Single-Cell Transcriptomics Reveal Altered Hematopoietic Mechanisms Driven By T21 and GATA1s
Authors: Takasaki Kaoru , Thom Christopher S. , Smith David , Kumar Sara S. , Sit Ying Ting , Gagne Alyssa , French Deborah L. , Chou Stella T. 
Published: December 2023

Introduction
Trisomy 21 (T21) is associated with baseline erythrocytosis and thrombocytopenia and risk of transient abnormal myelopoiesis (TAM) and acute megakaryoblastic leukemia of Down syndrome (ML-DS). TAM and ML-DS are characterized by mutations in the transcription factor GATA1, resulting in the truncated isoform GATA1s (G1S). We previously found that T21 increases erythropoiesis, while G1S severely impairs erythroid development but enhances megakaryocyte (MK) proliferation (Byrska-Bishop et al, JCI 2015). To better understand how T21 and G1S each impact hematopoiesis, we performed single-cell RNA sequencing (scRNA-seq) of multipotent and late hematopoietic progenitor cells (HPCs) differing only by chromosome 21 and/or G1S status.
Methods
Four isogenic human induced pluripotent stem cell (iPSC) lines differing only by chromosome 21 (euploid vs. T21) and/or GATA1 (WT vs. G1S) status underwent hematopoietic differentiation by embryoid body formation. Cells were collected at 3 timepoints: on day 7 (D7), CD41 +235 + multipotent HPCs were purified by flow cytometry and on days 9 (D9) and 11 (D11), late HPCs biased to a single lineage were collected. Cells were sequenced by scRNA-seq and underwent quality control followed by clustering and analysis using Seurat. Clusters were manually annotated based on expression of 141 key hematopoietic genes, including aggregated expression of lineage markers. Early vs. committed lineage clusters were determined based on the relative expression of lineage-specific markers. Within each lineage, differentially expressed genes (DEGs) associated with a genetic background were assessed using the Wilcoxon rank-sum test. The χ² test was used to compare categorical data. A 2-tailed p or adjusted p &lt;0.05 was considered statistically significant.
Results
Cluster annotation revealed HPC, early and committed erythroid and MK, and myeloid cells (Figure 1). D7 T21/G1S HPCs surprisingly showed an erythroid bias, with upregulation of genes such as GYPA and AHSP, while euploid/G1S HPCs showed a MK bias, with upregulation of genes including PF4V1 and PF4. With wtGATA1, neither euploid nor T21 D7 HPCs showed a significant lineage skew.
On D9 and D11, the euploid/WT, T21/WT, and T21/G1S cells developed subpopulations committing to the erythroid lineage, while the euploid/G1S cells demonstrated minimal erythroid potential (Figure 1). Across all 3 timepoints, early and committed erythrocytes from T21 and euploid cells with G1S had a greater number of combined DEGs than their wtGATA1 counterparts (G1S vs. WT, T21: 692 vs. 378; euploid: 1235 vs. 599). On D9, T21 was associated with a greater erythroid skew (38.6% of T21/WT and 32.2% of T21/G1S cells vs. 27.8% of euploid/WT cells; p &lt;0.00001). By D11, this erythroid drive was no longer apparent, and the T21/G1S erythrocytes were significantly less mature (Table 1; p &lt;0.00001). These data suggest that T21 initially enhances erythropoietic drive, while G1S results in a near-complete erythroid block in euploid cells but an incomplete developmental block with T21.
All 4 genotypes yielded MK subpopulations on D9 and D11 (Figure 1). With wtGATA1, euploid and T21 MKs were predominantly early MKs by D11 (Table 1). Interestingly, G1S was associated with enhanced MK commitment and maturation in the euploid context but arrest at the early stage with T21 (Table 1; p &lt;0.00001). G1S was again associated with increased DEGs compared to wtGATA1 (G1S vs. WT, T21: 594 vs. 454; euploid: 1689 vs. 1310). These data suggest that G1S in a euploid background promotes MK differentiation, but T21 and G1S cooperate to generate immature, abnormal MKs.
Conclusions
Although the hematopoietic abnormalities and leukemic risk of T21 have been well-described, the underlying developmental events remain unclear. This scRNA-seq timecourse of HPCs differentiated from isogenic iPSCs reveals the individual and synergistic effects of T21 and G1S. Overall, T21 enhances erythropoietic drive while G1S alone suppresses erythropoiesis. G1S in euploid cells enhances megakaryopoiesis, but with T21, megakaryopoietic commitment and maturation are uncoupled with a strong bias to early MKs. Together, the competing effects of T21 and G1S appear to result in abnormal, immature erythrocytes and MKs with impaired lineage commitment, consistent with a pro-leukemic phenotype.