IKZF1 Mutation Mediate Resistance to IMiDs in Human Hematopoietic Stem Cells
Authors: Li Shirong , Fu Jing , Wu Jing , Mapara Markus Y , Lentzsch Suzanne 
Published: October 2019

Abstract
Introduction: Previously we have shown that the immune modulatory drugs (IMiDs) downregulate GATA1 and PU.1 resulting in maturational arrest of granulocytes with accumulation of immature myeloid precursors and subsequent neutropenia. Our studies further revealed that similar to MM cells cereblon (CRBN) is critical for the mediation of the effects of IMiDS in hematopoietic stem cells (HSCs) and associated with decrease of IKZF1-dependent transcription factors such as GATA1 and PU.1, which are critical for development and maturation of neutrophils and erythrocytes as well as thrombocytes. Here we investigated the mechanism how IMIDs induce degradation of IKZF1 and confirmed our studies in vivo by using the humanized NOD/SCID/Gamma-c KO (NSG) mouse model.
Methods and Results After we had shown that knockdown of CRBN in HCS mediates resistance to IMIDs (2014 ASH abstract 418) we assessed the impact of IKZF1 inhibition using two different approaches. First, we knocked down IKZF1 expression in CD34+ cells by shRNA lentivirus transduction. As expected, IKZF1 knockdown in CD34+ cells mimicked the effects of IMiDs resulting in increased CD34+ cell proliferation, CD33+ cell expansion (flow cytometry) and shift of lineage commitment from BFU-E to CFU-G (colony assay). Knockdown of IKZF1 was associated with decreased GATA1 and PU.1 expression at both mRNA and protein levels. Next, we generated a mutant IKZF1 by substituting Glutamine Q146 to Histidine, which abrogates IKZF1 ubiquitination induced by CRBN. CD34+ cells were transduced with lentiviral constructs to overexpress IKZF1-WT or IKZF1-Q146H. POM failed to induce IKZF1 degradation in IKZF1-Q146H-OE CD34+ cells, indicating CRBN binding to IKZF1 and subsequent ubiquitination is critical in this process. Functional assays further confirmed that IKZF1-Q146H CD34+ cells were resistant to POM induced CD33+ cell expansion and shift in lineage commitment from BFU-E to CFU-G.
Since conventional mouse models are not applicable to test IMIDs in vivo due to the fact that IMIDs do not bind to mouse CRBN (Kronke, Fink et al. 2015), we established a humanized mouse model resembling human hematopoiesis. In this model, NOD/SCID/Gamma-c KO (NSG) mice received human fetal thymus grafts and 105 CD34+ fetal liver cells to generate human hematopoiesis including functional T-cells. After establishing human hematopoiesis mice were injected with POM (0.3 mg/kg) i.v every 2 days for 3 weeks. Analysis of bone marrow revealed that POM treatment significantly induced granulocyte/macrophage progenitor cells (CD34+ CD38+ CD45RA+ cells) at the expense of common lymphoid progenitors (CD34+ CD10+ cells). The shift into myelopoiesis is consistent with our in vitro finding that IMiDs affect lineage commitment.
Conclusion: In summary, our results demonstrate that IMiDs affect CD34+ cell fate via CRBN and IKZF1 mediated mechanism. These results will be helpful to elucidate the mechanism of IMiDs on lineage commitment and maturation in HSCs. Also establishment of the humanized xenograft mice model may provide an advanced platform for the analysis of human hematopoiesis and human immune responses to IMiDs as well development of secondary hematologic malignancies in vivo.

Disclosures
Lentzsch: Axiom: Speakers Bureau; Celgene: Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees; BMS: Membership on an entity's Board of Directors or advisory committees; Janssen: Membership on an entity's Board of Directors or advisory committees.