Quality assessment of LNP-RNA therapeutics with orthogonal analytical techniques

The availability of analytical methods for the characterization of lipid nanoparticles (LNPs) for in-vivo intracellular delivery of nucleic acids is critical for the fast development of innovative RNA therapies. In this study, analytical protocols to measure (i) chemical composition, (ii) drug loading, (iii) particle size, concentration, and stability as well as (iv) structure and morphology were evaluated and compared based on a comprehensive characterization strategy linking key physical and chemical properties to in-vitro efficacy and toxicity. Furthermore, the measurement protocols were assessed either by testing the reproducibility and robustness of the same technique in different laboratories, or by a correlative approach, comparing measurement results of the same attribute with orthogonal techniques. The characterization strategy and the analytical measurements described here will have an important role during formulation development and in determining robust quality attributes ultimately supporting the quality assessment of these innovative RNA therapeutics.

Systematic review: predictive value of organoids in colorectal cancer

While chemotherapy alone or in combination with radiotherapy and surgery are important modalities in the treatment of colorectal cancer, their widespread use is not paired with an abundance of diagnostic tools to match individual patients with the most effective standard-of-care chemo- or radiotherapy regimens. Patient-derived organoids are tumour-derived structures that have been shown to retain certain aspects of the tissue of origin. We present here a systematic review of studies that have tested the performance of patient derived organoids to predict the effect of anti-cancer therapies in colorectal cancer, for chemotherapies, targeted drugs, and radiation therapy, and we found overall a positive predictive value of 68% and a negative predictive value of 78% for organoid informed treatment, which outperforms response rates observed with empirically guided treatment selection.

Optimization of N-Piperidinyl-Benzimidazolone Derivatives as Potent and Selective Inhibitors of 8-Oxo-Guanine DNA Glycosylase 1

8-oxo Guanine DNA Glycosylase 1 is the initiating enzyme within base excision repair and removes oxidized guanines from damaged DNA. Since unrepaired 8-oxoG could lead to G : C→T : A transversion, base removal is of utmost importance for cells to ensure genomic integrity. For cells with elevated levels of reactive oxygen species this dependency is further increased. In the past we and others have validated OGG1 as a target for inhibitors to treat cancer and inflammation. Here, we present the optimization campaign that led to the broadly used tool compound TH5487. Based on results from a small molecule screening campaign, we performed hit to lead expansion and arrived at potent and selective substituted N-piperidinyl-benzimidazolones. Using X-ray crystallography data, we describe the surprising binding mode of the most potent member of the class, TH8535. Here, the N-Piperidinyl-linker adopts a chair instead of a boat conformation which was found for weaker analogues. We further demonstrate cellular target engagement and efficacy of TH8535 against a number of cancer cell lines.

Small-molecule activation of OGG1 increases oxidative DNA damage repair by gaining a new function

Oxidative DNA damage is recognized by 8-oxoguanine (8-oxoG) DNA glycosylase 1 (OGG1), which excises 8-oxoG, leaving a substrate for apurinic endonuclease 1 (APE1) and initiating repair. Here, we describe a small molecule (TH10785) that interacts with the phenylalanine-319 and glycine-42 amino acids of OGG1, increases the enzyme activity 10-fold, and generates a previously undescribed β,δ-lyase enzymatic function. TH10785 controls the catalytic activity mediated by a nitrogen base within its molecular structure. In cells, TH10785 increases OGG1 recruitment to and repair of oxidative DNA damage. This alters the repair process, which no longer requires APE1 but instead is dependent on polynucleotide kinase phosphatase (PNKP1) activity. The increased repair of oxidative DNA lesions with a small molecule may have therapeutic applications in various diseases and aging.

A Children's Health Perspective on Nano- and Microplastics

BACKGROUND

Pregnancy, infancy, and childhood are sensitive windows for environmental exposures. Yet the health effects of exposure to nano- and microplastics (NMPs) remain largely uninvestigated or unknown. Although plastic chemicals are a well-established research topic, the impacts of plastic particles are unexplored, especially with regard to early life exposures.

OBJECTIVES

This commentary aims to summarize the knowns and unknowns around child- and pregnancy-relevant exposures to NMPs via inhalation, placental transfer, ingestion and breastmilk, and dermal absorption.

