Soft Tissue Sarcoma In Children Research Articles

Abstract 1299: Transcriptomic analysis to unveil alveolar rhabdomyosarcoma metastatic process

Abstract Background: Rhabdomyosarcoma (RMS) is the most prevalent soft tissue sarcoma in children and adolescents. It is divided in two major histological subtypes, being the Alveolar (ARMS) the one associated to a poorer prognosis1. The cure rate for these tumors is up to 70% for children with localized disease, but this numbers decrease notably to a 20% 5-year survival rate for patients that show metastatic spreading. However, mechanisms leading to metastatic disease remain poorly understood. Hence, there is an urgent need to better understand this process, which will hopefully lead to novel targeted therapies and improved survival rates for these patients. Methods: ARMS cells (RH4) were injected in the gastrocnemius muscle of Athymic Nude-Foxn1nu mice2. Primary tumors (800mm3) and peritoneal metastases (visible by IVIS®) were collected, and RNA extracted. Xenograft-derived metastatic cell lines were obtained directly as a primary culture from mice metastases. Characterization of paired tumors and metastases was performed using Clariom™ D microarray (Applied Biosystems). After data normalization and comparative analyses (R; RMA and limma), a list of differentially expressed transcripts between sample groups was obtained. These are being validated by qPCR and Western Blot. Results: Transcriptomic profiling of tumors (n=3), paired metastases (n=3), xenograft-derived metastatic cell lines (n=3) and RH4 cell line (n=3) revealed 3420 differentially expressed transcripts (2747 increased in metastases, 673 in tumors). Several transcripts of interest are overexpressed in metastases samples compared to primary tumors as showed by microarray analysis and validated by qPCR. These targets are related in literature to metastasis in other cancers, as well as lymph nodes metastasis, which occurs to happen in our model and in ARMS clinics3. Additionally, some transcripts are overexpressed in tumor samples, which are linked to inhibition of proliferation and motility. Conclusions/Future Work: In vitro phenotypic characterization of the obtained targets will involve proliferation (MTT), migration (Transwell®) and clonogenic assays, including loss-of-function (siRNA and commercial inhibitors). Using stable knock-out and knock-in models of the genes of interest, their role in ARMS metastasis will be further studied in vitro and in vivo. The clinical relevance of these targets will be explored in several cohorts of patients by immunohistochemistry on Tissue Microarrays. Studying the profile of paired tumors and metastases provides a unique tool to decipher the mechanisms of tumor progression. Fully understanding the metastatic process will ultimately lead to the development of better targeted therapies and thus to improved outcomes for ARMS patients. Also, these same steps are being followed in our laboratory for Ewing Sarcoma, another developmental tumor with discouraging survival rates for metastatic patients (<30%)4. Citation Format: Sara Sanchez-Serra, Mariona Chicon-Bosch, Paola Monaco, Laura Santana-Viera, Susana Maqueda-Marcos, Manuel Gris-Lorente, Roser Lopez-Alemany, Oscar M. Tirado. Transcriptomic analysis to unveil alveolar rhabdomyosarcoma metastatic process [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 1299.

Abstract SY23-03: Rhabdomyosarcoma: New research tools for precision medicine

Abstract Rhabdomyosarcoma (RMS) is a cancer of skeletal muscle histogenesis and the most common soft tissue sarcoma of childhood. Despite decades of basic and clinical research, survival for patients with high-risk disease has not improved since the 1970s, remaining <30% at five years. Genomic landscape studies have identified the most frequently occurring alterations, parsing out fusion-negative RMS driven by RAS pathway mutations, and fusion-positive RMS driven by signature fusion genes including PAX3-FOXO1. However, this information has not yet translated into improved outcomes. Here, we highlight multi-institutional efforts to generate new tools for RMS precision medicine, including the NCI FusOnC2 Cancer Moonshot program to target PAX3-FOXO1, the NCI CCDI Molecular Characterization Initiative to analyze every child’s RMS tumor nucleic acids, and the application of machine learning to RMS tumor images to predict molecular subtype and improve risk stratification. Citation Format: Corinne Mary Linardic. Rhabdomyosarcoma: New research tools for precision medicine. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr SY23-03.

