Browsing by Subject "ATM"
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Item Open Access A pleiotropic ATM variant (rs1800057 C>G) is associated with risk of multiple cancers.(Carcinogenesis, 2021-10-13) Qian, Danwen; Liu, Hongliang; Zhao, Lingling; Luo, Sheng; Walsh, Kyle M; Huang, Jiaoti; Li, Chuan-Yuan; Wei, QingyiATM (ataxia-telangiectasia mutated) is an important cell-cycle checkpoint kinase required for cellular response to DNA damage. Activated by DNA double strand breaks, ATM regulates the activities of many downstream proteins involved in various carcinogenic events. Therefore, ATM or its genetic variants may have a pleiotropic effect in cancer development. We conducted a pleiotropic analysis to evaluate associations between genetic variants of ATM and risk of multiple cancers. With genotyping data extracted from previously published genome-wide association studies of various cancers, we performed multivariate logistic regression analysis, followed by a meta-analysis for each cancer site, to identify cancer risk-associated single-nucleotide polymorphisms (SNPs). In the ASSET two-sided analysis, we found that two ATM SNPs were significantly associated with risk of multiple cancers. One tagging SNP (rs1800057 C>G) was associated with risk of multiple cancers (two-sided P=5.27×10 -7). Because ATM rs1800057 is a missense variant, we also explored the intermediate phenotypes through which this variant may confer risk of multiple cancers and identified a possible immune-mediated effect of this variant. Our findings indicate that genetic variants of ATM may have a pleiotropic effect on cancer risk and thus provide an important insight into common mechanisms of carcinogenesis.Item Open Access Dissecting Tumor Response to Radiation Therapy Using Genetically Engineered Mouse Models(2015) Moding, Everett JamesApproximately 50% of all patients with cancer receive radiation therapy at some point during the course of their illness. Despite advances in radiation delivery and treatment planning, normal tissue toxicity often limits the ability of radiation to eradicate tumors. The tumor microenvironment consists of tumor cells and stromal cells such as endothelial cells that contribute to tumor initiation, progression and response to therapy. Although endothelial cells can contribute to normal tissue injury following radiation, the contribution of stromal cells to tumor response to radiation therapy remains controversial. To investigate the contribution of endothelial cells to the radiation response of primary tumors, we have developed the technology to contemporaneously mutate different genes in the tumor cells and stromal cells of a genetically engineered mouse model of soft tissue sarcoma. Using this dual recombinase technology, we deleted the DNA damage response gene Atm in sarcoma and heart endothelial cells. Although deletion of Atm increased cell death of proliferating tumor endothelial cells, Atm deletion in quiescent endothelial cells of the heart did not sensitize mice to radiation-induced myocardial necrosis. In addition, the ATM inhibitor NVP-BEZ235 selectively radiosensitized primary sarcomas, demonstrating a therapeutic window for inhibiting ATM during radiation therapy. Sensitizing tumor endothelial cells to radiation by deleting Atm prolonged tumor growth delay following a non-curative dose of radiation, but failed to increase local control. In contrast, deletion of Atm in tumor parenchymal cells increased the probability of tumor eradication. These results demonstrate that tumor parenchymal cells rather than endothelial cells are the critical targets that regulate tumor eradicaiton by radiation therapy.
Item Open Access Post-translational Regulation of RPA32, ATM and Rad17 Controls the DNA Damage Response(2009) Feng, JunjieThe eukaryotic genome integrity is safeguarded by the DNA damage response, which is composed of a network of signal transduction pathways that upon genotoxic stresses, arrest cell cycle progression, motivate repair processes, or induce apoptosis or senescence when cells incur irreparable DNA damage. During this process, DNA damage-induced post-translational modifications, most notably protein phosphorylation, of a variety of DNA damage-responsive proteins has been shown to mediate the initiation, transduction and reception of the DNA damage signals, resulting in alterations of their stability, activities or subcellular localizations, ultimately leading to activation of various downstream effector pathways.
