Abstract 3628: RAC1 aberrations in head and neck cancer affect immune microenvironments

Abstract RAC1 is a Rho GTPase well-identified as an oncogene overexpressed in various cancer types. RAC1 amplification or gain account for ~40% of head and neck squamous cell carcinoma (HNSCC) while mutation of RAC1 occurs at ~3% according to TCGA-HNSCC cohort. HNSCC patients with RAC1 aberrations, including RAC1 mutations and amplification or gain, are associated with poor overall and disease-free survival in HNSCC. Significantly higher tumor mutational burden is observed in RAC1-mutated group (p=0.0336) or in RAC1-amplified/gain group (p=0.0109) vs. wildtype patients in TCGA-HNSCC cohort. As increases in TMB are often associated with immune cell infiltration in cancers, we hypothesized that RAC1 aberrations impact the tumor immune microenvironment (TIME) of HNSCC, potentially contributing to disease progression.PD-L1 (CD274) is an important immunosuppressive immune checkpoint molecule expressed in tumor cells. Here, we first reported that ectopic overexpression of RAC1 p.P29S and p.A159V mutations in a human HNSCC cell line (PECAPJ41 clone D2, ATCC, USA) could cause upregulation of PD-L1 at both mRNA (by RNA-seq) and protein levels (by Western blotting), first demonstrating a direct effect of RAC1 mutations on potential HNSCC TIME modulation via PD-L1. Furthermore, by tumor immune estimation resources (TIMER) analysis, we found that RAC1-mutated tumors have significantly higher level of neutrophils immune infiltration of 38.63% (p=0.027) compared to the wildtype tumors. From the CIBERSORTx analysis, M2 Macrophages was significantly increased by 40.42% in RAC1-mutated tumors vs wildtype in the TCGA-HNSCC cohort (p=0.031). We also validated this finding by immunofluorescent staining of PD-L1 and M2 Macrophages in immunocompetent mouse HNSCC xenografts models expressing RAC1 p.P29S and p.A159V mutations as compared to controls. Our in silico, in vitro and in vivo findings first uncover an important role of RAC1 aberrations in HNSCC TIME immunosuppression by regulating PD-L1 expression, neutrophil and M2 macrophages infiltrations. Clinical activities of PD-L1 inhibitors in RAC1-mutated HNSCC patients worth future investigations. Acknowledgement: This research is funded by the General Research Fund (Ref. No.:14168517) and the Research Impact Fund (Ref. No.: R4015-19) from the Research Grants Council in Hong Kong. VWYL is supported by Start-up Fund, Georgia Cancer Center, Medical College of Georgia at Augusta University. Citation Format: Helen Hoi Yin Chan, Yuchen Liu, Hoi Lam Ngan, Vivian Wai Yan Lui. RAC1 aberrations in head and neck cancer affect immune microenvironments. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 3628.