When adult cells are re-programmed into induced pluripotent stem cells (iPS cells), they still have some characteristics of their past identities. This “hanging on their past” might explain why iPSCs have some limits on their ability to function as candidates for cell replacement therapies. These findings, which were published online July 19 in Nature, highlight a major challenge in developing clinical and scientific applications for iPSCs.
George Daley, Howard Hughes Medical Institute investigator and Director of the Stem Cell Transplantation Program at Children’s Hospital said, “iPS cells retain a ‘memory’ of their tissue of origin. iPS cells made from blood are easier to turn back into blood than, say, iPS cells made from skin cells or brain cells.” If we contrast this to another technique known as nuclear transfer, pluripotent stem cells made from cloned embryos apparently have no memory of their cells of origin, and can transform into several tissue types with equal ease. In iPS cells, however, the memory of the original donor tissue can be more fully erased with additional steps or drugs, the researchers found, which made those iPS cells as good as the nuclear-transfer stem cells at generating different types of early tissue cells in lab dishes.
The residual cellular memory comes in part from lingering genome-wide “epigenetic modifications” to the DNA that gives each cell a distinctive identity, such as skin or blood, despite otherwise identical genomes. In the study, the persistent bits of a certain type of epigenetic modification called methylation were so distinctive in iPS cells that their tissues of origin could be identified by their methylation signatures alone.
It turns out that the genomes of iPSCs were not as completely reprogrammed as those from nuclear transfer cells. DNA methylation was completely reset in nuclear transfer cells and incompletely reset in iPSCs. These epigenetic marks help explain the lineage restriction of iPSCs, since they leave an epigenetic memory of the tissue of origin that remains after reprogramming.
Mind you, epigenetic memory can be a very helpful thing when it comes to some clinical applications. For example, generating blood cells from iPS cells originally derived from a person’s own blood, etc. However, this epigenetic memory may interfere with efforts to engineer other tissues for treatment in diseases such as Parkinson’s or diabetes or to use the cells to study the same disease processes in laboratory dishes and test drugs for potential treatments and toxicities.
In the current study, Kitai Kim, PhD, postdoctoral fellow in the Daley lab, tested mice iPS cells head-to-head with pluripotent cells made through somatic cell nuclear transfer. Best known as the cloning method that created the sheep Dolly fourteen years ago, nuclear transfer reprograms an adult cell by transferring its nucleus into an unfertilized egg cell, or oocyte, whose nucleus has been removed. The process of transferring the nucleus immediately reprograms it epigenetically, replicating the same process that happens to sperm upon fertilization. They noted that stem cells generated by somatic cell nuclear transfer were, for the most part, very much like embryonic stem cells. Daley noted that there is still a need to study the mechanisms by which nuclear transfer reprograms cells, since such finding might help us make better iPS cells.”
However, Kim took this study one step further. He used older mice for this study (older mice – ages 1 to 2). His aim was to simulate future human clinical scenario, which is likely to involve older people. Older cells are more difficult to reprogram. Kim originally wanted to compare the transplantation success of blood cells made from three different pluripotent sources: iPS cells, embryonic stem cells (the gold standard), and nuclear transfer stem cells. He did not get as far as transplantation. Since they saw such striking differences in blood-forming potential, Kim and his co-workers examined this phenomenon instead.
They found that iPS cells from blood were best at making blood, and fibroblasts were best at differentiating into bone. However, if they reset the iPS cells more fully by differentiating them first into blood cells and then reprogramming them again, or by treating them with drugs that change their epigenetic profile, then they could differentiate into other cell types more effectively. In contrast, nuclear transfer stem cells from the same sources — blood cells and skin – were equally able to differentiate into blood and bone, Kim and his colleagues found. Like iPS cells, the nuclear transfer technique also creates patient-specific cells, but has not yet proven successful with human cells.
