Purpose: We have previously shown the presence of an activated platelet-derived growth factor (PDGF) receptor (PDGFR) B and its ligand PDGFB in a limited number of patients with clinical and radiological responses to imatinib mesylate treatment. This article describes the results of comprehensive molecular/biochemical analyses of the three receptors targeted by the drug (PDGFRB, PDGFRA, and KIT) in a series of 31 chordoma patients.
Experimental design: The presence and activation status of PDGFRB, PDGFRA, and KIT receptors were investigated by means of immunoprecipitation and Western blot analyses complemented by immunohistochemistry, their expression level was analyzed by means of real-time PCR, and the occurrence of activating point mutations was investigated by means of cDNA sequencing. The PDGFB, PDGFA, and stem cell factor cognate ligands were investigated by reverse transcription-PCR, and gene status was assessed by fluorescence in situ hybridization.
Results: The results show that PDGFRB was highly expressed and phosphorylated, whereas PDGFRA and KIT were less expressed but phosphorylated and thus activated. These findings, together with the absence of gain-of-function mutations and the presence of the cognate ligands, strongly support the hypothesis that the activation mechanism is the autocrine/paracrine loop. No role seems to be played by gene amplification.
Conclusions: In the light of our findings, the clinical benefit observed in chordoma patients treated with imatinib seems to be attributable to the switching off of all three receptors.