KIT and BRAF heterogeneous mutations in gastrointestinal stromal tumors after secondary imatinib resistance

Gastrointestinal stromal tumors (GISTs) are the most common mesenchymal tumor of the digestive tract and characterized by expression of KIT protein. Imatinib is the frontline therapy for metastatic and unresectable GIST patients showing clinical responses in 80 % of cases. Despite the often long-lasting clinical benefit seen in most patients treated with imatinib, many will eventually suffer disease progression. The most frequent mechanism of imatinib resistance in GIST is the acquisition of secondary mutations in either KIT or PDGFRA. There are also some imatinib-resistant GIST patients lacking an identifiable mechanism of treatment failure. Recently, activating BRAF mutation was detected in a small percentage of GISTs. In this study, we report a case of GIST with acquired resistance to imatinib during therapy. Histological, immunohistochemical, Western blot and mutational analyses were performed on GIST tissues before and after imatinib resistance. The imatinib-resistant tumor showed not only heterogeneous mutations of KIT and BRAF besides the primary mutation, but also transdifferentiation into a rhabdomyosarcoma phenotype. According to Western blot analysis, in imatinib-resistant GIST with both KIT V559D and BRAF V600E mutations, the inhibition of KIT V559D by imatinib caused a strong decrease of AKT phosphorylation, while ERK1/2 phosphorylation was not affected. This finding, in combination with the loss of KIT expression, suggests the possibility of activation of RAS-RAF-MEK-ERK pathways driven by a KIT-independent oncogenic mechanism. Understanding the genetic aberrations beyond KIT and PDGFRA may lead to the identification of additional therapeutic targets for GISTs.