Transformation by Mutant IDH and (R)-2HG Is Reversible.

Abstract 2413

Somatic mutations in IDH1 and IDH2 are common in normal karyotype AML as well as in other clonal myeloid disorders and several solid tumors. Mutant IDH overproduces the R-enantiomer of 2-hydroxyglutarate, (R)-2HG (Dang, et al Nature 462: 739, 2009), which is hypothesized to alter the epigenetic landscape of cancer cells by inhibiting the activity of α-ketoglutarate-dependent enzymes, including the TET family of 5-methylcytosine hydroxylases and the jumonji-domain-containing family of histone demethylases. There is considerable interest in developing inhibitors of mutant IDH in the hope that, by decreasing (R)-2HG production in cancer cells, their epigenetic regulation can be restored. However, there is, to date, no evidence that transformation by mutant IDH is reversible or that inhibiting production of (R)-2HG has any effect on cancers harboring IDH mutations. Herein we report that in two different myeloid transformation assays, transformation by (R)-2HG and mutant IDH1 is reversible by removal of (R)-2HG.

We previously reported that stable expression of a tumor-derived mutant IDH1 (IDH1R132H) induces growth factor independence and blocks EPO-induced differentiation in the human TF-1 erythroleukemia cell line, and that treatment of TF-1 cells with a cell-permeable form of (R)-2HG, TFMB-(R)-2HG, is sufficient to recapitulate this phenotype (Late Breaking Abstract #LBA-4, ASH 2011). We have extended these studies and found that transformation by TFMB-(R)-2HG is reversible and that this reversibility is influenced by the duration and intensity (dose) of (R)-2HG exposure. We developed a second myeloid transformation assay using a murine myeloid leukemia cell line that is transformed by expression of a HoxB8-ER fusion protein when cultured in the presence of estrogen. Upon estrogen withdrawal, the cells undergo monocytic differentiation and apoptosis. We expressed wild-type IDH1 or IDH1R132H in the cells and found that cells expressing wild-type IDH1 differentiate normally, but cells expressing IDH1R132H do not upregulate monocytic markers CD11b/Mac1 and Gr1 upon estrogen withdrawal. Furthermore, treatment of the IDH1R132H-expressing cells with an inhibitor of mutant IDH1 restores their ability to undergo monocytic differentiation upon estrogen withdrawal. Our findings suggest that continued exposure to (R)-2HG is required to maintain the cellular changes induced by mutant IDH, and further suggest that targeting (R)-2HG production may have therapeutic efficacy in the treatment of cancers harboring IDH mutations.