Mechanisms of Activation of the FIP1L1-PDGFRα Fusion Kinase.

We recently identified the FIP1L1-PDGFRα fusion protein as a frequent cause of hypereosinophilic syndrome (HES), a disorder characterized by persistent eosinophilia and organ dysfunction. ~60% of HES patients who are sensitive to imatinib, a selective tyrosine kinase inhibitor, harbor the FIP1L1-PDGFRα fusion. The fusion is expressed as a consequence of an interstitial chromosomal deletion of human chromosome 4 that fuses a novel protein FIP1L1 to the cytoplasmic domain of the tyrosine kinase PDGFRα, a type III receptor tyrosine kinase (RTK). The fusion protein is a constitutively active kinase and has transforming properties in vitro and in vivo that are inhibited by imatinib. In order to understand the mechanism of activation of the FIP1L1-PDGFRα fusion kinase, a series of deletions of FIP1L1 were fused to PDGFRα and the fusions were tested for transforming activity. The data suggested that the FIP1L1 moiety was in fact dispensable for PDGFRα activation. For example, substitution of FIP1L1 with the Myc-epitope fused to PDGFRA still resulted in PDGFRα autophosphorylation; transformation of Ba/F3 cells to IL-3 independent growth; and induction of a myeloproliferative disease in a murine bone marrow transplant assay. Structural analyses of other type III RTKs, such as FLT3, have identified an autoinhibitory function of the juxtamembrane (JM) domain. Thus, the lack of requirement for FIP1L1 suggests that activation of the FIP1L1-PDGFRa fusion kinase might be caused by disruption of an autoinhibitory JM domain in PDGFRα. Indeed, in patients with the FIP1L1-PDGFRα fusion, the breakpoints in PDGFRA are tightly clustered within exon 12, which encompasses the JM domain. Whereas activation of most known fusion kinases relies on enforced dimerization by a fusion partner, disruption of an autoinhibitory JM domain may be an alternative mode of fusion kinase activation.