Re-emergence of undifferentiated cells from transplants of human induced pluripotent stem cells as a possible risk factor of tumourigenesis

Although induced pluripotent stem cells (iPSCs) are a potential source for transplantation therapy, malignant transformation (tumourigenesis) remains a major concern in their safe clinical application. iPSCs are considered more tumourigenic than embryonic stem cells (ESCs) because of genetic and epigenetic manipulations. We generated 22 human iPSC lines from normal human fibroblasts and injected three of these cell lines into SCID mice, and produced three tumours, all of which were identified as teratomas with at least two germ layers. Using cells cultured from them, RT-PCR showed that the cells expressed undifferentiated cell markers, including OCT4 and NANOG. This suggests that some undifferentiated cells remain in the teratoma during its formation. We also found emergence of cells expressing undifferentiated cell markers from teratoma-derived cells during culturing with the ESC medium. Immunocytochemical analyses showed that NANOG-, OCT4- and SSEA4-positive cells appeared and increased with time in culture. These data indicate that iPSC-like undifferentiated cells can emerge from differentiated cells under certain condition and they may present a potential risk of tumourigenesis, as do residual iPSCs.