Evaluation of cMET aberration by immunohistochemistry and fluorescence in situ hybridization (FISH) in triple negative breast cancers

Purpose: To evaluate the incidence of cMET proto-oncogene aberration in a cohort of triple negative breast cancers using immunohistochemistry and fluorescence in situ hybridization (FISH) methods and correlated with patient outcome.

Patients and methods: One hundred and six female patients with diagnosis of triple negative invasive breast carcinoma at The University of Texas-M D Anderson Cancer Center from 1983 to 2009 were included in the study. Expression of cMET was assessed by IHC using rabbit monoclonal anti-total cMET antibody (SP44 from Ventana). Staining intensity was scored on a scale of 0, 1+, 2+ and 3+. cMET overexpression was defined as at least moderate membranous/cytoplasmic staining in ≥50% of tumor cells (score ≥ 2+). FISH analysis was performed using MET (7q31) specific probe (BAC clone RP11-95i20, Abbott Molecular Inc.) and the centromere probe (CEP7/D7Z1, Abbott Molecular Inc.) as internal control. cMET amplification was defined as gene copy numbers ≥4 per cell or cMET/CEP7 ratio ≥ 2. cMET status was tested for correlation using Fisher's exact test with other clinicopathological parameters. The Kaplan-Meier product limit method was used to estimate the survival outcomes. Cox proportional hazards models were fit to determine the association of cMET status by IHC, or by FISH, or by copy number with survival outcomes after adjustment for other patient and disease characteristics.

Results: Medium follow up is 69.4 months (range 9-317 months). cMET was successfully evaluated by both IHC and FISH methods in ninety-six patients. There were 13 patients whose tumors overexpressed cMET was by IHC. Two patients had cMET amplification by FISH using definitive of cMET/CEP7 ratio of ≥2 and four patients had cMET copy number >4. Only one patient showed cMET/CEP7 ratio of 2.53 and one was positive for cMET overexpression by IHC. No significant association between cMET overexpression by IHC and by FISH using cut-off of with either cMET/CEP7 ratio of ≥2 or cMET copy number of >4 (P = 1.0). There was no significant correlation between the cMET overexpression and other clinicopathological characteristics, such as patient demographics, tumor grade, stage, or chemotherapy treatment history. cMET overexpression and gene amplification did not correlate with the prognosis of TNBC regarding OS or DFS.

Conclusion: MET amplification is a rare incidence in TNBCs. cMET overexpression is infrequent in TNBCs and may not be driven by gene amplification. Neither have significant prognostic value nor do they correlate with other clinicopathological characteristics in this TNBC cohort.