Telomeres control human telomerase (TERT) expression through non-telomeric TRF2

The function of the human telomerase reverse transcriptase (referred hereafter as TERT) in the synthesis and maintenance of chromosome ends, or telomeres, is widely understood. Whether and how telomeres, on the other hand, influence TERT regulation is relatively less studied. We found TERT was transcriptionally altered depending on telomere length (TL). This resulted from TL-dependent binding of TRF2 between telomeres and the TERT promoter. TERT promoter-bound TRF2 was non-telomeric and did not involve the looping of telomeres to the TERT promoter. Cell lines from different tissue types fibrosarcoma (HT1080), colon cancer (HCT116), and breast cancer (MDA-MB-231), engineered for either telomere elongation/shortening, gave an increase/decrease in TERT, respectively. Mechanistically, we show TERT promoter-bound non-telomeric TRF2 recruits the canonical PRC2-complex, inducing repressor histone H3K27-trimethylation in a TL-dependent fashion. This was further supported by TL-dependent promoter activity from an exogenously inserted TERT reporter. Increase in TL over days followed by a gradual decline, resulted in activation followed by repression of TERT in a concerted manner, further implicating TL as a key factor for TERT regulation. Notably, on reprogramming primary fibroblasts to induced pluripotent stem cells (iPSCs), TRF2 loss from the TERT promoter was evident along with telomere elongation and TERT upregulation. Conversely, on telomere shortening in iPSCs, TERT promoter-bound TRF2 was restored with a marked reduction in TERT, further supporting the causal role of TL in TERT transcription. Mechanisms of tight control of TERT by TL shown here are likely to have major implications in telomere-related physiologies, particularly, cancer, ageing, and pluripotency.