Abstract
Neomorphic mutations in isocitrate dehydrogenase (IDH)1 and IDH2, but not in IDH3, have been identified in acute myelogenous leukemia, low-grade glioma, and secondary glioblastoma. In addition, as shown recently by our group, non-mutated wild-type IDH1 mRNA and protein are commonly overexpressed in primary GBM, and through enhanced production of cytoplasmic NADPH accelerate de novo lipid biosynthesis, regulate redox homeostasis, and enhance glioblastoma progression (Calvert et al., 2017; Cell Reports). While defined roles of mutated and wild-type IDH1 and IDH2 paralogs in cancer pathobiology emerged, little is known about IDH3, which catalyzes one of the rate-limiting, tightly controlled reactions of the TCA cycle. We demonstrate that IDH3a protein expression is elevated in Glioblastoma patient samples compared to normal brain tissue. In support of IDH3a pro-tumor effect, IDH3a over-expression (CRISPR/Cas9-mediated KO) enhanced (reduced) the tumorigenic potential of patient-derived xenografts, as demonstrated by accelerated (decreased) glioblastoma progression and decreased (increased) overall animal subject survival. On molecular levels, IDH3a binds serine hydroxymethyltransferase 1 (cSHMT1), a key regulator of one carbon metabolism. SHMT1 promotes the assembly of a multi-enzyme thymidylate synthesis complex at the nuclear lamina during S and G2/M phases, and appropriates folate metabolites between thymidylate synthesis and the methionine cycle. Through complex formation with SHTM1 at the nuclear lamina, IDH3a increases SHMT1 activity, and in so doing, increases the flux of folate metabolites through the thymidylate synthesis pathway, enhances nucleotide availability during DNA replication, promotes progression through the cell cycle, and decreases susceptibility to the anti-folate chemotherapeutic methotrexate, while shunting methionine cycle turnover and the methylation potential of cells. The identification of a nuclear IDH3-SHMT1 signaling axis point to the regulation of one carbon metabolism by cancer-associated IDH3a as a non-canonical mechanism of metabolic adaptation in glioblastoma, which promotes unabated tumor cell growth and resistance to anti-folate chemotherapy.