Role of PML nuclear bodies and of the histone chaperone complex HIRA in the regulation of chromatin dynamics

Description

Within the nucleus of eucaryotic cells, DNA is wrapped around histone proteins in order to form a structure called chromatin. This organization allows the compaction of a genome of 2 meters in a nucleus of around ten micrometers. It also allows the regulation of gene expression. Indeed, chromatin carries a source of information called epigenetic information and the modulation of its structure has important consequences on the transcriptional program. The histone chaperone complex HIRA is involved in the deposition of the histone variant H3.3 in euchromatin regions and in regions devoid of nucleosomes, in order to ensure chromatin and epigenetic integrity. Under normal conditions, the HIRA complex localizes homogeneously in the cell nucleus; on the other hand, under certain stressful conditions, such as during inflammation, the entry of cells into senescence or, as I showed in my thesis, during the induction of double-strand DNA breaks, the HIRA complex accumulates with H3.3 in spherical nuclear membrane-less organelles called PML nuclear bodies. The functional impact of such accumulation is still poorly understood. During my thesis, I sought to understand how and why the HIRA complex localizes in these organelles. In particular, I sought to analyse the functional impact of this localization on the deposition of H3.3 in particular regions of the chromatin.

First, I investigated the mechanisms and role of HIRA relocalization during inflammation response to type I interferon (IFN-I) treatment, a pro-inflammatory protein. My work has shown that the HIRA complex relocalizes within PML nuclear bodies in a manner dependent on interactions between SUMO modifications and a SUMO-interacting motif (SIM). In addition, I have shown that the HIRA complex and PML nuclear bodies are important in the regulation of the expression of IFN-I-stimulated genes (ISGs), by participating in the incorporation of the histone variant H3 .3 in these gene loci. Their action could allow better control of the inflammatory response in order to ensure its homeostasis. Secondly, I was interested in the relocalization of HIRA within PML nuclear bodies induced by double-strand breaks, never described before. I have shown that this relocalization is independent of IFN-I signaling and probably depends on signaling in response to DNA damage. Recruitment of HIRA is also dependent on SIM-SUMO interactions. Interestingly, the HIRA complex in PML nuclear bodies juxtaposes near the sites of damage, and this could serve to incorporate new histones after repair and thus ensure the reestablishment of the chromatin structure.