# IDH2: R140Q and R172K Mutations, Oncometabolite Accumulation, and Targeted Inhibitor Therapy ## Key Takeaways - Somatic mutations in *IDH2*, specifically at arginine hotspots R140 (most common, e.g., R140Q) and R172 (e.g., R172K), confer a neomorphic gain-of-function. This altered enzyme catalyzes the reduction of α-ketoglutarate (α-KG) to the oncometabolite D-2-hydroxyglutarate (D-2HG). - D-2HG accumulation competitively inhibits α-KG-dependent dioxygenases, notably TET family enzymes and JmjC-domain-containing histone demethylases. This leads to widespread DNA hypermethylation (CpG island methylator phenotype) and histone hypermethylation, driving a block in cellular differentiation and promoting tumorigenesis in malignancies like AML and gliomas. - The FDA-approved drug enasidenib (AG-221) is a selective small-molecule inhibitor targeting mutant IDH2. It binds to an allosteric site, stabilizing an inactive conformation and reducing D-2HG levels, thereby restoring differentiation potential in leukemic cells. - *IDH2* mutations are found in approximately 8-19% of AML cases, often associated with intermediate-risk cytogenetics, and are a key molecular marker for enasidenib therapy. In gliomas, they define a molecular subtype and are associated with a favorable prognosis. - Beyond cancer, germline *IDH2* mutations cause D-2-hydroxyglutaric aciduria, a metabolic disorder, and the successful use of enasidenib in these patients demonstrates the therapeutic potential of IDH2 inhibition in non-oncological settings. --- ## Executive Summary & Key Metadata Isocitrate dehydrogenase 2 (IDH2) is a mitochondrial matrix enzyme that catalyzes the oxidative decarboxylation of isocitrate to α-ketoglutarate (α-KG, also known as 2-oxoglutarate) while reducing NADP⁺ to NADPH. This reaction is central to the tricarboxylic acid (TCA) cycle and cellular redox homeostasis. Beyond its canonical metabolic role, IDH2 has emerged as a critical oncogene when somatically mutated at specific arginine hotspots (R140 and R172). These neomorphic mutations confer a gain-of-function activity that reduces α-KG to the oncometabolite D-2-hydroxyglutarate (D-2HG), which competitively inhibits α-KG-dependent dioxygenases, including the TET family of 5-methylcytosine hydroxylases and JmjC-domain-containing histone demethylases. The resulting epigenetic dysregulation—manifesting as a CpG island methylator phenotype (CIMP) and histone hypermethylation—blocks cellular differentiation and drives tumorigenesis in acute myeloid leukemia (AML), gliomas, cholangiocarcinoma, chondrosarcoma, and other malignancies. The clinical importance of IDH2 mutations has been solidified by the FDA approval of enasidenib (AG-221), a first-in-class, oral, selective small-molecule inhibitor of mutant IDH2, for the treatment of adult patients with relapsed or refractory AML harboring an IDH2 mutation. This therapeutic breakthrough, alongside the development of other targeted agents and combination regimens, underscores the necessity of a comprehensive understanding of IDH2 biology, from genomic architecture to structural enzymology and clinical pharmacology. | Attribute | Detail | | :--- | :--- | | **HGNC Symbol** | IDH2 | | **UniProt Accession** | P48735 | | **Representative PDB ID** | 5SVN (Human IDH2 R140Q mutant in complex with enasidenib) | | **Chromosomal Locus** | 15q26.1 [1] | | **Primary Molecular Function** | Oxidative decarboxylation of isocitrate to α-ketoglutarate (NADP⁺-dependent); production of NADPH; neomorphic production of D-2-hydroxyglutarate when mutated | | **Disease & Pathology Associations** | Acute myeloid leukemia (AML), myelodysplastic syndromes (MDS), gliomas (astrocytoma, oligodendroglioma, oligoastrocytoma), cholangiocarcinoma, chondrosarcoma, angioimmunoblastic T-cell lymphoma, sinonasal undifferentiated carcinoma, tall cell carcinoma with reversed polarity of the breast, D-2-hydroxyglutaric aciduria (germline) [1, 2, 3, 4, 5, 6, 7, 8] | --- ## 1. Genomic Locus, Chromosomal Organization & Isoforms ### 1.1 Chromosomal Localization and Gene Structure The human *IDH2* gene is located on the long (q) arm of chromosome 15 at cytogenetic band 15q26.1 [1]. This locus was initially mapped by fluorescence *in