Cell News 01/2017
34
Binding of MBD proteins to DNA blocks Tet1 function
thereby modulating transcriptional noise
Anne K. Ludwig, Peng Zhang, Stephanie Meyer, Henry D. Herce, Udo Mueller,
Florian D. Hastert, Anne Lehmkuhl, Heinrich Leonhardt and M. Cristina Cardoso
Presenting author: Anne Kathrin Ludwig
Department of Biology, Technische Universität Darmstadt,
Darmstadt, Germany
Aberrant DNA methylation is a hallmark of various severe
human disorders, indicating that the spatial and temporal
regulation of DNA methylation writers, readers and modifiers is
imperative for normal development and differentiation. While
the regulation of 5-methylcytosine writers and readers is well
characterized, yet little is known about how ten-eleven-trans-
location (Tet) proteins, a family of enzymes recently described
to initiate DNA demethylation, are regulated. Here, we show
that prior binding of methyl-CpG binding protein 2 (Mecp2)
and methyl-CpG binding domain protein 2 (Mbd2) to meth-
ylated DNA protects 5-methylcytosine from Tet1 mediated
oxidation in vitro and in vivo. The mechanism is not based on
competition for 5-methylcytosine binding but rather on Mecp2
and Mbd2 directly restricting Tet1 access to DNA. Furthermore,
we demonstrate that the efficiency of Tet1 repulsion highly de-
pends on the number of bound 5-methylcytosine guardians per
DNA molecule. Accordingly, we find the Tet oxidation product
5-hydroxymethylcytosine to be enriched in vivo at pericentric
heterochromatin of Mecp2 deficient neurons of a mouse model
for Rett syndrome. As a functional consequence, in the absence
of Mecp2, we found that Tet1 induces re-expression of epige-
netically silenced major satellite repeats, with the potential to
compromise genome stability. In summary, these results unveil
fundamental regulatory mechanisms of Tet enzymes and their
potential pathophysiological role in Rett syndrome. Important-
ly, the protective function of Mecp2 and Mbd2 shown here,
suggest that they may have an essential physiological role as
guardians of the epigenome.
OTHER TOPICS
The DREAM/MuvB component Lin37 cooperates with
Rb to initiate quiescence
Christina Mages, Axel Wintsche, Kurt Engeland, and Gerd A. Müller
Presenting author: Christina Mages
Molecular Oncology, University of Leipzig, Semmelweisstr. 14,
D-04103 Leipzig, Germany
Rb/E2F and MuvB-based complexes play an essential role in
the coordination of cell cycle-dependent gene expression. In
quiescent cells, the MuvB core (Lin54, Lin52, Lin37, Lin9 and
Rbbp4) interacts with E2f4/5, p130, and Dp1 to form the re-
pressing DREAM complex. During progression through the cell
cycle, p130, E2f4/5, and Dp1 dissociate from the MuvB core.
In S phase, MuvB components associate with B-Myb form-
ing the B-Myb-MuvB (MMB) complex required for recruiting
Foxm1. Finally, the Foxm1-MuvB complex stimulates maximum
expression of late cell cycle genes in G2/M.
The specific functions of MuvB core proteins are not well
understood. While Lin54 interacts with DNA through CHR
promoter elements and Lin52 recruits the p130/p107/E2f4/5/
Dp module, the function of Lin37 remained elusive. We applied
a CRISPR/Cas9 nickase system to create Lin37-deficient cells.
In contrast to cells lacking Lin54, Lin52 or Lin9 which exhibit
serious cell cycle defects, Lin37 knockout cells proliferate nor-
mally. Furthermore, expression of cell cycle genes in proliferat-
ing cells is not reduced showing that activating MuvB com-
plexes can function without Lin37. However, mRNA levels of
early and late expressed genes are elevated in G0/G1 because
Lin37 is essential for function of DREAM as a transcriptional
repressor complex. Nonetheless Lin37 knockout cells can still
arrest in response to serum deprivation. To test the hypothesis
that Rb/E2F complexes mediate G1 arrest in the absence of
Lin37 and DREAM function, we created Rb-/- and Lin37-/-/
Rb-/- cells. As Lin37-/- cells, Rb-/- cells still have the potential
to arrest in G0/G1. However, Lin37-/-/Rb-/- cells are essentially
unable to enter quiescence. This is consistent with the pheno-
type for pocket protein triple knockout cells.
Based on our results, it can be concluded that Lin37 is neces-
sary for repression of cell cycle genes, but not for activation,
and cooperates as a part of DREAM with Rb in initiating
quiescence.
