Multiple oligomerization domains of KANK1-PDGFRβ are required for JAK2-independent hematopoietic cell proliferation and signaling via STAT5 and ERK

Background KANK1-PDGFRB is a fusion gene generated by the t(5;9) translocation between KANK1 and the platelet-derived growth factor receptor beta gene PDGFRB. This hybrid was identified in a myeloproliferative neoplasm featuring severe thrombocythemia, in the absence of the JAK2 V617F mutation. Design and Methods KANK1-PDGFRB was transduced into Ba/F3 cells and CD34(+) human progenitor cells to gain insights into the mechanisms whereby this fusion gene transforms cells. Results Although platelet-derived growth factor receptors are capable of activating JAK2, KANK1-PDGFR beta did not induce JAK2 phosphorylation in hematopoietic cells and a JAK inhibitor did not affect KANK1-PDGFR beta-induced cell growth. Like JAK2 V617F, KANK1-PDGFR beta constitutively activated STAT5 transcription factors, but this did not require JAK kinases. In addition KANK1-PDGFR beta induced the phosphorylation of phospholipase C-gamma, ERK1 and ERK2, like wild-type PDGFR beta and TEL-PDGFR beta, another hybrid protein found in myeloid malignancies. We next tested various mutant forms of KANK1-PDGFR beta in Ba/F3 cells and human CD34(+) hematopoietic progenitors. The three coiled-coil domains located in the N-terminus of KANK1 were required for KANK1-PDGFR beta-induced cell growth and signaling via STAT5 and ERK. However, the coiled-coils were not essential for KANK1-PDGFR beta oligomerization, which could be mediated by another new oligomerization domain. KANK1-PDGFR beta formed homotrimeric complexes and heavier oligomers. Conclusions KANK1-PDGFRB is a unique example of a thrombocythemia-associated oncogene that does not signal via JAK2. The fusion protein is activated by multiple oligomerization domains, which are required for signaling and cell growth stimulation.