DNA is packaged into chromatin, a highly compacted DNA-protein complex; therefore, all cellular processes that use the DNA as a template, including DNA repair, require a high degree of coordination between the DNA-repair machinery and chromatin modification/remodelling, which regulates the accessibility of DNA in chromatin. Recent studies have implicated histone acetyltransferase (HAT) complexes and chromatin acetylation in DNA repair; however, the precise underlying mechanism remains poorly understood. Here, we show that the HAT cofactor Trrap and Tip60 HAT bind to the chromatin surrounding sites of DNA double-strand breaks (DSBs) in vivo. Trrap depletion impairs both DNA-damage-induced histone H4 hyperacetylation and accumulation of repair molecules at sites of DSBs, resulting in defective homologous recombination (HR) repair, albeit with the presence of a functional ATM-dependent DNA-damage signalling cascade. Importantly, the impaired loading of repair proteins and the defect in DNA repair in Trrap-deficient cells can be counteracted by chromatin relaxation, indicating that the DNA-repair defect that was observed in the absence of Trrap is due to impeded chromatin accessibility at sites of DNA breaks. Thus, these data reveal that cells may use the same basic mechanism involving HAT complexes to regulate distinct cellular processes, such as transcription and DNA repair.

SummaryActivated Ras GTPase signalling is a critical driver of oncogenic transformation and malignant disease. Cellular models of RAS-dependent cancers have been used to identify experimental small-molecules, such as SCH51344, but their molecular mechanism of action remains generally enigmatic. Here, using a chemical proteomic approach we identify the target of SCH51344 as the human mutT homologue MTH1, a nucleotide pool sanitising enzyme. Loss-of-function of MTH1 impaired growth of KRAS tumour cells whereas MTH1 overexpression mitigated sensitivity toward SCH51344. Searching for more drug-like inhibitors, we identified the kinase inhibitor crizotinib as a nanomolar suppressor of MTH1 activity. Surprisingly, the clinically used (R)-enantiomer of the drug was inactive, whereas the (S)-enantiomer selectively inhibited MTH1 catalytic activity. Enzymatic assays, chemical proteomic profiling, kinome-wide activity surveys, and MTH1 co-crystal structures of both enantiomers provided a rationale for this remarkable stereospecificity. Disruption of nucleotide pool homeostasis via MTH1 inhibition by (S)-crizotinib induced an increase in DNA single strand breaks, activated DNA repair in human colon carcinoma cells, and effectively suppressed tumour growth in animal models. Our results propose (S)-crizotinib as an attractive chemical entity for further pre-clinical evaluation and small molecule inhibitors of MTH1 in general as a promising novel class of anti-cancer agents.

CK2 was the first protein kinase identified and is required for the proliferation and survival of mammalian cells. Here, we have identified an unanticipated role for CK2. We show that this essential protein kinase phosphorylates the scaffold protein XRCC1 and thereby enables the assembly and activity of DNA single-strand break repair protein complexes in vitro and at sites of chromosomal breakage. Moreover, we show that inhibiting XRCC1 phosphorylation by mutation of the CK2 phosphorylation sites or preventing CK2 activity using a highly specific inhibitor ablates the rapid repair of cellular DNA single-strand breaks by XRCC1. These data identify a direct role for CK2 in the repair of chromosomal DNA strand breaks and in maintaining genetic integrity.

The diversity of somatic mutations in human cancers can be decomposed into individual mutational signatures, patterns of mutagenesis that arise because of DNA damage and DNA repair processes that have occurred in cells as they evolved towards malignancy. Correlations between mutational signatures and environmental exposures, enzymatic activities and genetic defects have been described, but human cancers are not ideal experimental systems—the exposures to different mutational processes in a patient’s lifetime are uncontrolled and any relationships observed can only be described as an association. Here, we demonstrate the proof-of-principle that it is possible to recreate cancer mutational signatures in vitro using CRISPR-Cas9-based gene-editing experiments in an isogenic human-cell system. We provide experimental and algorithmic methods to discover mutational signatures generated under highly experimentally-controlled conditions. Our in vitro findings strikingly recapitulate in vivo observations of cancer data, fundamentally validating the concept of (particularly) endogenously-arising mutational signatures.