The AID-induced DNA damage response in chromatin

Chromatin accessibility: methods, mechanisms, and biological insights

Access to DNA is a prerequisite to the execution of essential cellular processes that include transcription, replication, chromosomal segregation, and DNA repair. How the proteins that regulate these processes function in the context of chromatin and its dynamic architectures is an intensive field of study. Over the past decade, genome-wide assays and new imaging approaches have enabled a greater understanding of how access to the genome is regulated by nucleosomes and associated proteins. Additional mechanisms that may control DNA accessibility in vivo include chromatin compaction and phase separation - processes that are beginning to be understood. Here, we review the ongoing development of accessibility measurements, we summarize the different molecular and structural mechanisms that shape the accessibility landscape, and we detail the many important biological functions that are linked to chromatin accessibility.

Prospectively defined patterns of APOBEC3A mutagenesis are prevalent in human cancers

Mutational signatures defined by single base substitution (SBS) patterns in cancer have elucidated potential mutagenic processes that contribute to malignancy. Two prevalent mutational patterns in human cancers are attributed to the APOBEC3 cytidine deaminase enzymes. Among the seven human APOBEC3 proteins, APOBEC3A is a potent deaminase and proposed driver of cancer mutagenesis. In this study, we prospectively examine genome-wide aberrations by expressing human APOBEC3A in avian DT40 cells. From whole-genome sequencing, we detect hundreds to thousands of base substitutions per genome. The APOBEC3A signature includes widespread cytidine mutations and a unique insertion-deletion (indel) signature consisting largely of cytidine deletions. This multi-dimensional APOBEC3A signature is prevalent in human cancer genomes. Our data further reveal replication-associated mutations, the rate of stem-loop and clustered mutations, and deamination of methylated cytidines. This comprehensive signature of APOBEC3A mutagenesis is a tool for future studies and a potential biomarker for APOBEC3 activity in cancer.

The germinal center reaction depends on RNA methylation and divergent functions of specific methyl readers

Long-lasting immunity depends on the generation of protective antibodies through the germinal center (GC) reaction. N6-methyladenosine (m6A) modification of mRNAs by METTL3 activity modulates transcript lifetime primarily through the function of m6A readers; however, the physiological role of this molecular machinery in the GC remains unknown. Here, we show that m6A modifications by METTL3 are required for GC maintenance through the differential functions of m6A readers. Mettl3-deficient GC B cells exhibited reduced cell-cycle progression and decreased expression of proliferation- and oxidative phosphorylation-related genes. The m6A binder, IGF2BP3, was required for stabilization of Myc mRNA and expression of its target genes, whereas the m6A reader, YTHDF2, indirectly regulated the expression of the oxidative phosphorylation gene program. Our findings demonstrate how two independent gene networks that support critical GC functions are modulated by m6A through distinct mRNA binders.

Long-Range Control of Class Switch Recombination by Transcriptional Regulatory Elements

Immunoglobulin class switch recombination (CSR) plays a crucial role in adaptive immune responses through a change of the effector functions of antibodies and is triggered by T-cell-dependent as well as T-cell-independent antigens. Signals generated following encounter with each type of antigen direct CSR to different isotypes. At the genomic level, CSR occurs between highly repetitive switch sequences located upstream of the constant gene exons of the immunoglobulin heavy chain locus. Transcription of switch sequences is mandatory for CSR and is induced in a stimulation-dependent manner. Switch transcription takes place within dynamic chromatin domains and is regulated by long-range regulatory elements which promote alignment of partner switch regions in CSR centers. Here, we review recent work and models that account for the function of long-range transcriptional regulatory elements and the chromatin-based mechanisms involved in the control of CSR.

Noncoding RNA processing by DIS3 regulates chromosomal architecture and somatic hypermutation in B cells

Noncoding RNAs are exquisitely titrated by the cellular RNA surveillance machinery for regulating diverse biological processes. The RNA exosome, the predominant 3' RNA exoribonuclease in mammalian cells, is composed of nine core and two catalytic subunits. Here, we developed a mouse model with a conditional allele to study the RNA exosome catalytic subunit DIS3. In DIS3-deficient B cells, integrity of the immunoglobulin heavy chain (Igh) locus in its topologically associating domain is affected, with accumulation of DNA-associated RNAs flanking CTCF-binding elements, decreased CTCF binding to CTCF-binding elements and disorganized cohesin localization. DIS3-deficient B cells also accumulate activation-induced cytidine deaminase-mediated asymmetric nicks, altering somatic hypermutation patterns and increasing microhomology-mediated end-joining DNA repair. Altered mutation patterns and Igh architectural defects in DIS3-deficient B cells lead to decreased class-switch recombination but increased chromosomal translocations. Our observations of DIS3-mediated architectural regulation at the Igh locus are reflected genome wide, thus providing evidence that noncoding RNA processing is an important mechanism for controlling genome organization.

APOBEC1 cytosine deaminase activity on single-stranded DNA is suppressed by replication protein A

Many APOBEC cytidine deaminase members are known to induce ‘off-target’ cytidine deaminations in 5′TC motifs in genomic DNA that contribute to cancer evolution. In this report, we characterized APOBEC1, which is a possible cancer related APOBEC since APOBEC1 mRNA is highly expressed in certain types of tumors, such as lung adenocarcinoma. We found a low level of APOBEC1-induced DNA damage, as measured by γH2AX foci, in genomic DNA of a lung cancer cell line that correlated to its inability to compete in vitro with replication protein A (RPA) for ssDNA. This suggests that RPA can act as a defense against off-target deamination for some APOBEC enzymes. Overall, the data support the model that the ability of an APOBEC to compete with RPA can better predict genomic damage than combined analysis of mRNA expression levels in tumors and analysis of mutation signatures.

Towards a comprehensive view of 8-oxo-7,8-dihydro-2'-deoxyguanosine: Highlighting the intertwined roles of DNA damage and epigenetics in genomic instability

8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG), a major product of DNA oxidation, is a pre-mutagenic lesion which is prone to mispair, if left unrepaired, with 2'-deoxyadenosine during DNA replication. While unrepaired or incompletely repaired 8-oxodG has classically been associated with genome instability and cancer, it has recently been reported to have a role in the epigenetic regulation of gene expression. Despite the growing collection of genome-wide 8-oxodG mapping studies that have been used to provide new insight on the functional nature of 8-oxodG within the genome, a comprehensive view that brings together the epigenetic and the mutagenic nature of the 8-oxodG is still lacking. To help address this gap, this review aims to provide (i) a description of the state-of-the-art knowledge on both the mutagenic and epigenetic roles of 8-oxodG; (ii) putative molecular models through which the 8-oxodG can cause genome instability; (iii) a possible molecular model on how 8-oxodG, acting as an epigenetic signal, could cause the translocations and deletions which are associated with cancer.

