FULLY-PLURIPOTENT iPS CELLS: MOUSE TETRAPLOID COMPLEMENTATION, ETHICAL HUMAN ES-LIKE CELLS AND REPRODUCTIVE CLONING BAN

Full pluripotency of mouse induced pluripotent stem (iPS) cells was recently demonstrated using the tetraploid blastocyst complementation assay, thereby establishing transient transgenic expression of the "four Yamanaka transcription factors" as a bona fide reprogramming technique. Such a genesis of fertile adult "iPS" mice could revive the roguish temptation of banned human reproductive cloning currently doomed by the failure of human cell reprogramming by somatic cell nuclear transfer (SCNT). This major concern deals with a potential use of human iPS cells and contrasts with the fact that these cells stand as breakthrough ethical substitutes for current allogeneic human embryonic stem (ES) cells and planned patient-specific SCNT-ES cells. Indeed, unlike ES cells, iPS cells are generated without the need of an oocyte or a blastocyst and are therefore free of the human oocyte scarcity and embryo destruction problems. For patient-specific iPS cells, safety issues are thus the very limitation to the initiation of their bench-to-bedside translation. Full pluripotency is the first concern since it is the most stringent proof of an unbiased epigenetic reprogramming. Importantly, mouse tetraploid-complementing iPS cells have been shown to be few among the former deemed bona fide iPS cells and are thus expected to be instrumental in the identification of potential reprogramming caveats and of critical markers of the fully-pluripotent state. In this respect, hot-off-the-press data show that full pluripotency is correlated to the activity of the imprinted Dlk1-Dio3 multigenic region which is frequently aberrantly silenced in iPS cells. The stringency of the tetraploid complemention assay by mouse iPS cells is thus expected to translate soon into optimized protocols for the genesis/identification of fully-pluripotent human iPS cells. Such a full pluripotency is discussed in light of transgene-free reprogramming protocols aimed a clearing the second safety concern of iPS cell genesis: oncogenic hazards resulting from random integration of reprogramming transgenes into target-cell chromosomal DNA. In this respect, iPS cell genesis being the result of a transient gene therapy mechanism, the transient epigenetic gene therapy arm of our proposed universal stem cell gene therepy platform is presented together with concurrent approaches mediated by protein transduction, mRNA transfection and small molecules.