Cloning: experience from the mouse and other animals.

Cloning mammals has been successful for many years by splitting an early embryo or transferring embryonic cell nuclei into enucleated oocytes. Cloning is now possible with adult somatic cells. At present, cloning efficiency--as determined by the proportion of live offspring developed from all oocytes that received donor cell nuclei--is low regardless of the cell type (including, embryonic stem (ES) cells) and animal species used. In all animals, except of Japanese black beef cattle, the vast majority (>97%) of cloned embryos perish before reaching full term. Even in the Japanese cattle, less than 20% of cloned embryos reach the adulthood. This low efficiency of cloning seems to be due largely to faulty epigenetic reprogramming of donor cell nuclei after transfer into recipient oocytes. Cloned embryos with major epigenetic errors die before or soon after implantation. Those with relatively 'minor' epigenetic errors may survive birth and reach adulthood. We found that almost all fetuses of inbred mice die at birth from respiratory problems, while those of hybrid mice do not, suggesting that genomic heterogeneity masks-to some extent-faulty epigenetic errors. Thus far, the majority of cloned mice that survived birth, had a normal life span and were fertile. However, these animals may not be totally free of health problems. Postpubertal obesity in certain strains of mice is one example. A trial and error approach may discover better cells for cloning, but it would be wiser to understand the molecular mechanisms of epigenetic nuclear programming and reprogramming to find the way to make cloning safer and more efficient. The relatively high cloning success rate in the Japanese black cattle may provide us a clue of solving the problem of high mortality of cloned offspring.