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Human Telomerase: Biogenesis, Trafficking, Recruitment, and Activation

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Human Telomerase: Biogenesis, Trafficking, Recruitment, and Activation Downloaded from genesdev.cshlp.org on October 3, 2021 - Published by Cold Spring Harbor Laboratory Press REVIEW Human telomerase: biogenesis, trafficking, recruitment, and activation Jens C. Schmidt and Thomas R. Cech Howard Hughes Medical Institute, Department of Chemistry and Biochemistry, BioFrontiers Institute, University of Colorado Boulder, Boulder, Colorado 80309, USA Telomerase is the ribonucleoprotein enzyme that catalyz- et al. 2015). In the other direction, deficiencies in telome- es the extension of telomeric DNA in eukaryotes. Recent rase activity, its maturation, or recruitment to telomeres work has begun to reveal key aspects of the assembly of can lead to human diseases such as aplastic anemia and the human telomerase complex, its intracellular traffick- dyskeratosis congenita (Armanios and Blackburn 2012). ing involving Cajal bodies, and its recruitment to telo- Chromosome ends in human cells consist of 5–15 kb of meres. Once telomerase has been recruited to the telomeric TTAGGG repeats. The majority of these re- telomere, it appears to undergo a separate activation peats are double-stranded, but the very end of each chro- step, which may include an increase in its repeat addition mosome is made up of a single-stranded 3′ overhang processivity. This review covers human telomerase bio- (Blackburn 2005). This single-stranded region is the sub- genesis, trafficking, and activation, comparing key aspects strate or primer for telomerase. This DNA anneals with with the analogous events in other species. the template region in the telomerase RNA (TR) compo- nent, allowing the TERT to synthesize a telomeric repeat (Cech 2004). Once a telomeric repeat has been added, the ′ Human chromosomes are capped by telomeres, repetitive 3 end of the chromosome is repositioned, and telomerase DNA tracts bound by the six-protein shelterin complex. can synthesize additional repeats, a process called repeat The shelterin complex prevents telomeres from being rec- addition processivity (RAP) (Greider 1991). ognized as sites of DNA damage and recruits telomerase The double-stranded region of the human telomere is (Palm and de Lange 2008; Nandakumar and Cech 2013). bound by TRF1 and TRF2, which specifically recognize Telomerase is an RNA-containing reverse transcriptase the telomeric repeat sequence. TRF1 and TRF2 play criti- that adds telomeric repeat DNA to chromosome ends cal roles in suppressing DNA damage signaling at the telo- (Blackburn and Collins 2011). This prevents successive mere and recruit the rest of the shelterin complex (Palm shortening of telomeres caused by the failure of the and de Lange 2008). TIN2 binds to TRF1 and TRF2 and DNA replication machinery to duplicate the very end of serves as a linker between the double-stranded and sin- each chromosome, termed the end replication problem gle-stranded binding complexes of the telomere (Ye et al. (Levy et al. 1992). Once telomeres shrink to a critical 2004; Abreu et al. 2010). The shelterin component POT1 (Baumann and Cech 2001) specifically associates with length, cells enter replicative senescence or alternatively ′ undergo programmed cell death, a major tumor-suppres- the single-stranded 3 overhang of the telomere, preventing sive mechanism. Thus, continuously dividing cells such it from being recognized by the DNA damage machinery as germ cells, stem cells, and, importantly, most cancer (Denchi and de Lange 2007). POT1 is linked to the shelterin cells require telomerase activity for survival (Stewart complex by TPP1, which in turn binds to TIN2 (Liu et al. and Weinberg 2006; Shay and Wright 2011). The impor- 2004; Ye et al. 2004). In addition to promoting POT1 bind- tance of telomerase in oncogenesis is highlighted by the ing, TPP1 is required to recruit telomerase to the telomere recent identification of recurrent mutations in the pro- (Xin et al. 2007; Abreu et al. 2010; Tejera et al. 2010). In moter of the gene for the telomerase protein component summary, the six shelterin components integrate chromo- hTERT (human telomerase reverse transcriptase) (Horn some end protection and telomerase recruitment to allow et al. 2013; Huang et al. 2013), the most frequent mutation continuous cell division in many cell types. in some cancer types (Heidenreich et al. 2014). These pro- Telomere maintenance requires proper assembly of the moter mutations are associated with increased hTERT ex- protein and RNA components of telomerase into a ribonu- pression, telomerase activity, and telomere length (Borah cleoprotein (RNP) as well as a number of cofactors © 2015 Schmidt and Cech This article is distributed exclusively by Cold [Keywords: biogenesis; cancer; recruitment; telomerase; telomere; Spring Harbor Laboratory Press for the first six months after the full-issue trafficking] publication date (see http://genesdev.cshlp.org/site/misc/terms.xhtml). Corresponding author: [email protected] After six months, it is available under a Creative Commons License (At- Article is online at http://www.genesdev.org/cgi/doi/10.1101/gad.263863. tribution-NonCommercial 4.0 International), as described at http:// 115. creativecommons.org/licenses/by-nc/4.0/. GENES & DEVELOPMENT 29:1095–1105 Published by Cold Spring Harbor Laboratory Press; ISSN 0890-9369/15; www.genesdev.org 1095 Downloaded from genesdev.cshlp.org on October 3, 2021 - Published by Cold Spring Harbor Laboratory Press Schmidt and Cech involved in the maturation, stability, and subcellular lo- A H/ACA Domain 3’ calization of telomerase. This review is focused on the IO BOX biogenesis, trafficking, and activation of human telome- B ACA hTERT TCAB1 rase. We compare and contrast human telomerase with C X A O B telomerases in other species, highlight recent findings, B CR4/CR5 identify open questions, and describe new technologies Box H H/ACA Complex that promise to advance our understanding of telomere biology. Pseudoknot Biogenesis of human telomerase The human telomerase RNP is composed of the TR com- hTR 5’ Template ponent (hTR), the TERT (hTERT), and the accessory pro- teins dyskerin, NOP10, NHP2, and GAR1. The formation B of the telomerase RNP is a multistep process that requires hTERT additional cofactors. Mutations in many of the core telo- N TEN TRBD RT CTE C merase RNP subunits as well as the maturation factors lead to telomerase deficiency diseases such as dyskera- 100 AA tosis congenita or aplastic anemia, and the resultant Figure 1. Functional domains of hTR and hTERT. (A) Secondary genome instability can be oncogenic (Armanios and structure of the hTR. The pseudoknot and CR4/CR5 domains in- Blackburn 2012). teract with the hTERT. The template for telomeric repeat synthe- sis is indicated. Two copies of the H/ACA complex, each composed of dyskerin, NHP2, NOP10, and GAR1, associate TR synthesis and maturation with the H/ACA domain of hTR to stabilize the RNA in the nu- cleus. TCAB1 interacts with the CAB box to facilitate telomerase TRs are divergent in size and sequence between species, ∼ ∼ localization to Cajal bodies. (B) hTERT contains four functional ranging from 150 nucleotides (nt) in ciliates to 1150 domains. The telomerase N-terminal (TEN) domain participates nt in Saccharomyces cerevisiae and >2000 nt in Neuro- in catalysis and drives telomerase localization to telomeres. The spora crassa. While TR size and sequence are not highly TR-binding domain (TRBD) interacts with hTR. The reverse tran- conserved, four functional features are present in all scriptase (RT) and C-terminal extension (CTE) form the catalytic TRs: the template for reverse transcription, the pseudo- core of telomerase. Bar, 100 amino acids. knot domain, a stem–loop that interacts with TERT, and a3′ element required for RNA stability (Fig. 1A; Theimer and Feigon 2006). et al. 2006). Rapid association of this complex with hTR Fully mature hTR (451 nt) contains all of the functional is crucial for hTR accumulation. The importance of the features mentioned above (Zhang et al. 2011). Like other formation of the complex between hTR and dyskerin, vertebrate TRs, the 3′ stabilizing element is an H/ACA NOP10, NHP2, and NAF1 is highlighted by a number of motif consisting of two hairpins connected by a short telomerase deficiency diseases associated with mutations single-stranded stretch, the H-box, and a terminal ACA re- in these factors (Armanios and Blackburn 2012). These gion (Fig. 1A; Mitchell et al. 1999; Egan and Collins mutations are thought to affect either binding of these – – 2012a). Vertebrate TRs share this H/ACA motif with proteins to hTR or the formation of a dyskerin NOP10 – small nucleolar and small Cajal body (CB) RNAs (sno- NHP2 NAF1 complex capable of binding to hTR, either RNAs and scaRNAs) that associate with the cofactors dys- of which would cause reduced levels of hTR and thus of kerin, NOP10, NHP2, and GAR1 (Kiss et al. 2010). H/ telomerase activity. ACA scaRNAs and snoRNAs function as guides, specify- In addition to the canonical functional features de- scribed above, hTR contains more specialized sequence ing the targets for the pseudouridylase dyskerin (Kiss et al. ′ 2010). In contrast, the only known functions of the associ- elements in the terminal loop of its 3 hairpin (Fig. 1A). ation between hTR and dyskerin are RNA stabilization The BIO-box stimulates hTR stability by promoting H/ and nuclear retention, and no pseudouridylation targets ACA RNP formation (Egan and Collins 2012b). The for hTR have been identified. CAB-box, a motif that hTR shares with the scaRNAs, is Unlike scaRNAs and snoRNAs, whose precursors are required for trafficking hTR to the CB, where it receives contained in intronic regions of mRNAs, hTR is tran- its TMG cap, and NAF1 is replaced by GAR1 (Jády et al. scribed from its own promoter by RNA polymerase II 2004; Girard et al. 2008; Kiss et al. 2010). (Feng et al. 1995). The mature form of hTR is not polyaden- ylated; instead, its 3′ end is formed by exonucleolytic hTERT cleavage up to the boundary formed by the H/ACA domain, where further cleavage is prevented by dyskerin In most organisms, the TERT protein contains four func- associated with the RNA (Fu and Collins 2003).
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