Role of SH2 Domain in JAK2-V617F Mediated Transformation.

A point mutation in JAK2 (V617F) has been described recently in patients with myeloproliferative diseases like polycythemia vera (PV), essential thrombocythemia (ET) and chronic idiopathic myelofibrosis (IMF). This V617F point mutation in JAK2 has been shown to activate several downstream pathways including STAT5 and ERK. This mutation also renders haematopoietic progenitors cytokine-independent. The role of the V617F mutation in oncogenesis is not fully understood. In this study we aim to dissect the role of the SH2 domain in JAK2-V617F mediated transformation.

Stable Ba/F3 cell lines expressing JAK2-wild type (wt), JAK2-V617F, JAK2-R439K (SH2 domain mutation) and JAK2-V617F/R439K mutants were generated. Cell proliferation assays showed that JAK2-V617F transforms Ba/F3 cells and renders them IL3 independent, while wild type JAK2 and JAK2-R439K could not. Surprisingly, JAK2-V617F/R439K was not able to induce a transformed phenotype in Ba/F3 cells. Imunoblotting revealed strong activation of JAK2, STAT5 and ERK in cells expressing JAK2-V617F, whereas no such activation could be found in JAK2-wt, JAK2-R439K and in JAK2-V617F/R439K expressing cells. Thus the SH2 domain in JAK2-V617F seems to play a crucial role in the transformation of Ba/F3 cells containing a heterodimeric (IL-3) cytokine receptor.

It has been demonstrated that JAK2-V617F induces cellular transformation more efficiently in cells expressing a homodimeric cytokine receptor such as the erythropoetin receptor. We therefore established Ba/F3 cells overexpressing EpoR together with JAK2-wt, JAK2-V617F, JAK2-R439K and JAK2-V617F/R439K. In contrast to parental Ba/F3 cells, EpoR expressing Ba/F3 cells could be transformed by both JAK2-V617F as well as JAK2-V617F/R439K. Both the single and double mutant Ba/F3 cells showed strong activation of STAT5 and ERK. This suggests that an intact SH2 domain is not required for homodimeric cytokine receptor expressing cells.

These results show that transformation by JAK2-V617F requires an intact SH2 domain only in cells expressing a heterodimeric cytokine receptor. In contrast, cells containing a homodimeric cytokine receptor are able to induce transformation in the presence of JAK2-V617F with an additional SH2 mutation. Further progress in understanding the role of the SH2 domain in JAK2-V617F mediated transformation may help in delineating downstream signalling with therapeutic implications.