Dysfunctional DNA Double-Strand Break Repair Is Present in a Subset of Primary t-AML/t-MDS Myeloblasts

Abstract Abstract 2415 The chromosomal aberrations found in treatment-related acute myeloid leukemia/myelodysplastic syndrome (t-AML/t-MDS) cells suggest that disease initiation and progression may result from the inappropriate response to double-strand DNA breaks (DSBs) induced by prior exposure to radiation or chemotherapy. We hypothesized that dysregulation of DSB repair by homology-directed repair (HDR) or nonhomologous end joining (NHEJ) in t-AML/t-MDS may result from acquired mutations in HDR/NHEJ pathway genes. To test this possibility, we used next-generation sequencing technology to identify somatic genetic variants in 21 canonical HDR and 9 NHEJ DNA repair genes, as well as a subset of 7 DNA damage response genes using tumor DNA and paired normal DNA obtained from 25 t-AML/t-MDS patients. We identified 6 patients with somatic changes in 3 of these genes (RAD51L3, EME1, TP53). As dysfunctional DSB repair from epigenetic or post-translational modifications in DSB repair pathway genes or abnormalities in other DNA repair pathway genes would be missed using this approach, in parallel we performed functional studies of DSB repair using primary bone marrow cells from 16 of these t-AML/t-MDS patients and CD34+ cells from 5 normal donors. We evaluated DSB by measuring phosphorylated histone H2AX (pH2AX), a well established marker for DSB, in myeloblasts (CD45 dim, low side scatter) and lymphocytes (a surrogate for normal cells) in these samples. Baseline measurements of primary cells, coupled with a time course to measure pH2AX induction and decay after 2 Gy of irradiation (IR) were used to assess the basal DSB burden and response to acute damage, respectively. pH2AX levels were measured by flow cytometry and the geometric mean of the fluorescence intensity was converted to mean equivalent soluble fluorophore (MESF) through the use of standard beads included in each experiment. We found that 4 of 16 t-AML/t-MDS patients had myeloblasts that displayed baseline and post-damage pH2AX levels similar to normal CD34+ controls, while 12/16 patients had abnormal pH2AX levels which fell into one of three major patterns. 1) The first subset had myeloblasts in which baseline pH2AX levels were elevated compared to normal donor CD34+ (average MESF 23,107 vs 11,490, respectively; p<=0.002) suggesting an increased basal DSB burden in these cells. Furthermore, the myeloblasts showed impaired pH2AX induction (measured at 30 min. post IR) compared to CD34+ controls (1.53 vs 2.97 fold increase in pH2AX over baseline, p<=0.002), suggesting a defect in detecting DSB. This phenotype was unique to patients harboring trisomy 8 and was tumor specific, as their lymphocytes displayed baseline and post-induction pH2AX levels similar to lymphocytes from normal controls. No somatic (tumor) sequencing variants were present in the interrogated genes, raising the possibility that trisomy 8 could be driving an abnormal DNA damage response. 2) A second subset of patients had impaired pH2AX induction compared to normal donor CD34+ cells (1.44 vs 2.97 fold increase in pH2AX over baseline, p<=0.01), again suggesting a defect in detecting DSBs. These patients also lacked somatic changes in HDR/NHEJ pathway genes. 3) The final subset of patients had delayed resolution of pH2AX levels compared to CD34+ controls post IR either at 4 hours (average MESF 39,260 vs 25,480, p<0.05) or delayed resolution over the entire 24 hour period compared to controls (p<0.001). These data are consistent with a DSB repair defect and similar to our data showing cells lacking BRCA2, a gene central to the HDR pathway, have elevated pH2AX levels at 4–24 hours post DSB induction compared to BRCA2 sufficient cells (p=0.01). One of these patients had an acquired mutation in the HDR gene RAD51L3. We are currently determining the sensitivity of primary t-AML/t-MDS cells with abnormalities in pH2AX levels to a combination of DSB inducing chemotherapy and PARP inhibition, which is synthetically lethal in the setting of HDR defects. We show cell lines lacking RAD51L3 are more sensitive to PARP inhibition compared to isogenic controls (surviving fraction (SF)50 5 nM vs 20,000 nM). In total, this study confirms that DNA repair genes are mutated in t-AML/t-MDS, suggests that dysfunctional DSB repair is present in t-AML/t-MDS myeloblasts, and provides a rationale to test whether the abnormal DNA damage response can be exploited therapeutically using a synthetic lethal approach in this disease. Disclosures: No relevant conflicts of interest to declare.

