How DNA damage and non-canonical nucleotides alter the telomerase catalytic cycle

Abstract

Telomeres at the ends of linear chromosomes are essential for genome maintenance and sustained cellular proliferation, but shorten with each cell division. Telomerase, a specialized reverse transcriptase with its own integral RNA template, compensates for this by lengthening the telomeric 3' single strand overhang. Mammalian telomerase has the unique ability to processively synthesize multiple GGTTAG repeats, by translocating along its product and reiteratively copying the RNA template, termed repeat addition processivity (RAP). This unusual form of processivity is distinct from the nucleotide addition processivity (NAP) shared by all other DNA polymerases. In this review, we focus on the minimally active human telomerase catalytic core consisting of the telomerase reverse transcriptase (TERT) and the integral RNA (TR), which catalyzes DNA synthesis. We review the mechanisms by which oxidatively damaged nucleotides, and anti-viral and anti-cancer nucleotide drugs affect the telomerase catalytic cycle. Finally, we offer perspective on how we can leverage telomerase's unique properties, and advancements in understanding of telomerase catalytic mechanism, to selectively manipulate telomerase activity with therapeutics, particularly in cancer treatment.

Keywords: Aging; Cancer; DNA damage; DNA repair; Nucleoside analogs; Ribonucleotides; Telomerase; Telomerase therapeutics; Telomeres.

Figure 1.

Structures of TERT from (A) tcTERT (PDB: 6USO), (B) T. Thermophila (PDB: 6D6V), and (C) human telomerase (EMDB:EMD-7521). The TRBD (yellow), RT domain (orange), and CTE (red) form the TERT ring. The TEN domain and IFD are shown in blue and cyan, respectively. A segment of tetrahymena telomerase’s RNA template highlights how the adjacent 5’ template boundary element (TBE, pink), and the 3’ template recognition element (TRE, green) define the template region (grey).

Figure 2.

Structure of telomerase holoenzymes from (A) human (EMD-7521) and (B) T. Thermophila telomerase (PDB: 6D6V). Human telomerase has a bilobal structure with the catalytic core containing TERT (grey) and hTR (orange) in one lobe. The H/ACA lobe consists of NHP2 (purple), TCAB1 (yellow), NOP10 (blue), GAR1 (teal) , dyskerin (red), and hTR (orange). Tetrahymena telomerase is a compact structure consisting of TERT (grey), ttTR (orange), p65 (red), p50 (green), Teb1C (purple), Teb2N (yellow), and Teb3 (blue). The DNA substrate for both structures is shown in cyan.

Figure 3.. Telomerase nucleotide addition processivity.

The numbers represent each step in the cycle. Gray indicates the telomeric DNA primer. Purple indicates the telomerase RNA template and hTERT. Yellow indicates the newly added nucleotides.

Figure 4.. Telomerase repeat addition processivity.

The numbers represent each step in the cycle. Gray indicates the telomeric DNA primer. Purple indicates the telomerase RNA template and hTERT. Yellow indicates the newly added nucleotides.

Figure 5.. Modified nucleotides disrupt different steps of the telomerase catalytic cycle.

The gray headings indicate each step of the catalytic cycle shown in Figure 4. “Other” refers to mechanisms that remain to be defined or related to altered interactions with the telomerase active site. Red indicates the modified nucleotides. Purple indicates the telomerase RNA template and hTERT.