Figure 1: Left. Structure of the CXXC domain (yellow) that selectively binds to unmethylated CpG dinucleotides (red). Right. Multiple developmental abnormalities and pervasive apoptosis (blue cells) in mouse embryos disrupted for Fbxl10 (also termed CXXC2).
Figure 2: Example of ectopic de novo methylation at CpG-dense promoters and transcriptional silencing in cells lacking Fbxl10.
The Boulard group integrates genetic, molecular biology and genomic approaches to explore how cytosine methylation represses transcription.
Previous and current research
The human body is composed of hundreds of different cell types all bearing the same genetic material. Cell identity is specified by the coordinated action of transcription factors, which bind to specific DNA regulatory sequences and turn gene expression on. The paradigm of gene expression being controlled by simple binding interactions has some exceptions. For instance, the mono-allelic expression of imprinted genes shows that two alleles exposed to the same transcription factors can have opposite transcriptional states. Other examples of transcriptionally inactive loci include i) genes located on the inactive X chromosome and ii) parasitic elements such as retrotransposons or proviruses. The repression of these genes relies on a chemical modification of the DNA called cytosine methylation. Genomic methylation represses transcription even in the presence of the transcription factors necessary for their activation. Because the methyl mark is mitotically inherited, the inactive state resulting from genomic methylation is epigenetically propagated. Importantly, most of the promoters are protected from epigenetic silencing and are kept unmethylated in all cell types.
We previously investigated the mechanism that defends CpG-dense promoters against abnormal de novo methylation in the early embryo and uncovered the key role of the multidomain protein FBXL10 in this process. Our results highlighted how hyper-conserved CpG-dense promoters evade de novo methylation.
More recently we began addressing the long-standing question of the mechanism of repression. Our first insight came from the characterisation of a mutation that disrupts the silencing property of methylated cytosines. This finding enabled us to biochemically purify the methylation-dependent silencing complex and analyse its composition. Our future research is centered on the functional exploration of the proteins that belong to the silencing complex.
Future projects and goals
The biological question that drives our research is how cytosine methylation represses transcription and ultimately impacts phenotypes. The primary approach of our laboratory is genetic and we use the mouse as a mammalian model. Specifically, we are investigating:
- The biology of repressors of transcription in the context of reproduction and development.
- The function and the epigenomic distribution of post-translationally modified repressors.
- The factors that target the silencing complex to methylated genes.