Another study published online simultaneously in the journal Nature Biotechnology reports similar findings. “Our paper comes to a similar conclusion that a retention of memory reflects the cell of origin and affects the capacity of the iPS cell to differentiate into other cell types,” said senior author Konrad Hochedlinger, PhD, a stem cell biologist at the Massachusetts General Hospital Center for Regenerative Medicine and, like Daley, a member of the Harvard Stem Cell Institute. “When we let the cells go through a lot of cell divisions, they lose the memory,” he said.
Category: Cloning
Cloned embryos can’t fool a womb
See the following link for an interesting paper on the ability of the womb to discern between a cloned embryo and an embryo made by means of fertilization. See this link for the article.
Embryonic stem cells from cloned embryos are indistinguishable from those made from embryos made by fertilization. They express the same genes (see DJ Guo et al., Proteomics. 2009 Apr 22, and this article), show the same biological behaviors (see this link for this paper), show normal embryonic stem cell morphology, express key stem-cell markers, and can differentiate into multiple cell types in vitro and in vivo (JA Byrne, Nature 450 (2007): 497-502).
Since embryonic stem cells are made from the internal cells of the embryo (the inner cell mass), the inner cell mass cells from cloned embryos are rather normal (ML Condic, Cell Proliferation 41, suppl 1: 7-19). However, the outer layer of cells (trophectoderm) that engage the endometrium and work with it to implant the embryo into the inner layer of the uterus do not differentiate normally in cloned embryos (DR Arnold et al. Reproduction 132, no. 2 (2006): 279-90). Trophoblast cells in cloned embryos are normal at the early stages (S. Kishigami et al., FEBS Letters 580, no. 7 (2006): 1801-6), but they go on to make abnormal placentas (DR Arnold et al, Placenta 29 Suppl A (2007): S108-10).
Now this paper shows that the differentiating placenta of the cloned embryo does not interact normally with the surrogate mother’s uterus. This is probably one of the main reasons why cloned embryos and fetuses tend to die prior to birth. The endometrial cells of the mothers who were carried the cloned embryos showed substantial variation in the genes they expressed in comparison to endometria that carried in vitro fertilized embryos (S. Bauersachs et al., PNAS 106, no. 14 (2009): 5681-6). Thus cloned embryos fail to properly communicate with the mother’s uterus.
Implantation is a very complex process. It requires cross talk between the embryo and the uterus. Without this cross talk, implantation does not occur successfully. Without successful implantation, the embryo perishes.
Here again we find another reason to not clone humans. We are subjecting them to a process that is less robust than fertilization. The chances of the embryo surviving are far less than an embryo concieved in the usual manner (fertilization). We should simply ban this process in humans overall.
New NIH Guidelines for Embryonic Stem Cell Research
Melinda Penner evaluates the new NIH guidelines for embryonic stem cell research and this site: here. It is a very interesting evaluation.
The guidelines allow the use of surplus embryos from in vitro fertilization cycles for the production of embryonic stem cells. These embryos were originally made for reproductive purposes, but research that will end their existence is allowed on them. Embryos that were made by somatic cell nuclear transfer are usually made for the purpose of research. However the guidelines prohibit research on such embryos that were originally made for research. In others the guidelines allow research on embryos originally not made for research and prohibit funding for research on embryos made for the purpose of research. In this regard the guidelines are inconsistent.
However, the guidelines seem to regard embryos as expendable. That creates a society where the weakest members of our species are perpetually at risk. To justify killing them, we use arguments like “they are going to die anyway.” Such are argument was used by Scrooge in Charles Dickens “A Christmas Carol.” When asked for a donation to help the poor at Christmas time, Scrooge said that the poor and homeless should hurry up and die and “decrease the surplus population.” We would regard such an attitude and inhumane, but when it comes to those who are a little younger than the rest of us, it is somehow perfectly acceptable to destroy them. I refuse to call such reasoning “moral progress” or such a policy “wise.”
Is a cloned embryo a human person?
