|
HOME :: CHAPTER 0 :: 0. :: CLONING MAMMALS: WHAT DOES IT MEAN? |
Cloning Mammals: What Does It Mean?
(An opinion from Scott Gilbert)
Breaking News: Primate cloning might not be possible with existing methods
Current cloning techniques may not be able to clone humans and other primates. Despite the successful attempts to clone mice, sheep, cattles, and cats, no primates have been cloned. This is not due to lack of trying. In one study, nuclei from several different types of tissues (cell types whose nuclei can produce embryos in the other species) were placed into over 700 rhesus oocytes. None formed clones. Simerly and his colleagues in Gerald Schatten's laboratory (Simerly et al., 2003) found a physiological difference between the oocytes that could support cloning and those that couldn't. When nuclear transplantations were done with the rhesus oocytes, the chromosomes did not assort correctly to the mitotic spindle fibers, and the microtubules, themselves, were not arrayed properly. The cells became aneuploid. Simerly and colleagues further showed that these disruptions were due to the absence of two proteins, NuMA and HSET on the rhesus mitotic spindles. NuMA is a protein responsible for spindle pole assembly, and HSET part of the kinesin motor system along the microtubules. In the rhesus oocytes, these proteins aggregate at the centrosomes of the meiotic spindles. When those spindles are removed to enucleate the oocyte, no more HSET or NuMA is synthesized. Thus, these proteins are not present for the assembly of the mitotic spindles of the inserted nucleus. In cattle and mice, these proeins remain in the enucleated egg, so that they are present during the mitoses of the inserted nuclei. It seems that only in primates that NuMA and HSET become exclusively concentrated on the meiotic spindle. Thus, this reproductive cloning in humans might not be possible using the present level of technology.
Further News: Researchers find that clones are often developmentally abnormal
On Sunday, March 25, 2001, the New York Times published an article on mammalian cloning by science reporter Gina Kolata. Interviewing laboratory leaders who have successfully cloned mammals, she found that many, if not most all, of the cloned mammals were not doing well. The cow clones that were born had enlarged hearts or lungs that did not develop properly. Some of the cloned mice grew normally for about half their lifespan. They then became enormously obese. Dolly, herself, has grown so fat that she has to be kept on a separate diet. The proponents of human cloning have clung to the notion that cloning is not harmful. This idea has to be thoroughly re-evaluated. It appears that not only do most of the clones die in utero; but those who make it may not be healthy. Dr. Rudolph Jaenich, an MIT professor who studies chromatin reprogramming, noted, "With cloning, you are asking an egg to reprogram in minutes, or at most hours...That's where the major problem is." He called human cloning "reckless and irresponsible" and he predicted that cloned humans would be likely to have developmental anomalies. Based on such findings, Dr. Brigid Hogan, a professor at Vanderbilt University and the current president of the Society for Developmental Biology, said that cloning humans "would be morally indefensible."
New technologies can be permitted, forbidden, or regulated. On March 28, 2001, U.S. Representative James Greenwood of Bucks County, Pennsylvania convened the first congressional hearing on what policy, if any, the US government take on human cloning. As bioethicist Arthur Caplan observed "One look around the hearing room would tell you that Congress needs to act and act fast." Scattered among the scientists and legal experts were Rael, the spiritual leader of a Canadian cult who believe that they are the cloned descendents of extraterrestrials and would very much like to make more of "their own"; Randolph Wicker of Clone Rights United Front, who from his tiny apartment in New York promotes the virtues of cloning the dead; and Panos Zavos, a Kentucky scientist who has been leading a team of doctors from country to country around the Mediterranean in search of a home for his oddball human-cloning project...The media have given us all the impression that cloning works. Not true. The best way to describe the current state of cloning is reproductive carnage. We have all seen images of Dolly, the sheep cloned in Scotland, prancing about her pen. Few of us realize that she is much larger than she should be and is aging at an uncertain rate. Even fewer know that nearly 250 clone embryos failed in the effort to make this one sheep. Of those clone embryos that started to develop, far more produced dead and malformed animals than Dollies."
The following summarizes some of the mammalian cloning work.
I. Dolly the sheep
Individuals from two mammalian speciesosheep and cowsohave recently been cloned. The sheep "Dolly" has been generated asexually by fusing the nucleus of an adult mammary gland cell to an enucleated sheep egg (Wilmut et al., 1997). What does this mean? Scientifically, Dolly has not told us much. The cloning of an adult mammal from the transplanted nucleus of an adult mammalian cell is direct evidence for what developmental biologists have long held to be true—that the nuclei of most cells in an individual are genetically identical. The fact that the Scottish investigators do not know the developmental state of the cell from which the nucleus came makes it even less impressive scientifically than the amphibian experiments done 20 years ago. The experiments of Gurdon and colleagues (1975) and Orr and colleagues (1986) used amphibian cells that were known to be fully differentiated. Orr and her co-workers used frog erythrocytes, while Gurdon's laboratory used cultured keratinocytes whose differentiation was demonstrated by skin-specific cytokeratins. We do not know the differentiated state of the cell whose nucleus was used to generate Dolly. It could be that the nucleus was from a stem cell population which, while committed to a particular cell fate, is relatively undifferentiated. There are some valid scientific questions that could be asked. For instance, did the nucleus become "rejuvenated" by the egg cytoplasm. That is to say, will the lifespan of the cloned sheep approximate that of a normal sheep, or will it be shortened by the amount of time the nucleus has already spent as an adult. It would be interesting to know the state of its telomeres. Society has to answer its own questions, namely: (1) Is such scientific knowledge worth the risk of inventing a technique that could clone humans? and (2) When, if ever, is the cloning of human nuclei allowable? The mechanism by which Wilmut and colleagues cloned Dolly is not very efficient. Out of the 277 mammary gland nuclei fused to enucleated unfertilized eggs, only one live lamb was born. However, the claim that our inability to clone other mammals is a technical, rather than a physiological, problem suggests that more efficient cloning techniques can be developed. There may be physiological barriers against cloning in mice and humans. Sheep, for instance, activate their nuclei relatively lateoat the 16 cell stage. This might give the egg cytoplasm ample time to "reprogram" the chromatin. Mice, on the other hand, activate their nuclei at the late one-cell stage. Humans activate some genes (such as SRY) during the one-cell stage, but the global activation of the human genome does not occur until the 4-8 cell stage, shortly before compaction (occurring around the 10-cell stage in H. sapiens [Nikas et al., 1996]).
