From Crypts to Cancer: A Holistic Perspective on Colorectal Carcinogenesis and Therapeutic Strategies

Abstract

Colorectal cancer (CRC) represents a significant global health burden, with high incidence and mortality rates worldwide. Recent progress in research highlights the distinct clinical and molecular characteristics of colon versus rectal cancers, underscoring tumor location's importance in treatment approaches. This article provides a comprehensive review of our current understanding of CRC epidemiology, risk factors, molecular pathogenesis, and management strategies. We also present the intricate cellular architecture of colonic crypts and their roles in intestinal homeostasis. Colorectal carcinogenesis multistep processes are also described, covering the conventional adenoma-carcinoma sequence, alternative serrated pathways, and the influential Vogelstein model, which proposes sequential APC, KRAS, and TP53 alterations as drivers. The consensus molecular CRC subtypes (CMS1-CMS4) are examined, shedding light on disease heterogeneity and personalized therapy implications.

Keywords: colorectal cancer; epidemiology; molecular pathogenesis; risk factor; therapeutic strategy.

Figure 1

Conventional adenoma–carcinoma sequence pathway of colorectal cancer development. (A) The first stage involves hyperproliferation, where there is an increased rate of cell division and growth within the colonic epithelium. This is depicted as a slight thickening of the epithelial lining. (B) As the process progresses, small adenomatous polyps begin to form. These polyps are benign growths protruding into the colonic lumen and are represented as small, spherical structures attached to the epithelial lining. (C) Over time, some of these small polyps can grow larger, forming large adenomatous polyps. These are shown as larger spherical masses connected to the colonic wall. (D) Within these larger polyps, severe dysplasia occurs, characterized by abnormal cell growth and organization. This stage is visually depicted by the presence of an irregular, yellow-colored growth within the polyp structure. (E) The dysplastic cells in the polyp can then transform into an adenocarcinoma, which is an invasive malignant tumor. This stage is represented by a large, irregularly shaped mass protruding into the colonic lumen, with a distinct boundary separating it from the surrounding normal tissue. (F) In the final stage, the adenocarcinoma has progressed to a full-blown colorectal cancer. This is depicted as a large, irregular mass filling a significant portion of the colonic lumen, indicating advanced tumor growth and invasion. Figure created using BioRender (https://www.biorender.com/ accessed on 29 July 2024).

Figure 2

The Vogelstein model of colorectal cancer progression. This model was proposed by Bert Vogelstein and his colleagues at Johns Hopkins University in the early 1990s, based on their groundbreaking research on the genetic alterations involved in colorectal cancer (CRC). The initial stage shows a normal epithelium, representing a healthy colon lining with no visible abnormalities. (A) The first genetic event leads to the formation of a small benign growth polyp. This is a consequence of the inactivation of the APC gene, a critical tumor suppressor gene. (B) The next stage is associated with the progression from a small polyp to a large benign growth (early adenoma). The activation of the KRAS oncogene, a key driver of cellular proliferation, contributes to the growth and expansion of the adenomatous polyp. (C) This is followed by the loss of tumor suppressor gene DCC, which contributes to the development of a late adenoma or large benign growth. (D) The next step is the transition from a late adenoma to an invasive malignant tumor or carcinoma by the loss of tumor suppressor gene TP53. (E) The final stage represents a full-blown CRC, depicting an advanced, invasive malignant tumor mass. This stage may involve additional genetic alterations beyond the core events highlighted in the Vogelstein model. Figure created using BioRender.

Figure 3

The serrated neoplasia routes. (A) The traditional serrated pathway begins with a KRAS mutation in normal colonic mucosa, leading to the formation of a traditional serrated adenoma (TSA). Further MGMT methylation and other TSG (tumor suppressor gene) methylation events promote the progression to TSA-HGD (traditional serrated adenoma–high grade dysplasia). Subsequent accumulation of genetic alterations results in the development of serrated adenocarcinoma (SAC), which can exhibit either an MSI-L/CMP-L (microsatellite instability–low/CpG island methylator phenotype–low) or MSS/CMP-L (microsatellite stable/CpG island methylator phenotype–low) molecular profile, indicative of invasive colorectal cancer (CRC). (B) The sessile serrated pathway initiates with a BRAF mutation in normal colonic mucosa, leading to the formation of sessile serrated lesions (SSL). Further MLH1 methylation and other TSG methylation events promote the progression to SSL-HGD (sessile serrated lesion–high grade dysplasia). Subsequent accumulation of genetic alterations results in the development of serrated adenocarcinoma (SAC), which can exhibit either an MSI-L/CMP-H (microsatellite instability–low/CpG island methylator phenotype–high) or MSI-H/CMP-H (microsatellite instability–high/CpG island methylator phenotype–high) molecular profile, indicative of invasive CRC. Figure created using BioRender.

