DNA methyltransferase contributes to delayed ischemic brain injury

DNA methylation is important for controlling the profile of gene expression and is catalyzed by DNA methyltransferase (MTase), an enzyme that is abund ant in brain. Because significant DNA damage and alterations in gene expres sion develop as a consequence of cerebral ischemia, we measured MTase activ ity in vitro and DNA methylation in vivo after mild focal brain ischemia. A fter 30 min middle cerebral artery occlusion (MCAo) and reperfusion, MTase catalytic activity and the 190 kDa band on immunoblot did not change over t ime. However, [H-3]methyl-group incorporation into DNA increased significan tly in wildtype mice after reperfusion, but not in mutant mice heterozygous for a DNA methyltransferase gene deletion (Dnmt(S/+)). Dnmt(S/+) mice were resistant to mild ischemic damage, suggesting that increased DNA methylati on is associated with augmented brain injury after MCA occlusion. Consisten t with this formulation, treatment with the MTase inhibitor 5-aza-2'-deoxyc ytidine and the deacetylation inhibitor trichostatin A conferred stroke pro tection in wild-type mice. In contrast to mild stroke, however, DNA methyla tion was not enhanced, and reduced dnmt gene expression was not protective in an ischemia model of excitotoxic/necrotic cell death. In conclusion, our results demonstrate that MTase activity contributes to poor tissue outcome after mild ischemic brain injury.