Epigenetic suppression of human telomerase (hTERT) is mediated by the metastasis suppressor NME2 in a G-quadruplex–dependent fashion

Abstract

Transcriptional activation of the human telomerase reverse transcriptase (hTERT) gene, which remains repressed in adult somatic cells, is critical during tumorigenesis. Several transcription factors and the epigenetic state of the hTERT promoter are known to be important for tight control of hTERT in normal tissues, but the molecular mechanisms leading to hTERT reactivation in cancer are not well-understood. Surprisingly, here we found occupancy of the metastasis suppressor non-metastatic 2 (NME2) within the hTERT core promoter in HT1080 fibrosarcoma cells and HCT116 colon cancer cells and NME2-mediated transcriptional repression of hTERT in these cells. We also report that loss of NME2 results in up-regulated hTERT expression. Mechanistically, additional results indicated that the RE1-silencing transcription factor (REST)–lysine-specific histone demethylase 1 (LSD1) co-repressor complex associates with the hTERT promoter in an NME2-dependent way and that this assembly is required for maintaining repressive chromatin at the hTERT promoter. Interestingly, a G-quadruplex motif at the hTERT promoter was essential for occupancy of NME2 and the REST repressor complex on the hTERT promoter. In light of this mechanistic insight, we studied the effects of G-quadruplex–binding ligands on hTERT expression and observed that several of these ligands repressed hTERT expression. Together, our results support a mechanism of hTERT epigenetic control involving a G-quadruplex promoter motif, which potentially can be targeted by tailored small molecules.

Human Telomerase Expression is under Direct Transcriptional Control of the Telomere-binding-factor TRF2

“…Notably, we found about 20000 TRF2 binding sites spread throughout the genome where TRF2-mediated promoter epigenetics and gene regulation was evident. A large fraction of the TRF2 binding sites coincided with potential DNA secondary structure G-quadruplex-forming sequences 12 .Further, we observed interaction of TRF2 with G-quadruplexes at multiple promoters, consistent with the emerging role of G-quadruplexes as epigenetic regulatory motifs (reviewed in [13][14][15][16][17] ).Moreover, the hTERT near promoter was reported to harbor multiple G-quadruplexes [18][19][20] .Here, we asked if TRF2 directly associates with the hTERT promoter and affects regulation of hTERT expression. We found direct binding of TRF2 at the hTERT promoter and TRF2-mediated transcriptional repression of hTERT.…”

“…Because the hTERT promoter had multiple G-quadruplexes [18][19][20][21] (shown in Supplementary Figure 4A) we analyzed the TERT promoter across vertebrates. And, noted with interest that several vertebrate species have one or more G-quadruplexes within 500 bp of TERT TSS ( Figure 4A).…”

Androgen Receptor Enhances Hepatic Telomerase Reverse Transcriptase Gene Transcription After Hepatitis B Virus Integration or Point Mutation in Promoter Region

“…Therefore, some mechanism other than genetic changes might exist for the elevation of TERT in HCC without promoter genetic changes. For example, several mechanisms have been proposed to overcome TERT silencing, through either specific signaling pathways or specific epigenetic regulations targeting its promoter regulation . As a ribonucleoprotein that catalyzes the addition of telomeric repeats to the ends of chromosomes, elevation of TERT activity is critical for the immortality of liver cancer cells.…”

“…For example, several mechanisms have been proposed to overcome TERT silencing, through either specific signaling pathways or specific epigenetic regulations targeting its promoter regulation. (30,31) As a ribonucleoprotein that catalyzes the addition of telomeric repeats to the ends of chromosomes, elevation of TERT activity is critical for the immortality of liver cancer cells. Regardless of the mechanism, TERT is elevated in most HCCs, which, however, is silent in normal hepatocytes.…”

Structure and Function of Multimeric G-quadruplexes

“…does not indicate which model is correct, it does support the idea that a G-rich structure in this promoter regulates hTERT expression [112][113][114][115].…”