Abstract 6836: Single-cell RNA sequencing of FUS::DDIT3 myxoid/round cell liposarcoma delineates mechanisms of oncogenic progression and immune escape
Authors: Gularte-Mérida Rodrigo , Seffar Evan , Li George , Kim Young-Mi , Agaram Narasimhan P. , Socci Nicholas , Sanchez-Vega Francisco , Schultz Nikolaus , Singer Samuel 
Published: March 2024

Abstract
Myxoid/Round Cell Liposarcoma (MRCL) is an understudied soft tissue sarcoma that typically affects young adults and accounts for 30% of all liposarcomas. MRCL is characterized by oncogenic DDIT3 fusions, with FUS::DDIT3 being present in over 90% of cases. However, the cellular origins and oncogenic mechanisms of MRCL remain incompletely defined. Here we report oncogene transcription, and tumor-immune interactions in a cohort of 5 patient tumors with matched normal tissue for which we sequenced the transcriptomes of approximately 45,000 single cells using 10X Genomics, and full-length transcripts in approximately 20,000 single cells via Oxford Nanopore in 2 of these patients.
Our results reveal two distinct cancer cell populations: Early Cancer Cells (ECC) and Advanced Cancer Cells (ACC). These cells appear to derive from adipose stem progenitor cells (ASPC) and fall along an ASPC→ECC→ACC pseudo-time axis. Along this axis, a distinct pattern emerges where oncogenes tend to be upregulated and tumor suppressor genes (TSG) tend to be downregulated and these appear to be mutually exclusive. Comparing ECC to ASPC we see increased expression of HIPPO and PI3K pathway oncogenes, FGFR2 and IGF1R RTKs, and downregulation of the TSG CDKN2A and CDKN1A. ACC show upregulation of hedgehog, ErbB, WNT, HIPPO, PI3K, and MAPK signaling pathways compared to ECC. Full-length transcript sequencing at the single-cell level shows a consistent expression of the FUS::DDIT3 fusion transcript in both ECC and ACC populations, confirming that these populations are cancer cells. Furthermore, MRCL displays several immune escape mechanisms, including poor immune infiltration and a marked decline of HLA Class I expression along the pseudo-time axis from ECC to ACC, suggesting a complex interplay between oncogenic signaling and immune evasion.
These results suggest that MRCL likely originates from ASPC and expression of FUS:DDIT3 oncoprotein induces early HIPPO, PI3K, FGFR2, and insulin signaling pathway expression in ECC with further hedgehog, ErbB, MAPK, and WNT pathway activation in ACC. This progressive activation in oncogenic signaling is associated with immune evasion. This is the first single-cell RNA seq analysis to detect the presence of the FUS::DDIT3 fusion in ECC and ACC significantly advancing our understanding of the mechanisms of MRCL oncogenesis, immune escape, and identifies potential novel therapeutic targets.
Citation Format: Rodrigo Gularte-Mérida, Evan Seffar, George Li, Young-Mi Kim, Narasimhan P. Agaram, Nicholas Socci, Francisco Sanchez-Vega, Nikolaus Schultz, Samuel Singer. Single-cell RNA sequencing of FUS::DDIT3 myxoid/round cell liposarcoma delineates mechanisms of oncogenic progression and immune escape [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 6836.