Base excision repair-mediated resistance to cisplatin in KRAS(G12C) mutant NSCLC cells

KRAS inhibitors in drug resistance and potential for combination therapy

Kirsten Rat Sarcoma (KRAS) is a potent target for cancer therapy because it acts as a signaling hub, engaging in various signaling pathways and regulating a number of cellular functions like cell differentiation, proliferation, and survival. Recently, an emergency approval from the US-FDA has been issued for KRASG12C inhibitors (sotorasib and adagrasib) for metastatic lung cancer treatment. However, clinical studies on covalent KRASG12C inhibitors have rapidly confronted resistance in patients. Many methods are being assessed to overcome this resistance, along with various combinatorial clinical studies that are in process. Moreover, because KRASG12D and KRASG12V are more common than KRASG12C, focus must be placed on the therapeutic strategies for this type of patient, along with sustained efforts in research on these targets. In the present review, we try to focus on various strategies to overcome rapid resistance through the use of combinational treatments to improve the activity of KRASG12C inhibitors.

Impact of DNA ligase inhibition on the nick sealing of polβ nucleotide insertion products at the downstream steps of base excision repair pathway

DNA ligase (LIG) I and IIIα finalize base excision repair (BER) by sealing a nick product after nucleotide insertion by DNA polymerase (pol) β at the downstream steps. We previously demonstrated that a functional interplay between polβ and BER ligases is critical for efficient repair, and polβ mismatch or oxidized nucleotide insertions confound the final ligation step. Yet, how targeting downstream enzymes with small molecule inhibitors could affect this coordination remains unknown. Here, we report that DNA ligase inhibitors, L67 and L82-G17, slightly enhance hypersensitivity to oxidative stress-inducing agent, KBrO3, in polβ+/+ cells more than polβ-/- null cells. We showed less efficient ligation after polβ nucleotide insertions in the presence of the DNA ligase inhibitors. Furthermore, the mutations at the ligase inhibitor binding sites (G448, R451, A455) of LIG1 significantly affect nick DNA binding affinity and nick sealing efficiency. Finally, our results demonstrated that the BER ligases seal a gap repair intermediate by the effect of polβ inhibitor that diminishes gap filling activity. Overall, our results contribute to understand how the BER inhibitors against downstream enzymes, polβ, LIG1, and LIGIIIα, could impact the efficiency of gap filling and subsequent nick sealing at the final steps leading to the formation of deleterious repair intermediates.

An Overview of Altered Pathways Associated with Sensitivity to Platinum-Based Chemotherapy in Neuroendocrine Tumors: Strengths and Prospects

Neuroendocrine neoplasms (NENs) are a diverse group of malignancies with a shared phenotype but varying prognosis and response to current treatments. Based on their morphological features and rate of proliferation, NENs can be classified into two main groups with a distinct clinical behavior and response to treatment: (i) well-differentiated neuroendocrine tumors (NETs) or carcinoids (with a low proliferation rate), and (ii) poorly differentiated small- or large-cell neuroendocrine carcinomas (NECs) (with a high proliferation rate). For certain NENs (such as pancreatic tumors, higher-grade tumors, and those with DNA damage repair defects), chemotherapy is the main therapeutic approach. Among the different chemotherapic agents, cisplatin and carboplatin, in combination with etoposide, have shown the greatest efficacy in treating NECs compared to NETs. The cytotoxic effects of cisplatin and carboplatin are primarily due to their binding to DNA, which interferes with normal DNA transcription and/or replication. Consistent with this, NECs, which often have mutations in pathways involved in DNA repair (such as Rb, MDM2, BRCA, and PTEN), have a high response to platinum-based chemotherapy. Identifying mutations that affect molecular pathways involved in the initiation and progression of NENs can be crucial in predicting the response to platinum chemotherapy. This review aims to highlight targetable mutations that could serve as predictors of therapeutic response to platinum-based chemotherapy in NENs.

Strength of selection in lung tumors correlates with clinical features better than tumor mutation burden

Single nucleotide substitutions are the most common type of somatic mutations in cancer genome. The goal of this study was to use publicly available somatic mutation data to quantify negative and positive selection in individual lung tumors and test how strength of directional and absolute selection is associated with clinical features. The analysis found a significant variation in strength of selection (both negative and positive) among tumors, with median selection tending to be negative even though tumors with strong positive selection also exist. Strength of selection estimated as the density of missense mutations relative to the density of silent mutations showed only a weak correlation with tumor mutation burden. In the "all histology together" analysis we found that absolute strength of selection was strongly correlated with all clinically relevant features analyzed. In histology-stratified analysis selection was strongest in small cell lung cancer. Selection in adenocarcinoma was somewhat higher compared to squamous cell carcinoma. The study suggests that somatic mutation- based quantifying of directional and absolute selection in individual tumors can be a useful biomarker of tumor aggressiveness.

Collaboration of Hprt/K-RAS/c-Myc mutation in the oncogenesis of T-lymphocytic leukemia: a comparative study

Aim: Leukemia is a malignant clonal illness stem from the mutations of hematopoietic cells. Acute lymphoblastic leukemia is one of the utmost prevalent kinds of leukemia, is brought on by atypical lymphoid progenitor cell division in the bone marrow. Materials & methods: A comparative study between, titanium Nanoparticle-loaded doxorubicin or cisplatin and lactoferrin-loaded doxorubicin or cisplatin, on 7,12-dimethylbenz[a]-anthracene (DMBA)-induced leukemia was investigated and confirming the hypothesis that messenger RNA of Hprt/K-RAS/c-Myc/SAT-2/P53/JAK-2 is a forthcoming signaling pathways in leukemia. Results: A significant alteration in Hprt, K-RAS, C-Myc, P53, JAK-2 and SAT-2 genes was observed post DMBA intoxication the aforementioned Nanodrugs modulated these signaling pathways. Conclusion: The carrier-loaded drugs triggered cytotoxicity of cancer cells via enhancing drug efficacy and bio-availability.

Zinc Oxide Nanoparticles and Cancer Chemotherapy: Helpful Tools for Enhancing Chemo-sensitivity and Reducing Side Effects?

