c-Kit/PDGFRA gene status alterations possibly related to primary imatinib resistance in gastrointestinal stromal tumors.

Purpose: To correlate morphologic changes with molecular, biochemical, and cytogenetic profiles in gastrointestinal stromal tumor (GIST) patients before and after imatinib treatment. Experimental Design: We investigated 132 tumor samples obtained from 35 patients with advanced disease who underwent resective surgery after imatinib treatment according to the European Organization for Research and Treatment of Cancer-Soft Tissue and Bone Sarcoma Group protocol. On the basis of imaging findings, 27 patients were responders and 8 progressors, and retaining this radiological subdivision, we analyzed posttreatment morphologic changes correlating them with molecular, biochemical, and cytogenetic analyses. Results: On the basis of morphology (residual viable cellularity/proliferation markers), three subgroups were identified showing high, moderate, or low response. All of the progressing cases clustered in the low-response subgroup, whereas the responding cases were distributed in all three subgroups. The correlation between morphology and the molecular findings showed that secondary mutations segregated with the low-response subgroup, whereas c- Kit primary resistance mutations were randomly distributed in the three subgroups. Fluorescence in situ hybridization analysis of c- Kit/PDGFRA genes showed that all of the progressing cases were disomic. Referring to morphology, among the responding cases, a disomic pattern was mainly restricted to the high responders, whereas the moderate and low responders were aneusomic. Comparison of post-imatinib genomic profiles with the 23 available primary tumors showed that 17 cases carried the same cytogenetic pattern. Overall, 12 of the 27 primary tumors presented a gain/loss of c- Kit/PDGFRA gene copy number. Conclusions: Our findings show that c- Kit/PDGFRA genomic alterations were present at disease onset in 1/3 of the examined cases. They therefore represent an early event possibly related to primary imatinib resistance in GISTs.