FN-RMS is below 30%, highlighting the need for improved therapeutic strategies. CD73 is a 5’ ectonucleotidase that hydrolyzes AMP to adenosine and regulates the purinergic signaling pathway. We found that CD73 is elevated in FN-RMS tumors that express high levels of TWIST2. While high expression of CD73 contributes to the pathogenesis of multiple cancers, its role in FN-RMS has not been investigated. We found that CD73 knockdown decreased FN-RMS cell growth while upregulating the myogenic differentiation program. Moreover, mutation of the catalytic residues of CD73 rendered the protein enzymatically inactive and abolished its ability to stimulate FN-RMS growth. Overexpression of wildtype CD73, but not the catalytically inactive mutant, in CD73 knockdown FN-RMS cells, restored their growth capacity. Likewise, treatment with an adenosine receptor A2A-B agonist partially rescued FN-RMS cell proliferation and bypassed the CD73 knockdown defective growth phenotype. These results demonstrate that the catalytic activity of CD73 contributes to the pathogenic growth of FN-RMS through the activation of the purinergic signaling pathway. Therefore, targeting CD73 and the purinergic signaling pathway represents a potential therapeutic approach for FN-RMS patients. Citation Format: Karla Gabriela Cano Hernandez, Akansha Shah, Victor Lopez, Vincent Tagliabracci, Kenian Chen, Lin Xu, Rhonda Bassel-Duby, Eric Olson, Ning Liu. CD73 contributes to the pathogenesis of fusion-negative rhabdomyosarcoma through the purinergic signaling pathway [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 1097.

Abstract 1095: Prox1 transcription factor and its downstream signaling - A new therapeutic target for rhabdomyosarcoma

Abstract Rhabdomyosarcoma (RMS) stands as the predominant soft tissue sarcoma in children, characterized by a high-grade neoplasm composed of cells resembling skeletal myoblasts. Despite being prevalent, patients with metastatic or recurrent rhabdomyosarcoma still experience poor outcomes, underscoring the necessity for novel treatments. We have recently shown that PROX1 is a critical factor in RMS tumor growth and myogenic characteristics (Gizaw NY, et al. PNAS 2022). As PROX1 has previously been associated with tumor metabolism in colorectal cancer, we investigated further its role in RMS metabolism. Our findings demonstrate that PROX1 plays a crucial role in regulating RMS cell metabolism. Specifically, silencing of PROX1 resulted in decreased lipid and oxidative metabolism, which was accompanied by an increase in glycolysis and triglyceride accumulation. RNA sequencing and Gene Set Enrichment Analysis revealed downregulation of all genes involved in the cholesterol synthesis pathway in PROX1-silenced RMS cells. Our studies further reveal that inhibiting cholesterol synthesis, akin to PROX1 silencing, can effectively inhibit RMS cell growth. Our results provide novel insights into the role of PROX1 in RMS metabolism and highlight the potential of PROX1-regulated metabolic pathways as promising targets for the development of novel RMS therapies. Citation Format: Nebeyu Y. Gizaw, Tom Böhling, Mika Sampo, Kari Alitalo, Riikka M. Kivela. Prox1 transcription factor and its downstream signaling - A new therapeutic target for rhabdomyosarcoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 1095.

Abstract 4269: Effect of PAX3-FOXO1 fusion gene on cell-extracellular matrix interaction of Rhabdomyosarcoma

