Régulation de L'expression de Gènes
Chromatin is the template of transcriptional programs and gene expression regulations. Chromatin based regulations involve crosstalks between transcriptional activity with histone modifiers or modifications, binding of transcription factors to enhancers and promoters, or of architectural proteins to insulators, and chromatin topology. Genome is subdivided in euchromatin and heterochromatin domains. Euchromatin is mostly associated with active genes and enriched in H3K36 and H3K4 methylations while heterochromatin, contributing to genes silencing, is enriched in H3K27me3 when facultative heterochromatin or H3K9me3 when pericentromeric or telomeric heterochromatin. On top of this linear organisation, chromatin is further folded in topologically associated domains (TADs) and compartments that cluster spatially, which lead to the clustering of chromatin domains that share similar functions. Insulators are anchors that regulate chromatin topology by creating or stabilizing loops isolating TADs from each other. Evidences demonstrate that both chromatin state and topological structures can influence or dictate gene expression, and conversely, chromatin is modified by transcription-coupled processes and transcription factories may participate to the formation of compartments. Whether global rules govern such crosstalks at the genome-wide levels, remain unclear. Moreover, whether some kind of hierarchy exists among such influences of chromatin related processes remains poorly understood. During my thesis, I have analysed the respective influence of both H3K36 di-methylation and tri-methylation in maintaining euchromatin domains. I have further contributed to the development of a computational method aiming to understand the role of insulators. H3K36me2 and me3 are both enriched in euchromatin domains and are reported to globally counteract repressive H3K27me3 deposition; however, it is unclear whether di-methylation is sufficient for this blocking or is just an intermediary stage for tri-methylation. Those modifications are ensured by unique or multiple histone methyltransferases (HMTs), depending on species, which can hinder the characterisation of their respective roles. In Drosophila, H3K36me2 is specifically done by Mes-4/dNSD1 that is thought to be recruited at domain borders via insulators. In contrast, HypB/SETD2 may decorate genes with H3K36me3 co-transcriptionnally. Depletions of either Mes-4/dNSD1 or HypB/SETD2 lead to specific increases of H3K27me3, causing genes repression. These results indicate that both Mes 4/dNSD1 and HypB/SETD2 are able to protect genes from H3K27me3 spreading independently of each other. Of interest, our results highlight that a specific role of each of these HMTs depends on chromatin topological contexts. Mes-4/dNSD1 protects euchromatin genes at insulator-associated TAD borders while HypB/SETD2 repels H3K27me3 on active genes located on fuzzy hetero-euchromatin borders lacking insulator anchoring. In a second manuscript, we have developed tools to visualize long-range interactions (LRIs) detected in Hi-C contact maps to improve the detection of LRIs through quantilization of contact frequencies. We further developed Hi-C aggregation plots to enable the analysis of asymmetric LRIs such as enhancer-promoter or insulator-promoter contacts, by orientating the sub-matrices before aggregation. Our method allows to accurately distinguish insulators from enhancers depending on surrounding Hi-C contacts with potential "leaks" when LRIs are not block by an insulator. Depletion of CTCF confirms the existence of such leaks. Quantification of the Hi-C signal at single-locus levels allows LRIs detection, confirming that leaks occurring upon CTCF depletion are specific to CTCF binding sites. Thus, our method provides a comprehensive detection of differential LRIs surrounding insulators, rendering possible discoveries regarding insulators, mechanisms and associated factors.