During immune responses, B cells engaging a cognate antigen are recruited to GCs in secondary lymphoid organs where they will diversify their BCR to generate highly specific and adapted humoral ...responses. They do so, by inducing the expression of activation‐induced cytidine deaminase (AID), which initiates somatic hypermutation (SHM) and class switch recombination (CSR). AID deaminates cytosines in ss DNA, generating U:G mismatches that are processed to induce ds DNA break intermediates during CSR that result in the expression of a different antibody isotype. Interestingly, hypoxia regions have been reported in GCs and suggesting that hypoxia could modulate the humoral response. Furthermore, hypoxia inducible transcription factor (HIF) can bind to the AID promoter and induce AID expression in a non‐B‐cell setting, suggesting that it might be involved in the transcriptional induction of AID in B cells, hence, regulating SHM and CSR. We, thus, hypothesized that HIF could regulate the efficiency of CSR. Here, we show that the inactivation of both the HIF‐1α and HIF‐1β subunits of the HIF transcription factor in murine CH12 B cells results in defective CSR and that this is due to the suboptimal induction of AID expression.
During immune responses, B cells respond to BCR, cytokine, TLR and/or hypoxia signaling to activate the HIF transcription factor and induce optimal levels of Aicda mRNA expression and CSR. In the absence of HIF‐1α or HIF‐1β, Aicda mRNA expression is delayed and results in defective CSR.
Centromeric integrity is key for proper chromosome segregation during cell division
. Centromeres have unique chromatin features that are essential for centromere maintenance
. Although they are ...intrinsically fragile and represent hotspots for chromosomal rearrangements
, little is known about how centromere integrity in response to DNA damage is preserved. DNA repair by homologous recombination requires the presence of the sister chromatid and is suppressed in the G1 phase of the cell cycle
. Here we demonstrate that DNA breaks that occur at centromeres in G1 recruit the homologous recombination machinery, despite the absence of a sister chromatid. Mechanistically, we show that the centromere-specific histone H3 variant CENP-A and its chaperone HJURP, together with dimethylation of lysine 4 in histone 3 (H3K4me2), enable a succession of events leading to the licensing of homologous recombination in G1. H3K4me2 promotes DNA-end resection by allowing DNA damage-induced centromeric transcription and increased formation of DNA-RNA hybrids. CENP-A and HJURP interact with the deubiquitinase USP11, enabling formation of the RAD51-BRCA1-BRCA2 complex
and rendering the centromeres accessible to RAD51 recruitment and homologous recombination in G1. Finally, we show that inhibition of homologous recombination in G1 leads to centromeric instability and chromosomal translocations. Our results support a model in which licensing of homologous recombination at centromeric breaks occurs throughout the cell cycle to prevent the activation of mutagenic DNA repair pathways and preserve centromeric integrity.
Activation-induced cytidine deaminase (AID) initiates antibody gene diversification by creating U:G mismatches. However, AID is not specific for antibody genes; Off-target lesions can activate ...oncogenes or cause chromosome translocations. Despite its importance in these transactions little is known about how AID finds its targets. We performed an shRNA screen to identify factors required for class switch recombination (CSR) of antibody loci. We found that Spt5, a factor associated with stalled RNA polymerase II (Pol II) and single stranded DNA (ssDNA), is required for CSR. Spt5 interacts with AID, it facilitates association between AID and Pol II, and AID recruitment to its Ig and non-Ig targets. ChIP-seq experiments reveal that Spt5 colocalizes with AID and stalled Pol II. Further, Spt5 accumulation at sites of Pol II stalling is predictive of AID-induced mutation. We propose that AID is targeted to sites of Pol II stalling in part via its association with Spt5.
Display omitted
► The Pol II stalling factor, Spt5, is required for CSR ► Spt5 interaction with AID is important for AID-Pol II association ► Spt5 colocalizes with AID at sites of Pol II stalling genome-wide in B cells ► Sites of high-density Spt5 and Pol II stalling are predictive of AID-mediated mutation
p53-binding protein 1 (53BP1) regulates both the DNA damage response and p53 signaling. Although 53BP1’s function is well established in DNA double-strand break repair, how its role in p53 signaling ...is modulated remains poorly understood. Here, we identify the scaffolding protein AHNAK as a G1 phase-enriched interactor of 53BP1. We demonstrate that AHNAK binds to the 53BP1 oligomerization domain and controls its multimerization potential. Loss of AHNAK results in hyper-accumulation of 53BP1 on chromatin and enhanced phase separation, culminating in an elevated p53 response, compromising cell survival in cancer cells but leading to senescence in non-transformed cells. Cancer transcriptome analyses indicate that AHNAK-53BP1 cooperation contributes to the suppression of p53 target gene networks in tumors and that loss of AHNAK sensitizes cells to combinatorial cancer treatments. These findings highlight AHNAK as a rheostat of 53BP1 function, which surveys cell proliferation by preventing an excessive p53 response.