Abstract 1784: FGFR4 and CD276 dual targeting CAR T cells demonstrate synergistic antitumor activity in childhood rhabdomyosarcoma

Abstract Background Chimeric antigen receptor T cells (CAR T) are potential therapies for rhabdosarcoma (RMS), the most common soft tissue sarcoma in children, where patients with relapsed or refractory disease have a dismal cure rate, and effective therapies are urgently needed. RMS tumors express high levels of the cell surface receptors FGFR4 and CD276, and both are direct targets of the PAX3-FOXO1 chimeric oncogene. However, CAR T have shown poor performance in solid tumors due to T cell exhaustion, limited persistence, and heterogeneous expression of target antigens. We hypothesize that engineering CAR constructs targeting FGFR4 and CD276 will enhance their activity and dual targeting will overcome the heterogenous target expression to effectively eliminate RMS tumors. Methods We modified and tested different hinge and transmembrane domains (HTM) or intracellular domain (ICD) of FGFR4 CAR constructs and developed multiple dual targeting Bicistronic CARs (BiCisCARs) against FGFR4 and CD276. We tested these CARs in aggressive mouse models and performed Cellular Indexing of Transcriptomes and Epitopes by Sequencing (CITE-seq) of tumor infiltrating CAR T cells to functionally characterize these constructs. Results FGFR4 targeting CAR construct with CD8HTM and 4-1BB ICD could eliminate low burden RMS but was less effective against orthotopic tumors. Replacing CD8HTM with CD28HTM improved the efficacy in a moderate burden RMS model but did not show any benefit against a more aggressive RMS559 orthotopic model. Further modification using the CD28 ICD significantly enhanced the activity in a large burden RMS model but was unable to eradicate all tumors. Similarly, CAR T cells targeting CD276 alone, showed significant anti-tumor activity in moderate burden RMS models but could not eliminate all tumors. The BiCisCAR with FGFR4.CD28HTM.CD28ICD and CD276.CD8HTM.4-1BBICD was the most potent construct, eradicating 100% tumors in all tested orthotopic models, and those with heterogenous expression of target antigens. CITE-seq and flow cytometry assays demonstrated that this BiCisCAR showed the most significant tumor infiltration, and persistence, with limited exhaustion. This BiCisCAR exhibited a synergistic effect on cytokine production and anti-tumor activity, compared to single targeting CARs. Moreover, biochemical characterization revealed that the combined use of both CD28 and 4-1BB ICDs in the BiCisCAR resulted in the activation of three TCR downstream signaling pathways including AKT, Erk1/2 and p65. Conclusions and Future DirectionsThus, we have developed a potent BiCisCAR with dual targeting of FGFR4 and CD276 that shows optimal biochemical activity, persistence, and limited exhaustion, and addresses heterogenous expression of target antigens. This BiCisCAR will be further developed for future clinical trials in patients with high-risk RMS. Citation Format: Meijie Tian, Jun S. Wei, Adam Cheuk, David Milewski, Zhongmei Zhang, Yong Yuan Kim, Can Liu, Sherif Badr, Michael C. Kelly, Jerry T. Wu, Abdelrahman Rahmy, Hsien-Chao Chou, Xinyu Wen, Javed Khan. FGFR4 and CD276 dual targeting CAR T cells demonstrate synergistic antitumor activity in childhood rhabdomyosarcoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 1784.

Abstract 4750: TWIST2 mediated chromatin remodeling promotes fusion-negative rhabdomyosarcoma

Abstract Rhabdomyosarcoma (RMS) is a common soft tissue sarcoma in children that resembles the developing skeletal muscle. Unlike normal muscle cells, RMS cells fail to differentiate despite expression of the myogenic determination protein MYOD. The TWIST2 transcription factor is frequently overexpressed in fusion-negative RMS (FN-RMS). TWIST2 blocks differentiation by inhibiting MYOD activity in myoblasts, but its role in FN-RMS pathogenesis is incompletely understood. Here, we show that knockdown of TWIST2 enables FN-RMS cells to exit the cell cycle and undergo terminal myogenesis. TWIST2 knockdown also substantially reduces tumor growth in a xenograft model of FN-RMS. Mechanistically, TWIST2 controls H3K27 acetylation at distal enhancers by interacting with the chromatin remodelers SMARCA4 and CHD3 to activate growth-related and repress myogenesis-related target genes, respectively. These findings provide novel insights into the role of TWIST2 in maintaining an undifferentiated and tumorigenic state of FN-RMS and highlight the potential of suppressing TWIST2-regulated pathways to treat FN-RMS. Citation Format: Akansha M. Shah, Lei Guo, Maria Gabriella Morales, Priscilla Jaichander, Kenian Chen, Huocong Huang, Karla Cano Hernandez, Lin Xu, Rhonda Bassel-Duby, Eric N. Olson, Ning Liu. TWIST2 mediated chromatin remodeling promotes fusion-negative rhabdomyosarcoma. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 4750.

The natural origins of cytostatic compounds used in rhabdomyosarcoma therapy.

