Human induced pluripotent stem cells (hiPSCs) can be derived from somatic cells through a reprogramming process driven by overexpression of a defined set of transcription factors [1
]. These hiPSCs share the properties of self-renewal and pluripotency with human embryonic stem cells (hESCs), and can therefore be used to generate unlimited quantities of differentiated cell types of all three germ layers, including cardiac cells, neural cells, and hepatic cells. hiPSCs can be generated from patients of virtually any genetic background, including those with disease-conferring genetic mutations [3
]. In contrast, the derivation of hESCs from different genetic backgrounds is challenging because human embryo use is limited and ethically debated. The production of patient-specific and disease-specific hiPSCs enables a variety of downstream applications, including drug screening, disease modeling, pathogenesis studies, and regenerative medicine therapies.
However, traditional approaches to deriving hiPSCs require use of retroviruses or lentiviruses that integrate reprogramming genes into the host genome. Random integration of the reprogramming genes may result in insertional mutagenesis that causes malignant transformation of a clonal cell population [6
]. In addition, some of the genes used in the reprogramming process are known proto-oncogenes, and incomplete silencing of these transgenes may result in unknown adverse effects. These challenges have been partially circumvented in mice by the development of non-integrating viral [7
], non-viral episomal [8
], and excisional [9
] techniques for reprogramming. Yet clinical translation of these safer iPSC derivation techniques is challenging because human cells are relatively more resistant to non-viral transfection and are not immediately available in large quantities. Although hiPSCs may be generated by lentiviral transduction with subsequent Cre-loxP
excision of reprogramming factors [3
], residual vector sequences will be left behind in the genome. Transgene-free hiPSCs have been derived from neonatal foreskin fibroblasts using a combination of three episomal plasmids expressing seven reprogramming factors [11
]. Alternatively, transgene-free hiPSCs can be derived from fetal or neonatal cells by repeated transduction of proteins in the presence of chemical treatments (e.g., valproic acid) [12
]. However, none of the aforementioned techniques for transgene-free hiPSC derivation have been demonstrated using adult donors, a more clinically relevant population. Here, we describe in detail a protocol for the derivation of transgene-free hiPSCs with a non-viral minicircle DNA reprogramming construct used in conjunction with human adipose stromal cells (hASCs) [13
]. This technique is advantageous in translational studies because somatic cells from human adults
can be reprogrammed in the absence of genomic modification, viral sequences, or proto-oncogenes (such as c-Myc), effectively mitigating safety concerns [14
]. This protocol can be used to derive hiPSCs from human samples in ~4 weeks using standard molecular biology reagents and cell culture expertise ().
Schematic of hiPSC derivation protocol. Approximate time table of the hiPSC derivation process is shown with numbered steps above and cell culture media below.
Limitations of the protocol
Cells are not transduced with infectious viral particles in this protocol, ensuring a high likelihood of generating transgene-free hiPSCs. However, reprogramming efficiency using this protocol is substantially lower (~0.005%) compared to lentiviral techniques for overexpression of the transcription factors OCT4
, and LIN-28
. Further improvement of reprogramming efficiency may be achieved by treatment with small molecules (e.g. valproic acid) [15
] or cell signaling peptides (Wnt) [16
]. Also, users should be aware that the protocol as described here has not yet been successfully applied to the reprogramming of human dermal fibroblasts derived from adult sources. We have found that minicircle-based reprogramming of hASCs as described here is advantageous for deriving transgene-free hiPSCs from adult human donors, a clinically relevant cell source. Such methods offer the ability to develop patient-specific or disease-specific cell lines for exciting new translational and disease modeling studies.