Sirtuin 2 Inhibition Modulates Chromatin Landscapes Genome-Wide To Induce Senescence in ATRX-Deficient Malignant Glioma

BACKGROUND: Functional inactivation of ATRX characterizes large subgroups of malignant gliomas in adults and children. ATRX deficiency in glioma induces widespread chromatin remodeling, driving transcriptional shifts and oncogenic phenotypes. Effective strategies to therapeutically target these broad epigenomic sequelae remain undeveloped. METHODS: We utilized integrated multiomics and the Broad Institute Connectivity Map (CMAP) to identify drug candidates that could potentially revert ATRX-deficient transcriptional changes. We then employed disease-relevant experimental models to evaluate functional phenotypes, coupling these studies with epigenomic profiling to elucidate molecular mechanism(s). RESULTS: CMAP analysis and transcriptional/epigenomic profiling implicated the Class III HDAC Sirtuin2 (SIRT2) as a central mediator of ATRX-deficient cellular phenotypes and a driver of unfavorable prognosis in ATRX-deficient glioma. SIRT2 inhibitors reverted Atrx-deficient transcriptional signatures in murine neuroepithelial progenitor cells (mNPCs), impaired cell migration in Atrx/ATRX-deficient mNPCs and human glioma stem cells (GSCs), and increased expression of senescence markers in glioma models. Moreover, SIRT2 inhibition impaired growth and increased senescence in ATRX-deficient GSCs in vivo. These effects were accompanied by genome-wide shifts in enhancer-associated H3K27ac and H4K16ac marks, with the latter in particular demonstrating compelling transcriptional links to SIRT2-dependent phenotypic reversals. Motif analysis of these data identified the transcription factor KLF16 as a mediator of phenotype reversal in Atrx-deficient cells upon SIRT2 inhibition. CONCLUSIONS: Our findings indicate that SIRT2 inhibition selectively targets ATRX-deficient gliomas for senescence through global chromatin remodeling, while demonstrating more broadly a viable approach to combat complex epigenetic rewiring in cancer

Interplay Between Atrx and IDH1 Mutations Governs Innate Immune Responses in Diffuse Gliomas

Stimulating the innate immune system has been explored as a therapeutic option for the treatment of gliomas. Inactivating mutations in ATRX, defining molecular alterations in IDH-mutant astrocytomas, have been implicated in dysfunctional immune signaling. However, little is known about the interplay between ATRX loss and IDH mutation on innate immunity. To explore this, we generated ATRX-deficient glioma models in the presence and absence of the IDH1R132H mutation. ATRX-deficient glioma cells are sensitive to dsRNA-based innate immune agonism and exhibit impaired lethality and increased T-cell infiltration in vivo. However, the presence of IDH1R132H dampens baseline expression of key innate immune genes and cytokines in a manner restored by genetic and pharmacological IDH1R132H inhibition. IDH1R132H co-expression does not interfere with the ATRX deficiency-mediated sensitivity to dsRNA. Thus, ATRX loss primes cells for recognition of dsRNA, while IDH1R132H reversibly masks this priming. This work reveals innate immunity as a therapeutic vulnerability of astrocytomas

Characterizing and targeting the genomic consequences of ATRX deficiency in glioma

Mutational inactivation of histone chaperone ATRX (a-thalassaemia/mental retardation X-linked) represents a defining molecular feature in several cancers, including malignant glioma. As standard of care only leads to transient responses and poor outcomes, there is an unmet clinical need for developing new therapies that target ATRX-deficient glioma. Loss of ATRX gives rise to abnormal G-quadruplex DNA secondary structures at GC-rich sites of the genome, such as telomeric and pericentromeric regions, enhancing replication stress and genomic instability. These mutations are mutually exclusive with TERT promoter mutations, promoting the alternative lengthening of telomeres (ALT) pathway as a telomere maintenance mechanism in ATRX-deficient glioma. Recently, a class of agents known as G4 stabilizers demonstrated strong therapeutic promise, however, the genomic consequences and efficacy of this treatment are poorly understood. Studying the molecular changes induced by and efficacy of G4 stabilizers in the treatment of ATRX-deficient glioma will advance the development of novel targeted therapies for this invariably fatal cancer. Building upon earlier work, we evaluated the mechanisms of action and efficacy of the G4 stabilizer CX-5461 as both a single agent and in combination with ionizing radiation (IR), a mainstay in the current standard of care, using patient-derived glioma stem cell (GSC) preclinical models. We found that ATRX-deficient GSCs demonstrate dose-sensitive lethality to CX-5461, relative to ATRX-intact controls. Mechanistic studies revealed that CX-5461 disrupted histone variant H3.3 deposition, enhanced replication stress and DNA damage, activated p53-independent apoptosis, and induced G2/M arrest selectively in ATRX-deficient GSCs. These data were corroborated in vivo, where combinational treatment profoundly delayed tumor growth and prolonged survival exclusively in mice bearing ATRX-deficient GSC flank xenografts. Histopathological analyses revealed decreased proliferation, increased apoptosis, and significant induction of G4s, replication stress, and DNA damage in CX-5461-treated tumors, both alone and in combination with IR. Furthermore, systemic CX-5461 treatment induced tangible pharmacodynamic effects in ATRX-deficient intracranial GSC models, despite suboptimal central nervous system (CNS) penetration. Additionally, we conducted proof-of-concept studies in sarcoma models and found that CX-5461 induces dose-sensitive lethality, enhances replication stress and DNA damage, and induces G2/M cell cycle arrest in ATRX-deficient sarcoma cells, relative to ATRX-intact controls. These data implicate G4 stabilization as an effective treatment strategy with direct applications to other ATRX-deficient malignancies. Lastly, as DNA damage at telomeres is thought to drive the ALT pathway, we investigated the impact of G4 stabilization on ALT and found that CX-5461 does not inhibit ALT activity in ATRX-deficient glioma and sarcoma models. In its totality, this dissertation demonstrates efficacy and defines mechanisms of action for G-quadruplex stabilization as a novel therapeutic strategy targeting ATRX-deficient cancer, laying the groundwork for clinical translation

