Colorectal Cancer Molecular Pathways

The genetic/molecular routes by which colorectal cancer develops — relevant to understanding why some patients develop CRC quickly and others slowly, and increasingly relevant to treatment selection. Content below is from a textbook reference chapter (Sabiston Ch52 - Colorectal Neoplasia & Colon Cancer Surgery), not primary literature.

Chromosomal instability (suppressor) pathway — ~60% of CRC

The classic, best-understood pathway (Vogelstein et al.), based on evaluation of nearly 200 colorectal neoplasia samples ranging from polyps to invasive cancers. A step-wise model of carcinogenesis: activation of an oncogene plus loss of several tumor-suppressor genes. Most sporadic CRC is thought to arise this way over roughly 10 years from a precursor dysplastic adenoma (the classic adenoma-carcinoma sequence).

Molecular events: early APC (adenomatous polyposis coli) mutation → subsequent activating mutation in the oncogene KRAS → mutation inactivating the tumor suppressor TP53. “Chromosomal instability” refers to changes (gains/losses) in chromosome number (aneuploidy) as well as subchromosomal genomic amplifications and loss of heterozygosity.

CpG island methylator phenotype (CIMP) pathway

Initiating mutation involves the BRAF gene, resulting in inhibition of normal colon cell apoptosis. This leads to hyperplastic or sessile serrated polyps, which are prone to epigenetic silencing of genes within “CpG islands” (short DNA stretches rich in cytosine/guanine) in promoter regions via hypermethylation. hMLH1 (one of the Lynch-associated DNA repair genes) is one of the best-characterized genes silenced this way — resulting in an MSI-high (MSI-H) cancer if gene mutation/methylation goes further. Most cancers arising from sessile serrated adenomas have an MSI-H phenotype and are often located in the right colon.

Microsatellite instability (MSI) mutator pathway — ~15% of early-stage CRC

Due to mutation in genes responsible for repairing DNA base mismatches: MLH1, MLH3, MSH2, MSH3, MSH6, PMS2. Microsatellites are normally-occurring repeated DNA sequences (1–6 base pairs); when mismatch-repair mutations are present, replication mistakes at these microsatellites go uncorrected, producing MSI.

• MSI-H cancers are often characterized by proximal-colon location, large local tumor size, typical absence of metastatic disease, and poor tumor differentiation.
• Sporadic MSI-H cancer patients tend to be elderly; the hereditary form (Lynch syndrome — see Lynch Syndrome) tends to present younger (<50) and is associated with tumor-infiltrating lymphocytes.
• BRAF mutation testing differentiates the two: a BRAF mutation in an MSI-H CRC is evidence against Lynch syndrome (BRAF mutations occur in ~15% of sporadic CRCs but essentially not in Lynch tumors).

Hereditary CRCs overall account for ~5% of CRC genetically — inactivation of a tumor suppressor gene (e.g. APC in FAP) or a DNA repair gene (e.g. Lynch) via monoallelic germline expression plus a subsequent somatic “second hit,” which affects the function of the remaining allele.

Epithelial-mesenchymal transition (EMT)

CRC leads to death primarily if it metastasizes. EMT is the process by which cells lose epithelial functional/morphologic features and gain a “mesenchymal” phenotype — important in cancer but also a normal process in embryonic development and wound healing. Through EMT, locally growing cancer cells gain the ability to invade through the bowel wall and spread to regional lymph nodes: loss of epithelial cell polarity, loss of cell-cell adhesion, gain of a migratory/invasive phenotype. EMT is reversible — once cancer cells reach a metastatic site, they typically undergo the reverse process (mesenchymal-to-epithelial transition, MET) to establish there. Carcinoma cells only very rarely advance into a completely mesenchymal state.

Consensus molecular subtypes (CMS1–4)

A transcription-based cancer subtyping system developed through international data-sharing collaboration, intended to enable more individualized, targeted treatment.

• CMS1 — hypermutated, MSI-unstable, strong immune activation (JAK-STAT activation, caspases, DNA damage repair). More likely to correspond to MSI-H tumors; more often proximal colon.
• CMS2 — epithelial phenotype with marked WNT and MYC signaling activation.
• CMS3 — epithelial phenotype with metabolic dysregulation (glutaminolysis, lipidogenesis).
• CMS4 — mesenchymal phenotype, prominent TGF-β activation, stromal invasion, angiogenesis.

CMS2–4 are more likely to correspond to chromosomal instability (CIN) lesions; CMS1 more likely corresponds to MSI-H tumors. Tumor location correlates with subtype (CMS1 more proximal, CMS4 more distal per the chapter’s schematic), and stromal/immune microenvironment differs meaningfully across subtypes (highly immunogenic in CMS1 vs. poorly immunogenic/immunosuppressive in CMS4).

Open items / gaps

• No primary literature yet on how CMS subtyping is used prospectively in treatment selection — the chapter presents it as a promising framework, not a current standard-of-care decision tool.
• No detail yet on specific targeted therapies matched to each pathway/subtype (e.g. anti-EGFR therapy and RAS/BRAF status) — likely to come up when ingesting primary CRC treatment trials.