Figure 4

Model of the colitis-associated cancer pathway in inflammatory bowel disease. The figure depicts the multistage process by which chronic intestinal inflammation can lead to colorectal carcinoma development in the context of inflammatory bowel disease. The pathway is initiated by recurrent episodes of mucosal injury and inflammation in conditions such as ulcerative colitis (UC) or Crohn’s disease (CD). Prolonged inflammatory cell infiltration and cytokine/growth factor release results in accumulation of DNA damage and mutations in genes such as tumor suppressors (e.g., TP53) and oncogenes involved in Wnt/β-catenin signaling. Epigenetic alterations including DNA methylation changes also occur. This contributes to dysregulated epithelial proliferation and dysplasia. Immune system modulation favors an immunosuppressive microenvironment conducive to tumor growth. Through additional genetic and epigenetic changes, low- and high-grade dysplasia may develop, eventually progressing to adenocarcinoma, squamous cell carcinoma, or small cell carcinoma subtypes—the colitis-associated cancers (CAC). Figure created using BioRender.

Figure 5

Leveraging patient-derived models to investigate intratumoral heterogeneity in colorectal cancer subtypes. Primary tumor tissues dissociated from colorectal cancer (CRC) patients serve as the starting point for generating diverse experimental models. Single-cell suspensions allow enrichment of adherent cancer cells while depleting non-malignant stromal components, maintaining intratumoral heterogeneity. The enriched cancer cells can be directly cultured as patient-derived cell lines or used to derive three-dimensional organoid cultures that recapitulate aspects of the tumor microenvironment. Additionally, these patient-derived cells can be modified through transfection with siRNA, shRNA, cDNA, or CRISPR/Cas9 gene editing before utilization in downstream applications. One key application is injection into immunocompromised mouse models to establish patient-derived xenograft (PDX) tumors. Sample collection from these in vivo models provides primary tumors, metastases, and liquid biopsy samples for comprehensive molecular analyses. Established CRC cell lines like HT-29 offer an alternative source for generating xenograft models and modified sublines. Multi-omics data acquisition through techniques like RNA sequencing, mass spectrometry proteomics, and identification of differentially expressed genes enables biomarker discovery, pathway analysis, and biological interpretation of distinct CRC subtypes. Potential therapeutic targets derived from these analyses are validated through antibody/drug treatment studies in the PDX and cell line xenograft models. This multifaceted strategy integrating patient-derived models, genetic modifications, organoids, xenografts, and multi-omics profiling facilitates investigations into the complexity of intratumoral heterogeneity underlying CRC. Figure created using BioRender.

Figure 6

Mechanisms underlying the hallmarks of colorectal cancer progression. Colorectal cancer (CRC) is a complex and multifaceted disease characterized by the acquisition of various hallmark capabilities that enable tumor growth, progression, and metastasis. Sustained proliferative signaling in CRC tumors is driven by modifying mechanisms like cell cycle arrest, DNA repair, senescence, and apoptosis, while evasion of growth suppressors occurs through disruptions in tumor suppressors like TP53 and APC. Resistance to cell death is facilitated by dysregulation of apoptotic machinery and BCL-2 family members, and replicative immortality is enabled by deregulation of pathways like WNT, RAS, and PI3K. Angiogenesis is induced by factors like VEGF and hypoxic conditions, while invasion and metastasis involve epithelial–mesenchymal transition, altered cell–cell adhesion, and extracellular matrix remodeling. Deregulation of cellular energetics, such as the Warburg effect, provides a growth advantage, and immune evasion is mediated by mechanisms like PD-L1 upregulation. A tumor-promoting inflammatory microenvironment is created by cytokines, chemokines, and immune cell infiltration, while genomic instability and tumor progression are driven by the accumulation of mutations in genes like APC, KRAS, and TP53. The image further depicts emerging hallmarks specific to CRC, including the distinct molecular features of left-sided and right-sided tumors, unique characteristics of rectal cancer, and the involvement of signaling pathways like Wnt, Notch, Hedgehog, and TGF-β in disease progression. Figure created using BioRender.

