Title: Single-Cell Transcriptome Analyses Further Elucidate the Mechanism of Action of Eed Inhibition on HbF Induction

Mutations in the gene encoding the hemoglobin β (HBβ) subunit lead to red blood cell disorders such as Sickle Cell Disease (SCD) and β-thalassemia. Hemoglobin γ subunit, a fetal counterpart of HBβ, can relieve the disease symptoms by forming a functional complex with hemoglobin a subunit, thereby compensating for the mutated HBβ. The expression of β-like hemoglobins is tightly regulated during development, with a switch from the fetal ortholog (γ) to the adult ortholog (β) shortly after birth. Maintenance of HBγ is therapeutically beneficial for SCD patients and is observed clinically in individuals with mutations for both SCD and hereditary persistent fetal hemoglobin (HPFH) who exhibit less severe or even no SCD-related symptoms. Polycomb repressive complex (PRC) proteins have been shown to regulate the expression of β-like hemoglobins. Our previous research has demonstrated inhibitors targeting the PRC2 protein embryonic ectoderm development (EED) upregulate fetal hemoglobin (HbF) expression, leading to the formation of HbF complexes in both human CD34 ⁺ HSC-derived erythroid cells and the Townes SCD mouse model. However, the functional mechanisms of PRC complexes in this regulatory process and their target genes remain to be clarified.

In our study, we conducted single-cell transcriptome analyses on erythroid cells derived from CD34 ⁺ HSCs collected at various stages of differentiation. We identified several erythroid subpopulations, from early progenitors to maturing cells, within the CD34 ⁺ derived erythroid cell cultures. HbF ⁺ and HbF ^- cell populations were also identified using known markers.

To explore the mechanisms underlying HbF upregulation by EED inhibition, we compared single-cell transcriptome profiles of samples treated with and without an EED inhibitor. Our findings reveal that EED inhibitor treatment consistently leads to increased HbF ⁺ populations across multiple donors without impacting erythroid cell differentiation.

Differential gene expression analysis revealed sequential changes in the expression of previously identified HbF regulators during differentiation in the presence of EED inhibitor treatment. Through pseudotime analyses, we identified erythroid progenitor cells undergoing fate bifurcation, leading to an increase in the lineage of mature cells that express HBγ. Finally, by comparing the pseudotime trajectories of different samples, we identified genes whose expression changed along the differentiation trajectories in response to EED inhibitor treatment which influence cell fate. Overall, this study provides a deeper understanding of the relevant biological pathways that respond to EED inhibition and potential for the identification of novel molecular targets for HbF induction.