Functions In DNA Damage Response Research Articles

RNF126 Quenches RNF168 Function in the DNA Damage Response

DNA damage response (DDR) is essential for maintaining genome stability and protecting cells from tumorigenesis. Ubiquitin and ubiquitin-like modifications play an important role in DDR, from signaling DNA damage to mediating DNA repair. In this report, we found that the E3 ligase ring finger protein 126 (RNF126) was recruited to UV laser micro-irradiation-induced stripes in a RNF8-dependent manner. RNF126 directly interacted with and ubiquitinated another E3 ligase, RNF168. Overexpression of wild type RNF126, but not catalytically-inactive mutant RNF126 (CC229/232AA), diminished ubiquitination of H2A histone family member X (H2AX), and subsequent bleomycin-induced focus formation of total ubiquitin FK2, TP53-binding protein 1 (53BP1), and receptor-associated protein 80 (RAP80). Interestingly, both RNF126 overexpression and RNF126 downregulation compromised homologous recombination (HR)-mediated repair of DNA double-strand breaks (DSBs). Taken together, our findings demonstrate that RNF126 negatively regulates RNF168 function in DDR and its appropriate cellular expression levels are essential for HR-mediated DSB repair.

A PIM-CHK1 signaling pathway regulates PLK1 phosphorylation and function during mitosis.

Although the kinase CHK1 is a key player in the DNA damage response (DDR), several studies have recently provided evidence of DDR-independent roles of CHK1, in particular following phosphorylation of its S280 residue. Here, we demonstrate that CHK1 S280 phosphorylation is cell cycle-dependent and peaks during mitosis. We found that this phosphorylation was catalyzed by the kinase PIM2, whose protein expression was also increased during mitosis. Importantly, we identified polo-like kinase 1 (PLK1) as a direct target of CHK1 during mitosis. Genetic or pharmacological inhibition of CHK1 reduced the activating phosphorylation of PLK1 on T210, and recombinant CHK1 was able to phosphorylate T210 of PLK1 in vitro Accordingly, S280-phosphorylated CHK1 and PLK1 exhibited similar specific mitotic localizations, and PLK1 was co-immunoprecipitated with S280-phosphorylated CHK1 from mitotic cell extracts. Moreover, CHK1-mediated phosphorylation of PLK1 was dependent on S280 phosphorylation by PIM2. Inhibition of PIM proteins reduced cell proliferation and mitotic entry, which was rescued by expressing a T210D phosphomimetic mutant of PLK1. Altogether, these data identify a new PIM-CHK1-PLK1 phosphorylation cascade that regulates different mitotic steps independently of the CHK1 DDR function.This article has an associated First Person interview with the first author of the paper.

NRDE-2, the human homolog of fission yeast Nrl1, prevents DNA damage accumulation in human cells

ABSTRACTThe RNA helicase Mtr4 is a versatile protein that is a crucial component of several distinct RNA surveillance complexes. Here we describe a novel complex that contains Mtr4, but has a role distinct from any of those previously described. We found that Mtr4 association with the human homolog of fission yeast Nrl1, NRDE-2, defines a novel function for Mtr4 in the DNA damage response pathway. We provide biochemical evidence that Mtr4 and NRDE-2 are part of the same complex and show that both proteins play a role in the DNA damage response by maintaining low DNA double-strand break levels. Importantly, the DNA damage response function of the Mtr4/NRDE-2 complex does not depend on the formation of R loops. We show however that NRDE-2 and Mtr4 can affect R-loop signals at a subset of distinct genes, possibly regulating their expression. Our work not only expands the wide range of Mtr4 functions, but also elucidates an important role of the less characterized human NRDE-2 protein.

Abstract 3380: BRIP1/FANCJ is vital for efficient response to and repair of replication-associated DNA damage