Figure 7

Gut microbiome–host interactions in colorectal cancer development. The gut microbiome plays a crucial role in influencing colorectal cancer (CRC) development and progression. This schematic image illustrates the presence of both potentially pro-carcinogenic pathogens, including Fusobacterium nucleatum (F. nucleatum), Peptostreptococcus, Streptococcus, and Escherichia coli (E. coli), as well as beneficial probiotic species like Lactobacillus, Bifidobacterium, and Bacteroides fragilis (B. fragilis). These microbes interact with the colonic epithelium, influencing the proliferation of CRC cells and the process of tumorigenesis. For instance, B. fragilis and E. coli can promote CRC progression by activating the TLR/NF-κB signaling pathway. B. fragilis secretes polysaccharide A (PSA), which acts as a TLR2-specific agonist. The binding of PSA to TLR2 leads to downstream NF-κB activation, a key transcription factor that promotes CRC cell proliferation, survival, angiogenesis, and metastasis. NF-κB signaling induced by PSA enhances CRC growth and development by increasing pro-inflammatory cytokines like IL-6 and IL-8, leading to chronic inflammation and fostering CRC progression. PSA also stimulates TLR2 expression on colon and CRC cells, creating a positive feedback loop wherein higher TLR2 levels induce greater NF-κB responses to repeated PSA, driving cell proliferation. PSA protects CRC cells from chemotherapy and activates NF-κB survival signaling as well. Meanwhile, B. fragilis lipopolysaccharide (LPS) engages TLR4, stimulating NF-κB -mediated expression of genes for survival, invasion, and angiogenesis in CRC tissues with co-expressed TLR4 and NF-κB. LPS increases cytokines like IL-1β and IL-6 via NF-κB, fueling tumor growth and metastasis. It also induces COX-2 and EMT through NF-κB. E. coli LPS also can bind TLR4, triggering MyD88 recruitment and mitochondrial reactive oxygen species (ROS) generation via NOX1 upregulation in a NF-κB -dependent manner. Elevated mitochondrial ROS activates MAPKs and IKB oxidation, as well as nuclear translocating NF-κB. This underscores the importance of maintaining a balanced gut microbiome to modulate the tumor microenvironment and potentially prevent or manage CRC development. Figure created using BioRender.

Figure 8

Tumor–immune interactions and the dynamic interplay between tumor killing and immunosuppressive mechanisms. The image presents a comprehensive overview of the complex interactions between tumor cells and the immune system, highlighting the dynamic interplay between tumor-killing mechanisms and immunosuppressive pathways within the tumor microenvironment. Key effector cells like natural killer (NK) cells and cytotoxic T lymphocytes (CTLs) can directly eliminate tumor cells through the release of cytotoxic granules, while being activated by the presentation of tumor antigens by antigen-presenting cells (APCs) and the presence of pro-inflammatory cytokines like IFN-γ and TNF-α, which also enhance the expression of MHC molecules and tumor antigens, making them more susceptible to immune recognition. Chemokines like CXCL9, CXCL10, and CXCL11 promote the trafficking of these effector cells into the tumor site, facilitating their antitumor functions. Conversely, regulatory T cells (Tregs) and immunosuppressive cytokines like TGF-β, IL-10, and IL-4 create an inhibitory environment that dampens the activity of effector immune cells. Myeloid-derived suppressor cells (MDSCs) inhibit T cell responses through the production of enzymes like arginase (ARG1), inducible nitric oxide synthase (iNOS), and reactive oxygen species (ROS), which deplete essential nutrients and induce oxidative stress. Other enzymes like indoleamine 2,3-dioxygenase (IDO) and tryptophan 2,3-dioxygenase (TDO) deplete the essential amino acid tryptophan, leading to metabolic stress and suppression of T cell responses. Additionally, factors like VEGF, COX2, and PGE2 not only support tumor growth but also contribute to the recruitment and function of immunosuppressive cell types. This intricate balance and crosstalk between pro-inflammatory and anti-inflammatory signals within the tumor microenvironment ultimately determines the overall efficacy of the antitumor immune response or the establishment of an immunosuppressive state that favors tumor progression, with the balance often tipped towards progression in advanced stages of cancer. Figure created using BioRender.