Cancer chemotherapy is still a serious challenge. Chemo-resistance and destructive side effects of chemotherapy drugs are the most critical limitations of chemotherapy. Chemo-resistance is the leading cause of chemotherapy failure. Chemo-resistance, which refers to the resistance of cancer cells to the anticancer effects of chemotherapy drugs, is caused by various reasons. Among the most important of these reasons is the increase in the efflux of chemotherapy drugs due to the rise in the expression and activity of ABC transporters, the weakening of apoptosis, and the strengthening of stemness. In the last decade, a significant number of studies focused on the application of nanotechnology in cancer treatment. Considering the anti-cancer properties of zinc, zinc oxide nanoparticles have received much attention in recent years. Some studies have indicated that zinc oxide nanoparticles can target the critical mechanisms of cancer chemo-resistance and enhance the effectiveness of chemotherapy drugs. These studies have shown that zinc oxide nanoparticles can reduce the activity of ABC transporters, increase DNA damage and apoptosis, and attenuate stemness in cancer cells, leading to enhanced chemo-sensitivity. Some other studies have also shown that zinc oxide nanoparticles in low doses can be helpful in minimizing the harmful side effects of chemotherapy drugs. In this article, after a brief overview of the mechanisms of chemo-resistance and anticancer effects of zinc, we will review all these studies in detail.

Pan-Cancer Interrogation of MUTYH Variants Reveals Biallelic Inactivation and Defective Base Excision Repair Across a Spectrum of Solid Tumors

PURPOSE

Biallelic germline pathogenic variants of the base excision repair (BER) pathway gene MUTYH predispose to colorectal cancer (CRC) and other cancers. The possible association of heterozygous variants with broader cancer susceptibility remains uncertain. This study investigated the prevalence and consequences of pathogenic MUTYH variants and MUTYH loss of heterozygosity (LOH) in a large pan-cancer analysis.

MATERIALS AND METHODS

Data from 354,366 solid tumor biopsies that were sequenced as part of routine clinical care were analyzed using a validated algorithm to distinguish germline from somatic MUTYH variants.

RESULTS

Biallelic germline pathogenic MUTYH variants were identified in 119 tissue biopsies. Most were CRCs and showed increased tumor mutational burden (TMB) and a mutational signature consistent with defective BER (COSMIC Signature SBS18). Germline heterozygous pathogenic variants were identified in 5,991 biopsies and their prevalence was modestly elevated in some cancer types. About 12% of these cancers (738 samples: including adrenal gland cancers, pancreatic islet cell tumors, nonglioma CNS tumors, GI stromal tumors, and thyroid cancers) showed somatic LOH for MUTYH, higher rates of chromosome 1p loss (where MUTYH is located), elevated genomic LOH, and higher COSMIC SBS18 signature scores, consistent with BER deficiency.

CONCLUSION

This analysis of MUTYH alterations in a large set of solid cancers suggests that in addition to the established role of biallelic pathogenic MUTYH variants in cancer predisposition, a broader range of cancers may possibly arise in MUTYH heterozygotes via a mechanism involving somatic LOH at the MUTYH locus and defective BER. However, the effect is modest and requires confirmation in additional studies before being clinically actionable.

NSCLC Cells Resistance to PI3K/mTOR Inhibitors Is Mediated by Delta-6 Fatty Acid Desaturase (FADS2)

Hyperactivation of the phosphatidylinositol-3-kinase (PI3K) pathway is one of the most common events in human cancers. Several efforts have been made toward the identification of selective PI3K pathway inhibitors. However, the success of these molecules has been partially limited due to unexpected toxicities, the selection of potentially responsive patients, and intrinsic resistance to treatments. Metabolic alterations are intimately linked to drug resistance; altered metabolic pathways can help cancer cells adapt to continuous drug exposure and develop resistant phenotypes. Here we report the metabolic alterations underlying the non-small cell lung cancer (NSCLC) cell lines resistant to the usual PI3K-mTOR inhibitor BEZ235. In this study, we identified that an increased unsaturation degree of lipid species is associated with increased plasma membrane fluidity in cells with the resistant phenotype and that fatty acid desaturase FADS2 mediates the acquisition of chemoresistance. Therefore, new studies focused on reversing drug resistance based on membrane lipid modifications should consider the contribution of desaturase activity.

NKp44-Derived Peptide Used in Combination Stimulates Antineoplastic Efficacy of Targeted Therapeutic Drugs

Lung cancer cells in the tumor microenvironment facilitate immune evasion that leads to failure of conventional chemotherapies, despite provisionally decided on the genetic diagnosis of patients in a clinical setup. The current study follows three lung cancer patients who underwent "personalized" chemotherapeutic intervention. Patient-derived xenografts (PDXs) were subjected to tumor microarray and treatment screening with chemotherapies, either individually or in combination with the peptide R11-NLS-pep8; this peptide targets both membrane-associated and nuclear PCNA. Ex vivo, employing PDX-derived explants, it was found that combination with R11-NLS-pep8 stimulated antineoplastic effect of chemotherapies that were, although predicted based on the patient's genetic mutation, inactive on their own. Furthermore, treatment in vivo of PDX-bearing mice showed an exactly similar trend in the result, corroborating the finding to be translated into clinical setup.

The Prognostic and Therapeutic Potential of DNA Damage Repair Pathway Alterations and Homologous Recombination Deficiency in Lung Cancer

Lung cancer remains the second most commonly diagnosed cancer worldwide and the leading cause of cancer-related mortality. The mapping of genomic alterations and their role in lung-cancer progression has been followed by the development of new therapeutic options. Several novel drugs, such as targeted therapy and immunotherapy, have significantly improved outcomes. However, many patients with lung cancer do not benefit from existing therapies or develop progressive disease, leading to increased morbidity and mortality despite initial responses to treatment. Alterations in DNA-damage repair (DDR) genes represent a cancer hallmark that impairs a cell's ability to prevent deleterious mutation accumulation and repair. These alterations have recently emerged as a therapeutic target in breast, ovarian, prostate, and pancreatic cancers. The role of DDR alterations remains largely unknown in lung cancer. Nevertheless, recent research efforts have highlighted a potential role of some DDR alterations as predictive biomarkers of response to treatment. Despite the failure of PARP inhibitors (main class of DDR targeting agents) to improve outcomes in lung cancer patients, there is some evidence suggesting a role of PARP inhibitors and other DDR targeting agents in benefiting a distinct subset of lung cancer patients. In this review, we will discuss the existing literature on DDR alterations and homologous recombination deficiency (HRD) state as predictive biomarkers and therapeutic targets in both non-small cell lung and small cell lung cancer.

