Tyk2 is dispensable for induction of myeloproliferative disease by mutant FLT3

Internal tandem duplication of FLT3 tyrosine kinase (FLT3-ITD) is the most prevalent mutation found in acute myelogenous leukemia (AML), having been identified in 20% to 30% of all AML patients. We have previously shown that FLT3-ITD signals mainly through the signal transducer and activator of transcription 5 (STAT5) pathway and have suggested the possible involvement of Tyk2 in STAT5 activation by FLT3-ITD. The present study addressed the role of Tyk2 in FLT3-ITD signaling in a murine bone marrow transplantation (BMT) model. Transplantation of wild-type bone marrow cells transduced with the FLT3-ITD gene induced lethal myeloproliferative disease (MPD) in the recipient mice at a median latency of 89 days. Interestingly, some mice presented the proliferation of B- or T-lymphoid blasts in various organs, a presentation that resembled acute lymphoblastic leukemia (ALL). Mice that received Tyk2-deficient bone marrow cells transduced with FLT3-ITD developed lethal MPD with a disease latency (median, 100 days) and pathologic picture similar to those of mice that received wild-type bone marrow cells. These results indicate that (1) Tyk2 is not essential for MPD induction by FLT3-ITD and (2) FLT3-ITD by itself can induce ALL in a murine BMT model.