HER2 (ERBB2) in Colorectal Cancer: A Predictive Biomarker with Its Own Rules

What the Test Measures

HER2, encoded by the ERBB2 gene, is a receptor tyrosine kinase in the epidermal growth factor receptor family. Under normal conditions it participates in signaling that drives cell proliferation and survival. When the gene is amplified — meaning the tumor carries extra copies — the cell surface becomes crowded with HER2 protein, and the downstream signaling that promotes growth runs at an abnormally high level. In colorectal cancer (CRC), this happens in a small but clinically meaningful subset of tumors, on the order of a few percent, concentrated in RAS/BRAF wild-type disease.

The point of testing is to identify that subset. HER2 amplification is not a prognostic curiosity here; it's a predictive marker, meaning it tells us something about whether a tumor is likely to respond to a particular category of drug. That distinction matters, because a predictive biomarker earns its place in the workup only when a therapy is tied to the result.

How It's Tested

Three assay types are used, and in practice they're complementary rather than interchangeable. Immunohistochemistry (IHC) stains for HER2 protein on the cell membrane and is typically the first-line, low-cost screen. In-situ hybridization (ISH) counts ERBB2 gene copies relative to a chromosome 17 reference, and it resolves the cases that IHC leaves equivocal. Next-generation sequencing (NGS) detects amplification as part of a broader molecular panel, which is convenient because the same run can report the RAS and BRAF status that HER2 eligibility depends on.

All of these run on formalin-fixed, paraffin-embedded (FFPE) tissue, and the preanalytic constraints are the usual ones: adequate, promptly fixed tissue with enough viable tumor and minimal crush or necrosis. Degraded or poorly fixed specimens produce weak or artifactual membrane staining that's genuinely hard to interpret.

The scoring is where colorectal cancer parts ways with the more familiar breast and gastric frameworks. CRC uses its own criteria, developed and validated within colorectal trials, defining what counts as 3+ (positive), 2+ (equivocal, reflexed to ISH), and lower scores. In practice, the most common error I see is applying breast cancer IHC thresholds to a colorectal specimen — the membrane-staining patterns and the intensity cutoffs simply aren't the same, and importing the wrong rubric will misclassify tumors in both directions. A positive result requires either strong, consistent IHC positivity or ISH-confirmed amplification, following the CRC-specific definitions rather than any borrowed one.

What Each Result State Means

A HER2-positive result means the tumor shows amplification or overexpression at the level the CRC criteria require — strong IHC or confirmatory ISH. Biologically, that's a tumor whose growth signaling is being driven, at least in part, through HER2, and that's the population in which HER2-directed agents have shown activity. A HER2-negative result means the tumor doesn't meet those criteria; the HER2 pathway isn't a validated therapeutic handle, and management proceeds on other grounds. Equivocal IHC (2+) isn't a final answer — it's a signal to reflex to ISH, which adjudicates the case one way or the other. Reporting an equivocal stain as if it were positive or negative is a preventable diagnostic error.

What It Determines for Treatment Eligibility

Here the co-dependencies are strict. HER2 amplification informs eligibility for HER2-directed therapy only in metastatic CRC that is also RAS and BRAF wild-type. This constraint comes directly from how the trials were designed and read out. HERACLES tested dual HER2 blockade in HER2-amplified, KRAS wild-type metastatic CRC and demonstrated meaningful response, establishing HER2 as an actionable target in this setting [1]. MOUNTAINEER subsequently evaluated a HER2-directed antibody combination in HER2-positive, RAS wild-type metastatic disease and confirmed clinical activity, contributing to regulatory approval in this space [2]. A broader review of HER2-expressing CRC summarizes the current landscape of agents and approvals [3].

The framing here is deliberate: a positive HER2 result informs eligibility for a class of HER2-directed drugs. It does not, by itself, tell any individual patient what they should take. Eligibility is a gate, not a prescription — decisions about whether and when to use a given agent belong to the treating clinician and patient, weighing the whole clinical picture. A HER2-positive result in a tumor that carries a RAS or BRAF mutation does not open that gate, because the evidence supporting HER2-directed benefit was generated in wild-type disease.

Caveats and What's Evolving

The most contested question is HER2-low. In breast cancer, a HER2-low category has become therapeutically meaningful with antibody-drug conjugates, and there's an understandable temptation to extend that thinking to colon cancer. The evidence doesn't yet support doing so. The reasons are partly biological and partly evidentiary. The CRC IHC baseline scoring differs from the breast framework, so a stain called "low" in one organ doesn't map cleanly onto the other; prevalence differs as well; and, critically, prospective CRC-specific data defining a HER2-low population and demonstrating antibody-drug conjugate benefit in that population are not mature. Until such data exist, HER2-low in CRC should be treated as an open research question rather than a validated eligibility category.

Two further points are worth holding in mind. First, assay concordance isn't perfect — IHC, ISH, and NGS can disagree, particularly in the equivocal middle, which is exactly why the CRC-specific reflex algorithm exists. Second, this is a fast-moving area; cutoffs, approved agents, and companion-diagnostic definitions continue to evolve as trials read out.

Summary

HER2 testing in colorectal cancer works only when it's done on CRC's own terms — its own scoring criteria, its own reflex logic, and its tight dependence on concurrent RAS/BRAF status. As molecular testing becomes more integrated, the case for reporting HER2 alongside RAS and BRAF on a single panel grows stronger, because HER2 eligibility is meaningless without that context. The unresolved HER2-low question is where the field will likely move next, and how it's answered — with CRC-specific prospective data rather than borrowed frameworks — will determine whether the actionable HER2 population in colorectal cancer grows beyond the amplified subset we can confidently identify today.

References

1. Sartore-Bianchi A, Trusolino L, Martino C, et al. Dual-targeted therapy with trastuzumab and lapatinib in treatment-refractory, KRAS codon 12/13 wild-type, HER2-positive metastatic colorectal cancer (HERACLES): a proof-of-concept, multicentre, open-label, phase 2 trial. Lancet Oncol. 2016;17(6):738–746. PMID: 26980441.
2. Strickler JH, Cercek A, Siena S, et al. Tucatinib plus trastuzumab for chemotherapy-refractory, HER2-positive, RAS wild-type unresectable or metastatic colorectal cancer (MOUNTAINEER): a multicentre, open-label, phase 2 trial. Lancet Oncol. 2023;24(5):496–508. PMID: 36764279.
3. Treatment options for HER2-expressing colorectal cancer: updates and recent approvals. Peer-reviewed review, 2024. PMCID: PMC10798128.