Generation of cloned mice and application on the field of laboratory animal science

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In order to study the practical application of cloning technology to the field of laboratory animal science, we have attempted to generate cloned mice from fibroblasts and embryonic stem cells by a serial nuclear transfer technique, and also examined the characteristics of the cloned mice. We successfully generated 5 ($ % of blastocysts transferred) mice from metaphase-arrested CD1 or (CD1 x B6) F1 fetal fibroblasts, and 27 (6%) and 20 (2%) mice from two TT2 ES cell lines that were independently inactivated different genes, G9a and Oviduct-specific glycoprotein (OGP). In these cloned mice, the commonly observed morphological abnormality was hypertrophy of the placenta, which was over 2-5 times over than the controls. In addition, some cloned mice showed increased body weights and open eyelids at birth. However, these abnormalities have not transmitted the progeny by mating, indicating that the abnormalities seen in the cloned mice were caused by inappropriate reprogramming of the epigenetic modifications, but not by accumulative DMA mutations, and was correctly reprogrammed through the germ line. In fact, we clarified that the expression pattern of imprinting genes, i.e., Igf2 and H19, were distorted in the cloned mice and their placentas. However, the fact that the progeny from the cloned mice that showed various abnormalities were normal suggests the usefulness of cloning technology in the field of laboratory animal science. The possibilities in which transfer of mitochondria originated from donor cells may influence the phenotype of the clone mice were also demonstrated.