NBS1-CtIP-mediated DNA end resection suppresses cGAS binding to micronuclei

Cyclic guanosine monophosphate-adenosine monophosphate synthase (cGAS) is activated in cells with defective DNA damage repair and signaling (DDR) factors, but a direct role for DDR factors in regulating cGAS activation in response to micronuclear DNA is still poorly understood. Here, we provide novel evidence that Nijmegen breakage syndrome 1 (NBS1) protein, a well-studied DNA double-strand break (DSB) sensor-in coordination with Ataxia Telangiectasia Mutated (ATM), a protein kinase, and Carboxy-terminal binding protein 1 interacting protein (CtIP), a DNA end resection factor-functions as an upstream regulator that prevents cGAS from binding micronuclear DNA. When NBS1 binds to micronuclear DNA via its fork-head-associated domain, it recruits CtIP and ATM via its N- and C-terminal domains, respectively. Subsequently, ATM stabilizes NBS1's interaction with micronuclear DNA, and CtIP converts DSB ends into single-strand DNA ends; these two key events prevent cGAS from binding micronuclear DNA. Additionally, by using a cGAS tripartite system, we show that cells lacking NBS1 not only recruit cGAS to a major fraction of micronuclear DNA but also activate cGAS in response to these micronuclear DNA. Collectively, our results underscore how NBS1 and its binding partners prevent cGAS from binding micronuclear DNA, in addition to their classical functions in DDR signaling.

Immunomodulatory Effects of Radiotherapy

Radiation therapy (RT), an integral component of curative treatment for many malignancies, can be administered via an increasing array of techniques. In this review, we summarize the properties and application of different types of RT, specifically, conventional therapy with x-rays, stereotactic body RT, and proton and carbon particle therapies. We highlight how low-linear energy transfer (LET) radiation induces simple DNA lesions that are efficiently repaired by cells, whereas high-LET radiation causes complex DNA lesions that are difficult to repair and that ultimately enhance cancer cell killing. Additionally, we discuss the immunogenicity of radiation-induced tumor death, elucidate the molecular mechanisms by which radiation mounts innate and adaptive immune responses and explore strategies by which we can increase the efficacy of these mechanisms. Understanding the mechanisms by which RT modulates immune signaling and the key players involved in modulating the RT-mediated immune response will help to improve therapeutic efficacy and to identify novel immunomodulatory drugs that will benefit cancer patients undergoing targeted RT.

Dysfunctional telomeres trigger cellular senescence mediated by cyclic GMP-AMP synthase

Defective DNA damage response (DDR) signaling is a common mechanism that initiates and maintains the cellular senescence phenotype. Dysfunctional telomeres activate DDR signaling, genomic instability, and cellular senescence, but the links among these events remains unclear. Here, using an array of biochemical and imaging techniques, including a highly regulatable CRISPR/Cas9 strategy to induce DNA double strand breaks specifically in the telomeres, ChIP, telomere immunofluorescence, fluorescence in situ hybridization (FISH), micronuclei imaging, and the telomere shortest length assay (TeSLA), we show that chromosome mis-segregation due to imperfect DDR signaling in response to dysfunctional telomeres creates a preponderance of chromatin fragments in the cytosol, which leads to a premature senescence phenotype. We found that this phenomenon is caused not by telomere shortening, but by cyclic GMP-AMP synthase (cGAS) recognizing cytosolic chromatin fragments and then activating the stimulator of interferon genes (STING) cytosolic DNA-sensing pathway and downstream interferon signaling. Significantly, genetic and pharmacological manipulation of cGAS not only attenuated immune signaling, but also prevented premature cellular senescence in response to dysfunctional telomeres. The findings of our study uncover a cellular intrinsic mechanism involving the cGAS-mediated cytosolic self-DNA-sensing pathway that initiates premature senescence independently of telomere shortening.

Mitochondrial Substrate Utilization Regulates Cardiomyocyte Cell Cycle Progression

