Abstract LB227: Accelerated intra-clonal transcriptomic diversification of hematopoietic stem cells during aging fuels leukemia evolution
Authors: Naddaf Lamis , De Dominici Marco , Chen Xiaowen , Roeder Travis , Oguro Hideyuki , Karmakar Shilpita , Pietras Eric , DeGregori James , Li Sheng 
Published: April 2024

Abstract
Cancers are diseases of aging. Many cellular processes characteristic of cancer, e.g., epigenetic dysregulation, are similarly observed during aging. Acute myeloid leukemia (AML) is emblematic of this interplay. As a blood cancer stemming from hematopoietic stem and progenitors (HSPC), its median age of diagnosis is 65. AML is linked to age-related clonal hematopoiesis (CH), a condition resulting from the expansion of HSPC, marked by mutations in leukemia-associated genes. We aim to clarify the intrinsic role of aging in HSPC as it causes CH and leukemic transformation.
Hypothesis: aging amplifies the heritable phenotypic variations in hematopoietic stem cells (HSC), enhancing epigenetic heterogeneity. This acts as a substrate for natural selection, driving leukemic transformation.
Results: We found a significant increase in epigenetic and transcriptomic heterogeneity in long-term HSC (LT-HSC) from old mice compared to their younger counterparts via single-cell RNA-seq (scRNA-seq)s. This process occurs in an aged bone marrow with higher inflammation and altered HSPC support. Of relevance, we previously show that the rise of epigenetic and transcriptomic heterogeneity in HSC with Tet2 loss and Flt3 ITD precedes leukemic transformation. We further employed CellTag Indexing and scRNA-seq to track the fate of specific HSC subclones. We tracked HSPC cell culture (in vitro) after two-weeks and two-months following transplantation into recipient mice of matching ages (in vivo). Cells of the same clone manifest higher transcriptomic similarities than those from different clones, validating CellTag in mapping clonal evolution. We computed the progeny output activity (Ai) by the ratio of clone i abundance in the committed progenitors to its frequency in the HSC compartment. We found that young mice exhibit significantly higher variability and a higher mean of progeny output activity Ai, indicating the efficiency in self-renewal and cell differentiation. While both young and old mice have low-output (higher stemness) clones, only young mice had significant differential expression of key genes in low-output clones compared to high-output clones, e.g., Dpp4, Eps8, and Col5a1. Depite lower progeny output Ai variation, LT-HSCs from older mice exhibited greater clonal transcriptome diversification both in vitro and in vivo , evidenced by intra-clonal transcriptomic correlation. This pattern also existed in ST-HSC and MPP from older mice. Genes with higher expression variability in older mice highlighted pathways pivotal for HSPC fitness, e.g., ribosome synthesis (RS), inflammation response mechanisms (JAK-STAT), hematopoietic cell lineage, AML and cancer pathways (GSEA). The RS pathway were remarkably enriched in the megakaryocytic-erythroid progenitors, directly before differentiation where substantial protein translation is required.
Conclusion: our results suggest that the synergy of aging and somatic mutations in epigenetic regulators may enhance cell-to-cell variations, offering additional heritable phenotypic variations. These, in turn, may facilitate clonal hematopoiesis and leukemic transformation. Future work will delineate specific epigenetic and transcriptomic configurations for mutant HSPC clonal expansion. This study provides a cohesive framework, linking the principles governing aging to cancer.
Citation Format: Lamis Naddaf, Marco De Dominici, Xiaowen Chen, Travis Roeder, Hideyuki Oguro, Shilpita Karmakar, Eric Pietras, James DeGregori, Sheng Li. Accelerated intra-clonal transcriptomic diversification of hematopoietic stem cells during aging fuels leukemia evolution [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(7_Suppl):Abstract nr LB227.