Zeb2-Defficiency in the Adult Murine Hematopoietic Precursor Cells Leads to Differentiation Defects in Multiple Hematopoietic Lineages and a Myeloproliferative-Like Phenotype

Abstract 1199

The life long replenishment of highly specialized blood cells by a small number of hematopoietic cells (HSC) requires a strict regulation between self-renewal and differentiation in the immature compartment of the bone marrow. Perturbation of this equilibrium can result in stem cell loss or hematologic malignancies. This balance is at least in part controlled by a network of transcription factors. Zeb2 is a transcriptional repressor and plays an important role during the embryonic development as a modulator of the epithelial to mesenchymal transition (EMT) as well as tumor progression and metastasis. We have previously identified the essential role of Zeb2 in murine embryonic hematopoiesis, where selective Zeb2 deficiency in the hematopoietic stem cells resulted in early lethality around day 12.5. The aim of this study was to analyze whether Zeb2 plays a specific role in the regulation of homeostasis in the adult hematopoietic system.

Using the Mx1-Cre based inducible Zeb2 conditional knock out mouse model we analyzed the impact of Zeb2 loss on adult hematopoietic stem cell function. Upon the induction of Zeb2 deletion we found a significant decrease in most cell lineages of the peripheral blood, except the neutrophil granulocytes. However, the reduction of mature cells in the blood was not accompanied by reduced bone marrow cellularity, as the cellularity was similar between Zeb2^{Δ/Δ Mx1-Cre} (Zeb2 conditional KO) mice and the control animals (Zeb2^+/+Mx1-Cre). However, in the bone marrow of the Zeb2^{Δ/Δ Mx1-Cre} animals the granulocytic lineage was dominating, whereas other lineages e.g. red blood cell precursors and B-lymphoid precursors were drastically reduced. Histological sections of the bone marrow cavity revealed megacaryocytes with abnormal morphology reflecting maturation defects and an increased production of reticular fibers in the BM of Zeb2^{Δ/Δ Mx1-Cre} mice. In addition Zeb2^{Δ/Δ Mx1-Cre} mice displayed a two to three fold increase in spleen size compared to control animals due to an extramedullary hematopoiesis. Analysis of the primitive hematopoietic compartment in the bone marrow and spleens revealed that Zeb2 deletion resulted in a pronounced increase in the most immature hematopoietic cells, defined as Lin-Sca1+cKit+CD48-CD150+ population, and perturbation in different lineage restricted progenitor subpopulations. No difference in cell cycling or apoptotic rate in the stem cell enriched bone marrow population (Lin-Sca1+cKit+CD48-CD150+) was detectable between the genotypes. Upon transplantation into lethally irradiated wild type recipients, Zeb2 deficient stem cells demonstrated significantly reduced ability to differentiate into multiple hematopoietic lineages indicating a niche independent effect of Zeb2 in promoting differentiation of hematopoietic stem cells. On the molecular level, gene expression analysis of hematopoietic stem and progenitor cells using microarray approach revealed increased transcripts of downstream targets of Wnt/ß-Catenin signaling, suggesting increased Wnt signaling activity in absence of Zeb2 in the hematopoietic compartment, which at least in part might be responsible for the observed phenotype.

These data indicate that Zeb2 is involved in the regulation of the balance between self-renewal and differentiation at multiple stages of hematopoietic cell maturation. Furthermore the lack of Zeb2 in the hematopoietic compartment leads to a phenotype that resembles the features of human myeloproliferative disorders, especially the early stages of primary myelofibrosis with dominant granulopoiesis, production of reticular fibers in the bone marrow, and morphological abnormalities in megacaryocytes, accompanied by extramedullary hematopoiesis.