Pros and Cons of SCNT

 

Pros of SCNT Even though the technique of mammalian SCNT is just over a decade old it has already resulted in numerous significant advances. 1. Repr Reprod oduc uctiv tive e cloni cloning ng has been demonstrated in a number of agricultural agricultural animals and endangered species, and SCNT has also improved efficiencies in generating transgenic animals for biomedical applications . Proof-ofprinciple studies on therapeutic cloning have been conducted to generate autologous cells for cell transplantation therapy in a mouse model. The generation of human ES cell lines of disease disease ould ould be invaluable tools in medical research and drug discovery. Each of these points are e!panded on belo. "eproductive cloning using usi ng adult tissue as a donor cell type has been demonstrated in a number of agricultural species cattle #++&+(. #$%&$'(&/sgoat #$)&$*(& sheep #+,(& pigincluding #+&+$(& horse an alternative to artificial insemination and embryo transfer& SCNT can be directly applied in farm animal production in both the dairy and beef industries #+0(. "eproductive cloning can be used to create multiple copies of animals ith highly valued traits& such as cos ith high mil1 production or bulls ith ith high quality meat #+%(. 2urthermore& studies have revealed no significant difference in biological properties and nutritional value of mil1 and meat samples ta1en from clones generated using embryonic cells& and adult somatic cells& and non-cloned cattle #+'&+)(. This This suggests that products from cloned animals and especially their offspring do not seem to pose any ris1s to human health. The 23/ recently 45anuary $,,)6 reported reported that food from healthy clones is as safe as food from non-cloned animals #+*(. 2. Clonin Cloning g using using SCNT SCNT is also also a useful useful tool for conserving genetic diversity and for epanding populations of endangered animal animal species.  Somatic cells can be obtained easily& easily& e!panded in culture& cryopreserved& and thaed at a later date for use in SCNT. The cloning of ild animals using host oocytes from different species has been successful in species such as 7os gaurus

 

  #,(& 8vis orientalis musimon #(& 8vis candensis me!icana   #$( and 7udorcas ta!icolor    #+(. 9oever& the availability of host oocyte donors and:or suitable foster mothers still represents a challenge. The The progress in the area of reproductive cloning in both domesticated and endangered species is encouraging for the future of SCNT for conservation. !. SCNT is also a useful techni"ue techni"ue for generating generating transgenic animals animals for biomedical applications. Transgenesis by other methods such as pronuclei injection is not only very inefficient 4; <6& but also problematic because of the random integration of desired genes into the genome #-%(. SCNT is at present the most efficient method to produce genetically modified farm animals. 7y integrating desired gene constructs into established cell lines groing in vitro& and then using the donor nuclei from cells ith a confirmed integration site for cloning& precise genetic modifications can be introduced into the genome of farm animals. 8ne application of this method is to produce transgenic animals anim als that e!press human proteins that can be subsequently purified from the animal 4e.g.& in their mil16 for the treatment of a range of human diseases. =oats& cos& sheep and pigs have been genetically modified and cloned in attempts to create such proteins #'(. /nother application is to clonedisease animalsuch disease models similarities to human as sheep andith pigs pig s hich share similarities in physiology& including the si>e of their organs to humans. These These animals can be used to understand the manifestation of the disease in genetically identical individuals and also to develop and test treatments. =eneration of animals ith organs suitable for !eno-transplantation to humans is another use of cloning based transgenesis #)(. #. Therapeutic Therapeutic cloning cloning is the most most applic applicable able form of cloning for humans and has infinite potential in cell therapy. The first proof of principle study on the therapeutic application of SCNT derived cell lines as reported in $,,$ hereby ES cells ere derived

 

from SCNT blastocysts using donor cells from immune-deficient "ag$-:- mice #*(. These mutant mice lac1 mature 7 and T lymphocytes and do not have antibodies in their serum hich is the typical phenotype observed in humans ith severe combined immune deficiency 4SC?36. Targeted homologous homologo us recombination as used to correct the "ag$ recombinase gene in the SCNT derived ES cells& hich ere then differentiated in vitro into hematopoetic stem cells 49SCs6 for transplantation #*(. The SCNT SCNT ES cell derived 9SCs ere then engrafted into the donor mice and ere able to reconstitute the haematopoietic system including the formation of 7 and T lymphocytes. 9oever& it as observed that the 9SCs derived from repaired ES cells e!pressed lo levels of @9C-? molecules& hich made the them vulnerable to NA cell mediated immune attac1. 3espite these results that shoed that even genetically matched cells and their derivatives derived from SCNT SCNT may be ineffective for the treatment of some disorders& these results shoed that it is feasible to combine therapeutic cloning ith gene therapy to generate cells for transplantation. $. %part %part from the the generati generation on of SCNT SCNT &S cells cells as patient matched cells for cell therapy SCNT could be used to generate generate human &S cell lines of disease. Currently& ES cell lines have been isolated from embryos identified by pre-implantation genetic diagnosis to carry disease #0,(. Such cell lines provide a valuable in vitro tool to understand the pathology of the disease and potentially offer a means to investigate therapies. Btili>ation of somatic cells from patients ith diseases such as later Par1insons disease& hichcomple! are usually manifested in life& ould provide a source of ES cell lines hich carry a predisposition to the disease. This ould allo both the study of the disease progression and also therapies to alleviate or delay onset of the disease. This ould be highly advantageous in both medical research or could potentially be used for drug discovery.

