Browsing by Subject "Targeted therapy"
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Item Open Access Functional and Therapeutic Relevance of MTAP Deletion in Glioblastoma(2019) Hansen, Landon JohnPrimary glioblastoma (GBM) is the most common and lethal primary malignant brain tumor, with a median patient survival of only 15 months from the time of diagnosis. GBM is particularly challenging to treat due to its aggressive and invasive nature, and has proven resistant to therapeutic advances, with no significant improvement in outcomes over the past several decades. Understanding of the molecular characteristics of GBM, however, has improved dramatically, with genetic, epigenetic, and transcriptomic classifications now able to divide GBM into subtypes that provide prognostic information and guide the organization of clinical trials. One of the most frequent genetic alterations that has been identified in GBM is homozygous deletion of the methylthioadenosine phosphorylase (MTAP) gene, which occurs in 50% of all GBM cases. Despite its common occurrence, it is unclear what contribution MTAP loss makes in the pathogenesis of GBM or whether this genetic alteration can be used as a therapeutic target.
MTAP is a metabolic enzyme in the salvage pathway of adenine and methionine and its absence results in the accumulation of its metabolic substrate, methylthioadenosine (MTA), within and around tumor cells. MTA is known to inhibit activity of methyltransferases, raising the possibility that MTA accumulation is interfering with regulatory processes within the cell.
We utilized patient-derived GBM cell lines in vitro and GBM xenografts in vivo, to characterize consequence of MTAP deletion in GBM through analysis of DNA methylation, gene expression, and response to therapeutic agents. We show that MTAP loss promotes the formation of glioma stem-like cells through epigenomic dysregulation. We show these epigenetic changes influence gene expression patterns and alter the sensitivity to epigenome-modifying drugs. We also demonstrate that MTAP-null GBM cells are more tumorigenic in experimental models and that patients with MTAP deletion have poor disease outcomes. Finally, we show that targeting metabolic liabilities of MTAP-null cells through inhibition of de novo purine synthesis specifically depletes the therapy-resistant, stem-like cell subpopulation of GBM.
As the final component of this work, we explore the impact of MTA accumulation in the tumor microenvironment. We found that MTA alters the function of immune cells through adenosine receptor signaling, suggesting that modulation of adenosine receptor signaling in GBM may improve the native immune response and the efficacy of immunotherapeutics in the treatment of this disease.
This work thus establishes MTAP deletion as a pathogenic genetic alteration in the process of gliomagenesis by illustrating it’s contribution to the formation of the cancer cell epigenomic landscape, stemness characteristics, growth, and response to therapeutic agents.
Item Open Access Functional screening to define apoptosis-inducing precision cancer therapies(2018) Anderson, GraceCancer is a diverse set of diseases characterized by genetic and epigenetic alterations that permit growth across diverse environmental contexts. The last decade has led to an explosion of sequencing efforts to define the molecular drivers of proliferation across cancers. This effort has led to the development of small-molecule inhibitors that can block oncogenic drivers and the signaling pathways driving growth. These so called “targeted therapies” have led to better progression-free survival in patients. Despite this early success, it has become clear that with few exceptions, all patients treated with targeted therapies will ultimately relapse. Thus, there is an imminent need to define combination strategies that can be employed to suppress intrinsic or acquired resistance in cancer. Here, we combine functional screening approaches, both pharmacological and genetic, to define apoptosis-inducing precision cancer therapies. Specifically, we utilize a pharmacological screening approach to uncover that breast cancers rely on Mcl-1 and Bcl-XL for survival, and that we can leverage mTOR’s translational control over Mcl-1 to induce apoptosis in PIK3CA mutant breast cancers. Additionally, we utilize CRISPR-Cas9 loss-of-function screening to define the landscape of therapeutic cooperativity in KRAS -driven cancers across diverse tissue types. Further, we leverage this landscape to define principles to rationally design combination therapies to suppress resistance. Lastly, in an effort to define targeted therapeutic strategies for cancers that lack traditional oncogenic drivers, we utilized a pharmacological screening approach to define vulnerabilities associated with dysregulated mitochondrial dynamics proteins in cancer. Collectively, our work has demonstrated the power of functional screening approaches to define apoptosis-inducing anti-cancer precision therapies that combat intrinsic and acquired resistance.
Item Open Access Molecular Regulators of Stem Cell Fate and Tumor Development in the Cerebellum(2014) Brun, Sonja NicoleMedulloblastoma (MB) is a highly malignant brain tumor that occurs primarily in children. Although surgery, radiation and high-dose chemotherapy have led to increased survival, many MB patients still die from their disease, and patients who survive suffer severe long-term side effects as a consequence of treatment. Thus, more effective and less toxic therapies for MB are critically important. Identifying new treatments will require an understanding of early stages of tumor development - the cell types from which the tumors arise and the signals that regulate their growth - as well as identification of pathways that are critical for the growth and maintenance of established tumors.