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Trisomy 8 Is Associated with Increased DNA Double Strand Breaks in Primary Myeloblasts

Blood ◽

10.1182/blood.v120.21.3492.3492 ◽

2012 ◽

Vol 120 (21) ◽

pp. 3492-3492

Author(s):

Meagan A Jacoby ◽

Rigoberto de Jesus ◽

Matthew J. Walter

Keyword(s):

Dna Damage ◽

Fold Increase ◽

S Phase ◽

Chromosome 8 ◽

Normal Donor ◽

Apoptotic Cells ◽

Trisomy 8 ◽

Dsb Repair ◽

Cd34 Cells ◽

In Cells

Abstract Abstract 3492 We hypothesize that the frequent chromosomal aberrations found in treatment-related acute myeloid leukemia/myelodysplastic syndrome (t-AML/t-MDS) reflect dysregulation of DNA double strand break (DSB) repair. We performed functional studies of DSB repair using primary bone marrow cells from 15 t-AML/t-MDS patients and CD34+ cells from 5 normal donors. We evaluated DSB by measuring phosphorylated histone H2AX (pH2AX), a well-established marker for DSB, in myeloblasts (CD45 dim, low side scatter) and lymphocytes (a surrogate for normal cells). Baseline measurements of primary cells, coupled with a time course to measure pH2AX induction and decay after 2 Gy of irradiation (IR) were used to assess the basal DSB burden and response to acute damage, respectively. We found that 4 of 15 t-AML/t-MDS patients had myeloblasts that displayed baseline and post-damage pH2AX levels similar to normal CD34+ controls, while 11/15 patients had abnormal pH2AX levels which fell into one of three major patterns. 1) The first subset of patients had impaired pH2AX induction compared to normal donor CD34+ cells (1.44 vs 2.97 fold increase in pH2AX over baseline, respectively, p<=0.01), suggesting a defect in detecting DSBs. 2) A second subset of patients had delayed resolution of pH2AX levels compared to CD34+ controls post IR either at 4 hours (mean 1.54 fold higher than CD34+ control cells, p<0.05) or delayed resolution over 24 hours compared to controls (p<0.001). 3) The final subset had myeloblasts in which baseline pH2AX levels were elevated compared to CD34+ cells (2.01 fold, p<=0.002) suggesting an increased basal DSB burden in these cells. This phenotype was unique to patients with trisomy 8 and was tumor specific, as their lymphocytes displayed pH2AX levels similar to those from normal controls. The neutral Comet assay confirmed the presence of significantly elevated DSB in myeloblasts from 5/5 t-AML/t-MDS patients with trisomy 8 as compared to CD34+ controls (mean percent DNA in tail, 31.9 vs 11.8 respectively; p=0.002). Furthermore, myeloblasts from 4/5 of these patients had elevated basal DSB compared to those from the t-AML/t-MDS patients with normal pH2AX kinetics (p=0.004). Elevated basal DSB may be due to increased cells in S phase (replication fork collapse), increased apoptotic cells, or increased spontaneous and/or persistent DNA damage in cells harboring trisomy 8. The percent of myeloblasts in S phase from patients with trisomy 8 was not significantly different than those with normal pH2AX kinetics, either at baseline (mean 21vs 25.7, respectively, p=0.42) or 24 hours after mock irradiation (mean 49.7 vs 48.1, respectively, p=0.92). Furthermore, the percent of apoptotic (caspase+PI+) myeloblasts from the patients with trisomy 8 was not significantly different than those with normal pH2AX kinetics at baseline (mean 2.57 vs 1.87, respectively, p=0.77) or 24 hours after mock irradiation (mean 0.32 vs 0.94, respectively, p=0.39). Although it is possible that caspase negative early apoptotic cells could contribute to the elevated DSBs, there is no attrition of trisomy 8 myeloblasts compared to those with normal pH2AX kinetics after 24 hours (mean fold increase in cells 1.72 vs 0.88, respectively, p=0.13) in the setting of similar S phase percentages; further, the pH2AX levels in unirradiated cells after 24 hours remain significantly elevated in trisomy 8 myeloblasts versus those with normal pH2AX kinetics (2.01 fold, p<0.001) and CD34 controls (2.11 fold, p<0.001). Collectively, these results are most consistent with spontaneous and/or persistent DNA damage existing in myeloblasts harboring trisomy 8 and suggest that these cells may tolerate an elevated DSB burden. Of note, MYC is located on chromosome 8 and has been shown to induce DNA damage. Gene expression analysis showed that MYC is significantly overexpressed in samples with trisomy 8 as compared to CD34+ controls (p=0.01). To test whether overexpression of MYC in primary hematopoietic progenitor cells induces DSBs, we transduced mouse ckit+ cells with MSCV-MYC-ires-GFP or MSCV-ires-GFP retrovirus. MYC- overexpressing cells had elevated DSBs compared to control GFP+ cells (p<0.001), suggesting that MYC overexpression contributes to the spontaneous and/or persistent DNA damage in myeloblasts harboring trisomy 8. The ability of these cells to persist in the setting of elevated DSB may provide a mechanism for chemoresistance. Disclosures: No relevant conflicts of interest to declare.