METHODS

A comprehensive literature search to map the state of the science on NMPs found 37 primary research articles on the health relevance of NMPs during early life and revealed major knowledge gaps in the field. We discuss opportunities and challenges for quantifying child-specific exposures (e.g., NMPs in breastmilk or infant formula) and health effects, in light of global inequalities in baby bottle use, consumption of packaged foods, air pollution, hazardous plastic disposal, and regulatory safeguards. We also summarize research needs for linking child health and NMP exposures and address the unknowns in the context of public health action.

DISCUSSION

Few studies have addressed child-specific sources of exposure, and exposure estimates currently rely on generic assumptions rather than empirical measurements. Furthermore, toxicological research on NMPs has not specifically focused on child health, yet children's immature defense mechanisms make them particularly vulnerable. Apart from few studies investigating the placental transfer of NMPs, the physicochemical properties (e.g., polymer, size, shape, charge) driving the absorption, biodistribution, and elimination in early life have yet to be benchmarked. Accordingly, the evidence base regarding the potential health impacts of NMPs in early life remains sparse. Based on the evidence to date, we provide recommendations to fill research gaps, stimulate policymakers and industry to address the safety of NMPs, and point to opportunities for families to reduce early life exposures to plastic. https://doi.org/10.1289/EHP9086.

MTH1 Inhibitor TH1579 Induces Oxidative DNA Damage and Mitotic Arrest in Acute Myeloid Leukemia

Acute myeloid leukemia (AML) is an aggressive hematologic malignancy, exhibiting high levels of reactive oxygen species (ROS). ROS levels have been suggested to drive leukemogenesis and is thus a potential novel target for treating AML. MTH1 prevents incorporation of oxidized nucleotides into the DNA to maintain genome integrity and is upregulated in many cancers. Here we demonstrate that hematologic cancers are highly sensitive to MTH1 inhibitor TH1579 (karonudib). A functional precision medicine ex vivo screen in primary AML bone marrow samples demonstrated a broad response profile of TH1579, independent of the genomic alteration of AML, resembling the response profile of the standard-of-care treatments cytarabine and doxorubicin. Furthermore, TH1579 killed primary human AML blast cells (CD45⁺) as well as chemotherapy resistance leukemic stem cells (CD45⁺Lin^-CD34⁺CD38^-), which are often responsible for AML progression. TH1579 killed AML cells by causing mitotic arrest, elevating intracellular ROS levels, and enhancing oxidative DNA damage. TH1579 showed a significant therapeutic window, was well tolerated in animals, and could be combined with standard-of-care treatments to further improve efficacy. TH1579 significantly improved survival in two different AML disease models in vivo. In conclusion, the preclinical data presented here support that TH1579 is a promising novel anticancer agent for AML, providing a rationale to investigate the clinical usefulness of TH1579 in AML in an ongoing clinical phase I trial. SIGNIFICANCE: The MTH1 inhibitor TH1579 is a potential novel AML treatment, targeting both blasts and the pivotal leukemic stem cells while sparing normal bone marrow cells.

Small molecule inhibitor of OGG1 blocks oxidative DNA damage repair at telomeres and potentiates methotrexate anticancer effects

The most common oxidative DNA lesion is 8-oxoguanine which is mainly recognized and excised by the 8-oxoG DNA glycosylase 1 (OGG1), initiating the base excision repair (BER) pathway. Telomeres are particularly sensitive to oxidative stress (OS) which disrupts telomere homeostasis triggering genome instability. In the present study, we have investigated the effects of inactivating BER in OS conditions, by using a specific inhibitor of OGG1 (TH5487). We have found that in OS conditions, TH5487 blocks BER initiation at telomeres causing an accumulation of oxidized bases, that is correlated with telomere losses, micronuclei formation and mild proliferation defects. Moreover, the antimetabolite methotrexate synergizes with TH5487 through induction of intracellular reactive oxygen species (ROS) formation, which potentiates TH5487-mediated telomere and genome instability. Our findings demonstrate that OGG1 is required to protect telomeres from OS and present OGG1 inhibitors as a tool to induce oxidative DNA damage at telomeres, with the potential for developing new combination therapies for cancer treatment.