While a lot has been elucidated on the downstream events of the DNA damage response, little is known about how DNA damage is detected. Two still ongoing studies of this dissertation attempt to address this question. Our preliminary work on ATM indicates that serine 2546 is critical for its kinase activity. Substitution of this residue with phosphomimetic aspartate, but not nonphosphorylable alanine, abrogates the kinase activity of ATM and fails to rescue the checkpoint-deficient phenotype exhibited by the ATM-deficient cells, suggesting that removal of an inhibitory phospho group at S2546 might be required for the activation of ATM. In another study, we identified a novel DNA-damage responsive threonine residue (T622) in Rad17, which undergoes ATM/ATR-dependent phosphorylation in vitro and in vivo. Ectopic expression of a phosphodeficient mutant (T622A) of Rad17, but not its wild-type control, shows a pronounced defect in sustaining Chk1 phosphorylation and the corresponding G2/M checkpoint upon DNA damage, suggesting that phosphorylation at T622 might complement that on the two previously reported phosphorylation sites, S635 and S645, to mediate G2/M checkpoint activation while the latter is primarily responsible for intra-S phase checkpoint.
Although a large amount of knowledge has been accumulated about the initiation and activation process of the DNA damage response, how cells recover, the equally important flip side of the response, has remained poorly understood. We have found that in cells recovering from replication stress, RPA32 phosphorylation at ATM/ATR-responsive sites T21 and S33, which reportedly suppresses DNA replication and recruiting other checkpoint and repair proteins to the DNA lesions, is reversed by the serine/threonine protein phosphatase 2A (PP2A). Cells with a RPA32 persistent-phosphorylation mimic (T21D/S33D) exhibit normal checkpoint activation and re-enter the cell cycle normally after recovery, but display a pronounced defect in the repair of DNA breaks. These data indicate that PP2A-mediated RPA32 dephosphorylation may be a required event during the repair process in the DNA damage response.
In summary, these studies in this dissertation highlight the importance of reversible phosphorylation and dephosphorylation in the modulation of the DNA damage response. What's more, they also extend our knowledge and deepen our understanding of this process by revealing that dephosphorylation may positively regulate the activation of cell cycle checkpoints, which is seemingly dominated by protein phosphorylation upon DNA damage, that phosphorylation of certain checkpoint proteins at different sites may result in distinct consequences, and that dephosphorylation of some activated checkpoint/repair proteins may function as an important mechanism for cells to recover from the DNA damage response.
Item Open Access Tel1p and Mec1p Regulate Chromosome Segregation and Chromosome Rearrangements in Saccharomyces cerevisiae(2010) McCulley, Jennifer L.Cancer cells often have elevated frequencies of chromosomal aberrations, and it is likely that loss of genome stability is one driving force behind tumorigenesis. Deficiencies in DNA replication, DNA repair, or cell cycle checkpoints can all contribute to increased rates of chromosomal duplications, deletions and translocations. The human ATM and ATR proteins are known to participate in the DNA damage response and DNA replication checkpoint pathways and are critical to maintaining genome stability. The Saccharomyces cerevisiae homologues of ATM and ATR are Tel1p and Mec1p, respectively. Because Tel1p and Mec1p are partially functionally redundant, loss of both Tel1p and Mec1p in haploid yeast cells (tel1 mec1 strains) results in synergistically elevated rates of chromosomal aberrations, including terminal duplications, chromosomal duplications, and telomere-telomere fusions. To determine the effect of Tel1p and Mec1p on chromosome aberrations that cannot be recovered in haploid strains, such as chromosome loss, I investigated the phenotypes associated with the tel1 mec1 mutations in diploid cells. In the absence of induced DNA damage, tel1 mec1 diploid yeast strains exhibit extremely high rates of aneuploidy and chromosome rearrangements. There is a significant bias towards trisomy of chromosomes II, VIII, X, and XII, whereas the smallest chromosomes I and VI are commonly monosomic.
The telomere defects associated with tel1 mec1 strains do not cause the high rates of aneuploidy, as restoring wild-type telomere length in these strains by expression of the Cdc13p-Est2p fusion protein does not prevent cells from becoming aneuploid. The tel1 mec1 diploids are not sensitive to the microtubule-destabilizing drug benomyl, nor do they arrest the cell cycle in response to the drug, indicating that the spindle assembly checkpoint is functional. The chromosome missegregation phenotypes of tel1 mec1 diploids mimic those observed in mutant strains that do not achieve biorientation of sister chromatids during mitosis.
