November 30, 2007

scientists react to new paper on induced pluripotent stem cells

The research published in the journal Nature Biotechnology shed more light on which genetic factors are critical in the reprogramming of adult skin cells to become other types of cell. In this study the researchers achieved reprogramming of adult cells without the use of a gene which has been linked to the development of tumours.

Prof Robin Lovell-Badge, Head of Developmental Genetics, MRC National Institute For Medical Research, said:

“This is a very interesting follow up study from Yamanaka’ lab, where the main conclusion is that, of the four factors they originally used to reprogramme either mouse or human skin cells (fibroblasts) into pluripotent stem cells, the oncogene cMyc is dispensable. This is important because of their previous data showing that chimeric mice derived using iPS cells were prone to tumours, which would obviously compromise the use of human iPS cells in research and for therapies. Chimeric mice made with the “3-factor” mouse iPS cells did not develop obvious tumours, while those made also using cMyc did, which suggests that tumour formation was indeed due to abnormal cMyc expression rather than some general problem of the reprogramming method. However, they did not look thoroughly for tumours and screened relatively few animals, so more work is needed on this. Moreover, they find that the reprogramming is slower and the frequency significantly reduced when only the three factors (Oct4, Sox2 and Klf4) are used. Does this mean that it now only works with a rare cell type ? And will these turn out to be normal, for example an immature cell or even a skin stem cell, or are they abnormal cells carrying some mutation ? As always, many more questions are posed than answered.

“The finding that it is not necessary to add cMyc to obtain human iPS cells was also reported by Thomson’s lab in Science about a week ago, although this was with neonatal rather than adult fibroblasts.”

Dr Lyle Armstrong, International Centre For Life, Newcastle University, said:

“Induced pluripotency seems to be the current hot topic. This new paper demonstrates that one of the more controversial reprogramming factors (c-myc) is not absolutely necessary to produce iPS cells which gives us hope that methods may be developed soon that produce cells which can be used in human patients. However, there is still much work to be done to prove that the cells are safe particularly since even this newer method still uses retroviruses to tranfer the reprogramming factors.”

Dr Anna Krassowska, Research Manager, The UK Stem Cell Foundation, said:

“The present study is a continuation of Yamanaka’s recent work that contributes to our understanding of the basic science of cellular reprogramming, while also addressing one of the important issues to be tackled for this technology to be useful for medical research.”

Dr Krassowska also provided the following facts: Why is the iPS work exciting? This is beautiful science continuing directly from lasts week’s publications to identify genes or factors critical for reprogramming a differentiated adult cell back into a pluripotent stem cell. In the future this may be useful to generate patient specific cell lines. However, many important questions must be addressed before one could consider developing iPS cell technology for therapeutic use. This paper addresses one of these questions (see below). What are the limitations? – Previous studies by this group and the current study show that mouse iPS cells, when reprogrammed with the combination of genes that included c-myc, resulted in tumours in some of the adult chimeric mice generated by injecting iPS cells into early embryos (blastocysts) and also in some of their offspring. – This is one important reason why those cells would not be suitable for developing therapies. What is new in this paper? – iPS cells were generated with mouse and human cells without using the gene c-myc. In the mouse experiments no tumour formation was found (within 100 days). This experiment cannot be done with the human cells obviously. – NB: The Thomson paper from last week inserted four genes but did not use c-myc. They reprogrammed human skin cells from a new-born. This study reprogrammes skin cells from a 36 year old person (more mature cells). Other important findings? When c-myc is omitted and only the other 3 genes are inserted: – the reprogramming is slower and less efficient – but there is evidence that reprogramming may be more specific (ie of the total colonies formed, eventhough there were far far fewer, a higher percentage of these were actually iPS cells) – in half of the expts, no human iPS cells were formed Some things we still don’t know: – how similar are iPS cells to ES cells wrt gene expression, differentiation potential? – what effect does the site where the genes are inserted into the genome have, ie insertion is random so are crucial genes being disrupted in any way? – in the mice expts, would the mice develop tumours after 100 days? – how are these genes reprogramming the fibroblasts? The ideal future? Identifying small molecules that could be added to cells in culture to achieve reprogramming without the need for genetic modification. Note: None of this work could have been done without all the knowledge gained from research using ES cells from embryos, human and otherwise.