Genomic and epigenomic insights into the origin, pathogenesis, and clinical behavior of mantle cell lymphoma subtypes

Mantle cell lymphoma (MCL) is a mature B-cell neoplasm initially driven by CCND1 rearrangement with 2 molecular subtypes, conventional MCL (cMCL) and leukemic non-nodal MCL (nnMCL), that differ in their clinicobiological behavior. To identify the genetic and epigenetic alterations determining this diversity, we used whole-genome (n = 61) and exome (n = 21) sequencing (74% cMCL, 26% nnMCL) combined with transcriptome and DNA methylation profiles in the context of 5 MCL reference epigenomes. We identified that open and active chromatin at the major translocation cluster locus might facilitate the t(11;14)(q13;32), which modifies the 3-dimensional structure of the involved regions. This translocation is mainly acquired in precursor B cells mediated by recombination-activating genes in both MCL subtypes, whereas in 8% of cases the translocation occurs in mature B cells mediated by activation-induced cytidine deaminase. We identified novel recurrent MCL drivers, including CDKN1B, SAMHD1, BCOR, SYNE1, HNRNPH1, SMARCB1, and DAZAP1. Complex structural alterations emerge as a relevant early oncogenic mechanism in MCL, targeting key driver genes. Breakage-fusion-bridge cycles and translocations activated oncogenes (BMI1, MIR17HG, TERT, MYC, and MYCN), generating gene amplifications and remodeling regulatory regions. cMCL carried significant higher numbers of structural variants, copy number alterations, and driver changes than nnMCL, with exclusive alterations of ATM in cMCL, whereas TP53 and TERT alterations were slightly enriched in nnMCL. Several drivers had prognostic impact, but only TP53 and MYC aberrations added value independently of genomic complexity. An increasing genomic complexity, together with the presence of breakage-fusion-bridge cycles and high DNA methylation changes related to the proliferative cell history, defines patients with different clinical evolution.

Programmed DNA Damage and Physiological DSBs: Mapping, Biological Significance and Perturbations in Disease States

DNA double strand breaks (DSBs) are known to be the most toxic and threatening of the various types of breaks that may occur to the DNA. However, growing evidence continuously sheds light on the regulatory roles of programmed DSBs. Emerging studies demonstrate the roles of DSBs in processes such as T and B cell development, meiosis, transcription and replication. A significant recent progress in the last few years has contributed to our advanced knowledge regarding the functions of DSBs is the development of many next generation sequencing (NGS) methods, which have considerably advanced our capabilities. Other studies have focused on the implications of programmed DSBs on chromosomal aberrations and tumorigenesis. This review aims to summarize what is known about DNA damage in its physiological context. In addition, we will examine the advancements of the past several years, which have made an impact on the study of genome landscape and its organization.

Mechanism and regulation of class switch recombination by IgH transcriptional control elements

Class switch recombination (CSR) plays an important role in humoral immunity by generating antibodies with different effector functions. CSR to a particular antibody isotype is induced by external stimuli, and occurs between highly repetitive switch (S) sequences. CSR requires transcription across S regions, which generates long non-coding RNAs and secondary structures that promote accessibility of S sequences to activation-induced cytidine deaminase (AID). AID initiates DNA double-strand breaks (DSBs) intermediates that are repaired by general DNA repair pathways. Switch transcription is controlled by various regulatory elements, including enhancers and insulators. The current paradigm posits that transcriptional control of CSR involves long-range chromatin interactions between regulatory elements and chromatin loops-stabilizing factors, which promote alignment of partner S regions in a CSR centre (CSRC) and initiation of CSR. In this review, we focus on the role of IgH transcriptional control elements in CSR and the chromatin-based mechanisms underlying this control.

Uncoupling Splicing From Transcription Using Antisense Oligonucleotides Reveals a Dual Role for I Exon Donor Splice Sites in Antibody Class Switching

Class switch recombination (CSR) changes antibody isotype by replacing Cμ constant exons with different constant exons located downstream on the immunoglobulin heavy (IgH) locus. During CSR, transcription through specific switch (S) regions and processing of non-coding germline transcripts (GLTs) are essential for the targeting of activation-induced cytidine deaminase (AID). While CSR to IgG1 is abolished in mice lacking an Iγ1 exon donor splice site (dss), many questions remain regarding the importance of I exon dss recognition in CSR. To further clarify the role of I exon dss in CSR, we first evaluated RNA polymerase II (RNA pol II) loading and chromatin accessibility in S regions after activation of mouse B cells lacking Iγ1 dss. We found that deletion of Iγ1 dss markedly reduced RNA pol II pausing and active chromatin marks in the Sγ1 region. We then challenged the post-transcriptional function of I exon dss in CSR by using antisense oligonucleotides (ASOs) masking I exon dss on GLTs. Treatment of stimulated B cells with an ASO targeting Iγ1 dss, in the acceptor Sγ1 region, or Iμ dss, in the donor Sμ region, did not decrease germline transcription but strongly inhibited constitutive splicing and CSR to IgG1. Supporting a global effect on CSR, we also observed that the targeting of Iμ dss reduced CSR to IgG3 and, to a lesser extent, IgG2b isotypes. Altogether, this study reveals that the recognition of I exon dss first supports RNA pol II pausing and the opening of chromatin in targeted S regions and that GLT splicing events using constitutive I exon dss appear mandatory for the later steps of CSR, most likely by guiding AID to S regions.

The DNA-binding activity of USP1-associated factor 1 is required for efficient RAD51-mediated homologous DNA pairing and homology-directed DNA repair

USP1-associated factor 1 (UAF1) is an integral component of the RAD51-associated protein 1 (RAD51AP1)-UAF1-ubiquitin-specific peptidase 1 (USP1) trimeric deubiquitinase complex. This complex acts on DNA-bound, monoubiquitinated Fanconi anemia complementation group D2 (FANCD2) protein in the Fanconi anemia pathway of the DNA damage response. Moreover, RAD51AP1 and UAF1 cooperate to enhance homologous DNA pairing mediated by the recombinase RAD51 in DNA repair via the homologous recombination (HR) pathway. However, whereas the DNA-binding activity of RAD51AP1 has been shown to be important for RAD51-mediated homologous DNA pairing and HR-mediated DNA repair, the role of DNA binding by UAF1 in these processes is unclear. We have isolated mutant UAF1 variants that are impaired in DNA binding and tested them together with RAD51AP1 in RAD51-mediated HR. This biochemical analysis revealed that the DNA-binding activity of UAF1 is indispensable for enhanced RAD51 recombinase activity within the context of the UAF1-RAD51AP1 complex. In cells, DNA-binding deficiency of UAF1 increased DNA damage sensitivity and impaired HR efficiency, suggesting that UAF1 and RAD51AP1 have coordinated roles in DNA binding during HR and DNA damage repair. Our findings show that even though UAF1's DNA-binding activity is redundant with that of RAD51AP1 in FANCD2 deubiquitination, it is required for efficient HR-mediated chromosome damage repair.

Microhomology-mediated end joining drives complex rearrangements and overexpression of MYC and PVT1 in multiple myeloma

MYC is a widely acting transcription factor and its deregulation is a crucial event in many human cancers. MYC is important biologically and clinically in multiple myeloma, but the mechanisms underlying its dysregulation are poorly understood. We show that MYC rearrangements are present in 36.0% of newly diagnosed myeloma patients, as detected in the largest set of next generation sequencing data to date (n=1,267). Rearrangements were complex and associated with increased expression of MYC and PVT1, but not other genes at 8q24. The highest effect on gene expression was detected in cases where the MYC locus is juxtaposed next to super-enhancers associated with genes such as IGH, IGK, IGL, TXNDC5/BMP6, FAM46C and FOXO3 We identified three hotspots of recombination at 8q24, one of which is enriched for IGH-MYC translocations. Breakpoint analysis indicates primary myeloma rearrangements involving the IGH locus occur through non-homologous end joining, whereas secondary MYC rearrangements occur through microhomology-mediated end joining. This mechanism is different to lymphomas, where non-homologous end joining generates MYC rearrangements. Rearrangements resulted in overexpression of key genes and chromatin immunoprecipitation-sequencing identified that HK2, a member of the glucose metabolism pathway, is directly over-expressed through binding of MYC at its promoter.