The neonatal mammalian heart is capable of regeneration for a brief window of time after birth. However, this regenerative capacity is lost within the first week of life, which coincides with a postnatal shift from anaerobic glycolysis to mitochondrial oxidative phosphorylation, particularly towards fatty-acid utilization. Despite the energy advantage of fatty-acid beta-oxidation, cardiac mitochondria produce elevated rates of reactive oxygen species when utilizing fatty acids, which is thought to play a role in cardiomyocyte cell-cycle arrest through induction of DNA damage and activation of DNA-damage response (DDR) pathway. Here we show that inhibiting fatty-acid utilization promotes cardiomyocyte proliferation in the postnatatal heart. First, neonatal mice fed fatty-acid deficient milk showed prolongation of the postnatal cardiomyocyte proliferative window, however cell cycle arrest eventually ensued. Next, we generated a tamoxifen-inducible cardiomyocyte-specific, pyruvate dehydrogenase kinase 4 (PDK4) knockout mouse model to selectively enhance oxidation of glycolytically derived pyruvate in cardiomyocytes. Conditional PDK4 deletion resulted in an increase in pyruvate dehydrogenase activity and consequently an increase in glucose relative to fatty-acid oxidation. Loss of PDK4 also resulted in decreased cardiomyocyte size, decreased DNA damage and expression of DDR markers and an increase in cardiomyocyte proliferation. Following myocardial infarction, inducible deletion of PDK4 improved left ventricular function and decreased remodelling. Collectively, inhibition of fatty-acid utilization in cardiomyocytes promotes proliferation, and may be a viable target for cardiac regenerative therapies.

Werner Syndrome Protein and DNA Replication

Werner Syndrome (WS) is an autosomal recessive disorder characterized by the premature development of aging features. Individuals with WS also have a greater predisposition to rare cancers that are mesenchymal in origin. Werner Syndrome Protein (WRN), the protein mutated in WS, is unique among RecQ family proteins in that it possesses exonuclease and 3' to 5' helicase activities. WRN forms dynamic sub-complexes with different factors involved in DNA replication, recombination and repair. WRN binding partners either facilitate its DNA metabolic activities or utilize it to execute their specific functions. Furthermore, WRN is phosphorylated by multiple kinases, including Ataxia telangiectasia mutated, Ataxia telangiectasia and Rad3 related, c-Abl, Cyclin-dependent kinase 1 and DNA-dependent protein kinase catalytic subunit, in response to genotoxic stress. These post-translational modifications are critical for WRN to function properly in DNA repair, replication and recombination. Accumulating evidence suggests that WRN plays a crucial role in one or more genome stability maintenance pathways, through which it suppresses cancer and premature aging. Among its many functions, WRN helps in replication fork progression, facilitates the repair of stalled replication forks and DNA double-strand breaks associated with replication forks, and blocks nuclease-mediated excessive processing of replication forks. In this review, we specifically focus on human WRN's contribution to replication fork processing for maintaining genome stability and suppressing premature aging. Understanding WRN's molecular role in timely and faithful DNA replication will further advance our understanding of the pathophysiology of WS.

Abstract 1490: Role of sub-cellular specific reactive oxygen species in heart regeneration after cancer therapy

Introduction: Cardiovascular disease and cancer are the two leading causes of morbidity and mortality worldwide. As advancements in radiation therapy (RT) have significantly increased the number of cancer survivors, the risk of radiation-induced cardiovascular disease in this group is a growing concern. However, the molecular mechanism of radiation-induced heart failure is still elusive. Recently, it has been discovered that the reactive oxygen species (ROS)-mediated oxidative DNA damage is the primary upstream mechanism that prevents cardiomyocyte proliferation. Therefore, elucidating the spatial and temporal aspects of ROS production will lead to the development of countermeasures to prevent heart injury following chest radiotherapy.

Methods: We have generated GSH redox potential (Grx-roGFP) ROS probe targeted to cytoplasm, chromatin, nucleolus, telomere, nuclear inner membrane and heterochromatin. These probes have been inserted into cardiomyocytes specific AAV9 vectors which were used to infect cardiomyocytes both in vitro an in vivo. For in vivo study, we infected three months old mice with high-titer AAV particles via tail vein injection and then exposed to chest-only radiation (5-10 Gy). At different post-radiation times, fresh heart slices of 300-500 µM thickness were either mock- or treated with mitochondrial electron transport complex (ETC) inhibitors and then subjected to live tissue imaging using a confocal microscope.

Results: Interestingly, our results showed that the distribution of basal ROS levels is not uniform in different sub-nuclear compartments. Significantly, upon the induction of oxidative stress, the ROS levels was elevated in all the cellular compartments, but the extent of ROS level was significantly higher in the cytoplasm. Similarly, radiation altered ROS levels in all the cellular compartments; however the effect of radiation on the ROS levels was sub-nuclear compartment-specific. Intriguingly, we found that the complex IV of the ETC was critical for the maintenance of ROS levels in different cellular compartments as compared with complexes I and II.

Conclusion and Future Directions: Our data clearly indicate that the spatial and temporal levels of ROS are not uniform across the cell and the mitochondrial ETC plays a major role in regulation ROS levels in different sub-nuclear compartments. The results obtained from this study can be utilized to develop sub-cellular compartment specific targeted both genetic and pharmacological ROS scavengers that will help to regenerate adult heart by re-activating the proliferative capacity of cardiomyocytes following cancer therapy. Finally, our novel approach can be applied to assess alterations in ROS levels in different cancers.

