|Alarm bells for iPSC?|
by Lyle Armstrong
Induced pluripotent Stem Cells (iPSC) are widely believed to share many of the characteristics of Embryonic Stem Cells (ESC) and as such have been credited with the potential to revolutionise regenerative medicine. The potential benefit of iPSC exists because of their genetic similarity to the individual from whom they were derived, implying that if differentiated and clinically useful cell types produced from iPSC were transplanted back into the individual, the likelihood of immune rejection should be greatly reduced.
The way in which iPSC were created in early experiments cast doubts upon the clinical usefulness of these cells because integrating retroviruses or lentiviruses were used to reprogram somatic cells, but the development of non-integrating reprogramming methods had generated some degree of optimism that iPSC could be created that might be truly safe for clinical application. Now a new study has cast doubts upon the characteristics of iPSC themselves, since it seems that somatic cells produced from iPSC may have a tendency to undergo apoptosis or become senescent much earlier than similar cells derived from ESC.
This newly published research from the group of Robert Lanza in Stem Cells compared 25 human ESC (hESC) and 8 human iPSC (hiPSC) lines which had similar abilities to differentiate into blood precursor cells, blood vessels, and cells of the eye. Comparing the characteristics of cells derived from hiPSC and hESC, researchers found that blood and vascular derivatives from hiPSC display abnormal molecular and/or cellular processes compared to their corresponding hESC counterparts. The hiPSC showed notably decreased growth and differentiation efficiency, sometimes 1000 fold less than hESC when prompted to differentiate into haematopoietic stem cells (HSC). The differentiation method in this case made use of a well established technique that causes any HSC arising from the hiPSC to expand and differentiate further into colonies of more mature blood cell types such as granulocytes and macrophages, which appear as highly visible colonies in a soft agar medium. This method usually works well with most hESC lines, but the hiPSC lines examined typically gave much fewer and much smaller colonies.
Much more disturbing is the propensity for apoptosis in the cells that differentiate from hiPSC. Blast cells, an early type of haematopoietic progenitor sometimes called haemangioblasts, showed fragmented morphology and presence of the cleaved form of Caspase 3 that clearly indicate an apoptotic phenotype. Worse still, endothelial cells and cells of the retinal pigmented epithelium (RPE, which supports the photoreceptors of the retina) became senescent very soon after differentiating from iPSC. Over 50% of endothelial cells derived using the group´s well established protocol expressed senescence associated β-galactosidase and many, if not all, displayed flattened morphology typical of senescent cells. RPE develops quite slowly from hESC but can be routinely expanded for approximately five passages. This was not the case for RPE derived from hiPSC since they could only survive as far as the first passage before adopting a senescent morphology and β-galactosidase expression.
There have been numerous reports, both in the scientific literature and media, suggesting hiPSC may be identical to hESCs but these current data from Lanza´s group are among the first to suggest that this may not be absolutely true. Global gene expression profiling shows that many transcripts are common to both hESC and hiPSC but also that there are significant numbers of genes (approximately 4%) whose expression levels differ. The consequences of this are currently unclear, as are the mechanisms that might lead to the early senescence of hiPSC derived cells, but there have been other reports describing differences between hESC and hiPSC, such as aberrant maintenance of gene imprinting and expression of oncofetal antigens, so we would be well advised to investigate such differences in exhaustive detail before contemplating any clinical applications of hiPSCs.
It must be noted that the hiPSC used in Lanza´s study were all derived using integrating retroviral or lentiviral vectors, so it is possible that their observations reflect the genomic changes caused by viral insertions, which means that a similar investigation of hiPSCs derived using non integrative methods is both desirable and timely. If these future studies suggest that the problem lies with inefficient or incomplete reprogramming of the somatic genome then we must consider the possible applications of hiPSC with great care. To quote Robert Lanza, the senior author of this study, “Although there is excitement that iPSC can serve as an embryo-free source of stem cells, it would premature to abandon research using hESC until we fully understand what’s causing these problems.” We look forward to the insight gained from future developments in this field with great interest.