Colorectal Cancer (CRC) and High-Risk Adenoma (HRA)
A 90% sensitivity for Colorectal Cancer and 40% for High Risk Adenoma (HRA) in Average-Risk Adults is still insufficient! Colorectal cancer is the second-leading cause of cancer-related deaths in the U.S.1 Today, one in three adults have not completed the recommended CRC screening even though colorectal cancer is curable if caught early. Our multi-omics test which adds our powerful and proprietary customeBiome platform will be a revolutionary milestone and change the CRC health outcome.
The Human Microbiome and CRC
Colorectal cancer (CRC) is the second most fatal and the third most common cancer. In 2020, close to two million new colorectal cancer cases were estimated and short of one million deaths1. The known environmental risk factors for CRC include smoking, alcoholism, obesity, sedentary lifestyle, diabetes, and the Western diet, which explain the rising incidence of CRC in developing countries. Current therapies for CRC include surgical treatments, chemotherapy, and immunotherapy. Although the overall survival of patients has increased with the help of these therapies, CRC patients still face poor prognosis, late diagnosis, and low long-term survival 2,3. The American Cancer Society lowered the age for recommended screening to 45 years of age in 2018 to increase the likelihood of early detection in individuals at risk; however, a mass screening effort is not recommended due to the high costs of colonoscopies and poor execution of diagnostic services 1. Therefore, it is of great importance to find and/or refine less invasive and more economical methods to detect the disease, or even better, to act as a predictive biomarker, allowing possibilities to alleviate the growing burden of the disease.
The microbiome is involved in health and disease, affecting metabolism and immune functions. Together with environmental factors, changes in the gut microbiome are potentially supporting the initiation and development of CRC. Some studies have alluded to differences and patterns in overall composition and abundance of specific microbes between healthy individuals and CRC patients, yet there are still many inconsistencies between studies, warranting further research 4–7.
There are multiple stages of CRC, beginning with the growth of colorectal adenomas. In this early stage, there is evidence that the gut microbiome can be utilized to identify those at risk. Therefore, screening and early detection could be improved using gut microbiome changes as a prediction biomarker 4,6. Interplay between the Microbiome and the Immune System contributes to CRC
A healthy gut epithelium can develop pre-cancerous lesions, polyps, due to multiple genetic pathways and the accumulation of various mutations leading to mechanisms enabling tumorigenesis and carcinogenesis, while also compromising the immune system 7. The mechanisms involved between the microbiota and CRC are several and distinctive. The inflammation present in CRC is a result of both the microbiota and the immune system, whose intersection at the epithelial barrier of the colon can enable the development of bowel tumorigenesis 7,8. The colon epithelium can undergo mutagenesis due to exposure to bacterial toxins damaging DNA. Epithelial proliferation can occur by mimicking of ligands, contributing to neoplasia 8. The modulation of the immune system by the microbiota triggers immune responses and activating cells through immunomodulatory factors. These circumstances can affect the population, distribution and function of immune cell populations in the epithelium and underlying stroma, creating favorable microenvironments for tumor growth and promoting malignancy 8. Important to note is that the communication between the innate immune system and the microbiome is extensive and bi-directional 9; therefore, there could be domino effects due to changes in the homeostasis of either.
Another intriguing realization is that the induction of immunophenotypes and immunomodulatory functions do not correlate with microbial phylogeny 10–12. Researching this discrepancy further by analyzing gut microbes on the strain level could contribute to answering some of the questions relating to contradictions relating to composition and abundance of species in the gut microbiome that are often found. Especially since different strains of the same species can affect the host immune system in distinct and strikingly different ways. 10.
Changes throughout CRC development
Alterations in the microbiome in the presence of colorectal adenomas and CRC compared to healthy mucosa have been consistently found 13. The question of whether these changes cause or are a result of the cancerous growth is still being debated 4. Nevertheless, once defined decisively, such microbiome alterations are perfect candidates for early, non-invasive diagnosis 14,15 . The changes in the gut microbiome related to CRC are characterized by higher species richness, lower abundance of taxa known for their protective effects, increased abundance of taxa which have carcinogenic potential (Figure below), and colonic biofilms 15,16.
The bacteria, Fusobacterium nucleatum, is related to CRC through multiple pathways and throughout the disease's progression, metastasis, and treatment resistance 13,17. This bacteria can be measured in tumor tissue and fecal samples and has been suggested as a potential marker for CRC 15,18. Intriguingly, Fusobacterium nucleatum is an oral bacteria species which translocates to the gut and colon in certain circumstances, making it an excellent example of patient physiological changes which lead to microbiome changes in composition and behavior and eventually affect colon cancer progression and aggressiveness. Another example is the strain Escherichia coli pks+ which synthesizes a genotoxin, colibactin, known for causing DNA damage that elevates the risk of CRC18. This E. coli was hence shown to deteriorate colorectal carcinoma.
Microbiome involvement in CRC development
CRC carcinogenesis is promoted through interactions of the gut microbiome and immune response 7. An imbalance of the gut microbiome, dysbiosis, can disintegrate the mucus layer protecting the colonic epithelial barrier, creating an entry and direct pathway for pathogenic bacteria and their secretions to the underlying tissue. This invasion of bacteria stimulates immune cell-mediated responses increasing pro-inflammatory cytokines 7,19. Altogether, within the tumor microenvironment, the bacteria are classified as drivers, directly carcinogenic, or passenger, opportunistic bacteria 15.
A factor connecting the gut microbiota and CRC risk is microbial metabolites. These small molecules result from the metabolism of dietary and host-derived compounds and can influence the population dynamics in the gut and host cells with a range of effects through various pathways, including carcinogenesis. Therefore, dysbiosis changes the metabolite homeostasis, which has been shown to cause the onset and progression of CRC 18. The most influential metabolites are short-chain fatty acids (SCFAs), bile acids (BAs) and Tryptophan 3,18.
Understanding the development of polyps and the importance of surveilling them and their microenvironment could increase the prospects of earlier diagnoses and treatment targets 7.
Solutions and challenges between Microbiome and CRC
There are exciting ideas and new research on harnessing the relationship of CRC with the microbiome, both before the carcinogenesis cascade fully commences and after CRC is well established (Figure 3 below).
The most evident challenges with these solutions are the lack of optimal testing and implementation regimens and the uncertainty regarding sensitivity and specificity 15,20. Novel approaches to microbiome based computational analysis are necessary to decipher the microbiota-related diagnosis factors (whether microbial strains or functional attributes) and their role in the various phases of CRC. Important potential sources of innovation in this area are the fields of Artificial Intelligence (AI), Deep Learning (DL) and metagenomics analyses. Such approaches must be validated comprehensively using prospective clinical trials, like the ones performed by BiotaX.
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