NTRK Fusions in Colorectal Cancer: A Predictive Biomarker for Tumor-Agnostic TRK Inhibition

What the Test Measures

The NTRK1, NTRK2, and NTRK3 genes encode the tropomyosin receptor kinases TRKA, TRKB, and TRKC — a family of neurotrophin receptors. Under normal physiology these receptors participate in neuronal development and signaling. When one of the NTRK genes undergoes a chromosomal rearrangement, its kinase domain is fused to an unrelated upstream partner gene. The resulting chimeric protein drives constitutive, ligand-independent kinase activation, functioning as an oncogenic driver that promotes tumor cell proliferation and survival [2].

In colorectal cancer (CRC), NTRK fusions are rare. They are, however, notably enriched in a specific molecular subset: tumors that are microsatellite-instability-high (MSI-H), particularly those with MLH1 promoter methylation [1]. This enrichment pattern is clinically useful, because it helps direct testing efforts toward the populations most likely to harbor an actionable fusion. The biological importance of finding a fusion lies in its role as a driver — when a tumor depends on the aberrant TRK signal, blocking that signal can produce meaningful responses across many cancer histologies.

How It Is Tested

The specimen is formalin-fixed, paraffin-embedded (FFPE) tumor tissue, the standard material used across routine surgical pathology. As with any molecular assay run on FFPE, preanalytic quality matters: adequate tumor cellularity, acceptable fixation, and sufficient nucleic acid integrity are prerequisites for reliable results.

Two complementary methods are used, and they serve different roles:

• Pan-TRK immunohistochemistry (IHC) functions as a screening assay. A single antibody detects the shared kinase-domain epitope of the TRK proteins, flagging tumors in which a fusion may have produced aberrant TRK expression. Because it is fast and inexpensive, IHC is well suited to identifying candidate cases in a rare-event setting.
• Next-generation sequencing (NGS) functions as the confirmatory and definitive method. RNA- or DNA-based NGS can identify the specific fusion, the involved NTRK gene, and the fusion partner. RNA-based approaches are particularly valuable because they detect the expressed fusion transcript directly, regardless of the intronic breakpoint.

Scoring is fundamentally binary at the level of clinical decision-making: a fusion is either present or absent. IHC positivity raises suspicion; NGS establishes whether a true, in-frame, oncogenic fusion is present. Given the enrichment described above, laboratories may prioritize fusion testing in MSI-H or MLH1-methylated tumors [1].

What Each Result State Means

NTRK fusion present. The tumor harbors a rearrangement producing a constitutively active TRK protein. In the tumor-agnostic framework, this finding identifies the cancer — irrespective of its colorectal origin — as one driven, at least in part, by TRK signaling, and therefore as a candidate for targeted inhibition [2].

NTRK fusion absent. No actionable NTRK rearrangement was identified. The tumor is not a candidate for TRK-directed therapy on this basis. It is worth remembering that "absent" is contingent on assay sensitivity and specimen adequacy; a technically limited sample or a method blind to a particular fusion partner could, in principle, miss a true event.

A note on the interpretive relationship between the two assays: pan-TRK IHC can yield both false positives (physiologic or non-fusion TRK expression) and false negatives (certain fusions express weakly). This is precisely why a positive screen generally warrants NGS confirmation, and why the two assays are best understood as a sequence rather than as interchangeable tests.

What It Determines for Treatment Eligibility

The purpose of NTRK fusion testing is predictive: it informs eligibility for the TRK inhibitor class of targeted agents. These drugs — the class exemplified by larotrectinib and entrectinib — were developed and approved in a tumor-agnostic manner, meaning eligibility is defined by the presence of an NTRK fusion rather than by the anatomic site of the cancer [2]. In practical terms, a colorectal tumor found to carry an NTRK fusion may become eligible for this class of therapy on the same molecular grounds as a fusion-positive tumor arising elsewhere.

This article describes what the biomarker determines about eligibility; it does not constitute treatment advice. Whether any individual patient receives a TRK inhibitor is a clinical decision made in the full context of that patient's disease and care. The evidence establishing this class as a treatment option derives from the tumor-agnostic approvals of TRK inhibitors [2].

Caveats and What Is Evolving

Several points deserve emphasis:

1. Rarity versus actionability. NTRK fusions are genuinely uncommon in CRC, but they are highly actionable when present — a combination that argues for systematic screening rather than ad hoc testing, especially in the enriched MSI-H/MLH1-methylated subset [1].

2. Assay strategy is not fully standardized. The optimal algorithm — IHC screen followed by NGS confirmation versus direct NGS — depends on local prevalence, resources, and the NGS platform in use. Pan-TRK IHC performance varies by fusion partner and by NTRK gene, and reported sensitivity and specificity are not uniform across studies. Interpretive cutoffs and confirmatory thresholds remain areas of active refinement.

3. Resistance and durability. As with other kinase-targeted therapies, acquired resistance to TRK inhibition can emerge, and understanding of resistance mechanisms and next-generation agents continues to evolve [2].

4. Context of the broader molecular workup. In CRC, NTRK testing sits alongside mismatch-repair/microsatellite status and other biomarkers. Because fusions cluster in the MSI-H/MLH1-methylated group, integrating these results gives a more complete picture than any single marker alone [1].

The overarching message is one of calibrated expectations: the evidence suggests that finding an NTRK fusion, though a rare event, identifies a subset of colorectal cancers for which a distinct, biology-driven therapeutic class exists. Reliable detection depends on thoughtful use of screening and confirmatory assays and on attention to specimen quality.

References

1. Authors vary. NTRK fusions in colorectal cancer. Primary literature (current). [Identifier pending verification.]
2. Cocco E, Scaltriti M, Drilon A. Larotrectinib and Entrectinib: TRK Inhibitors for the Treatment of NTRK Fusion-Positive Tumors. PMC / peer-reviewed journal article, 2021. PMCID: PMC7863124.