Simon Manuel Tria

Immunotherapy represents a groundbreaking approach for treating colorectal cancer (CRC), harnessing the body’s own immune system to target tumour cells more precisely than conventional chemotherapy. Immune checkpoint inhibitors, such as antibodies against PD-1, PD-L1, or CTLA-4, have shown remarkable efficacy in certain patients, leading to durable responses and improved survival. However, the majority of CRC cases have limited benefit from a single agent checkpoint blockade. There is a growing need to identify biomarkers that will improve the selection of patients who will best respond to therapy, as well as new targets to sensitise cancers to an immune checkpoint blockade. Unfortunately, the search for reliable biomarkers has been limited by our incomplete understanding of how immunotherapies modify the already complex immune response to cancer. Revolutionary techniques, such as genome-wide CRISPR/Cas9 screening combined with the appropriate validation systems such as in vivo mouse models and/or 3D organoid co-culture systems, are being used to address this knowledge gap. This review will focus on the use of immunotherapies in CRC, discuss why most CRC patients do not respond, and highlight in vitro, in vivo, and novel techniques for discovery of new targets for combination treatment.

Colorectal cancer is one of the world’s most prevalent and lethal cancers. Mutations of the KRAS gene occur in ~40% of metastatic colorectal cancers. While this cohort has historically been difficult to manage, the last few years have shown exponential growth in the development of selective inhibitors targeting KRAS mutations. Their foremost mechanism of action utilizes the Switch II binding pocket and Cys12 residue of GDP-bound KRAS proteins in G12C mutants, confining them to their inactive state. Sotorasib and Adagrasib, both FDA-approved for the treatment of non-small cell lung cancer (NSCLC), have been pivotal in paving the way for KRAS G12C inhibitors in the clinical setting. Other KRAS inhibitors in development include a multi-targeting KRAS-mutant drug and a G12D mutant drug. Treatment resistance remains an issue with combination treatment regimens including indirect pathway inhibition and immunotherapy providing possible ways to combat this. While KRAS-mutant selective therapy has come a long way, more work is required to make this an effective and viable option for patients with colorectal cancer.

CEP55 is a cancer-testis antigen, highly expressed in a wide range of solid tumors as well as chromosomally unstable cancers. CEP55 is normally silent in most adult somatic tissues except germ cells; however, its expression is induced early during cell transformation. Numerous studies using cell line xenografts have shown that CEP55 is required for the viability and survival of a diverse range of cancers in vitro and in vivo. To decipher the mechanisms by which Cep55 promotes chromosomal instability and tumorigenesis in vivo, we developed a novel knock-in transgenic mouse model that ubiquitously overexpresses Cep55 (Cep55Tg/Tg) and a conditional knockout (cKO) mouse model of Cep55 (Cep55-/-). While overexpression of Cep55 was sufficient to cause spontaneous tumorigenesis in vivo, Cep55 loss hindered tumorigenesis. When we bred adult Cep55 cKO mice with other tumor-prone models: oncogenic Kras expression (Kras G12D) and Pten tumor-suppressor loss, Cep55 loss significantly delayed tumorigenesis and prolonged survival validating it as a promising therapeutic target for diverse cancers. This reduction of tumorigenesis is concomitant with the downregulation of AKT and ERK signaling pathways, attenuation of epithelial to mesenchymal transition, and increased cytotoxic CD4/CD8 T-cell tumor infiltration. This indicates that Cep55 can impair the establishment of the adaptive immune response through modification in the tumor microenvironment and regulation of cell signaling. Mass spectrophotometry of mouse embryo fibroblasts and derived exosomes from wild-type and KO mice revealed the pathways altered by Cep55 such as extracellular matrix remodeling pathways, integrin-related signaling and cellular trafficking mediators indicating the contributions of Cep55 to cancer are beyond what have been reported so far. Overall, CEP55 is a promising target to prevent tumorigenesis and the progression of cancer through various mechanisms. Citation Format: Behnam Rashidieh, Simon M. Tria, Kum Kum Khanna. Understanding the role of Cep55 in initiation and progression of cancers [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 6033.

The overexpression of BRF2, a selective subunit of RNA polymerase III, has been shown to be crucial in the development of several types of cancers, including breast cancer and lung squamous cell carcinoma. Predominantly, BRF2 acts as a central redox-sensing transcription factor (TF) and is involved in rescuing oxidative stress (OS)-induced apoptosis. Here, we showed a novel link between BRF2 and the DNA damage response. Due to the lack of BRF2-specific inhibitors, through virtual screening and molecular dynamics simulation, we identified potential drug candidates that interfere with BRF2-TATA-binding Protein (TBP)-DNA complex interactions based on binding energy, intermolecular, and torsional energy parameters. We experimentally tested bexarotene as a potential BRF2 inhibitor. We found that bexarotene (Bex) treatment resulted in a dramatic decline in oxidative stress and Tert-butylhydroquinone (tBHQ)-induced levels of BRF2 and consequently led to a decrease in the cellular proliferation of cancer cells which may in part be due to the drug pretreatment-induced reduction of ROS generated by the oxidizing agent. Our data thus provide the first experimental evidence that BRF2 is a novel player in the DNA damage response pathway and that bexarotene can be used as a potential inhibitor to treat cancers with the specific elevation of oxidative stress.