Evolution of Precision Medicine in HER2-Positive Breast Cancer
The management of HER2-positive (HER2+) breast cancer has undergone a radical transformation over the last two decades. Once associated with a poor prognosis, the advent of HER2-targeted therapies has significantly improved survival outcomes. However, HER2+ breast cancer remains a highly heterogeneous disease. While some patients achieve a pathologic complete response (pCR) with standard trastuzumab-based regimens, others require more intensive treatment or experience early relapse. Traditional biomarkers, such as hormone receptor (HR) status, tumor grade, and the Ki67 proliferation index, provide valuable but incomplete information regarding the biological behavior of these tumors.
The Emergence of HER2DX
To address this clinical gap, the HER2DX genomic test was developed and validated. As a specialized 27-gene expression assay, HER2DX provides three distinct scores: a relapse risk score, a likelihood of pCR score, and an ERBB2 mRNA expression score. While its prognostic and predictive value has been established in clinical trials, understanding how these genomic scores correlate with traditional histopathologic features in a real-world setting is crucial for clinical integration. A recent study published in Clinical Cancer Research provides a deep dive into these associations, reinforcing the test’s role as a cornerstone of personalized oncology.
Study Design and Methodology
The researchers conducted a multi-institutional study in Spain, analyzing 410 patients with newly diagnosed stage I-III HER2+ breast cancer between January 2022 and June 2025. The primary objective was to evaluate the association between HER2DX scores and a comprehensive suite of histopathologic features. These features included tumor grade, HR status, histological subtype, Ki67 index, HER2 immunohistochemistry (IHC) score, and detailed immune microenvironment markers such as stromal tumor-infiltrating lymphocytes (TILs), tertiary lymphoid structures (TLS), and spatial immune distribution.
Evaluating Neoadjuvant Outcomes
Of the total cohort, 250 patients underwent neoadjuvant trastuzumab-based therapy and had available surgical outcome data. The study employed univariate and multivariable logistic regression analyses to determine which factors—genomic or histopathologic—were the most reliable predictors of pCR. This rigorous approach allowed the investigators to isolate the unique contribution of the HER2DX pCR score relative to established clinical markers.
Key Findings: A Bridge Between Genomics and Histopathology
The results of the study underscore a high degree of concordance between HER2DX and the biological reality of the tumor. The HER2DX pCR score demonstrated a significant association with all eight histopathologic features analyzed. In contrast, the relapse risk score was associated with five features, and the ERBB2 mRNA score showed associations with only two.
Correlation with Immune and Proliferative Markers
One of the most compelling findings was the strong correlation between specific genomic signatures and phenotypic markers. The study found a correlation coefficient of 0.59 between stromal TILs and the HER2DX immune/immunoglobulin (IGG) signature. This suggests that the IGG signature is a precise molecular mirror of the tumor’s immune infiltration, particularly B-cell and plasma cell activity. Furthermore, the Ki67 index, a classic marker of cell proliferation, correlated significantly (r=0.50) with the HER2DX proliferation signature. These correlations provide biological plausibility for the test, confirming that it captures the essential hallmarks of tumor growth and immune engagement.
Predictive Power for Pathologic Complete Response
In the neoadjuvant subgroup, 36.0% of patients achieved a pCR. When the researchers performed a multivariable analysis—adjusting for factors such as HR status, TILs, and tumor size—the HER2DX pCR score emerged as the only independent predictor of response. With an odds ratio (OR) of 1.77 (95% CI 1.08-2.97, p=0.030), the genomic score provided predictive information that surpassed traditional histopathologic assessment. This indicates that the molecular nuances captured by the 27-gene assay are more indicative of treatment sensitivity than visual histopathologic markers alone.
Expert Commentary: Shifting the Paradigm
The findings from Sanfeliu and colleagues represent a significant step forward in the clinical utility of genomic testing for HER2+ breast cancer. Historically, genomic assays like Oncotype DX or MammaPrint have been standard in HR+/HER2- disease, but HER2+ management has relied heavily on IHC and FISH.
The Role of the Immune Microenvironment
The strong association between the IGG signature and TILs/TLS highlights the importance of the immune system in the efficacy of HER2-targeted therapy. Tumors with a ‘hot’ immune microenvironment are more likely to respond to trastuzumab and its derivatives. By quantifying this through the HER2DX score, clinicians can better identify patients who might be candidates for treatment de-escalation—potentially avoiding the toxicity of multi-agent chemotherapy while maintaining excellent outcomes.
Addressing Clinical Heterogeneity
The fact that HER2DX was the only independent predictor of pCR in multivariable analysis is particularly noteworthy. It suggests that even within the ‘HER2-positive’ category, there is a wide spectrum of ERBB2 expression and biological drive. Traditional IHC (2+ vs 3+) can be subjective and may not fully reflect the ERBB2 mRNA levels that drive tumor dependency on the HER2 pathway. HER2DX provides a standardized, objective measurement that refines our understanding of this dependency.
Study Limitations and Considerations
While the results are robust, the study authors and clinical experts note certain limitations. The analysis was conducted within a specific geographic population (Spain), and while the results are likely generalizable, continued validation in diverse global cohorts is beneficial. Additionally, the study focused on trastuzumab-based therapy; as newer agents like trastuzumab deruxtecan (T-DXd) and tucatinib move into earlier lines of therapy, the predictive value of HER2DX for these specific agents will require ongoing investigation.
Conclusion: Practical Implications for Clinicians
The study by Sanfeliu et al. confirms that the HER2DX genomic test is not merely a redundant measure of what can be seen under a microscope. Instead, it is a sophisticated integration of tumor proliferation, immune response, and HER2 signaling that provides superior predictive accuracy for pCR.For clinicians, these data support the integration of HER2DX into routine practice to assist in:
1. Risk Stratification
Identifying high-risk patients who may require treatment intensification (e.g., adding pertuzumab or switching to T-DM1 post-surgery if pCR is not achieved).
2. Treatment De-escalation
Identifying low-risk, highly responsive patients who may achieve excellent outcomes with less aggressive, less toxic regimens.
3. Enhanced Biological Understanding
Providing a clearer picture of the tumor’s immune landscape, which may guide future immunotherapy combinations.In summary, HER2DX bridges the gap between traditional pathology and modern genomics, offering a more precise map for navigating the complexities of HER2-positive breast cancer treatment.
References
Sanfeliu E, Martínez-Romero A, Marín-Aguilera M, et al. Associations of the HER2DX Genomic Test with Biological and Pathological Features in HER2-positive Breast Cancer. Clin Cancer Res. 2025 Dec 1. doi: 10.1158/1078-0432.CCR-25-3123. Epub ahead of print. PMID: 41324567.
