MutT homolog 1 counteracts the effect of anti-neoplastic treatments in adult and pediatric glioblastoma cells

Pediatric glioblastoma cells are sensitive to drugs that inhibit eIF2α dephosphorylation and its phosphomimetic S51D variant

We found that pediatric glioblastoma (PED-GBM) cell lines from diffuse intrinsic pontine glioma (DIPG) carrying the H3K27M mutation or from diffuse hemispheric glioma expressing the H3G34R mutation are sensitive to the combination of vorinostat (a histone deacetylase inhibitor) and PARP-1 inhibitors. The combined treatment increased the phosphorylation of eIF2α (P-eIF2α) relative to each drug alone and enhanced the decrease in cell survival. To explore the role played by increased P-eIF2α in modulating PED-GBM survival and response to treatments, we employed brain-penetrating inhibitors of P-eIF2α dephosphorylation: salubrinal and raphin-1. These drugs increased P-eIF2α, DNA damage, and cell death, similarly affecting the sensitivity of DIPG cells and derived neurospheres to PARP-1 inhibitors. Interestingly, these drugs also decreased the level of eIF2Bϵ (the catalytic subunit of eIF2B) and increased its phosphorylation, thereby enhancing the effect of increased P-eIF2α. Transient transfection with the S51D phosphomimetic eIF2α variant recapitulated the effect of salubrinal and raphin-1 on PED-GBM survival and sensitivity to PARP-1 inhibitors. Importantly, either salubrinal or raphin-1 dramatically increased the sensitivity of DIPG cells to radiation, the main treatment modality of PED-GBM. Finally, PED-GBM was more sensitive than normal human astrocytes to salubrinal, raphin-1, and the treatment combinations described herein. Our results indicate that combinations of histone deacetylase inhibitors and PARP-1 inhibitors should be evaluated for their toxicity and efficacy in PED-GBM patients and point to drugs that increase P-eIF2α or modulate its downstream effectors as a novel means of treating PED-GBM.

Inhibition of Oxidized Nucleotide Sanitation By TH1579 and Conventional Chemotherapy Cooperatively Enhance Oxidative DNA Damage and Survival in AML

Currently, the majority of patients with acute myeloid leukemia (AML) still die of their disease due to primary resistance or relapse toward conventional reactive oxygen species (ROS)- and DNA damage-inducing chemotherapy regimens. Herein, we explored the therapeutic potential to enhance chemotherapy response in AML, by targeting the ROS scavenger enzyme MutT homolog 1 (MTH1, NUDT1), which protects cellular integrity through prevention of fatal chemotherapy-induced oxidative DNA damage. We demonstrate that MTH1 is a potential druggable target expressed by the majority of patients with AML and the inv(16)/KITD816Y AML mouse model mimicking the genetics of patients with AML exhibiting poor response to standard chemotherapy (i.e., anthracycline & cytarabine). Strikingly, combinatorial treatment of inv(16)/KITD816Y AML cells with the MTH1 inhibitor TH1579 and ROS- and DNA damage-inducing standard chemotherapy induced growth arrest and incorporated oxidized nucleotides into DNA leading to significantly increased DNA damage. Consistently, TH1579 and chemotherapy synergistically inhibited growth of clonogenic inv(16)/KITD816Y AML cells without substantially inhibiting normal clonogenic bone marrow cells. In addition, combinatorial treatment of inv(16)/KITD816Y AML mice with TH1579 and chemotherapy significantly reduced AML burden and prolonged survival compared with untreated or single treated mice. In conclusion, our study provides a rationale for future clinical studies combining standard AML chemotherapy with TH1579 to boost standard chemotherapy response in patients with AML. Moreover, other cancer entities treated with ROS- and DNA damage-inducing chemo- or radiotherapies might benefit therapeutically from complementary treatment with TH1579.

Radiosensitization in Pediatric High-Grade Glioma: Targets, Resistance and Developments

Pediatric high-grade gliomas (pHGG) are the leading cause of cancer-related death in children. These epigenetically dysregulated tumors often harbor mutations in genes encoding histone 3, which contributes to a stem cell-like, therapy-resistant phenotype. Furthermore, pHGG are characterized by a diffuse growth pattern, which, together with their delicate location, makes complete surgical resection often impossible. Radiation therapy (RT) is part of the standard therapy against pHGG and generally the only modality, apart from surgery, to provide symptom relief and a delay in tumor progression. However, as a single treatment modality, RT still offers no chance for a cure. As with most therapeutic approaches, irradiated cancer cells often acquire resistance mechanisms that permit survival or stimulate regrowth after treatment, thereby limiting the efficacy of RT. Various preclinical studies have investigated radiosensitizers in pHGG models, without leading to an improved clinical outcome for these patients. However, our recently improved molecular understanding of pHGG generates new opportunities to (re-)evaluate radiosensitizers in these malignancies. Furthermore, the use of radio-enhancing agents has several benefits in pHGG compared to other cancers, which will be discussed here. This review provides an overview and a critical evaluation of the radiosensitization strategies that have been studied to date in pHGG, thereby providing a framework for improving radiosensitivity of these rapidly fatal brain tumors.