Targeting OGG1 arrests cancer cell proliferation by inducing replication stress

Altered oncogene expression in cancer cells causes loss of redox homeostasis resulting in oxidative DNA damage, e.g. 8-oxoguanine (8-oxoG), repaired by base excision repair (BER). PARP1 coordinates BER and relies on the upstream 8-oxoguanine-DNA glycosylase (OGG1) to recognise and excise 8-oxoG. Here we hypothesize that OGG1 may represent an attractive target to exploit reactive oxygen species (ROS) elevation in cancer. Although OGG1 depletion is well tolerated in non-transformed cells, we report here that OGG1 depletion obstructs A3 T-cell lymphoblastic acute leukemia growth in vitro and in vivo, validating OGG1 as a potential anti-cancer target. In line with this hypothesis, we show that OGG1 inhibitors (OGG1i) target a wide range of cancer cells, with a favourable therapeutic index compared to non-transformed cells. Mechanistically, OGG1i and shRNA depletion cause S-phase DNA damage, replication stress and proliferation arrest or cell death, representing a novel mechanistic approach to target cancer. This study adds OGG1 to the list of BER factors, e.g. PARP1, as potential targets for cancer treatment.

Novel Broad-Spectrum Antiviral Inhibitors Targeting Host Factors Essential for Replication of Pathogenic RNA Viruses

Recent RNA virus outbreaks such as Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and Ebola virus (EBOV) have caused worldwide health emergencies highlighting the urgent need for new antiviral strategies. Targeting host cell pathways supporting viral replication is an attractive approach for development of antiviral compounds, especially with new, unexplored viruses where knowledge of virus biology is limited. Here, we present a strategy to identify host-targeted small molecule inhibitors using an image-based phenotypic antiviral screening assay followed by extensive target identification efforts revealing altered cellular pathways upon antiviral compound treatment. The newly discovered antiviral compounds showed broad-range antiviral activity against pathogenic RNA viruses such as SARS-CoV-2, EBOV and Crimean-Congo hemorrhagic fever virus (CCHFV). Target identification of the antiviral compounds by thermal protein profiling revealed major effects on proteostasis pathways and disturbance in interactions between cellular HSP70 complex and viral proteins, illustrating the supportive role of HSP70 on many RNA viruses across virus families. Collectively, this strategy identifies new small molecule inhibitors with broad antiviral activity against pathogenic RNA viruses, but also uncovers novel virus biology urgently needed for design of new antiviral therapies.

Abstract 5204: Targeting OGG1 arrest cancer cell proliferationby mitochondrial dysfunction and replication stress

Altered oncogene expression in cancer cells causes lost redox homeostasis resulting in oxidative DNA damage, e.g., 8-oxoguanine (8-oxoG), repaired by base excision repair (BER). PARP1 coordinates BER, and relies on the upstream 8-oxoguanine-DNA glycosylase (OGG1) to recognise and excise 8-oxoG. Here we hypothesize that OGG1 may represent an attractive target to exploit reactive oxygen species (ROS) in cancer. Although OGG1 depletion is well tolerated in non-transformed cells, we report here that OGG1 depletion obstruct A3 T-cell lymphoblastic acute leukemia growth in vitro and in vivo, validating OGG1 as a potential anti-cancer target. In line with this hypothesis, we show OGG1 inhibitors (OGG1i) target a wide range of cancer cells, with a favourable therapeutic index to non-transformed cells. Mechanistically, OGG1i and shRNA depletion cause lost mitochondrial function leading to S-phase damage, replication stress and lost proliferation or cell death, representing a novel mechanistic approach to target cancer. This study adds OGG1 to the list of BER factors, e.g., PARP1, as potential targets for treatment of cancer.

Citation Format: Carlos Benitez-Buelga, Torkild Visnes, Kumar Sanjiv, Oliver Mortusewicz, Ulrika Warpman Berglund, Thomas Helleday. Targeting OGG1 arrest cancer cell proliferationby mitochondrial dysfunction and replication stress [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 5204.

Thermal Proteome Profiling Identifies Oxidative-Dependent Inhibition of the Transcription of Major Oncogenes as a New Therapeutic Mechanism for Select Anticancer Compounds

Identification of the molecular mechanism of action (MoA) of bioactive compounds is a crucial step for drug development but remains a challenging task despite recent advances in technology. In this study, we applied multidimensional proteomics, sensitivity correlation analysis, and transcriptomics to identify a common MoA for the anticancer compounds RITA, aminoflavone (AF), and oncrasin-1 (Onc-1). Global thermal proteome profiling revealed that the three compounds target mRNA processing and transcription, thereby attacking a cancer vulnerability, transcriptional addiction. This led to the preferential loss of expression of oncogenes involved in PDGF, EGFR, VEGF, insulin/IGF/MAPKK, FGF, Hedgehog, TGFβ, and PI3K signaling pathways. Increased reactive oxygen species level in cancer cells was a prerequisite for targeting the mRNA transcription machinery, thus conferring cancer selectivity to these compounds. Furthermore, DNA repair factors involved in homologous recombination were among the most prominently repressed proteins. In cancer patient samples, RITA, AF, and Onc-1 sensitized to poly(ADP-ribose) polymerase inhibitors both in vitro and ex vivo These findings might pave a way for new synthetic lethal combination therapies.Significance: These findings highlight agents that target transcriptional addiction in cancer cells and suggest combination treatments that target RNA processing and DNA repair pathways simultaneously as effective cancer therapies.