Abstract BRCA1-Interacting Protein 1 (BRIP1), a DNA helicase and tumor suppressor also known as FANCJ, is implicated in breast and ovarian cancers as well as the genetic instability syndrome Fanconi anemia (FA) which predisposes patients to multiple neoplasms. This study investigates the role of FANCJ in repair of replication-associated DNA damage which can lead to DNA strand breaks, chromosomal instability, and tumorigenesis. FANCJ has long been known to function in repair of DNA crosslinks; however, its role in protecting cells from replication-associated DNA damage is less well known. We have shown that loss of FANCJ sensitizes cells to damage caused by compounds that block DNA replication, such as hydroxyurea (HU) and the topoisomerase I poison camptothecin (CPT). Loss of FANCJ disrupts the cell's response to replication-associated DNA damage by impairing Chk1 and Chk2 phosphorylation and stability and activation of multiple DNA repair proteins. FANCJ-deficient cells exposed to low levels of CPT failed to progress through S-phase indicating that FANCJ is important for intra-S phase DNA damage checkpoint and recovery from replication fork blocks. FANCJ depletion leads to a 90% decrease in homologous recombination repair in assays designed to measure repair of DNA double strand breaks as well as a loss of expression of repair-associated proteins and DNA repair foci in response to damage from HU and CPT. Loss of FANCJ impairs DNA replication and leads to accumulation of gross chromosomal aberrations. Importantly, some of the DNA damage response functions of FANCJ are independent of its helicase activity, as a helicase-deficient mutant is able to compensate for loss of FANCJ in activation and stability of FANCD2 and other HR proteins. Collectively our findings indicate a novel role for FANCJ as a key regulator of the DNA damage response to errors in DNA replication potentially leading to mutagenesis and tumorigenesis. Citation Format: David W. Clark, Chinnadurai Mani, Komariah Palle. BRIP1/FANCJ is vital for efficient response to and repair of replication-associated DNA damage [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3380.

Recent advances in understanding hematopoiesis in Fanconi Anemia.

Fanconi anemia is an inherited disease characterized by genomic instability, hypersensitivity to DNA cross-linking agents, bone marrow failure, short stature, skeletal abnormalities, and a high relative risk of myeloid leukemia and epithelial malignancies. The 21 Fanconi anemia genes encode proteins involved in multiple nuclear biochemical pathways that effect DNA interstrand crosslink repair. In the past, bone marrow failure was attributed solely to the failure of stem cells to repair DNA. Recently, non-canonical functions of many of the Fanconi anemia proteins have been described, including modulating responses to oxidative stress, viral infection, and inflammation as well as facilitating mitophagic responses and enhancing signals that promote stem cell function and survival. Some of these functions take place in non-nuclear sites and do not depend on the DNA damage response functions of the proteins. Dysfunctions of the canonical and non-canonical pathways that drive stem cell exhaustion and neoplastic clonal selection are reviewed, and the potential therapeutic importance of fully investigating the scope and interdependences of the canonical and non-canonical pathways is emphasized.

ATM and ATR play complementary roles in the behavior of excitatory and inhibitory vesicle populations

ATM (ataxia-telangiectasia mutated) and ATR (ATM and Rad3-related) are large PI3 kinases whose human mutations result in complex syndromes that include a compromised DNA damage response (DDR) and prominent nervous system phenotypes. Both proteins are nuclear-localized in keeping with their DDR functions, yet both are also found in cytoplasm, including on neuronal synaptic vesicles. In ATM- or ATR-deficient neurons, spontaneous vesicle release is reduced, but a drop in ATM or ATR level also slows FM4-64 dye uptake. In keeping with this, both proteins bind to AP-2 complex components as well as to clathrin, suggesting roles in endocytosis and vesicle recycling. The two proteins play complementary roles in the DDR; ATM is engaged in the repair of double-strand breaks, while ATR deals mainly with single-strand damage. Unexpectedly, this complementarity extends to these proteins' synaptic function as well. Superresolution microscopy and coimmunoprecipitation reveal that ATM associates exclusively with excitatory (VGLUT1+) vesicles, while ATR associates only with inhibitory (VGAT+) vesicles. The levels of ATM and ATR respond to each other; when ATM is deficient, ATR levels rise, and vice versa. Finally, blocking NMDA, but not GABA, receptors causes ATM levels to rise while ATR levels respond to GABA, but not NMDA, receptor blockade. Taken together, our data suggest that ATM and ATR are part of the cellular "infrastructure" that maintains the excitatory/inhibitory balance of the nervous system. This idea has important implications for the human diseases resulting from their genetic deficiency.

Bromodomain proteins: repairing DNA damage within chromatin.

