Aberrant DNA methylation in 5' regions of DNA methyltransferase genes in aborted bovine clones.

High rate of abortion and developmental abnormalities is thought to be closely associated with inefficient epigenetic reprogramming of the transplanted nuclei during bovine cloning. It is known that one of the important mechanisms for epigenetic reprogramming is DNA methylation. DNA methylation is established and maintained by DNA methyltransferases (DNMTs), therefore, it is postulated that the inefficient epigenetic reprogramming of transplanted nuclei may be due to abnormal expression of DNMTs. Since DNA methylation can strongly inhibit gene expression, aberrant DNA methylation of DNMT genes may disturb gene expression. But presently, it is not clear whether the methylation abnormality of DNMT genes is related to developmental failure of somatic cell nuclear transfer embryos. In our study, we analyzed methylation patterns of the 5' regions of four DNMT genes including Dnmt3a, Dnmt3b, Dnmt1 and Dnmt2 in four aborted bovine clones. Using bisulfite sequencing method, we found that 3 out of 4 aborted bovine clones (AF1, AF2 and AF3) showed either hypermethylation or hypomethylation in the 5' regions of Dnmt3a and Dnmt3b, indicating that Dnmt3a and Dnmt3b genes are not properly reprogrammed. However, the individual AF4 exhibited similar methylation level and pattern to age-matched in vitro fertilized (IVF) fetuses. Besides, we found that the 5' regions of Dnmt1 and Dnmt2 were nearly completely unmethylated in all normal adults, IVF fetuses, sperm and aborted clones. Together, our results suggest that the aberrant methylation of Dnmt3a and Dnmt3b 5' regions is probably associated with the high abortion of bovine clones.