Human Erythroblasts with c-Kit Activating Mutations Have Reduced Cell Culture Costs and Remain Capable of Terminal Maturation and Enucleation

Abstract There is a constant need for red blood cells for transfusion therapy in the treatment of anemias and acute injury. As all blood products for transfusion come from donors, there are concerns over shortages and safety. Furthermore, many patients with transfusion-dependent anemias risk alloiumminization. The in vitro production of red blood cells would address these problems, especially as they can be genetically engineered to prevent alloimmunization. Numerous erythroid culture systems now exist for the in vitro production of red blood cells. Hematopoietic stem and progenitor cells (HSPCs) obtained from umbilical cord or peripheral blood can be differentiated into erythrocytes, however, they are limited in expansion. While umbilical cord HSPCs have greater expandability than peripheral blood, the resulting erythrocytes contain fetal globins. Pluripotent stem cells can also be used as a starting source, however only a small percentage of the cells can be differentiated into erythroblasts which also suffer from low enucleation rates. Presently, the cost of in vitro production of a unit of red cells is greater than an order of magnitude higher than obtaining it from a donor largely due to the medium and cytokine costs (Timmins & Nielsen, Trends Biotechnol, 2009). A relatively new approach of immortalizing early erythroblasts allowing unlimited expansion as well as terminal maturation and enucleation shows great therapeutic promise (Kurita et al., PLoS One, 2013; Huang et al., Mol Ther, 2014; Trakarnsanga et al., Nat Commun, 2017). However, these immortalized erythroblasts are still reliant on two costly cytokines: stem cell factor (SCF) and erythropoietin (Epo). Mutations in exon 17 of the receptor tyrosine kinase gene KIT are frequently seen in acute myeloid leukemias, gastrointestinal stromal tumors, and mast cells leading to mastocytosis. These mutations cause the c-Kit protein to spontaneously activate and transduce signal in the absence of SCF (Kit-ligand). To generate an SCF-independent HUDEP-2 cell line (Kurita et al., PLoS One, 2013), we used CRISPR/Cas9 to introduce missense and frameshifting mutations within the vicinity of Asp816 in exon 17 of the KIT gene. The resulting monoclonal cell lines were selected for by removing SCF from the expansion medium and were subsequently named KIT-CAT (KIT with Constitutively Activating Transformation). To better understand what KIT mutations allowed or impaired terminal maturation, monoclonal cell lines were genotyped by Sanger sequencing. Three cell lines with unique genotypes were chosen for further analysis. All three KIT-CAT lines had a shorter doubling time compared to HUDEP-2 cells (16.7 vs 18.9 hrs, p=0.020) and were no longer dependent on SCF or Epo. However, two of the three KIT-CAT lines showed more robust proliferation with Epo in the expansion medium. The addition of SCF to the medium caused no increase in c-Kit activation by Western blotting for phosphorylation at Tyr703. Furthermore, the low molecular weight and immature form of c-Kit is also phosphorylated in KIT-CAT cells, but not HUDEP-2 cells, indicating c-Kit activation occurs before trafficking to the cell membrane where SCF would bind (Tabone-Eglinger et al., Clin Cancer Res, 2008). Key features of erythroblast maturation are the decrease in cell and nuclear size which can be measured using imaging flow cytometry (McGrath et al., Methods, 2017). While in expansion phase, all 3 cell lines were larger in cell and nuclear area compared to the parental HUDEP-2 line. By day 6 of maturation, all three cell lines had statistically significant decreases in cell and nuclear size indicating maturation. By day 13 of culture, Wright-Giemsa staining showed that the majority of the cells were orthochromatic erythroblasts or enucleate reticulocytes. Reducing cell culture costs is needed for in vitro manufacturing of red blood cells to be economically feasible. These results show that a c-Kit activating mutations in human erythroblasts removes the cost of SCF and reduces the cost of Epo while still allowing for terminal maturation and enucleation. Disclosures No relevant conflicts of interest to declare.

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