The AKT isoforms 1 and 2 drive B cell fate decisions during the germinal center response

The PI3K pathway is integral for the germinal center (GC) response. However, the contribution of protein kinase B (AKT) as a PI3K effector in GC B cells remains unknown. Here, we show that mice lacking the AKT1 and AKT2 isoforms in B cells failed to form GCs, which undermined affinity maturation and antibody production in response to immunization. Upon B-cell receptor stimulation, AKT1/2-deficient B cells showed poor survival, reduced proliferation, and impaired mitochondrial and metabolic fitness, which collectively halted GC development. By comparison, Foxo1 T24A mutant, which cannot be inactivated by AKT1/2 phosphorylation and is sequestered in the nucleus, significantly enhanced antibody class switch recombination via induction of activation-induced cytidine deaminase (AID) expression. By contrast, repression of FOXO1 activity by AKT1/2 promoted IRF4-driven plasma cell differentiation. Last, we show that T-cell help via CD40, but not enforced expression of Bcl2, rescued the defective GC response in AKT1/2-deficient animals by restoring proliferative expansion and energy production. Overall, our study provides mechanistic insights into the key role of AKT and downstream pathways on B cell fate decisions during the GC response.

CUL7 E3 Ubiquitin Ligase Mediates the Degradation of Activation-Induced Cytidine Deaminase and Regulates the Ig Class Switch Recombination in B Lymphocytes

Activation-induced cytidine deaminase (AID) initiates class switch recombination and somatic hypermutation in Ig genes. The activity and protein levels of AID are tightly controlled by various mechanisms. In this study, we found that CUL7 E3 ubiquitin ligases specifically mediated AID ubiquitination. CUL7 overexpression or knockdown influenced the decay of AID, affecting AID protein levels and subsequently IgA class switching in CH12F3 cells, a mouse B lymphocyte cell line. Further analysis indicated that CUL7 mediated AID ubiquitination by forming a complex with FBXW11. In a CUL7 ^fl/fl CD19 ^cre+ mouse model, we demonstrated that CUL7 knockout significantly enhanced AID protein levels in B cells in the germinal center and increased both the IgG1 and IgA class switching. Collectively, our results reveal a subtle regulation mechanism for tightly controlling AID protein levels. The manipulation of this pathway may be useful for regulating AID abundance and efficiency of Ig class switching and is therefore a potential target for developing immunologic adjuvants for vaccines of various pathogens such as HIV-1 and influenza viruses.

CHK1 dosage in germinal center B cells controls humoral immunity

Germinal center (GC) B cells are among the fastest replicating cells in our body, dividing every 4-8 h. DNA replication errors are intrinsically toxic to cells. How GC B cells exert control over the DNA damage response while introducing mutations in their antibody genes is poorly understood. Here, we show that the DNA damage response regulator Checkpoint kinase 1 (CHK1) is essential for GC B cell survival. Remarkably, effective antibody-mediated immunity relies on optimal CHK1 dosage. Chemical CHK1 inhibition or loss of one Chk1 allele impairs the survival of class-switched cells and curbs the amplitude of antibody production. Mechanistically, active B cell receptor signaling wires the outcome of CHK1-inhibition towards BIM-dependent apoptosis, whereas T cell help favors temporary cell cycle arrest. Our results predict that therapeutic CHK1 inhibition in cancer patients may prove potent in killing B cell lymphoma and leukemia cells addicted to B cell receptor signaling, but will most likely dampen humoral immunity.

Signaling control of antibody isotype switching

Class switch recombination (CSR) generates isotype-switched antibodies with distinct effector functions essential for mediating effective humoral immunity. CSR is catalyzed by activation-induced deaminase (AID) that initiates DNA lesions in the evolutionarily conserved switch (S) regions at the immunoglobulin heavy chain (Igh) locus. AID-initiated DNA lesions are subsequently converted into DNA double stranded breaks (DSBs) in the S regions of Igh locus, repaired by non-homologous end-joining to effect CSR in mammalian B lymphocytes. While molecular mechanisms of CSR are well characterized, it remains less well understood how upstream signaling pathways regulate AID expression and CSR. B lymphocytes express multiple receptors including the B cell antigen receptor (BCR) and co-receptors (e.g., CD40). These receptors may share common signaling pathways or may use distinct signaling elements to regulate CSR. Here, we discuss how signals emanating from different receptors positively or negatively regulate AID expression and CSR.

Molecular regulation of peripheral B cells and their progeny in immunity

In this review, Boothby et al. summarize some salient advances toward elucidation of the molecular programming of the fate choices and function of B cells in the periphery. They also note unanswered questions that pertain to differences among subsets of B lymphocytes and plasma cells.

Mature B lymphocytes are crucial components of adaptive immunity, a system essential for the evolutionary fitness of mammals. Adaptive lymphocyte function requires an initially naïve cell to proliferate extensively and its progeny to have the capacity to assume a variety of fates. These include either terminal differentiation (the long-lived plasma cell) or metastable transcriptional reprogramming (germinal center and memory B cells). In this review, we focus principally on the regulation of differentiation and functional diversification of the “B2” subset. An overview is combined with an account of more recent advances, including initial work on mechanisms that eliminate DNA methylation and potential links between intracellular metabolites and chromatin editing.

RNF169 limits 53BP1 deposition at DSBs to stimulate single-strand annealing repair

53BP1 restrains DNA end resection, and its dosage imbalance upsets DNA double-strand break (DSB) repair pathway choice. Here, by monitoring 53BP1 distribution on DSB-flanking chromatin, we have established a dose-dependent role of the RING finger protein RNF169 in limiting 53BP1 DSB deposition. Moreover, we found that forced expression of RNF169 overcomes 53BP1 activity and stimulates mutagenic DSB repair via the single-strand annealing pathway. Our findings suggest that aberrant expression of RNF169 may represent a deleterious factor in DSB repair control and in maintenance of genome stability.

Unrestrained 53BP1 activity at DNA double-strand breaks (DSBs) hampers DNA end resection and upsets DSB repair pathway choice. RNF169 acts as a molecular rheostat to limit 53BP1 deposition at DSBs, but how this fine balance translates to DSB repair control remains undefined. In striking contrast to 53BP1, ChIP analyses of AsiSI-induced DSBs unveiled that RNF169 exhibits robust accumulation at DNA end-proximal regions and preferentially targets resected, RPA-bound DSBs. Accordingly, we found that RNF169 promotes CtIP-dependent DSB resection and favors homology-mediated DSB repair, and further showed that RNF169 dose-dependently stimulates single-strand annealing repair, in part, by alleviating the 53BP1-imposed barrier to DSB end resection. Our results highlight the interplay of RNF169 with 53BP1 in fine-tuning choice of DSB repair pathways.

DNA Damage and Chromatin Conformation Changes Confer Nonhost Resistance: A Hypothesis Based on Effects of Anti-cancer Agents on Plant Defense Responses

Over the last decades, medical research has utilized DNA altering procedures in cancer treatments with the objective of killing cells or suppressing cell proliferation. Simultaneous research related to enhancing disease resistance in plants reported that alterations in DNA can enhance defense responses. These two opposite perspectives have in common their effects on the center for gene transcription, the nuclear chromatin. A review of selected research from both anticancer- and plant defense-related research provides examples of some specific DNA altering actions: DNA helical distortion, DNA intercalation, DNA base substitution, DNA single cleavage by DNases, DNA alkylation/methylation, and DNA binding/exclusion. The actions of the pertinent agents are compared, and their proposed modes of action are described in this study. Many of the DNA specific agents affecting resistance responses in plants, e.g., the model system using pea endocarp tissue, are indeed anticancer agents. The tumor cell death or growth suppression in cancer cells following high level treatments may be accompanied with chromatin distortions. Likewise, in plants, DNA-specific agents activate enhanced expression of many genes including defense genes, probably due to the chromatin alterations resulting from the agents. Here, we propose a hypothesis that DNA damage and chromatin structural changes are central mechanisms in initiating defense gene transcription during the nonhost resistance response in plants.