Rhabdomyosarcoma (RMS) is the most common soft tissue sarcoma in children and represents a high-grade neoplasm of skeletal myoblast-like cells. About 40% of all registered soft tissue tumors are RMSs. This paper describes our current understanding of the RMS subtypes (alveolar (ARMS), embryonic (ERMS), pleomorphic (PRMS), and spindle cell/sclerosing (s/scRMS)), diagnostic methods, molecular bases, and characteristics. We also present the currently used treatment methods and the potential use of natural substances in the treatment of this type of cancer. Natural cytotoxic substances are compounds that have been the subject of numerous studies and discussions in recent years. Since anti-cancer therapies are often limited by a low therapeutic index and cancer resistance to pharmacotherapy, it is very important to search for new, effective compounds. Additionally, compounds of a natural origin are usually readily available and have a reduced cytotoxicity. Thus, the undiscovered potential of natural anti-cancer compounds makes this field of research a very important area. The introduction of model species into research examining the use of natural cytostatic therapies for RMS will allow for further assessment of the effects of these compounds on cancerous and healthy tissues.

Imaging in rhabdomyosarcoma: a patient journey

Rhabdomyosarcoma, although rare, is the most frequent soft tissue sarcoma in children and adolescents. It can present as a mass at nearly any site in the body, with most common presentations in the head and neck, genitourinary tract and extremities. The optimal diagnostic approach and management of rhabdomyosarcoma require a multidisciplinary team with multimodal treatment, including chemotherapy and local therapy. Survival has improved over the last decades; however, further improvement in management is essential with current 5-year overall survival ranging from 35% to 100%, depending on disease and patient characteristics. In the full patient journey, from diagnosis, staging, management to follow-up after therapy, the paediatric radiologist and nuclear physician are essential members of the multidisciplinary team. Recently, guidelines of the European paediatric Soft tissue sarcoma Study Group, the Cooperative Weichteilsarkom Studiengruppe and the Oncology Task Force of the European Society of Paediatric Radiology (ESPR), in an ongoing collaboration with the International Soft-Tissue Sarcoma Database Consortium, provided guidance for high-quality imaging. In this educational paper, given as a lecture during the 2022 postgraduate ESPR course, the multi-disciplinary team of our national paediatric oncology centre presents the journey of two patients with rhabdomyosarcoma and discusses the impact on and considerations for the clinical (paediatric) radiologist and nuclear physician. The key learning points of the guidelines and their implementation in clinical practice are highlighted and up-to-date insights provided for all aspects from clinical suspicion of rhabdomyosarcoma and its differential diagnosis, to biopsy, staging, risk stratification, treatment response assessment and follow-up.

Triggered release from thermosensitive liposomes improves tumor targeting of vinorelbine.

Triggered drug delivery strategies have been shown to enhance drug accumulation at target diseased sites in comparison to administration of free drug. In particular, many studies have demonstrated improved targetability of chemotherapeutics when delivered via thermosensitive liposomes. However, most studies continue to focus on encapsulating doxorubicin while many other drugs would benefit from this targeted and localized delivery approach. The proposed study explores the therapeutic potential of a thermosensitive liposome formulation of the commonly used chemotherapy drug vinorelbine in combination with mild hyperthermia (39-43°C) in a murine model of rhabdomyosarcoma. Rhabdomyosarcoma, the most common soft tissue sarcoma in children, is largely treated using conventional chemotherapy which is associated with significant adverse long-term sequelae. In this study, mild hyperthermia was pursued as a non-invasive, non-toxic means to improve the efficacy and safety profiles of vinorelbine. Thorough assessment of the pharmacokinetics, biodistribution, efficacy and toxicity of vinorelbine administered in the thermosensitive liposome formulation was compared to administration in a traditional, non-thermosensitive liposome formulation. This study shows the potential of an advanced formulation technology in combination with mild hyperthermia as a means to target an untargeted therapeutic agent and result in a significant improvement in its therapeutic index.

Surfaceome Profiling of Cell Lines and Patient-Derived Xenografts Confirm FGFR4, NCAM1, CD276, and Highlight AGRL2, JAM3, and L1CAM as Surface Targets for Rhabdomyosarcoma.

