The 9-1-1 DNA clamp is required for immunoglobulin gene conversion

PrimPol-mediated repriming elicits gap-filling by template switching and promotes cellular tolerance to cidofovir

A nucleoside analog, Cidofovir (CDV), is used for the treatment of viral diseases such as cytomegalovirus retinitis and herpes virus infection. CDV converts to its active diphosphate metabolite (CDVpp) through cellular kinases and acts as a competitive inhibitor for viral polymerase thereby interfering with viral replication. However, the effect of this drug on the replication of healthy host cells and the mechanisms involved in the cellular tolerance to CDV are yet to be fully understood. In this study, we explored the mechanisms underlying cellular tolerance to CDV by screening mutant cell lines exhibiting hypersensitivity to CDV from a collection of DT40 mutants deficient in various genome maintenance systems. We identified Rad17 and PrimPol as critical factors for CDV tolerance. We found that Rad17 plays a pivotal role in activating intra-S phase checkpoint by the phosphorylation of Chk1, a vital checkpoint mediator. We showed that PrimPol, a factor involved in the release of stalled replication, plays critical roles in CDV tolerance in tandem with Rad17. We found that PrimPol deficient cells showed slower replication on the CDV-incorporated template strand than did wild-type cells, indicating a critical role of PrimPol in the continuous replication fork progression on the CDV-incorporated damaged template. PrimPol releases replication arrest with its DNA-damage bypass function and its repriming function, we thus investigated which PrimPol function is involved in CDV tolerance using the separation of function mutant genes of PRIMPOL. The CDV hypersensitive phenotype of PrimPol deficient cells was restored by PRIMPOLY89D (primase active / reduced polymerase activity), indicating that the repriming function of PrimPol is required for maintaining replication on the CDV-damaged template. Moreover, we found that the number of sister chromatid exchange (SCE) was reduced in PrimPol-deficient cells. These data indicate that gaps generated by PrimPol-mediated repriming on CDV-damaged templates promote post-replicative gap-filing by template switching.

RAD18- and BRCA1-dependent pathways promote cellular tolerance to the nucleoside analog ganciclovir

Ganciclovir (GCV) is a clinically important drug as it is used to treat viral infections. GCV is incorporated into the DNA during replication, where it interferes with subsequent replication on GCV-incorporated templates. However, the effects of GCV on the host genome and the mechanisms underlying cellular tolerance to GCV remain unclear. In this study, we explored these mechanisms using a collection of mutant DT40 cells. We identified RAD17/-, BRCA1-/-, and RAD18-/- cells as highly GCV-sensitive. RAD17, a component of the alternative checkpoint-clamp loader RAD17-RFC, was required for the activation of the intra-S checkpoint following GCV treatment. BRCA1, a critical factor for promoting homologous recombination (HR), was required for suppressing DNA double-strand breaks (DSBs). Moreover, RAD18, an E3-ligase involved in DNA repair, was critical in suppressing the aberrant ligation of broken chromosomes caused by GCV. We found that BRCA1 suppresses DSBs through HR-mediated repair and template switching (TS)-mediated damage bypass. Moreover, the strong GCV sensitivity of BRCA1-/- cells was rescued by the loss of 53BP1, despite the only partial restoration in the sister chromatid exchange events which are hallmarks of HR. These results indicate that BRCA1 promotes cellular tolerance to GCV through two mechanisms, TS and HR-mediated repair.

REV3 promotes cellular tolerance to 5-fluorodeoxyuridine by activating translesion DNA synthesis and intra-S checkpoint

The drug floxuridine (5-fluorodeoxyuridine, FUdR) is an active metabolite of 5-Fluorouracil (5-FU). It converts to 5-fluorodeoxyuridine monophosphate (FdUMP) and 5-fluorodeoxyuridine triphosphate (FdUTP), which on incorporation into the genome inhibits DNA replication. Additionally, it inhibits thymidylate synthase, causing dTMP shortage while increasing dUMP availability, which induces uracil incorporation into the genome. However, the mechanisms underlying cellular tolerance to FUdR are yet to be fully elucidated. In this study, we explored the mechanisms underlying cellular resistance to FUdR by screening for FUdR hypersensitive mutants from a collection of DT40 mutants deficient in each genomic maintenance system. We identified REV3, which is involved in translesion DNA synthesis (TLS), to be a critical factor in FUdR tolerance. Replication using a FUdR-damaged template was attenuated in REV3-/- cells, indicating that the TLS function of REV3 is required to maintain replication on the FUdR-damaged template. Notably, FUdR-exposed REV3-/- cells exhibited defective cell cycle arrest in the early S phase, suggesting that REV3 is involved in intra-S checkpoint activation. Furthermore, REV3-/- cells showed defects in Chk1 phosphorylation, which is required for checkpoint activation, but the survival of FUdR-exposed REV3-/- cells was further reduced by the inhibition of Chk1 or ATR. These data indicate that REV3 mediates DNA checkpoint activation at least through Chk1 phosphorylation, but this signal acts in parallel with ATR-Chk1 DNA damage checkpoint pathway. Collectively, we reveal a previously unappreciated role of REV3 in FUdR tolerance.

