Abstract A023: ASPSCR1::TFE3 promotes angiogenesis via RAB27A/SYTL2 upregulation in alveolar soft part sarcoma

Abstract Prompt trafficking of growth factors and cytokines to the cytoplasmic membrane is essential to provide growth stimuli during cell-cell communication. Also, efficient trafficking of surface receptors is required for their renewal upon the binding with their specific ligands. The RAB27/SYTL/Myosin V/actin axis plays a central role in trafficking of cytoplasmic vesicles and surface receptors toward the plasma membrane. Abnormal enhancement of this pathway induces excessive intercellular signaling exchange in cancer, resulting in significant modifications of tumor microenvironment. Our previous study identified SYTL1, a binding partner of RAB27B, as a transcriptional target of MEIS1 that promotes homing and engraftment of leukemia stem cells in the hematopoietic niche by facilitating membrane trafficking of CXCR4 and FLT3. Alveolar soft part sarcoma (ASPS) is a rare neoplasm that affects adolescents and young adults with an invariable ASPSCR1::TFE3 (AT3) fusion. ASPS is characterized by the abundant vascular network forming the alveolus around tumor cell nests that causes the highly metastatic activity of the tumor. ChIP-seq analysis revealed that AT3 binding peaks are enriched at angiogenesis-associated enhancers, and inhibition of enhancer activity by the BRD4 inhibitor JQ1 suppressed angiogenesis and tumor growth in vivo. Subsequent in vivo epigenomic CRISPR/dCas9-KRAB screening identified RAB27A and SYTL2 as key molecules of angiogenesis in ASPS. Upregulation of the RAB27A/SYTL2 axis enhanced secretion of angiogenic factors including PDGFB, VWF, GPNMB, and ANGPTL2, promoting migration and proliferation of pericytes and endothelial cells to form the highly integrated vascular network characteristic of ASPS. A similar pathway activation is observed in translocation renal cell carcinoma (tRCC) harboring TFE3 rearrangements, generating a comparable vascular architecture. We have reconstructed vascular mimics in vitro using ASPS and tRCC cells in 3D microfluidic devices. We are currently exploring novel inhibitors targeting the protein-protein interaction (PPI) between RAB27A and SYTL2. To perform high throughput screening (HTS) of candidate inhibitors we constructed the cell-based Fluoppi and cell-free AlphaLISA systems. During our first HTS Compound A was identified as a specific inhibitor of RAB27A and SYTL2 PPI. This compound inhibited the secretion of pro-angiogenic factors and significantly suppressed angiogenesis and tumor growth in vivo. We are currently optimizing Compound A to enhance its PPI inhibitory activity and safety profile. Citation Format: Miwa Tanaka, Takuro Nakamura. ASPSCR1::TFE3 promotes angiogenesis via RAB27A/SYTL2 upregulation in alveolar soft part sarcoma [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Fusion-Positive Cancer: From Discovery to Therapy; 2026 Jan 13-15; Philadelphia PA. Philadelphia (PA): AACR; Cancer Res 2026;86(1_Suppl):Abstract nr A023.