Abstract 2496: The landscape of the pre-leukemic bone marrow niche in acute myeloid leukemia
Authors: Goda Chinmayee , Kulkarni Rohan , Rudich Alexander , Karunasiri Malith , Rajgolikar Girish , Mundy-Bosse Bethany , Garzon Ramiro , Mardis Elaine , Miller Katherine , Dorrance Adrienne M. 
Published: April 2023

Abstract
Acute myeloid leukemia (AML) is a heterogenous disease that develops due to genetic mutations occurring within hematopoietic stem/progenitor cells (HSPCs), giving rise to clonal proliferation. Mutated leukemic stem cells (LSCs) can remain dormant within the bone marrow (BM) for decades without disease, suggesting the possibility that LSCs could alter the normal crosstalk between HSCs and cells that comprise the BM niche. Therefore, remodeling of the BM niche, in addition to the acquisition of genetic mutations in HSPCs, may be required for full-blown leukemic transformation. Uncovering novel mechanisms by which LSCs remodel their microenvironment may be critical in the hopes of developing novel targeted therapies to completely eradicate the disease. Our lab has previously developed a murine model of AML (MllPTD/WT; Flt3ITD/WT (PTD; ITD)) that recapitulates important hallmarks of the human disease. This model develops AML sporadically with a preleukemic phase in which they display normal white blood counts (WBCs) and absence of leukemic blasts in the bone marrow. Recently, we observed a persistent decline in red blood cells (RBCs) in these mice that preceded leukemia development without any increases in BM cellularity. Thus, this unique model provides us with an opportunity to evaluate changes in the BM niche that could correlate with LSC emergence. Using this model we performed single cell RNA-sequencing on preleukemic BM niche cells compared to wild-type (WT) controls. Upon performing unbiased clustering, we found 17 stromal cell clusters, with 5 clusters of Lepr+ mesenchymal stromal cells, 3 clusters of endothelial cells, 4 fibroblast clusters, 3 osteo-lineage cell clusters 1 pericyte cluster, and 1 chondrocytes cluster. There was an increase in pericytes, with a shift away from the normal niche regulatory populations including LepR+ mesenchymal cells and endothelial cells in the preleukemic mice. Interestingly, in contrast to other studies examining the BM niche after full blown AML, we found an increase in fibroblasts, specifically CD55+ fibroblasts, that was confirmed by immunohistochemical microscopy. This increase in CD55+ fibroblasts was found to be a result of increased proliferation of this subpopulation. Differential gene expression analysis in revealed down-regulation of several collagen genes including: Col1a1, Col1a2, Col3a1, Col4a1, and Col6a1. Alterations of ECM proteins in preleukemic BM were confirmed by immunohistochemical analysis, suggesting remodeling of the ECM is an important mechanism by which LSCs lead to alterations in the BM niche and contributes to disease progression. Overall, we have confirmed that LSCs remodel their BM niche leading to an increase in CD55+ fibroblasts expressing decreased levels of critical ECM proteins. These changes might support the maintenance/generation of preleukemic clones at early stages that could ultimately evolve into leukemic transformation.
Citation Format: Chinmayee Goda, Rohan Kulkarni, Alexander Rudich, Malith Karunasiri, Girish Rajgolikar, Bethany Mundy-Bosse, Ramiro Garzon, Elaine Mardis, Katherine Miller, Adrienne M. Dorrance. The landscape of the pre-leukemic bone marrow niche in acute myeloid leukemia [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 2496.