October 5, 2011

expert reaction to new research on human embryonic stem cells, as published in Nature

New research used somatic cell nuclear transfer (SCNT) to produce early-stage embryos with three sets of chromosomes, which increased the efficiency with which stem cells could be derived.

Professor Mary Herbert, Institute for Ageing and Health, Newcastle University and Newcastle Fertility Centre, said:

“This study shows that the conventional approach to somatic cell nuclear transfer (SCNT) is inefficient in humans. However, the authors were able to increase the efficiency by leaving the host oocyte genome in place. While this approach does not in itself provide a solution, it takes us a step closer to understanding where the problems lie.”

Professor Azim Surani, Marshall-Walton Professor, Wellcome Trust/Cancer Research UK Gurdon Institute, University of Cambridge, said:

“This study suggests that somatic nuclei are not reprogrammed efficiently when transplanted into human oocytes in the absence of oocyte’s own genome. The latter may ensure that early development can proceed under the control of the host genome, allowing more time for reprogramming of the donor somatic nucleus. The donor genome may require pretreatment to improve the efficiency of the procedure if it is to be of practical use.”

Professor Chris Mason, Chair of Regenerative Medicine Bioprocessing, University College London, said:

“This novel research adds to the growing number of options for cell therapies of the future. Whilst not immediately life-changing for patients, it significantly adds to the scientific pool of knowledge that underpins progress towards advanced treatments. Which approach is the best? Only time will tell, but multiple routes forward may eventually speed the delivery of cell therapies for a broad range of unmet clinical needs.”

Professor Roger A. Pedersen, Director of Research, The Anne McLaren Laboratory for Regenerative Medicine, University of Cambridge, said:

“This work represents the first report of any development resembling normal embryogenesis after somatic cell nuclear transfer (SCNT) to a human oocyte. The authors found that progressive development occurred only when the oocyte’s own genetic material was not disturbed; this confirms previous reports on the difficulty of achieving SCNT in non-human primates (rhesus monkeys) and explains the previous failures of researchers to achieve SCNT using human oocytes. This obstacle appears to be unique to primates (both non-human and human) and contrasts with species (such as sheep, cows and other mammals) in which SCNT to oocytes can be accomplished using standard methods. Future studies can now be focused on understanding what goes wrong with the human oocyte when it is subjected to the SCNT procedures. If successful, future SCNT studies could yield human SCNT-derived stem cells that could then be compared carefully with human stem cells obtained using other methods, such as induced pluripotent stem cells.”

Professor Robin Lovell-Badge, Head of Division, National Institute for Medical Research, said:

“This paper will be seen as significant both by those who are trying to use SCNT (somatic cell nuclear transfer) to produce human patient-specific Embryonic Stem (ES) cell lines and by those who oppose human “cloning” experiments. However, it is a technically complex story, with some unexplained results, and an ending that still falls short of the original aim – they did not obtain useful cell lines. However, the work may reveal a way to overcome some problems.

“The use of SCNT methods to obtain blastocyst stage human embryos (the stage just before implantation, where the embryo is about 80 to 100 cells), has been reported by others, but at a relatively low efficiency. Using similar methods, where the host egg nuclear DNA is removed and replaced by a donor somatic cell nucleus, the current authors were in fact unable to obtain any embryo that developed beyond about the 6 to 10 cell stage. Instead the authors changed tack and deliberately left the host DNA in place, simply adding the donor nucleus to the one that was already there. Rather surprisingly – as this means that they are creating an embryo with too many copies of each chromosome – these constructs developed well and efficiently to the blastocyst stage. Furthermore, the authors were able to derive (apparently) pluripotent ES cell lines from these embryos – the first time that this has been achieved from embryos derived using SCNT methods. These results suggest that some factor associated with the nuclear DNA in the oocyte is required for reprogramming the somatic cell nucleus or at least for allowing the correct activation of genes required to get the embryo beyond the first few cell divisions.

“The authors could get successful development to blastocysts when they transferred a nucleus from an early embryo cell rather than an adult somatic (skin) cell into an enucleated egg. This suggests that the critical factor(s) has something to do with activating embryonic gene expression, because such early embryo nuclei will already have many of the necessary genes active. The SCNT embryos produced in this way are true clones of the donor embryo – but irrelevant with respect to the aim of deriving patient-specific ES cell lines.

“The SCNT embryos and ES cell lines derived by leaving the host genome in place, are not true clones of anything (they could be called intra-specific hybrids) – although the authors convincingly show that thr donor DNA is reprogrammed correctly. Instead they are equivalent to rare embryos obtained by fertilistion of an egg with a sperm carrying two sets of chromosomes rather than one (i.e. diploid rather than haploid). ES cell lines obtained by this method will not be ideal for studying genetic disease as they have both normal (host) and abnormal (donor) DNA, and I doubt they could be used clinically for any cell-based therapy. Furthermore, both ES cell lines obtained in this work appear to be triploid – with two sets of chromosomes from the donor cell and one from the host oocyte. Triploid cells and embryos usually develop poorly, therefore this is odd in itself. But the SCNT embryos they were derived from should have had four sets of chromosomes: two from the donor and two from the host. The host eggs were at a stage with two sets of chromosomes – normally at fertilisation (or activation) one set would end up in the (second) polar body – a small cell that fails to survive, leaving a haploid set within the much larger egg. Perhaps this polar body was formed – but there is no description of it in the paper. So where are the missing chromosomes?

“Despite all these questions and problems the work suggests that there is a DNA-associated factor present in human eggs (but apparently not in those of other animals) required to efficiently activate and reprogramme the somatic cell nucleus after it is transferred to an egg. Finding such a factor may help the desired aim of obtaining patient-specific ES cell lines.”

‘Human oocytes reprogram somatic cells to a pluripotent state’, Elgi et al, published in Nature on Weds 5 Oct 2011.