RAS oncogenic activity predicts response to chemotherapy and outcome in lung adenocarcinoma

Activating mutations in KRAS occur in 32% of lung adenocarcinomas (LUAD). Despite leading to aggressive disease and resistance to therapy in preclinical studies, the KRAS mutation does not predict patient outcome or response to treatment, presumably due to additional events modulating RAS pathways. To obtain a broader measure of RAS pathway activation, we developed RAS84, a transcriptional signature optimised to capture RAS oncogenic activity in LUAD. We report evidence of RAS pathway oncogenic activation in 84% of LUAD, including 65% KRAS wild-type tumours, falling into four groups characterised by coincident alteration of STK11/LKB1, TP53 or CDKN2A, suggesting that the classifications developed when considering only KRAS mutant tumours have significance in a broader cohort of patients. Critically, high RAS activity patient groups show adverse clinical outcome and reduced response to chemotherapy. Patient stratification using oncogenic RAS transcriptional activity instead of genetic alterations could ultimately assist in clinical decision-making.

NEDD9 Restrains dsDNA Damage Response during Non-Small Cell Lung Cancer (NSCLC) Progression

DNA damaging modalities are the backbone of treatments for non-small cell lung cancer (NSCLC). Alterations in DNA damage response (DDR) in tumor cells commonly contribute to emerging resistance to platinating agents, other targeted therapies, and radiation. The goal of this study is to identify the previously unreported role of NEDD9 scaffolding protein in controlling DDR processes and sensitivity to DNA damaging therapies. Using a siRNA-mediated approach to deplete NEDD9 in a group of human and murine KRAS/TP53-mutant NSCLC cell lines, coupled with a set of cell viability and clonogenic assays, flow cytometry analysis, and Western blotting, we evaluated the effects of NEDD9 silencing on cellular proliferation, DDR and epithelial-to-mesenchymal transition (EMT) signaling, cell cycle, and sensitivity to cisplatin and UV irradiation. Using publicly available NSCLC datasets (TCGA) and an independent cohort of primary NSCLC tumors, subsequent in silico and immunohistochemical (IHC) analyses were performed to assess relevant changes in NEDD9 RNA and protein expression across different stages of NSCLC. The results of our study demonstrate that NEDD9 depletion is associated with the increased tumorigenic capacity of NSCLC cells. These phenotypes were accompanied by significantly upregulated ATM-CHK2 signaling, shifting towards a more mesenchymal phenotype in NEDD9 depleted cells and elevated sensitivity to UV-irradiation. IHC analyses revealed an association between reduced NEDD9 protein expression and a decrease in overall (OS) and progression-free survival (PFS) of the NSCLC patients. These data, for the first time, identified NEDD9 as a negative regulator of ATM kinase activity and related DDR signaling in numerous KRAS/TP53 mutated NSCLC, with its effects on the regulation of DDR-dependent EMT signaling, sensitivity to DNA damaging modalities in tumor cells, and the survival of the patients.

The Influence of Oncogenic RAS on Chemotherapy and Radiotherapy Resistance Through DNA Repair Pathways

RAS oncogenes are chief tumorigenic drivers, and their mutation constitutes a universal predictor of poor outcome and treatment resistance. Despite more than 30 years of intensive research since the identification of the first RAS mutation, most attempts to therapeutically target RAS mutants have failed to reach the clinic. In fact, the first mutant RAS inhibitor, Sotorasib, was only approved by the FDA until 2021. However, since Sotorasib targets the KRAS G12C mutant with high specificity, relatively few patients will benefit from this therapy. On the other hand, indirect approaches to inhibit the RAS pathway have revealed very intricate cascades involving feedback loops impossible to overcome with currently available therapies. Some of these mechanisms play different roles along the multistep carcinogenic process. For instance, although mutant RAS increases replicative, metabolic and oxidative stress, adaptive responses alleviate these conditions to preserve cellular survival and avoid the onset of oncogene-induced senescence during tumorigenesis. The resulting rewiring of cellular mechanisms involves the DNA damage response and pathways associated with oxidative stress, which are co-opted by cancer cells to promote survival, proliferation, and chemo- and radioresistance. Nonetheless, these systems become so crucial to cancer cells that they can be exploited as specific tumor vulnerabilities. Here, we discuss key aspects of RAS biology and detail some of the mechanisms that mediate chemo- and radiotherapy resistance of mutant RAS cancers through the DNA repair pathways. We also discuss recent progress in therapeutic RAS targeting and propose future directions for the field.

Failure to EGFR-TKI-based therapy and tumoural progression are promoted by MEOX2/GLI1-mediated epigenetic regulation of EGFR in the human lung cancer

BACKGROUND

Mesenchyme homeobox-2 (MEOX2)-mediated regulation of glioma-associated oncogene-1 (GLI1) has been associated with poor overall survival, conferring chemoresistance in lung cancer. However, the role of MEOX2/GLI1 in resistance to epidermal growth factor receptor-tyrosine kinase inhibitors (EGFR-TKIs)-based therapy remains unexplored in human lung cancer.

METHODS

Functional assays using genetic silencing strategy by short hairpin RNAs, as well as cytotoxic (tetrazolium dye MTT) and clonogenic assays, were performed to evaluate MEOX2/GLI1-induced malignancy capacity in lung cancer cells. Further analysis performed includes western blot, qPCR and ChIP-qPCR assays to identify whether MEOX2/GLI1 promote EGFR/AKT/ERK activation, as well as EGFR overexpression through epigenetic mechanisms. Finally, preclinical tumour progression in vivo and progression-free disease interval analyses in patients treated with EGFR-TKI were included.

RESULTS

Overexpressed MEOX2/GLI1 in both EGFR wild-type and EGFR/KRAS-mutated lung cancer cells were detected and involved in the activation/expression of EGFR/AKT/ERK biomarkers. In addition, MEOX2/GLI1 was shown to be involved in the increased proliferation of tumour cells and resistance capacity to cisplatin, EGFR-TKIs (erlotinib and AZD9291 'osimertinib'), AZD8542-SMO, and AZD6244-MEKK1/2. In addition, we identified that MEOX2/GLI1 promote lung tumour cells progression in vivo and are clinically associated with poorer progression-free disease intervals. Finally, both MEOX2 and GLI1 were detected to be epigenetically involved in EGFR expression by reducing both repressive markers polycomb-EZH2 and histone H3K27me3, but, particularly, increasing an activated histone profile H3K27Ac/H3K4me3 at EGFR-gene enhancer-promoter sequences that probably representing a novel EGFR-TKI-based therapy resistance mechanism.