Genome surveillance and repair, termed the DNA damage response (DDR), functions within chromatin. Chromatin-based DDR mechanisms sustain genome and epigenome integrity, defects that can disrupt cellular homeostasis and contribute to human diseases. An important chromatin DDR pathway is acetylation signalling which is controlled by histone acetyltransferase (HAT) and histone deacetylase (HDAC) enzymes, which regulate acetylated lysines within proteins. Acetylated proteins, including histones, can modulate chromatin structure and provide molecular signals that are bound by acetyl-lysine binders, including bromodomain (BRD) proteins. Acetylation signalling regulates several DDR pathways, as exemplified by the preponderance of HATs, HDACs and BRD proteins that localize at DNA breaks to modify chromatin for lesion repair. Here, we explore the involvement of acetylation signalling in the DDR, focusing on the involvement of BRD proteins in promoting chromatin remodelling to repair DNA double-strand breaks. BRD proteins have widespread DDR functions including chromatin remodelling, chromatin modification and transcriptional regulation. We discuss mechanistically how BRD proteins read acetylation signals within chromatin to trigger DDR and chromatin activities to facilitate genome-epigenome maintenance. Thus, DDR pathways involving BRD proteins represent key participants in pathways that preserve genome-epigenome integrity to safeguard normal genome and cellular functions.This article is part of the themed issue 'Chromatin modifiers and remodellers in DNA repair and signalling'.

RA-Induced Transcriptional Silencing of Checkpoint Kinase-2 through Promoter Methylation by Dnmt3b Is Required for Neuronal Differentiation of P19 Cells

In a previous study, we identified several novel targets of Dnmt3b using a chromatin library from retinoic acid (RA)-treated P19 cells. The present study describes the regulation of expression and function of checkpoint kinase (Chk2), which was one of the target genes of Dnmt3b. Chromatin immunoprecipitation followed by quantitative PCR analysis showed that recruitment of Dnmt3b on Chk2 promoter is induced following RA treatment of P19 cells. Both bisulfite genomic sequence and COBRA analyses showed that the methylation level of Chk2 promoter is progressively increased during RA-induced neuronal differentiation of P19 cells. Concomitantly, both mRNA and protein expression of Chk2 are reduced as determined by real-time PCR and Western blot analysis, respectively. Suppression of Dnmt3b expression by lentiviral-mediated shRNA resulted in increased expression and reduced methylation of Chk2, which clearly showed that Dnmt3b is responsible for transcriptional silencing of Chk2 gene in RA-treated P19 cells. Neuronal differentiation of P19 cells was inhibited upon enforced Chk2 expression in P19 cells, which showed that the decrease in endogenous expression of Chk2 is essential for normal differentiation. Ectopic Chk2 expression also negatively regulated cell cycle arrest and apoptosis following RA treatment, which could also contribute to impaired neuronal differentiation. Together, this study described the regulation of Chk2 expression through promoter methylation and also presented a novel role of Chk2 during neuronal differentiation, which is independent of its previously known function in DNA damage response.

Abstract 1120: Chronopharmacology of cisplatin: role of the circadian rhythm in modulating cisplatin-induced toxicity in melanoma mouse model

Abstract Purpose: The goal of this research is to highlight novel and divergent molecular mechanisms of the circadian system in modulating the response of cisplatin therapy against melanoma tumors. Cisplatin is one of the most commonly used chemotherapeutic drugs in treating a variety of tumors including cancers of ovaries, testis, lungs, blood and solid tumors of the head and neck, and is more recently under clinical trials for potential application in melanoma tumors. However, the major limitation of cisplatin as a chemotherapeutic drug is its tumor resistance and nephrotoxicity. Hence, improving the effectiveness and reducing the toxicities associated with cisplatin therapy are desirable outcomes. Studies on human models have shown decreased renal and blood toxicity by time-of-the-day. Consequently, our project seeks to study the chronopharmacological effects and understand the mechanisms that have been successfully demonstrated in human models. We hypothesize a mechanistic, circadian rhythm-based cause for these outcomes. Experimental Design: Studies were done on B16F10 melanoma mouse models of wild-type and circadian disrupted Per1/2 -/- animals treated with 3 doses of 5 mg/kg cisplatin in the morning (7 AM) and evening (5 PM). Animal weights and tumor sizes were measured regularly post-treatment. Upon sacrifice, tissues (skin, kidney, and tumor) were harvested and analyzed for cisplatin-induced toxicities and DNA damage responses using H & E staining, western blot, and KIM-1 and immuno-slot blot assays. Results: Weight measurements show a clock-regulated response to cisplatin toxicity. AM-treated wild-type animals showed significant weight loss compared to PM-treated wild-type animals. This treatment time differential is lost in the Per1/2 -/-. On a molecular level, kidney tissue DNA samples showed clock-controlled cisplatin-DNA repair activity in wild-type animals compared to Per1/2 -/-. These initial findings strongly suggest that nephrotoxicity and DNA damage response function might be regulated by the circadian rhythm. Conclusions: These findings indicate a possible mechanism for the chronopharmacology of cisplatin in minimizing the toxicity associated with it and reveal a target for future study of additional mechanistic causes of circadian dosing changes in cisplatin and other genotoxic stress-mediated anti-cancer agents. Citation Format: Panshak Dakup, Kenneth Porter, Rajendra Gajula, Shobhan Gaddameedhi. Chronopharmacology of cisplatin: role of the circadian rhythm in modulating cisplatin-induced toxicity in melanoma mouse model [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 1120. doi:10.1158/1538-7445.AM2017-1120

