Dicer Ablations in Bone Marrow Niche Impair Hematopoietic Progenitor/Stem Cells and Induce Myelodysplasia in Young Mice but Are Dispensable for Adult Hematopoiesis

MicroRNAs (miRNAs) regulate hematopoietic cell fate and their global down-regulation by Dicer1 deletion promotes tumorigenesis in a cancer-cell-autonomous manner (Kumar M.S. et al, 2007). Raajimakers MH et al. (2010) using neonatal Osterix specific dicer deletion showed altered hematopoiesis and developed myelodysplasia. However, there is no study illustrating the role of the ablation of bone marrow (BM) niche specific miRNA processing machinery in the adult mice. Since expression and functions of different mesenchymal and osteoprogenitors vary from embryonic development to adulthood, studying the dicer ablation in adult mice may provide more insight about the role of miRNA processing in adult mice niche.

Here we investigate whether adult Osterix expressing cells play a similar role in the HSC niche compared to fetal Osterix expressing cells. We crossed Osx-GFP-tTA-Cre recombinase mice with mice with floxed Dicer1 allele. Crossing generated Osx- GFP-tTA-Cre+Dicer^fl/+ (OCD^fl/+control) and Osx-GFP- tTA-Cre+ Dicer^fl/fl (OCD^fl/fl mutant) mice. Osx-GFP-tTA-Cre expression was either activated during embryonic development (young dicer KO) or suppressed using tetracycline until mice were 6 weeks of age (adult dicer KO). We found young dicer KO mice had reduced weight (p=0.0031), leukopenia, anemia, reduced mature CD19+B220- B lymphocytes (p=0.0034) and increased CD11b+Gr- monocytes and CD11b+Gr+ neutrophils (p=0.02 and p=0.04 respectively) in peripheral blood compared to OCD^fl/+ control aged littler mates. The leucocytes and platelets showed dysplastic changes suggestive of myelodysplasia and had extra-medullary hematopoiesis. Adult dice KO, on the other hand, show no leukemia development 6 months after Cre activation. The number of BM hematopoietic progenitors (Lin-Sca1+ c-Kit+ cells, LSK) and long term hematopoietic stem cells (LT-HSCs, LSK CD150+CD48+ cells) in young dicer KO mice were significantly reduced compared to age matched control (OCD^fl/+ control) mice. We observed increased Annexin V positive LSK, LT-HSCs and megakaryocytes erythroid progenitors (MEP) in the young dicer KO mice indicating increased apoptosis. Adult dicer KO mice didn't have significant changes in apoptosis in different hematopoietic progenitors. In young dicer KO mice, BM derived LSK and LT-HSCs showed increased cycling (SG2M phase, p=0.0133) and less quiescenece (Go phase, p=0.013). However LT-HSC from adult dicer KO didn't show any difference in cell cycling (p=0.18 and 0.09 respectively). Together these results indicate that while Osterix expressing cells in fetal and young mice give rise to a variety of HSC niche supporting cells the adult expression is limited to more mature osteoblast that are not absolutely essential for HSC maintenance. Our study provides the rationale for further exploration of the complexity in hierarchy of activity within niche constituting mesenchymal stroma progenitors and their role in different developmental stages to maintain hematopoiesis.