CONCLUSION

MEOX2/GLI1 promote resistance to cisplatin and EGFR-TKI-based therapy in lung cancer cells, modulating EGFR/AKT/ERK signalling pathway activation, as well as inducing an aberrant epigenetic modulation of the EGFR-gene expression in human lung cancer.

Oncogenic KRAS drives radioresistance through upregulation of NRF2-53BP1-mediated non-homologous end-joining repair

KRAS-activating mutations are oncogenic drivers and are correlated with radioresistance of multiple cancers, including colorectal cancer, but the underlying precise molecular mechanisms remain elusive. Herein we model the radiosensitivity of isogenic HCT116 and SW48 colorectal cancer cell lines bearing wild-type or various mutant KRAS isoforms. We demonstrate that KRAS mutations indeed lead to radioresistance accompanied by reduced radiotherapy-induced mitotic catastrophe and an accelerated release from G2/M arrest. Moreover, KRAS mutations result in increased DNA damage response and upregulation of 53BP1 with associated increased non-homologous end-joining (NHEJ) repair. Remarkably, KRAS mutations lead to activation of NRF2 antioxidant signaling to increase 53BP1 gene transcription. Furthermore, genetic silencing or pharmacological inhibition of KRAS, NRF2 or 53BP1 attenuates KRAS mutation-induced radioresistance, especially in G1 phase cells. These findings reveal an important role for a KRAS-induced NRF2-53BP1 axis in the DNA repair and survival of KRAS-mutant tumor cells after radiotherapy, and indicate that targeting NRF2, 53BP1 or NHEJ may represent novel strategies to selectively abrogate KRAS mutation-mediated radioresistance.

The integrated stress response is tumorigenic and constitutes a therapeutic liability in KRAS-driven lung cancer

The integrated stress response (ISR) is an essential stress-support pathway increasingly recognized as a determinant of tumorigenesis. Here we demonstrate that ISR is pivotal in lung adenocarcinoma (LUAD) development, the most common histological type of lung cancer and a leading cause of cancer death worldwide. Increased phosphorylation of the translation initiation factor eIF2 (p-eIF2α), the focal point of ISR, is related to invasiveness, increased growth, and poor outcome in 928 LUAD patients. Dissection of ISR mechanisms in KRAS-driven lung tumorigenesis in mice demonstrated that p-eIF2α causes the translational repression of dual specificity phosphatase 6 (DUSP6), resulting in increased phosphorylation of the extracellular signal-regulated kinase (p-ERK). Treatments with ISR inhibitors, including a memory-enhancing drug with limited toxicity, provides a suitable therapeutic option for KRAS-driven lung cancer insofar as they substantially reduce tumor growth and prolong mouse survival. Our data provide a rationale for the implementation of ISR-based regimens in LUAD treatment.

The Importance of Being PI3K in the RAS Signaling Network

Ras proteins are essential mediators of a multitude of cellular processes, and its deregulation is frequently associated with cancer appearance, progression, and metastasis. Ras-driven cancers are usually aggressive and difficult to treat. Although the recent Food and Drug Administration (FDA) approval of the first Ras G12C inhibitor is an important milestone, only a small percentage of patients will benefit from it. A better understanding of the context in which Ras operates in different tumor types and the outcomes mediated by each effector pathway may help to identify additional strategies and targets to treat Ras-driven tumors. Evidence emerging in recent years suggests that both oncogenic Ras signaling in tumor cells and non-oncogenic Ras signaling in stromal cells play an essential role in cancer. PI3K is one of the main Ras effectors, regulating important cellular processes such as cell viability or resistance to therapy or angiogenesis upon oncogenic Ras activation. In this review, we will summarize recent advances in the understanding of Ras-dependent activation of PI3K both in physiological conditions and cancer, with a focus on how this signaling pathway contributes to the formation of a tumor stroma that promotes tumor cell proliferation, migration, and spread.

Olaparib enhances curcumin-mediated apoptosis in oral cancer cells by inducing PARP trapping through modulation of BER and chromatin assembly

Apart from inducing catalytic inhibition of PARP-1, PARP inhibitors can also trap PARP proteins at the sites of DNA damage and forming toxic PARP-DNA complexes. These complexes obstruct the DNA repair process, resulting in cancer cell death. To study the detailed mechanism of anti-cancer action through PARP trapping, we have treated oral cancer cells (H-357) with curcumin (Cur), olaparib (Ola) and their combination (Cur + Ola). Cur + Ola treatment triggered the expressions of PARP-1 and adenomatous polyposis coli (APC) and down regulated other base excision repair (BER) proteins in the chromatin fraction but not in the nuclear fraction. Cur + Ola treatment inhibited PARylation, altered interaction of PARP-1 with representative BER proteins and arrested cells in S-phase. We have for the first time provided direct evidence and measured the cellular PARP-1 trapping potentiality of Ola in Cur pretreated H-357 cells. Unchanged cellular PARP-1 trapping, unaltered expression of BER proteins and BER activity were found in APC silenced H-357 cells, which further confirmed that the DNA damage/repair response was APC-dependent. Interestingly, complete abolishment of the chromatin remodeler 'amplified in Liver Cancer 1' (ALC1), decreased expression of Histone H3 and histone acetyltransferase (P300) was noted in chromatin of Cur + Ola treated cells. Their expressions remained unchanged in APC silenced cells. Cur + Ola also altered the interaction of ALC1 with BER proteins including APC. Thus, the present study reveals that Cur + Ola treatment increased oral cancer cell death not only through catalytic inhibition of PARP-1 but also predominantly through PARP-1 trapping and indirect inhibition of chromatin remodeling.

Base excision repair and its implications to cancer therapy

Base excision repair (BER) has evolved to preserve the integrity of DNA following cellular oxidative stress and in response to exogenous insults. The pathway is a coordinated, sequential process involving 30 proteins or more in which single strand breaks are generated as intermediates during the repair process. While deficiencies in BER activity can lead to high mutation rates and tumorigenesis, cancer cells often rely on increased BER activity to tolerate oxidative stress. Targeting BER has been an attractive strategy to overwhelm cancer cells with DNA damage, improve the efficacy of radiotherapy and/or chemotherapy, or form part of a lethal combination with a cancer specific mutation/loss of function. We provide an update on the progress of inhibitors to enzymes involved in BER, and some of the challenges faced with targeting the BER pathway.

