(P 393) INVESTIGATING THE ROLE OF HDACS IN THE PROGRESSION OF METASTATIC UNDIFFERENTIATED PLEOMORPHIC SARCOMA

Objective: Undifferentiated Pleomorphic Sarcoma (UPS) is the most commonly diagnosed soft tissue sarcoma amongst adults. About ⅓ of patients will eventually develop metastatic UPS, with the lung as the most common metastatic site. UPS consists of a heterogeneous population of cells which can significantly impact the effect of therapeutic interventions. In mesenchymal tumors, it is poorly understood how these different tumor clones functionally impact tumor evolution, specifically how they may contribute to primary tumor growth and metastasis. In previous studies, histone deacetylases have been identified as drivers of metastasis. My objective is to identify and elucidate the epigenetic mechanisms of histone deacetylases (HDACs) on the metastatic potential of a specific subpopulation of cells driving metastatic UPS.

Methods: We model UPS tumors in mice by the activation of oncogenic KrasG12D and knockout of p53. CRISPR barcoding and fluorescent reporters enable us to label individual tumor clones, providing insight on the clonal dynamics of UPS. To initiate tumorigenesis, an adenovirus containing cre recombinase is injected into the hind limb of the mouse. The tumor bearing limb is amputated and metastasis is observed over a period of time. Using these lineage tracing methods, we revealed that the primary tumors are polyclonal in origin, while the metastatic lesions arise from a single clone. This specific subpopulation of cells that can migrate and form distant lesions are identified as metastatic initiating cells or MCs.

Results: Transcriptomic analysis of the MC and non-MC populations revealed over 800 genes that are differentially expressed in the MC population. Gene signatures associated with poor cancer outcomes, such as CINSARC, were also positively enriched in this population. Whole genome sequencing showed no significant differences between these two groups and the genetic variants specific to the MC population represented less than 2% of the total variants. In addition, copy number alterations were found to be highly similar. These results suggest the role of epigenetic factors in the metastatic potential of this specific cell population. In order to investigate these factors, a CRISPR-Cas9 screening was conducted in vivo where over 400 epigenetic modulator genes were targeted. This screening revealed that histone deacetylases (HDACs) played a regulatory role in the ability of the MC cells to metastasize to the lung. Preliminary drug screening data suggests that HDAC inhibitors, such as Domatinostat, have the potential to specifically target the MCs in order to inhibit metastasis in UPS.

Conclusion: HDACs have been identified as potential therapeutic targets for metastatic UPS. Previous data has shown that a single subpopulation of cells is responsible for the formation of these distant lesions. Ongoing studies aim to identify which HDACs are important for metastasis in addition to elucidating their mechanism of disease progression and further analysis of the effect of pharmacological inhibition.