Alternative sources of pluripotent stem cells: altered nuclear transfer

Abstract

Altered nuclear transfer (ANT) is one of several methods that have been suggested for obtaining pluripotent stem cells without destroying human embryos. ANT proposes to alter the nucleus of a somatic cell and/or the cytoplasm of an enucleated oocyte such that when the two are combined, they do not produce a zygote, but rather they form a cell capable of producing pluripotent stem cells without being an embryo. The ANT proposal raises the serious question of whether it is possible to know with confidence that this procedure generates a non-embryo, rather than merely an embryo with a deficiency. Here I address the question of how embryos can be distinguished from non-embryos using scientific criteria and apply these criteria to the two forms of ANT proposed thus far: ANT combined with oocyte-assisted reprogramming (ANT-OAR) or with gene deletion (ANT-GD). I propose that the first globally coordinated event in human development, the formation of trophoblast and inner cell mass (ICM) lineages via Cdx2-Oct3/4 mutual cross-repression, is the earliest act of the embryo qua embryo; it is an operation intrinsic to an embryo as such, and entities lacking the power (potentia) for such an act cannot be considered embryos. Thus, I will argue that formation of trophoblast-ICM lineages is a both necessary and sufficient criterion for determining whether ANT produces an embryo or a non-embryonic entity.

Figure 1

A schematic representation of normal development and possible ANT manipulations. (a) At the two cell stage, the blastomere that divides first (the leading blastomere) contributes predominantly to the ICM, while the blastomere that divides second (the lagging blastomere) predominantly generates trophectoderm (Zernicka‐Goetz, 2002; Gardner and Davies, 2003; Stanton et al., 2003; Rossant and Tam, 2004; Plusa et al., 2005). By the eight‐cell stage, Cdx2 and Oct3/4 mutual cross‐repression establishes a clear molecular difference between these two lineages. (b) In ANT‐OAR, over expression of factors that are required for ICM formation (for example, Oct3/4) and/or of later acting factors that are specific to Oct3/4 expressing ICM/ESCs [for example, Sox2, cMyc and K1f4; (Takahashi and Yamanaka, 2006; Maherali et at., 2007; Okita et al., 2007; Wernig et al., 2007)], produces a cell that is not capable of the essential function of the zygote. The ANT‐OAR cell immediately exhibits the properties of a pluripotent stem cell derived from the ICM (cf. the leading cell in Fig. 1a) (c) in ANT‐GD, deletion of factors that are required for the essential function of the zygote (for example, Cdx2) accomplishes the same goals as overexpressing ESC‐associated factors; the cell produced by Cdx2 deletion immediately has the properties of the ICM/ESC lineage (cf. the leading cell in Fig. 1a), and is not capable of the coordinated interactions that are essential to a zygote. Panel 1a adapted from (Niwa et al., 2005).