There may be a way to avoid the moral controversies surrounding the use of human embryos in stem cell research — induced pluripotent stem (iPS) cells. With the help of the laboratory mouse, the technologies for efficiently producing iPS cells that can safely be used to treat human diseases are evolving quickly. Several JAX® Mice strains may be particularly useful.
First produced in 2006 (Takahashi and Yamanaka 2006), iPS cells are a type of pluripotent stem cell generated by virally transducing certain transcription factors into non-pluripotent cells, forcing them to express certain genes. They appear to be identical to natural pluripotent cells – such as embryonic stem (ES) cells – in many ways: they express stem cell genes and proteins, exhibit the same DNA methylation patterns and doubling time as stem cells, and form embryoid bodies, teratomas and viable chimeras.
They are an important breakthrough in therapeutic stem cell research because of their potential to substitute for ES cells, eliminating the ethical dilemmas associated with using pluripotent stem cells derived from human embryos. Additionally, because they are derived entirely from a patient's somatic cells, they are less likely to be rejected than are ES cells.
Current technologies for producing iPS cells impart risks that must be overcome. Because viral transduction is generally used to introduce transcription factor genes into the host cell genome, the genome of the resulting iPS cells can be altered by integrating the exogenous viral DNA. Moreover, c-Myc, one of the transcription factor genes commonly used to produce iPS cells, is a proto-oncogene. Methods to reduce these risks are being developed (see Lysiottis et al. 2009).
Initially, reprogramming somatic cells to become pluripotent required combinations of four transcription factor genes – either Oct4 (new name is POU domain, class 5, transcription factor 1 – Pou5f1), Klf4, Sox2, and c-Myc or Oct4, Sox2, Nanog, and Lin28. Later, a three-gene combination of Oct4, Sox2, and Klf4 was developed, eliminating the need for c-Myc. More recent improvements include the use of endogenously expressed factors, excisable vectors, non-integrating vectors, and transient transfection approaches.
Particularly promising is the possibility of replacing transcription factors with chemicals that can force the expression of desired genes. For example, one new induction method uses valproic acid and requires only two factors – Oct4 and Sox2 – eliminating the need for both c-Myc and Klf4. Recently a high-throughput chemical screen revealed a suitable chemical substitute for Klf4 (Lysiottis et al. 2009). The identification of additional chemical substitutes will likely eliminate the risks of viral transduction, increase the efficiency with which iPS cells are produced, better our understanding of the reprogramming process and lead to new disease therapies.
The Jackson Laboratory is distributing several mouse strains that can be used for iPS cell research. Mouse embryonic fibroblasts (MEFs) derived from several of these strains can be used as high-throughput screens for discovering pluripotency-inducing chemicals.
Because the reprogramming factor genes in the previous three strains are carried on a single construct, the strains can be easily maintained and the transgenes can be easily transferred to other genetic backgrounds.
Carey BW, Markoulaki S, Beard C, Hanna J, Jaenisch R. 2010. Single-gene transgenic mouse strains for reprogramming adult somatic cells. Nat Methods 7:56-9.
Lengner CJ, Camargo FD, Hochedlinger K, Welstead GG, Zaidi S, Gokhale S, Scholer HR, Tomilin A, Jaenisch R. 2007. Oct4 expression is not required for mouse somatic stem cell self-renewal. Cell Stem Cell 1:403-415.
Lyssiotis CA, Foreman RK, Staerk J, Garcia M, Mathur D, Markoulaki S, Hanna J, Lairson LL, Charette BD, Bouchez LC, Bollong M, Kunick C, Brinker A, Cho CY, Schultz PG, Jaenisch R. 2009. Reprogramming of murine fibroblasts to induced pluripotent stem cells with chemical complementation of Klf4. Proc Natl Acad Sci U S A.106:8912-7.
Wernig M, Meissner A, Foreman R, Brambrink T, Ku M; Hochedlinger K, Bernstein BE, Jaenisch R. 2007. In vitro reprogramming of fibroblasts into a pluripotent ES-cell-like state. Nature 448:318-24.
Takahashi K, Yamanaka S. 2006. Induction of pluripotent stem cells from mouse embryonic and adult fibroblast cultures by defined factors. Cell 126:663–76.