High Throughput Multi‐omic Characterization of Clonal Hematopoietic Stem Cells During Progression of Myeloid Malignancies

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Hematopoietic stem cells (HSCs) are responsible for giving rise to all other lineages of blood cells in the body. Over time, mutations in HSCs can promote the outgrowth of clonal populations that outcompete other HSCs, resulting in a phenomenon called Clonal Hematopoiesis of Indeterminate Potential (CHIP). Though not cancerous in and of itself, CHIP can progress to more serious hematologic disorders, such as the Myelodysplastic Syndromes (MDS) and Acute Myeloid Leukemia (AML). The mechanisms of clonal expansion, by which certain mutant HSCs acquire a competitive advantage over other HSCs, currently remain largely undeciphered, as are the mechanisms by which clonal HSCs drive the initiation of MDS and contribute to the development of AML. Moreover, previous studies have shown that such mutant HSCs are resistant to conventional therapies and may act as reservoirs for disease relapse and progression. In the past, researchers were hindered by bulk cell analyses and the relative rarity of HSCs, but advances in single‐cell omics have now enabled us to explore the molecular heterogeneity of clonal HSCs and identify distinct clonal populations based on genotype and cell surface phenotype. We used single cell RNA sequencing to examine the transcriptional dynamics of purified HSCs from MDS and AML patients before and after treatment, as well as from age matched elderly controls. Interestingly, dimensionality reduction methods such as UMAP and tSNE revealed a reservoir of control HSCs that clustered with MDS HSCs. Upon comparison with other normal control HSCs, we found that genes associated with aging, mitochondrial function, and particular ion channels were strongly upregulated in these “MDS‐like” control HSCs, while genes involved in ribosomal and translation activity, along with certain surface markers, were substantially downregulated. Additionally, ribosomal transcripts were depleted in MDS HSCs from patients who did not respond to treatment. These results support the notion that the most immature HSCs, which impose the strictest constraints on translation, might clonally expand and initiate CHIP and/or MDS upon acquiring driver mutations, also serving as treatment resistant populations that underlie disease relapse. We also subjected purified MDS HSCs to simultaneous single cell targeted DNA sequencing and cell surface phenotyping, which allowed us to correlate the cell surface phenotype and genotype for specific clonal populations and identify cell surface markers that can be used to isolate HSCs with enhanced engraftment ability. Taken together, these results indicate that combined single cell genotyping and phenotyping can be used to track clonal populations across different stages of pathogenesis, providing further insight about the development of CHIP and its progression to MDS and AML.