MEK1/2 kinases cooperate with c-Myc:MAX to prevent polycomb repression of TERT in human pluripotent stem cells

Telomerase counteracts telomere shortening, enabling human embryonic stem cells (hESC) to undergo long-term proliferation. MAPK signaling plays a major role in regulating the self-renewal of hESC, and previous studies in induced pluripotent stem cells (iPSC) suggested that expression of TERT, the gene encoding the catalytic subunit of telomerase, relies on MAPK signaling. We examined whether MEK-ERK signaling regulated TERT transcription in a model of normal hESC. Kinase inhibitors of MEK1 and MEK2 (MEKi) or ERK1 and ERK2 (ERKi) significantly repressed TERT mRNA levels. Using chromatin immunoprecipitation (ChIP) we observed that MEKi induced the accumulation of the repressive histone mark histone 3 lysine 27 trimethylation (H3K27me3) at the TERT proximal promoter. This increase corresponded with a loss of histone 3 lysine 27 acetylation (H3K27ac) which is associated with transcriptionally active loci. Inhibition of the polycomb repressive complex 2 (PRC2), which deposits H3K27me3, partially rescued the loss of TERT expression, indicating that MEK1/2 activity can limit PRC2 activity at TERT. Inhibition of MEK/ERK kinases also repressed expression of c-Myc, a transcription factor reported to regulate TERT in other immortalized cells. Consistent with a key role for c-Myc in regulating TERT, low doses of a c-Myc:MAX dimerization inhibitor induced a striking and rapid gain of H3K27me3 at TERT and repressed TERT transcription in hESC. Inhibiting c-Myc:MAX dimerization also resulted in lower MAX recruitment to TERT, suggesting that this complex acts in cis at TERT. Our study using a model of normal human pluripotent stem cells identifies new regulators and mechanisms controlling transcription of an important, developmentally regulated gene involved in telomere protection.