RAC1 as a potential pan-cancer diagnostic, prognostic, and immunological biomarker

Background

Ras-related C3 botulinum toxin substrate 1 (RAC1) is an important member of the Rho GTPase family involved in tumorigenesis. However, its role and potential clinical utility across cancer entities in solid tumors is unknown.

Methods

We analyzed data from various databases, including The Cancer Genome Atlas (TCGA), Genotype-Tissue Expression Project, cBioPortal, Tumor Immune Estimation Resource 2 (TIMER2), and published articles. A prognostic nomogram for liver hepatocellular carcinoma (LIHC) patients was developed based on RAC1-guanosine triphosphate (GTP) gene expression levels, which were validated using immunohistochemistry (IHC).

Results

In this study, RAC1 was highly expressed in most cancers and correlated with prognosis and pathological stages. Furthermore, significant associations were observed between RAC1 and DNA methylation, immune cell infiltration, immune-related genes, tumor mutational burden, and microsatellite instability in most tumors. As a use case, we employed gene set enrichment analysis (GSEA) and gene set variation analysis (GSVA) to analyze the biologic importance of RAC1 expression and established a prognostic nomogram based on tumor stage and RAC1 expression, which can better predict the overall survival rate of patients with LIHC better than tumor stage alone. The gene expression results were validated with IHC, which confirmed a higher expression of the RAC1-GTP protein in LIHC compared to paracancerous tissues.

Conclusions

This extensive solid tumor analysis provides sound evidence that RAC1 can serve as both as an immunotherapy target and as a diagnostic and prognostic biomarker.

GTPase splice variants RAC1 and RAC1B display isoform-specific differences in localization, prenylation, and interaction with the chaperone protein SmgGDS

Identifying events that regulate the prenylation and localization of small GTPases will help define new strategies for therapeutic targeting of these proteins in disorders such as cancer, cardiovascular disease, and neurological deficits. Splice variants of the chaperone protein SmgGDS (encoded by RAP1GDS1) are known to regulate prenylation and trafficking of small GTPases. The SmgGDS-607 splice variant regulates prenylation by binding pre-prenylated small GTPases, but the effects of SmgGDS binding to the small GTPase RAC1 versus the splice variant RAC1B are not well defined. Here we report unexpected differences in the prenylation and localization of RAC1 and RAC1B, and their binding to SmgGDS. Compared to RAC1, RAC1B more stably associates with SmgGDS-607, is less prenylated, and accumulates more in the nucleus. We show that the small GTPase DIRAS1 inhibits binding of RAC1 and RAC1B to SmgGDS and reduces their prenylation. These results suggest that prenylation of RAC1 and RAC1B is facilitated by binding to SmgGDS-607, but the greater retention of RAC1B by SmgGDS-607 slows RAC1B prenylation. We show that inhibiting RAC1 prenylation by mutating the CAAX motif promotes RAC1 nuclear accumulation, suggesting that differences in prenylation contribute to the different nuclear localization of RAC1 versus RAC1B. Finally, we demonstrate RAC1 and RAC1B that cannot be prenylated bind GTP in cells, indicating that prenylation is not a prerequisite for activation. We report differential expression of RAC1 and RAC1B transcripts in tissues, consistent with these two splice variants having unique functions that might arise in part from their differences in prenylation and localization.