Title: FIP1L1-PDGFRalpha, a tyrosine kinase fusion protein involved in chronic eosinophilic leukemia
Description: This dissertation describes the discovery and characterization of FIP1L1-PDGFRA, a tyrosine kinase fusion gene present in a substantial fraction of chronic eosinophilic leukemia (CEL) patients. We identified the FIP1L1-PDGFRA fusion in a CEL patient and showed that it is due to an interstitial deletion on chromosome 4. The FIP1L1-PDGFRα fusion protein consists of an N-terminal portion of FIP1L1 fused to a C-terminal portion of PDGFRα, including the kinase domain. We demonstrated that FIP1L1-PDGFRα is a constitutively active kinase that is sensitive to inhibition by imatinib, a small molecule tyrosine kinase inhibitor that has clinical efficacy in CEL. A PDGFRA T674I mutation was identified in a CEL patient who relapsed while on imatinib therapy; this mutation conferred imatinib resistance in vitro and validated that FIP1L1-PDGFRα is a target of imatinib in CEL. We showed that FIP1L1-PDGFRα causes a myeloproliferative disease in a mouse bone marrow transplant (BMT) assay and used this model to assess the in vivo effectiveness of two kinase inhibitors, AMN107 and PKC412. We showed that FIP1L1-PDGFRα can be inhibited by AMN107 and PKC412 both in vitro and in the in vivo BMT model; PKC412 can also overcome imatinib resistance conferred by the T674I mutation. Finally, we investigated the mechanism of activation of FIP1L1-PDGFRα. Studies of deletions of FIP1L1 showed that FIP1L1 is not required for kinase activity of FIP1L1-PDGFRα in vitro and in vivo. Because the juxtamembrane (JM) domain is consistently disrupted in FIP1L1-PDGFRA fusions in patients, we investigated the role of the JM domain in FIP1L1-PDGFRα activation.

Partial or complete deletion of the PDGFRα JM domain resulted in an active kinase that transformed hematopoietic cells; in contrast, restoring the full length JM domain eliminated kinase activity and transformation, demonstrating that a complete JM domain is inhibitory to PDGFRα activity. Our results show that FIP1L1 is dispensable for FIP1L1-PDGFRα activation, but disruption of the autoinhibitory PDGFRα JM domain by FIP1L1-PDGFRα fusion is a critical event in activation of the fusion kinase. Our studies of FIP1L1-PDGFRα have important biological and clinical implications for CEL and other malignancies caused by aberrant tyrosine kinase activation.