A Constitutively-Active ABL Family Kinase, TEL-ARG, Induces Lethal Mastocytosis through Sensitizing Hematopoietic Stem/Progenitor Cells to IL-3

 ABL family kinases, ABL1 (ABL) and ABL2 (ARG), share functional domains such as SH2-, SH3- and kinase domains, and are highly homologous except their C-terminal domain. Fusions to TEL (ETV6), TEL-ABL and TEL-ARG, are constitutively-active kinases and have been reported in rare cases of human CML, AML or ALL. Although TEL-ABL is involved in leukemogenesis, the role of TEL-ARG has not been elucidated because this fusion protein has been always accompanied with other major translocations, such as PML-RARα.

 We have previously shown that although their kinase activities are comparable, TEL-ABL strongly transforms Ba/F3 cells, while TEL-ARG has a much lower transforming activity, and these differences are attributed to their distinct C-terminal domain (Okuda K and Hirai H, Open Journal of Blood Diseases 2013). At the last ASH annual meeting, we have shown that TEL-ABL induces myeloid leukemia in a short latency, whereas TEL-ARG induces lethal mastocytosis in a long latency in a mouse bone marrow (BM) transplantation model (Abstract number #2368, ASH 2014). Here we investigated the clonogenicity of mastocytosis and explored the detailed mechanism underlying the onset of mastocytosis induced by TEL-ARG.

 First, we performed a serial transplantation experiment to evaluate mastocytosis-initiating capacity of TEL-ARG-expressing cells. Hematopoietic stem/progenitor cells (HSPCs) from 5-FU-treated mice were retrovirally transduced with TEL-ARG and transplanted to the first recipient mice. BM cells from moribund mice due to mastocytosis were transplanted to the sublethally irradiated second recipients. On day 219 after transplantation, we detected mast cells circulating in the peripheral blood of these two recipients, and observed severe pancytopenia and body weight loss in one of them. In this mouse, mast cells engulfing blood cells were accumulated in the BM and spleen, and subcutaneous tissues were massively infiltrated by mast cells, all of which were characteristics of mastocytosis observed in the first recipients. These results indicate that TEL-ARG confers mastocytosis-initiating capacity on HSPCs.

 Next, we focused on the mechanisms why TEL-ARG induces mastocytosis, whereas TEL-ABL induces myeloid leukemia. HSPCs from 5-FU-treated mice were retrovirally transduced with TEL-ABL or TEL-ARG, and subjected to the in vitro mast cell differentiation assay in the presence of WEHI-conditioned medium, as a source of IL-3 (Figure). IL-3 enhanced differentiation and proliferation of empty-virus-transduced HSPCs toward mast cells in a dose-dependent manner. TEL-ARG induced mast cell differentiation in the absence of IL-3 to some extent, and IL-3 markedly increased mast cell number even at a lower concentration. TEL-ARG-expressing mast cells continue to proliferate for more than 4 months maintaining their phenotype as mast cells. In contrast, IL-3 did not enhance mast cell differentiation but support myeloid differentiation of TEL-ABL-expressing HSPCs. These data suggest that while TEL-ABL induces myeloid differentiation, TEL-ARG strongly promotes differentiation toward mast cells through sensitizing HSPCs to IL-3, an important factor for differentiation, survival and proliferation of mast cells. Furthermore, these results might account for differences in the phenotypes of diseases induced by TEL-ABL (myeloid leukemia) or TEL-ARG (mastocytosis).

 In conclusions, TEL-ABL strongly induces myeloid-skewed differentiation, whereas TEL-ARG promotes mast cell differentiation through increasing sensitivity to IL-3 and induces clonal mast cell disease. We are currently investigating the molecular mechanisms by which they activate distinct differentiation pathways toward myeloid cells or mast cells. We believe that further exploration of the underlying mechanisms will deepen our understanding of the molecular basis for ABL kinase-mediated leukemogenesis as well as mast cell disorders.