DNA-PK is activated by nucleosomes and phosphorylates H2AX within the nucleosomes in an acetylation-dependent manner

Eukaryotic DNA is organized into nucleosomes and higher order chromatin structure, which plays an important role in the regulation of many nuclear processes including DNA repair. Non-homologous end-joining, the major pathway for repairing DNA double-strand breaks (DSBs) in mammalian cells, is mediated by a set of proteins including DNA-dependent protein kinase (DNA-PK). DNA-PK is comprised of a large catalytic subunit, DNA-PKcs, and its regulatory subunit, Ku. Current models predict that Ku binds to the ends of broken DNA and DNA-PKcs is recruited to form the active kinase complex. Here we show that DNA-PK can be activated by nucleosomes through the ability of Ku to bind to the ends of nucleosomal DNA, and that the activated DNA-PK is capable of phosphorylating H2AX within the nucleosomes. Histone acetylation has little effect on the steps of Ku binding to nucleosomes and subsequent activation of DNA-PKcs. However, acetylation largely enhances the phosphorylation of H2AX by DNA-PK, and this acetylation effect is observed when H2AX exists in the context of nucleosomes but not in a free form. These results suggest that the phosphorylation of H2AX, known to be important for DSB repair, can be regulated by acetylation and may provide a mechanistic basis on which to understand the recent observations that histone acetylation critically functions in repairing DNA DSBs.     

Genome-wide screen of human bromodomain-containing proteins identifies Cecr2 as a novel DNA damage response protein

The formation of γ-H2AX foci after DNA double strand breaks (DSBs) is crucial for the cellular response to this lethal DNA damage. We previously have shown that BRG1, a chromatin remodeling enzyme, facilitates DSB repair by stimulating γ-H2AX formation, and this function of BRG1 requires the binding of BRGI to acetylated histone H3 on γ-H2AX-containing nucleosomes using its bromodomain (BRD), a protein module that specifically recognizes acetyl-Lys moieties. We also have shown that the BRD of BRG1, when ectopically expressed in cells, functions as a dominant negative inhibitor of the BRG1 activity to stimulate γ-H2AX and DSB repair. Here, we found that BRDs from a select group of proteins have no such activity, suggesting that the γ-H2AX inhibition activity of BRG1 BRD is specific. This finding led us to search for more BRDs that exhibit γ-H2AX inhibition activity in the hope of finding additional BRD-containing proteins involved in DNA damage responses. We screened a total of 52 individual BRDs present in 38 human BRD-containing proteins, comprising 93% of all human BRDs. We identified the BRD of cat eye syndrome chromosome region candidate 2 (Cecr2), which recently was shown to form a novel chromatin remodeling complex with unknown cellular functions, as having a strong γ-H2AX inhibition activity. This activity of Cecr2 BRD is specific because it depends on the chromatin binding affinity of Cecr2 BRD. Small interfering RNA knockdown experiments showed that Cecr2 is important for γ-H2AX formation and DSB repair. Therefore, our genomewide screen identifies Cecr2 as a novel DNA damage response protein.     

Next-generation bromodomain inhibitors of the SWI/SNF complex enhance DNA damage and cell death in glioblastoma

Glioblastoma (GBM) is an aggressive brain cancer with a poor prognosis. While surgical resection is the primary treatment, adjuvant temozolomide (TMZ) chemotherapy and radiotherapy only provide slight improvement in disease course and outcome. Unfortunately, most treated patients experience recurrence of highly aggressive, therapy-resistant tumours and eventually succumb to the disease. To increase chemosensitivity and overcome therapy resistance, we have modified the chemical structure of the PFI-3 bromodomain inhibitor of the BRG1 and BRM catalytic subunits of the SWI/SNF chromatin remodelling complex. Our modifications resulted in compounds that sensitized GBM to the DNA alkylating agent TMZ and the radiomimetic bleomycin. We screened these chemical analogues using a cell death ELISA with GBM cell lines and a cellular thermal shift assay using epitope tagged BRG1 or BRM bromodomains expressed in GBM cells. An active analogue, IV-129, was then identified and further modified, resulting in new generation of bromodomain inhibitors with distinct properties. IV-255 and IV-275 had higher bioactivity than IV-129, with IV-255 selectively binding to the bromodomain of BRG1 and not BRM, while IV-275 bound well to both BRG1 and BRM bromodomains. In contrast, IV-191 did not bind to either bromodomain or alter GBM chemosensitivity. Importantly, both IV-255 and IV-275 markedly increased the extent of DNA damage induced by TMZ and bleomycin as determined by nuclear γH2AX staining. Our results demonstrate that these next-generation inhibitors selectively bind to the bromodomains of catalytic subunits of the SWI/SNF complex and sensitize GBM to the anticancer effects of TMZ and bleomycin. This approach holds promise for improving the treatment of GBM.     