Germline-activating mutations in PIK3CD compromise B cell development and function

Gain-of-function (GOF) mutations in PIK3CD, encoding the p110δ subunit of phosphatidylinositide 3-kinase (PI3K), cause a primary immunodeficiency. Affected individuals display impaired humoral immune responses following infection or immunization. To establish mechanisms underlying these immune defects, we studied a large cohort of patients with PIK3CD GOF mutations and established a novel mouse model using CRISPR/Cas9-mediated gene editing to introduce a common pathogenic mutation in Pik3cd In both species, hyperactive PI3K severely affected B cell development and differentiation in the bone marrow and the periphery. Furthermore, PI3K GOF B cells exhibited intrinsic defects in class-switch recombination (CSR) due to impaired induction of activation-induced cytidine deaminase (AID) and failure to acquire a plasmablast gene signature and phenotype. Importantly, defects in CSR, AID expression, and Ig secretion were restored by leniolisib, a specific p110δ inhibitor. Our findings reveal key roles for balanced PI3K signaling in B cell development and long-lived humoral immunity and memory and establish the validity of treating affected individuals with p110δ inhibitors.

Nucleolus as an emerging hub in maintenance of genome stability and cancer pathogenesis

The nucleolus is the major site for synthesis of ribosomes, complex molecular machines that are responsible for protein synthesis. A wealth of research over the past 20 years has clearly indicated that both quantitative and qualitative alterations in ribosome biogenesis can drive the malignant phenotype via dysregulation of protein synthesis. However, numerous recent proteomic, genomic, and functional studies have implicated the nucleolus in the regulation of processes that are unrelated to ribosome biogenesis, including DNA-damage response, maintenance of genome stability and its spatial organization, epigenetic regulation, cell-cycle control, stress responses, senescence, global gene expression, as well as assembly or maturation of various ribonucleoprotein particles. In this review, the focus will be on features of rDNA genes, which make them highly vulnerable to DNA damage and intra- and interchromosomal recombination as well as built-in mechanisms that prevent and repair rDNA damage, and how dysregulation of this interplay affects genome-wide DNA stability, gene expression and the balance between euchromatin and heterochromatin. We will also present the most recent insights into how malfunction of these cellular processes may be a central driving force of human malignancies, and propose a promising new therapeutic approach for the treatment of cancer.

H2AX facilitates classical non-homologous end joining at the expense of limited nucleotide loss at repair junctions

Phosphorylated histone H2AX, termed 'γH2AX', mediates the chromatin response to DNA double strand breaks (DSBs) in mammalian cells. H2AX deficiency increases the numbers of unrepaired DSBs and translocations, which are partly associated with defects in non-homologous end joining (NHEJ) and contributing to genomic instability in cancer. However, the role of γH2AX in NHEJ of general DSBs has yet to be clearly defined. Here, we showed that despite little effect on overall NHEJ efficiency, H2AX deficiency causes a surprising bias towards accurate NHEJ and shorter deletions in NHEJ products. By analyzing CRISPR/Cas9-induced NHEJ and by using a new reporter for mutagenic NHEJ, we found that γH2AX, along with its interacting protein MDC1, is required for efficient classical NHEJ (C-NHEJ) but with short deletions and insertions. Epistasis analysis revealed that ataxia telangiectasia mutated (ATM) and the chromatin remodeling complex Tip60/TRRAP/P400 are essential for this H2AX function. Taken together, these data suggest that a subset of DSBs may require γH2AX-mediated short-range nucleosome repositioning around the breaks to facilitate C-NHEJ with loss of a few extra nucleotides at NHEJ junctions. This may prevent outcomes such as non-repair and translocations, which are generally more destabilizing to genomes than short deletions and insertions from local NHEJ.

PTIP chromatin regulator controls development and activation of B cell subsets to license humoral immunity in mice

To provide optimal host defense, the full spectrum of antibody-based immunity requires natural antibodies and immunization-induced antigen-specific antibodies. Here we show that the PTIP (Pax transactivation domain-interacting protein) chromatin regulator is induced by B cell activation to potentiate the establishment of steady-state and postimmune serum antibody levels. It does so by promoting activation-associated proliferation and differentiation of all the major B cell subsets, at least in part, through regulating the NF-κB pathway. With the genetic basis still unknown for a majority of patients with common variable immunodeficiency, further work investigating how PTIP controls cell signaling may generate valuable new insight for human health and disease.

B cell receptor signaling and downstream NF-κB activity are crucial for the maturation and functionality of all major B cell subsets, yet the molecular players in these signaling events are not fully understood. Here we use several genetically modified mouse models to demonstrate that expression of the multifunctional BRCT (BRCA1 C-terminal) domain-containing PTIP (Pax transactivation domain-interacting protein) chromatin regulator is controlled by B cell activation and potentiates steady-state and postimmune antibody production in vivo. By examining the effects of PTIP deficiency in mice at various ages during ontogeny, we demonstrate that PTIP promotes bone marrow B cell development as well as the neonatal establishment and subsequent long-term maintenance of self-reactive B-1 B cells. Furthermore, we find that PTIP is required for B cell receptor- and T:B interaction-induced proliferation, differentiation of follicular B cells during germinal center formation, and normal signaling through the classical NF-κB pathway. Together with the previously identified role for PTIP in promoting sterile transcription at the Igh locus, the present results establish PTIP as a licensing factor for humoral immunity that acts at several junctures of B lineage maturation and effector cell differentiation by controlling B cell activation.

RNA Exosome and Non-coding RNA-Coupled Mechanisms in AID-Mediated Genomic Alterations

The eukaryotic RNA exosome is a well-conserved protein complex with ribonuclease activity implicated in RNA metabolism. Various families of non-coding RNAs have been identified as substrates of the complex, underscoring its role as a non-coding RNA processing/degradation unit. However, the role of RNA exosome and its RNA processing activity on DNA mutagenesis/alteration events have not been investigated until recently. B lymphocytes use two DNA alteration mechanisms, class switch recombination (CSR) and somatic hypermutation (SHM), to re-engineer their antibody gene expressing loci until a tailored antibody gene for a specific antigen is satisfactorily generated. CSR and SHM require the essential activity of the DNA activation-induced cytidine deaminase (AID). Causing collateral damage to the B-cell genome during CSR and SHM, AID induces unwanted (and sometimes oncogenic) mutations at numerous non-immunoglobulin gene sequences. Recent studies have revealed that AID's DNA mutator activity is regulated by the RNA exosome complex, thus providing an example of a mechanism that relates DNA mutagenesis to RNA processing. Here, we review the emergent functions of RNA exosome during CSR, SHM, and other chromosomal alterations in B cells, and discuss implications relevant to mechanisms that maintain B-cell genomic integrity.

Kin17 facilitates multiple double-strand break repair pathways that govern B cell class switching

Class switch recombination (CSR) in B cells requires the timely repair of DNA double-stranded breaks (DSBs) that result from lesions produced by activation-induced cytidine deaminase (AID). Through a genome-wide RNAi screen, we identified Kin17 as a gene potentially involved in the maintenance of CSR in murine B cells. In this study, we confirm a critical role for Kin17 in CSR independent of AID activity. Furthermore, we make evident that DSBs generated by AID or ionizing radiation require Kin17 for efficient repair and resolution. Our report shows that reduced Kin17 results in an elevated deletion frequency following AID mutational activity in the switch region. In addition, deficiency in Kin17 affects the functionality of multiple DSB repair pathways, namely homologous recombination, non-homologous end-joining, and alternative end-joining. This report demonstrates the importance of Kin17 as a critical factor that acts prior to the repair phase of DSB repair and is of bona fide importance for CSR.