Rhabdomyosarcoma (RMS) is the most common soft tissue sarcoma in children. The prognosis for patients with high-grade and metastatic disease is still very poor, and survivors are burdened with long-lasting side effects. Therefore, more effective and less toxic therapies are needed. Surface proteins are ideal targets for antibody-based therapies, like bispecific antibodies, antibody-drug conjugates, or chimeric antigen receptor (CAR) T-cells. Specific surface targets for RMS are scarce. Here, we performed a surfaceome profiling based on differential centrifugation enrichment of surface/membrane proteins and detection by LC-MS on six fusion-positive (FP) RMS cell lines, five fusion-negative (FN) RMS cell lines, and three RMS patient-derived xenografts (PDXs). A total of 699 proteins were detected in the three RMS groups. Ranking based on expression levels and comparison to expression in normal MRC-5 fibroblasts and myoblasts, followed by statistical analysis, highlighted known RMS targets such as FGFR4, NCAM1, and CD276/B7-H3, and revealed AGRL2, JAM3, MEGF10, GPC4, CADM2, as potential targets for immunotherapies of RMS. L1CAM expression was investigated in RMS tissues, and strong L1CAM expression was observed in more than 80% of alveolar RMS tumors, making it a practicable target for antibody-based therapies of alveolar RMS.

A case report of vulvar embryonal rhabdomyosarcoma in an adult pregnant woman

Embryonal rhabdomyosarcoma (ERMS) is the most common soft tissue sarcoma in children, but it is rare in adults, where cases are reported as individual cases or series. We present an ERMS found in the vulva of a 24-year-old pregnant woman who complained of a painless soft tissue mass in her left vulva. The mass grew rapidly within a month, and abortion, internal iliac artery embolization, and chemotherapy were performed. Despite an initially good response, cancer unfortunately progressed, and the patient passed away within one year after diagnosis. Adults with ERMSs tend to have poorer outcomes than children, and our case suggests that in some cases, pregnancy may accelerate the deterioration of the tumor.

Baseline Characteristics, Prognostic Factors, and Treatment Outcomes for Adult Patients With Rhabdomyosarcoma (RMS).

Rhabdomyosarcoma (RMS) is the most common soft tissue sarcoma of childhood, while in adults it is one of the rarer tumors. Its prognosis is better in children with current treatment modalities; however, it carries poorer prognosis in adults. Recent data on adult RMS is scarce from our part of world. We report outcomes of adult patients with RMS, and with 40 patients; it is the first study to publish such a large data from Pakistan. This was a retrospective study that included 64 adult patients aged 18 years and older. After data extraction and scrutiny, a total of 40 patients were segregated with diagnosis of RMS of various varieties who were treated and followed up subsequently. International Business Machines (IBM) Statistical Package for Social Sciences (SPSS), version 26 (IBM Corp., Armonk, NY) was used to evaluate all of the gathered data. Embryonal RMS (ERMS) was the most common subtype. Factors favoring better overall survival (OS) at 5 years were absence of nodal and distal metastases, treatment with surgery, margin negative resection, and absence of residual disease on postoperative imaging. Adjuvant radiation therapy (XRT) for positive resection margins as well as for residual disease on postoperative imaging also favored better OS at 5 years. Chemotherapy did impart a trend towards better OS; however, it was not significant. Histopathologic subtype and tumor size did not have any significant impact on outcomes. Median progression free survival (PFS) was 11 months and median OS was 15 months. Adult RMS is a rare disease entity with widely heterogeneous clinical picture and poorer outcomes as compared to the disease of childhood and adolescence. Further prospective studies with larger sample size are required to establish role of patient, disease, and treatment-related factors affecting outcomes in our population.

The botanical drug PBI-05204, a supercritical CO2 extract of Nerium oleander, sensitizes alveolar and embryonal rhabdomyosarcoma to radiotherapy in vitro and in vivo.

Treatment of rhabdomyosarcoma (RMS), the most common a soft tissue sarcoma in childhood, provides intensive multimodal therapy, with radiotherapy (RT) playing a critical role for local tumor control. However, since RMS efficiently activates mechanisms of resistance to therapies, despite improvements, the prognosis remains still largely unsatisfactory, mainly in RMS expressing chimeric oncoproteins PAX3/PAX7-FOXO1, and fusion-positive (FP)-RMS. Cardiac glycosides (CGs), plant-derived steroid-like compounds with a selective inhibitory activity of the Na+/K+-ATPase pump (NKA), have shown antitumor and radio-sensitizing properties. Herein, the therapeutic properties of PBI-05204, an extract from Nerium oleander containing the CG oleandrin already studied in phase I and II clinical trials for cancer patients, were investigated, in vitro and in vivo, against FN- and FP-RMS cancer models. PBI-05204 induced growth arrest in a concentration dependent manner, with FP-RMS being more sensitive than FN-RMS, by differently regulating cell cycle regulators and commonly upregulating cell cycle inhibitors p21Waf1/Cip1 and p27Cip1/Kip1. Furthermore, PBI-05204 concomitantly induced cell death on both RMS types and senescence in FN-RMS. Notably, PBI-05204 counteracted in vitro migration and invasion abilities and suppressed the formation of spheroids enriched in CD133+ cancer stem cells (CSCs). PBI-05204 sensitized both cell types to RT by improving the ability of RT to induce G2 growth arrest and counteracting the RT-induced activation of both Non-Homologous End-Joining and homologous recombination DSBs repair pathways. Finally, the antitumor and radio-sensitizing proprieties of PBI-05204 were confirmed in vivo. Notably, both in vitro and in vivo evidence confirmed the higher sensitivity to PBI-05204 of FP-RMS. Thus, PBI-05204 represents a valid radio-sensitizing agent for the treatment of RMS, including the intrinsically radio-resistant FP-RMS.