Spatial function of the oxidative DNA damage response in radiation induced bystander effects in intra- and inter-system of Caenorhabditis elegans.

Though the signaling events involved in radiation induced bystander effects (RIBE) have been investigated both in vitro and in vivo, the spatial function of these communications, especially the related signaling pathways, is not fully elucidated. In the current study, significant increases of DNA damage were clearly observed in C. elegans germline upon irradiation to both intra-system of posterior pharynx and inter-system of vulva, in which more severe damage, even to F1 generation worms, was shown for vulva irradiation. Spatial function assay indicated the DDR key components of mrt-2/hus-1/cep-1/ced-4 were indispensable in germ cells for both sites irradiation, while those components in somatic cells were either not (cep-1/ced-4) or partially (mrt-2/hus-1) required to promote apoptosis. Moreover, production of reactive oxygen species (ROS) indicated by the superoxide dismutase expression and the unfolded protein response of the mitochondria was found systemically involved in the initiation of these processes for both two site irradiation. These results will give a better understanding of the RIBE mechanisms in vivo, and invaluable to assess the clinical relevance to radiotherapy.

Inhibitory effect of uranyl nitrate on DNA double-strand break repair by depression of a set of proteins in the homologous recombination pathway.

Occupational and environmental exposure to uranium has been confirmed to cause tissue injury and carcinogenesis. As a heavy metal from actinide series, the chemical and radiological toxicities of uranium jointly induce the detrimental effects. However, the mutual action and mechanism of both forms of toxicities still need to be further elucidated. DNA double-strand break (DSB) is a fundamental cause of cell death or genomic instability induced by ionizing radiation. Herein, we investigate the effect of uranyl nitrate on the cellular function of DNA damage response and intrinsic DSB repair on the aspect of chemical toxicity. The results indicated that uranyl ion increased the accumulation of nuclear DNA DSBs in a dose-dependent manner. Both homologous recombination (HR) and non-homologous end joining (NHEJ) pathways of DSB repair were affected by the uranyl ion. The inhibition of DSB repair efficiency is attributed to the depression of a set of critical repair proteins, particularly those for the HR pathway such as ATM, BRCA1, RPA80 and EXO1. The available data enable us to imagine that the chemical toxicity of uranium leads to inhibition of cellular DNA repair capability, which can further aggravate its radiological toxicity.

Abstract IA10: Targeting the DNA damage response in cancer: Progress in the clinic

