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IDH1 Mutations in Gliomas: When an Enzyme Loses Its Grip

2010; Cell Press; Volume: 17; Issue: 1 Linguagem: Inglês

10.1016/j.ccr.2009.12.031

1878-3686

Christian Frezza, Daniel A. Tennant, Eyal Gottlieb,

Epigenetics and DNA Methylation

The growing interest in cancer metabolism is best demonstrated by the rapid progress made in studying isocitrate dehydrogenase (IDH) mutations since their discovery just over a year ago. In a recent study published in Nature, Dang et al. identified 2-hydroxyglutarate as a product of tumor-associated IDH mutants with potential oncogenic activities. The growing interest in cancer metabolism is best demonstrated by the rapid progress made in studying isocitrate dehydrogenase (IDH) mutations since their discovery just over a year ago. In a recent study published in Nature, Dang et al. identified 2-hydroxyglutarate as a product of tumor-associated IDH mutants with potential oncogenic activities. It is now almost a century since the studies that first associated cellular metabolic changes with cancer. However, the recognition of a causal connection between metabolic alterations and cancer formation was revealed only this decade. Ironically, it was genetics, rather than biochemistry, that enabled this breakthrough when genes encoding mitochondrial enzymes of the tricarboxylic acid (TCA) cycle, succinate dehydrogenase (SDH) and fumarate hydratase (FH), were identified as bona fide tumor suppressors (King et al., 2006King A. Selak M.A. Gottlieb E. Oncogene. 2006; 25: 4675-4682Crossref PubMed Scopus (534) Google Scholar). Over the past year, new genetic studies placed another metabolic enzyme, isocitrate dehydrogenase (IDH), in the spotlight of cancer biology (Yan et al., 2009aYan H. Bigner D.D. Velculescu V. Parsons D.W. Cancer Res. 2009; 69: 9157-9159Crossref PubMed Scopus (125) Google Scholar). High-throughput sequencing revealed that two of the three isoforms of IDH (IDH1 and IDH2) are mutated in high proportions in gliomas (Parsons et al., 2008Parsons D.W. Jones S. Zhang X. Lin J.C. Leary R.J. Angenendt P. Mankoo P. Carter H. Siu I.M. Gallia G.L. et al.Science. 2008; 321: 1807-1812Crossref PubMed Scopus (4529) Google Scholar, Yan et al., 2009bYan H. Parsons D.W. Jin G. McLendon R. Rasheed B.A. Yuan W. Kos I. Batinic-Haberle I. Jones S. Riggins G.J. et al.N. Engl. J. Med. 2009; 360: 765-773Crossref PubMed Scopus (4215) Google Scholar). However, unlike SDH and FH, IDH mutations do not follow Knudson's two-hit model of tumor suppressor genes. In the new study, Dang et al., 2009Dang L. White D.W. Gross S. Bennett B.D. Bittinger M.A. Driggers E.M. Fantin V.R. Jang H.G. Jin S. Keenan M.C. et al.Nature. 2009; 462: 739-744Crossref PubMed Scopus (2617) Google Scholar demonstrated that although IDH1 mutants lose their normal enzymatic activity in tumors, they gain a new one, generating a new product, 2-hydroxyglutarate, with potentially tumor-supporting actions (making it an onco-metabolite). Eukaryotic cells contain two classes of IDH enzymes according to dependence on either NAD+ or NADP+. These enzymes normally convert isocitrate to α-ketoglutarate (aka 2-oxoglutarate), with the concurrent reduction of NAD(P)+ to NAD(P)H (Figure 1). The two NADP+-dependent forms, IDH1 and IDH2, are cytosolic and mitochondrial, respectively. IDH3, the only NAD+-dependent IDH, is located at the mitochondria and is part of the TCA cycle. Rapid cycling of metabolites between cytosol and mitochondria is a common feature of cellular metabolism. Metabolites entering the mitochondria can be processed for energy generation usually through the production of NADH in the TCA cycle whereas metabolites exported back to the cytosol take part in anabolic