Multimodality imaging evaluation of nasal sinus alveolar rhabdomyosarcoma: Two case reports

Rhabdomyosarcoma (RMS) is a common pleomorphic malignant soft tissue sarcoma in children and adolescents that originates from rhabdomyoblasts or mesenchymal precursor cells. Alveolar rhabdomyosarcoma (ARMS) mostly occurs in adolescents aged 10-15 years and is characterized by more aggressive behaviors and worse prognosis than other sarcomas, prone to lymphatic and hematogenous metastasis in the early stage as well as metastasizing to breast, testis, pancreas, and other parts. ARMS often occurs in the limbs and genitourinary system, however, head and neck ARMS are relatively rare when involving the nasal cavity or sinuses. The role of MRI and 18F-FDG positron emission tomography combined with computed tomography (PET/CT) remains to be established in ARMS. Case 1: An 18-year-old male was found with a left submandibular mass of approximately 1 cm in diameter 2 months ago, which gradually increased in size. CT showed multiple soft tissue masses in maxillofacial and neck regions and the lesions invaded the frontal lobe and the inner wall of the left orbital lobe. MRI showed the masses with hypointensity on T1WI, hyperintensity on T2WI, and diffusion-weighted imaging (DWI) with significant enhancement. 18F-FDG PET/CT showed multiple hypermetabolic lesions located in the maxillofacial, neck region, 3rd lumbar vertebra, and the right sacrum. A nasal endoscopic tumor biopsy and molecular testing finally helped to diagnose the ARMS. Case 2: A 14-year-old male presented with left maxillary pain with nasal congestion and left ocular swelling 15 days ago. CT demonstrated a soft tissue mass in the nasal cavity and sinuses with local protrusion into the left orbit. MRI showed the masses with a slightly low signal on T1WI, a high signal on T2WI, and DWI with significant heterogenous enhancement. 18F-FDG PET/CT showed hypermetabolic lesions in the left maxillofacial and neck regions. ARMS was finally diagnosed by a nasal endoscopic tumor biopsy and molecular testing. The patient had a recurrence of the lesion after chemotherapy and surgical resection and is currently undergoing radiation therapy. Nasal sinus ARMS is highly malignant with a poor prognosis. Accurate diagnosis relies not only on histopathology and immunohistochemistry examination but also on genetic detection of characteristic chromosomal translocations and fusion genes. Imaging methods, such as MRI and PET/CT can accurately assess the extent of the lesions and metastases, assist in the diagnosis of the disease and the selection of treatment regimens, provide precise localization for surgery, and help with treatment monitoring and follow-up.

Translational Implications for Radiosensitizing Strategies in Rhabdomyosarcoma.

Rhabdomyosarcoma (RMS) is the most common soft tissue sarcoma of childhood and adolescence that includes FP-RMS, harboring the fusion oncoprotein PAX3/7-FOXO1 and FN-RMS, often mutant in the RAS pathway. Risk stratifications of RMS patients determine different prognostic groups and related therapeutic treatment. Current multimodal therapeutic strategies involve surgery, chemotherapy (CHT) and radiotherapy (RT), but despite the deeper knowledge of response mechanisms underpinning CHT treatment and the technological improvements that characterize RT, local failures and recurrence frequently occur. This review sums up the RMS classification and the management of RMS patients, with special attention to RT treatment and possible radiosensitizing strategies for RMS tumors. Indeed, RMS radioresistance is a clinical problem and further studies aimed at dissecting radioresistant molecular mechanisms are needed to identify specific targets to hit, thus improving RT-induced cytotoxicity.

Surgical Resection for Pediatric Synovial Sarcoma: What Margin Is Adequate?