The importance of Ras in drug resistance in cancer

In this review, we analyse the impact of oncogenic Ras mutations in mediating cancer drug resistance and the progress made in the abrogation of this resistance, through pharmacological targeting. At a physiological level, Ras is implicated in many cellular proliferation and survival pathways. However, mutations within this small GTPase can be responsible for the initiation of cancer, therapeutic resistance and failure, and ultimately disease relapse. Often termed "undruggable," Ras is notoriously difficult to target directly, due to its structure and intrinsic activity. Thus, Ras-mediated drug resistance remains a considerable pharmacological problem. However, with advances in both analytical techniques and novel drug classes, the therapeutic landscape against Ras is changing. Allele-specific, direct Ras-targeting agents have reached clinical trials for the first time, indicating there may, at last, be hope of targeting such an elusive but significant protein for better more effective cancer therapy.

Establishment and Characterization of Patient-Derived Xenografts (PDXs) of Different Histology from Malignant Pleural Mesothelioma Patients

Simple Summary

Malignant pleural mesothelioma (MPM) is a rare tumor with unfavorable prognosis for which new therapeutic interventions are urgently needed. The aim of our study was to develop a preclinical model representative of the different histotypes found in human tumors that can be used as models for the discovery of new treatments and combinations. We successfully generated patient-derived xenografts (PDXs) from MPM, which strongly resembled the tumors of origin in terms of morphology and immunohistochemistry. These tumors, when growing in mice, poorly respond to cisplatin, a finding that aligned with the clinical results. From one of the PDXs, we generated 2D and 3D cultures maintaining the phenotypical characteristics of human tumors and PDXs. Altogether, these preclinical models represent a useful tool for the discovery of new targets and drug combinations.

Abstract

Background: Malignant pleural mesothelioma (MPM) is a very aggressive tumor originating from mesothelial cells. Although several etiological factors were reported to contribute to MPM onset, environmental exposure to asbestos is certainly a major risk factor. The latency between asbestos (or asbestos-like fibers) exposure and MPM onset is very long. MPM continues to be a tumor with poor prognosis despite the introduction of new therapies including immunotherapy. One of the major problems is the low number of preclinical models able to recapitulate the features of human tumors. This impacts the possible discovery of new treatments and combinations. Methods: In this work, we aimed to generate patient-derived xenografts (PDXs) from MPM patients covering the three major histotypes (epithelioid, sarcomatoid, and mixed) occurring in the clinic. To do this, we obtained fresh tumors from biopsies or pleurectomies, and samples were subcutaneously implanted in immunodeficient mice within 24 h. Results: We successfully isolated different PDXs and particularly concentrated our efforts on three covering the three histotypes. The tumors that grew in mice compared well histologically with the tumors of origin, and showed stable growth in mice and a low response to cisplatin, as was observed in the clinic. Conclusions: These models are helpful in testing new drugs and combinations that, if successful, could rapidly translate to the clinical setting.

Molecular determinants of response to PI3K/akt/mTOR and KRAS pathways inhibitors in NSCLC cell lines

Despite the impressive results obtained in the preclinical setting, all the inhibitors targeting two central cascades in cancer, the PI3K/akt/mTOR and the KRAS/MEK/ERK pathways, have shown, apart from very few exceptions, disappointing efficacy when translated to the clinic. One of the main reasons of their clinical failure seems to be the lack of a clear molecular determinant of response to these drugs. In this study, we tried to address this point by evaluating the cytotoxic activity of different inhibitors targeting the two pathways at different levels in a panel of ten NSCLC cell lines harboring alterations in PI3K, KRAS or both. We were not able to highlight a correlation between the presence of KRAS and PI3K mutations and a specific sensitivity to the different drugs used. Molecular analyses performed after equimolar treatments showed that, independently from the entity of the response, the drugs are able to modulate the activation of their targets. Interestingly, we found that p53 mutational status separates the cell lines according to their sensitivity to PI3K pathway inhibitors treatments. The alterations considered in the PI3K/akt/mTOR and in the KRAS/MEK/ERK pathways in the different NSCLC cell lines are not sufficient to drive treatment choice but rather p53 status is a potential biomarker for the activity of this class of drugs.

Activity of Birinapant, a SMAC Mimetic Compound, Alone or in Combination in NSCLCs With Different Mutations

Liver kinase B1 (LKB1/STK11) is the second tumor suppressor gene most frequently mutated in non-small-cell lung cancer (NSCLC) and its activity is impaired in about half KRAS-mutated NSCLCs. Nowadays, no effective therapies are available for patients having these mutations. To highlight new vulnerabilities of this subgroup of tumors exploitable to design specific therapies we screened an US FDA-approved drug library using an isogenic system of wild-type (WT) or deleted LKB1. Among eight hit compounds, Birinapant, an inhibitor of the Inhibitor of Apoptosis Proteins (IAPs), was the most active compound in LKB1-deleted clone only compared to its LKB1 WT counterpart. We validated the Birinapant cells response and its mechanism of action to be dependent on LKB1 deletion. Indeed, we demonstrated the ability of this compound to induce apoptosis, through activation of caspases in the LKB1-deleted clone only. Expanding our results, we found that the presence of KRAS mutations could mediate Birinapant resistance in a panel of NSCLC cell lines. The combination of Birinapant with Ralimetinib, inhibitor of p38α, restores the sensitivity of LKB1- and KRAS-mutated cell lines to the IAP inhibitor Birinapant. Our study shows how the use of Birinapant could be a viable therapeutic option for patients with LKB1-mutated NSCLCs. In addition, combination of Birinapant and a KRAS pathway inhibitor, as Ralimetinib, could be useful for patients with LKB1 and KRAS-mutated NSCLC.

