The Effect of Mismatch-Specific Thymine DNA Glycosylase (TDG) Deficiency on Spontaneous Mutation in Mice

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In mammalian cells, methylation of cytosine (5-methylcytosine) at CpG site is thought to play a pivotal role in control of gene expression, embryogenesis, genomic imprinting, and X chromosome inactivation. 5-methylcytosine is also known to be mutagenic because it is dearninated hydrolytically and converted to thyrnine resulting in G;T mismatch formation. Thus, this deamination of 5-methylcytosine is implied to be the main cause of G:C to A:T base substitution.TDG (mismatch-specific T__-hymine D__-NA G__-lycosylase) is one of the repair enzymes working in removal of thymine mispairing with G at CpG site. We generated TDG (-/-) ES cells by gene targeting in order to investigate its function in vivo. Although TDG (-/-) ES cells had little G-T mismatch glycosylase activity in nicking assay, there is no difference in mutant frequency at Hprt gene between TDG (+/+) and (-/-) ES cells. This result suggests that TDG might not have strong effects on mutation induction in ES cells.We have also generated TDG (+/-) mice. They are fertile and grow normally however, homozygous mutant (-/-) mice died before ll.5 days post coitum (dpc). Although there was no significant difference in spontaneous mutant frequency between wild type and TDG . (-/-) embryos, we found that the level of noradrenaline, necessary for normal embryogenesis, dramatically reduced in TDG (-/-) embryos. Furthermore, we could partially rescue TDG (-/-) embryos beyind 11.5 dpc by the administration of noradrenaline. These results suggest that the lethality of TDG (-/-) embryo is in parts due to noradrenaline reduction, and TDG is indispensable for embryonic development.