Dominant roles of BRCA1 in cellular tolerance to a chain-terminating nucleoside analog, alovudine

Alovudine is a chain-terminating nucleoside analog (CTNA) that is frequently used as an antiviral and anticancer agent. Generally, CTNAs inhibit DNA replication after their incorporation into nascent DNA during DNA synthesis by suppressing subsequent polymerization, which restricts the proliferation of viruses and cancer cells. Alovudine is a thymidine analog used as an antiviral drug. However, the mechanisms underlying the removal of alovudine and DNA damage tolerance pathways involved in cellular resistance to alovudine remain unclear. Here, we explored the DNA damage tolerance pathways responsible for cellular tolerance to alovudine and found that BRCA1-deficient cells exhibited the highest sensitivity to alovudine. Moreover, alovudine interfered with DNA replication in two distinct mechanisms: first: alovudine incorporated at the end of nascent DNA interfered with subsequent DNA synthesis; second: DNA replication stalled on the alovudine-incorporated template strand. Additionally, BRCA1 facilitated the removal of the incorporated alovudine from nascent DNA, and BRCA1-mediated homologous recombination (HR) contributed to the progressive replication on the alovudine-incorporated template. Thus, we have elucidated the previously unappreciated mechanism of alovudine-mediated inhibition of DNA replication and the role of BRCA1 in cellular tolerance to alovudine.

Fast-tracking antibody maturation using a B cell-based display system

Affinity maturation, an essential component of antibody engineering, is crucial for developing therapeutic antibodies. Cell display system coupled with somatic hypermutation (SHM) initiated by activation-induced cytidine deaminase (AID) is a commonly used technique for affinity maturation. AID introduces targeted DNA lesions into hotspots of immunoglobulin (Ig) gene loci followed by erroneous DNA repair, leading to biased mutations in the complementary determining regions. However, systems that use an in vivo mimicking mechanism often require several rounds of selection to enrich clones possessing accumulated mutations. We previously described the human ADLib® system, which features autonomous, AID-mediated diversification in Ig gene loci of a chicken B cell line DT40 and streamlines human antibody generation and optimization in one integrated platform. In this study, we further engineered DT40 capable of receiving exogenous antibody genes and examined whether the antibody could be affinity matured. The Ig genes of three representative anti-hVEGF-A antibodies originating from the human ADLib® were introduced; the resulting human IgG1 antibodies had up to 76.4-fold improvement in binding affinities (sub-picomolar K_D) within just one round of optimization, owing to efficient accumulation of functional mutations. Moreover, we successfully improved the affinity of a mouse hybridoma-derived anti-hCDCP1 antibody using the engineered DT40, and the observed mutations remained effective in the post-humanized antibody as exhibited by an 8.2-fold increase of in vitro cytotoxicity without compromised physical stability. These results demonstrated the versatility of the novel B cell-based affinity maturation system as an easy-to-use antibody optimization tool regardless of the species of origin.Abbreviations: ADLib®: Autonomously diversifying library, ADLib® KI-AMP: ADLib® knock-in affinity maturation platform, AID: activation-induced cytidine deaminase, CDRs: complementary-determining regions, DIVAC: diversification activator, ECD: extracellular domain, FACS: fluorescence-activated cell sorting, FCM: flow cytometry, HC: heavy chainIg: immunoglobulin, LC: light chain, NGS: next-generation sequencing, PBD: pyrrolobenzodiazepine, SHM: somatic hypermutation, SPR: surface plasmon resonance.

DNA Damage Tolerance Mechanisms Revealed from the Analysis of Immunoglobulin V Gene Diversification in Avian DT40 Cells

DNA replication is an essential biochemical reaction in dividing cells that frequently stalls at damaged sites. Homologous/homeologous recombination (HR)-mediated template switch and translesion DNA synthesis (TLS)-mediated bypass processes release arrested DNA replication forks. These mechanisms are pivotal for replication fork maintenance and play critical roles in DNA damage tolerance (DDT) and gap-filling. The avian DT40 B lymphocyte cell line provides an opportunity to examine HR-mediated template switch and TLS triggered by abasic sites by sequencing the constitutively diversifying immunoglobulin light-chain variable gene (IgV). During IgV diversification, activation-induced deaminase (AID) converts dC to dU, which in turn is excised by uracil DNA glycosylase and yields abasic sites within a defined window of around 500 base pairs. These abasic sites can induce gene conversion with a set of homeologous upstream pseudogenes via the HR-mediated template switch, resulting in templated mutagenesis, or can be bypassed directly by TLS, resulting in non-templated somatic hypermutation at dC/dG base pairs. In this review, we discuss recent works unveiling IgV diversification mechanisms in avian DT40 cells, which shed light on DDT mode usage in vertebrate cells and tolerance of abasic sites.

