Friday, February 23, 2007

Stem Cells Part VIII: Nanog, parthenotes and Cdx2

Is it possible to obtain hES cells without destroying an embryo? There are some variant methods out there which are touted as filling the bill, including embryo biopsy, as well as a group of methods called pluripotent stem cells derived from biological artifacts. We will look at each in turn.

To understand embryo biopsy, we first need to recall that the primary product of IVF is a zygote. Actually, many zygotes. These zygotes are then allowed to grow in vitro to the blastocyst stage, and the best ones among them are selected for implantation in the mother's womb, while the rest go into the freezer. There are two steps in the selection process: one step is a morphologic assessment where the developing embryos are examined under the microscope, and graded for "quality"[1]. The other step is "pre-implantation genetic diagnosis" (PGD), which is actually performed before the morphologic assessment. It works like this: the IVF zygote is allowed to divide to the 8 cell morula stage (day 3), where the individual cells are still morphologically similar, and known as blastomeres. One of these blastomeres is removed from the morula using established techniques, and its chromosomes checked. If they're OK, the morula from which the single blastomere was removed is allowed to grow to the blastocyst stage, and, if the morphologic grading is OK, implanted. The live birth rate from IVF is abysmal; it varies from 11% (women over 40) to about 35% (women under 35).[2] Nevertheless, advocates of PGD claim that this embryo biopsy, which removes 12% of the embryo's body, doesn't hurt its already slim chances of survival. Recently, researchers from Advanced Cell Technology of Worcester, MA used the PGD embryo biopsy technique to establish a new hES cell line.[3] They obtained cast off embryos from an IVF clinic, and removed blastomeres using standard procedures. These blastomeres were then cultured, and some of them developed into cells with hES cell characteristics. The cells didn't form new embryos. The authors' proposal is that, since a blastomere is clipped from an IVF embryo anyway as part of the PGD process, why not use that blastomere to develop a new hES cell line?[4] No embryo is killed (in theory, anyway) and researchers have a source from which to develop new hES cell lines.

The National Catholic Bioethics Center [5] lists three reasons embryo biopsy is unacceptable.

1. It requires nontherapeutic intervention on a human embryo which, in addition to presenting a serious risk to its life, reduces the "young human to commodities or manipulable products."
2. The method can only be used as an adjunct to IVF, which is itself intrinsically evil.
3. Human ES cells from killed embryos are necessary in co-culture with the blastomeres, so the need to kill embryos in not, in fact, obviated.

So much for embryo biopsy. In May, 2005, the President's Council on Bioethics published its "White Paper: Alternative Sources of Pluripotent Stem Cells",[6] which explored several alternatives for obtaining hES cells without harming the embryo. Interestingly, one of the alternatives rejected on ethical grounds (danger to the embryo) was embryo biopsy, though this didn't bother the folks at Advanced Cell Technology when they published in Nature the following year. However, two items of concern here were lumped together under the heading, "Pluripotent Stem Cells Derived from Biological Artifacts."[7] They are (1) oocyte assisted reprogramming - altered nuclear transfer (OAR-ANT) and (2) parthenotes.

OAR-ANT is basically a modification of SCNT. In regular SCNT, a normal somatic cell nucleus is transferred to an enucleated oocyte to produce an embryo who is a clone of the somatic cell's owner. In ANT, the nucleus is altered prior to transfer (hence the name) to knock out some genes (such as a gene called Cdx2) necessary for early embryonic development. The oocyte's cytoplasm is also modified (such as by the addition of the transcription factor Nanog) such that it will better assist the reprogramming of the transferred nucleus to make it grow up into an hES cell instead of an embryo; that's the OAR part of OAR-ANT. The idea is to create an entity which "would therefore lack organized development from the very earliest stages... Such an entity would be a 'biological artifact,' not an organism."[8] To put it more succinctly, the method would "use genetic engineering and somatic cell nuclear transfer to create an embryo-like entity that was designed from the beginning to self destruct after the blastocyst stage."[9]

OAR-ANT precipitated quite a debate among orthodox Catholic theologians. Some agree with the notion that the "artifact" is not, never was, and never will be an embryo, and therefore the method is licit.[10] Others argue that the artifact is, in fact, an embryo, albeit one severely crippled by design.[11] This is not a settled issue. My opinion lies with the latter interpretation.

The final candidate in this sad roundup is the parthenote. Parthenogenesis, where a single egg develops into an adult individual without a male contribution, is common in lower animals. But it doesn't occur naturally in placental mammals. However, experimental parthenogenesis has had limited success in a variety of mammals, including monkeys, though none of the experiments has resulted in a live birth.[12] There have not been, to my knowledge, any experimentally induced human parthenotes beyond the single cell stage, but the point of the exercise is to get a human parthenote to grow to at least the blastocyst stage, and then destroy it for its inner cell mass. Like the "artifact" produced by ANT, the human parthenote is believed to be unable to grow much beyond the blastocyst stage, and therefore is not really a human embryo. The White Paper points out that the only way to test that hypothesis is to create a human parthenote, implant it in a womb, and see what happens.[13] And here we shall let the matter rest.

[1] Veeck LL. An Atlas of Human Gametes and Conceptuses Parthenon, New York, 1999.
[2] 2003 Assisted Reproductive Technology Report Centers for Disease Control, Dept. Of Health and Human Services, available at See 2003 National Summary Table. This is the most recent data reported.
[3] Klimanskaya I, Chung Y, et al. Human embryonic cell lines derived from single blastomeres. Nature 444:481-485, 23 November 2006. Originally published on line in August.
[4] ibid. See also the clarifying letter: Simpson, JL. Blastomeres and stem cells. Nature 444:432-435. 23 November 2006.
[5] On Proposed Method for Extracting Embryonic Stem Cells by Embryo Biopsy. August, 2006. National Catholic Bioethics Center
[6] White Paper: Alternative Sources of Pluripotent Stem Cells The President's Council on Bioethics Available at
[7] ibid, Section III.
[8] ibid.
[9] Byrnes, WM. Partial Trajectory: The Story of the Altered Nuclear Transfer-Oocyte Assisted Reprogramming (ANT-OAR) Proposal. Linacre Quarterly 74(1): 50-59, February, 2007. Emphasis in the original.
[10] Austriaco, NPG. The Moral Case for ANT-Derived Pluripotent Stem Cell Lines. National Catholic Bioethics Quarterly 6(3):517-537. Fr. Austriaco references much of the moral literature, pro and con, in this recent publication.
[11] Byrnes, ibid. This paper is a good and up to the minute synopsis of the OAR-ANT story.
[12] Hipp J & Atala A: Tissue engineering, stem cells, cloning, and parthenogenesis: new paradigms for therapy. Journal of Experimental & Clinical Assisted Reproduction 2004:1:3
[13] White Paper, ibid, Section V D.

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