Display omitted
•AHNAK is a G1-enriched interactor of 53BP1•AHNAK controls 53BP1-mediated G1-S phase transition upon DNA damage•AHNAK restrains 53BP1 oligomerization and phase separation•AHNAK balances between apoptosis and senescence in cancer and non-transformed cells
Ghodke et al. identify the large scaffolding protein AHNAK as a G1-enriched interactor of 53BP1 that ensures optimal partitioning of 53BP1 into phase-separated condensates and limits excessive interaction with p53, which would otherwise lead to apoptosis in cancer cells and senescence in non-transformed cells.
Fluorescent labeling of endogenous proteins for live-cell imaging without exogenous expression of tagged proteins or genetic manipulations has not been routinely possible. We describe a simple ...versatile antibody-based imaging approach (VANIMA) for the precise localization and tracking of endogenous nuclear factors. Our protocol can be implemented in every laboratory allowing the efficient and nonharmful delivery of organic dye-conjugated antibodies, or antibody fragments, into different metazoan cell types. Live-cell imaging permits following the labeled probes bound to their endogenous targets. By using conventional and super-resolution imaging we show dynamic changes in the distribution of several nuclear transcription factors (i.e., RNA polymerase II or TAF10), and specific phosphorylated histones (γH2AX), upon distinct biological stimuli at the nanometer scale. Hence, considering the large panel of available antibodies and the simplicity of their implementation, VANIMA can be used to uncover novel biological information based on the dynamic behavior of transcription factors or posttranslational modifications in the nucleus of single live cells.
PARP3 has been shown to be a key driver of TGFβ-induced epithelial-to-mesenchymal transition (EMT) and stemness in breast cancer cells, emerging as an attractive therapeutic target. Nevertheless, the ...therapeutic value of PARP3 inhibition has not yet been assessed. Here we investigated the impact of the absence of PARP3 or its inhibition on the tumorigenicity of BRCA1-proficient versus BRCA1-deficient breast cancer cell lines, focusing on the triple-negative breast cancer subtype (TNBC). We show that PARP3 knockdown exacerbates centrosome amplification and genome instability and reduces survival of BRCA1-deficient TNBC cells. Furthermore, we engineered PARP3
BRCA1-deficient or BRCA1-proficient TNBC cell lines using the CRISPR/nCas9
gene editing technology and demonstrate that the absence of PARP3 selectively suppresses the growth, survival and in vivo tumorigenicity of BRCA1-deficient TNBC cells, mechanistically via effects associated with an altered Rictor/mTORC2 signaling complex resulting from enhanced ubiquitination of Rictor. Accordingly, PARP3 interacts with and ADP-ribosylates GSK3β, a positive regulator of Rictor ubiquitination and degradation. Importantly, these phenotypes were rescued by re-expression of a wild-type PARP3 but not by a catalytic mutant, demonstrating the importance of PARP3's catalytic activity. Accordingly, reduced survival and compromised Rictor/mTORC2 signaling were also observed using a cell-permeable PARP3-specific inhibitor. We conclude that PARP3 and BRCA1 are synthetic lethal and that targeting PARP3's catalytic activity is a promising therapeutic strategy for BRCA1-associated cancers via the Rictor/mTORC2 signaling pathway.
Mouse pericentromeric DNA is composed of tandem major satellite repeats, which are heterochromatinized and cluster together to form chromocenters. These clusters are refractory to DNA repair through ...homologous recombination (HR). The mechanisms by which pericentromeric heterochromatin imposes a barrier on HR and the implications of repeat clustering are unknown. Here, we compare the spatial recruitment of HR factors upon double-stranded DNA breaks (DSBs) induced in human and mouse pericentromeric heterochromatin, which differ in their capacity to form clusters. We show that while DSBs increase the accessibility of human pericentromeric heterochromatin by disrupting HP1α dimerization, mouse pericentromeric heterochromatin repeat clustering imposes a physical barrier that requires many layers of de-compaction to be accessed. Our results support a model in which the 3D organization of heterochromatin dictates the spatial activation of DNA repair pathways and is key to preventing the activation of HR within clustered repeats and the onset of chromosomal translocations.
Display omitted
•DSBs at human pericentric HC are positionally stable and recruit HR factors•Repeat clustering restricts Rad51 at the periphery of heterochromatin•Human pericentromeric HC is more accessible to HR factors than mouse HC•Inhibition of DSB relocation at clustered repeats leads to translocations
Mitrentsi et al. demonstrate that heterochromatic repeat clustering regulates the spatial activation of DNA repair pathways, and it is not conserved between mouse and human. Clustered repeats are refractory to HR factors, imposing a barrier to illegitimate recombination between repeats from different chromosomes that lead to the formation of translocations.