Funding: This work was supported by the NASA (NNX13AD57G/NNX15AE06G) CPRIT (RP160520) and NIH R01AG053341 grants.

Citation Format: Salim Abdisalaam, Souparno Bhattacharya, Kalayarasan Srinivasan, Shibani Mukherjee, Hesham A. Sadek, Aroumougame Asaithamby. Role of sub-cellular specific reactive oxygen species in heart regeneration after cancer therapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1490. doi:10.1158/1538-7445.AM2017-1490

Abstract 2490: Rad51 suppresses innate immune response by blocking MRE11-mediated degradation of newly replicated genome

Purpose of the study: Eukaryotic cells accrue DNA damage as a result of endogenous metabolic activities such as DNA replication, recombination errors or environmental exposures such as ionizing radiation, ultra-violet light and chemical mutagens. Unrepaired DNA damage leads to tumorigenesis. Rad51 is a multifunctional protein that plays a central role in DNA replication and homologous recombination repair. It is known that defects in Rad51 function can cause cancer. The goal of this study is to identify a novel role for Rad51 outside of its known functions in DSB repair and replication fork processing.

Methods: Since Rad51 knockout is lethal to cells, we generated an inducible system in which we can down-regulate Rad51 expression in HT1080 cells after Doxycycline treatment. To determine the effect of Rad51-knockdown in global gene expression pattern, we carried out unbiased microarray gene expression analysis and after induction of DNA damage and replication stress by radiation. ssDNA and dsDNA in the cytosolic fractions were quantified using Quant-iT OliGreen and PicoGreen Assay Kits. For cytoplasmic BrdU detection, exponentially grown cells were labeled with BrdU for 18-20 h and then immunostained with anti-BrdU antibody. Additionally, we measured the expression and post-translational modification of proteins involved in innate immune signaling by western blotting. We also employed DNA fiber assay to determine the role of Rad51 in replication fork processing.

Results: We found that defects in Rad51 lead to the accumulation of self-DNA in the cytoplasm, triggering a STING-mediated innate immune response after replication stress and DNA damage. Mechanistically, the unprotected newly replicated genome in the absence of Rad51 is degraded by the exonuclease activity of Mre11, and the fragmented nascent DNA accumulates in the cytosol, initiating an innate immune response. Our data revealed that in addition to playing roles in homologous recombination-mediated DNA double-strand break repair and replication fork processing, Rad51 is also implicated in the suppression of innate immunity.

Conclusion: Rad51 plays a novel role in immunity outside its known functions in DSB repair and replication fork processing. We discovered that the lack of Rad51 leads to the upregulation of innate immune response pathway genes upon DNA damage and replication induced by irradiation. We found that in the absence of Rad51 the newly replicated genome is degraded by the exonuclease activity of Mre11. We also showed that these degraded nascent DNA fragments are exported to the cytoplasm, triggering innate immune response signaling. Our study reveals a previously unidentified role for Rad51 in triggering an innate immune response, and places Rad51 at the hub of new interconnections between DNA replication, DNA repair, and immunity. Funding: This work was supported by NIH R01AG053341 grants.

Citation Format: Kalayarasan Srinivasan, Souparno Bhattacharya, Salim Abdisalaam, Shibani Mukherjee, Asaithamby Aroumougame. Rad51 suppresses innate immune response by blocking MRE11-mediated degradation of newly replicated genome [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 2490. doi:10.1158/1538-7445.AM2017-2490

RAD51 interconnects between DNA replication, DNA repair and immunity

RAD51, a multifunctional protein, plays a central role in DNA replication and homologous recombination repair, and is known to be involved in cancer development. We identified a novel role for RAD51 in innate immune response signaling. Defects in RAD51 lead to the accumulation of self-DNA in the cytoplasm, triggering a STING-mediated innate immune response after replication stress and DNA damage. In the absence of RAD51, the unprotected newly replicated genome is degraded by the exonuclease activity of MRE11, and the fragmented nascent DNA accumulates in the cytosol, initiating an innate immune response. Our data suggest that in addition to playing roles in homologous recombination-mediated DNA double-strand break repair and replication fork processing, RAD51 is also implicated in the suppression of innate immunity. Thus, our study reveals a previously uncharacterized role of RAD51 in initiating immune signaling, placing it at the hub of new interconnections between DNA replication, DNA repair, and immunity.

