Clinicopathological characteristics of Japanese patients with breast cancer and MET exon 14 skipping alterations.

Aims: In non-small cell lung cancer, alterations in mesenchymal-epithelial transition (MET) have been recognized as novel therapeutic targets. In particular, the MET exon 14 skipping mutation (METex14s) is a rare oncogenic driver. Targeted therapy with MET tyrosine kinase inhibitors has recently been approved for this mutation. However, c-MET expression and METex14s frequency and their clinicopathological effects in Japanese patients with breast cancer (BC) remain unknown.

Methods: Tissue microarray-based immunohistochemistry (IHC) was performed to measure c-MET expression in 930 patients with BC (808 with invasive and 122 with noninvasive BC). Reverse transcription polymerase chain reaction was performed to analyse METex14s in patients exhibiting c-MET expression. Clinicopathological characteristics, including patient prognosis based on c-MET expression and METex14s, were elucidated.

Results: IHC staining revealed c-MET expression in 91/930 (9.7%) patients. Notably, IHC expression was frequently observed in apocrine carcinomas (11/26 cases). Among the c-MET IHC-positive cases, METex14s frequency was 25.9% (14/54 cases) in invasive BC and 54.1% (20/37 cases) in noninvasive BC. Furthermore, 4/11 informative noninvasive and invasive BC cases with apocrine differentiation carried METex14s. The nuclear grade was significantly higher in the METex14s-positive group among invasive BC with c-met IHC expression. Furthermore, patients' age and negative rate for PgR IHC was significantly lower in the METex14s-positive group among noninvasive BC. Regarding the factors associated with patient outcomes, both c-MET IHC staining and METex14s expression did not affect survival, regardless of the hormone receptor status.

Conclusion: High c-MET expression and METex14s are common in apocrine carcinoma.