Cancer Research in Switzerland - Krebsliga Schweiz

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by TDG prevents efficient downstream processing of
repair intermediates, thereby mediating DNA­directed
cytotoxicity. The status of TDG expression in a cancer is
therefore likely to determine its response to 5­FU­based
chemotherapy.
Project coordinator
Prof. Dr. Primo Schär
Institut für Biochemie und Genetik
Departement Biomedizin
Universität Basel
Mattenstrasse 28
CH­4058 Basel
Phone +41 (0)61 267 07 67
Fax +41 (0)61 267 35 66
primo.schaer@unibas.ch
Schär Primo | The role of thymine DNA glycosylase
in the maintenance of genetic and epigenetic stability
and the suppression of tumorigenesis
(OCS 2193­02­2008)
Background of the study
To maintain the integrity of genetic information, every cell
of your body uses repair systems to remove damage occurring
to DNA. The most prevalent DNA lesions are small
chemical modifications of the DNA bases, and these are
recognized and removed from DNA by specialized enzymes.
TDG belongs to this class of enzymes. It removes
uracil (U) and thymine (T) when mispaired with guanine
(G). Such mispairs in DNA arise frequently by spontaneous
deamination of cytosine or 5­methylcytosine, respectively.
In addition to the repair of these lesions, TDG has
been implicated in the regulation of gene transcription
and in the control of cytosine methylation. Cytosine
methylation in DNA serves as a molecular tag to activate
or inactivate genes. In cancerous cells these tags are often
inaccurately set, thereby avoiding the expression of genes
that act against tumour development. Although its biological
function remains to be clarified, the available
evidence strongly suggests that TDG act against genetic
mutation and stabilizes gene expression, both of which
are important tumour suppressor functions. Thus, TDG is
likely to play a critical role in tumour suppression. This
study was designed to address the putative tumour suppressor
function for the first time directly in human cancers.
Aims
The aims of this study were to analyze the expression status
of TDG in different human cancers and the respective
normal tissue, to identify molecular causes of the potential
loss of TDG expression in cancer cells and to characterize
epigenetic and genetic instabilities in such cancers.
Methods and approach
We developed immunohistochemical methods for specific
staining of TDG in tissue sections. These methods were
used to compare presence and levels of TDG protein in
various human cancers with those in the respective nor­
mal tissues. In addition, we established methods allowing
the analysis of cytosine methylation in gene regulatory regions
and used these to assess the effect of TDG expression
on the status of cytosine methylation.
Findings of the study
The systematic analysis of TDG expression in normal and
cancerous tissues generated interesting results. In most
normal tissues, TDG was inhomogenously expressed and
present only in a subset of cells. The mean number of TDG
expressing cells varied in a tissue­dependent manner,
ranging from less than 10 % (breast) to over 80 % (brain).
In tumours, however, such patterns of TDG expression
were dramatically changed, most prominently in the colorectal
cancers. In comparison to normal colorectal epithelium,
which showed about 30 % TDG positive cells, the
amount of TDG expressing cells in the cancer tissue was
reduced to an average of 4 %. Strikingly, most colorectal
tumours presented with a complete absence of TDG protein.
Initial molecular analysis of these tumours revealed
an increase of cytosine methylation in the regulatory regions
of the Tdg gene and of other genes. Altogether, our
data suggest that the absence of TDG brings about instability
in gene expression, conferring a selective advantage
to cancer forming cells.
Project coordinator
Prof. Dr. Primo Schär
Institut für Biochemie und Genetik
Departement Biomedizin
Universität Basel
Mattenstrasse 28
CH­4058 Basel
Phone +41 (0)61 267 07 67
Fax +41 (0)61 267 35 66
primo.schaer@unibas.ch
Schübeler Dirk | Role and plasticity of DNA methylation
in stem cell pluripotency and cancer
(KLS 01865­02­2006)
In recent years, stem cells have been discovered in many
adult tissues throughout the body, where they differentiate
into specific cell types and help to regenerate the tissue.
In addition, scientific studies suggest that these adult
stem cells can transform into cancer stem cells. Next to
genetic mutations, epigenetics changes might play a role
in this aberration. In this study we determined the dynamics
of DNA methylation, an epigenetic modification, during
normal differentiation and explored its potential to
differentiate between normal and cancer stem cells.
Aim
The goal of the study was to identify epigenetic markers
of cancer stem cells, i.e. genomic regions that are specifically
DNA methylated or unmethylated in cancer stem
cells.
Method and Approach
We employed embryonic stem (ES) cells as an in vitro
model. We differentiated them into neuronal progenitors
and terminal neurons and furthermore compared them to
embryonic carcinoma (EC) cells. Using the MeDIP technique
to measure DNA methylation and chromatin immunoprecipitation
technique to measure histone modifica