Chromatin compartmentalization regulates the response to DNA damage.

The DNA damage response is essential to safeguard genome integrity. Although the contribution of chromatin in DNA repair has been investigated1,2, the contribution of chromosome folding to these processes remains unclear3. Here we report that, after the production of double-stranded breaks (DSBs) in mammalian cells, ATM drives the formation of a new chromatin compartment (D compartment) through the clustering of damaged topologically associating domains, decorated with γH2AX and 53BP1. This compartment forms by a mechanism that is consistent with polymer-polymer phase separation rather than liquid-liquid phase separation. The D compartment arises mostly in G1 phase, is independent of cohesin and is enhanced after pharmacological inhibition of DNA-dependent protein kinase (DNA-PK) or R-loop accumulation. Importantly, R-loop-enriched DNA-damage-responsive genes physically localize to the D compartment, and this contributes to their optimal activation, providing a function for DSB clustering in the DNA damage response. However, DSB-induced chromosome reorganization comes at the expense of an increased rate of translocations, also observed in cancer genomes. Overall, we characterize how DSB-induced compartmentalization orchestrates the DNA damage response and highlight the critical impact of chromosome architecture in genomic instability.

Switch Tandem Repeats Influence the Choice of the Alternative End-Joining Pathway in Immunoglobulin Class Switch Recombination.

Immunoglobulin class switch recombination (CSR) plays an important role in humoral imm\une responses by changing the effector functions of antibodies. CSR occurs between highly repetitive switch (S) sequences located upstream of immunoglobulin constant gene exons. Switch sequences differ in size, the nature of their repeats, and the density of the motifs targeted by the activation-induced cytidine deaminase (AID), the enzyme that initiates CSR. CSR involves double-strand breaks (DSBs) at the universal Sµ donor region and one of the acceptor S regions. The DSBs ends are fused by the classical non-homologous end-joining (C-NHEJ) and the alternative-NHEJ (A-NHEJ) pathways. Of the two pathways, the A-NHEJ displays a bias towards longer junctional micro-homologies (MHs). The Sµ region displays features that distinguish it from other S regions, but the molecular basis of Sµ specificity is ill-understood. We used a mouse line in which the downstream Sγ3 region was put under the control of the Eµ enhancer, which regulates Sµ, and analyzed its recombination activity by CSR-HTGTS. Here, we show that provision of Eµ enhancer to Sγ3 is sufficient to confer the recombinational features of Sµ to Sγ3, including efficient AID recruitment, enhanced internal deletions and robust donor function in CSR. Moreover, junctions involving Sγ3 display a bias for longer MH irrespective of sequence homology with switch acceptor sites. The data suggest that the propensity for increased MH usage is an intrinsic property of Sγ3 sequence, and that the tandem repeats of the donor site influence the choice of the A-NHEJ.

Non-canonical DNA/RNA structures during Transcription-Coupled Double-Strand Break Repair: Roadblocks or Bona fide repair intermediates?

Although long overlooked, it is now well understood that DNA does not systematically assemble into a canonical double helix, known as B-DNA, throughout the entire genome but can also accommodate other structures including DNA hairpins, G-quadruplexes and RNA:DNA hybrids. Notably, these non-canonical DNA structures form preferentially at transcriptionally active loci. Acting as replication roadblocks and being targeted by multiple machineries, these structures weaken the genome and render it prone to damage, including DNA double-strand breaks (DSB). In addition, secondary structures also further accumulate upon DSB formation. Here we discuss the potential functions of pre-existing or de novo formed nucleic acid structures, as bona fide repair intermediates or repair roadblocks, especially during Transcription-Coupled DNA Double-Strand Break repair (TC-DSBR), and provide an update on the specialized protein complexes displaying the ability to remove these structures to safeguard genome integrity.

Senataxin resolves RNA:DNA hybrids forming at DNA double-strand breaks to prevent translocations.