Impact of ERCC1, XPF and DNA Polymerase β Expression on Platinum Response in Patient-Derived Ovarian Cancer Xenografts

Platinum resistance is an unmet medical need in ovarian carcinoma. Molecular biomarkers to predict the response to platinum-based therapy could allow patient stratification and alternative therapeutic strategies early in clinical management. Sensitivity and resistance to platinum therapy are partially determined by the tumor’s intrinsic DNA repair activities, including nucleotide excision repair (NER) and base excision repair (BER). We investigated the role of the NER proteins—ERCC1, XPF, ERCC1/XPF complex—and of the BER protein DNA polymerase β, as possible biomarkers of cisplatin (DDP) response in a platform of recently established patient-derived ovarian carcinoma xenografts (OC-PDXs). ERCC1 and DNA polymerase β protein expressions were measured by immunohistochemistry, the ERCC1/XPF foci number was detected by proximity ligation assay (PLA) and their mRNA levels by real-time PCR. We then correlated the proteins, gene expression and ERCC1/XPF complexes with OC-PDXs’ response to platinum. To the best of our knowledge, this is the first investigation of the role of the ERCC1/XPF complex, detected by PLA, in relation to the response to DDP in ovarian carcinoma. None of the proteins in the BER and NER pathways studied predicted platinum activity in this panel of OC-PDXs, nor did the ERCC1/XPF foci number. These results were partially explained by the experimental evidence that the ERCC1/XPF complex increases after DDP treatment and this possibly better associates with the cancer cells’ abilities to activate the NER pathway to repair platinum-induced damage than its basal level. Our findings highlight the need for DNA functional assays to predict the response to platinum-based therapy.

Predicting the Role of DNA Polymerase β Alone or with KRAS Mutations in Advanced NSCLC Patients Receiving Platinum-Based Chemotherapy

Clinical data suggest that only a subgroup of non-small cell lung cancer (NSCLC) patients has long-term benefits after front-line platinum-based therapy. We prospectively investigate whether KRAS status and DNA polymerase β expression could help identify patients responding to platinum compounds. Prospectively enrolled, advanced NSCLC patients treated with a first-line regimen containing platinum were genotyped for KRAS and centrally evaluated for DNA polymerase β expression. Overall survival (OS), progression-free survival (PFS), and the objective response rate (ORR) were recorded. Patients with KRAS mutations had worse OS (hazard ratio (HR): 1.37, 95% confidence interval (95% CI): 0.70–2.27). Negative DNA polymerase β staining identified a subgroup with worse OS than patients expressing the protein (HR: 1.43, 95% CI: 0.57–3.57). The addition of KRAS to the analyses further worsened the prognosis of patients with negative DNA polymerase β staining (HR: 1.67, 95% CI: 0.52–5.56). DNA polymerase β did not influence PFS and ORR. KRAS may have a negative role in platinum-based therapy responses in NSCLC, but its impact is limited. DNA polymerase β, when not expressed, might indicate a group of patients with poor outcomes. KRAS mutations in tumors not expressing DNA polymerase β further worsens survival. Therefore, these two biomarkers together might well identify patients for whom alternatives to platinum-based chemotherapy should be used.

Prognostic value and therapeutic implications of expanded molecular testing for resected early stage lung adenocarcinoma

OBJECTIVES

This study aimed to evaluate the prognostic and potential therapeutic value of expanded molecular testing of resected early-stage lung ACA.

METHODS

We analyzed 324 patients who underwent lobectomy and lymphadenectomy for clinical Stage I&II lung ACA between 2011-2017. Molecular testing was routinely performed, first by PCR-based Sanger sequencing and FISH and then expanded to a 20 and then 50-gene next generation sequencing (NGS) panel. The frequency of mutations by testing method and their association with disease-free (DFS) and overall survival (OS) were tested.

RESULTS

A total of 241 patients (74.4%) had at least one somatic mutation detected, with KRAS exon 2 (38.1%) and EGFR (17.9%) being the most common. TP53 was the most frequent co-existing mutation. Detection of at least one mutation increased from 49% with selective PCR/FISH testing to 82% with limited NGS/FISH, and 91% with extended NGS/FISH (p < 0.001). The rate of actionable mutations increased from 18% to 32% and 45% with expansion of molecular testing, respectively (p = 0.001). Using NGS, an additional 10 cases with EGFR mutations, and other rare mutations were found, including BRAF (5.9%), MET (5.6%), ERBB2 (4.1%), PIK3CA (2.3%), and DDR2 (2.1%). The expansion of FISH testing resulted in one additional detection of ROS1 and RET (1%) rearrangement. KRAS mutation was associated with worse DFS (HR 1.87; 95%CI 1.14-3.06) and OS (HR 2.09; 95%CI 1.11-3.92). BRAF mutation detected in NGS tested patients was also associated with decreased DFS (HR3.80; 95%CI 1.46-9.89) and OS (HR 7.37; 95%CI 2.36-22.99) on multivariate analysis.

CONCLUSION

The expansion of molecular testing has resulted in a substantial increase in the detection of potentially therapeutically significant mutations in resected early-stage ACA. KRAS and BRAF mutation status by NGS was prognostic for relapse and survival. These data emphasize opportunities for clinical trials in a growing number surgical ACA patients with available targeted therapies.

PARP inhibitor Olaparib Enhances the Apoptotic Potentiality of Curcumin by Increasing the DNA Damage in Oral Cancer Cells through Inhibition of BER Cascade

Although Olaparib (Ola, a PARP-inhibitor), in combination with other chemotherapeutic agents, was clinically approved to treat prostate cancer, but cytotoxicity, off-target effects of DNA damaging agents limit its applications in clinic. To improve the anti-cancer activity and to study the detailed mechanism of anti-cancer action, here we have used bioactive compound curcumin (Cur) in combination with Ola. Incubation of Ola in Cur pre-treated cells synergistically increased the death of oral cancer cells at much lower concentrations than individual optimum dose and inhibited the topoisomerase activity. Short exposure of Cur caused DNA damage in cells, but more increased DNA damage was noticed when Ola has incubated in Cur pre-treated cells. This combination did not alter the major components of homologous recombination (HR) and non-homologous end-joining (NHEJ) pathways but significantly altered both short patch (SP) and long patch (LP) base excision repair (BER) components in cancer cells. Significant reduction in relative luciferase activity, expression of BER components and PARylation after Cur and Ola treatment confirmed this combination inhibit the BER activity in cells. Reduction of PARylation, decreased expression of BER components, decreased tumor volume and induction of apoptosis were also noticed in Cur + Ola treated Xenograft mice model. The combination treatment of Cur and Ola also helped in recovering the body weight of tumor-bearing mice. Thus, Cur + Ola combination increased the oral cancer cells death by not only causing the DNA damage but also blocking the induction of BER activity.