Computational and Experimental Druggability Assessment of Human DNA Glycosylases

Due to a polar or even charged binding interface, DNA-binding proteins are considered extraordinarily difficult targets for development of small-molecule ligands and only a handful of proteins have been targeted successfully to date. Recently, however, it has been shown that development of selective and efficient inhibitors of 8-oxoguanine DNA glycosylase is possible. Here, we describe the initial druggability assessment of DNA glycosylases in a computational setting and experimentally investigate several methods to target endonuclease VIII-like 1 (NEIL1) with small-molecule inhibitors. We find that DNA glycosylases exhibit good predicted druggability in both DNA-bound and -unbound states. Furthermore, we find catalytic sites to be highly flexible, allowing for a range of interactions and binding partners. One flexible catalytic site was rationalized for NEIL1 and further investigated experimentally using both a biochemical assay in the presence of DNA and a thermal shift assay in the absence of DNA.

Abstract 105: MTH1 promotes mitotic progression to avoid oxidative DNA damage in cancer cells

MTH1 prevents incorporation of oxidized deoxynucleoside triphosphates (e.g., 8-oxodGTP) into DNA. Earlier we demonstrated that MTH1 inhibitors TH588 and TH1579 possesses anti-cancer activity in various tumour types, while other structurally distinct MTH1 inhibitors failed to kill cancer cells or elevate the levels of oxidized nucleotides in DNA. In our current study, we found a novel function of MTH1 in mitosis, as it plays an important role in correct spindle assembly in cancer cells and progression through mitosis, and furthermore, we demonstrate MTH1 binds tubulin in vivo. We also observed that structurally distinct MTH1 inhibitors display differential abilities to break the MTH1 tubulin interaction, and also demonstrate that TH588 and TH1579 can directly inhibit tubulin polymerisation in vitro. Consistent with our results, we establish that TH588 and TH1579 kill cancer cells by a mitosis-dependent mechanism and implicate reactive oxygen species (ROS) accumulation and incorporation of oxidised nucleotides into DNA in this process. Furthermore, we found that non-cytotoxic MTH1 inhibitors can become cytotoxic and increase incorporation of oxidised nucleotides into DNA when they are combined with sub-therapeutic concentrations of mitotic inhibitors. In conclusion, MTH1 inhibitors killing cancer cells have a unique and complex mechanism of action: (1) causing a mitotic arrest, induced by targeting MTH1 which is required for mitotic progression, but also through direct inhibition of microtubule polymerisation, in-turn leading to an increase in ROS and likely 8-oxodGTP and (2) MTH1-dependent 8-oxodGTPase inhibitory activity causing incorporation of damaged nucleotides into DNA to kill cancer cells.

Citation Format: Kumar Sanjiv, Helge gad, Sean G. Rudd, Oliver Mortusewicz, Ailine Stolz, Nuno Amaral, Lars Brautigham, Linda Pudelko, Christina Kaldéren, Ann-Sofie Jemth, Ingrid Almlöf, Torkild Visnes, Niklas Schultz, Johan Boström, José Manuel Calderon Montano1, Anna Hagenkort, Petra Groth, Camilla Gokturk, Tobias Koolmeister, Prasad Wakchaure, Evert Homan, Mikael Altun, Cecilia Ström, Martin Scobie, Holger Bastians, Ulrika Warpman Berglund, Thomas Helleday. MTH1 promotes mitotic progression to avoid oxidative DNA damage in cancer cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 105.