SUMOylation of PCNA by PIAS1 and PIAS4 promotes template switch in the chicken and human B cell lines

DNA damage tolerance (DDT) releases replication blockage caused by damaged nucleotides on template strands employing two alternative pathways, error-prone translesion DNA synthesis (TLS) and error-free template switch (TS). Lys164 of proliferating cell nuclear antigen (PCNA) is SUMOylated during the physiological cell cycle. To explore the role for SUMOylation of PCNA in DDT, we characterized chicken DT40 and human TK6 B cells deficient in the PIAS1 and PIAS4 small ubiquitin-like modifier (SUMO) E3 ligases. DT40 cells have a unique advantage in the phenotypic analysis of DDT as they continuously diversify their immunoglobulin (Ig) variable genes by TLS and TS [Ig gene conversion (GC)], both relieving replication blocks at abasic sites without accompanying by DNA breakage. Remarkably, PIAS1 ^-/- /PIAS4 ^-/- cells displayed a multifold decrease in SUMOylation of PCNA at Lys164 and over a 90% decrease in the rate of TS. Likewise, PIAS1 ^-/- /PIAS4 ^-/- TK6 cells showed a shift of DDT from TS to TLS at a chemosynthetic UV lesion inserted into the genomic DNA. The PCNA ^K164R/K164R mutation caused a ∼90% decrease in the rate of Ig GC and no additional impact on PIAS1 ^-/- /PIAS4 ^-/- cells. This epistatic relationship between the PCNA ^K164R/K164R and the PIAS1 ^-/- /PIAS4 ^-/- mutations suggests that PIAS1 and PIAS4 promote TS mainly through SUMOylation of PCNA at Lys164. This idea is further supported by the data that overexpression of a PCNA-SUMO1 chimeric protein restores defects in TS in PIAS1 ^-/- /PIAS4 ^-/- cells. In conclusion, SUMOylation of PCNA at Lys164 promoted by PIAS1 and PIAS4 ensures the error-free release of replication blockage during physiological DNA replication in metazoan cells.

Warsaw breakage syndrome DDX11 helicase acts jointly with RAD17 in the repair of bulky lesions and replication through abasic sites

Warsaw breakage syndrome, a developmental disorder caused by mutations in the conserved DDX11/ChlR1 DNA helicase, shows features of genome instability partly overlapping with those of Fanconi anemia (FA). Here, using avian cellular models of DDX11 deficiency, we find that DDX11 functions as backup to the FA pathway and facilitates, jointly with the checkpoint clamp 9-1-1, a homologous recombination pathway of DNA bulky-lesion repair that does not affect replication fork speed and stalled fork stability. DDX11 also promotes diversification of the immunoglobulin-variable gene locus by facilitating hypermutation and gene conversion at programmed abasic sites that constitute endogenous replication blocks. The results suggest commonality between postreplicative gap filling and replication through abasic sites and pinpoint DDX11 as a critical player in both these processes.

Warsaw breakage syndrome, a developmental disorder caused by mutations in the DDX11/ChlR1 helicase, shows cellular features of genome instability similar to Fanconi anemia (FA). Here we report that DDX11-deficient avian DT40 cells exhibit interstrand crosslink (ICL)-induced chromatid breakage, with DDX11 functioning as backup for the FA pathway in regard to ICL repair. Importantly, we establish that DDX11 acts jointly with the 9-1-1 checkpoint clamp and its loader, RAD17, primarily in a postreplicative fashion, to promote homologous recombination repair of bulky lesions, but is not required for intra-S checkpoint activation or efficient fork progression. Notably, we find that DDX11 also promotes diversification of the chicken Ig-variable gene, a process triggered by programmed abasic sites, by facilitating both hypermutation and homeologous recombination-mediated gene conversion. Altogether, our results uncover that DDX11 orchestrates jointly with 9-1-1 and its loader, RAD17, DNA damage tolerance at sites of bulky lesions, and endogenous abasic sites. These functions may explain the essential roles of DDX11 and its similarity with 9-1-1 during development.

Functions of Multiple Clamp and Clamp-Loader Complexes in Eukaryotic DNA Replication

Proliferating cell nuclear antigen (PCNA) and replication factor C (RFC) were identified in the late 1980s as essential factors for replication of simian virus 40 DNA in human cells, by reconstitution of the reaction in vitro. Initially, they were only thought to be involved in the elongation stage of DNA replication. Subsequent studies have demonstrated that PCNA functions as more than a replication factor, through its involvement in multiple protein-protein interactions. PCNA appears as a functional hub on replicating and replicated chromosomal DNA and has an essential role in the maintenance genome integrity in proliferating cells.Eukaryotes have multiple paralogues of sliding clamp, PCNA and its loader, RFC. The PCNA paralogues, RAD9, HUS1, and RAD1 form the heterotrimeric 9-1-1 ring that is similar to the PCNA homotrimeric ring, and the 9-1-1 clamp complex is loaded onto sites of DNA damage by its specific loader RAD17-RFC. This alternative clamp-loader system transmits DNA-damage signals in genomic DNA to the checkpoint-activation network and the DNA-repair apparatus.Another two alternative loader complexes, CTF18-RFC and ELG1-RFC, have roles that are distinguishable from the role of the canonical loader, RFC. CTF18-RFC interacts with one of the replicative DNA polymerases, Polε, and loads PCNA onto leading-strand DNA, and ELG1-RFC unloads PCNA after ligation of lagging-strand DNA. In the progression of S phase, these alternative PCNA loaders maintain appropriate amounts of PCNA on the replicating sister DNAs to ensure that specific enzymes are tethered at specific chromosomal locations.

