| Description |
Glioblastoma (GBM) is the most common primary brain tumor in adults, and while advances in surgery, radiation, and standard chemotherapies have improved survival in this disease marginally over the years, the outcomes for most patients remain disappointing (Bhat, 2013). Recent studies have identified molecular subtypes of glioblastoma multiforme (GBM) that demonstrate differences in genetics and biology (Karsy, 2017). These subtypes display differences in outcomes, and may also point to opportunities for new treatments targeted at specific subtypes. One that generally has a worse prognosis and demonstrates increased resistance to standard treatment with temozolimide and radiation is the mesenchymal (MES) subtype (Stopschinski, 2013). While the variety of genetic alterations in GBM is diverse, recent studies have identified mutations and alterations in epigenetic-related genes as a common alteration seen in the majority of GBM (Phillips, 2006). These findings suggest that therapeutic targeting of epigenetic mechanisms in GBM may be an approach to improve therapeutic efficacy. One epigenetic gene of particular interest is the chromatin remodeler lysine demethylase 1 (LSD1) which is a frequent partner in multi-protein complexes associated with histones (McDonald, 2011). Inhibition of LSD1 has been shown to have a significant effect on altering gene expression in multiple tumor types and demonstrates anti-tumor effects in other cancers including Ewing Sarcoma which is genetically driven by alterations that focus on epigenetic mechanisms (Sankar, 2014). We thus investigated the role of LSD1 and LSD1 inhibition in regulating the survival, proliferation, and treatment resistance in GBM. The tumor model we utilized to examine effects of epigenetic targeting in GBM is the neurosphere culture approach with glioma stem cells (GSCs) which utilizes tumor stem-like cells derived from individual GBM tumors grown in stem-cell growth conditions (Bhat, 2013). We found that exposure of GBM neurospheres to a potent LSD1 inhibitor (HCI2509) resulted in alternations in specific biomarkers to resemble a more PRO phenotype. MES GSCs also showed a dose-dependent inhibition of proliferation when exposed to HCI2509. Inhibition of LSD1 in MES GSCs also increased sensitivity to radiation as measured by decreased cell survival and increased expression of the DNA repair marker gamma-H2AX. Immunohistochemical (IHC) assays performed on resected tumors from GBM patients with genetic alterations of either EGFR or NF1 (both of which demonstrate increased MES gene expression) showed marked increase in expression of LSD1 relative to both normal brain and gliomas with mutations in the gene IDH1 (Verhaak, 2010). To investigate the association with histone marks, we utilized IHC and found that lower H3K4me3 and H3K27me3 expression was associated with EGFR and NF1 mutations, while tumors with IDH mutation had higher expression of both of these histone marks. Epidermal growth factor receptor (EGFR) is a membrane protein with tyrosine-kinase function with broad downstream effects. Aberrant EGFR function, or EGFR amplification, identified in several types of cancer, including GBM, often impairs apoptosis and increases cell proliferation (Taylor, 2012). Likewise, neurofibromin 1 (NF1) mutations have been associated with a MES GBM (Verhaak, 2010). These findings suggest a link between genetic alterations, epigenetics, and tumor gene expression. We also identify LSD1 inhibition as a possible therapeutic target for a GBM phenotype that heretofore has proved exceedingly difficult to treat. |