situ* hybridization (FISH) and somatic cell hybrid analysis, which assigned the gene to a region distal to the *APRT* locus [1, 5]. The genomic coordinates (GRCh38/hg38) span approximately 20 kilobases (kb) on the plus strand, from roughly 90,083,045 to 90,102,850 base pairs. The gene consists of 11 exons and 10 introns, with the translation initiation codon located in exon 1 and the stop codon in exon 11. The coding sequence (CDS) is 1,374 nucleotides in length, encoding a precursor protein of 452 amino acids. The mature protein, after cleavage of the N-terminal mitochondrial targeting sequence (MTS), is 452 amino acids long, with the MTS spanning residues 1–40. The genomic organization is highly conserved across mammals, with the exon-intron boundaries maintained in orthologs such as mouse and rat. ### 1.2 Promoter Architecture and Transcriptional Regulation The *IDH2* promoter region lacks a canonical TATA box but contains a high GC content, characteristic of housekeeping genes. Several cis-regulatory elements have been identified *in silico* and experimentally, including binding sites for specificity protein 1 (Sp1), nuclear respiratory factor 1 (NRF-1), and the estrogen-related receptor α (ERRα). These transcription factors coordinate the expression of *IDH2* with mitochondrial biogenesis and oxidative metabolism. Epigenetic regulation of *IDH2* expression is a critical layer of control. The promoter and first exon reside within a CpG island. DNA methylation at this locus inversely correlates with expression in various cancer cell lines. Furthermore, histone modifications play a significant role; for instance, the WEE1 kinase has been shown to phosphorylate histone H2B at tyrosine 37 (pY37-H2B), which epigenetically modulates *IDH2* gene expression [6]. Specifically, WEE1-mediated pY37-H2B leads to transcriptional repression of *IDH2*, and inhibition of WEE1 results in increased *IDH2* mRNA and protein levels [6]. This finding links the cell cycle checkpoint machinery directly to metabolic gene expression. ### 1.3 Enhancer Elements and 3D Chromatin Architecture High-throughput chromatin conformation capture (Hi-C) data from ENCODE and other consortia indicate that the *IDH2* promoter interacts with several distal enhancer elements located within the same topologically associating domain (TAD). These enhancers are marked by H3K27ac and H3K4me1 in tissues with high oxidative metabolism, such as the heart, liver, and kidney. One notable enhancer region is located approximately 50 kb upstream of the transcription start site (TSS), within an intron of the neighboring gene *FURIN*. This intergenic interaction is thought to be cell-type-specific, contributing to the differential expression of IDH2 across tissues. ### 1.4 Alternative Splicing and Isoforms The primary *IDH2* transcript encodes the canonical 452-amino-acid mitochondrial isoform (UniProt P48735-1). Unlike *IDH1*, which has a well-characterized cytosolic isoform, *IDH2* does not have widely validated alternative protein-coding isoforms in normal human tissues. However, several expressed sequence tags (ESTs) and transcriptomic datasets suggest the existence of minor splice variants that may retain intronic sequences or utilize alternative 3' splice sites in the 5' UTR. These variants are predicted to be subject to nonsense-mediated decay (NMD) and are unlikely to produce stable, functionally distinct proteins. The absence of significant isoform diversity underscores the reliance on a single, tightly regulated gene product for mitochondrial NADPH production. --- ## 2. 3D Protein Domain Architecture & Structural Biology ### 2.1 Primary Structure and Domain Boundaries The IDH2 protein is synthesized as a precursor with an N-terminal mitochondrial targeting sequence (MTS) spanning residues 1–40. This amphipathic helix is cleaved upon import into the mitochondrial matrix by the mitochondrial processing peptidase (MPP). The mature protein (residues 41–452) folds into a highly conserved homodimeric structure. Each monomer of IDH2 is organized into three distinct structural domains: 1. **Large Domain (residues 41–140 and 269–360):** This domain forms the core of the enzyme and contains the majority of the active site residues. It adopts a classic α/β fold with a central parallel β-sheet flanked by α-helices. The large domain contributes to the dimer interface and harbors the binding site for the adenine dinucleotide phosphate (NADP⁺) cofactor. 