Biomarker & Molecular Overview: Colon Cancer

Why molecular testing matters here

Colon cancer is no longer treated as a single disease. Beneath the shared appearance of an adenocarcinoma under the microscope lie molecularly distinct tumors that behave differently and respond to different drug classes. Molecular testing is what converts that heterogeneity into actionable information — it identifies patients who will (or will not) benefit from a targeted or immune therapy, refines prognosis, and increasingly informs whether and when additional treatment is warranted after surgery.

The evidence supports a practical division of biomarkers by purpose: some are diagnostic/lineage clues, some are prognostic (they forecast outcome regardless of therapy), and many are predictive (they gate eligibility for a specific drug class). Several biomarkers serve more than one role. This page is an orientation and a hub; each biomarker named below links to its own detail article. Nothing here is treatment advice — biomarker results connect patients to eligibility for a drug class, and treatment decisions belong with a care team.

What gets tested, grouped by purpose

Diagnostic / lineage and hereditary triage

Mismatch repair status / microsatellite instability (dMMR/MSI-H) is the anchor test and is recommended universally on colorectal cancers [1]. It is assessed by immunohistochemistry for the four MMR proteins (MLH1, PMS2, MSH2, MSH6) and/or by PCR or NGS-based MSI testing [1][2]. Beyond its therapeutic role (below), dMMR carries a hereditary-triage function: loss of MLH1/PMS2 prompts reflex testing for BRAF V600E and MLH1 promoter methylation to distinguish sporadic (methylation-driven) tumors from those that should raise suspicion for Lynch syndrome. In this way, a single result branches into both germline and somatic workflows.

Prognostic markers

ctDNA for minimal residual disease (MRD) is the emerging prognostic test drawn from plasma after surgery [6]. Detectable post-operative circulating tumor DNA — whether measured by tumor-informed or tumor-agnostic NGS — is one of the strongest available predictors of recurrence [6][12]. This is a fast-moving, contested area; see the "what's emerging" section.

BRAF V600E is dual-purpose. As a prognostic marker it is associated with adverse outcomes in metastatic disease [3][7], and it also participates in the hereditary-triage reflex described above.

Predictive / therapy-gating markers

KRAS/NRAS mutation (expanded RAS testing) interrogates KRAS and NRAS exons 2, 3, and 4 [3]. It is predictive in a negative sense: RAS-mutant tumors do not benefit from anti-EGFR antibodies [3][4].

BRAF V600E additionally gates a targeted combination approach in metastatic disease [7].

HER2 (ERBB2) amplification/overexpression, scored with colorectal-specific criteria distinct from breast and gastric cancer, identifies a small subset eligible for HER2-directed therapy [5][8].

NTRK fusions are rare but tumor-agnostic actionable events, enriched in MSI-H/MLH1-methylated tumors [9][10].

How results steer treatment (result → drug class)

The following links a result to a therapy class — the eligibility relationship, not a recommendation.

• dMMR / MSI-H → immune checkpoint inhibitor class. MSI-H/dMMR status gates checkpoint inhibitor therapy, including in the first-line metastatic setting [2][11]. Conversely, pMMR/MSS tumors are generally not candidates for this class on the basis of MMR status alone.
• RAS wild-type → anti-EGFR antibody class. Only RAS wild-type tumors are candidates for cetuximab/panitumumab-type agents; RAS-mutant tumors are predicted not to benefit, and the mutation therefore excludes this class [3][4].
• BRAF V600E → BRAF-inhibitor-based combination class (typically paired with an anti-EGFR antibody) in metastatic disease [3][7].
• HER2-positive (in a RAS/BRAF wild-type background) → HER2-directed therapy class [5][8].
• NTRK fusion present → TRK-inhibitor class (tumor-agnostic) [9][10].
• ctDNA MRD-positive after surgery → informs adjuvant-therapy decision-making — an area of active investigation rather than a settled indication [6][12].

Two dependencies are worth emphasizing. First, RAS and BRAF results interact with HER2 interpretation, since HER2-directed eligibility is framed in the RAS/BRAF wild-type context [5]. Second, the MMR result triggers downstream BRAF/MLH1-methylation reflex testing whose purpose is hereditary triage, not therapy selection [1][3]. Testing is thus best ordered as a coordinated panel rather than as isolated single-gene assays.