Repriming by PrimPol is critical for DNA replication restart downstream of lesions and chain-terminating nucleosides

PrimPol is a DNA damage tolerance enzyme possessing both translesion synthesis (TLS) and primase activities. To uncover its potential role in TLS-mediated IgVλ hypermutation and define its interplay with other TLS polymerases, PrimPol(-/-) and PrimPol(-/-)/Polη(-/-)/Polζ (-/-) gene knockouts were generated in avian cells. Loss of PrimPol had no significant impact on the rate of hypermutation or the mutation spectrum of IgVλ. However, PrimPol(-/-) cells were sensitive to methylmethane sulfonate, suggesting that it may bypass abasic sites at the IgVλ segment by repriming DNA synthesis downstream of these sites. PrimPol(-/-) cells were also sensitive to cisplatin and hydroxyurea, indicating that it assists in maintaining / restarting replication at a variety of lesions. To accurately measure the relative contribution of the TLS and primase activities, we examined DNA damage sensitivity in PrimPol(-/-) cells complemented with polymerase or primase-deficient PrimPol. Polymerase-defective, but not primase-deficient, PrimPol suppresses the hypersensitivity of PrimPol(-/-) cells. This indicates that its primase, rather than TLS activity, is pivotal for DNA damage tolerance. Loss of TLS polymerases, Polη and Polζ has an additive effect on the sensitivity of PrimPol(-/-) cells. Moreover, we found that PrimPol and Polη-Polζ redundantly prevented cell death and facilitated unperturbed cell cycle progression. PrimPol(-/-) cells also exhibited increased sensitivity to a wide variety of chain-terminating nucleoside analogs (CTNAs). PrimPol could perform close-coupled repriming downstream of CTNAs and oxidative damage in vitro. Together, these results indicate that PrimPol's repriming activity plays a central role in reinitiating replication downstream from CTNAs and other specific DNA lesions.

In vivo evidence for translesion synthesis by the replicative DNA polymerase δ

The intolerance of DNA polymerase δ (Polδ) to incorrect base pairing contributes to its extremely high accuracy during replication, but is believed to inhibit translesion synthesis (TLS). However, chicken DT40 cells lacking the POLD3 subunit of Polδ are deficient in TLS. Previous genetic and biochemical analysis showed that POLD3 may promote lesion bypass by Polδ itself independently of the translesion polymerase Polζ of which POLD3 is also a subunit. To test this hypothesis, we have inactivated Polδ proofreading in pold3 cells. This significantly restored TLS in pold3 mutants, enhancing dA incorporation opposite abasic sites. Purified proofreading-deficient human Polδ holoenzyme performs TLS of abasic sites in vitro much more efficiently than the wild type enzyme, with over 90% of TLS events resulting in dA incorporation. Furthermore, proofreading deficiency enhances the capability of Polδ to continue DNA synthesis over UV lesions both in vivo and in vitro. These data support Polδ contributing to TLS in vivo and suggest that the mutagenesis resulting from loss of Polδ proofreading activity may in part be explained by enhanced lesion bypass.

The role of HERC2 and RNF8 ubiquitin E3 ligases in the promotion of translesion DNA synthesis in the chicken DT40 cell line

The replicative DNA polymerases are generally blocked by template DNA damage. The resulting replication arrest can be released by one of two post-replication repair (PRR) pathways, translesion DNA synthesis (TLS) and template switching by homologous recombination (HR). The HERC2 ubiquitin ligase plays a role in homologous recombination by facilitating the assembly of the Ubc13 ubiquitin-conjugating enzyme with the RNF8 ubiquitin ligase. To explore the role of HERC2 and RNF8 in PRR, we examined immunoglobulin diversification in chicken DT40 cells deficient in HERC2 and RNF8. Unexpectedly, the HERC2(-/-) and RNF8(-/-) cells and HERC2(-/-)/RNF8(-/-) double mutant cells exhibit a significant reduction in the rate of immunoglobulin (Ig) hypermutation, compared to wild-type cells. Further, the HERC2(-/-) and RNF8(-/-) mutants exhibit defective maintenance of replication fork progression immediately after exposure to UV while retaining proficient post-replicative gap filling. These mutants are both proficient in mono-ubiquitination of PCNA. Taken together, these results suggest that HERC2 and RNF8 promote TLS past abasic sites and UV-lesions at or very close to stalled replication forks.

