The discovery of mutations in JAK2, CALR, and MPL have uncovered activated JAK-STAT signaling as a primary driver of MF, supporting a rationale for JAK inhibition.
RQ-PCR experiments showed increased JAK2 expression in patients with the JAK2V617F mutation, with a significant difference between essential thrombocythemia (ET), polycythemia vera (PV), and myelofibrosis (MF) patients.
Here we report that expression of TEL-JAK2, a constitutively active variant of the JAK2 kinase, in lineage-depleted human umbilical cord blood cells results in erythropoietin-independent erythroid differentiation in vitro and induces the rapid development of myelofibrosis in an in vivo NOD/SCID xenotransplantation assay.
As such, CALR first, followed by MPL if CALR is absent, mutation screening is appropriate in the diagnostic work-up of JAK2-unmutated ET but does not replace the need for BM morphologic examination in (1) confirming the diagnosis in triple-negative ET and (2) distinguishing ET from other myeloproliferative neoplasms that share the same mutations, including masked PV and early/prefibrotic myelofibrosis.
Appreciation for the activation of JAK2 and the importance of increased levels of circulating proinflammatory cytokines in the pathogenesis and clinical manifestations of myelofibrosis has led to novel therapeutic agents targeting JAKs.
"Driver" mutations in JAK2, MPL and indels in CALR underlie the vast majority of cases of PMF and post-ET MF; the remainder (≈ 10%) lack identifiable driver mutations, but other clonal markers are usually detectable.
As mice carrying the hypomorphic Gata1<sup>low</sup> mutation, which reduces the levels of Gata1 mRNA in megakaryocytes, develop MF, we investigated whether the TPO axis is hyperactive in this model.
Detection of aberrant gene expression in CD34+ hematopoietic stem cells from patients with agnogenic myeloid metaplasia using oligonucleotide microarrays.
Pacritinib (PAC), a multi-kinase inhibitor with specificity for JAK2, FLT3, and IRAK1 but sparing JAK1, has demonstrated clinical activity in MF with minimal myelosuppression.
We also studied the same parameters in two mouse models of myelofibrosis, with genetic alterations affecting megakaryocyte differentiation (i.e. one model with low GATA-1 expression and the other with over-expression of thrombopoietin).
The expression of c-kit receptor (c-kit R; CD117) and CD34 was examined in acute myeloid leukemia (AML), acute lymphoid leukemia (ALL), chronic myeloid leukemia (CML) in blastic transformation (BT), and myelofibrosis (MF) in myeloid BT.
Our results show that: (i) the expression of SCL transcript is increased in peripheral blood mononuclear cells (PBMCs) from patients; (ii) SCL gene transcription is altered in MMMCD34+ progenitor cells sorted into CD34+CD41+ and CD34+CD41- subpopulations.
Furthermore, differentially methylated CpG sites in ASXL1 mutated MF cases are found in regulatory regions that could be associated with aberrant gene expression of ASXL1 target genes.
Expression of Hmga2 enhanced megakaryopoiesis, increased extramedullary hematopoiesis, and accelerated the development of MF in mice expressing Jak2<sup>V617F</sup> Mechanistically, the data show that expression of Hmga2 enhances the activation of transforming growth factor-β1 (TGF-β1) and Cxcl12 pathways in mice expressing Jak2<sup>V617F</sup> In addition, expression of Hmga2 causes upregulation of Fzd2, Ifi27l2a, and TGF-β receptor 2.
We found that mRNA levels of interleukin-8, one of the candidate cytokines related to the pathogenesis of myelofibrosis, was elevated predominantly in megakaryocytes derived from MF-iPS.