Report
Teaser, summary, work performed and final results
Periodic Reporting for period 1 - Epiherigans (Writing, reading and managing stress with H3K9me)
Teaser
Organismal development, somatic tissue differentiation, and germline immo rtality require both genome stability and the proper spatiotemporal regulation of gene expression. In addition, one of the dominant features of higher eukaryotic genomes is the prevalence of repetitive...
Summary
Organismal development, somatic tissue differentiation, and germline immo rtality require both genome stability and the proper spatiotemporal regulation of gene expression. In addition, one of the dominant features of higher eukaryotic genomes is the prevalence of repetitive sequences, which must be transcriptionally silenced to preserve genome stability (Zeller et al., 2016). Transcription is therefore tightly controlled by restricting the accessibility of transcription factors and RNA-polymerases to the DNA. One of the main mechanisms to restrict aberrant transcription of both genes and repetitive sequences is the compaction of parts of the genome into so called heterochromatin. DNA wrapped around histones marked by the methylation of histone H3 on lysine 9 (H3K9me), functions as an essential platform for the formation of heterochromatin (Padeken et al., 2015). The correct segregation of inactive heterochromatin and active euchromatin is essential for healthy development and both the perturbation of H3K9me resulting in the aberrant expression of repetitive elements (Ting et al., 2011), as well as the mistargeting of H3K9me resulting in the silencing of tumor suppressor genes (Chen et al., 2010; Hua et al., 2014) has been described for a variety of cancers. Its role in restricting the expression of genes to individual tissues, and/or developmental stages and its role in constitutively repressing the transcription of repetitive elements requires that H3K9me can be selectively established, maintained and erased, both in time and in space. The central goal of this project is to understand the mechanisms that target the enzymes that catalyze H3K9me during development and the impact of external stresses to these processes.
Work performed
Previous work in the lab has highlighted the advantage of studying heterochromatin function in C. elegans. An in depth genetic and phenotypic characterization of the two C. elegans H3K9 methyltransferases (MET-2 and SET-25) has highlighted the essential function of MET-2 in ensuring genomic stability and the absolute dependence of H3K9me-mutants on factors ensuring replication fork stability (Padeken et al., 2019). Further molecular and biochemical studies of the MET-2 enzyme identified a novel interaction partner that ensures its proper nuclear localization and functional activity (Delaney et al., 2019). This work also highlighted a critical role for MET-2 during temperature stress, an observation we are following up on. An ongoing microscopy-based screen is also looking to identify additional factors that mediate MET-2 targeting to specific regions in chromatin. A similar screen was established to investigate SET-25 targeting and allowed us to connect the small RNA pathway to H3K9methyl-mediated silencing of specific regions of the genome. Finally, we have been expanding our study trying to understand the functional role of chromatin compartmentalization in somatic tissues. In this regard, we believe we have unraveled the mechanism for our observation that perturbing euchromatic factors can affect the function of heterochromatin: our findings suggest that maintaining a euchromatic localization of histone acetyltransferases (HATs), particularly of the limiting CBP/p300, is key to protect heterochromatin from aberrant activation and detachment from the nuclear periphery in differentiated intestinal cells (Cabianca et al., in revision).
Final results
Our society is growing older and in the upcoming years this will impact health systems worldwide. Heterochromatin loss and gene deregulation is a feature of aging and can interfere with organismal functionality. To be able to intervene, we first must identify the factors involved in heterochromatin metabolism. Our work takes advantage of the nematode C. elegans to address fundamental questions of heterochromatin regulation. The use of this relatively simple model organism is allowing us to shed light on new pathways that are key for heterochromatin homeostasis in normal and in disease states.
Understanding how euchromatin and heterochromatin identities are maintained and segregated not only contributes to our basic understanding of differentiation but is also relevant to the field of cell fate reprogramming. In this perspective, this work could have an impact also on regenerative medicine and disease modelling approaches.