In these studies, we first explore the role of WNT signaling in cerebellar progenitors and their potential to serve as cells of origin for WNT-driven tumors. The WNT pathway plays multiple roles in neural development, is crucial for establishment of the embryonic cerebellum, and is highly expressed in a subset of MBs. However, the cell types within the cerebellum that are responsive to WNT signaling remain unknown. We show that expression of activated β-catenin promotes proliferation of cerebellar neural stem cells (NSCs) but not granule neuron precursors (GNPs). Although β-catenin expressing NSCs proliferate in vivo they do not undergo prolonged expansion or neoplastic growth; rather, WNT signaling markedly interferes with their capacity for self-renewal and differentiation. At a molecular level, mutant NSCs exhibit increased expression of c-Myc, which might account for their transient proliferation, but also express high levels of bone morphogenetic proteins and the cyclin-dependent kinase inhibitor p21, which might contribute to their altered self-renewal and differentiation. These studies suggest that the WNT pathway is a potent regulator of cerebellar stem cell growth and differentiation and that cooperating "second hits" may be required for neoplastic transformation.
In addition to understanding early stages of transformation, identifying vulnerabilities of established tumors will be critical for development of targeted therapies. Our studies in Chapter 3 are focused on the role of Survivin in SHH-driven MB and utility of survivin inhibition as a therapeutic approach for MB. Survivin is an inhibitor of apoptosis protein (IAP) that regulates cell cycle progression and resistance to apoptosis, is frequently expressed in human MB, and when expressed at high levels predicts poor clinical outcome. Here we show that Survivin is overexpressed in tumors from patched (Ptch) mutant mice, a model of Sonic hedgehog (SHH)-driven MB. Using genetic and pharmacological approaches, we demonstrate that inhibition of Survivin impairs proliferation and survival of both murine and human MB cells. Although Survivin antagonists do not cross the blood-brain barrier, they are capable of impeding growth of MB cells in flank allografts. These studies highlight the importance of Survivin in SHH-driven MB, and suggest that it may represent a novel therapeutic target in patients with this disease.
Item Open Access Overcoming Therapeutic Resistance by Targeting Oncogene-Driven and Targeted-Therapy Induced Cancer Dependencies(2020) Ali, MoiezTargeted therapies rarely yield complete tumor responses, and the residual cancer cells that survive upfront treatment act as a reservoir from which eventual resistant disease emerges. Here, we explore several clinically relevant models of disease resistance, with special attention placed on KRAS-driven colorectal cancer (CRC) and EGFR-driven non-small-cell lung cancer (NSCLC).
First, we note that KRAS mutations drive resistance to diverse targeted therapies, including EGFR inhibitors in colorectal cancer (CRC). Through genetic screens, we unexpectedly find that mutant HRAS, which is rarely found in CRC, is a stronger driver of resistance than mutant KRAS. This difference is ascribed to common codon bias in HRAS, which leads to much higher protein expression, and implies that the inherent poor expression of KRAS due to rare codons must be surmounted during drug resistance. In agreement, we demonstrate that primary resistance to cetuximab is dependent upon both KRAS mutational status and protein expression level, and acquired resistance is often associated with KRASQ61mutations that function even when protein expression is low. Finally, we show that cancer cells upregulate translation to facilitate KRASG12-driven acquired resistance, resulting in hypersensitivity to translational inhibitors. These findings demonstrate that codon bias plays a critical role in KRAS-driven resistance and provide a rationale for targeting translation to overcome resistance.
Next, we demonstrate that targeted therapies induce DNA double strand breaks and consequent, ATM-dependent DNA repair in tumor cells that survive upfront treatment. This DNA damage response, observed in both laboratory models and human patients, is driven by a pathway involving the sub-lethal activation of executioner caspases 3 and 7 and the downstream caspase-activated DNase (CAD). As a consequence, tumor cells that survive upfront treatment harbor a synthetic dependence on ATM, and combined treatment with targeted therapies and a selective ATM kinase inhibitor eradicates these cells, leading to more penetrant and durable responses in in vitro and in vivo models of EGFR-mutant NSCLC. Finally, rare patients with EGFR-mutant NSCLC harboring co-occurring, loss-of-function mutations in ATM show evidence of extended progression-free survival relative to patients lacking deleterious ATM mutations. Together, these findings establish a rationale for the mechanism-based integration of ATM inhibitors alongside existing targeted therapy paradigms.
Combined, these studies provide mechanistic-based rationale for pharmacological targeting of tumor-specific processes that may overcome intrinsic and/or acquired resistance states, serving as potential novel therapeutic options for genetically defined subsets of cancer patients.