UC San Diego

Isocitrate dehydrogenase 1 (IDH1) is a homodimeric enzyme that catalyzes the reversible NADP+-dependent oxidation of isocitrate (ICT) to α-ketoglutarate (αKG) in the cytoplasm and peroxisomes. Somatic mutations in IDH1 have been implicated in lower grade gliomas, chondrosarcomas, and intrahepatic cholangiocarcinomas, and while mutations typically ablate conventional activity, they can also drive a neomorphic function: the NADPH-dependent conversion of αKG to the oncometabolite, D-2-hydroxyglutarate (D2HG). D2HG can competitively inhibit αKG-dependent enzymes to result in widespread epigenetic changes. IDH1 mutants can differ substantially in catalytic efficiency for D2HG production. In this work, I first ask how IDH1 mutations affect the secretion of extracellular vesicles, and how their secretion is related to D2HG production. Using isogenic glioma and chondrosarcoma models, I found that mutant IDH1 expression increased sEV production, and that D2HG was physically packaged into sEVs. Treating wild type cells with cell-permeable octyl-D2HG reproduced the secretion phenotype in a dose-dependent manner, suggesting that D2HG accumulation is sufficient to drive vesicle release. sEVs were enriched for lysosomal and endoplasmic reticulum cargo, a pattern supported by RNAseq in the sEV-producing cells. Upon internalizing sEVs containing D2HG, I found that protein synthesis was acutely decreased. Building on our finding of D2HG dose dependence on sEV production, I next asked how the amount of D2HG produced by mutant cells affects tumor phenotypes by comparing two IDH1 mutants that have catalytically distinct activity. We saw that R132Q, a kinetically robust IDH1 mutant produced higher D2HG in both cells and in in vivo tumors. This work supports a model in which D2HG concentration both drives the export of D2HG and other cargo to neighboring cells and tunes tumor phenotype severity through mechanisms extending beyond CpG hypermethylation.