2. **Small Domain (residues 141–268):** This domain is primarily α-helical and is responsible for the conformational changes associated with substrate binding and catalysis. It contains the isocitrate binding pocket and undergoes a large domain closure upon substrate binding, moving by up to 10 Å to enclose the active site. 3. **Clasp Domain (residues 361–452):** The C-terminal clasp domain is unique to the homodimeric IDH2 and is critical for dimer stabilization. It forms a "clasp" that wraps around the large domain of the opposing monomer, creating a highly stable dimer interface. This domain also contributes to the allosteric regulation of the enzyme by citrate and other metabolites. ### 2.2 Active Site Architecture and Catalytic Mechanism The active site of IDH2 is located at the interface between the large and small domains of a single monomer. Key catalytic residues include: - **Arg140 (R140):** This residue is located in the small domain and is critical for the binding of the γ-carboxylate of isocitrate. Mutation of this residue to glutamine (R140Q) is the most common oncogenic alteration in AML. - **Arg172 (R172):** This residue is located in the large domain and coordinates the β-carboxylate of isocitrate. Mutation to lysine (R172K) is the most frequent alteration at this position and is more common in gliomas and other solid tumors. - **Asp275 (D275):** This residue acts as the catalytic base, abstracting a proton from the C2 hydroxyl group of isocitrate to initiate the oxidative decarboxylation reaction. - **Tyr194 (Y194):** This residue is involved in the stabilization of the transition state. The catalytic mechanism proceeds through a two-step process: 1. **Oxidation:** Isocitrate is oxidized to oxalosuccinate by the transfer of a hydride ion to NADP⁺, forming NADPH. 2. **Decarboxylation:** The β-carboxyl group of oxalosuccinate is lost as CO₂, yielding α-ketoglutarate. ### 2.3 The Neomorphic Active Site in Oncogenic Mutants The oncogenic mutations at R140 and R172 do not simply inactivate the enzyme; they fundamentally alter its catalytic activity. Structural studies of the R140Q mutant (PDB: 5SVN) reveal that the substitution of the bulky, positively charged arginine with a smaller, uncharged glutamine remodels the active site pocket. This remodeling creates a new binding pocket that can accommodate the γ-carboxylate of α-KG in a non-productive orientation. The mutant enzyme then catalyzes the NADPH-dependent reduction of α-KG to D-2-hydroxyglutarate (D-2HG). This neomorphic activity is a classic example of a gain-of-function mutation that introduces a new enzymatic activity rather than simply abolishing the original one. The structural basis for the differential activity of R140 and R172 mutants has been extensively studied. R172 mutants generally exhibit a higher catalytic efficiency for D-2HG production compared to R140 mutants, which may contribute to the distinct clinical phenotypes and prognostic implications associated with each mutation site [7]. ### 2.4 Dimerization and Allostery IDH2 functions as a strict homodimer. The dimer interface is extensive, burying over 3,000 Ų of solvent-accessible surface area per monomer. The clasp domain is essential for this dimerization; deletion of this domain results in monomeric, catalytically inactive protein. The dimeric architecture is also critical for the allosteric regulation of the enzyme. The binding of citrate, a TCA cycle intermediate, to an allosteric site at the dimer interface stabilizes the active conformation of the enzyme, promoting catalysis. This positive cooperativity ensures that IDH2 activity is tightly coupled to the metabolic state of the mitochondrion. --- **[Interactive 3D Protein Visualizer: Load IDH2 (PDB: 5SVN)](/tools/protein-structure-viewer?source=direct&pdbId=5SVN)** --- ## 3. Cellular Signaling Pathways & Molecular Function ### 3.1 The TCA Cycle and NADPH Production Within the mitochondrial matrix, IDH2 catalyzes the rate-limiting oxidative