ATP-dependent chromatin remodeling is required for base excision repair in conventional but not in variant H2A.Bbd nucleosomes

In eukaryotes, base excision repair (BER) is responsible for the repair of oxidatively generated lesions. The mechanism of BER on naked DNA substrates has been studied in detail, but how it operates on chromatin remains unclear. Here we have studied the mechanism of BER by introducing a single 8-oxo-7,8-dihydroguanine (8-oxoG) lesion in the DNA of reconstituted positioned conventional and histone variant H2A.Bbd nucleosomes. We found that 8-oxoguanine DNA glycosylase, apurinic/apyrimidinic endonuclease, and polymerase beta activities were strongly reduced in both types of nucleosomes. In conventional nucleosomes SWI/SNF stimulated the processing of 8-oxoG by each one of the three BER repair factors to efficiencies similar to those for naked DNA. Interestingly, SWI/SNF-induced remodeling, but not mobilization of conventional nucleosomes, was required to achieve this effect. A very weak effect of SWI/SNF on the 8-oxoG BER removal in H2A.Bbd histone variant nucleosomes was observed. The possible implications of our data for the understanding of in vivo mechanisms of BER are discussed.     

The p400 ATPase regulates nucleosome stability and chromatin ubiquitination during DNA repair

The complexity of chromatin architecture presents a significant barrier to the ability of the DNA repair machinery to access and repair DNA double-strand breaks (DSBs). Consequently, remodeling of the chromatin landscape adjacent to DSBs is vital for efficient DNA repair. Here, we demonstrate that DNA damage destabilizes nucleosomes within chromatin regions that correspond to the γ-H2AX domains surrounding DSBs. This nucleosome destabilization is an active process requiring the ATPase activity of the p400 SWI/SNF ATPase and histone acetylation by the Tip60 acetyltransferase. p400 is recruited to DSBs by a mechanism that is independent of ATM but requires mdc1. Further, the destabilization of nucleosomes by p400 is required for the RNF8-dependent ubiquitination of chromatin, and for the subsequent recruitment of brca1 and 53BP1 to DSBs. These results identify p400 as a novel DNA damage response protein and demonstrate that p400-mediated alterations in nucleosome and chromatin structure promote both chromatin ubiquitination and the accumulation of brca1 and 53BP1 at sites of DNA damage.     

Systematic Identification of Functional Residues in Mammalian Histone H2AX

The histone variant H2AX is a principal component of chromatin involved in the detection, signaling, and repair of DNA double-strand breaks (DSBs). H2AX is thought to operate primarily through its C-terminal S139 phosphorylation, which mediates the recruitment of DNA damage response (DDR) factors to chromatin at DSB sites. Here, we describe a comprehensive screen of 67 residues in H2AX to determine their contributions to H2AX functions. Our analysis revealed that H2AX is both sumoylated and ubiquitylated. Individual residues defective for sumoylation, ubiquitylation, and S139 phosphorylation in untreated and damaged cells were identified. Specifically, we identified an acidic triad region in both H2A and H2AX that is required in cis for their ubiquitylation. We also report the characterization of a human H2AX knockout cell line, which exhibits DDR defects, including p53 activation, following DNA damage. Collectively, this work constitutes the first genetic complementation system for a histone in human cells. Finally, our data reveal new roles for several residues in H2AX and define distinct functions for H2AX in human cells.     

Mutations to the histone H3 αN region selectively alter the outcome of ATP-dependent nucleosome-remodelling reactions

Mutational analysis of the histone H3 N-terminal region has shown it to play an important role both in chromatin function in vivo and nucleosome dynamics in vitro. Here we use a library of mutations in the H3 N-terminal region to investigate the contribution of this region to the action of the ATP-dependent remodelling enzymes Chd1, RSC and SWI/SNF. All of the enzymes were affected differently by the mutations with Chd1 being affected the least and RSC being most sensitive. In addition to affecting the rate of remodelling by RSC, some mutations prevented RSC from moving nucleosomes to locations in which DNA was unravelled. These observations illustrate that the mechanisms by which different ATP-dependent remodelling enzymes act are sensitive to different features of nucleosome structure. They also show how alterations to histones can affect the products generated as a result of ATP-dependent remodelling reactions.     

γH2A binds Brc1 to maintain genome integrity during S‐phase

ATM^Tel1 and ATR^Rad3 checkpoint kinases phosphorylate the C‐terminus of histone H2AX (H2A in yeasts) in chromatin flanking DNA damage, establishing a recruitment platform for checkpoint and repair proteins. Phospho‐H2A/X (γH2A/X)‐binding proteins at double‐strand breaks (DSBs) have been characterized, but those required for replication stress responses are unknown. Here, we present genetic, biochemical, small angle X‐ray scattering (SAXS), and X‐ray structural studies of the Schizosaccharomyces pombe Brc1, a 6‐BRCT‐domain protein that is structurally related to Saccharomyces cerevisiae Rtt107 and mammalian PTIP. Brc1 binds γH2A to form spontaneous and DNA damage‐induced nuclear foci. Spontaneous Brc1 foci colocalize with ribosomal DNA repeats, a region prone to fork pausing and genomic instability, whereas DNA damage‐induced Brc1 foci colocalize with DSB response factors. γH2A binding is critical for Brc1 function. The 1.45 Å resolution crystal structure of Brc1–γH2A complex shows how variable BRCT insertion loops sculpt tandem‐BRCT phosphoprotein‐binding pockets to facilitate unique phosphoprotein‐interaction specificities, and unveils an acidic DNA‐mimicking Brc1 surface. From these results, Brc1 docking to γH2A emerges as a critical chromatin‐specific response to replication‐associated DNA damage.