Inhibitor development of MTH1 via high-throughput screening with fragment based library and MTH1 substrate binding cavity

MutT Homolog 1 (MTH1) has been proven to hydrolyze oxidized nucleotide triphosphates during DNA repair. It can prevent the incorporation of wrong nucleotides during DNA replication and mitigate cell apoptosis. In a cancer cell, abundant reactive oxygen species can lead to substantial DNA damage and DNA mutations by base-pairing mismatch. MTH1 could eliminate oxidized dNTP and prevent cancer cells from entering cell death. Therefore, inhibition of MTH1 activity is considered to be an anti-cancer therapeutic target. In this study, high-throughput screening techniques were combined with a fragment-based library containing 2,313 compounds, which were used to screen for lead compounds with MTH1 inhibitor activity. Four compounds with MTH1 inhibitor ability were selected, and compound MI0639 was found to have the highest effective inhibition. To discover the selectivity and specificity of this action, several derivatives based on the MTH1 and MI0639 complex structure were synthesized. We compared 14 complex structures of MTH1 and the various compounds in combination with enzymatic inhibition and thermodynamic analysis. Nanomolar-range IC₅₀ inhibition abilities by enzyme kinetics and K_d values by thermodynamic analysis were obtained for two compounds, named MI1020 and MI1024. Based on structural information and compound optimization, we aim to provide a strategy for the development of MTH1 inhibitors with high selectivity and specificity.

hMTH1 and GPX1 expression in human thyroid tissue is interrelated to prevent oxidative DNA damage

Oxidative stress (OS) is recognized as disturbance of cellular equilibrium between reactive oxygen species (ROS) formation and their elimination by antioxidant defense systems. One example of ROS-mediated damage is generation of potentially mutagenic DNA precursor, 8-oxodGTP. In human cells genomic 8-oxodGTP incorporation is prevented by the MutT homologue 1 (MTH1 or hMTH1 for human MTH1) protein. It is well established that malignant cells, including thyroid cancer cells, require hMTH1 for maintaining proliferation and cancerous transformation phenotype. Above observations led to the development of hMTH1 inhibitors as novel anticancer therapeutics. In the current study we present extensive analysis of oxidative stress responses determining sensitivity to hMTH1 deficiency in cultured thyroid cells. We observe here that hMTH1 depletion results in downregulation of several glutathione-dependent OS defense system factors, including GPX1 and GCLM, making some of the tested thyroid cell lines highly dependent on glutathione levels. This is evidenced by the increased ROS burden and enhanced proliferation defect after combination of hMTH1 siRNA and glutathione synthesis inhibition. Moreover, due to the lack of data on hMTH1 expression in human thyroid tumor specimens we decided to perform detailed analysis of hMTH1 expression in thyroid tumor and peri-tumoral tissues from human patients. Our results allow us to propose here that anticancer activity of hMTH1 suppression may be boosted by combination with agents modulating glutathione pool, but further studies are necessary to precisely identify backgrounds susceptible to such combination treatment.

MutT Homolog1 has multifaceted role in glioma and is under the apparent orchestration by Hypoxia Inducible factor1 alpha

AIMS

The study focused on the expression and role of a recent potential cancer therapeutic target protein, MutT Homolog1 (MTH1). MTH1 gets activated in an increased reactive oxygen species (ROS) environment and removes the oxidized nucleotides from the cell. The study aimed to check the role of MTH1 in DNA damage and apoptosis, migration and angiogenesis and also to examine its regulation in glioma.

MAIN METHODS

The experiments were carried out in human glioma tissue samples and brain tissues of epilepsy patients (non-tumor control). We used two human glioblastomas cell lines, U87MG and U251MG cells. In order to study the role of MTH1 in glioma and to analyze the relation of MTH1 with Hif1α, we have used MTH1 siRNA and Hif1α siRNA respectively.

KEY FINDINGS

We found an increased expression of MTH1 in glioma tissues compared to the non-tumor brain tissues. Correlation analysis revealed that those samples showing reduced expression of MTH1 also had high levels of DNA damage and apoptotic markers, while diminished expression of angiogenesis regulators and levels of migration. MTH1 knockdown in vitro by siRNA in tumor cell lines corroborates the above observation. This justifies the emergence of MTH1 inhibitors as potential first-in-class drugs. Mechanistically, our observations suggest that Hif1α may modulate MTH1 expression.

SIGNIFICANCE

We found elevated MTH1 expression in glioma irrespective of their grades, while its inhibition affects multiple tumor progression pathways, and that targeting Hif1α could simulate the same.

