Problemas y perspectivas de la clonación de animales

Abstract

An efficient animal cloning technology would provide many new opportunities for livestock agriculture, human medicine, and animal conservation. Nuclear cloning involves the production of animals that are genetically identical to the donor cells used in a technique known as nuclear transfer (NT). However, at present it is an inefficient process: in cattle, only around 6% of the embryos transferred to the reproductive tracts of recipient cows result in healthy, longterm surviving clones. Of concern are the high losses throughout gestation, during birth and in the post-natal period through to adulthood. Many of the pregnancy losses relate to failure of the placenta to develop and function correctly. Placental dysfunction may also have an adverse influence on postnatal health. These anomalies are probably due to incorrect epigenetic reprogramming of the donor genome following NT, leading to inappropriate patterns of gene expression during the development of clones. Whilst some physiological tests on surviving clones suggest normality, other reports indicate a variety of post-natal clone-associated abnormalities. This variability in outcome may reflect species-specific and/or cloning methodological differences. Importantly, to date it appears that these clone-associated phenotypes are not transmitted to offspring following sexual reproduction. This indicates that they represent epigenetic errors, rather than genetic errors, which are corrected during gametogenesis. Whilst this needs confirmation at the molecular level, it provides initial confidence in the first application of NT in agriculture, namely, the production of small numbers of cloned sires from genetically elite bulls, for natural mating, to effectively disseminate genetic gain. In addition to the animal welfare concerns with the technology, the underlying health of the animals and the consequential effect on food safety are critical aspects that require investigation to gain regulatory and consumer acceptance. Future improvements in animal cloning will largely arise from a greater understanding of the molecular mechanisms of reprogramming.

Production of light-coloured, low heat-absorbing Holstein Friesian cattle by precise embryo-mediated genome editing

“…The fate of calf 15 highlighted that our biopsy screening was not accurate enough to completely avoid the transfer of embryos with unintended on-target edits that can result in calves with compromised health. As discussed earlier, the alternative cell-mediated approach using SCNT offers better control over on-target edits but also carries an animal welfare burden as it can compromise the development and health of cloned calves (Wells 2005). Scope and continued efforts for improving the design of editors, detection of HDR editing outcomes in embryos and cloning efficiencies will help to minimise or avoid undesirable editing outcomes that adversely affect the welfare of animals in the future.…”

Food Safety, Animal Health and Welfare and Environmental Impact of Animals derived from Cloning by Somatic Cell Nucleus Transfer (SCNT) and their Offspring and Products Obtained from those Animals

“…Initial pregnancy rates (at day 50 of gestation after transfer) in cattle serving as surrogate dams were found to be similar between those carrying clones (65 %) and those produced through the use of other artificial methods such as embryo transfer (58 %) and artificial insemination (67 %) (Heyman et al, 2002;Lee et al, 2004). However, there is a continued pregnancy loss throughout the entire gestation period in those surrogate dams carrying clones which is not observed in other ARTs, and embryo survival is only one-third of that following in vitro embryo production (Lee et al, 2004;Wells, 2005).…”