decarboxylation step of the TCA cycle, converting isocitrate to α-KG. This reaction is a major source of the α-KG that fuels the cycle. However, the most critical function of IDH2 in cellular physiology is the production of NADPH. NADPH is the primary reducing currency of the cell, essential for: - **Antioxidant Defense:** NADPH is a cofactor for glutathione reductase and thioredoxin reductase, which regenerate reduced glutathione (GSH) and thioredoxin, respectively. These molecules are crucial for neutralizing reactive oxygen species (ROS) and maintaining the cellular redox balance. IDH2-deficient cells exhibit increased mitochondrial oxidative stress and are more susceptible to apoptosis [1, 8]. - **Biosynthesis:** NADPH is required for reductive biosynthesis, including fatty acid synthesis and cholesterol synthesis. - **Detoxification:** NADPH is a cofactor for cytochrome P450 enzymes and other detoxification systems. The role of IDH2 in redox homeostasis is particularly important in the kidney, where it protects against ischemia-reperfusion injury and fibrosis [2, 8]. Studies have shown that IDH2 deficiency accelerates unilateral ureteral obstruction-induced kidney inflammation through oxidative stress and activation of macrophages [8]. Similarly, IDH2 plays a critical role in cisplatin-induced nephrotoxicity, where its loss exacerbates oxidative damage [3]. ### 3.2 The Oncometabolite Hypothesis: D-2-Hydroxyglutarate The discovery that mutant IDH1/2 enzymes produce D-2HG was a paradigm shift in cancer biology. D-2HG is a structural analog of α-KG and acts as a competitive inhibitor of a large family of α-KG-dependent dioxygenases. The accumulation of D-2HG to millimolar concentrations in IDH-mutant tumors leads to the inhibition of several key enzymes: 1. **TET Family of 5-Methylcytosine Hydroxylases:** TET1, TET2, and TET3 catalyze the oxidation of 5-methylcytosine (5mC) to 5-hydroxymethylcytosine (5hmC) and further oxidized derivatives, which are intermediates in active DNA demethylation. Inhibition of TET enzymes by D-2HG leads to DNA hypermethylation, particularly at CpG islands, resulting in the CpG island methylator phenotype (CIMP) [4, 5]. This hypermethylation silences tumor suppressor genes and blocks cellular differentiation. 2. **JmjC-Domain-Containing Histone Demethylases:** These enzymes remove methyl groups from lysine and arginine residues on histone tails. Their inhibition by D-2HG leads to histone hypermethylation, which alters chromatin structure and gene expression, contributing to the differentiation block [5]. 3. **Collagen Prolyl-4-Hydroxylases (P4HA):** These enzymes are required for proper collagen maturation and secretion. Their inhibition by D-2HG may contribute to the altered tumor microenvironment. 4. **Hypoxia-Inducible Factor (HIF) Prolyl Hydroxylases (PHDs):** PHDs target HIF-1α for proteasomal degradation under normoxic conditions. D-2HG can inhibit PHDs, leading to HIF-1α stabilization and activation of a pseudo-hypoxic gene expression program, which promotes angiogenesis and metabolic reprogramming [6, 7]. ### 3.3 Epigenetic Dysregulation and Differentiation Block The inhibition of TET enzymes and histone demethylases by D-2HG is the central mechanism by which IDH2 mutations drive leukemogenesis and gliomagenesis. The resulting hypermethylation of DNA and histones leads to a global block in cellular differentiation. In hematopoietic stem and progenitor cells, this manifests as an accumulation of immature blast cells, the hallmark of AML [1, 3, 8]. The differentiation block is mediated, in part, by the hypermethylation of genes involved in hematopoietic differentiation, such as *CEBPA* and *PU.1*. The cooperation between IDH2 mutations and other genetic alterations is critical for full malignant transformation. For example, IDH2 mutations frequently co-occur with mutations in *DNMT3A*, *NPM1*, *FLT3*, and *TET2* [1, 2, 3, 4]. Studies have shown that *Dnmt3a* loss and *Idh2* neomorphic mutations mutually potentiate malignant hematopoiesis [1]. Similarly, IDH2 and NPM1 mutations cooperate to activate *Hoxa9/Meis1* and hypoxia pathways in AML [4]. ### 3.4 Protein-Protein Interaction Networks IDH2 interacts with a network of proteins