The chromosome rearrangements in tel1 mec1 cells reflect both homologous recombination between non-allelic Ty elements, as well as non-homologous end joining (NHEJ) events. Restoring wild-type telomere length with the Cdc13p-Est2p fusion protein substantially reduces the levels of chromosome rearrangements (terminal additions and deletions of chromosome arms, interstitial duplications, and translocations). This result suggests that most of the rearrangements in tel1 mec1 diploids are initiated by telomere-telomere fusions. One common chromosome rearrangement in tel1 mec1 strains is an amplification of sequences on chromosome XII between the left telomere and rDNA sequences on the right arm. I have termed this aberration a "schromosome." Preliminary evidence indicates that the schromosome exists in the tel1 mec1 cells as an uncapped chromosome fragment that gets resected over time.
Item Open Access The Intersection of DNA Damage Response and Ferroptosis-A Rationale for Combination Therapeutics.(Biology, 2020-07-23) Chen, Po-Han; Tseng, Watson Hua-Sheng; Chi, Jen-TsanFerroptosis is a novel form of iron-dependent cell death characterized by lipid peroxidation. While the importance and disease relevance of ferroptosis are gaining recognition, much remains unknown about its interaction with other biological processes and pathways. Recently, several studies have identified intricate and complicated interplay between ferroptosis, ionizing radiation (IR), ATM (ataxia-telangiectasia mutated)/ATR (ATM and Rad3-related), and tumor suppressor p53, which signifies the participation of the DNA damage response (DDR) in iron-related cell death. DDR is an evolutionarily conserved response triggered by various DNA insults to attenuate proliferation, enable DNA repairs, and dispose of cells with damaged DNA to maintain genome integrity. Deficiency in proper DDR in many genetic disorders or tumors also highlights the importance of this pathway. In this review, we will focus on the biological crosstalk between DDR and ferroptosis, which is mediated mostly via noncanonical mechanisms. For clinical applications, we also discuss the potential of combining ionizing radiation and ferroptosis-inducers for synergistic effects. At last, various ATM/ATR inhibitors under clinical development may protect ferroptosis and treat many ferroptosis-related diseases to prevent cell death, delay disease progression, and improve clinical outcomes.Item Open Access Using Genetically Engineered Mouse Models to Dissect the Critical Cellular Target of Radiation Therapy(2018) Castle, Katherine DelandApproximately half of all cancer patients will receive radiation as part of their treatment regimen in either a palliative or curative setting. Conventionally, radiation therapy has been administered to patients in small 1.8-2 Gy daily fractions over the span of 1 to 2 months. However, advances in treatment planning and radiation delivery methods have enabled tumors to be irradiated with a small number of large radiation doses between 15 and 24 Gy. This innovative radiation delivery method has been employed for the treatment of various cancers, resulting in improved rates of local control. However, the mechanism mediating enhanced tumor eradication with high dose radiation therapy remains elusive. Some studies have suggested that the tumor vasculature is functionally damaged or destroyed following high doses of radiation, resulting in a second wave of tumor cell killing due to the depletion of nutrients. Other studies suggest that although high doses of radiation may disrupt the tumor vasculature, this phenotype is not a critical regulator of the enhanced rates of local control obtained with high dose radiation therapy. A better understanding of the mechanisms by which high radiation dose per fraction improves the efficacy of radiation therapy will help to develop strategies to improve patient outcomes.
To dissect the critical cellular target(s) of radiation therapy that regulates tumor response, we utilized genetically engineered mouse models of soft tissue sarcoma, non-small cell lung cancer, and brainstem glioma to manipulate the radiosensitivity of various cellular compartments. Dual recombinase technology enabled the deletion of the DNA damage response gene Atm in tumor endothelial cells. In primary sarcomas, radiosensitization of the tumor vasculature by disruption of ATM signaling resulted in improved tumor growth delay, but not rates of local control in response to radiation. The radiosensitization of endothelial cells did not improve tumor radiation responses in primary models of lung cancer and brainstem glioma. In stark contrast, deletion of Atm within the tumor parenchymal cells specifically resulted in improved radiation therapy outcomes in all three mouse models. Interestingly, loss of Atm was only able to improve tumor outcome after radiotherapy in brainstem gliomas lacking p53 which has potential significance for the translation of ATM inhibitors into the clinic. Together, these results identify tumor parenchymal cells, rather than endothelial cells, as the critical targets that promote tumor response to radiation therapy.