This study confirms that rituximab induces responses in a majority of previously treated patients with AIHA and ITP. Response duration generally exceeds 1 year. Retreatment with rituximab in responding patients is most often successful. The outcome of patients who fail on rituximab is poor. We were not able to identify pretreatment patient characteristics predicting for response.
The online version of this article contains a supplementary appendix.
BackgroundChimeric oncogenes encoding constitutively active protein tyrosine kinases are associated with chronic myeloid neoplasms. TEL-PDGFRβ (TPβ, also called ETV6-PDGFRB) is a hybrid protein produced by the t(5;12) translocation, FIP1L1-PDGFRα (FPα) 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 MethodsWe used Ba/F3 as a model cell line, as well as leukocytes from two patients, to analyze hybrid protein degradation.
ResultsIn contrast to the corresponding wild-type receptors, which are quickly degraded upon activation, we observed that TPβ, FPα and the ZNF198-FGFR1 hybrids escaped down-regulation in Ba/F3 cells. The high stability of TPβ and FPα hybrid proteins was confirmed in leukocytes from leukemia patients. Ubiquitination of TPβ and FPα 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 TPβ induced protein degradation. Instability was reverted by adding the destabilizing domain ligand, Shield1. The destabilization of this modified TPβ reduced cell transformation and STAT5 activation.
ConclusionsWe have shown that chimeric receptor tyrosine kinases escape ubiquitination and downregulation and that their stabilization is critical to efficient stimulation of cell proliferation.Key words: PDGF receptor, oncogenes, protein degradation, ubiquitin.Citation: Toffalini F, Kallin A, Vandenberghe P, Pierre P, Michaux L, Cools J, and Demoulin J-B. The fusion proteins TEL-PDGFRβ and FIP1L1-PDGFRα escape ubiquitination and degradation Haematologica 2009;94:1085-1093.doi:10.3324/haematol.2008 This is an open-access paper.
The fusion proteins TEL-PDGFRβ and FIP1L1-PDGFRα escape ubiquitination and degradation
Anaplastic lymphoma kinase (ALK)-positive anaplastic large cell lymphoma is characterized by 2p23/ALK aberrations, including the classic t(2;5)(p23;q35)/NPM1-ALK rearrangement present in ~80% of cases and several variant t(2p23/ALK) occurring in the remaining cases. The ALK fusion partners play a key role in the constitutive activation of the chimeric protein and its subcellular localization. Using various molecular technologies, we have characterized ALK fusions in eight recently diagnosed anaplastic large cell lymphoma cases with cytoplasmic-only ALK expression. The identified partner genes included EEF1G (one case), RNF213/ALO17 (one case), ATIC (four cases) and TPM3 (two cases). Notably, all cases showed copy number gain of the rearranged ALK gene, which is never observed in NPM1-ALK-positive lymphomas. We hypothesized that this could be due to lower expression levels and/or lower oncogenic potential of the variant ALK fusions. Indeed, all partner genes, except EEF1G, showed lower expression in normal and malignant T cells, in comparison with NPM1. In addition, we investigated the transformation potential of endogenous Npm1-Alk and Atic-Alk fusions generated by clustered regularly interspaced short palindromic repeats/Cas9 genome editing in Ba/F3 cells. We found that Npm1-Alk has a stronger transformation potential than Atic-Alk, and we observed a subclonal gain of Atic-Alk after a longer culture period, which was not observed for Npm1-Alk. Taken together, our data illustrate that lymphomas driven by the variant ATIC-ALK fusion (and likely by RNF213-ALK and TPM3-ALK), but not the classic NPM1-ALK, require an increased dosage of the ALK hybrid gene to compensate for the relatively low and insufficient expression and signaling properties of the chimeric gene.
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