TGS1 mediates 2,2,7-trimethyl guanosine capping of the human telomerase RNA to direct telomerase dependent telomere maintenance

Abstract

Pathways that direct the selection of the telomerase-dependent or recombination-based, alternative lengthening of telomere (ALT) maintenance pathway in cancer cells are poorly understood. Using human lung cancer cells and tumor organoids we show that formation of the 2,2,7-trimethylguanosine (TMG) cap structure at the human telomerase RNA 5′ end by the Trimethylguanosine Synthase 1 (TGS1) is central for recruiting telomerase to telomeres and engaging Cajal bodies in telomere maintenance. TGS1 depletion or inhibition by the natural nucleoside sinefungin impairs telomerase recruitment to telomeres leading to Exonuclease 1 mediated generation of telomere 3′ end protrusions that engage in RAD51-dependent, homology directed recombination and the activation of key features of the ALT pathway. This indicates a critical role for 2,2,7-TMG capping of the RNA component of human telomerase (hTR) in enforcing telomerase-dependent telomere maintenance to restrict the formation of telomeric substrates conductive to ALT. Our work introduces a targetable pathway of telomere maintenance that holds relevance for telomere-related diseases such as cancer and aging.

Methods that shaped telomerase research

“…As mentioned above, telomerase RNAs are dynamic and ever-changing molecules, undergoing 3’ and 5’-end processing steps, association with transport proteins, and assembly with telomerase components before acting at telomeres. Using microscopy-based methods, telomerase RNAs have been found to localise in different cellular compartments to undergo these maturation processes (Gallardo et al 2008 , 2011 ; Nguyen et al 2015 ; Vasianovich et al 2020 ; Buemi et al 2022 ). FISH with telomerase RNA specific fluorescent probes yielded details about cell compartment distribution of the RNA, and in combination with recombination-based molecular tools, was used to identify new components involved in early-RNA shuttling (Zhu et al 2004 ; Gallardo et al 2008 ; Vasianovich et al 2020 ).…”

Sequestering the 5′‐cap for viral RNA packaging

“…[33,34] The extra methyl group(s) in the hypermethylated cap is added by a unique methyltransferase (MTase). [32,33] Viruses have evolved multiple mechanisms for capping their RNAs. [11,35] The mRNAs of most nucleus-replicating DNA viruses (HBV) and retroviruses (HIV-1) are synthesized by the host Pol II and capped by the normal cellular capping apparatus.…”

“…[ 33,34 ] The extra methyl group(s) in the hypermethylated cap is added by a unique methyltransferase (MTase). [ 32,33 ]…”

“…For a small number of viruses, the 5′-cap is hypermethylated with additional modification(s) at the N2 position of the cap residue [26][27][28][29][30] which is a hallmark of the cellular small nuclear, small nucleolar and telomerase RNAs. [31,32] The 2,2,7-trimethyl guanosine (TMG) cap of a subset of HIV-1 mRNAs was shown to initiate translation in a CBCdependent manner rather than requiring eIF4E binding. [33,34] The extra methyl group(s) in the hypermethylated cap is added by a unique methyltransferase (MTase).…”

Telomerase-activated Au@DNA nanomachine for targeted chemo-photodynamic synergistic therapy

“…[30][31][32] Human telomerase is a special ribonucleoprotein polymerase that contains RNA sequences with short templates to guide the synthesis of telomere repeats at the ends of chromosomes, and it protects chromosomes from fusion, recombination, and degradation. [33][34][35] Telomerase is found in more than 85% of human tumours, whereas telomerase is hardly contained in normal cells. [36][37][38][39] Therefore, the huge difference in telomerase expression between tumour cells and normal cells makes telomerase an attractive tumour marker, and potential therapeutic target for chemotherapy.…”