Fibroblasts obtained from skin biopsies are typically used in the generation of iPSCs, although keratinocytes from hair follicles and peipheral blood mononuclear cells (PBMCs) have also been used as a more non-invasive source of iPSCs^1,2. Once a somatic cell source has been cultured, the cells are then reprogrammed. Reprogramming is traditionally achieved by infecting cells with viruses (also called viral vectors) carrying reprogramming genes. These reprogramming genes encode for transcription factors that influence the expression of certain genes in the somatic cell. Once the virus delivers the reprogramming genes to the somatic cells, the reprogramming genes are then expressed. After a period of time (approximately one month), the expression of the reprogramming genes results in the “de-differentiation” of the somatic cell into a pluripotent, stem cell-like state^3-5.

However, this commonly used approach for generating induced pluripotent stem cells has its drawbacks. In general, the efficiency of reprogramming can be quite low (<1%).  Furthermore, the use of viral infection in gene delivery can lead to an increased chance of cancer formation for two reasons.  One, some of the commonly used viral vectors (retroviruses and lentiviruses) can randomly insert genetic material into the target genome, which increases the risk of mutations.  Two, some transcription factors themselves can act as oncogenes that increase the risk of cancer.  To resolve these issues, some modifications to the original reprogramming methodology have been developed:

1. Use of different viral vectors, such as adenoviruses, that have a reduced risk of cancer formation can be used to reprogram human fibroblasts into iPSCs⁶.
2. Small, drug-like molecules can be introduced to somatic cells to mimic the effects of the transcription factors encoding by the reprogramming genes and promote reprogramming⁷.
3. Introduction of small RNA molecules (called microRNAs) was originally shown to improve the efficiency of reprogramming by viral gene delivery. It has since been shown that microRNAs themselves can reprogram somatic cells to pluripotency⁸; such an approach would remove the need for viral gene delivery altogether.
4. Direct addition of purified proteins (the transcriptional factors encoded by the reprogramming genes) circumvents the need to use viruses altogether^9,10.

References

1. Petit I, et al. (2011) Induced pluripotent stem cells from hair follicles as a cellular model for neurodevelopmental disorders. Stem Cell Res. 8(1): 134-140. PMID: 22099027.
2. DeRosa BA, et al. (2012) Derivation of autism spectrum disorder-specific induced pluripotent stem cells from peripheral blood mononuclear cells. Neurosci. Lett. Epub ahead of print. PMID: 22405972.
3. Takahashi K, Yamanaka S (2006) Induction of pluripotent stem cells from mouse embryonic and adult fibroblast cultured by defined factors. Cell 126(4): 664-676. PMID: 16904174.
4. Yu J, et al. (2007) Induced pluripotent stem cell lines derived from human somatic cells. Science 318(5858): 1917-1920. PMID: 18029452.
5. Takahashi K, et al. (2007) Induction of pluripotent stem cells from adult human fibroblasts by defined factors. Cell 131(5): 861-872. PMID: 18035408.
6. Zhou W, Freed CR (2009) Adenoviral gene delivery can reprogram human fibroblasts to induced pluripotent stem cells. Stem Cells 27(11): 2667-2674. PMID: 19697349.
7. Shi Y, et al. (2008) A combined chemical and genetic approach for the generation of induced pluripotent stem cells. Cell Stem Cell 2(6): 525-528. PMID: 18522845.
8. Miyoshi N, et al. (2011) Reprogramming of mouse and human cells to pluripotency using mature microRNAs. Cell Stem Cell 8(6): 633-638. PMID: 21620789.
9. Zhou H, et al. (2009) Generation of induced pluripotent stem cells using recombinant proteins. Cell Stem Cell 4(5): 381-384. PMID: 19398399.
10. Kim D, et al. (2009) Generation of human induced pluripotent stem cells by direct delivery of reprogramming proteins. Cell Stem Cell 4(6): 472-476. PMID: 19481515.

iPSCs are thought to be similar to embryonic stem cells. However, it is possible that there are differences between these two cell types that may be important when they are used clinically. The reprogramming process itself may introduce genetic mutations to the iPSCs that would make them behave differently. Furthermore, iPSCs from patients with disorders related to epigenetic defects may avoid reprogramming, as was shown in a study comparing DNA modifications in embryonic stem cells and iPSCs from Fragile X patients. These differences could result in different clinical findings in cells derived from iPSCs as opposed to those derived from embryonic stem cells.