INTRODUCTION: Synovial sarcoma is the most common nonrhabdomyosarcoma soft tissue sarcoma in children. Guidelines concerning the size of resection margins are lacking. We sought to evaluate if negative microscopic resection margins predicted overall survival. METHODS: A retrospective chart review showed 22 patients with synovial sarcoma from September 2011 to November 2019 at Texas Children’s Hospital. Pearson’s chi-square test and multivariate regression analyses were used. RESULTS: Relapsed free survival was 59% and overall survival was 72% with a median follow-up of 3 years (with interquartile range [IQR] 1.6 to 4.9 years) from initial resection (Figure). The patients with negative margins had a median resection margin of 1.5 mm (IQR 1 to 27mm). Positive microscopic margins were noted in 71% (5/7) of the patients who died compared with 12% (2/16) of patients who survived (p = 0.01). Survival in patients with surgical margins of <5 mm was 100% (7/7) compared with the 28% (2/7) of patients with positive surgical margins (p = 0.02). Positive microscopic margin was an independent risk factor for death (odds ratio 40, 95% CI 1.98 to 807; p = 0.02) when adjusting for adjuvant chemotherapy and radiation.FigureCONCLUSION: This study revealed that a negative microscopic surgical margin in pediatric synovial sarcoma is an independent prognostic predictor of overall survival.

Radioresistance in rhabdomyosarcomas: Much more than a question of dose.

Management of rhabdomyosarcoma (RMS), the most common soft tissue sarcoma in children, frequently accounting the genitourinary tract is complex and requires a multimodal therapy. In particular, as a consequence of the advancement in dose conformity technology, radiation therapy (RT) has now become the standard therapeutic option for patients with RMS. In the clinical practice, dose and timing of RT are adjusted on the basis of patients’ risk stratification to reduce late toxicity and side effects on normal tissues. However, despite the substantial improvement in cure rates, local failure and recurrence frequently occur. In this review, we summarize the general principles of the treatment of RMS, focusing on RT, and the main molecular pathways and specific proteins involved into radioresistance in RMS tumors. Specifically, we focused on DNA damage/repair, reactive oxygen species, cancer stem cells, and epigenetic modifications that have been reported in the context of RMS neoplasia in both in vitro and in vivo studies. The precise elucidation of the radioresistance-related molecular mechanisms is of pivotal importance to set up new more effective and tolerable combined therapeutic approaches that can radiosensitize cancer cells to finally ameliorate the overall survival of patients with RMS, especially for the most aggressive subtypes.

Congenital alveolar rhabdomyosarcoma - case report

Rhabdomyosarcoma is the most common soft tissue sarcoma of childhood and is very rare in the neonatal period. At this age, the alveolar type is a remarkably uncommon variety. We report a 56 days old female with alveolar RMS of the right eye noted since the age of 7 days with fast progression and unfavorable prognosis. Congenital alveolar RMS is an important cause of neonatal onset rapidly progressive proptosis. Early onset, alveolar type, and late diagnosis were poor prognostic factor.

Abstract IA013: Rhabdomyosarcoma: visions through the looking glass

Abstract Rhabdomyosarcoma (RMS) is the most common soft tissue sarcoma in children. Despite aggressive treatment clinical outcomes for RMS have not improved for three decades, emphasizing the need to uncover the molecular underpinnings of the disease. RMS has been presumed to originate from derailed muscle progenitors based on the histologic appearance and gene expression pattern of the tumors resembling embryonic developing skeletal muscle. However, an origin restricted to skeletal muscle does not explain RMS occurring in tissues devoid of skeletal muscle such as the prostate, bladder, and salivary gland. Previously, we described that activation of Sonic Hedgehog signaling through expression of a conditional, constitutively active Smoothened allele, SmoM2, under control of a presumed adipocyte-restricted adipose protein 2 (aP2)-Cre recombinase transgene in mice gives rise to aggressive skeletal muscle tumors. These tumors display the histologic and molecular characteristics of human embryonal fusion-negative RMS (FN-RMS). This model suggested a potential non-myogenic origin of FN-RMS and an avenue to explain FN-RMS development in anatomic sites devoid of skeletal muscle. Lineage tracing showed that RMS can originate from cell reprogramming and transdifferentiation of endothelial progenitor cells. Hedgehog pathway activation in committed endothelial progenitors results in Tbx1 expression and subsequent Myod1 expression driving a partially myogenic program characteristic of FN-RMS. Our work identifies reprogramming cell fate as a mechanism of transformation in pediatric sarcoma and illustrates that it is dangerous to assume the cell of origin from the characteristics of the tumor cell. The cell-reprogramming mechanism that shifts endothelial progenitors to muscle-like cells provides a unique system to define the core regulatory circuitry controlling RMS cell fate and to determine in vivo if targeting this network is a therapeutic vulnerability. Genomic profiling of human FN-RMS failed to uncover a unique mutation that drives oncogenesis. However, the PTEN cis-regulatory region is hypermethylated in more than 90% of all human FN-ERMS tumors, resulting in decreased expression. However, inhibiting the PI3K/AKT/mTOR pathway has had varied efficacy in RMS. In our RMS mouse model, PTEN localizes to the cytoplasm and nucleus, suggesting that PTEN could have functions other than regulating the PI3K/AKT/mTOR pathway. We demonstrate that Pten loss cooperates in RMS tumorigenesis and results in tumors more reflective of human FN-RMS. We show that Pten loss drives expression of the transcription factor PAX7 and identified PAX7 as a dependency in human FN-RMS. Furthermore, Pax7 deletion completely rescues the deleterious effects of Pten loss but also alters tumor cell fate, giving rise to a smooth muscle tumor. Thus, PTEN loss drives the expression of PAX7, a key member of the RMS core regulatory circuitry dictating tumor cell fate. This highlights a synthetic essential relationship between PTEN and PAX7 in FN-RMS tumor maintenance and tumor-fate decisions. Citation Format: Mark E. Hatley, Casey G. Langdon, Katherine E. Gadek, Matthew R. Garcia, Catherine J. Drummond, Jason A. Hanna, Hongjian E. Jin, Jerold E. Rehg. Rhabdomyosarcoma: visions through the looking glass [abstract]. In: Proceedings of the AACR Special Conference: Sarcomas; 2022 May 9-12; Montreal, QC, Canada. Philadelphia (PA): AACR; Clin Cancer Res 2022;28(18_Suppl):Abstract nr IA013.