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A Commentary on TDP-43 and DNA Damage Response in Amyotrophic Lateral Sclerosis

Journal of Experimental Neuroscience ◽

10.1177/1179069519880166 ◽

2019 ◽

Vol 13 ◽

pp. 117906951988016 ◽

Cited By ~ 1

Author(s):

Joy Mitra ◽

Muralidhar L Hegde

Keyword(s):

Dna Damage ◽

Dna Repair ◽

Amyotrophic Lateral Sclerosis ◽

Dna Damage Response ◽

Motor Neurons ◽

Dsb Repair ◽

Spinal Motor Neurons ◽

Spinal Motor ◽

Damage Response ◽

Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a devastating, motor neuron degenerative disease without any cure. About 95% of the ALS patients feature abnormalities in the RNA/DNA-binding protein, TDP-43, involving its nucleo-cytoplasmic mislocalization in spinal motor neurons. How TDP-43 pathology triggers neuronal apoptosis remains unclear. In a recent study, we reported for the first time that TDP-43 participates in the DNA damage response (DDR) in neurons, and its nuclear clearance in spinal motor neurons caused DNA double-strand break (DSB) repair defects in ALS. We documented that TDP-43 was a key component of the non-homologous end joining (NHEJ) pathway of DSB repair, which is likely the major pathway for repair of DSBs in post-mitotic neurons. We have also uncovered molecular insights into the role of TDP-43 in DSB repair and showed that TDP-43 acts as a scaffold in recruiting the XRCC4/DNA Ligase 4 complex at DSB damage sites and thus regulates a critical rate-limiting function in DSB repair. Significant DSB accumulation in the genomes of TDP-43-depleted, human neural stem cell-derived motor neurons as well as in ALS patient spinal cords with TDP-43 pathology, strongly supported a TDP-43 involvement in genome maintenance and toxicity-induced genome repair defects in ALS. In this commentary, we highlight our findings that have uncovered a link between TDP-43 pathology and impaired DNA repair and suggest potential possibilities for DNA repair-targeted therapies for TDP-43-ALS.

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Thrombopoietin-Increased DNA-PK-Dependent DNA Repair Limits Hematopoietic STEM and Progenitor CELL Mutagenesis in Response to Irradiation

Blood ◽

10.1182/blood.v120.21.3447.3447 ◽

2012 ◽

Vol 120 (21) ◽

pp. 3447-3447

Author(s):

Bérengère de Laval ◽

Patrycja Pawlikowska ◽

Benoit Roch ◽

Laurence Petit-Cocault ◽

Chrystele Bilhou-Nabera ◽

...

Keyword(s):