LKB1 Deficiency Renders NSCLC Cells Sensitive to ERK Inhibitors

INTRODUCTION

Serine/threonine kinase 11 (LKB1/STK11) is one of the most mutated genes in NSCLC accounting for approximately one-third of cases and its activity is impaired in approximately half of KRAS-mutated NSCLC. At present, these patients cannot benefit from any specific therapy.

METHODS

Through CRISPR/Cas9 technology, we systematically deleted LKB1 in both wild-type (WT) and KRAS-mutated human NSCLC cells. By using these isogenic systems together with genetically engineered mouse models we investigated the cell response to ERK inhibitors both in vitro and in vivo.

RESULTS

In all the systems used here, the loss of LKB1 creates vulnerability and renders these cells particularly sensitive to ERK inhibitors both in vitro and in vivo. The same cells expressing a WT LKB1 poorly respond to these drugs. At the molecular level, in the absence of LKB1, ERK inhibitors induced a marked inhibition of p90 ribosomal S6 kinase activation, which in turn abolished S6 protein activation, promoting the cytotoxic effect.

CONCLUSIONS

This work shows that ERK inhibitors are effective in LKB1 and LKB1/KRAS-mutated tumors, thus offering a therapeutic strategy for this prognostically unfavorable subgroup of patients. Because ERK inhibitors are already in clinical development, our findings could be easily translatable to the clinic. Importantly, the lack of effect in cells expressing WT LKB1, predicts that treatment of LKB1-mutated tumors with ERK inhibitors should have a favorable toxicity profile.

Platinum Resistance in Ovarian Cancer: Role of DNA Repair

Epithelial ovarian cancer (EOC) is the most lethal gynecological cancer. It is initially responsive to cisplatin and carboplatin, two DNA damaging agents used in first line therapy. However, almost invariably, patients relapse with a tumor resistant to subsequent treatment with platinum containing drugs. Several mechanisms associated with the development of acquired drug resistance have been reported. Here we focused our attention on DNA repair mechanisms, which are fundamental for recognition and removal of platinum adducts and hence for the ability of these drugs to exert their activity. We analyzed the major DNA repair pathways potentially involved in drug resistance, detailing gene mutation, duplication or deletion as well as polymorphisms as potential biomarkers for drug resistance development. We dissected potential ways to overcome DNA repair-associated drug resistance thanks to the development of new combinations and/or drugs directly targeting DNA repair proteins or taking advantage of the vulnerability arising from DNA repair defects in EOCs.

Platinum Resistance in Ovarian Cancer: Role of DNA Repair

Epithelial ovarian cancer (EOC) is the most lethal gynecological cancer. It is initially responsive to cisplatin and carboplatin, two DNA damaging agents used in first line therapy. However, almost invariably, patients relapse with a tumor resistant to subsequent treatment with platinum containing drugs. Several mechanisms associated with the development of acquired drug resistance have been reported. Here we focused our attention on DNA repair mechanisms, which are fundamental for recognition and removal of platinum adducts and hence for the ability of these drugs to exert their activity. We analyzed the major DNA repair pathways potentially involved in drug resistance, detailing gene mutation, duplication or deletion as well as polymorphisms as potential biomarkers for drug resistance development. We dissected potential ways to overcome DNA repair-associated drug resistance thanks to the development of new combinations and/or drugs directly targeting DNA repair proteins or taking advantage of the vulnerability arising from DNA repair defects in EOCs.

Platinum Resistance in Ovarian Cancer: Role of DNA Repair

Epithelial ovarian cancer (EOC) is the most lethal gynecological cancer. It is initially responsive to cisplatin and carboplatin, two DNA damaging agents used in first line therapy. However, almost invariably, patients relapse with a tumor resistant to subsequent treatment with platinum containing drugs. Several mechanisms associated with the development of acquired drug resistance have been reported. Here we focused our attention on DNA repair mechanisms, which are fundamental for recognition and removal of platinum adducts and hence for the ability of these drugs to exert their activity. We analyzed the major DNA repair pathways potentially involved in drug resistance, detailing gene mutation, duplication or deletion as well as polymorphisms as potential biomarkers for drug resistance development. We dissected potential ways to overcome DNA repair-associated drug resistance thanks to the development of new combinations and/or drugs directly targeting DNA repair proteins or taking advantage of the vulnerability arising from DNA repair defects in EOCs.

Co-occurring KRAS mutation/LKB1 loss in non-small cell lung cancer cells results in enhanced metabolic activity susceptible to caloric restriction: an in vitro integrated multilevel approach

Background

Non–small-cell lung cancer (NSCLC) is a heterogeneous disease, with multiple different oncogenic mutations. Approximately 25–30% of NSCLC patients present KRAS mutations, which confer poor prognosis and high risk of tumor recurrence. About half of NSCLCs with activating KRAS lesions also have deletions or inactivating mutations in the serine/threonine kinase 11 (LKB1) gene. Loss of LKB1 on a KRAS-mutant background may represent a significant source of heterogeneity contributing to poor response to therapy.

Methods

Here, we employed an integrated multilevel proteomics, metabolomics and functional in-vitro approach in NSCLC H1299 isogenic cells to define their metabolic state associated with the presence of different genetic background. Protein levels were obtained by label free and single reaction monitoring (SRM)-based proteomics. The metabolic state was studied coupling targeted and untargeted mass spectrometry (MS) strategy. In vitro metabolic dependencies were evaluated using 2-deoxy glucose (2-DG) treatment or glucose/glutamine nutrient limitation.

Results

Here we demonstrate that co-occurring KRAS mutation/LKB1 loss in NSCLC cells allowed efficient exploitation of glycolysis and oxidative phosphorylation, when compared to cells with each single oncologic genotype. The enhanced metabolic activity rendered the viability of cells with both genetic lesions susceptible towards nutrient limitation.

Conclusions

Co-occurrence of KRAS mutation and LKB1 loss in NSCLC cells induced an enhanced metabolic activity mirrored by a growth rate vulnerability under limited nutrient conditions relative to cells with the single oncogenetic lesions. Our results hint at the possibility that energy stress induced by calorie restriction regimens may sensitize NSCLCs with these co-occurring lesions to cytotoxic chemotherapy.

Electronic supplementary material

The online version of this article (10.1186/s13046-018-0954-5) contains supplementary material, which is available to authorized users.

Generation and characterization of MEK and ERK inhibitors- resistant non-small-cells-lung-cancer (NSCLC) cells

BACKGROUND

The RAS/RAF/MEK/ERK pathway is one of the most downregulated pathway in cancer. Inhibitors of RAF and MEK have established clinical use while ERK inhibitors recently faced the clinic. We aimed to generate resistant cell lines which could be helpful for defining new combinations able to overcome resistance.

