Drug Discovery


Figure 1


Pluripotent stem cell generation and utility.


Embryonic stem cells are isolated from early stage


embryos, whereas induced pluripotent stem (iPS) cells are reprogrammed from terminally differentiated cells types such as those found in blood and skin. Pluripotent stem cell populations are maintained


and propagated indefinitely in culture. Experimental intervention causes


differentiation to terminal cell types, which can then be used for a variety of investigational endpoints


embryonic stem cells. Of more utility is the poten- tial to introduce population diversity into early and basic research programmes, as iPS cells can be gen- erated from any individual. This iPS cell capability can provide researchers with tissue from virtually any genotype, ranging from the broad spectrum of ‘healthy’ individuals to clinical and disease cohorts to individuals exhibiting specific side-effects and/or idiosyncratic toxicities. Induced pluripotent stem cells will advance the concept of personalised med- icine closer to reality.


iPS cell generation: Several ways to reprogramme a cell


As noted above, the initial iPS cell derivations utilised four-independent factors delivered via viral vectors to reprogramme the starting fibrob- last populations. Since those seminal reports, reprogramming of cells has been successfully accomplished with a variety of delivery systems utilising variations of the initial four-factor cocktail, related transcription factor family members, endogenously expressed factors, small molecules, and soluble factors4,5. The use of non-genomic, small molecule factors is an attractive method for generating iPS cells, but the specificity with which they faithfully recapit- ulate transcriptional pathways is unknown, and it is unclear whether they can replace traditional reprogramming factors4.


The potential to use autologous iPS cells as ther- apeutic medicines holds tremendous promise. Cellular physiological integrity is, however, para- mount and will require delivery systems that do


32


not cause oncogenic transformation or alter gene function. Lentiviral- and transposon-based sys- tems, in which the inserted transgene is subse- quently excised using the Cre recombinase/lox site system or transposases respectively6,7, remain problematic, but recent technologies that do not require genome integration look more promising. These technologies include episomal vectors8, recombinant proteins9, viral RNA vectors10, and synthetic modified mRNAs11 (Table 1). Another important consideration for repro- gramming is the parental tissue source. The most popular starting material is fibroblasts, though other cell types have been successfully used, including keratinocytes12, mesenchymal cells13, adipose stem cells14, and melanocytes15. While to date there are no reports of cell types refrac- tory to reprogramming efforts, a more desirable source of starting material may be human peripheral blood, which can be readily obtained through routine and relatively non-invasive clin- ical procedures. Efforts to develop such methods have yielded iPS cells derived from human


CD34+ blood stem cells and T lymphocytes16- 18. Of particular note is a recent report demon- strating efficient human iPS cell derivation from T-lymphocytes collected in as little as 1ml of whole blood19.


There is considerable work ahead in the charac- terisation of iPS cells and their progeny. For exam- ple, recent studies indicate that iPS cells carry epi- genetic memory of their source tissue and exhibit a propensity to differentiate into their parental cell type20,21. However, this epigenetic memory


Drug Discovery World Winter 2010/11


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