Functional analysis of the DNA repair protein MBD4

• Main Author: MacDougall, Eilidh Fiona
• Published: University of Edinburgh 2006
• Subjects: 572.6
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.654229

The methylation of cytosine plays a fundamental role in mediating transcriptional repression. However, 5-methylcytosine can undergo spontaneous hydrolytic delamination to form thymine. It the resulting T:G mismatch is replicated prior to being repaired, a C:G to T:A transition mutation will be present in one of the two daughter DNA molecules. Methyl-CpG-binding domain protein 4 (MBD4) is a DNA glycosylase that can excise thymine from mismatches with guanine, and that acts preferentially on T:G mismatches within CpG dinucleotides <i>in vitro</i>. In order to test the hypothesis that MBD4 repairs the product of 5-methylcytosine delamination <i>in vivo</i>, MBD4-deficient mice were crossed onto the Big Blue genetic background. This background enables the frequency and spectrum of <i>in vivo</i> mutations in a bacteriophage lambda <i>cII</i> transgene to be determined. As predicted, <i>Mbd4^-/-</i> mice have a significantly increased frequency of C:G to T:A mutation sat CpG dinucleotides. T:G mismatch-specific thymine DNA glycosine (TDG) can also attempt to excise thymine from T:G mismatches within CpG dinucleotides <i>in vitro</i>. In an attempt to determine the relative contributions of MBD4 and TDG to the repair of 5-methylcytosine delamination-induced T:G mismatches <i>in vivo</i>, the mutation frequencies and spectra in cell lines lacking MBD4 and/or TDG were measured. An additional line of research focused on potential mechanisms by which the DNA repair activity of MBD4 may be regulated. A novel protein has previously been shown to interact with MBD4 in a yeast two-hybrid screen that used MBD4 as the bait protein. This interaction was further characterised by mapping of the interaction domains using the yeast two-hybrid assay, and by immunocytochemistry. Finally, it was also shown that MBD4 may be post-translationally modified by sumoylation.