Jarid2 Restricts Long-Term Repopulating Stem Cell Capacity in Multipotent Progenitors and Acts As Tumor Suppressor in Chronic Myeloid Neoplasms

Myeloid disorders including myeloproliferative neoplasms (MPN) are clonal diseases of hematopoietic stem or progenitor cells that arise from genetic and epigenetic alterations which perturb key processes such as self-renewal, proliferation and differentiation. The most common genetic mutations found in the MPN initiating clone occur in the genes JAK2, MPL and CALR, and these mutations are likely responsible for initiation of disease. Acquisition of additional co-operating mutations in genes such as TET2, IDH1/2, and NRAS in the MPN clone can give rise to secondary acute myeloid leukemia (sAML), a much more aggressive disease with an average survival time after transformation less than six months. Recent genome sequencing studies identified chromosomal deletions of JARID2, a Polycomb Repressive Complex 2 (PRC2) co-factor involved in implementing H3K27me3 marks, in the leukemic phase of post-MPN sAML but not in chronic MPN phase of the disease. These data indicate that JARID2 might act as a tumor suppressor in chronic myeloid disorders and genetic deletion of JARID2 could be a sAML-specific transforming event.

To investigate the role of Jarid2 as a tumor suppressor, we utilized an inducible mouse model of the prototypical MPN driver mutation Jak2^V617F. The absence of Jarid2 in a Jak2^V617F/+ background accelerated MPN progression, with a median survival of 23 and 43 days post Jarid2 -deletion for Jarid2^-/-- Jak2^V617F/+ and Jarid2^+/-- Jak2^V617F/+ mice, respectively, compared to 113 days for Jak2^V617F/+ control mice. Together, these data suggest that loss of Jarid2 in Jak2^V617F/+ background accelerates MPN pathogenesis.

To more completely understand the tumor suppressor role of Jarid2 in MPNs, we aimed to define its role in normal hematopoiesis. We deleted Jarid2 using the Vav-CRE driver to facilitate conditional inactivation of Jarid2 in hematopoietic system. Competitive transplantation was employed to assess the hematopoietic stem cell (HSC) potential. Either 100 phenotypically defined HSCs (Lineage^-cKit⁺ScaI⁺CD48^-CD150⁺) or 2.5x10⁵ whole bone marrow (WBM) cells were transplanted into lethally irradiated recipient mice along with 2.5 x 10⁵ WBM cells from genetically distinguishable WT mice. At 16 weeks post-transplant, the analysis of test cell contribution in recipient mice revealed that Vav-CRE: Jarid2 -KO HSCs did not contribute to peripheral blood (PB) chimerism. In contrast, Vav-CRE: Jarid2 -KO WBM contributed to PB lineages at levels comparable to control WBM. These data suggested that a population of cell(s) outside phenotypically-defined HSCs contribute(s) to hematopoiesis in the absence of Jarid2 . To find the population(s) that reside in Vav-CRE: Jarid2 -KO WBM with long-term multi-lineage capacity, we transplanted the following sorted progenitor populations: multipotent progenitors (50 MPPs; Lineage^-cKit⁺ScaI⁺CD48^-CD150^-) and restricted progenitors (250 RPs; Lineage ^-cKit⁺ScaI⁺CD48⁺CD150^-) in a competitive setting as described above. As expected, neither control MPPs or RPs were capable of long-term multi-lineage reconstitution. Strikingly, Vav-CRE: Jarid2 -KO MPPs engrafted in primary and secondary recipients, generating tri-lineage hematopoiesis effectively, whereas Vav-CRE: Jarid2 -KO RPs did not engraft long-term in any recipients. Additionally, MPPs isolated from Jak2^V617F - Jarid2^KO mice can propagate MPN in transplanted recipients, unlike MPPs transplanted from Jak2^V617F mice. RNA-seq expression profiling revealed upregulation of Runx1t1 and Mycn in Jarid2-KO MPPs, genes which have been used to reprogram committed blood cells to a multipotent state. We hypothesized that the inability to silence Runx1t1 and Mycn in Jarid2-KO MPPs was a mechanism that permits self-renewal in this normally transiently-repopulating cell fraction. Enforced expression of Runx1t1 and Mycn in WT MPPs endowed long-term repopulating capacity in vivo, phenocopying the behavior of Jarid2-KO MPPs. Cumulatively, our data suggest that the normal role of Jarid2 in hematopoiesis is to restrict self-renewal capacity to long-term HSCs, and that dysregulation of this process can lead to hematopoietic malignancies.