Comparison of the RNA polymerase I-, II- and III-dependent transcript levels between nuclear transfer and in vitro fertilized embryos at the blastocyst stage.

Cloned animals have been produced in several mammalian species so far, although success rates to term are very low. Aberrations in gene expression derived from abnormal epigenetic status have been thought to be a cause of developmental abnormalities in clones, and several abnormalities in gene expression have already been detected in cloned animals and embryos. In this study, we examined the hypothesis that the poor survival rates of nuclear transfer (NT) embryos are partly due to aberrations in the regulation of expression of genes transcribed by RNA polymerases I and III, in addition to polymerase II. We produced cloned and in vitro fertilized mouse embryos that developed to the blastocyst stage, and the amounts of several genes were analyzed using individual embryos. We found that the amounts of mature 18S ribosomal RNA (rRNA) transcribed by RNA polymerase I were lower in NT embryos than in IVF embryos, but that the amounts of 47S rRNA and intermediates of mature rRNAs were higher in NT embryos. In addition, the amount of 7SK RNA transcribed by RNA polymerase III was lower in NT embryos than in IVF embryos. The transcripts of all but one of the genes transcribed by RNA polymerase II were not noticeably different between NT and IVF embryos. These results suggest that some of the transcripts produced by RNA polymerases I, II and III are aberrantly regulated in NT embryos.