Genetic analyses of DNA methyltransferase genes in mouse model system.

DNA methylation regulates important biological processes and is involved in tumorigenesis and several human diseases, such as Rett and immunodeficiency, centromeric instability and facial anomalies (ICF). The major objective of our research is to investigate the roles of DNA methylation in mammals through genetic analysis of DNA methyltransferase genes in mouse and human. Previously, we found that Dnmt1 knockout embryonic stem (ES) cells are capable of methylating retroviral DNA de novo. In search of enzymes responsible for de novo methylation, we have cloned a novel family of mammalian DNA methyltransferase genes, Dnmt3a and Dnmt3b. Although extensive sequence similarity was found between Dnmt3a and Dnmt3b, little homology was observed between Dnmt1 and Dnmt3a/3b in the catalytic domain as well as in the N-terminal domain. Additionally, biochemical analysis revealed that, unlike Dnmt1, neither Dnmt3a nor Dnmt3b had a strong preference to hemimethylated DNA substrates. Genetic analysis demonstrated that Dnmt3a and Dnmt3b were required for de novo methylation activities in ES cells and during early embryogenesis and were essential for early development. Interestingly, phenotype analyses of single homozygous mice for either Dnmt3a or Dnmt3b suggested that the functions of Dnmt3a and Dnmt3b also were required at the late developmental stage and even at the adult stage.