Glioblastoma stem cell growth requires DOT1L-MED23 control of enhancer accessibility

Harboring a low mutational burden, glioblastoma relies on various epigenetic regulators to fuel its development and progression, several of which remain mechanistically enigmatic. Here, we show that the histone methyltransferase DOT1L shapes chromatin accessibility of glioblastoma stem cell enhancer elements to reversibly regulate fate- and growth-related transcriptional programs. A genome-wide chemogenomic knockout screen reveals that the mediator complex tail module subunit, MED23, is essential for glioblastoma stem cell growth arrest following DOT1L inhibition, critically relying on CCND2 repression. MED23 knockout (KO) glioblastoma stem cells do not display the chromatin accessibility changes at enhancer elements following DOT1L inhibition. Consequently, MED23-KO rescues glioblastoma stem cell growth by enabling CCND2 transcription in the context of DOT1L inhibition. Our results uncover valuable mechanistic insights involving DOT1L cooperation with the MED23-driven mediator complex to regulate chromatin accessibility and coordinate transcriptional programs governing glioblastoma stem cell growth.