Abacavir, an anti-HIV-1 drug, targets TDP1-deficient adult T cell leukemia

Adult T cell leukemia (ATL) is an aggressive T cell malignancy caused by human T cell leukemia virus type 1 (HTLV-1) and has a poor prognosis. We analyzed the cytotoxic effects of various nucleoside analog reverse transcriptase inhibitors (NRTIs) for HIV-1 on ATL cells and found that abacavir potently and selectively kills ATL cells. Although NRTIs have minimal genotoxicities on host cells, the therapeutic concentration of abacavir induced numerous DNA double-strand breaks (DSBs) in the chromosomal DNA of ATL cells. DSBs persisted over time in ATL cells but not in other cell lines, suggesting impaired DNA repair. We found that the reduced expression of tyrosyl-DNA phosphodiesterase 1 (TDP1), a repair enzyme, is attributable to the cytotoxic effect of abacavir on ATL cells. We also showed that TDP1 removes abacavir from DNA ends in vitro. These results suggest a model in which ATL cells with reduced TDP1 expression are unable to excise abacavir incorporated into genomic DNA, leading to irreparable DSBs. On the basis of the above mechanism, we propose abacavir as a promising chemotherapeutic agent for ATL.

The POLD3 subunit of DNA polymerase δ can promote translesion synthesis independently of DNA polymerase ζ

The replicative DNA polymerase Polδ consists of a catalytic subunit POLD1/p125 and three regulatory subunits POLD2/p50, POLD3/p66 and POLD4/p12. The ortholog of POLD3 in Saccharomyces cerevisiae, Pol32, is required for a significant proportion of spontaneous and UV-induced mutagenesis through its additional role in translesion synthesis (TLS) as a subunit of DNA polymerase ζ. Remarkably, chicken DT40 B lymphocytes deficient in POLD3 are viable and able to replicate undamaged genomic DNA with normal kinetics. Like its counterpart in yeast, POLD3 is required for fully effective TLS, its loss resulting in hypersensitivity to a variety of DNA damaging agents, a diminished ability to maintain replication fork progression after UV irradiation and a significant decrease in abasic site-induced mutagenesis in the immunoglobulin loci. However, these defects appear to be largely independent of Polζ, suggesting that POLD3 makes a significant contribution to TLS independently of Polζ in DT40 cells. Indeed, combining polη, polζ and pold3 mutations results in synthetic lethality. Additionally, we show in vitro that POLD3 promotes extension beyond an abasic by the Polδ holoenzyme suggesting that while POLD3 is not required for normal replication, it may help Polδ to complete abasic site bypass independently of canonical TLS polymerases.

Rad17, the clamp loader that loads more than clamps

Rad17 is best known as a checkpoint clamp loader in the activation of ATR kinase signaling. A new study in The EMBO Journal suggests that it also plays a role in initial recruitment of the MRN complex to sites of DNA double-strand breaks, thereby promoting early ATM checkpoint responses and homologous recombination repair.

Clamping down on mammalian meiosis

The RAD9A-RAD1-HUS1 (9-1-1) complex is a PCNA-like heterotrimeric clamp that binds damaged DNA to promote cell cycle checkpoint signaling and DNA repair. While various 9-1-1 functions in mammalian somatic cells have been established, mounting evidence from lower eukaryotes predicts critical roles in meiotic germ cells as well. This was investigated in 2 recent studies in which the 9-1-1 complex was disrupted specifically in the mouse male germline through conditional deletion of Rad9a or Hus1. Loss of these clamp subunits led to severely impaired fertility and meiotic defects, including faulty DNA double-strand break repair. While 9-1-1 is critical for ATR kinase activation in somatic cells, these studies did not reveal major defects in ATR checkpoint pathway signaling in meiotic cells. Intriguingly, this new work identified separable roles for 9-1-1 subunits, namely RAD9A- and HUS1-independent roles for RAD1. Based on these studies and the high-level expression of the paralogous proteins RAD9B and HUS1B in testis, we propose a model in which multiple alternative 9-1-1 clamps function during mammalian meiosis to ensure genome maintenance in the germline.

Establishment of an in vitro system representing the chicken gut-associated lymphoid tissue

The bursa of Fabricius is critical for B cell development and differentiation in chick embryos. This study describes the production in vitro, from dissociated cell suspensions, of cellular agglomerates with functional similarities to the chicken bursa. Co-cultivation of epithelial and lymphoid cells obtained from embryos at the appropriate developmental stage regularly led to agglomerate formation within 48 hours. These agglomerates resembled bursal tissue in having lymphoid clusters overlaid by well organized epithelium. Whereas lymphocytes within agglomerates were predominantly Bu-1a⁺, a majority of those emigrating onto the supporting membrane were Bu-1a⁻ and IgM⁺. Both agglomerates and emigrant cells expressed activation-induced deaminase with levels increasing after 24 hours. Emigrating cells were actively proliferating at a rate in excess of both the starting cell population and the population of cells remaining in agglomerates. The potential usefulness of this system for investigating the response of bursal tissue to avian Newcastle disease virus (strain AF2240) was examined.

