METABOLIC IMAGING BIOMARKERS FOR MUTANT IDH1 GLIOMAS

Abstract

BACKGROUND: The IDH1 mutation is now recognized as an important driver in the etiology of low-grade brain tumors. In contrast to wild-type IDH1, which catalyzes the conversion of isocitrate to α-ketoglutarate (αKG), mutant IDH1 converts αKG into the oncometabolite 2-hydroxygluatrate (2HG). In an effort to noninvasively image the activity of mutant IDH1, we have previously shown that we can use hyperpolarized αKG together with 13C magnetic resonance spectroscopy (MRS) to noninvasively detect the production of hyperpolarized 2HG in vivo. In more recent studies we have focused on identifying additional metabolic alterations associated with the presence of the IDH1 mutation in order to develop complementary metabolic imaging approaches that might serve to inform on the presence of mutant IDH1 and its modulation with targeted therapies. METHODS: Two mutant and wild-type IDH1 cell models were generated by using either U87 glioblastoma cells or E6/E7/hTERT immortalized normal human astrocytes (NHAs) and lentivirally transducing with either wild-type IDH1 (wt cells) or wild-type and mutant IDH1 (mutant cells). Using 1H MRS we investigated cell extracts to determine steady state metabolite levels. We also determined metabolic fluxes by using 13C MRS and probing the fate of 13C-labeled precursors (at thermal equilibrium or hyperpolarized) in live cells grown in a bioreactor or in orthotopic tumors in rats. RESULTS: We found that in both U87 and NHA-based cell models not only was the IDH1 mutation associated with elevated levels of 2HG, but other metabolite levels were also altered. In particular, steady state glutamate levels were lower. This was associated with an increased flux of glutamine to 2-HG and a concomitantly reduced flux of glutamine to glutamate. The flux of glucose into the TCA cycle and towards glutamate was also reduced. At the same time the activity and expression of BCAT1 and aspartate aminotransferase dropped. Pyruvate dehydrogenase activity was also reduced, associated with an increase in its phosphorylation and the expression of pyruvate dehydrogenase kinase 3. Collectively our findings likely explain the lower glutamate levels observed in mutant IDH1 cells. Importantly, we were also able to detect a drop in the potentially translatable in vivo imaging of hyperpolarized glutamate produced from hyperpolarized αKG in mutant IDH1 orthotopic tumors. CONCLUSIONS: In addition to the production of 2HG, the presence of mutant IDH1 leads to MRS-detectable metabolic changes that could serve as complementary imaging biomarkers of mutant IDH1 activity or its inhibition with treatment. SECONDARY CATEGORY: n/a.