There are three main sources of transplantable cells for hematopoietic cell transplantation (HCT): bone marrow, mobilized peripheral blood and umbilical cord blood [1,2], but it can be very difficult finding an appropriate donor, and numbers of hematopoietic stem cells (HSCs) isolated from these sources could be limited, such as for cord blood HCT [3,4▪▪]. Various ways to expand HSCs ex vivo have been evaluated, but there are limitations to these efforts [5,6]. To develop a patient-specific cell therapeutic agent, investigators have been evaluating pluripotent stem cells [(PSCs); especially reprogrammed induced pluripotent stem cells (iPSCs)]  and their capacity to differentiate into HSCs/hematopoietic progenitor cells (HPCs) . We highlight recent approaches to achieve functional HSCs and other hematopoietic cells from PSCs, ranging from ectopic expression of transcription factors, to the use of gene editing technology for correcting mutated genes in PSCs (Fig. 1).
Human embryonic stem cells and induced pluripotent stem cells
Human embryonic stem cells (hESCs) were established from the inner cell mass of the human embryo . As these cells can undergo differentiation into most cell types in the body, they have been explored for cell-based therapy. However, ethical controversies of using embryos have in part prevented our realizing the full potential of these cells for clinical use. A breakthrough overcoming some ethical concerns of utilizing hESCs has been discovery of iPSCs . By introducing four reprogramming genes (Oct4, Sox2, Klf4 and c-Myc) into mature somatic cells, a new type of PSCs was established. To establish clinically applicable iPSCs, reprogramming efficiency has been increased [9▪,10], a nongenome integrating gene delivery system has been devised  and efforts have been made to differentiate these cells into tissue-specific transplantable cells. As an example, iPSC-derived retinal pigment epithelium cell sheets generated from autologous fibroblasts were successfully transplanted into patients suffering from age-related macular degeneration . However, generating functional HSCs with high engraftment efficiency remains an ongoing target, the promise of which is suggested by the following interesting studies.
Direct differentiation of hematopoietic cells from hPSCS using ectopic expression of transcription factors
Hematopoietic lineage differentiation of human induced pluripotent stem cell (hiPSCs) has been most commonly carried out by two conventional methods: a two-dimensional differentiation protocol utilizing coculture of PSCs with stromal cells [13,14] or embryoid body-mediated three-dimensional differentiation . CD34+ hematopoietic precursor cells expressing hematopoietic transcription factors were derived from iPSCs using a coculture method in the presence of either primary human bone marrow-derived stromal cells or an assortment of stromal cell lines, including OP9, OP9-DL4, M2-10B4 and FH-B-hTERT [16–18]. As an alternative to this approach, embryoid body-mediated hematopoietic lineage differentiation of human pluripotent stem cells (hPSCs) was developed [15,19]. The first such studies incorporated a mixture of defined cytokines and fetal bovine serum. The most widely used differentiation methods of hPSCs into hematopoietic cells have involved addition of cytokines or induction of spontaneous differentiation during embryoid body formation . To achieve higher differentiation efficiency, there is a need for identifying specific intrinsic signals that regulate the production of hematopoietic cells in vitro. To generate more HSCs/HPCs from hPSCs, unlike classic differentiation methods (coculture and embryoid body formation), hematopoietic cells derived were during in-vivo teratoma formation [21,22]. Human PSCs were transplanted into immunodeficient mice to form teratomas (typically comprising three germ layer derivatives, including endodermal and neuronal lineage cells) from which CD34+CD45+ cells were isolated, suggesting that in-vivo microenvironmental cues are important for directed differentiation and development of immature blood cells [4▪▪]. It is unclear if such derived cells would be a viable option for clinical use.
Enhancing directed differentiation by ectopic expression of a single transcription factor
Direct differentiation by introducing ectopic expression of transcription factors represents an alternative strategy over classical differentiation methods. This approach involving candidate transcription factors such as T-cell acute lymphocytic leukemia 1 (TAL1), stem cell leukemia (SCL), runt-related transcription factor 1 (RUNX1), SRY-Box17 and Homeobox B4 (HOXB4) as key regulators of mesodermal and HSC developments from hPSCs has been recently introduced to control stem cell fate determination [23–27,28▪▪].