Ataxia with oculomotor apraxia 2 (AOA-2) and amyotrophic lateral sclerosis (ALS4) are neurological disorders caused by mutations in the gene encoding for senataxin (SETX), a putative RNA:DNA helicase involved in transcription and in the maintenance of genome integrity. Here, using ChIP followed by high throughput sequencing (ChIP-seq), we report that senataxin is recruited at DNA double-strand breaks (DSBs) when they occur in transcriptionally active loci. Genome-wide mapping unveiled that RNA:DNA hybrids accumulate on DSB-flanking chromatin but display a narrow, DSB-induced, depletion near DNA ends coinciding with senataxin binding. Although neither required for resection nor for timely repair of DSBs, senataxin was found to promote Rad51 recruitment, to minimize illegitimate rejoining of distant DNA ends and to sustain cell viability following DSB production in active genes. Our data suggest that senataxin functions at DSBs in order to limit translocations and ensure cell viability, providing new insights on AOA2/ALS4 neuropathies.

Complete cis Exclusion upon Duplication of the Eµ Enhancer at the Immunoglobulin Heavy Chain Locus.

Developing lymphocytes somatically diversify their antigen-receptor loci through V(D)J recombination. The process is associated with allelic exclusion, which results in monoallelic expression of an antigen receptor locus. Various cis-regulatory elements control V(D)J recombination in a developmentally regulated manner, but their role in allelic exclusion is still unclear. At the immunoglobulin heavy chain locus (IgH), the Eµ enhancer plays a critical role in V(D)J recombination. We generated a mouse line with a replacement mutation in the constant region of the locus that duplicates the Eµ enhancer and allows premature expression of the γ3 heavy chain. Strikingly, IgM expression was completely and specifically excluded in cis from the mutant allele. This cis exclusion recapitulated the main features of allelic exclusion, including differential exclusion of variable genes. Notably, sense and antisense transcription within the distal variable domain and distal V(H)-DJ(H) recombination were inhibited. cis exclusion was established and stably maintained despite an active endogenous Eµ enhancer. The data reveal the importance of the dynamic, developmental stage-dependent interplay between IgH locus enhancers and signaling in the induction and maintenance of allelic exclusion.

Insertion of an imprinted insulator into the IgH locus reveals developmentally regulated, transcription-dependent control of V(D)J recombination.

The assembly of antigen receptor loci requires a developmentally regulated and lineage-specific recombination between variable (V), diversity (D), and joining (J) segments through V(D)J recombination. The process is regulated by accessibility control elements, including promoters, insulators, and enhancers. The IgH locus undergoes two recombination steps, D-J(H) and then V(H)-DJ(H), but it is unclear how the availability of the DJ(H) substrate could influence the subsequent V(H)-DJ(H) recombination step. The Eµ enhancer plays a critical role in V(D)J recombination and controls a set of sense and antisense transcripts. We epigenetically perturbed the early events at the IgH locus by inserting the imprinting control region (ICR) of the Igf2/H19 locus or a transcriptional insulator devoid of the imprinting function upstream of the Eµ enhancer. The insertions recapitulated the main epigenetic features of their endogenous counterparts, including differential DNA methylation and binding of CTCF/cohesins. Whereas the D-J(H) recombination step was unaffected, both the insulator insertions led to a severe impairment of V(H)-DJ(H) recombination. Strikingly, the inhibition of V(H)-DJ(H) recombination correlated consistently with a strong reduction of DJ(H) transcription and incomplete demethylation. Thus, developmentally regulated transcription following D-J(H) recombination emerges as an important mechanism through which the Eµ enhancer controls V(H)-DJ(H) recombination.

Tissue-specific inactivation of HAT cofactor TRRAP reveals its essential role in B cells.

The transformation/transcription domain-associated protein (TRRAP) is a common component of many histone acetyltransferase (HAT) complexes. Targeted-deletion of the Trrap gene led to early embryonic lethality and revealed a critical function of TRRAP in cell proliferation. Here, we investigate the function of TRRAP in murine B cells. To this end, we ablated Trrap gene in a B cell-restricted manner and studied its impact on B-cell development and proliferation, a pre-requisite for class switch recombination (CSR), the process that allows IgM-expressing B lymphocytes to switch to the expression of IgG, IgE, or IgA isotypes. We show that TRRAP deficiency impairs B-cell development but does not directly affect CSR. Instead, cells induced to proliferate undergo apoptosis. Our findings demonstrate a central and general role of TRRAP in cell proliferation.

