The DNA methylation (5-MeC) and the enzymes for DNA methylation (DNMTs) were also present in quiescent NSCs during maintenance. This observation indicates that DNA methylation was dynamic and active during renewal and maintaining progenitor cells. The DNA methylation marks generally associated with gene repression are likely associated with the genes for differentiation, resulting in the maintenance of the quiescent or undifferentiating state of the neurosphere. However, MBD1, which recognizes methyl-CpG and represses transcription, was expressed minimally in the undifferentiated neurospheres. It is reported that MBD1 can facilitate repression during DNA replication and have a silencing role during S phase of the cell cycle (21). Thus, the low MBD1 binding to the specific cell cycle genes would favor the continuous renewal of the NSCs. The role of MBD1 in mediating the suppression of cell proliferation in NSCs is in agreement with our finding that expression of MBD1 increased in the differentiating neurosphere as proliferation declined and differentiation began.