MicroRNAs (miRNAs) are small noncoding RNAs that regulate vast networks of genes that share miRNA target sequences. To examine the physiologic effects of an individual miRNA-mRNA interaction in vivo, ...we generated mice that carry a mutation in the putative microRNA-155 (miR-155) binding site in the 3′-untranslated region of activation-induced cytidine deaminase (AID), designated
Aicda
155
mice. AID is required for immunoglobulin gene diversification in B lymphocytes, but it also promotes chromosomal translocations.
Aicda
155
caused an increase in steady-state
Aicda mRNA and protein amounts by increasing the half-life of the mRNA, resulting in a high degree of
Myc-Igh translocations. A similar but more pronounced translocation phenotype was also found in miR-155-deficient mice. Our experiments indicate that miR-155 can act as a tumor suppressor by reducing potentially oncogenic translocations generated by AID.
DNA breaks are complex DNA lesions that can be repaired by two alternative mechanisms: non-homologous end-joining and homologous recombination. The decision between them depends on the activation of ...the DNA resection machinery, which blocks non-homologous end-joining and stimulates recombination. On the other hand, post-translational modifications play a critical role in DNA repair. We have found that the SUMO E3 ligase CBX4 controls resection through the key factor CtIP. Indeed, CBX4 depletion impairs CtIP constitutive sumoylation and DNA end processing. Importantly, mutating lysine 896 in CtIP recapitulates the CBX4-depletion phenotype, blocks homologous recombination and increases genomic instability. Artificial fusion of CtIP and SUMO suppresses the effects of both the non-sumoylatable CtIP mutant and CBX4 depletion. Mechanistically, CtIP sumoylation is essential for its recruitment to damaged DNA. In summary, sumoylation of CtIP at lysine 896 defines a subpopulation of the protein that is involved in DNA resection and recombination.The choice between non-homologous end-joining and homologous recombination to repair a DNA double-strand break depends on activation of the end resection machinery. Here the authors show that SUMO E3 ligase CBX4 sumoylates subpopulation of CtIP to regulate recruitment to breaks and resection.
Maintenance of genome integrity requires the functional interplay between Fanconi anemia (FA) and homologous recombination (HR) repair pathways. Endogenous acetaldehyde, a product of cellular ...metabolism, is a potent source of DNA damage, particularly toxic to cells and mice lacking the FA protein FANCD2. Here, we investigate whether HR‐compromised cells are sensitive to acetaldehyde, similarly to FANCD2‐deficient cells. We demonstrate that inactivation of HR factors BRCA1, BRCA2, or RAD51 hypersensitizes cells to acetaldehyde treatment, in spite of the FA pathway being functional. Aldehyde dehydrogenases (ALDHs) play key roles in endogenous acetaldehyde detoxification, and their chemical inhibition leads to cellular acetaldehyde accumulation. We find that disulfiram (Antabuse), an ALDH2 inhibitor in widespread clinical use for the treatment of alcoholism, selectively eliminates BRCA1/2‐deficient cells. Consistently, Aldh2 gene inactivation suppresses proliferation of HR‐deficient mouse embryonic fibroblasts (MEFs) and human fibroblasts. Hypersensitivity of cells lacking BRCA2 to acetaldehyde stems from accumulation of toxic replication‐associated DNA damage, leading to checkpoint activation, G2/M arrest, and cell death. Acetaldehyde‐arrested replication forks require BRCA2 and FANCD2 for protection against MRE11‐dependent degradation. Importantly, acetaldehyde specifically inhibits in vivo the growth of BRCA1/2‐deficient tumors and ex vivo in patient‐derived tumor xenograft cells (PDTCs), including those that are resistant to poly (ADP‐ribose) polymerase (PARP) inhibitors. The work presented here therefore identifies acetaldehyde metabolism as a potential therapeutic target for the selective elimination of BRCA1/2‐deficient cells and tumors.
Synopsis
Treatment with acetaldehyde or with the alcohol‐deterrent disulfiram, which enhances acetaldehyde levels, selectively eliminates BRCA1/2‐deficient cells and tumors. Increasing cellular acetaldehyde might thus benefit cancer patients with BRCA1/2 mutations.
Acetaldehyde and disulfiram increased the levels of RPA foci and decreased replication fork progression, leading to accumulation of replication‐associated DNA damage specifically in BRCA2‐deficient cells.
The Aldh2 gene encodes an aldehyde dehydrogenase with key roles in endogenous acetaldehyde detoxification.
Aldh2 gene deletion or its point mutation E487K associated with the ethanol‐induced flushing syndrome in humans causes proliferation arrest in cells lacking BRCA1/2 expression.
Growth of BRCA1/2‐defective tumors, including those that have acquired resistance to PARP inhibitors, is suppressed by acetaldehyde treatment.
Treatment with acetaldehyde or with the alcohol‐deterrent disulfiram, which enhances acetaldehyde levels, selectively eliminates BRCA1/2‐deficient cells and tumors. Increasing cellular acetaldehyde might thus benefit cancer patients with BRCA1/2 mutations.
PDF
