Abstract
Cloning by nuclear transfer using mammalian somatic cells has enormous potential application. However, somatic cloning has been inefficient in all species in which live clones have been produced. High abortion and fetal mortality rates are commonly observed. These developmental defects have been attributed to incomplete reprogramming of the somatic nuclei by the cloning process. Various strategies have been used to improve the efficiency of nuclear transfer, however, significant breakthroughs are yet to happen. In this review we will discuss studies conducted, in our laboratories and those of others, to gain a better understanding of nuclear reprogramming. Because cattle are a species widely used for nuclear transfer studies, and more laboratories have succeeded in cloning cattle than any other specie, this review will be focused on somatic cell cloning of cattle.
Highlights
Somatic cell cloning is a technique in which the nucleus (DNA) of a somatic cell is transferred into an enucleated metaphase-II oocyte for the generation of a new individual, genetically identical to the somatic cell donor (Figure 1)
Because 56% of cycling cells in that study were in G1 stage, it is likely that all cloned animals produced in this study were from donor cells at G1 stage
This study demonstrated that cells at G1 stage can produce live cloned animals and G0 induction is not essential
Summary
Somatic cell cloning (cloning or nuclear transfer) is a technique in which the nucleus (DNA) of a somatic cell is transferred into an enucleated metaphase-II oocyte for the generation of a new individual, genetically identical to the somatic cell donor (Figure 1). Despite the fact that Kasinathan's study did not produce live clones from G0 cells, a high nuclear transfer success rate was obtained by Cho et al [55] who subjected donor cells to serum starvation and found no improvement in blastocyst development from adult donor cells, but resulted in a 27.3% calving rate. Jones et al [50] and Zhou et al [51] treated bovine fetal fibroblast cells and mouse stem cells with much higher doses of 5-aza-C (1 or 5 μm) and found that blastocyst development of cloned embryos were reduced The consensus from these studies [49,50,51] suggests that lowering the levels of DNA methylation in donor cells does not always improve development of cloned embryos. The detrimental effect of a higher dose of TSA on embryo development may be explained by the fact that treatment of cells with high concentrations of TSA causes chromatin breaks and apoptosis [66]
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