David Kirsch

Sarcomas are malignant tumours that arise from skeletal and extra-skeletal connective tissues, including the peripheral nervous system. The term ‘sarcomas of soft tissues’ embraces all of the malignant tumours which arise from the mesenchymal tissues excluding bone, i.e. malignant fibrous histiocytoma, liposarcoma, leiomyosarcoma, synovial sarcoma, rhabdomyosarcoma, epithelioid sarcoma, angiosarcoma, fibrosarcoma, etc. In addition, malignant tumours of peripheral nerve sheaths are included despite being ectodermal in origin, as their clinical behaviour is not measurably different from the other sarcomas. Gastrointestinal stromal tumours are derived from cells (interstitial cells of Cajal) with neural and smooth muscle features and are also considered soft tissue sarcomas. The relative frequency of different histological subtypes of sarcomas is shown in Table 40.1. This chapter discusses current practice in the management of patients with soft tissue sarcoma (other than rhabdomyosarcoma of the paediatric age group) as well as selected benign, infiltrative soft tissue tumours such as desmoids whose management is similar.

BACKGROUND: In ultra-rare sarcomas (URS) the conduction of prospective, randomized trials is challenging. Data from retrospective observational studies (ROS) may represent the best evidence available. ROS implicit limitations led to poor acceptance by the scientific community and regulatory authorities. In this context, an expert panel from the Connective Tissue Oncology Society (CTOS), agreed on the need to establish a set of minimum requirements for conducting high-quality ROS on the activity of systemic therapies in URS. METHODS: Representatives from > 25 worldwide sarcoma reference centres met in November 2020 and identified a list of topics summarizing the main issues encountered in ROS on URS. An online survey on these topics was distributed to the panel; results were summarized by descriptive statistics and discussed during a second meeting (November 2021). RESULTS: Topics identified by the panel included the use of ROS results as external control data, the criteria for contributing centers selection, modalities for ensuring a correct pathological diagnosis and radiologic assessment, consistency of surveillance policies across centers, study end-points, risk of data duplication, results publication. Based on the answers to the survey (55 of 62 invited experts) and discussion the panel agreed on 18 statements summarizing principles of recommended practice. CONCLUSIONS: These recommendations will be disseminated by CTOS across the sarcoma community and incorporated in future ROS on URS, to maximize their quality and favor their use as control data when results from prospective studies are unavailable. These recommendations could help the optimal conduction of ROS also in other rare tumors.

Rhabdomyosarcoma (RMS), a cancer characterized by features of skeletal muscle, is the most common soft-tissue sarcoma of childhood. With 5-year survival rates among high-risk groups at < 30%, new therapeutics are desperately needed. Previously, using a myoblast-based model of fusion-negative RMS (FN-RMS), we found that expression of the Hippo pathway effector transcriptional coactivator YAP1 (YAP1) permitted senescence bypass and subsequent transformation to malignant cells, mimicking FN-RMS. We also found that YAP1 engages in a positive feedback loop with Notch signaling to promote FN-RMS tumorigenesis. However, we could not identify an immediate downstream impact of this Hippo-Notch relationship. Here, we identify a HES1-YAP1-CDKN1C functional interaction, and show that knockdown of the Notch effector HES1 (Hes family BHLH transcription factor 1) impairs growth of multiple FN-RMS cell lines, with knockdown resulting in decreased YAP1 and increased CDKN1C expression. In silico mining of published proteomic and transcriptomic profiles of human RMS patient-derived xenografts revealed the same pattern of HES1-YAP1-CDKN1C expression. Treatment of FN-RMS cells in vitro with the recently described HES1 small-molecule inhibitor, JI130, limited FN-RMS cell growth. Inhibition of HES1 in vivo via conditional expression of a HES1-directed shRNA or JI130 dosing impaired FN-RMS tumor xenograft growth. Lastly, targeted transcriptomic profiling of FN-RMS xenografts in the context of HES1 suppression identified associations between HES1 and RAS-MAPK signaling. In summary, these in vitro and in vivo preclinical studies support the further investigation of HES1 as a therapeutic target in FN-RMS.