Lineage-specific STAT5 target gene activation in hematopoietic progenitor cells predicts the FLT3+-mediated leukemic phenotype

Abstract

Mutations that activate FMS-like tyrosine kinase 3 (FLT3) are frequent occurrences in acute myeloid leukemia. Two distinct types of mutations have been described: internal duplication of the juxtamembranous domain (ITD) and point mutations of the tyrosine kinase domain (TKD). Although both mutations lead to constitutive FLT3 signaling, only FLT3-ITD strongly activates signal transducer and activator of transcription 5 (STAT5). In a murine transplantation model, FLT3-ITD induces a myeloproliferative neoplasm, whereas FLT3-TKD leads to a lymphoid malignancy with significantly longer latency. Here we report that the presence of STAT5 is critical for the development of a myeloproliferative disease by FLT3-ITD in mice. Deletion of Stat5 in FLT3-ITD-induced leukemogenesis leads not only to a significantly longer survival (82 vs 27 days) of the diseased mice, but also to an immunophenotype switch with expansion of the lymphoid cell compartment. Interestingly, we were able to show differential STAT5 activation in FLT3-ITD(+) myeloid and lymphoid murine progenitors. STAT5 target genes such as Oncostatin M were highly expressed in FLT3-ITD(+) myeloid but not in FLT3-ITD(+) lymphoid progenitor cells. Strikingly, FLT3-TKD expression in combination with Oncostatin M is sufficient to reverse the phenotype to a myeloproliferative disease in FLT3-TKD mice. Thus, lineage-specific STAT5 activation in hematopoietic progenitor cells predicts the FLT3(+)-mediated leukemic phenotype in mice.

The oncostatin M‐stromal cell axis in health and disease

“…In contrast, expression of a tyrosine kinase domain‐mutated (TKD) Flt3 causes lymphoproliferative disease without marked OSM expression. In this model, ectopic OSM expression in Flt3‐TKD bone marrow switched the disease to a myeloproliferative phenotype similar to that observed with Flt3‐ITD . Thus, aberrant OSM expression appears to preferentially promote myeloproliferative neoplasia in mice, but not lymphoproliferative disease.…”

“…In the context of myeloproliferative neoplasms, elevated OSM production downstream of STAT5‐activating oncogenes may activate bone marrow stromal cells to promote a leukaemic niche. Bone marrow cells that overexpress mouse OSM rapidly generate lethal myeloproliferative neoplasia and bone marrow fibrosis . Notably, OSM expression had no effect on myeloid colony formation in vitro, suggesting a microenvironment‐conditioning effect in vivo .…”

“…Notably, OSM expression had no effect on myeloid colony formation in vitro, suggesting a microenvironment‐conditioning effect in vivo . Expression of Flt3 with an internal tandem duplication (ITD; a gain‐of‐function mutation found frequently in human acute myeloid leukaemia) in mouse bone marrow causes a myeloproliferative neoplasia in which OSM is highly expressed . In contrast, expression of a tyrosine kinase domain‐mutated (TKD) Flt3 causes lymphoproliferative disease without marked OSM expression.…”

The Cytokine Flt3-Ligand in Normal and Malignant Hematopoiesis

“…The reason for the difference in lineage outcome of the disease was differential activation of STAT5 from the two types of mutations, since the FLT3-TKD mutation expressed in this model did not result in STAT5 phosphorylation, as was the case with FLT3-ITD. Intriguingly, STAT5 phosphorylation seemed to be the decisive factor as to the lineage phenotype of the disorder, since deletion of STAT5 in FLT3-ITD-mediated malignancy significantly increased survival and switched the immunophenotype of the disease from a myeloid to a lymphoid one [135]. The differences in the in vivo effects of FLT3-ITD and FLT3-TKD were also demonstrated in a knock-in mouse model expressing the FLT3-TKD most commonly found in AML [136].…”

“…Therefore, further investigations are needed in this direction—not only at the gene expression level, since a lot of important differences between wild-type and FLT3-ITD might occur at a post-translational level. Recent evidence would suggest that the activation of STAT5 specifically by FLT3-ITD might be very important in this context [135]. Further research into the mechanistic aspects of this lineage-specific leukemogenic transformation by mutated FLT3 is required, with the final aim to assist the development of specific inhibitors for therapeutic treatment.…”