TAL1, also known as SCL, is a transcript in the emerging hemangioblast during hESC hematopoietic differentiation [23,29]. As TAL1-overexpressing hESC-derived embryoid bodies was used to accelerate the formation of erythro-megakaryocytic progenitors , TAL1 has been shown to support a hematopoietic program in hPSCs. Overexpression of TAL1 in hPSC enhanced the emergence of megakaryocytic precursors, mature megakaryocytes and platelets in vitro[30▪], however, these cells failed to engraft in vivo[30▪].
RUNX1 is a key transcription factor regulating differentiation of hematopoietic stem cell into mature blood cells . Overexpression of RUNX1A in hESC-derived/hiPSC-derived embryoid bodies significantly enhances hematopoietic differentiation and accelerates generation of hemato-endothelial cells . RUNX1 controls lineage specification of hPSCs into mesoderm and specifically enhances hemogenic differentiation as well as production of definitive HSCs. However, ectopic RUNX1 expression may entail risk of mediating transformation of hPSC-derived cells, as it is known to contribute to leukemogenesis .
Two types of inducible fusion proteins have been developed, including HOXB4-ERT2 and Kruppel like factor1 (KLF1)-ERT2 that can be induced at a defined time point during differentiation of hPSC to RBCs [33▪,34▪]. Activation of HOXB4 increases progenitor cell (CD43+/CD34+) populations and proportions of immature CD235a+/CD71+ erythroid cells from hPSC [33▪]. HOXB4 activity promotes the generation of embryonic (ε)/fetal (γ) globins rather than more mature adult (β) globin, a definitive phenotype, suggesting that HOXB4 induces production of progenitors. But, it does not overcome the transitioning barrier for production of enucleated RBCs. Activation of KLF1 at day 10 of differentiation of hiPSCs enhanced erythroid commitment and differentiation. Extended in-vitro culture resulted in the generation of more enucleated cells, but not expression of adult (β) globin [34▪]. Thus, HOXB4 or KLF1 plays important roles in hematopoietic development, but there are still limitations to producing mature RBC from hPSC.
Enhancing directed differentiation using ectopic expression of multiple transcription factors
Overexpression of a single transcription factor alone in hPSCs is not enough to yield hematopoietic cells that engraft in vivo. Concomitant ectopic expression of multiple transcription factors has been employed to overcome this barrier [24,26]. The most noticeable findings suggest that HOXA9, ETS-related gene and RAR related Orphan Receptor A confer self-renewal capacity of myeloid precursor cells in vitro, and addition of SOX4 and myeloblastosis oncogene (MYB) to these transcription factors confers short-term engraftment of myeloid and erythroid lineages in vivo. GATA binding protein2 (GATA2) and ETS variant2 promoted pan myeloid differentiation, whereas GATA2 and TAL1 enhanced erythro-megakaryocytic differentiation from hPSCs .
Large-scale production of megakaryocytes and platelets from PSCs has been achieved by simultaneously overexpressing GATA1, friend leukemia integration1 (FLI1) and TAL1 during early stages of differentiation in chemically defined conditions [35▪▪]. Functional platelets were generated throughout the culture, allowing prospective collection of several transfusion units from as few as 1 million starting hPSCs. It remains to be determined how closely these three transcription factors recapitulate normal hematopoietic development from hPSCs.
Ectopic overexpression of transcription factors in hiPSCs is a relatively quick and efficient tool for induction of HSCs hematopoietic cells, and this system can be used for identifying specific transcription factors that are required for endothelial and hematopoietic specification. However, current protocols have limited utility for studies of extracellular signaling involved in hematopoietic development, as it uses transcription factors to bypass surface receptor-mediated signaling. If we are to produce HSCs in vitro with the capacity for efficacious engrafting short-term and long-term, we must understand the exact orchestration of both intrinsic and extrinsic signals which mimic the complex environment of the human embryo. Understanding transcription factor programming is in its infancy. We are still a way from achieving large-scale production of engraftable HSCs and functional mature blood cells that can be used safely in the clinic [28▪▪].
Direct conversion of somatic cells into hematopoietic cells
Direct conversion methods that introduce lineage-restricted transcription factors into somatic cells and induce them into tissue-specific cells have generated attention [36–38]. This method bypasses intermediate stages where cells acquire pluripotency; these directly induced cells are not able to form teratomas. Attempts have been made to directly induce hematopoietic cell production by overexpressing early developmental hematopoietic associated transcription factors along with transcription factors controlling cell fate in somatic cells.
