Wild-Type Idh1: A Molecular Target In Idh1 Mutant Cancers

Neomorphic mutations targeting R132 of the TCA cycle enzyme, IDH1, have been identified in multiple cancer types and lead to a build up of (R)-2-hydroxyglutarate ((R)-2HG). Several mechanisms have been proposed to account for mutant-IDH1-mediated transformation: due to its structure similarity with alpha-ketoglutarate (α-KG), (R)-2HG can inhibit α-KG-dependent enzymes that act as tumour suppressors such as TET2 or EGLN, (R)-2HG might inhibit electron transport chain function, or rapid (R)-2HG generation may deplete the cellular pool of α-KG leading to depletion of NADPH. Recently, it has also emerged that IDH1 may be required to catalyse a series of back reactions through TCA cycle components (reductive decarboxylation) to facilitate fatty acid synthesis from media glutamine, particularly under hypoxic conditions. According to some reports in the literature, heterodimer formation between mutant an wild-type alleles of IDH1 is important for the production of high levels of (R)-2HG. We were interested in exploring novel ways to target tumour cells bearing mutant IDH1 alleles that were distinct from the obvious opportunity available to identify mutant-specific IDH1 inhibitors. The potential metabolic vulnerabilities of mutant IDH1 cancers raised the possibility that wild-type IDH1 might be essential for tumourigenesis or tumour maintenance in this context. We therefore employed Horizon9s rAAV-mediated homologous recombination gene engineering technology to generate conditional knockouts of the IDH1+ or IDH1R132C alleles in the fibrosarcoma cell line, HT1080. Despite several attempts, we did not recover a correctly targeted conditional knock-out of the IDH1R132C allele. However, we did recover a derivative cell line where the wild-type allele had been converted into a loxP-flanked IDH1R132C allele, and two independent clones where the wild-type allele had been flanked with loxP sites. Interestingly, in these latter two clones, no expression of wild-type IDH1 was detectable. Here we present data characterising the growth of these clones under a variety of culture conditions, including the hypoxic state where IDH1 has been proposed to be important for enabling lipid synthesis from glutamine. We also demonstrate that HT1080 cells that express no detectable levels of wild-type IDH1 retain the ability to be tumorigenic in immuno-compromised mice. These data suggest that the wild-type allele of IDH1 is dispensable for cancer formation. Citation Format: Julie A. Wickenden, Paul Russell, Amy Smith, Tom Henley, Jane Elliott, Dan Gitterman, Mark Stockdale, Christine Schofield, Chris Torrance, Jonathan D. Moore. Wild-type IDH1: A molecular target in IDH1 mutant cancers. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 4620. doi:10.1158/1538-7445.AM2014-4620