Small-molecule inhibitor of OGG1 suppresses proinflammatory gene expression and inflammation

The onset of inflammation is associated with reactive oxygen species and oxidative damage to macromolecules like 7,8-dihydro-8-oxoguanine (8-oxoG) in DNA. Because 8-oxoguanine DNA glycosylase 1 (OGG1) binds 8-oxoG and because Ogg1-deficient mice are resistant to acute and systemic inflammation, we hypothesized that OGG1 inhibition may represent a strategy for the prevention and treatment of inflammation. We developed TH5487, a selective active-site inhibitor of OGG1, which hampers OGG1 binding to and repair of 8-oxoG and which is well tolerated by mice. TH5487 prevents tumor necrosis factor-α-induced OGG1-DNA interactions at guanine-rich promoters of proinflammatory genes. This, in turn, decreases DNA occupancy of nuclear factor κB and proinflammatory gene expression, resulting in decreased immune cell recruitment to mouse lungs. Thus, we present a proof of concept that targeting oxidative DNA repair can alleviate inflammatory conditions in vivo.

Targeting BER enzymes in cancer therapy

Base excision repair (BER) repairs mutagenic or genotoxic DNA base lesions, thought to be important for both the etiology and treatment of cancer. Cancer phenotypic stress induces oxidative lesions, and deamination products are responsible for one of the most prevalent mutational signatures in cancer. Chemotherapeutic agents induce genotoxic DNA base damage that are substrates for BER, while synthetic lethal approaches targeting BER-related factors are making their way into the clinic. Thus, there are three strategies by which BER is envisioned to be relevant in cancer chemotherapy: (i) to maintain cellular growth in the presence of endogenous DNA damage in stressed cancer cells, (ii) to maintain viability after exogenous DNA damage is introduced by therapeutic intervention, or (iii) to confer synthetic lethality in cancer cells that have lost one or more additional DNA repair pathways. Here, we discuss the potential treatment strategies, and briefly summarize the progress that has been made in developing inhibitors to core BER-proteins and related factors.

mRNA Quantification of NIPBL Isoforms A and B in Adult and Fetal Human Tissues, and a Potentially Pathological Variant Affecting Only Isoform A in Two Patients with Cornelia de Lange Syndrome

Cornelia de Lange syndrome (CdLS) is a congenital developmental disorder characterized by craniofacial dysmorphia, growth retardation, limb malformations, and intellectual disability. Approximately 60% of patients with CdLS carry a recognizable pathological variant in the NIPBL gene, of which two isoforms, A and B, have been identified, and which only differ in the C-terminal segment. In this work, we describe the distribution pattern of the isoforms A and B mRNAs in tissues of adult and fetal origin, by qPCR (quantitative polymerase chain reaction). Our results show a higher gene expression of the isoform A, even though both seem to have the same tissue distribution. Interestingly, the expression in fetal tissues is higher than that of adults, especially in brain and skeletal muscle. Curiously, the study of fibroblasts of two siblings with a mild CdLS phenotype and a pathological variant specific of the isoform A of NIPBL (c.8387A > G; P.Tyr2796Cys), showed a similar reduction in both isoforms, and a normal sensitivity to DNA damage. Overall, these results suggest that the position of the pathological variant at the 3´ end of the NIPBL gene affecting only isoform A, is likely to be the cause of the atypical mild phenotype of the two brothers.

hMYH and hMTH1 cooperate for survival in mismatch repair defective T-cell acute lymphoblastic leukemia

hMTH1 is an 8-oxodGTPase that prevents mis-incorporation of free oxidized nucleotides into genomic DNA. Base excision and mismatch repair pathways also restrict the accumulation of oxidized lesions in DNA by removing the mis-inserted 8-oxo-7,8-dihydro-2'-deoxyguanosines (8-oxodGs). In this study, we aimed to investigate the interplay between hMYH DNA glycosylase and hMTH1 for cancer cell survival by using mismatch repair defective T-cell acute lymphoblastic leukemia (T-ALL) cells. To this end, MYH and MTH1 were silenced individually or simultaneously using small hairpin RNAs. Increased sub-G1 population and apoptotic cells were observed upon concurrent depletion of both enzymes. Elevated cell death was consistent with cleaved caspase 3 accumulation in double knockdown cells. Importantly, overexpression of the nuclear isoform of hMYH could remove the G1 arrest and partially rescue the toxicity observed in hMTH1-depleted cells. In addition, expression profiles of human DNA glycosylases were generated using quantitative reverse transcriptase-PCR in MTH1 and/or MYH knockdown cells. NEIL1 DNA glycosylase, involved in repair of oxidized nucleosides, was found to be significantly downregulated as a cellular response to MTH1-MYH co-suppression. Overall, the results suggest that hMYH and hMTH1 functionally cooperate for effective repair and survival in mismatch repair defective T-ALL Jurkat A3 cells.