Studies succeeded in directly converting human fibroblasts into immature blood cells by overexpressing the transcription factor, OCT4 . Although succeeding in directly converting fibroblasts into cells capable of differentiating into granulocytic, erythrocytic, monocytic and megakaryotic colonies, they had limited in self-renewal potential, hematopoietic reconstitution and differentiation capacity into lymphoid cells. Some have used a parallel strategy [40,41,42▪▪,43▪▪]. Overexpression of GATA1, TAL1, LMO2 and MYC proto-oncogene (GTLM) genes in mouse/human fibroblast cells dedifferentiated them into primitive erythroid progenitors [42▪▪]. An induced erythroid progenitor was established with an adult-type globin expression pattern by additionally overexpressing KLF1 and MYB with GTLM [42▪▪]. GATA2 and RUNX1 were used with GTLM gene to produce megakaryocytes and platelets cells from patients with Fanconi anemia [43▪▪]. These studies [42▪▪,43▪▪] exemplify how direct conversion is emerging as a promising tool to generate blood cells in vitro. Efforts are needed to obtain therapeutic-scale production and to design other gene introduction methods, possibly using nonviral systems. Although direct conversion methods may not have a high risk of producing cells that form teratomas, mechanisms of direct conversion have not been clarified yet, and there is a possibility of mutation because of the introduced gene; safety validations are required for their clinical application.
Improving hematopoietic differentiation of induced pluripotent stem cells by CRISPR/Cas9
Since development of the CRISPR-Cas9 gene-editing system , this technology evaluated for its potential to treat various genetic diseases . A number of groups have successfully applied CRISPR-Cas9 technology to correct β-thalassemia mutations in patient-derived iPSCs [46▪,47▪,48,49]. A disease-causing mutation in the β-globin gene (HBB) was corrected using CRISPR-Cas9 in iPSCs derived from a β-thablassemia patient with minimal off-target effects . HBB mutations in the patient-derived iPSCs were completely corrected, with increased production of HPCs. In another study, mutations of CD41/42 (–CTTT) in iPSCs from a β-thalassemia patient were successfully corrected using a combination of single-strand oligodeoxynucleotides with CRISPR/Cas9 [45,49]. The corrected iPSCs were selected for erythroblast differentiation and manifested restored expression of HBB protein. Sickle cell disease, which has a homozygous missense point mutation in the HBB gene encoding adult β-globin protein, is a severe incurable chronic anemia. HBB 20 bp downstream to the β mutation was corrected in human iPSCs using CRSPR/Cas9, with the 16-kDa β-globin protein expressed from the corrected HBB allele in erythrocytes that were differentiated from the genome-edited iPSCs [46▪].
Hemophilia, which affects about 400 000 people worldwide, is a hemorrhagic disease caused by a lack of protein that hardens blood due to a genetic mutation. The inverted FVIII gene in hemophilia patient-derived iPSCs was corrected using CRISPR/Cas9, without detectable off-target mutations in other genome locations. The corrected cells were then induced to differentiate into vascular endothelial cells, producing blood coagulation factors. These cells were transplanted into hemophilia mice, with symptom improvement [47▪].
The CRISPR/Cas9 system for precise genome editing may be a useful tool for removing and correcting genes or mutations involved in inherited hematological disorders. However, before use of CRISPR/Cas9-mediated gene correction in humans, appropriate delivery systems with higher efficiency and specificity must be identified [50,51].
Some believe that hiPSCs hold better promise than hESCs for clinical translation . Human iPSCs may in the future help to generate larger numbers of histocompatible cells for HCT and other organ transplants, possibly as a customized cell therapeutic agent. We are not there yet, but if iPSCs established from patient somatic cells can be differentiated into functional somatic cells after correcting the mutant genes with CRISPR/Cas9 or other technologies, this would be a major health advance. This is especially of interest if hematopoietic differentiation of iPSCs and production of engrafting HSCs and HPCs are successfully demonstrated by using a combination of intrinsic and extrinsic influences. Currently, transplantation of iPSC-derived hematopoietic cells is still limited to animal testing models. Low hematopoietic differentiation efficiency with low engrafting capability and the potential to form cancer in vivo are limiting factors that must be overcome for this field to progress.
Special thanks to STEMOPIA for continued support and CREKA for illustration.
Financial support and sponsorship
M.R.L.'s studies are supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2015R1A6A1A03032522), and work supported by the Soon Chun Hyang University Research Fund.
Conflicts of interest
The authors have no competing financial interests and potential conflicts of interest.
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