TMPRSS2::ERG Fusion and ERG Immunohistochemistry in Prostate Cancer

Introduction

Among the recurrent molecular events in prostatic adenocarcinoma, the fusion of the androgen-regulated gene TMPRSS2 with the ETS-family transcription factor ERG is the most common. This gene rearrangement is present in roughly half of prostate cancers and has become a widely studied biomarker, both as a lineage-defining feature of neoplastic prostatic epithelium and as a subject of prognostic investigation [1,2]. This article reviews what the assay measures, how it is performed and interpreted, what each result means, and where the evidence remains unsettled — framed for a mixed audience of pathologists, trainees, clinicians, and informed patients.

What the Test Measures

TMPRSS2 encodes a transmembrane serine protease whose expression is driven by androgen signaling — a physiologically relevant detail in the prostate, where androgen receptor activity is central. When a chromosomal rearrangement juxtaposes the androgen-responsive promoter/regulatory region of TMPRSS2 upstream of the ERG coding sequence, the result is aberrant, androgen-driven overexpression of the ERG transcription factor in tumor cells that would not normally express it. This overexpressed ERG protein is the basis for detection by immunohistochemistry (IHC) [2].

Because the fusion drives ERG protein to levels detectable by antibody-based staining, ERG IHC serves as a practical surrogate for the underlying genetic rearrangement. The concordance between ERG protein expression by IHC and the presence of the fusion by molecular methods is high enough that IHC is used as a convenient stand-in in routine practice [2].

How It Is Tested

Specimen and preanalytics. The assay is performed on formalin-fixed, paraffin-embedded (FFPE) tissue — the standard material generated from prostate biopsies and radical prostatectomy specimens. As with all FFPE-based immunohistochemistry, adequate and consistent fixation is important; suboptimal preanalytic handling can degrade antigenicity and complicate interpretation.

Assay platforms. Two complementary approaches are recognized:

• ERG immunohistochemistry (IHC): an antibody-based stain that detects ERG protein overexpression in the nuclei of tumor cells. This is the pragmatic front-line method because it is inexpensive, widely available, and directly visualized on tissue architecture [2].
• FISH or NGS: fluorescence in situ hybridization or next-generation sequencing detects the TMPRSS2::ERG rearrangement directly at the DNA/RNA level. These molecular methods confirm the fusion itself rather than its protein surrogate.

Scoring. ERG IHC is interpreted as nuclear expression in tumor epithelium, read as a surrogate for the presence of the fusion. Because ERG is normally expressed in endothelial (vascular) cells, those cells provide a convenient built-in internal positive control on every slide — a useful quality-assurance feature. Approximately 50% of prostate cancers are ERG-positive, reflecting the prevalence of the fusion [1,2].

What Each Result State Means

ERG positive (fusion). Nuclear ERG expression in prostatic epithelial cells indicates the presence of the TMPRSS2::ERG fusion. In diagnostic terms, ERG positivity in atypical glands supports a determination that the epithelium is neoplastic (of prostatic origin), because benign prostatic glands do not express ERG. This makes ERG a useful confirmatory, lineage-related marker when the differential includes benign mimics [2].

ERG negative. Absence of ERG staining means the fusion is not detected by this surrogate. Importantly, a negative result does not exclude prostate cancer: roughly half of prostatic adenocarcinomas lack this fusion and are driven by other molecular events. ERG negativity is therefore non-informative for the diagnosis of malignancy on its own and must be interpreted in the full morphologic and clinical context [1,2].

What It Determines for Treatment Eligibility

It is important to be direct about this point. TMPRSS2::ERG / ERG IHC is currently a lineage and diagnostic marker and is biologically defining but not therapy-gating. ERG positivity confirms neoplastic prostatic epithelium; it does not, in current practice, select patients for a specific drug class. In other words, unlike predictive biomarkers that inform eligibility for a defined therapeutic class, ERG status is not used to determine access to any particular treatment. Any future therapeutic linkage would require validation beyond the biological and molecular pathology evidence that supports its diagnostic use today.

For patients reading this: the presence or absence of the ERG fusion is a feature of the tumor's biology and its diagnosis, not an instruction about therapy. Treatment decisions rest with the treating clinical team and integrate many factors.

Caveats and What Is Evolving

Several points warrant epistemic caution:

• Prognostic role remains under study. The fusion is biologically defining, and its prognostic significance has been investigated extensively in the molecular pathology literature; however, the evidence for using ERG status as an independent prognostic determinant in routine practice is not settled. It should be interpreted as one biological feature among many, not a standalone prognostic verdict.
• IHC is a surrogate, not the fusion itself. ERG protein expression stands in for the genetic event. Discordant cases can occur, and molecular confirmation by FISH or NGS may be warranted in selected diagnostic scenarios [2].
• Negativity has limited meaning. Because only about half of prostate cancers carry the fusion, ERG-negative status cannot be used to argue against malignancy [1,2].
• Internal controls matter. Endothelial ERG expression aids interpretation, but staining quality is preanalytically dependent, reinforcing the need for standardized fixation and validated protocols.

Summary

ERG IHC provides a rapid, tissue-based readout of the TMPRSS2::ERG fusion present in approximately half of prostate cancers. Its principal established value is diagnostic and lineage-confirming: ERG positivity supports a neoplastic prostatic epithelial origin, while negativity is non-exclusionary. Molecular assays (FISH/NGS) confirm the fusion directly. The biomarker is biologically defining and is not, at present, used to gate therapy.

References

1. WHO Classification of Tumours Editorial Board. Urinary and Male Genital Tumours, 5th ed. IARC, Lyon; 2022. ISBN 978-92-832-4512-4.
2. Tomlins SA et al. Immunohistochemistry for ERG Expression as a Surrogate for TMPRSS2-ERG Fusion Detection in Prostatic Adenocarcinoma. The American Journal of Surgical Pathology; 2012. PMCID: PMC3505676.