Fail-Safe Therapy by Gamma-Ray Irradiation Against Tumor Formation by Human-Induced Pluripotent Stem Cell-Derived Neural Progenitors

Abstract

Cell replacement therapy holds great promise for Parkinson's disease (PD), but residual undifferentiated cells and immature neural progenitors in the therapy may cause tumor formation. Although cell sorting could effectively exclude these proliferative cells, from the viewpoint of clinical application, there exists no adequate coping strategy in the case of their contamination. In this study, we analyzed a component of proliferative cells in the grafts of human-induced pluripotent stem cell-derived neural progenitors and investigated the effect of radiation therapy on tumor formation. In our differentiating protocol, analyses of neural progenitors (day 19) revealed that the proliferating cells expressed early neural markers (SOX1, PAX6) or a dopaminergic neuron progenitor marker (FOXA2). When grafted into the rat striatum, these immature neurons gradually became postmitotic in the brain, and the rosette structures disappeared at 14 weeks. However, at 4-8 weeks, the SOX1(+)PAX6(+) cells formed rosette structures in the grafts, suggesting their tumorigenic potential. Therefore, to develop a fail-safe therapy against tumor formation, we investigated the effect of radiation therapy. At 4 weeks posttransplantation, when KI67(+) cells comprised the highest ratio, radiation therapy with (137)Cs Gammacell Exactor for tumor-bearing immunodeficient rats showed a significant decrease in graft volume and percentage of SOX1(+)KI67(+) cells in the graft, thus demonstrating the preventive effect of gamma-ray irradiation against tumorigenicity. These results give us critical criteria for the safety of future cell replacement therapy for PD.

Efficient Neural Differentiation of hPSCs by Extrinsic Signals Derived from Co-cultured Neural Stem or Precursor Cells

“…hPSCs remains a serious and critical hurdle for broader clinical implementation. [1][2][3] A variety of approaches to selectively ablate the undiff. cells and mitigate the tumor risks of hPSC-based cell therapy have been examined, including: (1) small-molecule targeting of cell death or survival pathways, (2) genetic engineering to introduce a suicide gene or microRNA switch, (3) antibodies targeting a surfacespecific antigen (or antibody-guided toxins), and (4) induction of selective phototoxicity.…”

“…hESC or iPSC markers frequently remain after efficient neural induction, 1,10,11 which can potentially induce tumor formation in the grafted brain. [1][2][3] In addition, the neural induction efficiency of hESCs/hiPSCs varies considerably depending on the cell line, 12,13 type of feeder culture, passage number of hESCs, 14 and other unidentified factors. Specifically, the neural differentiation propensities of hESCs are variable depending on the level of miR371-3 expression in the cells, and certain cell lines resist induction of differentiation by any of the available methods.…”

Fail-Safe System against Potential Tumorigenicity after Transplantation of iPSC Derivatives

“…It is impractical to identify all these risks through pre-clinical studies, and the development of pre-transplantation safety measures that eliminate all risk of tumorigenesis is unrealistic. For these reasons, post-transplantation safety measures, such as surgery and γ-ray irradiation (Katsukawa et al., 2016), are also extremely important. In the present study, we explored the efficacy of a suicide gene introduced into iPSCs as a fail-safe against post-transplantation tumorigenic transformation.…”

Elimination of proliferating cells from CNS grafts using a Ki67 promoter-driven thymidine kinase

“…Given the potential toxicity of CNS irradiation, we do not think that this approach is the most promising for future application in patients. 24 …”