Abstract Genomic instability is a recognized hallmark of cancer and is associated with a predilection for cumulative DNA damage. In contrast to cytotoxic chemotherapy and radiotherapy, targeting critical aberrations involved in the DNA damage response (DDR) of cancer cells permits greater tumor selectivity against tumors with deficient DDR function, while sparing normal tissue. Poly(ADP-ribose) polymerase (PARP) inhibitors are the first class of antitumor agents to exploit cellular weaknesses in DNA repair through the concept of synthetic lethality. Since the pivotal studies in 2005 that demonstrated the exquisite sensitivity of BRCA1 and BRCA2 (BRCA1/2) mutant cell lines and xenografts to PARP inhibition, great progress has been made in developing single agent PARP inhibitors for use in the Clinic. Early phase clinical trials demonstrated that olaparib was well tolerated, with single agent activity in patients with BRCA1/2 mutation ovarian, breast, pancreatic and castration-resistant prostate cancers. In 2014, olaparib (Lynparza; AstraZeneca) was the first PARP inhibitor to be approved by the European Medicines Agency (EMA) as maintenance therapy for responding patients with BRCA1/2 mutant ovarian cancer following platinum-based chemotherapy, and the first to obtain accelerated approval by the US Food and Drug Administration (FDA) for advanced BRCA1/2 mutant ovarian cancer. There are also multiple other potent and bona fide PARP inhibitors, including rucaparib (AGO14699; Clovis), talazoparib (BMN673; Medivation), niraparib (MK4827; Tesaro) and veliparib (ABT-888; Abbvie), which are all currently in late phase clinical trial development. A number of critical questions remain in terms of optimizing and widening the clinical efficacy and utility of PARP inhibitors, respectively. While benefits are observed following PARP inhibitor treatment in patients with a range of tumor types with germline BRCA1/2 mutations, it is now clear that clinical efficacy is not restricted to this niche population of patients. Potential antitumor strategies may include successfully characterizing other predictive biomarkers of response or defining unique PARP inhibitor combination regimens. Such novel combinations include those with molecularly targeted agents against the PI3K-AKT and angiogenic pathways, as well as immune checkpoint inhibitors. Apart from PARP inhibitors, other novel antitumor agents against key DDR targets such as ATR and Wee1 kinase are showing promise in early phase clinical trials. The first-in-class ATR inhibitor VX-970 (Vertex Pharmaceuticals) was well tolerated as monotherapy and in combination with carboplatin chemotherapy, with preliminary evidence of target modulation and early signals of antitumor activity in DDR aberrant cancers. VX-970 is currently being explored in early phase I/II studies involving multiple tumor types, including triple-negative breast cancer and non-small cell lung cancer; and in patients with DDR aberrations. AZD1775 (AstraZeneca) is a first-in-class Wee1 inhibitor, which was well tolerated with preliminary single-agent activity in patients harboring BRCA1/2 mutations. AZD1775 was also well tolerated in combination with carboplatin chemotherapy, with promising antitumor activity in p53 mutated ovarian cancer refractory or resistant to standard first line therapy. Citation Format: Timothy A. Yap. Targeting the DNA damage response in cancer: Progress in the clinic. [abstract]. In: Proceedings of the AACR Precision Medicine Series: Targeting the Vulnerabilities of Cancer; May 16-19, 2016; Miami, FL. Philadelphia (PA): AACR; Clin Cancer Res 2017;23(1_Suppl):Abstract nr IA10.

A Novel Role of Chromodomain Protein CBX8 in DNA Damage Response

Induction of DNA damage induces a dynamic repair process involving DNA repair factors and epigenetic regulators. Chromatin alterations must occur for DNA repair factors to gain access to DNA lesions and restore original chromatin configuration to preserve the gene expression profile. We characterize the novel role of CBX8, a chromodomain-containing protein with established roles in epigenetic regulation in DNA damage response. CBX8 protein rapidly accumulates at the sites of DNA damage within 30 s and progresses to accumulate until 4 min before gradually dispersing back to its predamage distribution by 15 min. CBX8 recruitment to the sites of DNA damage is dependent upon PARP1 activation and not dependent on ATM activation. CBX8 biochemically interacts with TRIM33, and its recruitment to DNA damage is also dependent on the presence of TRIM33. Knockdown of CBX8 using siRNA significantly reduces the efficiency of both homologous and the other non-homologous recombination, as well as increases sensitivity of cells to ionizing radiation. These findings demonstrate that CBX8 functions in the PARP-dependent DNA damage response partly through interaction with TRIM33 and is required for efficient DNA repair.

Functional Crosstalk between the PP2A and SUMO Pathways Revealed by Analysis of STUbL Suppressor, razor 1-1.

Posttranslational modifications (PTMs) provide dynamic regulation of the cellular proteome, which is critical for both normal cell growth and for orchestrating rapid responses to environmental stresses, e.g. genotoxins. Key PTMs include ubiquitin, the Small Ubiquitin-like MOdifier SUMO, and phosphorylation. Recently, SUMO-targeted ubiquitin ligases (STUbLs) were found to integrate signaling through the SUMO and ubiquitin pathways. In general, STUbLs are recruited to target proteins decorated with poly-SUMO chains to ubiquitinate them and drive either their extraction from protein complexes, and/or their degradation at the proteasome. In fission yeast, reducing or preventing the formation of SUMO chains can circumvent the essential and DNA damage response functions of STUbL. This result indicates that whilst some STUbL "targets" have been identified, the crucial function of STUbL is to antagonize SUMO chain formation. Herein, by screening for additional STUbL suppressors, we reveal crosstalk between the serine/threonine phosphatase PP2A-Pab1B55 and the SUMO pathway. A hypomorphic Pab1B55 mutant not only suppresses STUbL dysfunction, but also mitigates the phenotypes associated with deletion of the SUMO protease Ulp2, or mutation of the STUbL cofactor Rad60. Together, our results reveal a novel role for PP2A-Pab1B55 in modulating SUMO pathway output, acting in parallel to known critical regulators of SUMOylation homeostasis. Given the broad evolutionary functional conservation of the PP2A and SUMO pathways, our results could be relevant to the ongoing attempts to therapeutically target these factors.

