Abstract A27: Cellular barcoding of the leukemic niche reveals an apelin-mediated clonal expansion of niche endothelial and mesenchymal stromal cell clones
Authors: Baron Chloé S , Avagyan Serine , Yang Song , McKenna Aaron , Zon Leonard 
Published: May 2023

Abstract
Hematopoietic stem and progenitor cells (HSPCs) reside in niches that provide regulatory signals for their function. Perturbations such as acute leukemia induce cellular and molecular insults to the niche to support disease progression. Mutation of the MYC oncogene family is a frequent event leading to leukemogenesis. We developed a new zebrafish model of acute erythroid leukemia (AEL) by overexpression of human CMYC under the blood specific promotor draculin (drl). Analyses of drl:CMYC marrows demonstrated a significant expansion of progenitors and a decrease of erythroid, lymphoid and myeloid mature cells (fc=4.8, -4.5, -3.3, -6.5; p&lt;0.000001). RNA-Sequencing of drl:CMYC marrows revealed an upregulation of the erythroid master regulator gata1a (fc=1.4, p=0.01) and fetal hemoglobins hbbe1.1/2 (fc=4.7, 2.9; p=0.0004). Primary and secondary transplantation of drl:CMYC marrows resulted in engraftment and disease propagation (7/7; 17/18). We used the cellular barcoding GESTALT technique to uniquely barcode single cells using CRISPR-CAS9 during embryonic development. We induced a round of barcoding at the one-cell stage and another one at 28 hours post-fertilization, the time of HSC birth. We injected these GESTALT embryos with drl:CMYC to induce AEL, barcode HSPCs and their niche to perform clone tracing. The number of HSPC clones was decreased by half compared to controls (p=0.008) indicative of a clonal expansion of the disease. We performed barcode and single-cell transcriptome profiling of flk1:GFP+ niche endothelial cells and found a significant decrease in the number of endothelial cells clones (fc= -3.5, p&lt;0.05) paired with the identification of a novel AEL venous endothelial population upregulating 99 genes (fc&gt;1; p&lt;0.05) suggestive of angiogenesis likely supporting leukemogenesis. We sorted cxcl12a:dsRed+ niche stromal cells and found that AEL marrows have significantly less stromal clones (fc= -2.1, p&lt;0.01) that are selectively amplified (&gt;20% of the stromal compartment). We hypothesized that AEL expands a subset of stromal cells to promote disease progression and scRNA-Seq of 3,263 cxcl12a:dsRed+ stromal cells revealed an increased fraction of lepr+ mesenchymal stromal cells (MSCs, 66 vs 24% in controls). We hypothesized that AEL progenitors secrete a signal that can specifically remodel the marrow niche and we mined our scRNA-Sequencing data for a candidate ligand significantly upregulated in AEL cells compared to control HSPCs (fc&gt;1; p&lt;0.05) and paired receptors expressed on more than 20% of AEL associated endothelial and/or stromal cells. We identified the peptide hormone apelin expressed by the leukemia and tested if apelin alone could remodel the normal marrow niche endothelial and stromal cells by overexpressing apelin under the drl promotor. Adult marrow analyses revealed a decrease in the absolute number of stromal cells per marrow (fc= -2.2, p&lt;0.001). Together our data support a model in which leukemia induces a clonal expansion of HSPC niche clones via apelin signaling to promote disease progression.
Citation Format: Chloé S Baron, Serine Avagyan, Song Yang, Aaron McKenna, Leonard Zon. Cellular barcoding of the leukemic niche reveals an apelin-mediated clonal expansion of niche endothelial and mesenchymal stromal cell clones [abstract]. In: Proceedings of the AACR Special Conference: Acute Myeloid Leukemia and Myelodysplastic Syndrome; 2023 Jan 23-25; Austin, TX. Philadelphia (PA): AACR; Blood Cancer Discov 2023;4(3_Suppl):Abstract nr A27.