Top-level MET gene copy number gain defines a subtype of poorly differentiated pulmonary adenocarcinomas with poor prognosis

Abstract

Background: MET amplifications occur in human tumors, including non-small cell lung cancer (NSCLC). MET inhibitors have demonstrated some clinical activity in MET amplified NSCLC, presumably with a gene dose effect. However, the definition of MET positivity or MET amplification as a potential oncogenic driver is still under debate. In this study, we aimed to establish the molecular subgroup of NSCLC with the highest unequivocal MET amplification level and to describe the prevalence, and histologic and clinical phenotype of this subgroup.

Methods: A total of 373 unselected patients with NSCLC were consecutively tested for MET gene copy number (GCN) by FISH. Mean GCN, MET/CEN7 ratio and other FISH parameters were identified and correlated with morphological and molecular pathological characteristics of the tumors as well as with clinical data.

Results: Based on the variability of obtained data a top-level category of MET amplification was newly defined (>90th percentile of average GCN; ≥10 MET gene copies per tumor cell). This criterion was fulfilled in 2% of analyzed tumors. These tumors were exclusively poorly differentiated adenocarcinomas with a predominant solid subtype and pleomorphic features. Rarely, co-alterations were detected (KRAS mutation or MET exon 14 skipping mutation). In this top-level group, there were no EGFR mutations or ALK or ROS1 alterations. The most important clinical feature was a significantly shortened overall survival (HR 3.61; median OS 8.2 vs. 23.6 months). Worse prognosis did not depend on initial stage or treatment.

Conclusions: The newly defined top-level category of MET amplification in NSCLC defines a specific subgroup of pulmonary adenocarcinoma with adverse prognosis and characteristic morphological features. Lower levels of MET gene copy number seem to have probably no specific value as a prognostic or predictive biomarker.

Keywords: Mesenchymal-epithelial transition receptor (MET); amplification; fluorescence in situ hybridization (FISH); lung cancer; non-small cell lung cancer (NSCLC).

Figure 1

Overall survival analysis. (A) Comparing NSCLC patients of all stages at the time of initial diagnosis, top-level MET copy number gain (level 4, n=8) was associated with a significantly shorter overall survival (HR 3.61; median OS 8.2 vs. 23.6 months for MET Level 0–3, n=363). (B) Notably, this difference was also significant if MET levels are split up. There was no significant survival difference between level 3 (n=27) and lower MET amplification levels (level 1, n=84; level 2, n=22) or MET negative patients (level 0, n=230). This underlines the specific prognostic value of MET level 4 (top-level gain; defined by ≥10 gene copies per cell) over former definitions of high-level MET amplification which are included in level 3. (C) Comparison of OS for the subgroup of stage IV patients who did not receive anti-MET tyrosine kinase inhibitors (TKI) (n=182); median OS of MET level 4 patients (n=4) was significantly shorter than for level 0–3 patients (n=178; 3.5 vs. 14.1 months). (D) This statistical significance disappears if patients are included who were treated with anti-MET-TKI (see Figure 2 for details). Median OS of level 4 patients (n=5) was 5.1 months vs. 14.1 for level 0–3 (n=182).

Figure 2

Individual swimmer plots for NSCLC patients with MET amplification levels 3 and 4 (high and top-level copy number gain). Maximum survival for level 4 patients was 13.3 months from initial diagnosis, compared to 73.8 months in the group of level 3 patients. MET TKI treatment contributed to a prolonged survival in one patient with top-level gain (31.5 months). Each bar represents one patient with individual progress of disease since initial diagnosis; the color of the bars indicates the therapy applied; for description of colors and symbols used in this figure see legend; Level 4 patients listed in red; ¹, patients with MET mutation. OS, overall survival.

Figure 3

Morphologic phenotype of MET top-level amplified lung cancers. (A,B,C) All tumors were adenocarcinomas with minor features of pleomorphic carcinomas (descriptively referred to as adenocarcinomas with pleomorphic features) (A: HE, ×400; B: HE, ×400; C: HE, ×200). Histologically, solid growth pattern predominates. However, also gland forming structures were seen (arrows). Cases showed interspersed enlarged “pleomorphic” tumor cells (B, C, arrowheads) which were aggregated in focal tumor areas in some cases (C, bottom). (D) TTF1 was absent or weakly and unevenly expressed. All samples expressed strongly cytokeratin 7 but were negative for p40 and neuroendocrine markers (not shown) (D, TTF1 immunohistochemistry, ×400). (E) Fluorescence in situ hybridization reveals more than 10 MET gene copies on average per tumor cell (orange: centromere 7) (MET FISH, ×630).