Dna Damage ◽

Dna Repair ◽

Dna Damage Response ◽

Dna Damage Repair ◽

Dsb Repair ◽

Hematopoietic Stem ◽

Damage Repair ◽

Damage Response ◽

Radiation Induced

Abstract Abstract 3447 Radiation-induced double-strand breaks (DSBs) represent a serious threat to the preservation of genetic information when it reaches hematopoietic stem cells (HSCs). Residual loss of HSC functions and increased risk of developing hematopoietic malignancies are two concerning complications of anti-cancer radiotherapy. Management of acute myelosuppression following radio- or chemotherapy has been significantly improved in recent years by the use of growth factors. However, how cytokine/environmental signals integrate the DNA damage responses in HSCs and regulate the long-term residual HSC defects following radio-or chemotherapy is unknown. Notably, the contribution of cytokines regulating HSC functions to HSC intrinsic DNA damage repair processes remains to be delineated. Thrombopoietin (TPO) and its receptor, Mpl, are critical factors supporting HSC self-renewal, survival and expansion posttransplantation. In this study, we uncover an unknown and unique function for TPO/Mpl in the regulation the DNA damage response. We show that DSB repair, measured by both γH2Ax foci resolution and neutral comet assays, following γ-irradiation (IR) or topoisomerase II inhibitor treatments, is defective in Mpl−/− and Mpl+/− HS and progenitor cells (HSPCs). Similar defects were found in wild-type cells treated in the absence of TPO. This indicates that the impaired DNA repair of Mpl−/− and Mpl+/− cells results from a specific loss of TPO-mediated DNA damage response signaling at the time of IR rather than from intrinsic constitutive differences. TPO stimulates DNA repair by increasing IR-induced DNA-PK phosphorylation at Ser2056 and Thr2609 and non-homologous end joining (NHEJ) efficiency in both HSPCs and the human UT7-Mpl cell line. This is to our knowledge the first demonstration that a cytokine involved in the homeostatic maintenance of HSCs may also regulate their response to external DNA damaging insults by controlling the DSB repair machinery. Short TPO treatment in vitro or single TPO injection to TPO/Mpl proficient mice prior to sublethal total body IR reduced IR-induced HSC genomic instability and loss of long-term reconstitution ability. This may open new avenues for administration of TPO agonists before radiotherapy to minimize radiation-induced HSC injury and mutagenesis. In addition, since Mpl is haploinsufficient in the regulation of DNA damage repair, these data suggest that Mpl might also act as a tumor suppressor in response to radiotherapy. Disclosures: No relevant conflicts of interest to declare.

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Moving From Poly (ADP-Ribose) Polymerase Inhibition to Targeting DNA Repair and DNA Damage Response in Cancer Therapy

Journal of Clinical Oncology ◽

10.1200/jco.18.02050 ◽

2019 ◽

Vol 37 (25) ◽

pp. 2257-2269 ◽

Cited By ~ 28

Author(s):

Charlie Gourley ◽

Judith Balmaña ◽

Jonathan A. Ledermann ◽

Violeta Serra ◽

Rebecca Dent ◽

...

Keyword(s):

Dna Damage ◽

Dna Repair ◽

Dna Damage Response ◽

Ataxia Telangiectasia ◽

Parp Inhibitor ◽

Single Agent ◽

Effector Proteins ◽

Parp Inhibition ◽

Ataxia Telangiectasia Mutated ◽

Damage Response

The DNA damage response (DDR) pathway coordinates the identification, signaling, and repair of DNA damage caused by endogenous or exogenous factors and regulates cell-cycle progression with DNA repair to minimize DNA damage being permanently passed through cell division. Severe DNA damage that cannot be repaired may trigger apoptosis; as such, the DDR pathway is of crucial importance as a cancer target. Poly (ADP-ribose) polymerase (PARP) is the best-known element of the DDR, and several PARP inhibitors have been licensed. However, there are approximately 450 proteins involved in DDR, and a number of these other targets are being investigated in the laboratory and clinic. We review the most recent evidence for the clinical effect of PARP inhibition in breast and ovarian cancer and explore expansion into the first-line setting and into other tumor types. We critique the evidence for patient selection techniques and summarize what is known about mechanisms of PARP inhibitor resistance. We then discuss what is known about the preclinical rationale for targeting other members of the DDR pathway and the associated tumor cell genetics that may confer sensitivity to these agents. Examples include DNA damage sensors (MLH1), damage signaling molecules (ataxia-telangiectasia mutated; ataxia-telangiectasia mutated–related and Rad3-related; CHK1/2; DNA-dependent protein kinase, catalytic subunit; WEE1; CDC7), or effector proteins for repair (POLQ [also referred to as POLθ], RAD51, poly [ADP-ribose] glycohydrolase). Early-phase clinical trials targeting some of these molecules, either as a single agent or in combination, are discussed. Finally, we outline the challenges that must be addressed to maximize the therapeutic opportunity that targeting DDR provides.