Definition of mutations in polyautoimmunity

OBJECTIVES

Familial autoimmunity and polyautoimmunity represent extreme phenotypes ideal for identifying major genomic variants contributing to autoimmunity. Whole exome sequencing (WES) and linkage analysis are well suited for this purpose due to its strong resolution upon familial segregation patterns of functional protein coding and splice variants. The primary objective of this study was to identify potentially autoimmune causative variants using WES data from extreme pedigrees segregating polyautoimmunity phenotypes.

METHODS

DNA of 47 individuals across 10 extreme pedigrees, ascertained from probands affected with polyautoimmunity and familial autoimmunity, were selected for WES. Variant calls were obtained through Genome Analysis Toolkit. Filtration and prioritization framework to identify mutation(s) were applied, and later implemented for genetic linkage analysis. Sanger sequencing corroborated variants with significant linkage.

RESULTS

Novel and mostly rare variants harbored in SRA1, MLL4, ABCB8, DHX34 and PLAUR showed significant linkage (LOD scores are >3.0). The strongest signal was in SRA1, with a LOD score of 5.48. Network analyses indicated that SRA1, PLAUR and ABCB8 contribute to regulation of apoptotic processes.

CONCLUSIONS

Novel and rare variants in genetic linkage with polyautoimmunity were identified throughout WES. Genes harboring these variants might be major players of autoimmunity.

The SAGA Deubiquitination Module Promotes DNA Repair and Class Switch Recombination through ATM and DNAPK-Mediated γH2AX Formation

Class switch recombination (CSR) requires activation-induced deaminase (AID) to instigate double-stranded DNA breaks at the immunoglobulin locus. DNA breaks activate the DNA damage response (DDR) by inducing phosphorylation of histone H2AX followed by non-homologous end joining (NHEJ) repair. We carried out a genome-wide screen to identify CSR factors. We found that Usp22, Eny2, and Atxn7, members of the Spt-Ada-Gcn5-acetyltransferase (SAGA) deubiquitination module, are required for deubiquitination of H2BK120ub following DNA damage, are critical for CSR, and function downstream of AID. The SAGA deubiquitinase activity was required for optimal irradiation-induced γH2AX formation, and failure to remove H2BK120ub inhibits ATM- and DNAPK-induced γH2AX formation. Consistent with this effect, these proteins were found to function upstream of various double-stranded DNA repair pathways. This report demonstrates that deubiquitination of histone H2B impacts the early stages of the DDR and is required for the DNA repair phase of CSR.

Related Mechanisms of Antibody Somatic Hypermutation and Class Switch Recombination

The primary antibody repertoire is generated by mechanisms involving the assembly of the exons that encode the antigen-binding variable regions of immunoglobulin heavy (IgH) and light (IgL) chains during the early development of B lymphocytes. After antigen-dependent activation, mature B lymphocytes can further alter their IgH and IgL variable region exons by the process of somatic hypermutation (SHM), which allows the selection of B cells in which SHMs resulted in the production of antibodies with increased antigen affinity. In addition, during antigen-dependent activation, B cells can also change the constant region of their IgH chain through a DNA double-strand-break (DSB) dependent process referred to as IgH class switch recombination (CSR), which generates B cell progeny that produce antibodies with different IgH constant region effector functions that are best suited for a elimination of a particular pathogen or in a particular setting. Both the mutations that underlie SHM and the DSBs that underlie CSR are initiated in target genes by activation-induced cytidine deaminase (AID). This review describes in depth the processes of SHM and CSR with a focus on mechanisms that direct AID cytidine deamination in activated B cells and mechanisms that promote the differential outcomes of such cytidine deamination.

A PTIP-PA1 subcomplex promotes transcription for IgH class switching independently from the associated MLL3/MLL4 methyltransferase complex

Transcription at the immunoglobulin heavy chain (Igh) locus targets class switch recombination (CSR)-associated DNA damage and is promoted by the BRCT domain-containing PTIP protein. Starnes et al. found that PTIP functions in transcription and CSR separately from its association with the MLL3/MLL4 complex and from its localization to sites of DNA damage.

Class switch recombination (CSR) diversifies antibodies for productive immune responses while maintaining stability of the B-cell genome. Transcription at the immunoglobulin heavy chain (Igh) locus targets CSR-associated DNA damage and is promoted by the BRCT domain-containing PTIP (Pax transactivation domain-interacting protein). Although PTIP is a unique component of the mixed-lineage leukemia 3 (MLL3)/MLL4 chromatin-modifying complex, the mechanisms for how PTIP promotes transcription remain unclear. Here we dissected the minimal structural requirements of PTIP and its different protein complexes using quantitative proteomics in primary lymphocytes. We found that PTIP functions in transcription and CSR separately from its association with the MLL3/MLL4 complex and from its localization to sites of DNA damage. We identified a tandem BRCT domain of PTIP that is sufficient for CSR and identified PA1 as its main functional protein partner. Collectively, we provide genetic and biochemical evidence that a PTIP–PA1 subcomplex functions independently from the MLL3/MLL4 complex to mediate transcription during CSR. These results further our understanding of how multifunctional chromatin-modifying complexes are organized by subcomplexes that harbor unique and distinct activities.

MCM3AP and POMP Mutations Cause a DNA-Repair and DNA-Damage-Signaling Defect in an Immunodeficient Child

Immunodeficiency patients with DNA repair defects exhibit radiosensitivity and proneness to leukemia/lymphoma formation. Though progress has been made in identifying the underlying mutations, in most patients the genetic basis is unknown. Two de novo mutated candidate genes, MCM3AP encoding germinal center-associated nuclear protein (GANP) and POMP encoding proteasome maturation protein (POMP), were identified by whole-exome sequencing (WES) and confirmed by Sanger sequencing in a child with complex phenotype displaying immunodeficiency, genomic instability, skin changes, and myelodysplasia. GANP was previously described to promote B-cell maturation by nuclear targeting of activation-induced cytidine deaminase (AID) and to control AID-dependent hyperrecombination. POMP is required for 20S proteasome assembly and, thus, for efficient NF-κB signaling. Patient-derived cells were characterized by impaired homologous recombination, moderate radio- and cross-linker sensitivity associated with accumulation of damage, impaired DNA damage-induced NF-κB signaling, and reduced nuclear AID levels. Complementation by wild-type (WT)-GANP normalized DNA repair and WT-POMP rescued defective NF-κB signaling. In conclusion, we identified for the first time mutations in MCM3AP and POMP in an immunodeficiency patient. These mutations lead to cooperative effects on DNA recombination and damage signaling. Digenic/polygenic mutations may constitute a novel genetic basis in immunodeficiency patients with DNA repair defects.

Integrative genomic analysis reveals functional diversification of APOBEC gene family in breast cancer

Background

The human APOBEC protein family plays critical but distinct roles in host defense. Recent studies revealed that APOBECs mediate C-to-T mutagenesis in multiple cancers, including breast cancer. It is still unclear whether APOBEC gene family shows functional diversification involved in cancer mutagenesis.

Results

We performed an integrated analysis to characterize the functional diversification of APOBEC gene family associated with breast cancer mutagenesis relative to estrogen receptor (ER) status. Among the APOBEC family, we found that both APOBEC3B and APOBEC3C mRNA levels were significantly higher in estrogen receptor negative (ER−) subtype compared with estrogen receptor positive (ER+) subtype (P < 2.2 × 10⁻¹⁶ and P < 3.1 × 10⁻⁵, respectively). Epigenomic data further reflected the distinct chromatin states of APOBEC3B and APOBEC3C relative to ER status. Notably, we observed the significantly positive correlation between the APOBEC3B-mediated mutagenesis and APOBEC3B expression levels in ER+ cancers but not in ER− cancers. In contrast, we discovered the negative correlation of APOBEC3C mRNA levels with base-substitution mutations in ER− tumors. Meanwhile, we observed that breast cancers in carriers of germline deletion of APOBEC3B gene harbor similar mutation patterns, but higher mutation rates in the TCW motif (W corresponds to A or T) than cancers in non-carriers, indicating additional factors may also induce carcinogenic mutagenesis.