processes. The transport of metabolites is also coupled to electron exchange between mitochondrial and cytosolic NADH and NADPH, both of which cannot move across the mitochondrial inner membrane (Figure 1). Because mitochondrial NADH operates in energy metabolism and cytosolic NADPH functions in anabolic processes and redox control, it is reasonable to expect changes in one or all of these processes in tumors carrying an IDH mutation. Until now, only mutations in IDH1 and 2 were found in cancers, therefore leaving the TCA cycle untouched (Yan et al., 2009aYan H. Bigner D.D. Velculescu V. Parsons D.W. Cancer Res. 2009; 69: 9157-9159Crossref PubMed Scopus (125) Google Scholar). IDH1 mutations form the lion's share of IDH mutations found in cancer, with IDH2 mutation being much less common. So far, gliomas have been shown as the cancer type most likely to contain IDH mutations. Interestingly, they seem to arise early in the development of a glioma, suggesting that it confers advantage early on in tumor progression. One of the most striking features of IDH1 and 2 mutations is that it is always the same residue that is mutated: R132 in IDH1 and R172 in IDH2. These residues create the hydrophilic interactions that allow the binding of isocitrate (Xu et al., 2004Xu X. Zhao J. Xu Z. Peng B. Huang Q. Arnold E. Ding J. J. Biol. Chem. 2004; 279: 33946-33957Crossref PubMed Scopus (295) Google Scholar). The residues that are substituted for arginine are wide ranging, which strongly suggests that it is not the new residue, but the replacement of the arginine, which supports tumorigenesis by impairing isocitrate binding. Indeed, loss of IDH function was reported for these mutants and therefore IDH was suggested to be a tumor suppressor (Zhao et al., 2009Zhao S. Lin Y. Xu W. Jiang W. Zha Z. Wang P. Yu W. Li Z. Gong L. Peng Y. et al.Science. 2009; 324: 261-265Crossref PubMed Scopus (937) Google Scholar). However, the fact that mutations were observed only on specific arginine residues and only on one allele of IDH1/2 with the other remaining wild-type (WT) led to the hypothesis that these are, in fact, gain- rather than loss-of-function mutations with oncogenic potential. The new work (Dang et al., 2009Dang L. White D.W. Gross S. Bennett B.D. Bittinger M.A. Driggers E.M. Fantin V.R. Jang H.G. Jin S. Keenan M.C. et al.Nature. 2009; 462: 739-744Crossref PubMed Scopus (2617) Google Scholar) started with large-scale metabolite quantification (metabolomics) of cells expressing either WT or tumor-derived mutant of IDH1 (R132H). Only one significant metabolic change was observed in mutant-IDH1-expressing cells, which was a large accumulation of 2-hydroxyglutarate, a reduced form of α-ketoglutarate (Figure 1). Indeed, Dang et al. confirmed that the carbon backbone of the accumulated 2-hydroxyglutarate is derived from glutamine, the major source of α-ketoglutarate in these cells (Figure 1). These results suggest that the mutant IDH1 changed its substrate specificity and directionality. In vitro enzymatic analysis confirmed this; whereas WT IDH1 converted isocitrate to α-ketoglutarate, several tumor-associated mutants of IDH1 could no longer catalyze this reaction and instead reduced α-ketoglutarate to 2-hydroxyglutarate (but not to isocitrate). Structural comparison of the mutant and WT IDH1 revealed that mutations in R132 change the orientation of the catalytic site so the enzyme binds NADPH with higher affinity, a feature that supports reductase rather than oxidase activity. Furthermore, modeling α-ketoglutarate into the structure suggests a new orientation of the binding to α-ketoglutarate that can explain the formation of a new product, rather than simply running the reaction in reverse. Finally, Dang et al. demonstrated that 