Validation and development of MTH1 inhibitors for treatment of cancer

BACKGROUND

Previously, we showed cancer cells rely on the MTH1 protein to prevent incorporation of otherwise deadly oxidised nucleotides into DNA and we developed MTH1 inhibitors which selectively kill cancer cells. Recently, several new and potent inhibitors of MTH1 were demonstrated to be non-toxic to cancer cells, challenging the utility of MTH1 inhibition as a target for cancer treatment.

MATERIAL AND METHODS

Human cancer cell lines were exposed in vitro to MTH1 inhibitors or depleted of MTH1 by siRNA or shRNA. 8-oxodG was measured by immunostaining and modified comet assay. Thermal Proteome profiling, proteomics, cellular thermal shift assays, kinase and CEREP panel were used for target engagement, mode of action and selectivity investigations of MTH1 inhibitors. Effect of MTH1 inhibition on tumour growth was explored in BRAF V600E-mutated malignant melanoma patient derived xenograft and human colon cancer SW480 and HCT116 xenograft models.

RESULTS

Here, we demonstrate that recently described MTH1 inhibitors, which fail to kill cancer cells, also fail to introduce the toxic oxidized nucleotides into DNA. We also describe a new MTH1 inhibitor TH1579, (Karonudib), an analogue of TH588, which is a potent, selective MTH1 inhibitor with good oral availability and demonstrates excellent pharmacokinetic and anti-cancer properties in vivo.

CONCLUSION

We demonstrate that in order to kill cancer cells MTH1 inhibitors must also introduce oxidized nucleotides into DNA. Furthermore, we describe TH1579 as a best-in-class MTH1 inhibitor, which we expect to be useful in order to further validate the MTH1 inhibitor concept.

Discovery of the First Potent and Selective Inhibitors of Human dCTP Pyrophosphatase 1

The dCTPase pyrophosphatase 1 (dCTPase) regulates the intracellular nucleotide pool through hydrolytic degradation of canonical and noncanonical nucleotide triphosphates (dNTPs). dCTPase is highly expressed in multiple carcinomas and is associated with cancer cell stemness. Here we report on the development of the first potent and selective dCTPase inhibitors that enhance the cytotoxic effect of cytidine analogues in leukemia cells. Boronate 30 displays a promising in vitro ADME profile, including plasma and mouse microsomal half-lives, aqueous solubility, cell permeability and CYP inhibition, deeming it a suitable compound for in vivo studies.

A novel mouse model for the hyper-IgM syndrome: a spontaneous activation-induced cytidine deaminase mutation leading to complete loss of Ig class switching and reduced somatic hypermutation

We describe a spontaneously derived mouse line that completely failed to induce Ig class switching in vitro and in vivo. The mice inherited abolished IgG serum titers in a recessive manner caused by a spontaneous G → A transition mutation in codon 112 of the aicda gene, leading to an arginine to histidine replacement (AID(R112H)). Ig class switching was completely reconstituted by expressing wild-type AID. Mice homozygous for AID(R112H) had peripheral B cell hyperplasia and large germinal centers in the absence of Ag challenge. Immunization with SRBCs elicited an Ag-specific IgG1 response in wild-type mice, whereas AID(R112H) mice failed to produce IgG1 and had reduced somatic hypermutation. The phenotype recapitulates the human hyper-IgM (HIGM) syndrome that is caused by point mutations in the orthologous gene in humans, and the AID(R112H) mutation is frequently found in HIGM patients. The AID(R112H) mouse model for HIGM provides a powerful and more precise tool than conventional knockout strategies.

The PARP inhibitor Olaparib disrupts base excision repair of 5-aza-2'-deoxycytidine lesions

Decitabine (5-aza-2′-deoxycytidine, 5-azadC) is used in the treatment of Myelodysplatic syndrome (MDS) and Acute Myeloid Leukemia (AML). Its mechanism of action is thought to involve reactivation of genes implicated in differentiation and transformation, as well as induction of DNA damage by trapping DNA methyltranferases (DNMT) to DNA. We demonstrate for the first time that base excision repair (BER) recognizes 5-azadC-induced lesions in DNA and mediates repair. We find that BER (XRCC1) deficient cells are sensitive to 5-azadC and display an increased amount of DNA single- and double-strand breaks. The XRCC1 protein co-localizes with DNMT1 foci after 5-azadC treatment, suggesting a novel and specific role of XRCC1 in the repair of trapped DNMT1. 5-azadC-induced DNMT foci persist in XRCC1 defective cells, demonstrating a role for XRCC1 in repair of 5-azadC-induced DNA lesions. Poly (ADP-ribose) polymerase (PARP) inhibition prevents XRCC1 relocation to DNA damage sites, disrupts XRCC1–DNMT1 co-localization and thereby efficient BER. In a panel of AML cell lines, combining 5-azadC and Olaparib cause synthetic lethality. These data suggest that PARP inhibitors can be used in combination with 5-azadC to improve treatment of MDS and AML.