Conclusions

These results suggest that functional potential of APOBEC3B and APOBEC3C involved in cancer mutagenesis is associated with ER status.

Electronic supplementary material

The online version of this article (doi:10.1186/s40246-015-0056-9) contains supplementary material, which is available to authorized users.

Epigenetics of Peripheral B-Cell Differentiation and the Antibody Response

Epigenetic modifications, such as histone post-translational modifications, DNA methylation, and alteration of gene expression by non-coding RNAs, including microRNAs (miRNAs) and long non-coding RNAs (lncRNAs), are heritable changes that are independent from the genomic DNA sequence. These regulate gene activities and, therefore, cellular functions. Epigenetic modifications act in concert with transcription factors and play critical roles in B cell development and differentiation, thereby modulating antibody responses to foreign- and self-antigens. Upon antigen encounter by mature B cells in the periphery, alterations of these lymphocytes epigenetic landscape are induced by the same stimuli that drive the antibody response. Such alterations instruct B cells to undergo immunoglobulin (Ig) class switch DNA recombination (CSR) and somatic hypermutation (SHM), as well as differentiation to memory B cells or long-lived plasma cells for the immune memory. Inducible histone modifications, together with DNA methylation and miRNAs modulate the transcriptome, particularly the expression of activation-induced cytidine deaminase, which is essential for CSR and SHM, and factors central to plasma cell differentiation, such as B lymphocyte-induced maturation protein-1. These inducible B cell-intrinsic epigenetic marks guide the maturation of antibody responses. Combinatorial histone modifications also function as histone codes to target CSR and, possibly, SHM machinery to the Ig loci by recruiting specific adaptors that can stabilize CSR/SHM factors. In addition, lncRNAs, such as recently reported lncRNA-CSR and an lncRNA generated through transcription of the S region that form G-quadruplex structures, are also important for CSR targeting. Epigenetic dysregulation in B cells, including the aberrant expression of non-coding RNAs and alterations of histone modifications and DNA methylation, can result in aberrant antibody responses to foreign antigens, such as those on microbial pathogens, and generation of pathogenic autoantibodies, IgE in allergic reactions, as well as B cell neoplasia. Epigenetic marks would be attractive targets for new therapeutics for autoimmune and allergic diseases, and B cell malignancies.

Epigenetic Codes Programing Class Switch Recombination

Class switch recombination imparts B cells with a fitness-associated adaptive ­advantage during a humoral immune response by using a precision-tailored DNA excision and ligation process to swap the default constant region gene of the antibody with a new one that has unique effector functions. This secondary diversification of the antibody repertoire is a hallmark of the adaptability of B cells when confronted with environmental and pathogenic challenges. Given that the nucleotide sequence of genes during class switching remains unchanged (genetic constraints), it is logical and necessary therefore, to integrate the adaptability of B cells to an epigenetic state, which is dynamic and can be heritably modulated before, after, or even during an antibody-dependent immune response. Epigenetic regulation encompasses heritable changes that affect function (phenotype) without altering the sequence information embedded in a gene, and include histone, DNA and RNA modifications. Here, we review current literature on how B cells use an epigenetic code language as a means to ensure antibody plasticity in light of pathogenic insults.

Checkpoint kinase 2 is required for efficient immunoglobulin diversification

Maintenance of genome integrity relies on multiple DNA repair pathways as well as on checkpoint regulation. Activation of the checkpoint kinases Chk1 and Chk2 by DNA damage triggers cell cycle arrest and improved DNA repair, or apoptosis in case of excessive damage. Chk1 and Chk2 have been reported to act in a complementary or redundant fashion, depending on the physiological context. During secondary immunoglobulin (Ig) diversification in B lymphocytes, DNA damage is abundantly introduced by activation-induced cytidine deaminase (AID) and processed to mutations in a locus-specific manner by several error-prone DNA repair pathways. We have previously shown that Chk1 negatively regulates Ig somatic hypermutation by promoting error-free homologous recombination and Ig gene conversion. We now report that Chk2 shows opposite effects to Chk1 in the regulation of these processes. Chk2 inactivation in B cells leads to decreased Ig hypermutation and Ig class switching, and increased Ig gene conversion activity. This is linked to defects in non-homologous end joining and increased Chk1 activation upon interference with Chk2 function. Intriguingly, in the context of physiological introduction of substantial DNA damage into the genome during Ig diversification, the 2 checkpoint kinases thus function in an opposing manner, rather than redundantly or cooperatively.

Orientation-specific joining of AID-initiated DNA breaks promotes antibody class switching

During B-cell development, RAG endonuclease cleaves immunoglobulin heavy chain (IgH) V, D, and J gene segments and orchestrates their fusion as deletional events that assemble a V(D)J exon in the same transcriptional orientation as adjacent Cμ constant region exons. In mice, six additional sets of constant region exons (CHs) lie 100-200 kilobases downstream in the same transcriptional orientation as V(D)J and Cμ exons. Long repetitive switch (S) regions precede Cμ and downstream CHs. In mature B cells, class switch recombination (CSR) generates different antibody classes by replacing Cμ with a downstream CH (ref. 2). Activation-induced cytidine deaminase (AID) initiates CSR by promoting deamination lesions within Sμ and a downstream acceptor S region; these lesions are converted into DNA double-strand breaks (DSBs) by general DNA repair factors. Productive CSR must occur in a deletional orientation by joining the upstream end of an Sμ DSB to the downstream end of an acceptor S-region DSB. However, the relative frequency of deletional to inversional CSR junctions has not been measured. Thus, whether orientation-specific joining is a programmed mechanistic feature of CSR as it is for V(D)J recombination and, if so, how this is achieved is unknown. To address this question, we adapt high-throughput genome-wide translocation sequencing into a highly sensitive DSB end-joining assay and apply it to endogenous AID-initiated S-region DSBs in mouse B cells. We show that CSR is programmed to occur in a productive deletional orientation and does so via an unprecedented mechanism that involves in cis Igh organizational features in combination with frequent S-region DSBs initiated by AID. We further implicate ATM-dependent DSB-response factors in enforcing this mechanism and provide an explanation of why CSR is so reliant on the 53BP1 DSB-response factor.

Ectopic expression of RNF168 and 53BP1 increases mutagenic but not physiological non-homologous end joining

DNA double strand breaks (DSBs) formed during S phase are preferentially repaired by homologous recombination (HR), whereas G₁ DSBs, such as those occurring during immunoglobulin class switch recombination (CSR), are repaired by non-homologous end joining (NHEJ). The DNA damage response proteins 53BP1 and BRCA1 regulate the balance between NHEJ and HR. 53BP1 promotes CSR in part by mediating synapsis of distal DNA ends, and in addition, inhibits 5’ end resection. BRCA1 antagonizes 53BP1 dependent DNA end-blocking activity during S phase, which would otherwise promote mutagenic NHEJ and genome instability. Recently, it was shown that supra-physiological levels of the E3 ubiquitin ligase RNF168 results in the hyper-accumulation of 53BP1/BRCA1 which accelerates DSB repair. Here, we ask whether increased expression of RNF168 or 53BP1 impacts physiological versus mutagenic NHEJ. We find that the anti-resection activities of 53BP1 are rate-limiting for mutagenic NHEJ but not for physiological CSR. As heterogeneity in the expression of RNF168 and 53BP1 is found in human tumors, our results suggest that deregulation of the RNF168/53BP1 pathway could alter the chemosensitivity of BRCA1 deficient tumors.

