Technical Comments

Response to Comment on "Molecular Correlates of Primate Nuclear Transfer Failures"

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Science  12 Sep 2003:
Vol. 301, Issue 5639, pp. 1482
DOI: 10.1126/science.1087756

Space constraints precluded publication of bovine nuclear transfer (NT) data (Fig. 1) that may allay the concerns of Lanza et al (1). After primate NT, disorganized spindles with chaotically distributed chromosomes are always observed (2). Bovine NTs, however, organize a single microtubule aster emanating from the juxtanuclear NT centrosome at first interphase (Fig. 1A). Mitotic bovine NTs frequently assemble symmetrical bipolar spindles with precisely aligned chromosomes [Fig. 1B, blue and (2)], a situation never observed after primate NT. These and centrosome and kinesin NT data are subjects of a forthcoming report (3).

Fig. 1.

Chromosomes align on symmetrical mitotic spindles after bovine nuclear transfer, unlike the chaotic spindles detected after primate NT (4). (A) Somatic cell nuclear transfer in cattle oocytes results in a single radially arrayed microtubule aster at first interphase. (B) At first mitosis, bovine NTs organize accurate bipolar mitotic apparatus (green) with aligned chromosomes (blue). Scale bar: A, 10 μm; B, 5 μm.

Without benefit of this additional knowledge, Lanza et al. fairly raise the uncertainty in interpreting NT results, especially in extrapolating assisted reproductive technology (ART) successes or failures between species. For example, >90% fertilization is routinely achieved using intracytoplasmic sperm injection (ICSI) in both humans and nonhuman primates (resulting in the global adoption of ICSI by ART clinics), whereas ICSI has produced few mice or cattle (4). Because NT successes vary, comparisons between Dolly's birth after 29 embryo transfers into 13 ewes (5) and our lack of established pregnancies after 33 rhesus embryo transfers are not reasonable. Primate ART, including NT, is uniquely challenging (6): Oocyte availability and quality, synchronized surrogates, cleavage-stage embryo transfers versus blastocysts transfers, and reproductive seasonality all present practical limitations. And unlike mice and domestic mammals, primates carry singletons. Finally, while the protocols for mouse and bovine NT are species-specific, primate laboratories must extrapolate from human ART clinics for simian-specific optimizations.

Although human embryonic stem cells (hESCs) derived after NT might overcome immune rejection, NT epigenetic abnormalities (7) and microchimerism in transplantation patients (8) encourage alternate strategies for tolerance or managing incompatibilities. In light of the available evidence, we maintain our original conclusion (2) that “[w]ith current approaches, [primate] NT to produce embryonic stem cells may prove difficult—and reproductive cloning unachievable.” We agree with Lanza et al. (1) that interpretations regarding both NT and hESC data in primates, human (9, 10) and nonhuman (5, 11) alike, should be “tempered with abundant caution.”


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