HMGA2 Orchestrates the Tumorgenesis of Myeloproliferative Neoplasms (MPN) in Corporation with JAK2V617F

MPN harbors altered hematopoietic stem cell (HSC) function, resulting in skewed hematopoiesis and extramedullary hematopoiesis with splenomegaly. Mutations such as JAK2V617F and insertion/deletion of CALR exon9 have been established as phenotypic drivers of MPN. In addition, mutations in epigenetic modifiers and aberrant expressions of microRNAs play a crucial role in disease progression and clonal expansion. We have shown that almost all patients with myelofibrosis (MF) highly express HMGA2 (Harada-Shirado et al, BJH, 2015) and that transgenic mice expressing HMGA2 without 3'UTR including let7 binding sites (∆Hmga2 : H) (Ikeda et al, Blood, 2011) develop mild MPN. Moreover, we (Sashida et al, JEM, 2016) and other groups (Shimizu et al, JEM, 2016; Yang et al, Blood, 2016) showed that loss of EZH2 induces endogenous expression of HMGA2 and provokes severe MF in mice carrying JAK2V617F. Thus, we hypothesized that HMGA2 plays a central role in the disease progression of MPN. To clarify this, we generated ∆Hmga2/JAK2V617F transgenic mice (HJ), which developed severe leukocytosis, thrombocytosis, anemia, giant splenomegaly and shorter survival period, but did not progress fibrosis compared with transgenic mice carrying JAK2V617F alone (J) (Ueda et al, ASH, 2015). Compared with J, HJ also showed an increased bone marrow (BM) lineage^-Sca1⁺Kit⁺(LSK) cells and growth advantage in competitive serial BM transplants.

Encouraged by these findings, we further investigated the mechanism that HMGA2 exacerbates disease phenotype, and elucidated up-stream and down-stream factors of HMGA2. First, we sought cause of aggressive phenotypes. In BM cells of HJ mice, STAT3 and STAT5 were drastically upregulated in both expression and phosphorylation. Despite severer anemia, formations of EPO-independent erythroid colonies and proportions of CD71+Ter119+ erythroblasts in BM were not different between HJ and J. To explain this discrepancy, we measured spontaneous apoptosis. Erythroblasts were more susceptible to apoptosis in HJ as well as aged H, compared with J, suggesting that HMGA2 contributes to apoptosis under stressed condition.

Next, we compared gene expression profiles of BM LSK cells between HJ and J by RNA sequence to seek genes altered by addition of HMGA2 expression to JAK2V617F. To clarify the role of endogenous expression of HMGA2 induced by EZH2 deletion in hematopoiesis with JAK2V617F in addition to direct effects of EZH2 deletion, we also studied RNA sequence in BM LSK cells of Ezh2-/-JAK2V617F-Tg mice (EJ). Upregulated genes were highly overlapped in between HJ and EJ. Relative to J, 200 genes including oncogenic Lmo1 were commonly upregulated, while 314 and 739 were exclusively upregulated in HJ and EJ, respectively. Thus, some of the EZH2 targets may be also targets of HMGA2. Pathway analysis revealed that chaperon-related pathways involving SCAP1 and SREBF1/2 were commonly upregulated. On the other hand, TGFB pathway was upregulated in EJ but not in HJ, and it may explain the discrepancy that addition of HMGA2 in JAK2V617F hematopoiesis did not progress fibrosis despite severe phenotype of MPN, but deletion of EZH2 exacerbated fibrosis as well as MF patients with high expression of HMGA2. To verify this finding, we measured expression of HMGA2 and its main suppressor let7, and performed target sequencing in blood samples from 16 MF patients. Because all of them highly expressed HMGA2, we also evaluated selected essential thrombocythemia (ET) patients (N=10 for HMGA2 low, 10 for HMGA2 high). While high HMGA2 expression was consistently correlated with low let-7 expression in ET, only half of MF showed low expression of let7. The other half of MF harbored mutations in components or modifiers of polycomb, including EZH2, ASXL1 and spliceosomes. These findings are compatible with the finding that loss of polycomb function upregulates HMGA2.

In fact, the genetic depletion of HMGA2 or administration of HMGA2 inhibitor neuropsin diminished phenotype of EJ mice and prolonged survival. Our findings suggest importance of HMGA2 as a therapeutic target of MPN.