Replication stress induced site-specific phosphorylation targets WRN to the ubiquitin-proteasome pathway

Faithful and complete genome replication in human cells is essential for preventing the accumulation of cancer-promoting mutations. WRN, the protein defective in Werner syndrome, plays critical roles in preventing replication stress, chromosome instability, and tumorigenesis. Herein, we report that ATR-mediated WRN phosphorylation is needed for DNA replication and repair upon replication stress. A serine residue, S1141, in WRN is phosphorylated in vivo by the ATR kinase in response to replication stress. ATR-mediated WRN S1141 phosphorylation leads to ubiquitination of WRN, facilitating the reversible interaction of WRN with perturbed replication forks and subsequent degradation of WRN. The dynamic interaction between WRN and DNA is required for the suppression of new origin firing and Rad51-dependent double-stranded DNA break repair. Significantly, ATR-mediated WRN phosphorylation is critical for the suppression of chromosome breakage during replication stress. These findings reveal a unique role for WRN as a modulator of DNA repair, replication, and recombination, and link ATR-WRN signaling to the maintenance of genome stability.

Tumor suppressor protein DAB2IP participates in chromosomal stability maintenance through activating spindle assembly checkpoint and stabilizing kinetochore-microtubule attachments

Defects in kinetochore-microtubule (KT-MT) attachment and the spindle assembly checkpoint (SAC) during cell division are strongly associated with chromosomal instability (CIN). CIN has been linked to carcinogenesis, metastasis, poor prognosis and resistance to cancer therapy. We previously reported that the DAB2IP is a tumor suppressor, and that loss of DAB2IP is often detected in advanced prostate cancer (PCa) and is indicative of poor prognosis. Here, we report that the loss of DAB2IP results in impaired KT-MT attachment, compromised SAC and aberrant chromosomal segregation. We discovered that DAB2IP directly interacts with Plk1 and its loss inhibits Plk1 kinase activity, thereby impairing Plk1-mediated BubR1 phosphorylation. Loss of DAB2IP decreases the localization of BubR1 at the kinetochore during mitosis progression. In addition, the reconstitution of DAB2IP enhances the sensitivity of PCa cells to microtubule stabilizing drugs (paclitaxel, docetaxel) and Plk1 inhibitor (BI2536). Our findings demonstrate a novel function of DAB2IP in the maintenance of KT-MT structure and SAC regulation during mitosis which is essential for chromosomal stability.

COMT val158met polymorphism and molecular alterations in the human dorsolateral prefrontal cortex: Differences in controls and in schizophrenia

The single nucleotide val158met polymorphism in catechol o-methyltransferase (COMT) influences prefrontal cortex function. Working memory, dependent on the dorsolateral prefrontal cortex (DLPFC), has been repeatedly shown to be influenced by this COMT polymorphism. The high activity COMT val isoform is associated with lower synaptic dopamine levels. Altered synaptic dopamine levels are expected to lead to molecular adaptations within the synapse and within DLPFC neural circuitry. In this human post mortem study using high quality DLPFC tissue, we first examined the influence of the COMT val158met polymorphism on markers of dopamine neurotransmission, N-methyl-d-aspartate (NMDA) receptor subunits and glutamatic acid decarboxylase 67 (GAD67), all known to be critical to DLPFC circuitry and function. Next, we compared target gene expression profiles in a cohort of control and schizophrenia cases, each characterized by COMT genotype. We find that the COMT val allele in control subjects is associated with significant upregulation of GluN2A and GAD67 mRNA levels compared to met carriers. Comparisons between control and schizophrenia groups reveal that GluN2A, GAD67 and DRD2 are differentially regulated between diagnostic groups in a genotype specific manner. Chronic antipsychotic treatment in rodents did not explain these differences. These data demonstrate an association between COMTval158met genotype and gene expression profile in the DLPFC of controls, possibly adaptations to maintain DLPFC function. In schizophrenia val homozygotes, these adaptations are not seen and could reflect pathophysiologic mechanisms related to the known poorer performance of these subjects on DLPFC-dependent tasks.

Human ventricular unloading induces cardiomyocyte proliferation

BACKGROUND

The adult mammalian heart is incapable of meaningful regeneration after substantial cardiomyocyte loss, primarily due to the inability of adult cardiomyocytes to divide. Our group recently showed that mitochondria-mediated oxidative DNA damage is an important regulator of postnatal cardiomyocyte cell cycle arrest. However, it is not known whether mechanical load also plays a role in this process. We reasoned that the postnatal physiological increase in mechanical load contributes to the increase in mitochondrial content, with subsequent activation of DNA damage response (DDR) and permanent cell cycle arrest of cardiomyocytes.

