Cell and Molecular Biology of DNA Methyltransferase 1 - International Review of Cell and Molecular Biology - page 8

unmethylated DNA ( Bestor, 1992 ). Interestingly, a fusion protein con-
taining the intact amino terminus (

1000 amino acids) of mouse DNMT1
and most of the coding sequence of the Hha I bacterial cytosine methyl-
transferase (DNMT1- Hha I) increases (by 2.5-fold) the preference for
hemimethylated DNA over unmethylated DNA ( Pradhan and Roberts,
2000 ). A preference for hemimethylated DNA templates was not observed
for the parental Hha I methyltransferase. These experimental findings are
consistent with the notion that the noncatalytic region of DNMT1 stimu-
lates methyltransferase activity, presumably by specifically inducing mainte-
nance methylation of hemimethylated DNA. These in vitro studies of
isolated DNMT1 suggest that the intracellular catalytic function of DNMT1
is to maintain nuclear genomic methylation patterns by recognizing
hemimethylated DNA after replication and then enzymatically converting
it to fully methylated DNA.
Support for the existence of maintenance methylation in mammalian
cells comes from the evaluation of the developmental functions of different
isoforms of the DNMT1 cytosine methyltransferase. Analysis of DNMT1 in
different tissues in mouse and human revealed that there are two forms of
DNMT1 with differences in the primary amino acid sequence: a long form
of DNMT1 referred to as DNMT1s is produced in all somatic tissues,
whereas the short form of DNMT1, designated DNMT1o, is produced
exclusively in the oocytes ( Mertineit et al., 1998 ). These two different forms
are products of two different promoters, which produce alternative tran-
scripts leading to an exclusion of 118 amino acids at the N-terminus of
the mouse DNMT1o protein.
The effects of the engineered removals (by targeted mutations of the
Dnmt1 gene) of the constitutively expressed somatic form of DNMT1
(DNMT1s) in mice and ES cells and the oocyte-derived DNMT1o form
in mice are very supportive of a primarily maintenance methylation function
of DNMT1 during embryogenesis ( Howell et al., 2001; Li et al., 1992 ).
Removal of DNMT1 in mouse ES cells reduces genomic methylation by

80%, with the remaining 20% of normal methylation attributed to the
activities of two methyltransferases DNMT3a and DNMT3b ( Lei et al.,
1996; Schermelleh et al., 2007; Tsumura et al., 2006 ). Interestingly,
DNMT3a and DNMT3b have predominantly de novo methyltransferase
activity when studied in vitro ( Xie et al., 1999 ). Significant reduction in
the steady-state level of genomic methylation in the absence of DNMT1
and the presence of the DNMT3a and DNMT3b de novo methyltransferases
is consistent with a primary maintenance methyltransferase activity for