Intronic cleavage and polyadenylation regulates gene expression during DNA damage response through U1 snRNA.

The DNA damage response involves coordinated control of gene expression and DNA repair. Using deep sequencing, we found widespread changes of alternative cleavage and polyadenylation site usage on ultraviolet-treatment in mammalian cells. Alternative cleavage and polyadenylation regulation in the 3ʹ untranslated region is substantial, leading to both shortening and lengthening of 3ʹ untranslated regions of genes. Interestingly, a strong activation of intronic alternative cleavage and polyadenylation sites is detected, resulting in widespread expression of truncated transcripts. Intronic alternative cleavage and polyadenylation events are biased to the 5ʹ end of genes and affect gene groups with important functions in DNA damage response and cancer. Moreover, intronic alternative cleavage and polyadenylation site activation during DNA damage response correlates with a decrease in U1 snRNA levels, and is reversible by U1 snRNA overexpression. Importantly, U1 snRNA overexpression mitigates ultraviolet-induced apoptosis. Together, these data reveal a significant gene regulatory scheme in DNA damage response where U1 snRNA impacts gene expression via the U1-alternative cleavage and polyadenylation axis.

Hyperactivation of ATM upon DNA-PKcs inhibition modulates p53 dynamics and cell fate in response to DNA damage.

A functional DNA damage response is essential for maintaining genome integrity in the presence of DNA double-strand breaks. It is mainly coordinated by the kinases ATM, ATR, and DNA-PKcs, which control the repair of broken DNA strands and relay the damage signal to the tumor suppressor p53 to induce cell cycle arrest, apoptosis, or senescence. Although many functions of the individual kinases have been identified, it remains unclear how they act in concert to ensure faithful processing of the damage signal. Using specific inhibitors and quantitative analysis at the single-cell level, we systematically characterize the contribution of each kinase for regulating p53 activity. Our results reveal a new regulatory interplay in which loss of DNA-PKcs function leads to hyperactivation of ATM and amplification of the p53 response, sensitizing cells for damage-induced senescence. This interplay determines the outcome of treatment regimens combining irradiation with DNA-PKcs inhibitors in a p53-dependent manner.

Terminal differentiation induction as DNA damage response in hematopoietic stem cells by GADD45A.

Hematopoietic stem cells (HSCs) sustain lifelong blood cell regeneration by balancing their ability for self-renewal with their ability to differentiate into all blood cell types. To prevent organ exhaustion and malignant transformation, long-lived HSCs, in particular, must be protected from exogenous and endogenous stress, which cause severe DNA damage. When DNA is damaged, distinct DNA repair mechanisms and cell fate controls occur in adult HSCs compared with committed cells. Growth arrest and DNA damage-inducible 45 alpha (GADD45A) is known to coordinate a variety of cellular stress responses, indicating the molecule is an important stress mediator. So far, the function of GADD45A in hematopoietic stem and progenitor cells is controversial and appears highly dependent on the cell type and stress stimulus. Recent studies have analyzed its role in cell fate decision control of prospectively isolated HSCs and have revealed unexpected functions of GADD45A, as discussed here. The upregulation of GADD45A by DNA damage-causing conditions results in enhanced HSC differentiation, probably to efficiently eliminate aberrant HSCs from the system. These findings, in concert with a few studies on other stem cell systems, have led us to propose DNA damage-induced differentiation as a novel DNA damage response mechanism in stem cells that circumvents the fatal consequences of cumulative DNA damage in the stem cell compartment.

Unique epigenetic influence of H2AX phosphorylation and H3K56 acetylation on normal stem cell radioresponses.