Abstract A015: Biochemical and preclinical anti-tumor activity of a bi-steric mTORC1-selective inhibitor in fusion positive rhabdomyosarcoma

Abstract Rhabdomyosarcoma (RMS) is the most common soft tissue sarcoma in children. Although FOXO1 fusions portend a dismal prognosis for patients with both localized (5-year survival 65%) and metastatic (19%) disease, current therapy is largely agnostic to fusion status. The allosteric mTORC1 inhibitor temsirolimus delays disease progression in relapsed RMS but does not improve survival. We hypothesized that this incomplete response is due to inadequate suppression of critical downstream effectors of the mTORC1 complex. Recently described bi-steric mTORC1-selective inhibitors combine allosteric and catalytic modes of kinase inhibition, and have shown promise in diverse preclinical cancer models with early evidence of clinical activity. We therefore set out to compare the activity of the bi-steric mTORC1-selective inhibitor RMC-6272 to the allosteric mTOR inhibitor rapamycin in cell line and xenograft models of fusion positive RMS. Methods: We compared the in vitro anti-proliferative activity of RMC-6272 and rapamycin in FOXO1 fusion positive RMS cell lines at 72 and 144 hours. We assessed the ability of these inhibitors to suppress mTORC1 outputs, using immunoblot to quantify phosphorylation of p70S6K, RPS6, and 4EBP1, and m7-GTP pulldowns to assess repression of cap-dependent translation. In vivo studies were conducted in patient-derived xenograft (PDX) models subcutaneously implanted in NSG mice. We tested preclinical anti-tumor activity of RMC-6272 at two doses and used immunoblots and m7-GTP assays in tumors harvested from treated mice to assess target engagement. Results: We observed superior anti-proliferative effects of RMC-6272 when compared to rapamycin in vitro, with the latter having only modest effect. Biochemically, treatment of RMS cell lines with either RMC-6272 or rapamycin diminished phosphorylation of p70S6K and RPS6. However, RMC-6272 blocked 4EBP1 phosphorylation and cap-dependent translation much more potently than rapamycin. Based on these findings, we conducted in vivo studies using weekly intraperitoneal injections of RMC-6272 in PDX-harboring mice. Both doses were associated with tumor regressions in two fusion positive RMS PDX models: 2 CR and 1 PR at 8 mg/kg, and 3 CR, 1 PR, 1 SD, and 2 PD at 6 mg/kg. NSG mice treated at 8 mg/kg demonstrated more weight loss than those at 6 mg/kg, suggesting strain-specific determinants of tolerability. We confirmed in vivo suppression of p70S6K phosphorylation and cap-dependent translation in tumors. Conclusions: The mTORC1 inhibitor RMC-6272 exhibits greater anti-tumor activity than the allosteric inhibitor rapamycin in vitro, and is capable of inducing complete remissions as monotherapy in FOXO1 fusion positive RMS PDX. Increased anti-tumor activity was associated with suppression of cap-dependent translation as assessed by m7-GTP assays, nominating dephosphorylation of 4EBP1 as a potential response biomarker. Preclinical studies are ongoing to assess mechanism-informed combination therapeutic strategies incorporating this highly active agent in FOXO1 fusion positive RMS. Citation Format: Jacqueline Morales, Kristen Kwong, W. Clay Gustafson, Amit J. Sabnis. Biochemical and preclinical anti-tumor activity of a bi-steric mTORC1-selective inhibitor in fusion positive rhabdomyosarcoma [abstract]. In: Proceedings of the AACR Special Conference: Sarcomas; 2022 May 9-12; Montreal, QC, Canada. Philadelphia (PA): AACR; Clin Cancer Res 2022;28(18_Suppl):Abstract nr A015.