involved in metabolism, redox regulation, and cell survival. Key interactions identified through affinity purification-mass spectrometry (AP-MS) and yeast two-hybrid screens include: - **SIRT3:** This mitochondrial sirtuin deacetylates IDH2 at lysine residues, including K413, enhancing its enzymatic activity. The SIRT3-IDH2 axis is a critical regulator of mitochondrial redox homeostasis and has been implicated in macrophage lipid accumulation and β-cell maturation [5, 6]. - **WEE1:** As mentioned earlier, WEE1 indirectly regulates IDH2 expression through epigenetic mechanisms [6]. - **Mitofusin 2 (MFN2):** MFN2 promotes β-cell maturation from mouse embryonic stem cells via the Sirt3/Idh2 activation pathway [5]. - **p53:** IDH2 has been shown to interact with p53, and its expression is regulated by p53 in response to oxidative stress. ### 3.5 IDH2 in Non-Cancer Physiology Beyond cancer, IDH2 plays critical roles in various physiological processes: - **Cardiovascular Health:** IDH2-mediated mitophagy and mitochondrial unfolded protein response (mtUPR) are emerging as protective mechanisms in atherosclerosis [7]. Genetic variations in IDH2 have been associated with an increased risk of acute myocardial infarction [8]. - **Kidney Function:** IDH2 protects against kidney fibrosis and inflammation [2, 8]. - **Immune Cell Function:** IDH2 regulates macrophage polarization and inflammatory responses [8]. --- ## 4. Pathogenic Hotspot Mutations & Clinical Differentials ### 4.1 The R140 and R172 Hotspots Somatic mutations in IDH2 are almost exclusively missense mutations that affect one of two arginine residues: R140 (in the small domain) and R172 (in the large domain). These mutations are mutually exclusive with IDH1 mutations and are heterozygous, consistent with a gain-of-function mechanism. - **R140Q (c.419G>A):** This is the most common IDH2 mutation, accounting for approximately 75% of all IDH2 mutations in AML [7]. The substitution of arginine with glutamine at codon 140 is the canonical neomorphic alteration. - **R172K (c.515G>A):** This is the second most common mutation and is more frequently observed in gliomas, cholangiocarcinoma, and other solid tumors. R172K mutations are associated with a more aggressive clinical course in AML compared to R140Q mutations [1, 7]. Other, less frequent mutations at these codons include R140W, R140L, R172M, R172S, and R172G. All of these mutations result in the production of D-2HG. ### 4.2 IDH2 Mutations in Acute Myeloid Leukemia (AML) IDH2 mutations are found in approximately 8–19% of patients with AML, making them one of the most common recurrent genetic alterations in this disease [2, 3, 7, 8]. They are particularly enriched in cytogenetically normal AML (CN-AML) and are often associated with older age and intermediate-risk cytogenetics [1, 3]. The prognostic impact of IDH2 mutations in AML is complex and depends on the specific mutation site and the co-occurring genetic alterations. In some studies, IDH2 mutations have been associated with an adverse prognosis, particularly when present with *FLT3-ITD* or when the R172K mutation is present [1]. However, other studies have shown that IDH2 mutations, particularly R140Q, may be associated with a more favorable outcome in the absence of other high-risk mutations [3]. The presence of IDH2 mutations is a key molecular marker for the use of targeted therapy with enasidenib. ### 4.3 IDH2 Mutations in Gliomas IDH2 mutations are found in a subset of diffuse gliomas, including astrocytomas, oligodendrogliomas, and oligoastrocytomas [3, 4, 5, 6]. They are more common in grade II and III gliomas and secondary glioblastomas than in primary glioblastomas. In gliomas, IDH2 mutations are strongly associated with the 1p/19q codeletion and the *TERT* promoter mutation, defining the oligodendroglioma molecular subtype [6]. The presence of IDH1/2 mutations is a favorable prognostic marker in gliomas, and is now a defining criterion in the WHO classification of central nervous system tumors [4, 5, 7]. ### 4.4 IDH2 Mutations in Other Solid Tumors IDH2 mutations have been identified in a growing list of solid tumors, including: - **Cholangiocarcinoma:** Approximately 10–20% of intrahepatic cholangiocarcinomas