Understanding the role of bone marrow niche in myeloid malignancies

Abstract

Normal hematopoiesis is generated and maintained by rare hematopoietic stem cells (HSCs) through their capacity to self-renew and differentiate. This process is rigorously controlled, both by HSC-intrinsic molecular programs and extrinsic signals emitted by the local bone marrow (BM) microenvironment, the so-called HSC niche. The BM niche consists of many cellular elements, including mesenchymal stem cells (MSCs), and soluble factors secreted by the cells. The niche homeostasis is critical for maintenance of normal hematopoiesis, and disruption of this BM niche may lead to malignant hematopoiesis, including leukemia. On the other hand, once malignant hematopoiesis is established, the niche structure and composition can be altered to protect leukemia-initiating stem cell (LSC). The aims of the presented thesis were to investigate the role of the BM niche in development of myeloid malignancies.

In study I, we analyzed expression of leukotriene (LT) signaling molecules in LSCs derived from chronic myeloid leukemia (CML) patients, and tested their response to pharmacological inhibition of LT signaling. By using single cell PCR, we found only low expression of ALOX5 in patient BCR-ABL+ LSCs and BCR-ABL-HSCs. Moreover, in contrast to previous observations in mice and in liquid cultures in vitro, pharmacological inhibition of ALOX5 did not result in any significant growth suppression of CML LSCs in long-term culture initiating cell (LTC-IC) assay on a stromal cell layer. Furthermore, although expression of CYSLT1 was detected in the majority of analyzed LSCs, treatment with its antagonist, montelukast, did not significantly reduce the LTC-IC activity of LSCs. Thus, these results suggest that pharmacological inhibition of the LT pathway might not be sufficient to eradicate LSCs, particularly in the presence of BM stromal cells.

In study II, we investigated the role of BM niche in pathogenesis of MDS/MPN by using a Sipa1-/- mouse model. We found that Sipa1 was expressed in BM stromal cells from mice and healthy humans, but was downregulated in these cells from patients with MPN and MDS/MPN. Additionally, Sipa1 deficiency in mice led to phenotypical and functional alterations in the BM cellular niche prior to disease development, and reciprocal transplantation experiments further confirmed that Sipa1-/- BM niche was a prerequisite for MDS/MPN development. Moreover, RNA sequencing analysis showed dysregulated expression of inflammatory cytokines and growth factors in the BM stromal cells from young, disease-free Sipa1-/- mice. Altogether, our data suggest that Sipa1 expression in the BM stromal cells is critical for maintaining BM niche homeostasis, and that Sipa1 deficiency in BM niche plays an instructive role in development of MDS/MPN in mice.

Finally in study III, we prospectively characterized BM stromal cells in newly-diagnosed patients with CML. First of all, we discovered that patient’s BM stromal cells share similar immunophenotype as normal BM (NBM) counterparts, but that the CML BM niche composition was changed, showing increased frequency of endothelial cells. Moreover, we found alterations in functional properties of CML-derived MSCs, e.g. an impaired osteochondrogenic differentiation potential, and an enhanced capacity to support NBM hematopoietic stem and progenitor cells in vitro. Even though no BCR-ABL fusion gene was detected in CML BM stromal cells, the RNA sequencing revealed cytokine dysregulation, particularly loss of CXCL14 in CML BM niche. Interestingly, restoration of CXCL14 expression in stromal cells suppressed the growth of LSCs in LTC-IC assays, but promoted their differentiation. These results indicate that CXCL14 might help to eradicate LSCs and therefore serve as a new therapeutic candidate for CML treatment.

To conclude, we herein showed that BM niche might contribute to myeloid malignancies in mice and human. Thus, targeting the dysregulated BM niche factors and the abnormal interaction between BM niche and LSCs could be a promising therapeutic approach to treat patients with myeloid malignancies.