II. The calves ACT3, ACT4, and ACT5
James Robl's laboratory at the University of Massachusetts (Cibelli et al., 1998) has taken cloning a step further. First, their success rate is much better. Out of 28 embryos derived from dividing bovine embryonic fibroblast nuclei, three healthy and genetically identical calves were produced. The source of the nuclei—dividing fibroblasts from a 55-day old bull fetus&mdashwas important. These fibroblasts, while mesodermal in origin, are not considered to be differentiated while they are dividing. Moreover, they have a long G1 state, a stage of the cell cycle that has been thought to be conducive to the remodeling by the oocyte. About 58% of these dividing fibroblasts are in the G1 state. The nuclei were marked by the insertion of a beta-galactosidease/neomycin resistance fusion gene. The fibroblasts were selected for the expression of this gene (by growing them in neomycin), and the nuclei from these fibroblasts were inserted into the enucleated mature oocytes. Each of the three calves had this inserted gene and had the gene in the same place in the chromosomes. Since several individuals were produced from the descendents of a single embryo and were from the same modified fibroblast cell, this is truly "cloning." Each of the calves is genetically identical to the other.
III. Consequences
Commercially, the production of cloned lines of "elite" breeders has been a plan of agricultural biologists for a long time. Champion milk cows and genetically modified sheep that produce large amounts of pharmaceutically valuable human proteins are extremely rare. To breed them to others means that their special abilities might not be passed to their offspring. This technique enables the propagation of that special ability. This had formerly been the special advantage that plant breeders had, since vegetative propagation by cuttings could enable the propagation of a rarely colored flower or a genetically improved cereal.
Ecologically, this may create a bottleneck in the genetic variety of various types of livestock. It may also create a larger disparity between the "haves" and "have-nots." It also may be a mechanism for reproducing endangered animals who have already gone through a severe genetic bottleneck and whose fertility is limited.
Socially, the cloning of a "large charismatic mammal" may be very important. First, it may be a wake-up call needed to pass some laws prohibiting both medical and industrial laboratories from certain types of experimentation. Second, it calls into question the proprietary relationship that companies can have to organisms. Is Dolly "nature" or "culture"? Is she the product of natural circumstances or specifically the artifact of technology? If the latter, Dolly becomes patentable. The research to produce Dolly, after all, was done in a pharmaceutical company, not in a university or medical research center. They want returns on capital investments. Patents become more important than papers. Biotechnology has blurred the distinction between nature and culture to such a degree that "nature" is seen as a genetic engineer and transgenic and cloned organisms are seen as cultural productions (see Haraway, 1997).
Human cloning and germline manipulation may be nearer than we think. In recent months, proposals have been forthcoming saying that cloning should be legalized and that germline gene therapy and enhancement should not be stopped. For more on this, click here.
For more information and reports of animal cloning and its consequences, click here.
Click here for further discussions concerning human germline manipulation and human cloning.
Click here to have a bit of cloning fun.
|
| Figure 1 It had to be ewe. Dolly, a cloned sheep. |
Literature Cited
Cibelli, J. B., Stice, S. L., Golueke, P. J., Kane, J. J., Jerry, J., Blackwell, C., Ponce de Leon, F. A., and Robl, J. M. 1998. Cloned transgenic calves produced from nonquiescent fetal fibroblasts. Science 280: 1256-1258.
Gurdon, J. B., Laskey, R. A., and Reeves, O. R. 1975. The developmental capacity of nuclei transplanted from keratinized cells of adult frogs. J. Ebryol. Exper. Morphol. 34: 93-112.
Haraway, D. J. 1997. modest_witness@second_millenium. Routledge, London.
Nikas, G., Ao, A., Winston, R. M. L., and Handyside, A. H. 1996. Compaction and surface polarity in the human embryo in vitro. Biol. Reprod. 55: 32-37.
Orr, N. H., DiBerardino, M. A., and McKinnell, R. G. 1986. The genome of frog erythrocytes displays centuplicate replications. Proc. Natl. Acad. Sci. USA 83: 1369-1373.
Simerly, C. and 11 others. 2003. Molecular correlates of primate nuclear transfer failures. Science 300: 297.
Wilmut, I., Schnieke, A. E., McWhir, J., Kind, A. J., and Campbell. K. H. S. 1997. Viable offspring derived from fetal and adult mammalian cells. Nature 385: 810-813.
© All the material on this website is protected by copyright. It may not be reproduced in any form without permission from the copyright holder.
|
HOME :: CHAPTER 0 :: 0. :: CLONING MAMMALS: WHAT DOES IT MEAN? |