Seeking sense of antisense switch transcripts.

In B lymphocytes, class switch recombination (CSR) machinery targets highly repetitive sequences, called switch (S) sequences, in the constant domain of the immunoglobulin heavy chain (IgH) locus. Cotranscriptional generation of R loops at S sequences provides the substrate for the mutagenic enzyme AID (Activation-Induced cytidine Deaminase), which initiates the DNA breaks at the transcribed sequences. Both sense and antisense transcripts across the S regions have been reported. Our recent work shows that, unlike its sense counterpart, antisense transcription of S sequences is dispensable for CSR in vivo.

Sense transcription through the S region is essential for immunoglobulin class switch recombination.

Class switch recombination (CSR) occurs between highly repetitive sequences called switch (S) regions and is initiated by activation-induced cytidine deaminase (AID). CSR is preceded by a bidirectional transcription of S regions but the relative importance of sense and antisense transcription for CSR in vivo is unknown. We generated three mouse lines in which we attempted a premature termination of transcriptional elongation by inserting bidirectional transcription terminators upstream of Sµ, upstream of Sγ3 or downstream of Sγ3 sequences. The data show, at least for Sγ3, that sense transcriptional elongation across S region is absolutely required for CSR whereas its antisense counterpart is largely dispensable, strongly suggesting that sense transcription is sufficient for AID targeting to both DNA strands.

Physical interaction between the histone acetyl transferase Tip60 and the DNA double-strand breaks sensor MRN complex.

Chromatin modifications and chromatin-modifying enzymes are believed to play a major role in the process of DNA repair. The histone acetyl transferase Tip60 is physically recruited to DNA DSBs (double-strand breaks) where it mediates histone acetylation. In the present study, we show, using a reporter system in mammalian cells, that Tip60 expression is required for homology-driven repair, strongly suggesting that Tip60 participates in DNA DSB repair through homologous recombination. Moreover, Tip60 depletion inhibits the formation of Rad50 foci following ionizing radiation, indicating that Tip60 expression is necessary for the recruitment of the DNA damage sensor MRN (Mre11-Rad50-Nbs1) complex to DNA DSBs. Moreover, we found that endogenous Tip60 physically interacts with endogenous MRN proteins in a complex which is distinct from the classical Tip60 complex. Taken together, our results describe a physical link between a DNA damage sensor and a histone-modifying enzyme, and provide important new insights into the role and mechanism of action of Tip60 in the process of DNA DSB repair.

Replacement of Imu-Cmu intron by NeoR gene alters Imu germ-line expression but has no effect on V(D)J recombination.

The NeoR gene has often been used to unravel the mechanisms underlying long-range interactions between promoters and enhancers during V(D)J assembly and class switch recombination (CSR) in the immunoglobulin heavy chain (IgH) locus. This approach led to the notion that CSR is regulated through competition of germ-line (GL) promoters for activities displayed by the 3' regulatory region (3'RR). This polarized long-range effect of the 3'RR is disturbed upon insertion of NeoR gene in the IgH constant (C(H)) region, where only GL transcription derived from upstream GL promoters is impaired. In the context of V(D)J recombination, replacement of Emu enhancer or Emu core enhancer (cEmu) by NeoR gene fully blocked V(D)J recombination and mu0 GL transcription which originates 5' of DQ52 and severely diminished Imu GL transcription derived from Emu/Imu promoter, suggesting a critical role for cEmu in the regulation of V(D)J recombination and of mu0 and Imu expression. Here we focus on the effect of NeoR gene on mu0 and Imu GL transcription in a mouse line in which the Imu-Cmu intron was replaced by a NeoR gene in the sense-orientation. B cell development was characterized by a marked but incomplete block at the pro-B cell stage. However, V(D)J recombination was unaffected in sorted pro-B and pre-B cells excluding an interference with the accessibility control function of Emu. mu0 GL transcription initiation was relatively normal but the maturation step seemed to be affected most likely through premature termination at NeoR polyadenylation sites. In contrast, Imu transcription initiation was impaired suggesting an interference of NeoR gene with the IgH enhancers that control Imu expression. Surprisingly, in stark contrast with the NeoR effect in the C(H) region, LPS-induced NeoR expression restored Imu transcript levels to normal. The data suggest that Emu enhancer may be the master control element that counteracts the down-regulatory "Neo effect" on Imu expression upon LPS stimulation. More importantly, they reveal a complex and developmentally regulated interplay between IgH enhancers in the control of Imu expression.

