Does Retroviral Insertional Mutagenesis Play a Role in the Generation of Induced Pluripotent Stem Cells?

In their recent report, Aoi et al.1 described the establishment of induced pluripotent stem (iPS) cells from adult mouse hepatocytes and gastric epithelial cells (designated iPS-Hep and iPS-Stm cells, respectively) by retroviral transduction of cDNAs encoding the Oct 3/4, Sox2, Klf4 and c-Myc transcription factors. The authors noted that no common retroviral integration sites (CISs) were detected among four iPS cells analyzed and therefore concluded that retroviral integration into specific sites was not required for iPS cell generation. Examination of their data (Figures S7-S11 in Aoi et al.1) indicates that the iPS cell clones contain integrations into CISs previously identified by retroviral-mediated tumor induction in mice, as listed in the Mouse Retrovirus Tagged Cancer Gene Database (RTCGD) (summarized in Supplementary Table S1).2 This is particularly noteworthy in the cases of iPS-Stm cell clones 99-1 and 99-3. In the iPS-Stm cell clone 99-1, the c-Myc retroviral vector was found to have integrated downstream of the Zfp36l2 gene (originally designated the Evi134 CIS) associated with oncogenesis in a number of tumor models, and required for the progression of mouse embryos beyond the two-cell stage of development.3 Among the retroviral integration sites in the iPS-Stm cell clone 99-3, the c-Myc retroviral vector was demonstrated to hav