Isocitrate dehydrogenase 1 (IDH1) plays a crucial role in cellular metabolism through catalyzing the conversion of isocitrate to α-ketoglutarate (α-KG) in the cytosol, simultaneously producing NADPH. Mutations in IDH1, especially at residue R132, are implicated in the pathogenesis of various cancers and other diseases, predominantly through a gain-of-function mechanism where the mutant enzyme produces the oncometabolite D-2-hydroxyglutarate (2-HG). The accumulation of 2-HG acts as a competitive inhibitor of α-KG-dependent dioxygenases, resulting in widespread epigenetic alterations and metabolic dysregulation contributing to oncogenesis and disease progression.

Role of IDH1 in Cancer Pathophysiology

Gliomas (including low-grade gliomas and glioblastomas)

• IDH1 mutations are early events in gliomagenesis, occurring in the majority of World Health Organization (WHO) grade II/III gliomas and secondary glioblastomas but are rare in primary glioblastomas (Dimitrov, 2015).
• The mutation results in neomorphic activity causing elevated D-2-HG levels, leading to hypermethylation of DNA and histones, establishing a CpG island methylator phenotype (CIMP) that disrupts normal differentiation of glioma precursor cells (De Botton, 2016).
• IDH1 mutant gliomas exhibit alterations in hypoxia-inducible factor-1 (HIF-1) signaling via reduced α-KG levels, stabilizing HIF-1α, promoting angiogenesis, and tumor progression (Zhao, 2009).
• IDH1 mutations also alter tumor-associated immune responses, suppressing leukocyte chemotaxis and cytotoxic T cell accumulation, which contributes to immune evasion in gliomas (Amankulor, 2017), (Kohanbash, 2017).
• Clinically, IDH1 mutant gliomas often present with brain tumor-related epilepsy, where 2-HG structurally mimics glutamate, an excitatory neurotransmitter implicated in seizure pathogenesis (Liubinas, 2014), (Wang, 2022).
• Patients with IDH1 mutant gliomas generally have better prognosis and longer survival compared to wild-type, possibly due to distinct molecular and metabolic vulnerabilities (Hartmann, 2011), (Wefel, 2016).

Acute Myeloid Leukemia (AML)

• IDH1 mutations are identified in approximately 8-12% of AML cases, causing production of 2-HG, which inhibits α-KG dependent enzymes involved in epigenetic regulation, contributing to leukemogenesis (Gr, 2010), (Popovici-Mller, 2018).
• The mutation is often associated with normal karyotype AML and frequently co-occurs with NPM1 mutations, correlating with particular clinical outcomes (El-Lateef, 2021).
• Therapies targeting mutant IDH1, such as ivosidenib (AG-120), have demonstrated the ability to reverse 2-HG accumulation, promote differentiation of leukemic cells, and show clinical efficacy (Popovici-Mller, 2018), (Dhillon, 2018).
• A recent report suggests mutations in IDH1 may also be linked to acute coagulopathy and disseminated intravascular coagulation during treatment in AML, potentially due to rapid leukemic cell lysis and inflammatory mediator release (Iyer, 2024).

Other Cancers

• IDH1 mutations have been observed at lower frequencies in other tumors such as chondrosarcomas (including in Maffucci and Ollier syndromes), thyroid carcinomas, and adrenocortical carcinomas, although the role may differ by tissue type (Nejo, 2019), (Hemerly, 2010), (Pennanen, 2020).
• In Maffucci syndrome, IDH1 R132C mutations are found in associated enchondromas and central nervous system tumors, implicating the mutation in multiple tumor origins within a patient (Nejo, 2019).
• In chondrosarcomas and enchondromatosis, mutant IDH1 fosters epigenetic reprogramming leading to oncogenic transformation (Hughes, 2013).
• The mutation is rarely detected in other solid tumors outside the central nervous system and leukemias, highlighting tissue-specific pathogenesis (Bleeker, 2009).

Pathophysiological Mechanisms

• 2-Hydroxyglutarate accumulation: Mutant IDH1 converts α-KG to 2-HG, which competitively inhibits α-KG-dependent dioxygenases such as histone demethylases and TET family DNA hydroxylases. This leads to a hypermethylated state, altered gene expression, impaired cellular differentiation, and tumorigenesis (Yang, 2012), (Rohle, 2013).
• Metabolic Reprogramming: IDH1 mutations disrupt normal mitochondrial metabolism and redox balance, increasing oxidative mitochondrial metabolism dependence and reactive oxygen species generation, influencing cancer cell survival and proliferation (Grassian, 2014).
• Altered Cellular Signaling: Inhibition of prolyl hydroxylases stabilizes HIF-1α, promoting hypoxia-related gene expression that drives angiogenesis and metabolic adaptation under tumor hypoxic conditions (Zhao, 2009).
• Immune Modulation: Mutant IDH1 suppresses chemokine production (e.g., CXCL10), diminishing immune cell infiltration and fostering an immunosuppressive tumor microenvironment (Amankulor, 2017), (Kohanbash, 2017).

Other Diseases Linked to IDH1 Dysregulation

• Myelodysplastic Syndrome (MDS) and Sweet's Syndrome: IDH1 mutations contribute to epigenetic modifications implicated in leukemogenesis and may be associated with Sweet's syndrome, a febrile neutrophilic dermatosis linked to hematological malignancies (Snyder, 2018).
• Rare Metabolic Disorders: Although not directly IDH1, mutations in related enzymes in the Krebs cycle (such as malate dehydrogenase MDH2) cause severe encephalopathy with epilepsy, highlighting the importance of metabolic enzyme integrity in neurological diseases (Ait-El-Mkadem, 2016).

Summary Table of IDH1-Associated Diseases and Pathophysiological Features

Therapeutic Implications

• Mutant IDH1 inhibitors, such as ivosidenib (AG-120), have been developed and approved for treatment of IDH1-mutant AML, with ongoing trials in gliomas and other cancers (Popovici-Mller, 2018), (Dhillon, 2018).
• The inhibitors reduce 2-HG levels, restore normal differentiation, and show tolerable safety profiles.
• Immunotherapeutic strategies including vaccines targeting mutant IDH1 protein are under exploration (Dimitrov, 2015).
• Understanding IDH1 mutations' impact on tumor metabolism and immunity may guide combinatory therapies and improve prognostic stratification in affected patients (Molenaar, 2018).

In conclusion, IDH1 mutations act as metabolic and epigenetic drivers in gliomas, AML, and select other cancers. Their neomorphic enzymatic activity producing 2-HG disrupts multiple cellular pathways including DNA/histone methylation, hypoxia signaling, and immune surveillance, leading to oncogenic transformation and disease progression. Besides cancer, IDH1 mutations are implicated in other disease contexts through similar molecular disruptions including inflammatory conditions and rare metabolic disorders. Targeting mutant IDH1 offers promising therapeutic avenues.

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