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Simple Summary: Mutations of isocitrate dehydrogenase (IDH) genes are the distinctive genetic feature of lower-grade gliomas (LGGs). Tumor-associated IDH1/2 mutations result in a loss of normal enzymatic function and the abnormal production of 2-hydroxyglutarate (2-HG), which acts as an oncometabolite causing widespread changes in histone and DNA methylation and altering cellular metabolism. In the present review, we examine the "truncal" role of IDH mutations in gliomagenesis, giving hints on the different therapeutic strategies targeting IDH1/2-mutated gliomas. We analyze in detail, preclinical and, when available, data from clinical trials of specific inhibitors blocking the mutant enzyme, IDH-targeted immunotherapeutic approaches, and agents exploiting cellular metabolic and epigenetic vulnerabilities associated with the IDH mutant phenotype. Mutations in isocitrate dehydrogenase (IDH)1 and its homolog IDH2 are considered an earliest "driver" genetic event during gliomagenesis, representing now the molecular hallmark of lower-grade gliomas (LGGs). IDH-mutated genes encode for a neomorphic enzyme that converts α-ketoglutarate to the oncometabolite D-2-hydroxyglutarate (2-HG), which accumulates to high concentrations and alters cellular epigenetics and metabolism. Targeting IDH mutations is the first attempt to apply "precision oncology" in LGGs. Two distinct strategies have been proposed so far and are under intense clinical investigation: (i) reducing the amount of intratumoral 2-HG by directly blocking the function of mutant IDH enzyme; (ii) exploiting the selective epigenetic and metabolic cellular vulnerabilities as a consequence of 2-HG accumulation. The present review describes the physiopathological mechanisms by which IDH mutations lead to tumorigenesis, discussing their prognostic significance and pivotal role in the gliomas diagnostic classification system. We critically review preclinical evidence and available clinical data of first-generation mutant-selective IDH inhibitors and novel IDH-targeted vaccines. Finally, as an alternative and attractive approach, we present the rationale to take advantage of selective 2-HG related epigenetic and metabolic weaknesses. The results of ongoing clinical trials will help us clarify the complex scenario of IDH-targeted therapeutic approaches in gliomas. [ABSTRACT FROM AUTHOR]

Simple Summary: The behavior of diffuse lower-grade glioma (LGG) is changing over time, spontaneously, and in reaction to therapies. Due to genomic instability and clonal expansion, although LGG initially progresses slowly, the growth accelerates at the time of malignant transformation. Furthermore, its progression pattern may change by switching from a proliferative towards a more migratory profile. Along with glioma plasticity, the brain itself is constantly adapting to the tumor and treatment(s) thanks to reconfiguration within and between neural networks. The pattern of reallocation can also evolve, especially by shifting from perilesional to contrahemispheric functional reorganization: this reorientation of cerebral reshaping is related to metaplasticity. The interplay between LGG mutations and reactional connectomal rearrangement leads to perpetual modulations in the glioma–neural equilibrium, explaining the possible preservation of quality of life. An original model of these dynamic interactions across LGG plasticity and the brain metanetwork is proposed to guide a tailored step-by-step individualized management over years. The behavior of lower-grade glioma (LGG) is changing over time, spontaneously, and in reaction to treatments. First, due to genomic instability and clonal expansion, although LGG progresses slowly during the early period of the disease, its growth velocity will accelerate when this tumor will transform to a higher grade of malignancy. Furthermore, its pattern of progression may change following therapy, e.g., by switching from a proliferative towards a more diffuse profile, in particular after surgical resection. In parallel to this plasticity of the neoplasm, the brain itself is constantly adapting to the tumor and possible treatment(s) thanks to reconfiguration within and between neural networks. Furthermore, the pattern of reallocation can also change, especially by switching from a perilesional to a contrahemispheric functional reorganization. Such a reorientation of mechanisms of cerebral reshaping, related to metaplasticity, consists of optimizing the efficiency of neural delocalization in order to allow functional compensation by adapting over time the profile of circuits redistribution to the behavioral modifications of the glioma. This interplay between LGG mutations and reactional connectomal instability leads to perpetual modulations in the glioma–neural equilibrium, both at ultrastructural and macroscopic levels, explaining the possible preservation of quality of life despite tumor progression. Here, an original model of these dynamic interactions across LGG plasticity and the brain metanetwork is proposed to guide a tailored step-by-step individualized therapeutic strategy over years. Integration of these new parameters, not yet considered in the current guidelines, might improve management of LGG patients. [ABSTRACT FROM AUTHOR]

Targeting the epigenome has been considered a compelling treatment modality for several cancers, including gliomas. Nearly 80% of the lower-grade gliomas and secondary glioblastomas harbor recurrent mutations in isocitrate dehydrogenase (IDH). Mutant IDH generates high levels of 2-hydroxyglutarate (2-HG) that inhibit various components of the epigenetic machinery, including histone and DNA demethylases. The encouraging results from current epigenetic therapies in hematological malignancies have reinvigorated the interest in solid tumors and gliomas, both preclinically and clinically. Here, we summarize the recent advancements in epigenetic therapy for lower-grade gliomas and discuss the challenges associated with current treatment options. A particular focus is placed on therapeutic mechanisms underlying favorable outcome with epigenetic-based drugs in basic and translational research of gliomas. This review also highlights emerging bridges to combination treatment with respect to epigenetic drugs. Given that epigenetic therapies, particularly DNA methylation inhibitors, increase tumor immunogenicity and antitumor immune responses, appropriate drug combinations with immune checkpoint inhibitors may lead to improvement of treatment effectiveness of immunotherapy, ultimately leading to tumor cell eradication. [ABSTRACT FROM AUTHOR]