Topoisomerase degradation, DSB repair, p53 and IAPs in cancer cell resistance to camptothecin-like topoisomerase I inhibitors

Topoisomerase I (TOP1) inhibitors applied in cancer therapy such as topotecan and irinotecan are derivatives of the natural alkaloid camptothecin (CPT). The mechanism of CPT poisoning of TOP1 rests on inhibition of the re-ligation function of the enzyme resulting in the stabilization of the TOP1-cleavable complex. In the presence of CPTs this enzyme-DNA complex impairs transcription and DNA replication, resulting in fork stalling and the formation of DNA double-strand breaks (DSB) in proliferating cells. As with most chemotherapeutics, intrinsic and acquired drug resistance represents a hurdle that limits the success of CPT therapy. Preclinical data indicate that resistance to CPT-based drugs might be caused by factors such as (a) poor drug accumulation in the tumor, (b) high rate of drug efflux, (c) mutations in TOP1 leading to failure in CPT docking, or (d) altered signaling triggered by the drug-TOP1-DNA complex, (e) expression of DNA repair proteins, and (f) failure to activate cell death pathways. This review will focus on the issues (d-f). We discuss degradation of TOP1 as part of the repair pathway in the processing of TOP1 associated DNA damage, give a summary of proteins involved in repair of CPT-induced replication mediated DSB, and highlight the role of p53 and inhibitors of apoptosis proteins (IAPs), particularly XIAP and survivin, in cancer cell resistance to CPT-like chemotherapeutics.

Does DNA repair occur during somatic hypermutation?

Activation-induced deaminase (AID) initiates a flood of DNA damage in the immunoglobulin loci, leading to abasic sites, single-strand breaks and mismatches. It is compelling that some proteins in the canonical base excision and mismatch repair pathways have been hijacked to increase mutagenesis during somatic hypermutation. Thus, the AID-induced mutagenic pathways involve a mix of DNA repair proteins and low fidelity DNA polymerases to create antibody diversity. In this review, we analyze the roles of base excision repair, mismatch repair, and mutagenesis during somatic hypermutation of rearranged variable genes. The emerging view is that faithful base excision repair occurs simultaneously with mutagenesis, whereas faithful mismatch repair is mostly absent.

Competition between PARP-1 and Ku70 control the decision between high-fidelity and mutagenic DNA repair

Affinity maturation of antibodies requires a unique process of targeted mutation that allows changes to accumulate in the antibody genes while the rest of the genome is protected from off-target mutations that can be oncogenic. This targeting requires that the same deamination event be repaired either by a mutagenic or a high-fidelity pathway depending on the genomic location. We have previously shown that the BRCT domain of the DNA-damage sensor PARP-1 is required for mutagenic repair occurring in the context of IgH and IgL diversification in the chicken B cell line DT40. Here we show that immunoprecipitation of the BRCT domain of PARP-1 pulls down Ku70 and the DNA-PK complex although the BRCT domain of PARP-1 does not bind DNA, suggesting that this interaction is not DNA dependent. Through sequencing the IgL variable region in PARP-1(-/-) cells that also lack Ku70 or Lig4, we show that Ku70 or Lig4 deficiency restores GCV to PARP-1(-/-) cells and conclude that the mechanism by which PARP-1 is promoting mutagenic repair is by inhibiting high-fidelity repair which would otherwise be mediated by Ku70 and Lig4.

Simultaneous disruption of two DNA polymerases, Polη and Polζ, in Avian DT40 cells unmasks the role of Polη in cellular response to various DNA lesions

Replicative DNA polymerases are frequently stalled by DNA lesions. The resulting replication blockage is released by homologous recombination (HR) and translesion DNA synthesis (TLS). TLS employs specialized TLS polymerases to bypass DNA lesions. We provide striking in vivo evidence of the cooperation between DNA polymerase η, which is mutated in the variant form of the cancer predisposition disorder xeroderma pigmentosum (XP-V), and DNA polymerase ζ by generating POLη^−/−/POLζ^−/− cells from the chicken DT40 cell line. POLζ^−/− cells are hypersensitive to a very wide range of DNA damaging agents, whereas XP-V cells exhibit moderate sensitivity to ultraviolet light (UV) only in the presence of caffeine treatment and exhibit no significant sensitivity to any other damaging agents. It is therefore widely believed that Polη plays a very specific role in cellular tolerance to UV-induced DNA damage. The evidence we present challenges this assumption. The phenotypic analysis of POLη^−/−/POLζ^−/− cells shows that, unexpectedly, the loss of Polη significantly rescued all mutant phenotypes of POLζ^−/− cells and results in the restoration of the DNA damage tolerance by a backup pathway including HR. Taken together, Polη contributes to a much wide range of TLS events than had been predicted by the phenotype of XP-V cells.