BRCA1 recruitment to transcriptional pause sites is required for R-loop-driven DNA damage repair

The mechanisms contributing to transcription-associated genomic instability are both complex and incompletely understood. Although R-loops are normal transcriptional intermediates, they are also associated with genomic instability. Here, we show that BRCA1 is recruited to R-loops that form normally over a subset of transcription termination regions. There it mediates the recruitment of a specific, physiological binding partner, senataxin (SETX). Disruption of this complex led to R-loop-driven DNA damage at those loci as reflected by adjacent γ-H2AX accumulation and ssDNA breaks within the untranscribed strand of relevant R-loop structures. Genome-wide analysis revealed widespread BRCA1 binding enrichment at R-loop-rich termination regions (TRs) of actively transcribed genes. Strikingly, within some of these genes in BRCA1 null breast tumors, there are specific insertion/deletion mutations located close to R-loop-mediated BRCA1 binding sites within TRs. Thus, BRCA1/SETX complexes support a DNA repair mechanism that addresses R-loop-based DNA damage at transcriptional pause sites.

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Endogenous BRCA1 and senataxin (SETX) interact in a BRCA1-driven process

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BRCA1/SETX complexes are recruited to R-loop-associated termination regions (TRs)

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BRCA1/SETX complexes suppress transcriptional DNA damage arising at nearby R-loops

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BRCA1 breast cancers reveal indel mutations near BRCA1 TR binding regions

Cell cycle regulation of human DNA repair and chromatin remodeling genes

Maintenance of a genome requires DNA repair integrated with chromatin remodeling. We have analyzed six transcriptome data sets and one data set on translational regulation of known DNA repair and remodeling genes in synchronized human cells. These data are available through our new database: www.dnarepairgenes.com. Genes that have similar transcription profiles in at least two of our data sets generally agree well with known protein profiles. In brief, long patch base excision repair (BER) is enriched for S phase genes, whereas short patch BER uses genes essentially equally expressed in all cell cycle phases. Furthermore, most genes related to DNA mismatch repair, Fanconi anemia and homologous recombination have their highest expression in the S phase. In contrast, genes specific for direct repair, nucleotide excision repair, as well as non-homologous end joining do not show cell cycle-related expression. Cell cycle regulated chromatin remodeling genes were most frequently confined to G1/S and S. These include e.g. genes for chromatin assembly factor 1 (CAF-1) major subunits CHAF1A and CHAF1B; the putative helicases HELLS and ATAD2 that both co-activate E2F transcription factors central in G1/S-transition and recruit DNA repair and chromatin-modifying proteins and DNA double strand break repair proteins; and RAD54L and RAD54B involved in double strand break repair. TOP2A was consistently most highly expressed in G2, but also expressed in late S phase, supporting a role in regulating entry into mitosis. Translational regulation complements transcriptional regulation and appears to be a relatively common cell cycle regulatory mechanism for DNA repair genes. Our results identify cell cycle phases in which different pathways have highest activity, and demonstrate that periodically expressed genes in a pathway are frequently co-expressed. Furthermore, the data suggest that S phase expression and over-expression of some multifunctional chromatin remodeling proteins may set up feedback loops driving cancer cell proliferation.

MAD2L2 controls DNA repair at telomeres and DNA breaks by inhibiting 5' end resection

Appropriate repair of DNA lesions and the inhibition of DNA repair activities at telomeres are critical to prevent genomic instability. By fuelling the generation of genetic alterations and by compromising cell viability, genomic instability is a driving force in cancer and aging^{1, 2}. Here we identify MAD2L2 (also known as MAD2B or REV7) through functional genetic screening as a novel factor controlling DNA repair activities at mammalian telomeres. We show that MAD2L2 accumulates at uncapped telomeres and promotes non-homologous end-joining (NHEJ)-mediated fusion of deprotected chromosome ends and genomic instability. MAD2L2 depletion causes elongated 3′ telomeric overhangs, implying that MAD2L2 inhibits 5′ end-resection. End-resection blocks NHEJ while committing to homology-directed repair (HDR) and is under control of 53BP1, RIF1 and PTIP³. Consistent with MAD2L2 promoting NHEJ-mediated telomere fusion by inhibiting 5′ end-resection, knockdown of the nucleases CTIP or EXO1 partially restores telomere-driven genomic instability in MAD2L2-depleted cells. Control of DNA repair by MAD2L2 is not limited to telomeres. MAD2L2 also accumulates and inhibits end-resection at irradiation (IR)-induced DNA double-strand breaks (DSBs) and promotes end-joining of DSBs in multiple settings, including during immunoglobulin class switch recombination (CSR). These activities of MAD2L2 depend on ATM kinase activity, RNF8, RNF168, 53BP1 and RIF1, but not on PTIP, REV1 and REV3, the latter two acting with MAD2L2 in translesion synthesis (TLS)⁴. Together our data establish MAD2L2 as a critical contributor to the control of DNA repair activity by 53BP1 that promotes NHEJ by inhibiting 5′ end-resection downstream of RIF1.

Aberrant recombination and repair during immunoglobulin class switching in BRCA1-deficient human B cells

Breast cancer type 1 susceptibility protein (BRCA1) has a multitude of functions that contribute to genome integrity and tumor suppression. Its participation in the repair of DNA double-strand breaks (DSBs) during homologous recombination (HR) is well recognized, whereas its involvement in the second major DSB repair pathway, nonhomologous end-joining (NHEJ), remains controversial. Here we have studied the role of BRCA1 in the repair of DSBs in switch (S) regions during immunoglobulin class switch recombination, a physiological, deletion/recombination process that relies on the classical NHEJ machinery. A shift to the use of microhomology-based, alternative end-joining (A-EJ) and increased frequencies of intra-S region deletions as well as insertions of inverted S sequences were observed at the recombination junctions amplified from BRCA1-deficient human B cells. Furthermore, increased use of long microhomologies was found at recombination junctions derived from E3 ubiquitin-protein ligase RNF168-deficient, Fanconi anemia group J protein (FACJ, BRIP1)-deficient, or DNA endonuclease RBBP8 (CtIP)-compromised cells, whereas an increased frequency of S-region inversions was observed in breast cancer type 2 susceptibility protein (BRCA2)-deficient cells. Thus, BRCA1, together with its interaction partners, seems to play an important role in repairing DSBs generated during class switch recombination by promoting the classical NHEJ pathway. This may not only provide a general mechanism underlying BRCA1's function in maintaining genome stability and tumor suppression but may also point to a previously unrecognized role of BRCA1 in B-cell lymphomagenesis.