Abstract Background Rhabdomyosarcoma (RMS) is the most common childhood soft tissue sarcoma, categorized by two subtypes, fusion protein-positive (FPRMS) and negative (FNRMS), based on the existence of PAX3-FOXO1 fusion gene (P3F). Different features between the two subtypes could guide the use of distinct treatment approaches. We discovered that FNRMS exhibited enrichment of gene sets related to cell-extracellular matrix (ECM) interaction than FPRMS. ECM, providing physical support as a scaffold, regulates cancer development. We hypothesize that a distinct molecular feature, P3F, may cause the difference in cell-ECM interaction between FPRMS and FNRMS. Methods To confirm the higher cell-ECM interaction of FNRMS vs. FPRMS, we used confocal reflection microscopy (CRM), enabling us to monitor the fibrous structures of ECM. We generated the spheroids with FNRMS and FPRMS cell lines by hanging drop method and embedded them into collagen type I. Then, we assessed orientation and dynamic changes in the structure of collagen fibers surrounding spheroids. Also, we analyze the transcriptome of FPRMS cells with P3F knockdown to confirm if P3F regulates cell-ECM interaction negatively via RNA seq. Additionally, we examined ECM surrounding spheroids of P3F knockdown FPRMS cells if P3F modulates fibrous structures of ECM, indicating cell-ECM interaction. Results We showed that collagen fibers are oriented more perpendicular to the FNRMS spheroid surface, indicating the strong pulling and pushing effect of cells. However, fibers surrounding FPRMS spheroids showed parallel alignment to their surfaces, reflecting passively pushed by the spheroids. Additionally, fibers near FNRMS spheroids showed greater displacement than those near FNRMS spheroids, which suggests stronger cell-ECM interaction in FNRMS than in FPRMS. Furthermore, P3F knockdown FPRMS cells exhibited significant enrichment of gene sets related to cell-ECM interaction. Their CRM results showed that collagen fiber orientation was significantly perpendicular to the surfaces of spheroids of P3F knockdown FPRMS cells than those of control cells. P3F knockdown cell spheroids also displayed enhanced displacement of ECM fibers than control cells. These results suggest that P3F suppresses cell-ECM interaction, consistent with higher cell-ECM interaction of FNRMS than FPRMS. Conclusions The PAX3-FOXO1 gene may regulate cell-ECM interaction negatively, causing distinct features between FPRMS and FNRMS, which is crucial for various cellular functions and suggest different treatment between the two RMS subtypes. Citation Format: Chandra Kaladhar Vemula, Ivan Chavez, Anthony Chronopoulos, Jinseok Park. Effect of PAX3-FOXO1 fusion gene on cell-extracellular matrix interaction of Rhabdomyosarcoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 4269.

Abstract 131: Utilizing human induced pluripotent stem cells (hiPSCs) as a model to study the genetic drivers of rhabdomyosarcoma

Abstract Rhabdomyosarcoma (RMS) is a childhood soft tissue sarcoma composed of skeletal myoblast-like cells that lack the capacity to differentiate despite their high expression of terminal differentiation markers like myogenin. There are two main types of RMS: fusion-positive RMS (FP-RMS), which is associated with a chromosomal translocation resulting in an oncogenic fusion protein (PAX3-FOXO1), and the more common form, fusion-negative RMS (FN-RMS) which lacks this translocation and is instead thought to be driven by numerous other oncogenes. Our lab used a Bayesian-based computational algorithm to analyze NGS sequencing data from 290 RMS samples and identified 25 potential genetic drivers of RMS. We aim to use induced pluripotent stem cells as a new model to study how these drivers, including the PAX3-FOXO1 fusion gene, influence RMS formation and progression. To this aim, we have successfully made a doxycycline inducible lentiviral vector containing the full-length sequence of human PAX3-FOXO1 which we used to ectopically express PAX3-FOXO1 in two well-characterized iPSC lines. We subcutaneously injected these iPSCs into NOD/SCID mice and treated them with doxycycline to form teratomas, non-malignant tumors containing tissue from all three germ layers. We hypothesized that PAX3-FOXO1 induction in these differentiating teratomas, may have transforming capabilities and may deregulate transcriptional programs to form FP-RMS-like tumors. We have found that PAX3-FOXO1 overexpressing teratomas grew faster and histologically look more “immature” than their control counterparts but do not seem to give rise to rhabdo-like tumors. We want to further understand what other critical factors are necessary to drive RMS formation by focusing on one of our drivers of interest EZH2, a histone methyltransferase implicated as either an oncogene or a tumor suppressor in certain forms of cancer. We have data suggesting that EZH2-dependent histone methylation activity may act as a proto-oncogene with PAX3-FOXO1 to silence the expression of key tumor suppressor genes CDKN2A and CDKN2B in RMS. To this aim, we reasoned that iPSCs differentiated into myogenic progenitors, one of the potential cells of origin of RMS, will be a novel way to study the role of oncogenes like PAX3-FOXO1 and EZH2 in RMS. We have derived myogenic progenitors from iPSCs (iPSC-MPCs) that express key myogenic factors like PAX7 and MYOD and differentiate and fuse to form mature myofibers. We plan to next explore how ectopic expression of EZH2 and/or PAX3-FOXO1 alters gene expression and histone methylation and whether these alterations can drive RMS formation in these iPSC-MPCs. Using this model system of iPSC-MPCs to drive expression of these oncogenes has the potential to offer new and exciting ways to identify the most important genes in driving cancer formation hopefully providing insight into better targets for treatment of RMS. Citation Format: Celeste Romero, Yanbin Zheng, Lin Xu, Stephen Skapek. Utilizing human induced pluripotent stem cells (hiPSCs) as a model to study the genetic drivers of rhabdomyosarcoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 131.