Human transcription factors responsive to initial reprogramming are predominantly legitimate during iPSC reprogramming

The four transcription factors OCT4, SOX2, KLF4 and MYC (OSKM) together can convert human fibroblasts to induced pluripotent stem cells (iPSCs). It is, however, perplexing that they can do so only for a rare population of the starting cells with a long latency. Transcription factors (TFs) define identities of both the starting fibroblasts and the end product, iPSCs, and are of paramount importance for the reprogramming process as well. It is critical to upregulate or activate iPSC-enriched TFs while downregulate or silence the fibroblast-enriched TFs. This report explores the initial TF responses to OSKM as the molecular underpinnings for both the potency aspect and limitation sides of the OSKM reprogramming. The author first defined the TF reprogramome, i.e., the full complement of TFs to be reprogrammed. Most TFs were resistant to OSKM reprogramming at the initial stages, which agrees with the inefficiency and long latency of iPSC reprogramming. Surprisingly, the current analyses revealed that the majority of the TFs (83 genes) that did respond to OSKM induction underwent legitimate reprogramming. The initial legitimate transcriptional responses of TFs to OSKM reprogramming were observed from fibroblasts from a different individual. Such early biased legitimate reprogramming of the responsive TFs aligns well with the robustness aspect of the otherwise inefficient and stochastic OSKM reprogramming