Abstract A018: Investigating the fibroblastic phenotype of dermatofibrosarcoma protuberans in primary tumors and non-fusion-bearing fibroblasts

Abstract Dermatofibrosarcoma protuberans (DFSP) is the most common cutaneous sarcoma and is defined by chromosomal translocations, high level PDGF signaling, and a characteristic fibroblastic phenotype. The most common translocation is t(17;22) encoding the fusion protein COL1A1::PDGFB. DFSP demonstrates significant local invasion and recurrence, yet little is known about how the local tumor microenvironment impacts its behavior. In particular, it is unclear if DFSP harbors separate populations of non-fusion-harboring fibroblasts among tumor cells. Cancer-associated fibroblasts (CAFs) are well-described in other malignancies and regulate tumor behavior including growth, invasion, and immune responses. The presence and role of non-fusion-bearing fibroblasts and/or CAFs in DFSP is unknown, likely in part due to the overlapping phenotypes of DFSP and fibroblasts, making the ability to distinguish distinct populations more challenging. Here, we sought to further investigate the fibroblastic phenotype of DFSP and the possibility of associated CAFs by enriching for fibroblastic cells in tissue culture, and then comparing transcriptomes to human DFSP tumors and normal human fibroblasts. Fibroblastic outgrowths from primary DFSP tumors were enriched by culturing tumor specimens in media containing DMEM and 10% FBS. We found primary tumors harbored COL1A1::PDGFB fusions, but after 2-4 passages in tissue culture, the COL1A1::PDGFB fusion transcript was undetectable in cultured cells by RNA sequencing or PCR. Cultures demonstrated homogenous spindle cell morphology. Bulk RNA sequencing revealed that cultured cells resembled normal human fibroblasts more closely than their corresponding primary DFSP tumors, implicating a selective growth advantage of non-fusion-bearing cells in culture. These cells, which we termed CAFs, revealed differences from both primary DFSP tumors and normal human fibroblasts. Whereas primary DFSP tumors expressed fusion transcript and strong expression of DFSP markers CD34, PDGFB, NES, and APOD, cultured CAFs demonstrated low expression of typical DFSP markers but significantly elevated levels of fibroblast markers VIM, THY1 (CD90), SMA, and FN1 relative to their primary tumors, with the caveat that more tissue-matched specimens are needed to confirm these findings. Further, we identified elevated expression of several genes in CAFs encoding secreted factors including FGF5, DKK1, and IL6. Taken together, these studies reveal that in addition to higher expression of typical fibroblast markers than primary DFSP tumor cells, CAFs may express secreted factors that may influence DFSP tumor cell behavior through a paracrine mechanism. Further studies are ongoing to validate and define the specificity and biologic significance of these expression profiles in the tumor microenvironment, and will serve to provide new insights into local signaling that occurs in this invasive and highly recurrent malignancy. Citation Format: Jawad Aqeel, Olena Kondrachuk, Talha A. Syed, Claire E. Holtz, Kelly L. Harms, Christina V. Angeles, Andrzej A. Dlugosz, Elisabeth A. Pedersen. Investigating the fibroblastic phenotype of dermatofibrosarcoma protuberans in primary tumors and non-fusion-bearing fibroblasts [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 A018.