OBJECTIVES

The purpose of this study was to test the effect of mechanical unloading on mitochondrial mass, DDR, and cardiomyocyte proliferation.

METHODS

We examined the effect of human ventricular unloading after implantation of left ventricular assist devices (LVADs) on mitochondrial content, DDR, and cardiomyocyte proliferation in 10 matched left ventricular samples collected at the time of LVAD implantation (pre-LVAD) and at the time of explantation (post-LVAD).

RESULTS

We found that post-LVAD hearts showed up to a 60% decrease in mitochondrial content and up to a 45% decrease in cardiomyocyte size compared with pre-LVAD hearts. Moreover, we quantified cardiomyocyte nuclear foci of phosphorylated ataxia telangiectasia mutated protein, an upstream regulator of the DDR pathway, and we found a significant decrease in the number of nuclear phosphorylated ataxia telangiectasia mutated foci in the post-LVAD hearts. Finally, we examined cardiomyocyte mitosis and cytokinesis and found a statistically significant increase in both phosphorylated histone H3-positive, and Aurora B-positive cardiomyocytes in the post-LVAD hearts. Importantly, these results were driven by statistical significance in hearts exposed to longer durations of mechanical unloading.

CONCLUSIONS

Prolonged mechanical unloading induces adult human cardiomyocyte proliferation, possibly through prevention of mitochondria-mediated activation of DDR.

Cytoskeletal regulation of CD44 membrane organization and interactions with E-selectin

Interactions of CD44 on neutrophils with E-selectin on activated endothelial cells mediate rolling under flow, a prerequisite for neutrophil arrest and migration into perivascular tissues. How CD44 functions as a rolling ligand despite its weak affinity for E-selectin is unknown. We examined the nanometer scale organization of CD44 on intact cells. CD44 on leukocytes and transfected K562 cells was cross-linked within a 1.14-nm spacer. Depolymerizing actin with latrunculin B reduced cross-linking. Fluorescence resonance energy transfer (FRET) revealed tight co-clustering between CD44 fused to yellow fluorescent protein (YFP) and CD44 fused to cyan fluorescent protein on K562 cells. Latrunculin B reduced FRET-reported co-clustering. Number and brightness analysis confirmed actin-dependent CD44-YFP clusters on living cells. CD44 lacking binding sites for ankyrin and for ezrin/radixin/moesin (ERM) proteins on its cytoplasmic domain (ΔANKΔERM) did not cluster. Unexpectedly, CD44 lacking only the ankyrin-binding site (ΔANK) formed larger but looser clusters. Fluorescence recovery after photobleaching demonstrated increased CD44 mobility by latrunculin B treatment or by deleting the cytoplasmic domain. ΔANKΔERM mobility increased only modestly, suggesting that the cytoplasmic domain engages the cytoskeleton by an additional mechanism. Ex vivo differentiated CD44-deficient neutrophils expressing exogenous CD44 rolled on E-selectin and activated Src kinases after binding anti-CD44 antibody. In contrast, differentiated neutrophils expressing ΔANK had impaired rolling and kinase activation. These data demonstrate that spectrin and actin networks regulate CD44 clustering and suggest that ankyrin enhances CD44-mediated neutrophil rolling and signaling.

Scanning fluorescence correlation spectroscopy techniques to quantify the kinetics of DNA double strand break repair proteins after γ-irradiation and bleomycin treatment

A common feature of DNA repair proteins is their mobilization in response to DNA damage. The ability to visualizing and quantifying the kinetics of proteins localizing/dissociating from DNA double strand breaks (DSBs) via immunofluorescence or live cell fluorescence microscopy have been powerful tools in allowing insight into the DNA damage response, but these tools have some limitations. For example, a number of well-established DSB repair factors, in particular those required for non-homologous end joining (NHEJ), do not form discrete foci in response to DSBs induced by ionizing radiation (IR) or radiomimetic drugs, including bleomycin, in living cells. In this report, we show that time-dependent kinetics of the NHEJ factors Ku80 and DNA-dependent protein kinase catalytic subunits (DNA-PKcs) in response to IR and bleomycin can be quantified by Number and Brightness analysis and Raster-scan Image Correlation Spectroscopy. Fluorescent-tagged Ku80 and DNA-PKcs quickly mobilized in response to IR and bleomycin treatments consistent with prior reports using laser-generated DSBs. The response was linearly dependent on IR dose, and blocking NHEJ enhanced immobilization of both Ku80 and DNA-PKcs after DNA damage. These findings support the idea of using Number and Brightness and Raster-scan Image Correlation Spectroscopy as methods to monitor kinetics of DSB repair proteins in living cells under conditions mimicking radiation and chemotherapy treatments.