Abstract 1903: RAC1 mutations are associated with immunosuppressive tumor microenvironment in head and neck squamous cell carcinoma
Authors: Chan Helen Hoi Yin , Ngan Hoi-Lam , Liu Yuchen , Lui Vivian Wai Yan 
Published: July 2021

RAC1 is a well-known oncogene involved in the promotion of invasion, EMT, intravasation and metastasis in different cancer types. Extensive research has been carried out in melanoma regarding the P29S mutation, which still remains undruggable as of today. We found that RAC1 mutations (C18X, K116X, G15S, A159V), which account for ~3% (15/510) of head and neck squamous cell carcinoma (HNSCC), are associated with significant poorer disease-free survival (DFS) and overall survival (OS) (vs. patients with RAC1-wildtype status; P&lt;0.01). RAC1 amplification and gain, which accounts for 39.5% of HNSCC are also significantly associated with poorer OS and DFS (P&lt;0.01).

While HNSCC is one of the most highly immune infiltrated cancer types with promising results in clinical trials of immune checkpoint inhibitors, e.g. nivolumab, we examined the potential involvement of immune-related events to the dismal outcomes of RAC1-mutated HNSCC patients. Assessment of the immune tumor microenvironment by CIBERSORTx analysis revealed a 40.42% increase in the infiltrating level of M2 macrophage in RAC1-mutated vs. RAC1-wildtype HNSCC tumors (p=0.03). Surprisingly, this elevated M2 infiltration was not found in P29S-mutant melanomas, suggestive of a likely M2-polarized tumor immune microenvironment in RAC1-mutated tumors. Gene set enrichment analysis (GSEA) of RAC1-mutated HNSCC tumors also demonstrated a statistically significant enrichment of the gene set “GO Positive Regulation of Interleukin-6 Production” with a normalized enrichment score (NES) of 2.191 (p&lt;0.01). As IL6 is a pro-tumorigenic inflammatory cytokine essential signal for M2 macrophage polarization and known to be involved in tumor progression, it may contribute to RAC1-driven M2 polarization. We also examined the mRNA expression of the immunomodulatory genes in RAC1-mutated vs. RAC1-wildtype HNSCC tumors. RAC1-mutated patients have significantly higher expression of pro-inflammatory CXCL9 and CXCL10 (p=0.01) as well as druggable targets CD274 (PD-L1) (p=0.014) and IL2 (p&lt;0.01).

Taken together, RAC1 mutations may contribute to an immunosuppressive tumor microenvironment in HNSCC, first highlighting potential immunological roles of RAC1 aberrations in HNSCC.

Acknowledgments: This research is funded by the General Research Fund, Research Grant Council, Hong Kong SAR (#14168517). V.W.Y.L. also receives fundings from General Research Fund, Research Grant Council, Hong Kong Government, Hong Kong SAR (#17121616), Research Impact Fund (#R4015-19F, R4017-18), the Health and Medical Research Fund (HMRF#15160691; the Health and Medical Research Fund, the Food and Health Bureau, the Government of the Hong Kong Special Administrative Region), University-Industry Collaboration Programme (UIM/329; Innovation and Technology Fund, Hong Kong Government, Hong Kong SAR), and the Hong Kong Cancer Fund, Hong Kong SAR.

Citation Format: Helen Hoi Yin Chan, Hoi-Lam Ngan, Yuchen Liu, Vivian Wai Yan Lui. RAC1 mutations are associated with immunosuppressive tumor microenvironment in head and neck squamous cell carcinoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1903.