Abstract A039: IGFBP5 is induced by tumor-suppressive naïve stromal cells and regulates GAS2-associated apoptosis in rhabdomyosarcoma

Abstract Stromal cells can have either suppressive or stimulatory effects on tumor progression. However, this diverse collection of cells is still poorly characterized in soft-tissue sarcomas. Rhabdomyosarcoma (RMS) is the most common soft-tissue sarcoma in childhood with molecular features of skeletal muscle differentiation. In this study, we explored tumor growth-regulatory signaling between differentially educated non-malignant mesenchymal stromal cells and malignant tumor cells in RMS. Xenograft experiments in mice were designed to provide functional evidence for alterations in tumor behavior linked to tumor-associated stromal cells. Molecular characterization was performed on the Clariom D platform, by gene set enrichment analysis, cell culture validation of deregulated genes, and finally, by clinicopathological validation in a discovery cohort (N=101) and a validation cohort (N=147). The results revealed transient tumor growth-suppressive paracrine effects of naïve orthotopic stromal cells derived from skeletal muscle. These effects were easily abolished if the stromal cells were pre-exposed to RMS cells, either short-term in vitro, or long-term in hindlimb muscle in vivo. Explorative analyses of RMS cells exposed to naïve stromal cells identified insulin-like growth factor binding protein 5 (IGFBP5) as the top upregulated gene, with an ability to induce Caspase 3/7 activation and growth arrest. The gene coding for connective tissue growth factor (CTGF) was identified as the major prosurvival factor being downregulated. Analysis of patient cohorts consistently demonstrated an association between IGFBP5, growth arrest specific protein 2 (GAS2), lower disease stage and favorable survival. Altogether, our study identifies a novel anti-tumor growth mechanism with conditionally deregulated genes linked to apoptosis and differentiation in RMS. The results provide support for continued studies on tumor growth-regulatory mechanisms induced by tumor-surrounding stromal cell subsets at primary and metastatic sites in sarcoma. Citation Format: Monika Ehnman, Karim Katkhada, Liu Zhen Meng, Yue Zhang, Binbin Zhao, Shanlin Tong, Aditi Kallai, Arne Östman, Nick Tobin. IGFBP5 is induced by tumor-suppressive naïve stromal cells and regulates GAS2-associated apoptosis in rhabdomyosarcoma [abstract]. In: Proceedings of the AACR Special Conference: Sarcomas; 2022 May 9-12; Montreal, QC, Canada. Philadelphia (PA): AACR; Clin Cancer Res 2022;28(18_Suppl):Abstract nr A039.

Clinical and Molecular Characteristics, Management and Outcome of Infantile Fibrosarcoma: A Retrospective 18-Year Single-Institution Review

Infantile fibrosarcoma (IFS) is the most common non-rhabdomyosarcoma soft-tissue sarcoma in children under 1 year of age, with local aggressiveness, but a high cure rate with conservative treatment. We report our experience with 15 patients with histological diagnosis of IFS treated at our Institution from January-2003 to December-2020. The median age was 6.72 months (range: 0-36); 66% were males. The extremities were the most common tumor site (66.7%). The tumor size was &gt; 5 cm in 10 patients. Only one patient had metastatic disease (lung). Molecular studies were performed in 14 patients and were positive in ten: nine ETV6/NTRK3 and one RAF1. Initial non-mutilating surgery was performed in six patients. A delayed surgery could be performed in 4/8 patients after neoadjuvant chemotherapy with Vincristine - Actinomycin-D, and in one patient a watch-and-wait strategy was adopted. With a median follow-up of 58.67 months, all patients are alive except for the patient with metastatic disease. The 5-year overall and event-free survival were 93.5% and 86.5% respectively. Although IFS is locally aggressive, it has a high rate of overall survival due to its good response to chemotherapy and non-mutilating surgery even with involved margins. Advances in molecular genetics have improved diagnosis and refined classification.