Distinct global DNA methylation status in B-cell lymphomas: immunohistochemical study of 5-methylcytosine and 5-hydroxymethylcytosine

Lymphomas are malignant neoplasms composed of lymphoid cells at various developmental stages and lineages. Recent advances in comprehensive genomic analyses in acute myeloid leukemia have revealed prevalent mutations in regulators of epigenetic phenomena including global DNA methylation status. The examples include mutations in isocitrate dehydrogenase 1 (IDH1), IDH2, and ten-eleven translocation 2. These mutations are proposed to inhibit conversion of 5-methylcytosine (5 mC) to 5-hydroxymethylcytosine (5 hmC), leading to global accumulation of 5 mC. These changes in global DNA methylation status can be visualized immunohistochemically using specific antibodies against 5 mC and 5 hmC. We examined the global DNA methylation status of B-cell lymphomas and that of their normal counterparts by immunohistochemistry for 5 mC and 5 hmC. Non-tumor lymphoid cells inside germinal centers (GC) in reactive lymphoid hyperplasia (RLH) were stained positive for 5 mC, but they were negative for 5 hmC. Similarly, follicular lymphomas, whose postulated normal counterparts are centrocytes in GCs, were 5 mC-positive but 5 hmC-negative by immunohistochemistry. This immunostaining pattern was also observed in Burkitt lymphoma. In contrast, non-tumor lymphoid cells in mantle zones were stained positive for 5 mC as well as for 5 hmC. Likewise, most mantle cell lymphomas, whose postulated normal counterparts are mantle zone B cells in RLH, were stained positive for 5 mC as well as for 5 hmC. This immunostaining pattern was also observed in chronic lymphocytic leukemia/small lymphocytic lymphoma. These results suggest that, in terms of 5 mC/5 hmC immunohistochemistry, B-cell lymphomas with different histological subtypes are associated with distinct global DNA methylation statuses that resemble those of their postulated normal counterparts.

Ubiquitous transcription factors display structural plasticity and diverse functions: NusG proteins - Shifting shapes and paradigms

Numerous accessory factors modulate RNA polymerase response to regulatory signals and cellular cues and establish communications with co-transcriptional RNA processing. Transcription regulators are astonishingly diverse, with similar mechanisms arising via convergent evolution. NusG/Spt5 elongation factors comprise the only universally conserved and ancient family of regulators. They bind to the conserved clamp helices domain of RNA polymerase, which also interacts with non-homologous initiation factors in all domains of life, and reach across the DNA channel to form processivity clamps that enable uninterrupted RNA chain synthesis. In addition to this ubiquitous function, NusG homologs exert diverse, and sometimes opposite, effects on gene expression by competing with each other and other regulators for binding to the clamp helices and by recruiting auxiliary factors that facilitate termination, antitermination, splicing, translation, etc. This surprisingly diverse range of activities and the underlying unprecedented structural changes make studies of these "transformer" proteins both challenging and rewarding.

Inosine in DNA and RNA

Deamination of the nucleobases in DNA and RNA is a result of spontaneous hydrolysis, endogenous or environmental factors as well as deaminase enzymes. Adenosine is deaminated to inosine which is miscoding and preferentially base pairs with cytosine. In the case of DNA, this is a premutagenic event that is counteracted by DNA repair enzymes specifically engaged in recognition and removal of inosine. However, in RNA, inosine is an essential modification introduced by specialized enzymes in a highly regulated manner to generate transcriptome diversity. Defect editing is seen in various human disease including cancer, viral infections and neurological and psychiatric disorders. Enzymes catalyzing the deaminase reaction are well characterized and recently an unexpected function of Endonuclease V in RNA processing was revealed. Whereas bacterial Endonuclease V enzymes are classified as DNA repair enzymes, it appears that the mammalian enzymes are involved in processing of inosine in RNA. This yields an interesting yet unexplored, link between DNA and RNA processing. Further work is needed to gain understanding of the impact of inosine in DNA and RNA under normal physiology and disease progression.

AID and APOBEC deaminases: balancing DNA damage in epigenetics and immunity

DNA mutations and genomic recombinations are the origin of oncogenesis, yet parts of developmental programs as well as immunity are intimately linked to, or even depend on, such DNA damages. Therefore, the balance between deleterious DNA damages and organismal survival utilizing DNA editing (modification and repair) is in continuous flux. The cytosine deaminases AID/APOBEC are a DNA editing family and actively participate in various biological processes. In conjunction with altered DNA repair, the mutagenic potential of the family allows for APOBEC3 proteins to restrict viral infection and transposons propagation, while AID can induce somatic hypermutation and class switch recombination in antibody genes. On the other hand, the synergy between effective DNA repair and the nonmutagenic potential of the DNA deaminases can induce local DNA demethylation to support epigenetic cellular identity. Here, we review the current state of knowledge on the mechanisms of action of the AID/APOBEC family in immunity and epigenetics.

DNA-binding studies of AV-153, an antimutagenic and DNA repair-stimulating derivative of 1,4-dihydropiridine

The ability to intercalate between DNA strands determines the cytotoxic activity of numerous anticancer drugs. Strikingly, intercalating activity was also reported for some compounds considered to be antimutagenic. The aim of this study was to determine the mode of interaction of DNA with the antimutagenic and DNA repair-stimulating dihydropyridine (DHP) AV-153. DNA and AV-153 interactions were studied by means of UV/VIS spectroscopy, fluorimetry and infrared spectroscopy. Compound AV-153 is a 1,4 dihydropyridine with ethoxycarbonyl groups in positions 3 and 5. Computer modeling of AV-153 and DNA interactions suggested an ability of the compound to dock between DNA strands at a single strand break site in the vicinity of two pyrimidines, which was confirmed in the present study. AV-153 evidently interacted with DNA, as addition of DNA to AV-153 solutions resulted in pronounced hyperchromic and bathochromic effects on the spectra. Base modification in a plasmid by peroxynitrite only minimally changed binding affinity of the compound; however, induction of single-strand breaks using Fenton's reaction greatly increased binding affinity. The affinity did not change when the ionic strength of the solution was changed from 5 to 150 mM NaCl, although it increased somewhat at 300 mM. Neither was it influenced by temperature changes from 25 to 40°C, however, it decreased when the pH of the solution was changed from 7.4 to 4.7. AV-153 competed with EBr for intercalation sites in DNA: 116 mM of the compound caused a two-fold decrease in fluorescence intensity. FT-IR spectral data analyses indicated formation of complexes between DNA and AV-153. The second derivative spectra analyses indicated interaction of AV-153 with guanine, cytosine and thymine bases, but no interaction with adenine was detected.

CONCLUSIONS

The antimutagenic substance AV-153 appears to intercalate between the DNA strands at the site of a DNA nick in the vicinity of two pyrimidines.

Generation and repair of AID-initiated DNA lesions in B lymphocytes

Activation-induced deaminase (AID) initiates the secondary antibody diversification process in B lymphocytes. In mammalian B cells, this process includes somatic hypermutation (SHM) and class switch recombination (CSR), both of which require AID. AID induces U:G mismatch lesions in DNA that are subsequently converted into point mutations or DNA double stranded breaks during SHM/CSR. In a physiological context, AID targets immunoglobulin (Ig) loci to mediate SHM/CSR. However, recent studies reveal genome-wide access of AID to numerous non-Ig loci. Thus, AID poses a threat to the genome of B cells if AID-initiated DNA lesions cannot be properly repaired. In this review, we focus on the molecular mechanisms that regulate the specificity of AID targeting and the repair pathways responsible for processing AID-initiated DNA lesions.

Kruppel-associated box (KRAB) proteins in the adaptive immune system

The ability of adaptive immune system to protect higher vertebrates from pathogens resides in the ability of B and T cells to express different antigen specific receptors and to respond to different threats by activating distinct differentiation and/or activation pathways. In the past 10 years, the major role of epigenetics in controlling molecular mechanisms responsible for these peculiar features and, more in general, for lymphocyte development has become evident. KRAB-ZFPs is the widest family of mammalian transcriptional repressors, which function through the recruitment of the co-factor KRAB-Associated Protein 1 (KAP1) that in turn engages histone modifiers inducing heterochromatin formation. Although most of the studies on KRAB proteins have been performed in embryonic cells, more recent reports highlighted a relevant role for these proteins also in adult tissues. This article will review the role of KRAB-ZFP and KAP1 in the epigenetic control of mouse and human adaptive immune cells.