Molecular mechanisms of secondary imatinib resistance in patients with gastrointestinal stromal tumors

Abstract

Most gastrointestinal stromal tumor (GIST) patients respond to KIT inhibition therapy of imatinib, but eventually become resistant with a median time to progression of 2 years. The mechanism of acquired resistance to imatinib and oncogenic KIT signal transduction in GISTs has not been well defined. We sought to investigate the spectrum of molecular and genomic changes in imatinib-resistant GIST patients. KIT and PDGFRA mutations were evaluated in 48 samples obtained from 32 GIST patients who underwent surgery after imatinib treatment. KIT downstream signaling profiles were also investigated in eight specimens of five patients who were clinically responsive or resistant to imatinib therapy. Biochemical inhibition of KIT, mitogen-activated protein kinase (MAPK), mammalian target of rapamycin (MTOR), AKT, proliferating cell nuclear antigen (PCNA) and BCL-2 were determined by western blotting for protein activation. In all 32 GIST patients, activating mutations in the KIT gene were seen in 26 (81.3%) patients, PDGFRA gene mutations were seen in 2 (6.2%) patients and no primary mutations were found in 4 (12.5%) patients. Secondary KIT mutations were identified in 11/14 (78.6%) imatinib-acquired-resistance patients, with nine patients in KIT gene exon17, and the other two in exon 13. The expressions of p-KIT, p-AKT, PCNA and BCL-2 were higher in the samples of imatinib-resistant GISTs than those of imatinib-responsive ones. P-KIT, p-AKT expressions were higher in imatinib acquired-resistance GISTs with secondary KIT mutations than imatinib-responsive ones with primary mutation. Total KIT, MAPK, p-MAPK, p-MTOR expressions were comparable in all varied GISTs. Novel additional mutations of KIT gene exon 13 or exon 17 indicate the likely mechanism of secondary resistance to imatinib. The PI3-K/AKT pathway might be more relevant than MEK/MAPK for therapeutic targeting in imatinib-resistant GIST patients with secondary mutation.