LMTK3 to Regulate the Translation of Oncogenic KIT in GIST Regardless of Imatinib Sensitivity.

11535 Background: The majority of gastrointestinal stromal tumors (GIST) have been shown to be caused by somatic activating mutations in the receptor tyrosine kinase KIT. The major cause of death in patients with advanced KIT-mutant GIST is due to the development of KIT tyrosine kinase inhibitor-resistant (TKI-resistant) metastatic disease. Drug resistance arises almost exclusively from secondary mutations within KIT, highlighting the importance of KIT in the proliferation and survival of these tumors. Methods: We performed a human kinase siRNA screen in multiple KIT-mutant cancer cell lines, using viability as a read out. We defined candidate targets as those whose knockdown decreased viability in all cell lines. Validation and mechanistic studies were done using a library of KIT-mutant GIST cells. Results: We identified lemur tyrosine kinase 3 (LMTK3) as candidate target in three KIT-mutant cell lines. LMTK3 silencing reduced the viability of all KIT-mutant GIST cells tested to date, including cell lines with KIT TKI-resistance mutations. Importantly, LMTK3 silencing decreased the viability of KIT-mutant cells specifically, but not that of KIT-independent cells. LMTK3 knockdown also reduced tumor growth in vivo in a GIST xenograft model. Further, we found that decreased cell viability after LMTK3 silencing was due to induction of apoptosis. Because these cells depend so heavily on KIT and the loss of KIT signaling results in cell death, we hypothesized that LMTK3 silencing may affect this pathway. Indeed, LMTK3 silencing decreased total KIT protein across all cell lines. The reduction in KIT protein was not the result of changes in KIT transcript or KIT protein stability, but translation rate of KIT was significantly reduced after LMTK3 knockdown. Conclusions: The protein kinase LMTK3 is an important translational regulator of oncogenic KIT expression in KIT-mutant GIST regardless of drug sensitivity and represents a novel, tractable target, particularly in drug-resistant tumors.