p53 Is a Crucial Regulator of Hemogenic Endothelial Cell Differentiation from Human Induced Pluripotent Stem Cells

De novo generation of hematopoietic stem cells (HSCs) from human induced pluripotent stem cells (hiPSCs) could provide a virtually unlimited supply of autologous HSCs for clinical transplantation, and offer various approaches that enable gene therapy, drug discovery, disease modeling, and in vitro modeling of human hematopoietic development. However, the derivation of long-term self-renewing HSCs from hiPSCs in culture remains elusive. The tumor suppressor protein p53 plays important roles in normal and malignant hematopoiesis, and Trp53-deficient mice exhibit increased number of HSCs. Although activation of p53 is known to promote differentiation of hPSCs and hPSCs recurrently acquire TP53 dominant negative mutations, its role in hematopoietic differentiation of hiPSCs has not been explored.

To differentiate hiPSCs into hematopoietic stem and progenitor cells (HSPCs), we used embryoid body (EB) formation method to first differentiate hiPSCs into hemogenic endothelial (HE) cells that express the CD34 ^highCD144 ⁺CD73 ^-CD184 ^-CD43 ^-CD235a ^- cell-surface markers. HE cells were then transferred onto a Matrigel-coated plate to undergo endothelial-to-hematopoietic transition (EHT) to generate HSPCs that express the CD34 ^midCD45 ^mid cell-surface markers. Developed HSPCs were functionally evaluated by colony forming assay.

We observed that the expression of CDKN1A, a p53 target gene, was upregulated in hiPSC-derived EBs and HSPCs over the course of differentiation. To investigate the role of p53 in the generation of HSPCs from hiPSCs, we genetically deleted TP53 in hiPSCs followed by hematopoietic differentiation. While TP53 deletion increased the growth of EBs, it resulted in severe impairment of differentiation into HE cells and overall production of HSPCs that can form colonies. During HE differentiation from hiPSCs, TP53-deficient EBs showed significant reduction of endothelial-lineage gene expression, such as ETV2, CDH5, and PECAM1, as well as expression of RUNX1, a master transcription factor required for HE specification. These results indicate the indispensable role of p53 in HE differentiation from hiPSCs.

We then examined the effect of p53 activation on HE differentiation from hiPSCs by pharmacological activation of p53 in hiPSC-derived cells. Transient activation of p53 by Nutlin-3, a small molecule that inhibits the p53-HDM2 interaction and protects p53 from proteasomal degradation, only during HE differentiation but not during EHT significantly promoted HSPC generation as compared to the vehicle treated control.

Our findings shed light on the importance of selecting hiPSC lines that retain normal p53 activity for HE differentiation, and provide an approach to promote hematopoietic differentiation of hiPSCs by transiently activating p53 during HE differentiation.