harbor IDH1/2 mutations [2]. - **Chondrosarcoma:** IDH1/2 mutations are found in over 50% of central and periosteal chondrosarcomas [1, 4, 8]. - **Sinonasal Undifferentiated Carcinoma (SNUC):** A subset of these aggressive tumors harbor IDH2 R172 mutations [8]. - **Tall Cell Carcinoma with Reversed Polarity (TCCRP) of the Breast:** This rare breast cancer subtype is characterized by recurrent IDH2 R172 mutations [1, 2, 3]. - **Angioimmunoblastic T-cell Lymphoma (AITL):** IDH2 R172 mutations are a defining feature of this peripheral T-cell lymphoma subtype [3]. - **Carotid Body Paraganglioma:** IDH2 mutations have been identified in these rare neuroendocrine tumors [4]. - **Renal Cell Carcinoma:** A novel subtype of multifocal renal cell carcinoma with somatic IDH2 mutations has been described [5]. ### 4.5 Germline IDH2 Mutations and D-2-Hydroxyglutaric Aciduria Germline mutations in IDH2, typically at R140, cause D-2-hydroxyglutaric aciduria type II, a rare inherited metabolic disorder characterized by the accumulation of D-2HG in the urine, plasma, and cerebrospinal fluid. Affected individuals present with a variable phenotype, including developmental delay, epilepsy, hypotonia, and cardiomyopathy. The recent use of enasidenib in two individuals with this condition demonstrated proof-of-concept for targeted therapy in a non-cancer setting [6]. ### 4.6 Clonal Hematopoiesis and Secondary Malignancies IDH2 mutations are among the most common mutations found in clonal hematopoiesis of indeterminate potential (CHIP), a pre-malignant state that increases the risk of developing hematologic malignancies. The presence of an IDH2 mutation in CHIP is a strong predictor of future AML development. Furthermore, IDH2 mutations have been identified in therapy-related myeloid neoplasms and in B-ALL that arises after lenalidomide therapy for multiple myeloma [6]. --- ## 5. Host-Pathogen & Viral Interactions (If applicable) The direct interaction of viral oncoproteins with the IDH2 protein is not a well-established mechanism of viral oncogenesis. However, several indirect connections exist between viral infections and the IDH2 pathway. ### 5.1 Epstein-Barr Virus (EBV) and Metabolic Reprogramming EBV infection of B cells induces a profound metabolic reprogramming to support the proliferation of latently infected cells. This reprogramming includes an upregulation of genes involved in the TCA cycle and oxidative phosphorylation. While the direct modulation of IDH2 expression by EBV-encoded proteins (e.g., LMP1, EBNA2) has not been definitively shown, the virus's ability to activate MYC and HIF-1α can indirectly influence the expression of metabolic enzymes, including IDH2. ### 5.2 Human Papillomavirus (HPV) and the Warburg Effect HPV-driven cancers, such as cervical and head and neck cancers, exhibit a strong Warburg effect, characterized by increased aerobic glycolysis. This metabolic shift is driven by the viral oncoproteins E6 and E7, which stabilize HIF-1α and activate MYC. While IDH2 is not a direct target of these oncoproteins, the altered metabolic landscape may affect the demand for mitochondrial NADPH, potentially influencing the selective pressure for IDH2 mutations in certain contexts. ### 5.3 Hepatitis B and C Viruses (HBV/HCV) and Hepatocellular Carcinoma (HCC) Chronic HBV and HCV infections are major risk factors for HCC. While IDH1/2 mutations are rare in HCC, the chronic inflammation and oxidative stress induced by these viruses can lead to mitochondrial dysfunction. The role of IDH2 in mitigating this oxidative stress may be relevant to the progression of liver disease, although direct viral-IDH2 interactions have not been reported. ### 5.4 Bacterial Effectors and Immune Evasion There is no evidence to suggest that bacterial effectors directly target IDH2. However, the metabolic state of immune cells, including macrophages, is a critical determinant of their anti-bacterial activity. IDH2-mediated NADPH production is essential for the respiratory burst in macrophages, which generates ROS to kill phagocytosed bacteria. Pathogens that can modulate host cell metabolism to suppress this response may indirectly affect IDH2 function. --- ## 