Human DNA polymerase eta is required for common fragile site stability during unperturbed DNA replication.

Human DNA polymerase eta (Pol eta) modulates susceptibility to skin cancer by promoting translesion DNA synthesis (TLS) past sunlight-induced cyclobutane pyrimidine dimers. Despite its well-established role in TLS synthesis, the role of Pol eta in maintaining genome stability in the absence of external DNA damage has not been well explored. We show here that short hairpin RNA-mediated depletion of Pol eta from undamaged human cells affects cell cycle progression and the rate of cell proliferation and results in increased spontaneous chromosome breaks and common fragile site expression with the activation of ATM-mediated DNA damage checkpoint signaling. These phenotypes were also observed in association with modified replication factory dynamics during S phase. In contrast to that seen in Pol eta-depleted cells, none of these cellular or karyotypic defects were observed in cells depleted for Pol iota, the closest relative of Pol eta. Our results identify a new role for Pol eta in maintaining genomic stability during unperturbed S phase and challenge the idea that the sole functional role of Pol eta in human cells is in TLS DNA damage tolerance and/or repair pathways following exogenous DNA damage.

Role of TLS DNA polymerases eta and kappa in processing naturally occurring structured DNA in human cells.

Accurate DNA replication during S-phase is fundamental to maintain genome integrity. During this critical process, replication forks frequently encounter obstacles that impede their progression. While the regulatory pathways which act in response to exogenous replication stress are beginning to emerge, the mechanisms by which fork integrity is maintained at naturally occurring endogenous replication-impeding sequences remains obscure. Notably, little is known about how cells replicate through special chromosomal regions containing structured non-B DNA, for example, G4 quartets, known to hamper fork progression or trigger chromosomal rearrangements. Here, we have investigated the role in this process of the human translesion synthesis (TLS) DNA polymerases of the Y-family (pol eta, pol iota, and pol kappa), specialized enzymes known to synthesize DNA through DNA damage. We show that depletion by RNA interference of expression of the genes for Pol eta or Pol kappa, but not Pol iota, sensitizes U2OS cells treated with the G4-tetraplex interactive compound telomestatin and triggers double-strand breaks in HeLa cells harboring multiple copies of a G-rich sequence from the promoter region of the human c-MYC gene, chromosomally integrated as a transgene. Moreover, we found that downregulation of Pol kappa only raises the level of DSB in HeLa cells containing either one of two breakage hotspot structured DNA sequences in the chromosome, the major break region (Mbr) of BCL-2 gene and the GA rich region from the far right-hand end of the genome of the Kaposi Sarcoma associated Herpesvirus. These data suggest that naturally occurring DNA structures are physiological substrates of both pol eta and pol kappa. We discuss these data in the light of their downregulation in human cancers.

Distinct roles of chromatin-associated proteins MDC1 and 53BP1 in mammalian double-strand break repair.