Normal tissue injury resulting from cancer radiotherapy is often associated with diminished regenerative capacity. We examined the relative radiosensitivity of normal stem cell populations compared with non-stem cells within several radiosensitive tissue niches and culture models. We found that these stem cells are highly radiosensitive, in contrast to their isogenic differentiated progeny. Of interest, they also exhibited a uniquely attenuated DNA damage response (DDR) and muted DNA repair. Whereas stem cells exhibit reduced ATM activation and ionizing radiation-induced foci, they display apoptotic pannuclear H2AX-S139 phosphorylation (γH2AX), indicating unique radioresponses. We also observed persistent phosphorylation of H2AX-Y142 along the DNA breaks in stem cells, which promotes apoptosis while inhibiting DDR signaling. In addition, down-regulation of constitutively elevated histone-3 lysine-56 acetylation (H3K56ac) in stem cells significantly decreased their radiosensitivity, restored DDR function, and increased survival, signifying its role as a key contributor to stem cell radiosensitivity. These results establish that unique epigenetic landscapes affect cellular heterogeneity in radiosensitivity and demonstrate the nonubiquitous nature of radiation responses. We thus elucidate novel epigenetic rheostats that promote ionizing radiation hypersensitivity in various normal stem cell populations, identifying potential molecular targets for pharmacological radioprotection of stem cells and hopefully improving the efficacy of future cancer treatment.

AarF Domain Containing Kinase 3 (ADCK3) Mutant Cells Display Signs of Oxidative Stress, Defects in Mitochondrial Homeostasis and Lysosomal Accumulation.

Autosomal recessive ataxias are a clinically diverse group of syndromes that in some cases are caused by mutations in genes with roles in the DNA damage response, transcriptional regulation or mitochondrial function. One of these ataxias, known as Autosomal Recessive Cerebellar Ataxia Type-2 (ARCA-2, also known as SCAR9/COQ10D4; OMIM: #612016), arises due to mutations in the ADCK3 gene. The product of this gene (ADCK3) is an atypical kinase that is thought to play a regulatory role in coenzyme Q10 (CoQ10) biosynthesis. Although much work has been performed on the S. cerevisiae orthologue of ADCK3, the cellular and biochemical role of its mammalian counterpart, and why mutations in this gene lead to human disease is poorly understood. Here, we demonstrate that ADCK3 localises to mitochondrial cristae and is targeted to this organelle via the presence of an N-terminal localisation signal. Consistent with a role in CoQ10 biosynthesis, ADCK3 deficiency decreased cellular CoQ10 content. In addition, endogenous ADCK3 was found to associate in vitro with recombinant Coq3, Coq5, Coq7 and Coq9, components of the CoQ10 biosynthetic machinery. Furthermore, cell lines derived from ARCA-2 patients display signs of oxidative stress, defects in mitochondrial homeostasis and increases in lysosomal content. Together, these data shed light on the possible molecular role of ADCK3 and provide insight into the cellular pathways affected in ARCA-2 patients.

Agonists of the TRAIL Death Receptor DR5 Sensitize Intestinal Stem Cells to Chemotherapy-Induced Cell Death and Trigger Gastrointestinal Toxicity.

The combination of TRAIL death receptor agonists and radiochemotherapy to treat advanced cancers continues to be investigated in clinical trials. We previously showed that normal cells with a functional DNA damage response (DDR) upregulate the expression of death-inducing receptor DR5/TRAILR2/TNFRSF10B in a p53-dependent manner that sensitizes them to treatment with DR5 agonists. However, it is unclear if targeting DR5 selectively sensitizes cancer cells to agonist treatment following exposure to DNA-damaging chemotherapy, and to what extent normal tissues are targeted. Here, we show that the combined administration of the DR5 agonistic monoclonal antibody (mAb) and chemotherapy to wild-type mice triggered synergistic gastrointestinal toxicities (GIT) that were associated with the death of Lgr5(+) crypt base columnar stem cells in a p53- and DR5-dependent manner. Furthermore, we confirmed that normal human epithelial cells treated with the human DR5-agonistic mAb and chemotherapeutic agents were also greatly sensitized to cell death. Interestingly, our data also indicated that genetic or pharmacologic targeting of Chk2 may counteract GIT without negatively affecting the antitumor responses of combined DR5 agonist/chemotherapy treatment, further linking the DDR to TRAIL death receptor signaling in normal cells. In conclusion, the combination of DR5-targeting agonistic mAbs with DNA damaging chemotherapy may pose a risk of developing toxicity-induced conditions, and the effects of mAb-based strategies on the dose-limiting toxicity of chemotherapy must be considered when establishing new combination therapies.