High replication stress and limited Rad51-mediated DNA repair capacity, but not oxidative stress, underlie oligodendrocyte precursor cell radiosensitivity.

Cranial irradiation is part of the standard of care for treating pediatric brain tumors. However, ionizing radiation can trigger serious long-term neurologic sequelae, including oligodendrocyte and brain white matter loss enabling neurocognitive decline in children surviving brain cancer. Oxidative stress-mediated oligodendrocyte precursor cell (OPC) radiosensitivity has been proposed as a possible explanation for this. Here, however, we demonstrate that antioxidants fail to improve OPC viability after irradiation, despite suppressing oxidative stress, suggesting an alternative etiology for OPC radiosensitivity. Using systematic approaches, we find that OPCs have higher irradiation-induced and endogenous γH2AX foci compared to neural stem cells, neurons, astrocytes and mature oligodendrocytes, and these correlate with replication-associated DNA double strand breakage. Furthermore, OPCs are reliant upon ATR kinase and Mre11 nuclease-dependent processes for viability, are more sensitive to drugs increasing replication fork collapse, and display synthetic lethality with PARP inhibitors after irradiation. This suggests an insufficiency for homology-mediated DNA repair in OPCs-a model that is supported by evidence of normal RPA but reduced RAD51 filament formation at resected lesions in irradiated OPCs. We therefore propose a DNA repair-centric mechanism of OPC radiosensitivity, involving chronically-elevated replication stress combined with 'bottlenecks' in RAD51-dependent DNA repair that together reduce radiation resilience.

The SWI/SNF chromatin remodelling complex: Its role in maintaining genome stability and preventing tumourigenesis.

Genes encoding subunits of the two SWI/SNF chromatin remodelling complexes (BAF and PBAF) are mutated in almost 20% of all human cancers. In addition to a role in regulating transcription, recent work from our laboratory and others identified roles for both complexes in DNA damage responses and the maintenance of sister chromatid cohesion, which may have profound impacts on genome stability and contribute to its role as a tumour suppressor. Here, we review some of the transcription-independent functions of the SWI/SNF chromatin remodelling complex and discuss these in light of their potential relevance to tumourigenesis.

BAF180 promotes cohesion and prevents genome instability and aneuploidy.

BAF180, a subunit of the PBAF chromatin remodeling complex, is frequently mutated in cancer. Although PBAF regulates transcription, it remains unclear whether this is what drives tumorigenesis in cells lacking BAF180. Based on data from yeast, we hypothesized that BAF180 may prevent tumorigenesis by promoting cohesion. Here, we show BAF180 is required for centromeric cohesion in mouse and human cells. Mutations identified in tumor samples are unable to support this activity, and also compromise cohesion-dependent functions in yeast. We provide evidence of genome instability in line with loss of cohesion, and importantly, we find dynamic chromosome instability following DNA damage in cells lacking BAF180. These data demonstrate a function for BAF180 in promoting genome stability that is distinct from its well-characterized role in transcriptional regulation, uncovering a potent mechanism for its tumor-suppressor activity.

Cancer and the bromodomains of BAF180.

Chromatin remodelling complexes alter the structure of chromatin and have central roles in all DNA-templated activities, including regulation of gene expression and DNA repair. Mutations in subunits of the PBAF (polybromo/Brg1-associated factor) or SWI/SNF-B remodelling complex, including BAF180, are frequently associated with cancer. There are six potential acetyl-lysine-binding BDs (bromodomains) in BAF180, which may function to target the PBAF complex to promoters or sites of DNA repair. In the present review, we discuss what is currently known about the BDs of BAF180 and their potential significance in cancer.

The two different isoforms of the RSC chromatin remodeling complex play distinct roles in DNA damage responses.

The RSC chromatin remodeling complex has been implicated in contributing to DNA double-strand break (DSB) repair in a number of studies. Both survival and levels of H2A phosphorylation in response to damage are reduced in the absence of RSC. Importantly, there is evidence for two isoforms of this complex, defined by the presence of either Rsc1 or Rsc2. Here, we investigated whether the two isoforms of RSC provide distinct contributions to DNA damage responses. First, we established that the two isoforms of RSC differ in the presence of Rsc1 or Rsc2 but otherwise have the same subunit composition. We found that both rsc1 and rsc2 mutant strains have intact DNA damage-induced checkpoint activity and transcriptional induction. In addition, both strains show reduced non-homologous end joining activity and have a similar spectrum of DSB repair junctions, suggesting perhaps that the two complexes provide the same functions. However, the hypersensitivity of a rsc1 strain cannot be complemented with an extra copy of RSC2, and likewise, the hypersensitivity of the rsc2 strain remains unchanged when an additional copy of RSC1 is present, indicating that the two proteins are unable to functionally compensate for one another in DNA damage responses. Rsc1, but not Rsc2, is required for nucleosome sliding flanking a DNA DSB. Interestingly, while swapping the domains from Rsc1 into the Rsc2 protein does not compromise hypersensitivity to DNA damage suggesting they are functionally interchangeable, the BAH domain from Rsc1 confers upon Rsc2 the ability to remodel chromatin at a DNA break. These data demonstrate that, despite the similarity between Rsc1 and Rsc2, the two different isoforms of RSC provide distinct functions in DNA damage responses, and that at least part of the functional specificity is dictated by the BAH domains.