Development of an inducible caspase-9 safety switch for pluripotent stem cell-based therapies

Induced pluripotent stem cell (iPSC) therapies offer a promising path for patient-specific regenerative medicine. However, tumor formation from residual undifferentiated iPSC or transformation of iPSC or their derivatives is a risk. Inclusion of a suicide gene is one approach to risk mitigation. We introduced a dimerizable-inducible caspase-9 (iCasp9) suicide gene into mouse iPSC (miPSC) and rhesus iPSC (RhiPSC) via a lentivirus, driving expression from either a cytomegalovirus (CMV), elongation factor-1 alpha (EF1 alpha) or pluripotency-specific EOS-C(3+) promoter. Exposure of the iPSC to the synthetic chemical dimerizer, AP1903, in vitro induced effective apoptosis in EF1 alpha-iCasp9-expressing (EF1 alpha)-iPSC, with less effective killing of EOS-C(3+)-iPSC and CMV-iPSC, proportional to transgene expression in these cells. AP1903 treatment of EF1 alpha-iCasp9 miPSC in vitro delayed or prevented teratomas. AP1903 administration following subcutaneous or intravenous delivery of EF1 alpha-iPSC resulted in delayed teratoma progression but did not ablate tumors. EF1 alpha-iCasp9 expression was downregulated during in vitro and in vivo differentiation due to DNA methylation at CpG islands within the promoter, and methylation, and thus decreased expression, could be reversed by 5-azacytidine treatment. The level and stability of suicide gene expression will be important for the development of suicide gene strategies in iPSC regenerative medicine.