2-hydroxyglutarate levels are 100-fold higher in human gliomas that carry R132 mutations of IDH1 than in tumors with WT IDH1. These results revealed a new gain-of-function activity of the tumor-derived IDH1 mutants and strongly correlated the levels of 2-hydroxyglutarate with tumorigenesis. However, does this grant 2-hydroxyglutarate the title "onco-metabolite" as Dang et al. proposed? What might be these oncogenic functions of 2-hydroxyglutarate? The loss of activity of two other TCA cycle enzymes mentioned earlier, SDH or FH, supports tumor formation by increasing the levels of their respective TCA cycle substrates, succinate or fumarate. These substrates inhibit the oxygen-sensing enzymes hypoxia-inducible factor prolyl hydroxylases (PHDs) by competing with their cosubstrate α-ketoglutarate (MacKenzie et al., 2007MacKenzie E.D. Selak M.A. Tennant D.A. Payne L.J. Crosby S. Frederiksen C.M. Watson D.G. Gottlieb E. Mol. Cell. Biol. 2007; 27: 3282-3289Crossref PubMed Scopus (279) Google Scholar). PHD inhibition leads to the activation of the HIF transcription factor among other, less characterized, effects (King et al., 2006King A. Selak M.A. Gottlieb E. Oncogene. 2006; 25: 4675-4682Crossref PubMed Scopus (534) Google Scholar). It was previously demonstrated that PHDs are inhibited in cells carrying mutant IDH1 (Zhao et al., 2009Zhao S. Lin Y. Xu W. Jiang W. Zha Z. Wang P. Yu W. Li Z. Gong L. Peng Y. et al.Science. 2009; 324: 261-265Crossref PubMed Scopus (937) Google Scholar). Therefore, it is possible that like succinate and fumarate, 2-hydroxyglutarate inhibits PHD activity by competing with α-ketoglutarate (Figure 1). The observation that cell-permeable α-ketoglutarate esters prevent HIF activation in cells expressing mutant IDH1 (Zhao et al., 2009Zhao S. Lin Y. Xu W. Jiang W. Zha Z. Wang P. Yu W. Li Z. Gong L. Peng Y. et al.Science. 2009; 324: 261-265Crossref PubMed Scopus (937) Google Scholar) supports this model. The normal metabolic role of 2-hydroxyglutarate is not completely understood but 2-hydroxyglutarate is not unnatural to cells. It can be generated by specific α-ketoglutarate reductase enzymes (Struys, 2006Struys E.A. J. Inherit. Metab. Dis. 2006; 29: 21-29Crossref PubMed Scopus (65) Google Scholar) and oxidized back to α-ketoglutarate by 2-hydroxyglutarate dehydrogenases (2HGD) (Figure 1). The picture is further complicated by the existence of two enantiomers of 2-hydroxyglutarate with specific 2HGD for each. Mutations in 2HGD cause pathological accumulation of 2-hydroxyglutarate with different clinical features based on the enantiomer involved. Pathological accumulation of the L-2-hydroxyglutarate enantiomer is characterized by progressive neuronal defects and was recently linked to increased risk of brain tumors including gliomas (Aghili et al., 2009Aghili M. Zahedi F. Rafiee E. J. Neurooncol. 2009; 91: 233-236Crossref PubMed Scopus (132) Google Scholar). This is strong support for the potential oncogenic role of 2-hydroxyglutarate, but with one caveat: Dang et al. demonstrated that mutant IDH1 generates D-2-hydroxyglutarate and not the L enantiomer. Accumulation of D-2-hydroxyglutarate is observed in D-2HGD-deficient patients and is associated with encephalopathy, cardiomyopathy, and more—but, so far, not with tumors (Struys, 2006Struys E.A. J. Inherit. Metab. Dis. 2006; 29: 21-29Crossref PubMed Scopus (65) Google Scholar). It is possible that D-2-hydroxyglutarate, when reaching very high levels, is too toxic to have tumorigenic potential. This could have therapeutic significance because it may suggest that a small and transient pharmacological inhibition of 2HGD, by raising the levels of 2-hydroxyglutarate from protumorigenic to toxic, could specifically kill gliomas with IDH1 mutations.