Phosphorylated histone H2AX ("gamma-H2AX") recruits MDC1, 53BP1, and BRCA1 to chromatin near a double-strand break (DSB) and facilitates efficient repair of the break. It is unclear to what extent gamma-H2AX-associated proteins act in concert and to what extent their functions within gamma-H2AX chromatin are distinct. We addressed this question by comparing the mechanisms of action of MDC1 and 53BP1 in DSB repair (DSBR). We find that MDC1 functions primarily in homologous recombination/sister chromatid recombination, in a manner strictly dependent upon its ability to interact with gamma-H2AX but, unexpectedly, not requiring recruitment of 53BP1 or BRCA1 to gamma-H2AX chromatin. In contrast, 53BP1 functions in XRCC4-dependent nonhomologous end-joining, likely mediated by its interaction with dimethylated lysine 20 of histone H4 but, surprisingly, independent of H2AX. These results suggest a specialized adaptation of the "histone code" in which distinct histone tail-protein interactions promote engagement of distinct DSBR pathways.

Molecular analysis of sister chromatid recombination in mammalian cells.

Sister chromatid recombination (SCR) is a potentially error-free pathway for the repair of double-strand breaks arising during replication and is thought to be important for the prevention of genomic instability and cancer. Analysis of sister chromatid recombination at a molecular level has been limited by the difficulty of selecting specifically for these events. To overcome this, we have developed a novel "nested intron" reporter that allows the positive selection in mammalian cells of "long tract" gene conversion events arising between sister chromatids. We show that these events arise spontaneously in cycling cells and are strongly induced by a site-specific double-strand break (DSB) caused by the restriction endonuclease, I-SceI. Notably, some I-SceI-induced sister chromatid recombination events entailed multiple rounds of gene amplification within the reporter, with the generation of a concatemer of amplified gene segments. Thus, there is an intimate relationship between sister chromatid recombination control and certain types of gene amplification. Dysregulated sister chromatid recombination may contribute to cancer progression, in part, by promoting gene amplification.

Control of sister chromatid recombination by histone H2AX.

Histone H2AX has a role in suppressing genomic instability and cancer. However, the mechanisms by which it performs these functions are poorly understood. After DNA breakage, H2AX is phosphorylated on serine 139 in chromatin near the break. We show here that H2AX serine 139 enforces efficient homologous recombinational repair of a chromosomal double-strand break (DSB) by using the sister chromatid as a template. BRCA1, Rad51, and CHK2 contribute to recombinational repair, in part independently of H2AX. H2AX(-/-) cells show increased use of single-strand annealing, an error-prone deletional mechanism of DSB repair. Therefore, the chromatin response around a chromosomal DSB, in which H2AX serine 139 phosphorylation plays a central role, "shapes" the repair process in favor of potentially error-free interchromatid homologous recombination at the expense of error-prone repair. H2AX phosphorylation may help set up a favorable disposition between sister chromatids.

DNA polymerase beta overexpression stimulates the Rad51-dependent homologous recombination in mammalian cells.

Overexpression of DNA polymerase beta (polbeta), an error-prone DNA repair enzyme, has been shown to result in mutagenesis, aneuploidy and tumorigenesis. To further investigate the molecular basis leading to cancer-associated genetic changes, we examined whether the DNA polbeta could affect homologous recombination (HR). Using mammalian cells carrying an intrachromosomal recombination marker we showed that the DNA polbeta overexpression increased the HR mostly by enhancing gene conversion. Concomitantly, we observed the generation of DNA strand breaks as well as a DNA polbeta-dependent formation of Rad51 foci. The stimulation of HR was abolished by the coexpression of a dominant negative form of Rad51, suggesting that the Rad51 was involved in the increased HR events. The expression of different DNA polbeta mutants lacking polymerase activity did not result in HR stimulation, indicating that the DNA synthesis activity of DNA polbeta was related to this phenotype. These results provide new insights into the molecular mechanisms of the genetic instability observed in DNA polbeta overexpressing tumour cells.

BRCA1 and BRCA2 in hereditary breast cancer.

The hereditary breast and ovarian cancer susceptibility genes, BRCA1 and BRCA2, have established roles in genome integrity maintenance and in the control of homologous recombination. Recent work has produced valuable insights into the mechanisms of action of the gene products. This review summarizes some of these advances, and attempts to place them in the context of known functions of the genes.