A novel enhancer regulates MGMT expression and promotes temozolomide resistance in glioblastoma

Temozolomide (TMZ) has been used for the treatment of glioblastoma (GBM) since last decade, but its treatment benefits are limited by acquired resistance, a process that remains incompletely understood. Here we report that a novel enhancer, located between the promoters of Ki67 and O6-methylguanine-DNA-methyltransferase (MGMT) genes, is activated in TMZ-resistant patient-derived xenograft (PDX) lines as well as in recurrent tumor samples. Activation of the enhancer correlates with increased MGMT expression, a major known mechanism for TMZ resistance. We show that forced activation of the enhancer in cell lines with low MGMT expression results in elevated MGMT expression. Deletion of this enhancer in cell lines with high MGMT expression leads to reduced levels of MGMT and Ki67, increased TMZ sensitivity and impaired proliferation. Together, these studies uncover a novel mechanism that regulates MGMT expression, confers TMZ resistance and potentially regulates tumor proliferation. To analyze how epigenetic landscapes are altered during the development of TMZ resistance, we performed H3K4me1, H3K4me3, H3K9me3, H3K9ac, H3K27ac and H3K36me3 ChIP-seq on GBM12-5199 and GBM12-3080 tissue samples. We also performed RNA-seq on GBM12-5199 and GBM12-3080 tissue samples.