Essential Role for Stat5a/b in Myeloproliferative Neoplasms Induced by BCR-ABL1 and Jak2 V617F.

Abstract 312

The Signal Transducer and Activator of Transcription 5 (STAT5a/b) proteins are latent transcription factors that are constitutively activated in malignant cells from patients with a variety of hematologic malignancies, including the myeloproliferative neoplasms (MPNs) chronic myeloid leukemia (CML) and polycythemia vera (PV), but the role of STAT5a/b in the pathogenesis of these diseases has not been fully elucidated. While therapy with tyrosine kinase inhibitors like imatinib mesylate has revolutionized the treatment of CML, the emergence of clinical resistance to imatinib has rekindled interest in STAT5a/b as a promising therapeutic target downstream of dysregulated tyrosine kinases such as BCR-ABL1 and Jak2^V617F.

In this study, we used mice with a conditional (floxed, fl) null mutation in the Stat5a/b gene locus (Cui et al., Mol. Cell. Biol. 2004;24:8037) to determine the requirement for Stat5a/b in MPNs induced by BCR-ABL1 and Jak2^V617F in retroviral bone marrow (BM) transduction/transplantation models of CML and PV, respectively. Two distinct strategies were used to express Cre recombinase in the mutant hematopoietic cells: retroviral expression of a GFP-Cre fusion protein, and interferon-inducible expression from an Mx-Cre transgene.

In recipients of BCR-ABL1-transduced BM, loss of one Stat5a/b allele resulted in attenuation of CML-like MPN and the appearance of acute B-lymphoblastic leukemia (B-ALL). Complete deletion of the Stat5a/b gene resulted in the absence of any myeloid or lymphoid leukemia in recipients, with normal spleen weight and organ histopathology. However, these recipients had significant populations of GFP⁺ myeloid cells in peripheral blood and marrow with expression of BCR-ABL1 and phosphorylation of CrkL, indicative of engraftment with BCR-ABL1⁺ stem cells. Serial transplantation revealed that the BCR-ABL1-expressing Stat5a/b-deficient HSC can self-renew and progress to acute lymphoid leukemia in secondary recipients. These results demonstrate that Stat5a/b signaling is required for BCR-ABL1 to induce the massive expansion of myeloid progenitors that is characteristic of CML. However, our results also argue that inhibition of Stat5a/b signaling may be insufficient for eradication of leukemic stem cells in CML patients, or for preventing progression to blast crisis.

To address the role of Stat5a/b in the pathophysiology of PV, we utilized a similar model of PV induced by retroviral transduction of Jak2^V617F into mouse BM, followed by transplantation into recipient mice. Such recipients develop nonfatal polycythemia, reticulocytosis, and leukocytosis, and eventually progress to myelofibrosis (Zaleskas et al., PLoS One 2006;1:e18). Recipients of Stat5a/b^fl/− donor transduced with Jak2^V617F retrovirus that exhibited efficient Cre-mediated Stat5a/b deletion failed to develop polycythemia despite engraftment with transduced donor-derived stem cells and evidence of circulating GFP⁺ cells. Importantly, recipients of Jak2^V617F-transduced Stat5a/b^fl/− BM who engrafted with Jak2^V617F+ cells had evidence of subclinical MPN, with infiltration of spleen and liver with myeloerythroid cells and substantial myelofibrosis in the BM. These results suggest that inhibition of Stat5a/b signaling in PV patients may normalize their red cell mass, but might be less effective at preventing progression to myelofibrosis.

Together, these results demonstrate that Stat5a/b is necessary for the pathogenesis of CML-like disease induced by BCR-ABL1 and of polycythemia by Jak2^V617F, and validate the Stat5a/b pathway as a target for therapy in MPNs associated with dysregulated TKs. However, targeting Stat5 alone may not eradicate CML stem cells or prevent Jak2^V617F-induced myelofibrosis.