DNA replication is a fragile biochemical reaction, as the replicative DNA polymerases are readily stalled by DNA lesions. The resulting replication blockage is released by translesion DNA synthesis (TLS), which employs specialized TLS polymerases to bypass DNA lesions. There are at least seven TLS polymerases known in vertebrates. However, how they cooperate in vivo remains one of central questions in the field. We analyzed this functional interaction by genetically disrupting two of major TLS polymerases, Polη and Polζ, in the unique genetic model organism, chicken DT40 cells. Currently, it is widely believed that Polη plays a very specific role in cellular tolerance to ultraviolet light–induced DNA damage. Polζ, on the other hand, plays a key role in cellular tolerance to a very wide range of DNA–damaging agents, as POLζ^−/− cells are hypersensitivity to a number of DNA damaging agents. Our phenotypic analysis of POLη^−/−/POLζ^−/− cells shows that, unexpectedly, the loss of Polη significantly rescued all mutant phenotypes of POLζ^−/− cells. The genetic interaction shown here reveals a previously unappreciated role of human Polη in cellular response to a wide variety of DNA lesions and two-step collaborative action of Polymerase η and ζ.

The BRCT domain of PARP-1 is required for immunoglobulin gene conversion

During affinity maturation, genomic integrity is maintained through specific targeting of DNA mutations. The DNA damage sensor PARP-1 helps determine whether a DNA lesion results in faithful or mutagenic repair.

Genetic variation at immunoglobulin (Ig) gene variable regions in B-cells is created through a multi-step process involving deamination of cytosine bases by activation-induced cytidine deaminase (AID) and their subsequent mutagenic repair. To protect the genome from dangerous, potentially oncogenic effects of off-target mutations, both AID activity and mutagenic repair are targeted specifically to the Ig genes. However, the mechanisms of targeting are unknown and recent data have highlighted the role of regulating mutagenic repair to limit the accumulation of somatic mutations resulting from the more widely distributed AID-induced lesions to the Ig genes. Here we investigated the role of the DNA damage sensor poly-(ADPribose)-polymerase-1 (PARP-1) in the repair of AID-induced DNA lesions. We show through sequencing of the diversifying Ig genes in PARP-1^−/− DT40 B-cells that PARP-1 deficiency results in a marked reduction in gene conversion events and enhanced high-fidelity repair of AID-induced lesions at both Ig heavy and light chains. To further characterize the role of PARP-1 in the mutagenic repair of AID-induced lesions, we performed functional analyses comparing the role of engineered PARP-1 variants in high-fidelity repair of DNA damage induced by methyl methane sulfonate (MMS) and the mutagenic repair of lesions at the Ig genes induced by AID. This revealed a requirement for the previously uncharacterized BRCT domain of PARP-1 to reconstitute both gene conversion and a normal rate of somatic mutation at Ig genes, while being dispensable for the high-fidelity base excision repair. From these data we conclude that the BRCT domain of PARP-1 is required to initiate a significant proportion of the mutagenic repair specific to diversifying antibody genes. This role is distinct from the known roles of PARP-1 in high-fidelity DNA repair, suggesting that the PARP-1 BRCT domain has a specialized role in assembling mutagenic DNA repair complexes involved in antibody diversification.

To produce a limitless diversity of antibodies within the constraints of a finite genome, activated B cells introduce random mutations into antibody genes through a process of targeted DNA damage and subsequent mutagenic repair. At the same time, the rest of the genome must be protected from mutagenesis to prevent off-target mutations which can lead to the development of lymphoma or leukemia. How antibody genes are specifically targeted is still largely unknown. A potential player in this process is the DNA-damage-sensing enzyme PARP-1, which recruits DNA repair enzymes to sites of damage. Using a chicken B cell lymphoma cell line because it has only a single PARP isoform and constitutively mutates its antibody genes, we compared the types of mutations accumulated in PARP-1^−/− cells to wild type. We found that in cells lacking PARP-1, the major pathway of mutagenic repair was disrupted and fewer mutations than normal were introduced into their antibody genes. To identify what might be important for mutagenesis, we tested different factors for their ability to rescue this mutagenic deficiency and found a role for the BRCT (BRCA1 C-terminal) domain of PARP-1, a consensus protein domain known to be involved in directing protein-protein interactions. Our evidence suggests that PARP-1 may be interacting with another hypothetical protein via its BRCT domain that is required for the mutagenic rather than faithful repair of DNA lesions in the antibody genes.

DNA polymerases nu and theta are required for efficient immunoglobulin V gene diversification in chicken

The chicken DT40 B lymphocyte line diversifies its immunoglobulin (Ig) V genes through translesion DNA synthesis-dependent point mutations (Ig hypermutation) and homologous recombination (HR)-dependent Ig gene conversion. The error-prone biochemical characteristic of the A family DNA polymerases Polnu and Pol led us to explore the role of these polymerases in Ig gene diversification in DT40 cells. Disruption of both polymerases causes a significant decrease in Ig gene conversion events, although POLN(-/-)/POLQ(-/-) cells exhibit no prominent defect in HR-mediated DNA repair, as indicated by no increase in sensitivity to camptothecin. Poleta has also been previously implicated in Ig gene conversion. We show that a POLH(-/-)/POLN(-/-)/POLQ(-/-) triple mutant displays no Ig gene conversion and reduced Ig hypermutation. Together, these data define a role for Polnu and Pol in recombination and suggest that the DNA synthesis associated with Ig gene conversion is accounted for by three specialized DNA polymerases.

