Modeling Congenital Amegakaryocytic Thrombocytopenia Using Patient-Specific Induced Pluripotent Stem Cells

Abstract 703

Patient-specific, induced pluripotent stem cells (iPSCs) enable us to study disease mechanisms and drug screening. To clarify the phenotypic alterations caused by the loss of c-MPL, the thrombopoietin (TPO) receptor, we established iPSCs derived from skin fibroblasts of a patient who received curative bone marrow transplantation for congenital amegakarycytic thrombocytopenia (CAMT) caused by the loss of the TPO receptor gene, MPL. The resultant CAMT-iPSCs exhibited mutations corresponding to the original donor skin. Then using an in vitro culture system yielding hematopoietic progenitor cells (HPCs), we evaluated the role of MPL on the early and late phases of human hematopoiesis.

Although CAMT-iPSCs generated CD34+ HPCs, per se, their colony formation capability was impaired, as compared to control CD34+ HPCs. Intriguingly, both Glycophorin A (GPA)+ erythrocyte development and CD41+ megakaryocyte yields from CAMT-iPSCs were also impaired, suggesting that MPL is indispensable for MEP (megakaryocyte erythrocyte progenitors) development. Prospective analysis along with the hematopoietic hierarchy revealed that, in CAMT-iPSCs but not control iPSCs expressing MPL, mRNA expression and phosphorylation of putative signaling molecules downstream of MPL are severely impaired, as is the transition from CD34+CD43+CD41-GPA- MPP (multipotent progenitors) to CD41+GPA+ MEP. Additional analysis also indicated that c-MPL is required for maintenance of a consistent supply of megakaryocytes and erythrocytes from MEPs.

Conversely, complimentary transduction of MPL into CAMT-iPSCs using a retroviral vector restored the defective erythropoiesis and megakaryopoiesis; however, excessive MPL signaling appears to promote aberrant megakaryopoiesis with CD42b (GPIba)-null platelet generation and impaired erythrocyte production.

Taken together, our findings demonstrate the usefulness of CAMT-iPSCs for validation of functionality in the human hematopoiesis system. For example, it appears that MPL is not indispensable for the emergence of HPCs, but is indispensible for their maintenance, and for subsequent MEP development. Our results also strongly indicate that an appropriate expression level of an administered gene is necessary to achieve curative gene correction / therapy using patient-derived iPSCs.