The three receptor tyrosine kinases c-KIT, VEGFR2 and PDGFRα, closely spaced at 4q12, show increased protein expression in triple-negative breast cancer

Abstract

Background: Triple-negative breast cancer (TNBC) is a heterogeneous subgroup of breast cancer with poor prognosis and no targeted therapy available. Receptor tyrosine kinases (RTKs) are emerging targets in anticancer therapy and many RTK-inhibiting drugs are currently being developed. The aim of this study was to elucidate if there is a correlation between the protein expression of three RTKs c-KIT, VEGFR2 and PDGFRα, their gene copy number, and prognosis in TNBC compared to non-TNBC.

Methods: Tumor tissue samples from patients diagnosed with primary breast cancer were stained with immunohistochemistry (IHC) for protein assessment, and with fluorescence in situ hybridization (FISH) for gene copy number determination. Breast cancer mortality (BCM), measured from the date of surgery to death, was used as endpoint.

Results: The cohort included 464 patients, out of which 34 (7.3%) had a TNBC. High expression of the three RTKs was more common in TNBC compared to non-TNBC: c-KIT 49% vs. 10% (P<0.001), PDGFRα 32% vs. 19% (P = 0.07) and VEGFR2 32% vs. 6% (P<0.001). The odds ratio (OR) of c-KIT, VEGFR2 and PDGFRα positivity, adjusted for tumor characteristics, was 6.8, 3.6 and 1.3 times higher for TNBC than for non-TNBC. 73.5% of the TNBC had high expression of at least one of the three investigated receptors, compared to 30.0% of the non-TNBC (P<0.001). Survival analysis showed no significant difference in BCM for TNBC patients with high vs. low c-KIT, PDGFRα or VEGFR2 protein expression. 193 (42%) tumors were evaluated with FISH. No correlation was seen between increased gene copy number and TNBC, or between increased gene copy number and high protein expression of the RTK.

Conclusion: c-KIT, VEGFR2 and PDGFRα show higher protein expression in TNBC compared to non-TNBC. Further investigation clarifying the importance of these RTKs in TNBC is encouraged, as they are possible targets for anticancer therapy.

Figure 1. Flow-chart of the patient cohort included in this study.

^aThese patients were excluded because they did not meet the inclusion criteria (e. g. patients with local breast cancer recurrence, bilateral breast cancer, no breast cancer or too few cancer cells in tissue samples). ^bTNBC = Triple-negative breast cancer.

Figure 2. The left panels show examples of positive immunohistochemical staining with strong intensity for c-KIT (A), VEGFR2 (C) and PDGFRα (E).

Negative controls are shown to the right, c-KIT (B), VEGFR2 (D) and PDGFRα (F). Original magnification ×40.

Figure 3. Examples of FISH. A shows a normal cell with two copies of each gene (green = c-KIT, yellow/gold = VEGFR2 and red = PDGFRα), and two centromeres (blue).

B shows a cell with a gene and centromere copy number gain. Original magnification ×63.

Figure 4. Breast cancer mortality (BCM) stratified for positive vs negative status of c-KIT (A, B and C), VEGFR2 (D, E and F) and PDGFRα (G, H and I) for all (A, D, G), non triple-negative (B, E, H) and triple-negative patients (C, F, I).