Cell and Molecular Biology of DNA Methyltransferase 1 - International Review of Cell and Molecular Biology - page 15

about multiple conformational changes and ensures high fidelity of
DNMT1-mediated maintenance of cytosine methylation. Of course, deri-
vation of molecular mechanism from crystallographic structural information
of a portion of a protein can potentially be misleading. In this regard, it is
interesting that deletions of the CXXC domain rescued DNA methylation
patterns in Dnmt1-null ES cells just as efficiently as wild-type DNMT1
( Frauer et al., 2011 ).
Another approach to elucidate function of structured regions of an
enzyme such as DNMT1 is to identify and study functions residing outside
its catalytic activity. For example, recent studies show that biological func-
tions of DNMT1 could be independent of its catalytic activity and some of
this regulation seems to involve well-defined features of DNMT1 at or near
the amino terminus ( Espada et al., 2011 ). Deletion of DMAP1-binding and
PCNA-binding domains of DNMT1 results in downregulation of
E-cadherin, suggesting that DNMT1 enables derepression of E-cadherin.
This derepression is rescued by expression of a DNMT1 mutant lacking
the catalytic domain but not by a mutant that lacks the DMAP1- and
PCNA-binding domains. Biochemically, DNMT1 was found to interact
with SNAIL1, a repressor of E-cadherin to prevent access of the latter to
the E-cadherin promoter leading to derepression. These results identify a
previously unknown function of DNMT1 in modulating gene expression
that is independent of its catalytic activity.
Further evidence in favor of catalytic-independent functions of DNMT1
comes from experiments in Xenopus laevis ( Dunican et al., 2008 ). Early
embryos in Xenopus undergo gene silencing in an xDNMT1-dependent
manner and inactivation of xDnmt1 by morpholinos results in premature
activation of genes before midblastula transition. Because this premature
gene activation occurs without any global changes in DNA methylation,
at least some genes regulated by xDNMT1 do not appear to involve the pro-
cess of maintenance methylation. Consistent with this possibility, repression
of genes can be achieved in the morphants by expression of catalytically inac-
tive human DNMT1. Subsequent experiments have shown that xDnmt1
can localize to nonmethylated target promoters and repress transcription.
However, the precise regions of xDNMT1 responsible for this catalytic
activity-independent repression have not yet been defined.
In a recent study aimed at identification of target genes repressed by cat-
alytically inactive DNMT1, Clements et al. (2012) studied HCT116 colon
cancer cells that are genetically disrupted for Dnmt1 . When these mutant
cells were transfected with wild-type and catalytically inactive DNMT1,