Activation of FIP1L1-PDGFRα requires disruption of the juxtamembrane domain of PDGFRα and is FIP1L1-independent

Abstract

Genetic abnormalities that result in expression of chimeric tyrosine kinase proteins such as BCR-ABL1 and ETV6-PDGFR␤ are common causes of hematopoietic malignancies. The paradigm for constitutive activation of these fusion tyrosine kinases is enforced homodimerization by self-association domains present in the fusion partner proteins. The unique interstitial deletion on chromosome 4q12 that leads to expression of the FIP1L1-PDGFR␣ fusion tyrosine kinase was recently identified as a cause of chronic eosinophilic leukemia. In this report, we demonstrate that FIP1L1 is completely dispensable for PDGFR␣ activation in vitro and in vivo. Instead, truncation of PDGFR␣ between two conserved tryptophan residues in the juxtamembrane (JM) domain is required for kinase activation and transforming potential of FIP1L1-PDGFR␣. The presence of a complete JM domain in FIP1L1-PDGFR␣ is inhibitory, but this autoinhibition can be overcome by enforced homodimerization. Similar effects of the JM domain in the context of PDGFR␤ were observed. These results suggest that disruption of the autoinhibitory JM domain is an alternative, dimerization-independent mechanism by which chimeric tyrosine kinases are constitutively activated and induce leukemogenesis.fusion gene ͉ leukemia ͉ oncogene ͉ tyrosine kinase P rotein tyrosine kinase genes are frequent targets of chromosomal rearrangements in hematopoietic malignancies and, to a minor extent, in some solid tumors (1, 2). As a consequence, such chromosomal abnormalities result in expression of fusion tyrosine kinases, in which the tyrosine kinase domains are fused in-frame to distinct N-terminal partner proteins that contain homotypic oligomerization domains. Homodimerization of the fusion tyrosine kinases by the N-terminal oligomerization domains leads to autophosphorylation at tyrosine residues and constitutive kinase activation, as well as activation of critical downstream signaling effectors that induce cellular proliferation (1). Well studied examples include BCR-ABL, ETV6-PDGFRB, ZNF198-FGFR1, and NPM-ALK, in which the presence of the oligomerization domains of the fusion partners have been shown to be indispensable for kinase activation and for transforming properties of the fusion proteins (2-6).FIP1L1-PDGFRA is a fusion gene that has been identified in patients with hypereosinophilic syndrome͞chronic eosinophilic leukemia (CEL) and systemic mast cell disease (7,8). FIP1L1 is fused to PDGFRA due to an interstitial deletion on chromosome 4q12, in contrast to other fusion tyrosine kinases that are typically generated by chromosomal translocations (7-13). We and others have shown that the FIP1L1-PDGFR␣ fusion protein is a constitutively activated tyrosine kinase that confers growth factor-independent growth to hematopoietic cells and is sensitive to inhibition by imatinib (7,12,14). FIP1L1 is a protein involved in polyadenylation, but it is unknown whether FIP1L1 confers dimerization properties, because it does not harbor any known protein-protein interaction domains (15).Interestingly...

FIP1L1 presence in FIP1L1-RARA or FIP1L1-PDGFRA differentially contributes to the pathogenesis of distinct types of leukemia

“…We generated FLAG-tagged Altogether, these results indicate that the C-terminal portion of PDGFRA, by itself, possesses the ability to homodimerize and is a constitutively active kinase. These results are consistent with a previous report, describing that the FIP1L1 portion is dispensable for activation of FIP1L1-PDGFRA for its kinase activity [7].…”

“…These results were consistent with previous reports, stating that the FIP1L1 portion is dispensable for the activation of FIP1L1-PDGFRA kinase activity [7]. The FIP1L1 portion is, however, a contributing factor to the higher proliferating activity, induced by FIP1L1-PDGFRA.…”

“…Constitutive activation of FIP1L1-PDGFRA is reported to be dependent on disruption of the juxtamembrane domain of PDGFRA [7]; however, because our results clearly indicate that FIP1L1 possesses a homodimerization domain, we next examined whether FIP1L1 also plays a role in FIP1L1-PDGFRA homodimerization.…”

“…A previous report indicated that disruption of the juxtamembrane domain of PDGFRA is critical for constitutive activation of the kinase, whereas the FIP1L1 portion is dispensable [7]. Recently, it has been found that the FIP1L1 portion is also necessary for activating STAT5 and PKB/Akt in human hematopoietic progenitor cells [8].…”

Five years since the discovery of FIP1L1–PDGFRA: what we have learned about the fusion and other molecularly defined eosinophilias

“…However, we have observed that interruption of the juxtamembrane of PDGFRa is indispensable for kinase activation in the context of FIP1L1-PDGFRa. 58 Indeed, it was previously shown that mutations or duplications within the juxtamembrane region of the PDGFR family of tyrosine kinases can cause constitutive activation of their kinase activity. 59,60 Table 4 PDGFRB fusion genes in addition to ETV6-PDGFRB identified in patients with MPNs and eosinophilia a Not a chronic MPN, but diagnosed in a case of AML at relapse.…”