CBIO-18. G-QUADRUPLEX STABILIZATION TARGETS ATRX-DEFICIENT HIGH-GRADE GLIOMA VIA INDUCTION OF p53-INDEPENDENT APOPTOSIS

Abstract Gliomas are the most common primary malignant brain tumor in adults and mutational inactivation of histone chaperone ATRX is a critical molecular marker in the classification of high-grade glioma (HGG). ATRX loss occurs with concurrent mutations in TP53 and IDH1/2, altering genome-wide accessibility of chromatin and inducing replication stress and DNA damage via accumulations of abnormal G-quadruplex (G4) DNA secondary structures. While G4 stabilizers in particular hold strong therapeutic promise, the genomic consequences and efficacy of this treatment are poorly understood. We previously showed that chemical stabilization of G4s in ATRX-deficient normal human astrocytes (NHAs) results in lethality due to induction of replication stress, but it is unknown what drives this lethality in ATRX-deficient patient-derived preclinical models. We therefore sought to evaluate the mechanisms that underlie cell death in ATRX- and p53-deficient preclinical in vitro models following G4 stabilization. We found that ATRX-deficient glioma stem cells (GSCs) demonstrated dose-dependent enhanced sensitivity to G4 stabilization, compared to ATRX-intact controls. Evaluation of cell death mechanisms following G4 stabilization revealed a significant increase in cleaved caspase 3 expression and no p21 expression in ATRX-deficient GSCs, suggesting p53-independent apoptotic activation. Cell cycle flow analysis demonstrated G2/M checkpoint arrest in ATRX-deficient GSCs upon G4 stabilization, suggesting that p53 is nonfunctional at the G1/S checkpoint. Our preliminary findings now suggest that p73, a functional and structural homologue of p53, is activated and drives apoptosis in these ATRX-deficient GSCs. Furthermore, ATRX-deficient GSCs demonstrated upregulated expression of both pATR/pChk1 and pATM/pChk2, indicating enhanced replication stress and DNA damage via double-stranded breaks, respectively. These findings indicate that G4 stabilizers could potentially synergize with ionizing radiation, the current standard of care, as both therapies are DNA-damaging. Taken together, this study elucidates mechanisms of cytotoxicity and efficacy of a novel therapeutic strategy in ATRX-deficient preclinical models.</jats:p

CSIG-09. ATRX DEFICIENCY IN GLIOMA IMPACTS TRANSCRIPTIONAL PROFILES AND THE IMMUNE MICROENVIRONMENT IN VIVO

Abstract Current treatment for diffuse astrocytoma fails to address its underlying molecular mechanisms leading to inevitable disease progression and eventual patient death. Genomic studies have implicated ATRX alterations as critical to low grade glioma biology. Our lab has previously shown in vitro that ATRX influences glioma motility, cellular differentiation state, and epigenetic programming, however, the influence of ATRX alterations in vivo remains unclear. Here, we leveraged an RCAS/tva mouse tumor model to probe the role of ATRX deficiency in glioma. Atrx deficient murine tumors exhibited lower histopathological grade and were associated with longer survival than Atrx-intact counterparts, and syngeneic allografts of cell lines derived from primary tumors mirrored the differential degrees of aggressiveness seen in primary tumors. Tumor-derived Atrx-deficient cell lines showed increased susceptibility to G-quadruplex stabilizing compounds, pointing to increased replication stress and recapitulating a key phenotype of ATRX-mutant gliomas in humans. Transcriptional profiling revealed enrichments in MYC target genes, E2F targets as well as G2/M checkpoint pathways in Atrx-intact tumors and cells, and enrichment in RAS signaling in Atrx-deficient tumors and cells. Finally, Atrx deficient murine gliomas displayed increased levels of NK cells, a phenotype recapitulated in ATRX-mutant human gliomas, and primary Atrx-deficient glioma lines exhibited increased levels of NK cell-attracting cytokines. These latter findings suggest that ATRX deficiency could influence interactions between glioma cells and their immune microenvironment by way of phenotypically relevant molecular mechanisms.</jats:p