A regulatory role for the cohesin loader NIPBL in nonhomologous end joining during immunoglobulin class switch recombination

DNA double strand breaks (DSBs) are mainly repaired via homologous recombination (HR) or nonhomologous end joining (NHEJ). These breaks pose severe threats to genome integrity but can also be necessary intermediates of normal cellular processes such as immunoglobulin class switch recombination (CSR). During CSR, DSBs are produced in the G1 phase of the cell cycle and are repaired by the classical NHEJ machinery. By studying B lymphocytes derived from patients with Cornelia de Lange Syndrome, we observed a strong correlation between heterozygous loss-of-function mutations in the gene encoding the cohesin loading protein NIPBL and a shift toward the use of an alternative, microhomology-based end joining during CSR. Furthermore, the early recruitment of 53BP1 to DSBs was reduced in the NIPBL-deficient patient cells. Association of NIPBL deficiency and impaired NHEJ was also observed in a plasmid-based end-joining assay and a yeast model system. Our results suggest that NIPBL plays an important and evolutionarily conserved role in NHEJ, in addition to its canonical function in sister chromatid cohesion and its recently suggested function in HR.

UNG-initiated base excision repair is the major repair route for 5-fluorouracil in DNA, but 5-fluorouracil cytotoxicity depends mainly on RNA incorporation

Cytotoxicity of 5-fluorouracil (FU) and 5-fluoro-2′-deoxyuridine (FdUrd) due to DNA fragmentation during DNA repair has been proposed as an alternative to effects from thymidylate synthase (TS) inhibition or RNA incorporation. The goal of the present study was to investigate the relative contribution of the proposed mechanisms for cytotoxicity of 5-fluoropyrimidines. We demonstrate that in human cancer cells, base excision repair (BER) initiated by the uracil–DNA glycosylase UNG is the major route for FU–DNA repair in vitro and in vivo. SMUG1, TDG and MBD4 contributed modestly in vitro and not detectably in vivo. Contribution from mismatch repair was limited to FU:G contexts at best. Surprisingly, knockdown of individual uracil–DNA glycosylases or MSH2 did not affect sensitivity to FU or FdUrd. Inhibitors of common steps of BER or DNA damage signalling affected sensitivity to FdUrd and HmdUrd, but not to FU. In support of predominantly RNA-mediated cytotoxicity, FU-treated cells accumulated ~3000- to 15 000-fold more FU in RNA than in DNA. Moreover, FU-cytotoxicity was partially reversed by ribonucleosides, but not deoxyribonucleosides and FU displayed modest TS-inhibition compared to FdUrd. In conclusion, UNG-initiated BER is the major route for FU–DNA repair, but cytotoxicity of FU is predominantly RNA-mediated, while DNA-mediated effects are limited to FdUrd.

Uracil-DNA glycosylase in base excision repair and adaptive immunity: species differences between man and mouse

Genomic uracil is a DNA lesion but also an essential key intermediate in adaptive immunity. In B cells, activation-induced cytidine deaminase deaminates cytosine to uracil (U:G mispairs) in Ig genes to initiate antibody maturation. Uracil-DNA glycosylases (UDGs) such as uracil N-glycosylase (UNG), single strand-selective monofunctional uracil-DNA glycosylase 1 (SMUG1), and thymine-DNA glycosylase remove uracil from DNA. Gene-targeted mouse models are extensively used to investigate the role of these enzymes in DNA repair and Ig diversification. However, possible species differences in uracil processing in humans and mice are yet not established. To address this, we analyzed UDG activities and quantities in human and mouse cell lines and in splenic B cells from Ung(+/+) and Ung(-/-) backcrossed mice. Interestingly, human cells displayed ∼15-fold higher total uracil excision capacity due to higher levels of UNG. In contrast, SMUG1 activity was ∼8-fold higher in mouse cells, constituting ∼50% of the total U:G excision activity compared with less than 1% in human cells. In activated B cells, both UNG and SMUG1 activities were at levels comparable with those measured for mouse cell lines. Moreover, SMUG1 activity per cell was not down-regulated after activation. We therefore suggest that SMUG1 may work as a weak backup activity for UNG2 during class switch recombination in Ung(-/-) mice. Our results reveal significant species differences in genomic uracil processing. These findings should be taken into account when mouse models are used in studies of uracil DNA repair and adaptive immunity.

