Lack of evidence for substrate channeling or flux between wildtype and mutant isocitrate dehydrogenase to produce the oncometabolite 2-hydroxyglutarate

• Published in: The Journal of biological chemistry 2018-12, Vol.293 (52), p.20051-20061
• Main Authors: Dexter, Joseph P., Ward, Patrick S., Dasgupta, Tathagata, Hosios, Aaron M., Gunawardena, Jeremy, Vander Heiden, Matthew G.
• Format: Article
• Language: English
• Subjects: Cell Line, Tumor; HEK293 Cells; Humans; Hydroxybutyrates - metabolism; Isocitrate Dehydrogenase - genetics; Isocitrate Dehydrogenase - metabolism; Metabolism; Models, Biological; Mutation; NADP - genetics; NADP - metabolism; Neoplasm Proteins - genetics; Neoplasm Proteins - metabolism; Neoplasms - genetics; Neoplasms - metabolism; Neoplasms - pathology; Protein Multimerization
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.RA118.004278

Monoallelic point mutations in the gene encoding the cytosolic, NADP+-dependent enzyme isocitrate dehydrogenase 1 (IDH1) cause increased production of the oncometabolite 2-hydroxyglutarate (2-HG) in multiple cancers. Most IDH1 mutant tumors retain one wildtype (WT) IDH1 allele. Several studies have proposed that retention of this WT allele is protumorigenic by facilitating substrate channeling through a WT–mutant IDH1 heterodimer, with the WT subunit generating a local supply of α-ketoglutarate and NADPH that is then consumed by the mutant subunit to produce 2-HG. Here, we confirmed that coexpression of WT and mutant IDH1 subunits leads to formation of WT–mutant hetero-oligomers and increases 2-HG production. An analysis of a recently reported crystal structure of the WT–R132H IDH1 heterodimer and of in vitro kinetic parameters for 2-HG production, however, indicated that substrate channeling between the subunits is biophysically implausible. We also found that putative carbon-substrate flux between WT and mutant IDH1 subunits is inconsistent with the results of isotope tracing experiments in cancer cells harboring an endogenous monoallelic IDH1 mutation. Finally, using a mathematical model of WT–mutant IDH1 heterodimers, we estimated that the NADPH:NADP+ ratio is higher in the cytosol than in the mitochondria, suggesting that NADPH is unlikely to be limiting for 2-HG production in the cytosol. These findings argue against supply of either substrate being limiting for 2-HG production by a cytosolic IDH1 mutant and suggest that the retention of a WT allele in IDH1 mutant tumors is not due to a requirement for carbon or cofactor flux between WT and mutant IDH1.