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DNA Damage Response in Multiple Myeloma: The Role of the Tumor Microenvironment

Cancers ◽

10.3390/cancers13030504 ◽

2021 ◽

Vol 13 (3) ◽

pp. 504

Author(s):

Takayuki Saitoh ◽

Tsukasa Oda

Keyword(s):

Multiple Myeloma ◽

Dna Damage ◽

Dna Repair ◽

Tumor Microenvironment ◽

Dna Damage Response ◽

Genetic Instability ◽

Dna Repair Mechanisms ◽

Damage Response ◽

Repair Mechanisms

Multiple myeloma (MM) is an incurable plasma cell malignancy characterized by genomic instability. MM cells present various forms of genetic instability, including chromosomal instability, microsatellite instability, and base-pair alterations, as well as changes in chromosome number. The tumor microenvironment and an abnormal DNA repair function affect genetic instability in this disease. In addition, states of the tumor microenvironment itself, such as inflammation and hypoxia, influence the DNA damage response, which includes DNA repair mechanisms, cell cycle checkpoints, and apoptotic pathways. Unrepaired DNA damage in tumor cells has been shown to exacerbate genomic instability and aberrant features that enable MM progression and drug resistance. This review provides an overview of the DNA repair pathways, with a special focus on their function in MM, and discusses the role of the tumor microenvironment in governing DNA repair mechanisms.

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Harnessing the Nucleolar DNA Damage Response in Cancer Therapy

Genes ◽

10.3390/genes12081156 ◽

2021 ◽

Vol 12 (8) ◽

pp. 1156

Author(s):

Jiachen Xuan ◽

Kezia Gitareja ◽

Natalie Brajanovski ◽

Elaine Sanij

Keyword(s):

Dna Damage ◽

Dna Repair ◽

Cancer Therapy ◽

Dna Damage Response ◽

Rrna Genes ◽

Rrna Synthesis ◽

Pol I ◽

Clinical Investigations ◽

Damage Response ◽

Synthesis And Processing

The nucleoli are subdomains of the nucleus that form around actively transcribed ribosomal RNA (rRNA) genes. They serve as the site of rRNA synthesis and processing, and ribosome assembly. There are 400–600 copies of rRNA genes (rDNA) in human cells and their highly repetitive and transcribed nature poses a challenge for DNA repair and replication machineries. It is only in the last 7 years that the DNA damage response and processes of DNA repair at the rDNA repeats have been recognized to be unique and distinct from the classic response to DNA damage in the nucleoplasm. In the last decade, the nucleolus has also emerged as a central hub for coordinating responses to stress via sequestering tumor suppressors, DNA repair and cell cycle factors until they are required for their functional role in the nucleoplasm. In this review, we focus on features of the rDNA repeats that make them highly vulnerable to DNA damage and the mechanisms by which rDNA damage is repaired. We highlight the molecular consequences of rDNA damage including activation of the nucleolar DNA damage response, which is emerging as a unique response that can be exploited in anti-cancer therapy. In this review, we focus on CX-5461, a novel inhibitor of Pol I transcription that induces the nucleolar DNA damage response and is showing increasing promise in clinical investigations.

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Profilin-1; a novel regulator of DNA damage response and repair machinery in keratinocytes

Molecular Biology Reports ◽

10.1007/s11033-021-06210-6 ◽

2021 ◽

Author(s):

Chang-Jin Lee ◽

Min-Ji Yoon ◽

Dong Hyun Kim ◽

Tae Uk Kim ◽

Youn-Jung Kang

Keyword(s):

Dna Damage ◽

Dna Repair ◽

Dna Damage Response ◽

Recovery Time ◽

Actin Polymerization ◽

Loss Of Function ◽

Dna Double Strand Breaks ◽

Pten Loss ◽

Damage Response ◽

Specific Regulation

AbstractProfilin-1 (PFN1) regulates actin polymerization and cytoskeletal growth. Despite the essential roles of PFN1 in cell integration, its subcellular function in keratinocyte has not been elucidated yet. Here we characterize the specific regulation of PFN1 in DNA damage response and repair machinery. PFN1 depletion accelerated DNA damage-mediated apoptosis exhibiting PTEN loss of function instigated by increased phosphorylated inactivation followed by high levels of AKT activation. PFN1 changed its predominant cytoplasmic localization to the nucleus upon DNA damage and subsequently restored the cytoplasmic compartment during the recovery time. Even though γH2AX was recruited at the sites of DNA double strand breaks in response to DNA damage, PFN1-deficient cells failed to recruit DNA repair factors, whereas control cells exhibited significant increases of these genes. Additionally, PFN1 depletion resulted in disruption of PTEN-AKT cascade upon DNA damage and CHK1-mediated cell cycle arrest was not recovered even after the recovery time exhibiting γH2AX accumulation. This might suggest PFN1 roles in regulating DNA damage response and repair machinery to protect cells from DNA damage. Future studies addressing the crosstalk and regulation of PTEN-related DNA damage sensing and repair pathway choice by PFN1 may further aid to identify new mechanistic insights for various DNA repair disorders.