Nudix hydrolase 1 is a prognostic biomarker in hepatocellular carcinoma

We investigated the prognostic significance of Nudix hydrolase 1 (NUDT1) in hepatocellular carcinoma (HCC). NUDT1 mRNA and protein levels were significantly higher in HCC tissues than normal liver tissues. The level of NUDT1 expression correlated with tumor grade, stage, size, differentiation, degree of vascular invasion, overall survival (OS), and disease-free survival (DFS) in HCC patients. Multivariate analysis showed that NUDT1 expression was an independent prognostic factor for OS and DFS in HCC patients. We constructed a prognostic nomogram with NUDT1 expression, AFP levels, vascular invasion, Child-Pugh classification, age, sex, AJCC staging, and tumor differentiation as variables. This nomogram was highly accurate in predicting the 5-year OS of HCC patients (c-index= 0.709; AUC= 0.740). NUDT1 silencing in HCC cells significantly reduced their survival, colony formation, migration, and invasiveness. Gene set enrichment analysis showed that biological pathways related to cell cycle, fatty acid metabolism, bile acid and bile salt metabolism, and PLK1 signaling were associated with NUDT1, as were the gene ontology terms "DNA binding transcription activator activity," "RNA polymerase II," "nuclear division," and "transmembrane transporter activity." Our study thus demonstrates that NUDT1 is a prognostic biomarker with therapeutic potential in HCC patients.

High iodine induces DNA damage in autoimmune thyroiditis partially by inhibiting the DNA repair protein MTH1

This study aims to investigate the level of DNA damage in high iodine (HI)-induced autoimmune thyroiditis (AIT), and to explore the role of DNA repair protein MutT homolog-1 (MTH1) in this process. The levels of pro-inflammatory cytokines tumor necrosis factor (TNF)-α, interleukin (IL)-6, and IL-8 were measured using qRT-PCR and ELISA. The apoptosis was evaluated using TUNEL staining. The pathological changes of thyroid tissues were evaluated using hematoxylin and eosin (HE) staining. The DNA damage was assessed by determining the expression of 8-hydroxy-2'deoxyguanosine (8-OHdG; an indicator of oxidative DNA damage) and performing the Comet assay. Our results showed that both the HI-treated NOD.H-2^h4 mice (experimental AIT mice) and the HI-treated mouse thyroid follicular epithelial cells showed enhanced inflammation, apoptosis, and DNA damage level, accompanied by decreased MTH1 expression. Importantly, overexpression of MTH1 effectively abrogated the HI-induced enhancement of inflammation, apoptosis, and DNA damage in mouse thyroid follicular epithelial cells. In conclusion, HI treatment induces DNA damage in AIT, at least in part, by inhibiting the DNA repair protein MTH1.

Mechanisms of MTH1 inhibition-induced DNA strand breaks: The slippery slope from the oxidized nucleotide pool to genotoxic damage

Unlike normal tissues, tumor cells possess a propensity for genomic instability, resulting from elevated oxidant levels produced by oncogenic signaling and aberrant cellular metabolism. Thus, targeting mechanisms that protect cancer cells from the tumor-inhibitory consequences of their redox imbalance and spontaneous DNA-damaging events is expected to have broad-spectrum efficacy and a high therapeutic index. One critical mechanism for tumor cell protection from oxidant stress is the hydrolysis of oxidized nucleotides. Human MutT homolog 1 (MTH1), the mammalian nudix (nucleoside diphosphate X) pyrophosphatase (NUDT1), protects tumor cells from oxidative stress-induced genomic DNA damage by cleansing the nucleotide pool of oxidized purine nucleotides. Depletion or pharmacologic inhibition of MTH1 results in genomic DNA strand breaks in many cancer cells. However, the mechanisms underlying how oxidized nucleotides, thought mainly to be mutagenic rather than genotoxic, induce DNA strand breaks are largely unknown. Given the recent therapeutic interest in targeting MTH1, a better understanding of such mechanisms is crucial to its successful translation into the clinic and in identifying the molecular contexts under which its inhibition is likely to be beneficial. Here we provide a comprehensive perspective on MTH1 function and its importance in protecting genome integrity, in the context of tumor-associated oxidative stress and the mechanisms that likely lead to irreparable DNA strand breaks as a result of MTH1 inhibition.

hMTH1 is required for maintaining migration and invasion potential of human thyroid cancer cells

Cancer cells, including thyroid cancer cells, suffer from oxidative stress damaging multiple cellular targets, such as DNA and the nucleotide pool. The human MutT homologue 1 (hMTH1) controls the oxidative DNA damage load by sanitizing the nucleotide pool from the oxidized DNA precursor, 8-oxodGTP. It has previously been shown that hMTH1 is essential for cancer cell proliferation and survival, therefore hMTH1 inhibition has been proposed as a novel anticancer therapeutic strategy. Here we show that thyroid cancer cells respond to siRNA mediated hMTH1 depletion with increased DNA damage load and moderately reduced proliferation rates, but without detectable apoptosis, cell-cycle arrest or senescence. Importantly, however, hMTH1 depletion significantly reduced migration and invasion potential of the thyroid cancer cells. Accordingly, our results allow us to propose that hMTH1 may be a therapeutic target in thyroid malignancy, especially for controlling metastasis.