Uracil in DNA and its processing by different DNA glycosylases

Uracil in DNA may result from incorporation of dUMP during replication and from spontaneous or enzymatic deamination of cytosine, resulting in U:A pairs or U:G mismatches, respectively. Uracil generated by activation-induced cytosine deaminase (AID) in B cells is a normal intermediate in adaptive immunity. Five mammalian uracil-DNA glycosylases have been identified; these are mitochondrial UNG1 and nuclear UNG2, both encoded by the UNG gene, and the nuclear proteins SMUG1, TDG and MBD4. Nuclear UNG2 is apparently the sole contributor to the post-replicative repair of U:A lesions and to the removal of uracil from U:G contexts in immunoglobulin genes as part of somatic hypermutation and class-switch recombination processes in adaptive immunity. All uracil-DNA glycosylases apparently contribute to U:G repair in other cells, but they are likely to have different relative significance in proliferating and non-proliferating cells, and in different phases of the cell cycle. There are also some indications that there may be species differences in the function of the uracil-DNA glycosylases.

The rate of base excision repair of uracil is controlled by the initiating glycosylase

Uracil in DNA is repaired by base excision repair (BER) initiated by a DNA glycosylase, followed by strand incision, trimming of ends, gap filling and ligation. Uracil in DNA comes in two distinct forms; U:A pairs, typically resulting from replication errors, and mutagenic U:G mismatches, arising from cytosine deamination. To identify proteins critical to the rate of repair of these lesions, we quantified overall repair of U:A pairs, U:G mismatches and repair intermediates (abasic sites and nicked abasic sites) in vitro. For this purpose we used circular DNA substrates and nuclear extracts of eight human cell lines with wide variation in the content of BER proteins. We identified the initiating uracil-DNA glycosylase UNG2 as the major overall rate-limiting factor. UNG2 is apparently the sole glycosylase initiating BER of U:A pairs and generally initiated repair of almost 90% of the U:G mismatches. Surprisingly, TDG contributed at least as much as single-strand selective monofunctional uracil-DNA glycosylase 1 (SMUG1) to BER of U:G mismatches. Furthermore, in a cell line that expressed unusually high amounts of TDG, this glycosylase contributed to initiation of as much as approximately 30% of U:G repair. Repair of U:G mismatches was generally faster than that of U:A pairs, which agrees with the known substrate preference of UNG-type glycosylases. Unexpectedly, repair of abasic sites opposite G was also generally faster than when opposite A, and this could not be explained by the properties of the purified APE1 protein. It may rather reflect differences in substrate recognition or repair by different complex(es). Lig III is apparently a minor rate-regulator for U:G repair. APE1, Pol beta, Pol delta, PCNA, XRCC1 and Lig I did not seem to be rate-limiting for overall repair of any of the substrates. These results identify damaged base removal as the major rate-limiting step in BER of uracil in human cells.

Mitochondrial base excision repair of uracil and AP sites takes place by single-nucleotide insertion and long-patch DNA synthesis

Base excision repair (BER) corrects a variety of small base lesions in DNA. The UNG gene encodes both the nuclear (UNG2) and the mitochondrial (UNG1) forms of the human uracil-DNA glycosylase (UDG). We prepared mitochondrial extracts free of nuclear BER proteins from human cell lines. Using these extracts we show that UNG is the only detectable UDG in mitochondria, and mitochondrial BER (mtBER) of uracil and AP sites occur by both single-nucleotide insertion and long-patch repair DNA synthesis. Importantly, extracts of mitochondria carry out repair of modified AP sites which in nuclei occurs through long-patch BER. Such lesions may be rather prevalent in mitochondrial DNA because of its proximity to the electron transport chain, the primary site of production of reactive oxygen species. Furthermore, mitochondrial extracts remove 5' protruding flaps from DNA which can be formed during long-patch BER, by a "flap endonuclease like" activity, although flap endonuclease (FEN1) is not present in mitochondria. In conclusion, combined short- and long-patch BER activities enable mitochondria to repair a broader range of lesions in mtDNA than previously known.