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900 Depleting non-canonical, cell-intrinsic PD-L1 signals induces synthetic lethality to small molecule DNA damage response inhibitors in an immune independent and dependent manner

Journal for ImmunoTherapy of Cancer ◽

10.1136/jitc-2021-sitc2021.900 ◽

2021 ◽

Vol 9 (Suppl 3) ◽

pp. A944-A944

Author(s):

Anand Kornepati ◽

Clare Murray ◽

Barbara Avalos ◽

Cody Rogers ◽

Kavya Ramkumar ◽

...

Keyword(s):

Ovarian Cancer ◽

Dna Damage ◽

Dna Repair ◽

Cell Lines ◽

Dna Damage Response ◽

Small Molecule ◽

Treatment Resistance ◽

Programmed Death ◽

Damage Response ◽

Response Biomarker

BackgroundTumor surface-expressed programmed death-ligand 1 (PD-L1) suppresses immunity when it engages programmed death-1 (PD-1) on anti-tumor immune cells in canonical PD-L1/PD-1.1 Non-canonical, tumour-intrinsic PD-L1 signals can mediate treatment resistance2–6 but mechanisms remain incompletely understood. Targeting non-canonical, cell-intrinsic PD-L1 signals, especially modulation of the DNA damage response (DDR), remains largely untapped.MethodsWe made PD-L1 knockout (PD-L1 KO) murine transplantable and human cell lines representing melanoma, bladder, and breast histologies. We used biochemical, genetic, and cell-biology techniques for mechanistic insights into tumor-intrinsic PD-L1 control of specific DDR and DNA repair pathways. We generated a novel inducible melanoma GEMM lacking PD-L1 only in melanocytes to corroborate DDR alterations observed in PD-L1 KO of established tumors.ResultsGenetic tumor PD-L1 depletion destabilized Chk2 and impaired ATM/Chk2, but not ATR/Chk1 DDR. PD-L1KO increased DNA damage (γH2AX) and impaired homologous recombination DNA repair (p-RPA32, BRCA1, RAD51 nuclear foci) and function (DR-GFP reporter). PD-L1 KO cells were significantly more sensitive versus controls to DDR inhibitors (DDRi) against ATR, Chk1, and PARP but not ATM in multiple human and mouse tumor models in vitro and in vivo in NSG mice. PD-1 independent, intracellular, not surface PD-L1 stabilized Chk2 protein with minimal Chek2 mRNA effect. Mechanistically, PD-L1 could directly complex with Chk2, protecting it from PIRH2-mediated polyubiquitination. PD-L1 N-terminal domains Ig-V and Ig-C but not the PD-L1 C-terminal tail co-IP’d with Chk2 and restored Chk1 inhibitor (Chk1i) treatment resistance. Tumor PD-L1 expression correlated with Chk1i sensitivity in 44 primary human small cell lung cancer cell lines, implicating tumor-intrinsic PD-L1 as a DDRi response biomarker. In WT mice, genetic PD-L1 depletion but not surface PD-L1 blockade with αPD-L1, sensitized immunotherapy-resistant, BRCA1-WT 4T1 tumors to PARP inhibitor (PARPi). PARPi effects were reduced on PD-L1 KO tumors in RAG2KO mice indicating immune-dependent DDRi efficacy. Tumor PD-L1 depletion, likely due to impaired DDR, enhanced PARPi induced tumor-intrinsic STING activation (e.g., p-TBK1, CCL5) suggesting potential to augment immunotherapies.ConclusionsWe challenge the prevailing surface PD-L1 paradigm and establish a novel mechanism for cell-intrinsic PD-L1 control of the DDR and gene product expression. We identify therapeutic vulnerabilities from tumor PD-L1 depletion utilizing small molecule DDRi currently being tested in clinical trials. Data could explain αPD-L1/DDRi treatment resistance. Intracellular PD-L1 could be a pharmacologically targetable treatment target and/or response biomarker for selective DDRi alone plus other immunotherapies.ReferencesTopalian SL, Taube JM, Anders RA, Pardoll DM. Mechanism-driven biomarkers to guide immune checkpoint blockade in cancer therapy. Nat Rev Cancer 16:275–287, doi:10.1038/nrc.2016.36 (2016).Clark CA, et al. Tumor-intrinsic PD-L1 signals regulate cell growth, pathogenesis and autophagy in ovarian cancer and melanoma. Canres 0258.2016 (2016).Gupta HB et al. Tumor cell-intrinsic PD-L1 promotes tumor-initiating cell generation and functions in melanoma and ovarian cancer. 1, 16030 (2016).Zhu H, et al. BET bromodomain inhibition promotes anti-tumor immunity by suppressing PD-L1 expression. Cell Rep 16:2829–2837, doi:10.1016/j.celrep.2016.08.032 (2016)Wu B, et al. Adipose PD-L1 modulates PD-1/PD-L1 checkpoint blockade immunotherapy efficacy in breast cancer. Oncoimmunology 7:e1500107, doi:10.1080/2162402X.2018.1500107 (2018)Liang J, et al. Verteporfin inhibits PD-L1 through autophagy and the STAT1-IRF1-TRIM28 signaling axis, exerting antitumor efficacy. Cancer Immunol Res 8:952–965, doi:10.1158/2326-6066.CIR-19-0159 (2020)