Cons of SCNT The increasing body of literature surrounding mammalian SCNT has revealed a number of potential limitations ith this method of reprogramming. They include abnormalities associated ith clones& technical difficulties associated ith SCNT and the

 

limited success s uccess in humans hum ans and primates. p rimates. ?n addition& the ethical and practical difficulties associated ith obtaining good quality oocytes for human therapeutic cloning is perceived as possibly the greatest hurdle of this technology. 1. Reproductive Reproductive cloning cloning by SCNT often results results in abnormal phenotypes in clones due to incomplete reprogramming. /bnormalities have been detected in clones of a number of species in the placenta #0-0+(& in fetuses #0$& 0& 00(& and after af ter birth #00-0)(. # 00-0)(.  /lthough  /lthoug h these abnormalities abnormalities may not be applicable applicable to therapeutic cloning in humans& they raise concerns about transplanted cells cells potentially harboring partially-reprogrammed or altered transcriptional profiles and the potential for malignant groth or other complications. 2. Therapeutic Therapeutic cloning cloning in in humans humans is shroude shrouded d in in controversy as it re"uires the destruction of SCNT blastocysts. These ethical issues have been addressed in a number of countries hich have legali>ed therapeutic cloning including the Bnited Aingdom& Singapore& Seden& 7elgium& 5apan& Spain& ?srael& 9useyin Sumer& 5un Diu& Pollyanna Tat et al. 0 China& South Aorea& and /ustralia. hilst SCNT remains legal in other countries such as the Bnited States of /merica and ?ndia& it has not been introduced into federal la. ?nterestingly& individual BS state la prohibits SCNT in federally-funded public and private institutions such as universities& hoever recent political changes may see this la

being revised #0*(. 3espite the a legality legalit y can in these countries& the process of obtaining license be an arduous process& and sanctioned laboratory activities are tightly regulated. There is also the additional comple!ity of obtaining an ethically and legally accepted source of good quality oocytes& as the short and long-term effects of ovarian ovar ian stimulation and the perceived e!ploitation e!ploi tation of omen are emotive em otive issues& hich compounds the feasibility of obtaining oocytes. !. There are a number number of of technical technical hurdles hurdles that need to be addressed before SCNT can be a viable source of pluripotent cells for human cell therapy . SCNT is a very resource intensive procedure& the technique is performed manually using speciali>ed equipment including a micromanipulation enabled microscope.

 

The success rate of SCNT is e!tremely lo ith the cloning of 3olly resulting from manipulation manipul ation of , oocytes& hich gave rise to $* blastocysts hich on transfer to a recipient ee resulted in the birth of the no famous F3olly #+,(. Since then the success rates of generating offspring has been reported to range from ,G$,<& typically beteen , and $<   #%,(. 2urthermore& reports of successful therapeutic cloning in primates and humans are limited& possibly indicating a higher degree of difficulty in reprogramming these species. ?n primate there has been one reports of successful SCNT in the "hesus macaque mon1ey here $ ES cell lines ere created using +, oocytes ith a derivation efficiency efficiency of ,.'< #%(. hile in humans there have only been three reports of human SCNT embryos reaching blastocyst stage 4blastocyst development efficiency *.$<-$).%<6 #%$-%(& ith no reports of ES cell derivation. To circumvent the need for large numbers of human oocytes for therapeutic cloning& oocytes from other species have been e!plored in interspecies SCNT #%0&%%(. 3espite the successful generation of human NT ES cells from the interspecies SCNT using rabbit oocytes #%0(& this procedure remains illegal in some countries due to both ethical concerns and biological issues such as the inter-species heteroplasmy of mitochondrial 3N/ 4also discussed belo6 and potential safety ris1 of viral crossinfection across species.