Bloom DNA helicase facilitates homologous recombination between diverged homologous sequences

Bloom syndrome caused by inactivation of the Bloom DNA helicase (Blm) is characterized by increases in the level of sister chromatid exchange, homologous recombination (HR) associated with cross-over. It is therefore believed that Blm works as an anti-recombinase. Meanwhile, in Drosophila, DmBlm is required specifically to promote the synthesis-dependent strand anneal (SDSA), a type of HR not associating with cross-over. However, conservation of Blm function in SDSA through higher eukaryotes has been a matter of debate. Here, we demonstrate the function of Blm in SDSA type HR in chicken DT40 B lymphocyte line, where Ig gene conversion diversifies the immunoglobulin V gene through intragenic HR between diverged homologous segments. This reaction is initiated by the activation-induced cytidine deaminase enzyme-mediated uracil formation at the V gene, which in turn converts into abasic site, presumably leading to a single strand gap. Ig gene conversion frequency was drastically reduced in BLM(-/-) cells. In addition, BLM(-/-) cells used limited donor segments harboring higher identity compared with other segments in Ig gene conversion event, suggesting that Blm can promote HR between diverged sequences. To further understand the role of Blm in HR between diverged homologous sequences, we measured the frequency of gene targeting induced by an I-SceI-endonuclease-mediated double-strand break. BLM(-/-) cells showed a severer defect in the gene targeting frequency as the number of heterologous sequences increased at the double-strand break site. Conversely, the overexpression of Blm, even an ATPase-defective mutant, strongly stimulated gene targeting. In summary, Blm promotes HR between diverged sequences through a novel ATPase-independent mechanism.

Genetic evidence for single-strand lesions initiating Nbs1-dependent homologous recombination in diversification of Ig v in chicken B lymphocytes

Homologous recombination (HR) is initiated by DNA double-strand breaks (DSB). However, it remains unclear whether single-strand lesions also initiate HR in genomic DNA. Chicken B lymphocytes diversify their Immunoglobulin (Ig) V genes through HR (Ig gene conversion) and non-templated hypermutation. Both types of Ig V diversification are initiated by AID-dependent abasic-site formation. Abasic sites stall replication, resulting in the formation of single-stranded gaps. These gaps can be filled by error-prone DNA polymerases, resulting in hypermutation. However, it is unclear whether these single-strand gaps can also initiate Ig gene conversion without being first converted to DSBs. The Mre11-Rad50-Nbs1 (MRN) complex, which produces 3′ single-strand overhangs, promotes the initiation of DSB-induced HR in yeast. We show that a DT40 line expressing only a truncated form of Nbs1 (Nbs1^p70) exhibits defective HR-dependent DSB repair, and a significant reduction in the rate—though not the fidelity—of Ig gene conversion. Interestingly, this defective gene conversion was restored to wild type levels by overproduction of Escherichia coli SbcB, a 3′ to 5′ single-strand–specific exonuclease, without affecting DSB repair. Conversely, overexpression of chicken Exo1 increased the efficiency of DSB-induced gene-targeting more than 10-fold, with no effect on Ig gene conversion. These results suggest that Ig gene conversion may be initiated by single-strand gaps rather than by DSBs, and, like SbcB, the MRN complex in DT40 may convert AID-induced lesions into single-strand gaps suitable for triggering HR. In summary, Ig gene conversion and hypermutation may share a common substrate—single-stranded gaps. Genetic analysis of the two types of Ig V diversification in DT40 provides a unique opportunity to gain insight into the molecular mechanisms underlying the filling of gaps that arise as a consequence of replication blocks at abasic sites, by HR and error-prone polymerases.

An important class of chemotherapeutic drugs used in the treatment of cancer induces DNA damage that interferes with DNA replication. The resulting block to replication results in the formation of single-strand gaps in DNA. These gaps can be filled by specialized DNA polymerases, a process associated with the introduction of mutations or by recombination with an undamaged segment of DNA with an identical or similar sequence. Our work shows that diversification of the antibody genes in the chicken B cell line DT40, which is initiated by localized replication-stalling DNA damage, proceeds by formation of a single-strand intermediate. These gaps are generated by the action of a specific nuclease complex, comprising the Mre11, Rad50, and Nbs1 proteins, which have previously been implicated in the initiation of homologous recombination from double-strand breaks. However, in this context, their dysfunction can be reversed by the expression of a bacterial single-strand–specific nuclease, SbcB. Antibody diversification in DT40 thus provides an excellent model for studying the process of replication-stalling DNA damage and will allow a more detailed understanding of the mechanisms underlying gap repair and cellular tolerance of chemotherapeutic agents.