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Mutant huntingtin impairs PNKP and ATXN3, disrupting DNA repair and transcription

eLife ◽

10.7554/elife.42988 ◽

2019 ◽

Vol 8 ◽

Cited By ~ 15

Author(s):

Rui Gao ◽

Anirban Chakraborty ◽

Charlene Geater ◽

Subrata Pradhan ◽

Kara L Gordon ◽

...

Keyword(s):

Dna Damage ◽

Dna Repair ◽

Rna Polymerase Ii ◽

Dna Damage Response ◽

Functional Decline ◽

Transcriptional Elongation ◽

Damage Repair ◽

Damage Response ◽

Cyclic Amp Response Element ◽

Atm Signaling

How huntingtin (HTT) triggers neurotoxicity in Huntington’s disease (HD) remains unclear. We report that HTT forms a transcription-coupled DNA repair (TCR) complex with RNA polymerase II subunit A (POLR2A), ataxin-3, the DNA repair enzyme polynucleotide-kinase-3'-phosphatase (PNKP), and cyclic AMP-response element-binding (CREB) protein (CBP). This complex senses and facilitates DNA damage repair during transcriptional elongation, but its functional integrity is impaired by mutant HTT. Abrogated PNKP activity results in persistent DNA break accumulation, preferentially in actively transcribed genes, and aberrant activation of DNA damage-response ataxia telangiectasia-mutated (ATM) signaling in HD transgenic mouse and cell models. A concomitant decrease in Ataxin-3 activity facilitates CBP ubiquitination and degradation, adversely impacting transcription and DNA repair. Increasing PNKP activity in mutant cells improves genome integrity and cell survival. These findings suggest a potential molecular mechanism of how mutant HTT activates DNA damage-response pro-degenerative pathways and impairs transcription, triggering neurotoxicity and functional decline in HD.

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The SF3B1 K700E Mutation Induces R-Loop Accumulation and Associated DNA Damage

Blood ◽

10.1182/blood-2019-126981 ◽

2019 ◽

Vol 134 (Supplement_1) ◽

pp. 4219-4219 ◽

Cited By ~ 1

Author(s):

Shalini Singh ◽

Doaa Ahmed ◽

Hamid Dolatshad ◽

Dharamveer Tatwavedi ◽

Ulrike Schulze ◽

...

Keyword(s):

