The fusion proteins TEL-PDGFRβ and FIP1L1-PDGFRα escape ubiquitination and degradation

Background Chimeric oncogenes encoding constitutively active protein tyrosine kinases are associated with chronic myeloid neoplasms. TEL-PDGFR beta (TR beta also called ETV6-PDGFR beta) is a hybrid protein produced by the t(5;12) translocation, FIP1L1-PDGFR alpha (FP alpha) results from a deletion on chromosome 4q12 and ZNF198-FGFR1 is created by the t(8,13) translocation. These fusion proteins are found in patients with myeloid neoplasms associated with eosinophilia. Wild-type receptor tyrosine kinases are efficiently targeted for degradation upon activation, in a process that requires Cbl-mediated monoubiquitination of receptor lysines. Since protein degradation pathways have been identified as useful targets for cancer therapy, the aim of this study was to compare the degradation of hybrid and wild-type receptor tyrosine kinases. Design and Methods We used Ba/F3 as a model cell line, as well as leukocytes from two patients, to analyze hybrid protein degradation. Results In contrast to the corresponding wild-type receptors, which are quickly degraded upon activation, we observed that TP beta, FP alpha and the ZNF198-FGFR1 hybrids escaped down-regulation in Ba/F3 cells. The high stability of TP beta and FP alpha hybrid proteins was confirmed in leukocytes from leukemia patients. Ubiquitination of TP beta and FP alpha was much reduced compared to that of wild-type receptors, despite marked Cbl phosphorylation in cells expressing hybrid receptors. The Fusion of a destabilizing domain to TP beta induced protein A degradation. Instability was reverted by adding the destabilizing domain ligand, Shield1. The destabilization of this modified TP beta reduced cell transformation and STAT5 activation. Conclusions We have shown that chimeric receptor tyrosine kinases escape ubiquitination and downregulation and that their stabilization is critical to efficient stimulation of cell proliferation.