A03: The role of chromatin structure regulation in the susceptibility to transcription-blocking lesions

A major challenge for the fidelity of biological processes is damage to the nuclear DNA. Despite its role as carrier of all genetic information, DNA is surprisingly unstable and subjected to an estimated tens of thousands to genotoxic events per day in a human cell. When incorrectly repaired, DNA damage can lead to mutations that can alter gene function and cause cancer development (Schumacher et al. 2021). The lesions might also persist, resulting in cellular dysfunction and degeneration thus contributing to the aging process (da Silva and Schumacher, 2019). UV-induced DNA lesions, cyclobutane pyrimidine dimers (CPDs) and 6-4 pyrimidine-pyrimidone photoproducts (6-4PPs) are paradigmatic helix-distorting lesion types that lead to stalling of RNA polymerases until transcription-coupled nucleotide excision repair (TC-NER) removes the lesions. Defects in the TC-NER genes CSA and CSB cause growth retardation and premature aging in Cockayne syndrome (CS) patients (Rieckher, Garinis & Schumacher, 2021). Due to the complexity of the CS pathologies in humans and mouse models, we have in recent years developed C. elegans as model for studying TC-NER outcomes in a metazoan organism (Mueller…Müller…Schumacher, 2014). In addition to the actual DNA repair machineries epigenetic modifications, transcription elongation, and the regulation of gene expression are important determinants of the consequences of transcription-blocking DNA damage. Aiming to gain a more complete understanding of the mechanisms regulating genome stability in the context of gene expression, we have conducted a forward genetic screen for CS-like UV sensitivity in the nematode system. We identified a point mutation in ssl-1 the ortholog of the human Snf2 related CREBBP activator protein (SRCAP) that leads to a similar UV sensitivity phenotype as a mutation in csb-1 or csa-1. SSL-1 is required for loading the histone variant HTZ-1, which is the worm ortholog of H2A.Z (Whittle et al. 2008). H2A.Z function in the regulation of gene expression most prominently at the transcription start site and, less well understood, along gene bodies (Mylonas et al. 2021). Our preliminary data shows accelerated RNAPII elongation and an underexpression of DNA repair genes in ssl-1 mutants, which might hypersensitize the ssl-1 mutant animals to DNA damage-induced transcription impediments. We propose here to investigate how SSL-1 and its regulation of HTZ-1 incorporation regulates the fidelity of gene expression. We aim to gain a more complete understanding of how epigenetic regulators of chromatin composition impact the robustness of transcription through regulating the organism’s DNA repair capacities.