Dna Damage ◽

Dna Damage Response ◽

K562 Cells ◽

Splicing Factor ◽

Leukemia Cells ◽

Cd34 Cells ◽

Damage Response ◽

Healthy Control ◽

Atr Activation ◽

Mutant Cells

The myelodysplastic syndromes (MDS) are common myeloid malignancies. Mutations in genes involved in pre-mRNA splicing (SF3B1, SRSF2, U2AF1 and ZRSR2) are the most common mutations found in MDS. There is evidence that some spliceosomal components play a role in the maintenance of genomic stability. Splicing is a transcription coupled process; splicing factor mutations affect transcription and may lead to the accumulation of R-loops (RNA-DNA hybrids with a displaced single stranded DNA). Mutations in the splicing factors SRSF2 and U2AF1 have been recently shown to increase R-loops formation in leukemia cell lines, resulting in increased DNA damage, replication stress and activation of the ATR-Chk1 pathway. SF3B1 is the most frequently mutated splicing factor gene in MDS, but a role for mutated SF3B1 in R-loop accumulation and DNA damage has not yet been reported in hematopoietic cells. We have investigated the effects of the common SF3B1 K700E mutation on R-loop formation and DNA damage response in MDS and leukemia cells. R-loop signals and the DNA damage response were measured by immunofluorescence staining using S9.6 and anti-γ-H2AX antibodies respectively. Firstly, we studied K562 (myeloid leukemia) cells with the SF3B1 K700E mutation and isogenic SF3B1 K700K wildtype (WT) K562 cells. K562 cells with SF3B1 mutation showed a significant increase in the number of S9.6 foci [Fold change (FC) 2.01, p<0.001] and in the number of γ-H2AX foci (FC 2.32, p<0.001), indicating increased R-loops and DNA damage, compared to SF3B1 WT K562 cells. Moreover, we observed increased Chk1 phosphorylation at Ser345, a hallmark of activation of the ATR pathway, in SF3B1 mutant K562 cells. Next, we analyzed induced pluripotent stem cells (iPSCs) that we generated from the bone marrow cells of one SF3B1 mutant MDS patient and of one healthy control. A significant increase in R-loops and DNA damage response was observed in an iPSC clone harboring SF3B1 mutation compared to another iPSC clone without SF3B1 mutation obtained from same MDS patient (S9.6 mean fluorescence intensity - FC 1.72, p<0.001; γ-H2AX foci - FC 1.34, p=0.052) and to iPSCs from the healthy control (S9.6 mean fluorescence intensity - FC 1.53, p<0.001; γ-H2AX foci - FC 1.61, p=0.006). In addition, bone marrow CD34+ cells from a SF3B1 mutant MDS patient showed increased R-loops (as measured by number of S9.6 foci) compared to CD34+ cells from a MDS patient without splicing factor mutations (FC 1.9) and from a healthy control (FC 2.6). To investigate whether the observed DNA damage and ATR activation in SF3B1 mutant K562 cells result from induced R-loops, we overexpressed RNASEH1 (encoding an enzyme that degrades the RNA in RNA:DNA hybrids) to resolve R-loops in these cells. RNASEH1 overexpression significantly reduced the number of S9.6 (FC 0.51, p<0.001) and γ-H2AX foci (FC 0.63, p=0.035) in SF3B1 mutant K562 cells compared to SF3B1 WT K562 cells. RNASEH1 overexpression also resulted in decreased Chk1 phosphorylation, indicating suppression of ATR pathway activation in SF3B1 mutant K562 cells. To determine the functional importance of ATR activation associated with SF3B1 mutation, we evaluated the sensitivity of SF3B1 mutant cells towards the ATR inhibitor VE-821. SF3B1 mutant K562 cells showed preferential sensitivity towards VE-821 compared to SF3B1 WT K562 cells. Chk1 is a critical substrate of ATR, and we next investigated the effects of Chk1 inhibition in SF3B1 mutant cells. Interestingly, SF3B1 mutant K562 cells demonstrated preferential sensitivity towards the Chk1 inhibitor UCN-1 (IC50 61.8 nM) compared to SF3B1 WT K562 cells (IC50 267 nM), suggesting that ATR activation is important for the survival of SF3B1 mutant cells. SF3B1 mutant K562 cells were preferentially sensitive to the splicing modulator Sudemycin D6 (IC50 53.2 nM) compared to SF3B1 WT K562 cells (IC50 130.7 nM). The effects of VE-821 and UCN-1 on SF3B1 mutant K562 cells were enhanced by Sudemycin D6 (Combination index <1), indicating synergy. In summary, our results show that the SF3B1 mutation leads to accumulation of R-loops and associated DNA damage resulting in activation of the ATR pathway in MDS and leukemia cells. Thus different mutated splicing factors have convergent effects on R-loop elevation leading to DNA damage. Moreover, our data suggest that Chk1 inhibition, alone or in combination with splicing modulators, may represent a novel therapeutic strategy to target SF3B1 mutant cells. Disclosures Schuh: Janssen: Speakers Bureau; Verastem: Speakers Bureau; Kite: Speakers Bureau; Gilead: Speakers Bureau; Seattle Genetics: Speakers Bureau; Jazz Pharmaceuticals: Speakers Bureau; Bristol-Myers Squibb: Research Funding; AbbVie: Consultancy, Speakers Bureau; Genentech: Consultancy, Speakers Bureau; Pharmacyclics: Consultancy, Speakers Bureau. Wiseman:Novartis, Celgene: Consultancy, Honoraria.

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