6. [Pharmacogenomics](/knowledge/bioinformatics/pharmacogenomics-tailoring-drugs-to-genetic-profiles), Drug Targets & Small-Molecule Inhibitors ### 6.1 Enasidenib (AG-221): The First-in-Class IDH2 Inhibitor Enasidenib is an oral, selective, small-molecule inhibitor of the mutant IDH2 enzyme. It was the first targeted therapy approved by the FDA (August 2017) for the treatment of adult patients with relapsed or refractory AML and an IDH2 mutation. **Mechanism of Action:** Enasidenib binds to the allosteric site of the mutant IDH2 homodimer, stabilizing the enzyme in an inactive "open" conformation. This prevents the conformational changes required for catalysis, thereby inhibiting the production of D-2HG. By reducing D-2HG levels, enasidenib relieves the inhibition of TET2 and histone demethylases, promoting DNA demethylation and allowing leukemic blasts to differentiate into mature myeloid cells [1, 2, 7, 8]. **Clinical Efficacy:** In the pivotal phase I/II clinical trial, enasidenib induced overall response rates of approximately 40% in patients with relapsed/refractory IDH2-mutant AML, with a complete remission (CR) rate of approximately 20% [8]. The median time to first response was 1.9 months, and the median time to CR was 3.7 months. Responses were observed across all IDH2 mutation subtypes, including R140Q and R172K. The IDHENTIFY phase III trial compared enasidenib to conventional care regimens in patients with late-stage mutant-IDH2 relapsed/refractory AML. While the trial did not meet its primary endpoint of improved overall survival in the overall population, a pre-specified analysis suggested a potential survival benefit in the R172K mutant subgroup [7]. **Safety and Tolerability:** The most common adverse events associated with enasidenib include nausea, diarrhea, hyperbilirubinemia, and differentiation syndrome. Differentiation syndrome is a potentially fatal complication characterized by fever, dyspnea, pulmonary infiltrates, and pleural/pericardial effusions, and requires prompt treatment with corticosteroids and diuretics. ### 6.2 Combination Therapies The success of enasidenib as a single agent has led to the investigation of combination regimens to improve response rates and overcome resistance. - **Enasidenib + Venetoclax:** Venetoclax is a BCL-2 inhibitor that has shown significant activity in AML. The combination of enasidenib and venetoclax has been evaluated in patients with IDH2-mutated relapsed/refractory myeloid malignancies, with promising results [7]. The combination is generally well-tolerated, with a safety profile consistent with the individual agents. - **Enasidenib + Azacitidine:** Azacitidine is a hypomethylating agent that is a standard of care for older patients with AML who are unfit for intensive chemotherapy. The combination of enasidenib and azacitidine has shown high response rates in newly diagnosed IDH2-mutant AML. - **Enasidenib + Intensive Chemotherapy:** The addition of enasidenib to standard induction and consolidation chemotherapy is being evaluated in younger, fit patients with newly diagnosed IDH2-mutant AML. ### 6.3 Other IDH2 Inhibitors and Investigational Agents While enasidenib is the only FDA-approved IDH2 inhibitor, several other agents are in development: - **Olutasidenib (FT-2102):** This is a potent, oral, selective inhibitor of mutant IDH1. It is not an IDH2 inhibitor but is relevant to the broader IDH-targeted therapy landscape. - **Vorasidenib (AG-881):** This is a brain-penetrant, dual inhibitor of mutant IDH1 and IDH2. It is being developed for the treatment of IDH-mutant gliomas, where blood-brain barrier penetration is critical. ### 6.4 Resistance Mechanisms Resistance to enasidenib can emerge through several mechanisms: - **Second-Site Mutations:** Mutations in the IDH2 gene itself, such as Q316E or I319M, can occur in *cis* with the original R140Q mutation. These second-site mutations can restore the production of D-2HG by altering the binding of enasidenib to the allosteric site. - **MAPK Pathway Activation:** Activation of the RAS-MAPK pathway through mutations in *NRAS*, *KRAS*, or *PTPN11* can bypass the differentiation block imposed by the IDH2 mutation, leading to resistance. - **Clonal Evolution:** The emergence of a