Importance of Extracellular Vesicle Derived RNAs as Critical Colorectal Cancer Biomarkers

Abstract

Colorectal cancer typically begins from a nonmalignant polyp formation in the large intestine that, over time, develops into colorectal cancer. The growth of benign polyps can be checked if detected in the early stages of the disease. Doctors usually recommend colonoscopy to average and high-risk individuals for colorectal cancer screening. Elevated carcinoembryonic antigen (CEA) is a broadly used biomarker for colorectal cancer. The genetic and epigenetic alteration of genes such as p53, BRAF, APC, and PIK3CA is also correlated with colorectal cancer in various clinical studies. In general, tissue biopsy is most frequently used for colorectal cancer diagnosis, but the whole tumor heterogeneity cannot be accessed by this technique. Furthermore, such a highly invasive technique is not suitable for repeated testing. Recently, extracellular vesicles (EVs), lipid bilayer enclosed sacs secreted from colorectal cancer cells, are emerging as a diagnostic tool for colon cancer detection. The major advantages of using EVs for colon cancer diagnosis are (i) EVs can be isolated in a noninvasive manner from the body fluid and (ii) EV incorporated cargoes (mostly RNAs) reveal various aspects of colorectal cancer. EV-RNAs are also implicated in tumor invasion and influence the immune system for the further spread of tumors. However, due to the lack of standardized EV detection strategies, diagnostic applicability is limited. Herein, we review the recent literature on the pathobiological dependence of colorectal cancer on EV-RNAs. Further, we present the advantages of identification and characterization of EV-RNAs to explore the connection between differential expression of extracellular vesicle incorporated RNAs and colorectal cancer. How this approach may potentially translate into point of care colorectal cancer diagnostics is also discussed.

Figure 1

Biogenesis and sorting process of miRNA inside the EVs (exosomes, macrovesicles, and large oncosomes are shown). miRNA biogenesis comes with a series of molecular events. At first, pri-miRNAs are trapped by RNase III enzyme Drosha and the DiGeorge Critical Region 8 (DGCR8) RNA binding protein, and the pre-miRNA display a stem-loop structure and are exported to the cytoplasm by Exportin-5. After that, dsRNA is formed by cleaving the pre-miRNA loop and 5′/3′ nucleotide. One of the loops is then attached with RNA-induced Silencing Complex (RISC), which have members of the Argonaute (Ago) protein family. Specific Exo motifs allow protein recognition and exosome sorting.

Figure 2

Crucial role of exosomal miRNAs in tumor initiation and development are presented. Colorectal cancer cells can transfer miRNAs through the exosomes to the nearby cells. These exosomal RNAs can control cell proliferation, invasion, and metastasis by orchestrating their downstream targets.^,

Figure 3

Colorectal cancer cell derived exosome secretion and isolation using the gold standard ultracentrifugation approach and subsequent RNA isolation and RNA-seq data acquisition and analysis^, for exosomal RNA based clinical applications. Colorectal cancer cell derived EVs are enriched with RNAs (shown in the pie chart, reproduced from ref (82) under the terms of the Creative Commons Attribution License, copyright 2021, publisher name: BMJ Journals) which offers next generation therapeutics and new biomarker for diagnosis and prognosis of cancer.