Psychiatrist Paul McHugh has argued that embryos made by means of somatic cell transfer (SCNT) are not human persons even though those made by fertilization are. According to McHugh, SCNT is a “biological manufacturing process” that is used to make, not babies, but embryonic stem cell lines, and “resembles tissue culture” more than fertilization. McHugh has even fashioned the name “clonotes” for SCNT-derived embryos to distinguish them from embryo made by fertilization with sperm.
What is the substantive difference between embryos made by fertilization and those made by SCNT? McHugh’s main argument is as follows:
…If one used the notion of “potential” to protect cells developed through SCNT because with further manipulation they might become a living clone, then every somatic cell would deserve some protection because it has the potential to follow the same path (Paul R. McHugh, “Zygote and ‘Clonote’ – The Ethical Use of Embryonic Stem Cells,” New England Journal of Medicine 351 (2004): 209-11).
In other words, because nuclei from almost any somatic cell can be used to form a clonote, almost any somatic cell has the potential to become a clonote. It is absurd to regard all the somatic cells of our bodies as human persons. As Robert Lanza of Advanced Cell Technologies stated, “research advances are making all cells embryonic, but if you consider these cells human life, then 100 souls are lost every time I sneeze” (Joannie Fischer, “The First Clone,” U.S. News and World Report (3 December, 2001). Since it is untenable to regard somatic cells, which have the ability to form clonotes, as human persons, it is equally untenable to regard clonotes as human persons.
McHugh’s second argument notes that the vast majority of clonotes are grossly abnormal and die very early during development. Thus clonotes are not human persons, which make the production of ESCs from them morally justifiable.
McHugh’s second point is overstated. While many cloned animals develop into animals with a variety of developmental abnormalities, not all of then do. To classify cloned animals as a distinct kind of creature because they possess abnormalities ignores those cloned animals that either do not possess such abnormalities or whose health overlaps with animals that were not made by the process of cloning. If the abnormalities are part of the reason for assigning cloned animals into a different category, then that classification fails for normal cloned animals.
Secondly, molecular comparisons of cloned embryos with embryos made from in vitro fertilization have revealed extensive similarities. The abnormalities only arise later, once the embryo implants into the uterus. Thus the abnormalities that McHugh uses to disqualify cloned embryos as human persons have yet to arise.
Thirdly, if cloned embryos differ in kind from embryos made by fertilization, then what of those cloned animals that survived to term: Are such animals a different kind of animal? Consider Dolly, the cloned sheep. Was she so different as to not be considered a Suffolk Blackface sheep? This seems patently absurd. If relegating the cloned adult to a lower status is fallacious, then it is just as fallacious to demote cloned embryos to a similar status.
Finally, even if cloned embryos have abnormalities, so what? Do we really want to dismiss the humanity of an individual because they carry some sort of handicap? Dismissing the humanity of cloned embryos because of their potential abnormalities is tantamount to dismissing abnormal children and allowing medical research on them since their death is immanent (Robert P. George and Christopher Tollefson, Embryo: A Defense of Human Life (New York: Doubleday, 2008): 184-9). We should find such a proposal revolting.
What of McHugh’s first point; that is, nuclei from any somatic cell can produce a cloned embryo, and therefore, all somatic cells are potential embryos and deserve protection, which is absurd? On this point, the analogy of somatic cells with embryos seems hopelessly flawed. In fact, the entire category of “potential embryos” is simply nonsensical (Robert P. George and Patrick Lee, “Acorns and Embryos,” The New Atlantis (Fall 2004/Winter 2005): 90-100). The term embryo refers to a very specific entity in the life of an organism. Something is either an embryo or not. Secondly, somatic cells are not similar to embryos. Instead they are similar to sperm and eggs, the cells that are used to make embryos. Once the sperm and the egg fuse and complete conception, they no longer exist. Instead a new entity, the embryo, which did not exist before hand, begins it existence. The embryo is a “distinct, complete, self-integrating organism.” Somatic cells are no such thing, but are, instead, part of an organism. SCNT or fertilization makes an embryo. Thus McHugh’s first point also fails.
Beyond the Dish 