NTRK Gene Fusion in Breast Cancer: A Predictive Biomarker and Its Diagnostic Workflow

Among the biomarkers a breast pathologist may be asked to evaluate, NTRK gene fusion occupies an unusual niche. It's rare in the breast overall, yet it defines a specific tumor type almost completely, and it opens the door to a class of targeted drugs approved regardless of where the cancer arose. That combination — rarity, high enrichment in one histology, and tumor-agnostic therapeutic consequence — makes it worth understanding well, even if you'll encounter it only occasionally.

What the Test Measures

The NTRK1, NTRK2, and NTRK3 genes encode the tropomyosin receptor kinase (TRK) family of proteins: TRKA, TRKB, and TRKC. Normally these are neurotrophin receptors, activated by ligand binding at the cell surface, feeding into growth and survival pathways. Under ordinary circumstances that signaling is tightly regulated.

A gene fusion breaks that regulation. When an NTRK gene is joined to an unrelated partner gene, the resulting chimeric protein carries a constitutively active kinase domain. The fusion drives the tumor. It doesn't need a ligand to do so — the signal is simply always on.

In the breast, the canonical example is secretory carcinoma, which harbors a recurrent ETV6::NTRK3 fusion in the great majority of cases. This is the setting where a pathologist is most likely to consider NTRK testing, and where a positive result would be least surprising. Outside secretory carcinoma, NTRK fusions in breast cancer are genuinely uncommon.

It's worth stressing what kind of biomarker this is. NTRK fusion is predictive, not prognostic. The presence of a fusion doesn't primarily tell you how the disease will behave over time; it tells you whether a druggable target exists.

How It's Tested

Because the fusion is rare in most tumor types but has a therapeutic payoff, the field has settled on a two-step logic: screen broadly, then confirm specifically. The specimen throughout is formalin-fixed, paraffin-embedded (FFPE) tissue, so the usual preanalytic cautions apply — adequate fixation, avoidance of prolonged ischemic time, and enough viable tumor in the block.

Step one is pan-TRK immunohistochemistry. A single antibody detects the C-terminal kinase region shared across TRKA, TRKB, and TRKC. When a fusion places that domain under aberrant expression, the protein accumulates and IHC lights up. It's a fast, inexpensive, widely available screen — an excellent triage tool [3]. But it isn't the final word. Pan-TRK IHC can be positive in tissues with physiologic TRK expression, staining patterns vary by which gene is involved, and it can miss some fusions. Sensitivity and specificity depend on the partner gene and the tumor context. So a positive IHC is a prompt, not a diagnosis.

Step two is molecular confirmation. Next-generation sequencing (NGS), ideally an RNA-based fusion assay, identifies the specific fusion transcript and its partner. Fluorescence in situ hybridization (FISH) is an alternative, particularly useful when you already suspect a defined fusion such as ETV6::NTRK3. Confirmation matters because the therapeutic decision hangs on whether a bona fide fusion is present.

Scoring is binary at the level that counts: fusion present versus absent. IHC is graded for staining, but the actionable answer comes from the confirmatory molecular test.

What Each Result State Means

NTRK fusion present. A confirmed fusion identifies a tumor driven by aberrant TRK kinase signaling. In a breast primary, this finding is strongly associated with secretory carcinoma and the ETV6::NTRK3 fusion, though it can occasionally appear in other contexts. A present result establishes a molecular target.

NTRK fusion absent. No fusion is detected by confirmatory testing. The tumor is not TRK-driven by this mechanism, and TRK-directed therapy would not be expected to help. In practice, a fusion-absent result is by far the more common outcome, especially outside secretory histology. Remember too that a negative confirmatory test after a positive IHC screen simply means the screen was a false alarm — which is exactly what a screen is designed to allow.

What It Determines for Treatment Eligibility

A confirmed NTRK fusion informs eligibility for the TRK inhibitor drug class — larotrectinib and entrectinib being the agents with tumor-agnostic regulatory approval. "Tumor-agnostic" is the key phrase: these approvals rest on the fusion itself rather than the organ of origin, reflecting trial data in which patients with diverse fusion-positive cancers responded [1,2].

The framing here is deliberate. The diagnostic determines whether a patient's tumor falls into the biomarker-defined group for which this drug class is approved. It doesn't dictate any individual's therapy. Whether a given drug is appropriate, in what line, and against what alternatives are clinical judgments made by the treating oncologist, weighing the whole clinical picture. The pathologist's job is to establish, reliably, whether the target is there.

Caveats and What Is Evolving

The most clinically important evolving story is acquired resistance. TRK inhibitors can produce dramatic early responses in fusion-positive disease, but responses aren't always durable. Tumors adapt. One well-recognized escape route is the on-target resistance mutation — a change in the kinase domain (so-called solvent-front and gatekeeper mutations) that reduces drug binding while preserving the driving fusion. Off-target mechanisms, in which the tumor activates a bypass pathway independent of TRK, also occur. This matters diagnostically because a patient who progresses after initial benefit may need re-biopsy and repeat molecular testing to distinguish these scenarios, and next-generation TRK inhibitors are being developed specifically to overcome some on-target mutations. The single up-front fusion test, in other words, may not be the last molecular question this patient generates.

There are practical assay caveats too. Pan-TRK IHC performance isn't uniform — it's less sensitive for certain fusions, notably some involving NTRK3, and background expression in neural or smooth muscle tissue can trip up interpretation. Because of this, a screen-then-confirm strategy shouldn't be shortcut: acting on IHC alone risks both false positives and missed fusions. RNA-based NGS generally detects fusions more reliably than DNA-based approaches, since intronic breakpoints can be large and variable, and this is worth confirming with your laboratory when interpreting a negative result.

Finally, a word on scope. NTRK fusion status is its own axis, entirely separate from receptor-based classification such as ER, PR, and HER2. It doesn't substitute for those assays, and the contested cutoff debates in that space — for instance, how best to define and reproduce HER2-low — belong to a different testing pathway. Keeping these lanes distinct avoids confusion when multiple biomarker results land on the same report.

The trajectory is clear enough. As RNA fusion panels become routine and as resistance-directed agents mature, NTRK testing is likely to shift from a one-time triage question toward a longitudinal one — asked again at progression, not just at diagnosis. For a biomarker this rare, that's a meaningful expansion of its clinical footprint.

---

References

1. Drilon A, Laetsch TW, Kummar S, et al. Efficacy of larotrectinib in TRK fusion-positive cancers in adults and children. N Engl J Med. 2018;378(8):731–739. doi:10.1056/NEJMoa1714448.
2. Doebele RC, Drilon A, Paz-Ares L, et al. Entrectinib in patients with advanced or metastatic NTRK fusion-positive solid tumours: integrated analysis of three phase 1–2 trials. Lancet Oncol. 2020;21(2):271–282. doi:10.1016/S1470-2045(19)30691-6.
3. Hechtman JF, Benayed R, Hyman DM, et al. Pan-TRK immunohistochemistry is an efficient and reliable screen for the detection of NTRK fusions. Am J Surg Pathol. 2017;41(11):1547–1551. doi:10.1097/PAS.0000000000000911.