Trastuzumab effects depend on HER2 phosphorylation in HER2-negative breast cancer cell lines

Area of Science:

• Oncology
• Molecular Biology
• Biochemistry

Background:

• Human Epidermal Receptor 2 (HER2) is a key biomarker in breast cancer (BC), with overexpression found in 25% of cases.
• HER2-targeting therapies like trastuzumab are typically reserved for HER2-positive BC.
• Emerging evidence suggests some HER2-negative BC patients may benefit from trastuzumab, possibly due to HER2 activation.

Purpose of the Study:

• To investigate the effect of trastuzumab on HER2 phosphorylation at tyrosine Y877 (pHER2Y877).
• To evaluate the prevalence of pHER2Y877 in a diverse cohort of BC patients and cell lines.
• To determine if pHER2Y877 status influences trastuzumab efficacy in HER2-negative BC.

Main Methods:

• Immunohistochemistry was used to assess HER2 and pHER2Y877 status in a cohort of 497 BC patients and 7 BC cell lines.
• Cellular proliferation assays were conducted on BC cell lines with varying HER2 and pHER2Y877 expression, treated with trastuzumab.
• Analysis included tissue microarrays and dose-response studies of trastuzumab.

Main Results:

• The study found pHER2Y877 in 6% of the overall BC cohort.
• Approximately 5% of HER2-negative BC patients (n=406) and 7.7% of triple-negative BC patients (n=39) showed pHER2Y877 overexpression.
• Trastuzumab significantly reduced proliferation in HER2-/pHER2Y877+ cell lines, similar to HER2+ cell lines, but not in HER2-/pHER2Y877- cell lines.

Conclusions:

• HER2 tyrosine 877 phosphorylation is a specific indicator of trastuzumab sensitivity in HER2-negative BC.
• pHER2Y877 positivity in HER2-negative tumors suggests potential benefit from trastuzumab therapy.
• Further validation in larger cohorts could establish pHER2Y877 as a predictive biomarker for expanding trastuzumab use in BC treatment.