Abstract
Mutant isocitrate dehydrogenase 1 (mtIDH1) is commonly found in adult gliomas, particularly the lower grade variety, and appears predictive of therapeutic response. Despite better outcomes in tumors harboring these mutations, mtIDH1, through production of the onco-metabolite 2-hydroxyglutarate (2HG), appears sufficient to drive astrocyte transformation potentially through alterations in the epigenetic state of tumor cells. Inhibitors of histone deacetylases (HDACis) are drugs that affect the epigenetic status potentially reverting chromatin structure of tumor cells to a non-transforming configuration. Therefore, we hypothesized that gliomas that harbor mtIDH1 may show enhanced response to epigenetic drugs such as HDACis. To test this hypothesis, mtIDH1 was exogenously expressed in an immortalized normal human astrocyte line as well as the LN229 glioma cell line. Both constitutive and inducible expression of mtIDH1 was accomplished in these lines with confirmation of expression by western blot and production of 2HG in the media by mass spectrometry. Treatment with the HDACi belinostat (BeleodaqTM, Spectrum Pharmaceuticals) readily increases acetylation of histone H4 and tubulin in our cell lines. Moreover, this drug can cross the blood-brain barrier and we have demonstrated its single agent activity in a rodent orthotopic glioma model. Using our isogenic cell lines differing in mtIDH1 expression, we found by MTS assay that growth inhibition with belinostat treatment was significantly more pronounced when mtIDH1 was present. In assessing potential mechanisms for differential response to belinostat, we found higher drug-induced apoptosis by Annexin V assay in our cell lines expressing mtIDH1. Furthermore, the induction of caspase activity by belinostat was consistently 2-fold higher as measured by colorimetric assay in the mtIDH1 expressors compared with corresponding controls. Our data suggest that gliomas with mtIDH1 may be particularly sensitive to HDACis such as belinostat. Clinical testing of this drug in brain tumors harboring mtIDH1 such as lower grade gliomas may be warranted.