pre-existing or new subclone that does not depend on the IDH2 mutation for survival can lead to resistance. ### 6.5 IDH2 as a Target in Non-Cancer Diseases The successful use of enasidenib in D-2-hydroxyglutaric aciduria [6] has opened the door for repurposing IDH2 inhibitors for non-cancer indications. Furthermore, the role of wild-type IDH2 in protecting against oxidative stress suggests that enhancing its activity (e.g., through SIRT3 activation) could be a therapeutic strategy for diseases characterized by mitochondrial dysfunction, such as ischemia-reperfusion injury, neurodegeneration, and atherosclerosis [7, 8]. --- ## 7. Bioinformatic Resources & Database Accessions The following table provides key database accessions and identifiers for the IDH2 gene and protein. | Database | Identifier | Description | | :--- | :--- | :--- | | **NCBI Gene** | 3418 | Gene ID for human IDH2 | | **Ensembl** | ENSG00000182054 | Gene ID for human IDH2 | | **UniProt** | P48735 | Primary accession for human IDH2 protein | | **RCSB PDB** | 5SVN | Representative structure of human IDH2 R140Q mutant with enasidenib | | **OMIM** | 147650 | Online Mendelian Inheritance in Man entry | | **HGNC** | 5383 | HUGO Gene Nomenclature Committee symbol | | **ClinVar** | Varied | Database of clinically relevant variants | | **COSMIC** | IDH2 | Catalogue of Somatic Mutations in Cancer | | **STRING** | 9606.ENSP00000298604 | Protein-protein interaction network | | **BioGRID** | 108006 | Biological General Repository for Interaction Datasets | | **Gene Ontology (GO)** | GO:0004450 (catalytic activity), GO:0005739 (mitochondrion), GO:0006749 (NADPH regeneration) | Functional annotations | --- ## 8. Conclusion IDH2 is a bifunctional metabolic enzyme whose canonical role in the TCA cycle and NADPH production is essential for cellular redox homeostasis. However, its neomorphic mutation at R140 and R172 converts it into an oncogene that produces the oncometabolite D-2HG, leading to profound epigenetic dysregulation and a block in cellular differentiation. The clinical significance of IDH2 mutations is now firmly established across a wide spectrum of malignancies, and the development of targeted inhibitors like enasidenib has revolutionized the treatment of IDH2-mutant AML. The ongoing research into combination therapies, resistance mechanisms, and the role of IDH2 in non-cancer diseases promises to further expand the therapeutic and diagnostic utility of this remarkable enzyme. --- ## Related Clinical & Scientific Guides * [PIK3CA (PI3K Alpha): Helical and Kinase Domain Hotspot Mutations and Isoform-Specific Inhibition](/knowledge/bioinformatics/genes/cancer-genomics/pik3ca-gene-structure-function-pathway) * [ENTPD5 Gene: Structure, Function, and Clinical Significance](/knowledge/bioinformatics/genes/cancer-genomics/entpd5-gene-structure-function-pathway) * [PDGFB Gene: Structure, Function, and Clinical Significance](/knowledge/bioinformatics/genes/cancer-genomics/pdgfb-gene-structure-function-pathway) ## References [1] Al Abri, Y., Al Huneini, M., Al Zadjali, S., & Al Rawahi, M. (2024). IDH1 and IDH2 Gene Mutations in Omani Patients with Acute Myeloid Leukemia: Prognostic Significance and Clinic-pathologic Features. *Oman Medical Journal*. URL: https://www.semanticscholar.org/paper/eab0c14f7b5a10c0711ef47194cdbc6f31f18c81 [2] Ji, L., Yang, W., Xu, X., & Xu, Y. (2023). A case report of complete remission of acute myeloid leukemia combined with DNMT3A, FLT3-TKD, and IDH2 gene mutations and active pulmonary tuberculosis treated with homeharringtonine + venetoclax + azacytidine. *Frontiers in Medicine*. URL: https://www.semanticscholar.org/paper/50e62bd78ea8da73297701bcfa8ec8a720c46156 [3] Kim, J. I., Noh, M., Yoon, G., Jang, H., Kong, M., & Park, K. (2021). IDH2 gene deficiency accelerates unilateral ureteral obstruction-induced kidney inflammation through oxidative stress and activation of macrophages. *Korean Journal of Physiology and Pharmacology*. URL: https://www.semanticscholar.org/paper/28850b0c1319128abbf3c474f0e3ae9b03516fe1 [4] Mahajan, N., Malla, P., Bhagwat, S., Sharma, V., Sarnaik, A., Kim, J., Pilon-Thomas, S., Weber, J., & Mahajan, K. (2017). 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