Abstract IA23: Leukemia development and the microenvironment
Authors: Sánchez-Aguilera Abel , Stavropoulou Vaia , Martín-Pérez Daniel , Tzankov Alexandar , Schwaller Jürg , Méndez-Ferrer Simón 
Published: May 2016

Abstract
Myeloproliferative neoplasms (MPNs) are hematological malignancies originated by hematopoietic stem cells (HSCs) carrying mutations, most frequently in JAK2, CALR or MPL genes. These diseases, which cannot be cured in most cases, are characterized by overproduction of myeloid and erythroid-megakaryocytic lineages. However, it remains unclear how some mutations, that are frequently common in disparate MPNs, can cause different clinical outcomes. Moreover, current treatments are mostly symptomatic and are only capable of slowing down the disease, but generally they cannot eradicate it. This suggests that other factors critically contribute to MPN progression. Our research group has demonstrated specific alterations of the microenvironment of HSCs in MPN. We have shown that mutated HSCs can only expand and trigger disease after damaging bone marrow (BM) nerve terminals and mesenchymal stem cells (BMSCs), overcoming the control that this environment can exert over mutated HSC proliferation and migration. Therefore, preserving the HSC niche might be a complementary strategy to treat these diseases. This approach is being currently tested in a multicenter phase-II clinical trial (clinicaltrials.gov identifier NCT02311569).
MPNs can be considered a pre-leukemic state since MPN patients have a higher risk of developing acute myeloid leukemia (AML), which is highly aggressive and exhibits poor response to chemotherapy. Work from other groups and our preliminary data suggest that acute leukemic cells can transform the BM microenvironment to promote leukemogenesis. We have studied the contribution of nestin+ BMSCs to the development of acute myeloid leukemia (AML) induced by the MLL-AF9 oncogene. For this purpose we have used a doxycycline-inducible rtTA;MLL-AF9 mouse strain, which closely mimics the human disease. Development of AML in rtTA;MLL-AF9 mice caused a significant reduction of stromal cells in the BM but, contrasting our previous findings in MPN, it did not seem to affect the number of BM nestin+ cells. To study the possible contribution of nestin+ cells to leukemia development, we partially depleted nestin+ cells by inducing diphtheria toxin expression in these cells using Nestin-creERT2;iDTR mice previously transplanted in a competitive fashion with preleukemic and normal BM cells. We have shown previously that nestin+ cell depletion using this strategy can accelerate MPN. In contrast, nestin+ cell depletion had the opposite effect in AML, since it selectively diminished leukemia burden in BM, spleen and blood, and reduced the number of primitive leukemic cells, without affecting normal residual hematopoiesis. Combined nestin+ cell depletion and standard chemotherapy further exaggerated the elimination of BM leukemic cells.
Among primitive leukemic cells, MLL-AF9+ lin- ckitlow sca1- cells were most significantly expanded in leukemia and were most significantly decreased upon nestin+ cell ablation, an effect that seemed to be further enhanced by simultaneous chemotherapy. This was not associated with detectable changes in the cell cycle profile of leukemic progenitors. Moreover, non-proliferative (ki67-) lin- cells were not found enriched in the vicinity of nestin+ cells, and Cxcl12 deletion in nestin+ cells, which increased thrombocytosis in our previous MPN studies, did not affect AML progression. In order to identify changes in leukemic progenitors following Nes+ cell depletion, we compared the gene expression profiles of lin- ckitlow sca1- cells isolated from control and Nes+ cell-depleted, MLL-AF9 expressing mice. Next-generation sequencing assays identified downregulation of gene sets related to HSC self-renewal, protein synthesis and degradation, and mitochondrial respiration in leukemic progenitors obtained from Nes+ cell-depleted bone marrow. The data suggests that AML-AF9+ leukemic progenitors impose changes in Nes+ cells that promote leukemogenesis. Using a novel co-culture system, we are currently testing possible mediators of the pro-survival effect of BMSCs over leukemic progenitors.
Citation Format: Abel Sánchez-Aguilera, Vaia Stavropoulou, Daniel Martín-Pérez, Alexandar Tzankov, Jürg Schwaller, Simón Méndez-Ferrer. Leukemia development and the microenvironment. [abstract]. In: Proceedings of the Fourth AACR International Conference on Frontiers in Basic Cancer Research; 2015 Oct 23-26; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2016;76(3 Suppl):Abstract nr IA23.