Specimen and testing realities

Most of these biomarkers are assessed on formalin-fixed, paraffin-embedded (FFPE) tissue — the standard specimen for IHC, ISH, PCR, and NGS [1][3][5]. Because multiple alterations must be characterized (MMR/MSI, RAS, BRAF, HER2, NTRK), broad NGS panels are efficient, conserving limited tissue while covering point mutations, amplifications, and fusions in one workflow.

Reflex testing is a recurring theme. Universal MMR/MSI testing on all colorectal cancers is the entry point [1]; MLH1/PMS2 loss reflexes to BRAF V600E and MLH1 methylation for Lynch triage [1][3]. Screening-then-confirm strategies are also common: pan-TRK IHC can screen for NTRK fusions with NGS confirmation [9], and VE1 IHC can screen for BRAF V600E [3]. These stepwise designs conserve tissue and turnaround time.

Liquid biopsy (plasma ctDNA) has two distinct uses. For genotyping, RAS and BRAF can be interrogated in plasma when tissue is insufficient or when a rapid result is needed [3][7]. For MRD, plasma is the required specimen and the entire application is intrinsically blood-based [6]. A negative liquid-biopsy genotyping result should be interpreted cautiously, as low tumor-DNA shedding can produce false negatives; tissue remains the reference standard for definitive genotyping.

What's emerging

The most dynamic frontier is ctDNA-based MRD. The evidence suggests post-operative ctDNA positivity is a powerful recurrence predictor, and trials such as DYNAMIC and its successors have begun testing whether ctDNA can guide adjuvant-therapy decisions — escalating treatment for MRD-positive patients or safely de-escalating for those who are ctDNA-negative [6][12]. This remains a work in progress; whether it is ready for routine standard-of-care use is explicitly debated [12], and clinicians should treat it as an evolving, trial-driven area rather than an established gate.

HER2 in colorectal cancer is also advancing rapidly, with treatment options and approvals in this space updated recently [8]; the colorectal-specific scoring criteria underscore that HER2 interpretation is not transferable from other tumor types [5]. NTRK-directed and other tumor-agnostic strategies continue to expand the actionable landscape for rare fusions [9][10]. And the first-line role of checkpoint inhibitors in MSI-H/dMMR disease reflects how a lineage/diagnostic marker has become a decisive predictive one [11].

Taken together, the trajectory is toward broad, panel-based, and increasingly blood-inclusive molecular profiling — with several biomarkers doing double duty across diagnosis, prognosis, and prediction. Each named marker above has a dedicated detail article; use this hub to navigate to the specifics.

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References

1. WHO Classification of Tumours Editorial Board. Digestive System Tumours, 5th ed. IARC, Lyon; 2019. ISBN 978-92-832-4499-8.
2. dMMR/MSI testing and immunotherapy in colorectal cancer (primary literature; e.g., KEYNOTE-177) — verify.
3. Expanded RAS testing and anti-EGFR therapy (primary literature; e.g., PRIME, CRYSTAL) — verify.
4. Douillard JY, et al. Panitumumab–FOLFOX4 Treatment and RAS Mutations in Colorectal Cancer. N Engl J Med. 2013. PMID: 24024839 / DOI: 10.1056/NEJMoa1305275.
5. HER2-amplified metastatic colorectal cancer (primary literature; e.g., HERACLES, MOUNTAINEER) — verify.
6. Tie J, et al. ctDNA-guided adjuvant therapy in colon cancer (DYNAMIC). N Engl J Med. 2022. DOI: 10.1056/NEJMoa2200075 — verify.
7. BRAF V600E and combination therapy in mCRC (primary literature; e.g., BEACON) — verify.
8. Treatment options for HER2-expressing colorectal cancer: updates and recent approvals. PMC; 2024. PMCID: PMC10798128.
9. NTRK fusions in colorectal cancer (primary literature) — verify.
10. Cocco E, Scaltriti M, Drilon A. Larotrectinib and Entrectinib: TRK Inhibitors for the Treatment of NTRK Fusion-Positive Tumors. PMC; 2021. PMCID: PMC7863124.
11. FDA. FDA approves pembrolizumab for first-line treatment of MSI-H/dMMR colorectal cancer. Drugs@FDA; 2020.
12. ctDNA/MRD Testing for Colon Cancer: A Work in Progress or Ready for Prime-Time Standard of Care? PubMed; 2024. PMID: 39681074.

This overview is educational and does not constitute medical advice. Biomarker results establish eligibility for therapy classes; treatment decisions require a qualified care team.