METHODS

the human NSCLC cell line NCI-H727, sensitive to both MEK and ERK inhibitors, was treated with increasing concentrations of MEK162 (as MEK inhibitor) or SCH772984 as ERK inhibitor.

RESULTS

we successfully obtained a MEK resistant subline (H727/MEK, after 40 passages) as well as an ERK resistant subline (H727/SCH, after 18 passages). The two resistant sublines H727/MEK and H727/SCH were cross-resistant to ERK and MEK inhibitors, respectively, but not to RAF inhibitors. The sublines maintained the responsiveness to inhibitors of the parallel PI3K/akt/mTOR pathway as well as to agents with different mechanism of action. Mechanistically, treatment of sensitive and resistant cells with MEK or ERK inhibitors was able to induce a similar inhibition of ERK phosphorylation, while only in parental cells the drugs were able to induce a downregulation of S6 and RSK phosphorylation.

CONCLUSIONS

these resistant cells represent an important tool for further studies on the mechanisms of resistance and ways to overcome it.

Targeting BER enzymes in cancer therapy

Base excision repair (BER) repairs mutagenic or genotoxic DNA base lesions, thought to be important for both the etiology and treatment of cancer. Cancer phenotypic stress induces oxidative lesions, and deamination products are responsible for one of the most prevalent mutational signatures in cancer. Chemotherapeutic agents induce genotoxic DNA base damage that are substrates for BER, while synthetic lethal approaches targeting BER-related factors are making their way into the clinic. Thus, there are three strategies by which BER is envisioned to be relevant in cancer chemotherapy: (i) to maintain cellular growth in the presence of endogenous DNA damage in stressed cancer cells, (ii) to maintain viability after exogenous DNA damage is introduced by therapeutic intervention, or (iii) to confer synthetic lethality in cancer cells that have lost one or more additional DNA repair pathways. Here, we discuss the potential treatment strategies, and briefly summarize the progress that has been made in developing inhibitors to core BER-proteins and related factors.

Management of KRAS-Mutant Non-Small Cell Lung Cancer in the Era of Precision Medicine

OPINION STATEMENT

The discovery of genomic alterations that drive the development and progression of non-small cell lung cancer (NSCLC) has transformed how we treat metastatic disease. However, the promise of precision medicine remains elusive for the most commonly mutated oncogene in NSCLC, KRAS. This is perhaps due to the substantial heterogeneity within the broader genomic context of KRAS-mutant NSCLC. At this time, approaches for treating metastatic KRAS-mutant NSCLC mirror those for treating NSCLC that lacks a known driver mutation, including standard chemotherapeutic and immunotherapeutic approaches. Ongoing research aims to define further subgroups of KRAS-mutant NSCLC based on mutation subtype and co-occurring mutations. These efforts offer the potential to optimize standard-of-care regimens within these emerging subgroups and harness innovative strategies to realize precision medicine in this setting.

Different metabolic responses to PI3K inhibition in NSCLC cells harboring wild-type and G12C mutant KRAS

KRAS mutations in non-small-cell lung cancer (NSCLC) patients are considered a negative predictive factor and indicate poor response to anticancer treatments. KRAS mutations lead to activation of the PI3K/akt/mTOR pathway, whose inhibition remains a challenging clinical target. Since the PI3K/akt/mTOR pathway and KRAS oncogene mutations all have roles in cancer cell metabolism, we investigated whether the activity of PI3K/akt/mTOR inhibitors (BEZ235 and BKM120) in cells harboring different KRAS status is related to their metabolic effect. Isogenic NSCLC cell clones expressing wild-type (WT) and mutated (G12C) KRAS were used to determine the response to BEZ235 and BKM120. Metabolomics analysis indicated the impairment of glutamine in KRAS-G12C and serine metabolism in KRAS-WT, after pharmacological blockade of the PI3K signaling, although the net effect on cell growth, cell cycle distribution and caspase activation was similar. PI3K inhibitors caused autophagy in KRAS-WT, but not in KRAS-G12C, where there was a striking decrease in ammonia production, probably a consequence of glutamine metabolism impairment.

These findings lay the grounds for more effective therapeutic combinations possibly distinguishing wild-type and mutated KRAS cancer cells in NSCLC, exploiting their different metabolic responses to PI3K/akt/mTOR inhibitors.

Wee1 inhibitor MK1775 sensitizes KRAS mutated NSCLC cells to sorafenib

Non-Small-Cell Lung Cancer (NSCLC) is a poorly chemosensitive tumor and targeted therapies are only used for about 15% of patients where a specific driving and druggable lesion is observed (EGFR, ALK, ROS). KRAS is one of the most frequently mutated genes in NSCLC and patients harboring these mutations do not benefit from specific treatments. Sorafenib, a multi-target tyrosine kinase inhibitor, was proposed as a potentially active drug in KRAS-mutated NSCLC patients, but clinical trials results were not conclusive. Here we show that the NSCLC cells’ response to sorafenib depends on the type of KRAS mutation. KRAS G12V cells respond less to sorafenib than the wild-type counterpart, in vitro and in vivo. To overcome this resistance, we used high-throughput screening with a siRNA library directed against 719 human kinases, and Wee1 was selected as a sorafenib response modulator. Inhibition of Wee1 by its specific inhibitor MK1775 in combination with sorafenib restored the KRAS mutated cells’ response to the multi-target tyrosine kinase inhibitor. This combination of the Wee1 inhibitor with sorafenib, if confirmed in models with different genetic backgrounds, might be worth investigating further as a new strategy for KRAS mutated NSCLC.

Treating KRAS-mutant NSCLC: latest evidence and clinical consequences

KRAS mutations represent one of the most prevalent oncogenic driver mutations in non-small cell lung cancer (NSCLC). For many years we have unsuccessfully addressed KRAS mutation as a unique disease. The recent widespread use of comprehensive genomic profiling has identified different subgroups with prognostic implications. Moreover, recent data recognizing the distinct biology and therapeutic vulnerabilities of different KRAS subgroups have allowed us to explore different treatment approaches. Small molecules that selectively inhibit KRAS G12C or use of immune checkpoint inhibitors based on co-mutation status are some examples which anticipate that personalized treatment for this challenging disease is finally on the horizon.