CBIO-04. G-QUADRUPLEX STABILIZATION ENHANCES REPLICATION STRESS AND DNA DAMAGE IN ATRX-DEFICIENT HIGH-GRADE GLIOMA

Abstract Loss of function mutations in α-thalassaemia/mental retardation X-linked (ATRX) are a critical molecular hallmark for invariably fatal high-grade glioma (HGG). Mutational inactivation of histone chaperone ATRX leads to accumulations of abnormal DNA secondary structures known as G-quadruplexes (G4s), thereby inducing replication stress and DNA damage. As G4s arise at GC-rich regions (i.e., pericentromeric and telomeric regions), ATRX-deficiency alters genome-wide accessibility of chromatin, leads to transcriptional dysregulation, and induces alternative lengthening of telomeres (ALT). Our goal is to target ATRX deficiency through G4 stabilizers, which represent a class of novel small molecule compounds that selectively bind to and stabilize G4 structures. However, the genomic consequences and efficacy of this therapy for ATRX-deficient HGG are poorly understood. We therefore sought to evaluate the molecular mechanisms that drive selective lethality in patient-derived ATRX-deficient glioma stem cells (GSCs), following G4 stabilization. We found that ATRX-deficient GSCs demonstrate dose-dependent enhanced sensitivity to G4 stabilization, compared to ATRX-intact controls. Cell viability assays confirmed the specificity of our G4 stabilizer in comparison to other commonly used G4 stabilizers. Interestingly, G4 stabilization activated p53-independent apoptosis in ATRX-deficient GSCs. Furthermore, ATRX-deficient GSCs exhibit upregulated expression of both ATR and ATM pathways upon G4 stabilization, indicating enhanced replication stress and DNA damage via double-stranded breaks, respectively. Our preliminary findings suggest that ATR and ATM activation leads to the inhibition of transcription factor NF-κB, which in turn drives apoptosis. Lastly, our data indicate that G4 stabilization perturbs the ALT phenotype in ATRX-deficient GSCs, likely contributing to telomeric dysfunction. Taken together, these findings suggest that G4 stabilizers could synergize with ionizing radiation, the standard of care, as they are both DNA-damaging therapies. Our work defines mechanisms of action and efficacy of a novel therapeutic strategy for ATRX-deficient HGG, with strong implications for other ATRX-deficient cancers.</jats:p

Sirtuin 2 inhibition modulates chromatin landscapes genome-wide to induce senescence in ATRX-deficient malignant glioma

ABSTRACTInactivating mutations inATRXcharacterize large subgroups of malignant gliomas in adults and children. ATRX deficiency in glioma induces widespread chromatin remodeling, driving transcriptional shifts and oncogenic phenotypes. Effective strategies to therapeutically target these broad epigenomic sequelae remain undeveloped. We utilized integrated mulit-omics and the Broad Institute Connectivity Map (CMAP) to identify drug candidates that could potentially revert ATRX-deficient transcriptional changes. We then employed disease-relevant experimental models to evaluate functional phenotypes, coupling these studies with epigenomic profiling to elucidate molecular mechanim(s). CMAP analysis and transcriptional/epigenomic profiling implicated the Class III HDAC Sirtuin2 (Sirt2) as a central mediator of ATRX-deficient cellular phenotypes and a driver of unfavorable prognosis in ATRX-deficient glioma. Sirt2 inhibitors reverted Atrx-deficient transcriptional signatures in murine neuroprogenitor cells (mNPCs) and impaired cell migration in Atrx/ATRX-deficient mNPCs and human glioma stem cells (GSCs). While effects on cellular proliferation in these contexts were more modest, markers of senescence significantly increased, suggesting that Sirt2 inhibition promotes terminal differentiation in ATRX-deficient glioma. These phenotypic effects were accompanied by genome-wide shifts in enhancer-associated H3K27ac and H4K16ac marks, with the latter in particular demonstrating compelling transcriptional links to Sirt2-dependent phenotypic reversals. Motif analysis of these data identified the transcription factor KLF16 as a mediator of phenotype reversal in Atrx-deficient cells upon Sirt2 inhibition. Finally, Sirt2 inhibition impaired growth and increased senescence in ATRX-deficient GSCsin vivo. Our findings indicate that Sirt2 inhibition selectively targets ATRX-deficient gliomas through global chromatin remodeling, while demonstrating more broadly a viable approach to combat complex epigenetic rewiring in cancer.Graphical AbstractOne Sentence SummaryOur study demonstrates that SIRT2 inhibition promotes senescence in ATRX-deficient glioma model systems through global epigenomic remodeling, impacting key downstream transcriptional profiles.</jats:sec