Correlation of HER2 Protein Level With mRNA Level Quantified by RNAscope in Breast Cancer

Area of Science:

• Oncology
• Molecular Diagnostics
• Biomarker Research

Background:

• Trastuzumab deruxtecan (T-DXd) is approved for HER2-positive and HER2-low metastatic breast cancer.
• Current HER2 immunohistochemical (IHC) assays struggle to accurately assess HER2-low breast cancers for T-DXd eligibility.
• Improved methods are needed to identify patients who will benefit from T-DXd therapy.

Purpose of the Study:

• To evaluate RNAscope as a novel assay for quantifying HER2 expression in breast cancer.
• To compare RNAscope results with existing HER2 IHC assays (PATHWAY and HercepTest).
• To correlate HER2 expression levels measured by RNAscope with patient response to T-DXd treatment.

Main Methods:

• HER2 expression was assessed in 526 breast cancer tissue microarray cores using PATHWAY and HercepTest IHC assays.
• RNAscope was used to measure HER2 RNA levels in the same cores.
• HER2 protein levels were quantified using immunofluorescence and mass spectrometry in a subset of cores.
• RNAscope was performed on metastatic biopsies from 23 patients treated with T-DXd, correlating results with response rates.

Main Results:

• HER2 RNA levels by RNAscope strongly correlated with HER2 protein levels and IHC H-scores (P < .0001).
• No significant difference in protein or RNA levels was observed for IHC scores of 0, ultralow, and 1+.
• Significantly higher RNA levels were found in T-DXd responders (6.4 dots/cell) compared to non-responders (2.6 dots/cell) (P = .030).

Conclusions:

• RNAscope is a reliable and objective assay for quantifying HER2 expression in breast cancer.
• RNAscope demonstrates potential as an alternative to current IHC assays, particularly for HER2-low breast cancer.
• This assay may improve patient selection for trastuzumab deruxtecan (T-DXd) therapy.