Prediction of response to anti-HER2 therapy using a multigene assay

HER2-positive breast cancer (BC), which constitutes 13-15% of cases, shows variable response to anti-HER2 therapies. HER2-positivity, defined as protein overexpression (immunohistochemistry (IHC) score 3+) or equivocal expression (IHC 2+) with evidence of HER2 gene amplification, determines the eligibility to anti-HER2 therapy. MammaTyper® assay (Cerca Biotech GmbH) is a RT-qPCR BC subtyping platform based on the mRNA expression of ERBB2, ESR1, PGR, and MKI67. This study aims to evaluate the accuracy of the MammaTyper® assay in predicting the response of HER2-positive patients to therapy. A well-characterized HER2-positive BC cohort of 287 cases diagnosed at Nottingham University hospitals between 2006 and 2018 was included. The cohort was divided into 2 groups: a trastuzumab-treated group (n=159) and a chemotherapy-only treated group (n=128). Tumor clinicopathologic characteristics were matched between the two groups. Cases with discordant HER2 status were validated through staining of surgical excision specimens. ERBB2 mRNA identified 251/287 (87.5%) cases as HER2-positive, 10.8% (31/287) as HER2 low and 1.7% (5/287) as HER2-negative. According to MammaTyper® assay, ERBB2-positive patients treated with anti-HER2 therapy had significantly prolonged 5-year disease (DFS) and distant metastasis (DMFS) free survival (HR=0.56, p=0.003 and HR=0.62, p=0.023, respectively). MammaTyper®-defined HER2-Enriched subtype showed better response to anti-HER2 therapy compared to IHC-defined subtypes, with significant differences in both 5-year DFS and BCSS (p=0.01 and <0.001, respectively). ERBB2-negative patients did not show survival difference between the group of patients who were treated with trastuzumab and those who were treated with chemotherapy only (p>0.05). Validation analysis revealed that 11/36 ERBB2-negative cases were IHC 2+/ISH positive with very low level of gene amplification and 25 cases were false classified as HER2 positive using current protocols. Combining MammaTyper® assay with IHC to assess HER2 status improves the identification of HER2-positive BC patients who would benefit from anti-HER2 therapy.