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Telomerase Biogenesis and Activities from the Perspective of Its Direct Interacting Partners

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Telomerase Biogenesis and Activities from the Perspective of Its Direct Interacting Partners cancers Review Telomerase Biogenesis and Activities from the Perspective of Its Direct Interacting Partners Kathryn T. T. T. Nguyen and Judy M. Y. Wong * Faculty of Pharmaceutical Sciences, University of British Columbia, Vancouver, BC V6T 1Z3, Canada; [email protected] * Correspondence: [email protected] Received: 1 June 2020; Accepted: 22 June 2020; Published: 24 June 2020 Abstract: Telomerase reverse transcriptase (TERT)—the catalytic subunit of telomerase—is reactivated in up to 90% of all human cancers. TERT is observed in heterogenous populations of protein complexes, which are dynamically regulated in a cell type- and cell cycle-specific manner. Over the past two decades, in vitro protein–protein interaction detection methods have discovered a number of endogenous TERT binding partners in human cells that are responsible for the biogenesis and functionalization of the telomerase holoenzyme, including the processes of TERT trafficking between subcellular compartments, assembly into telomerase, and catalytic action at telomeres. Additionally, TERT have been found to interact with protein species with no known telomeric functions, suggesting that these complexes may contribute to non-canonical activities of TERT. Here, we survey TERT direct binding partners and discuss their contributions to TERT biogenesis and functions. The goal is to review the comprehensive spectrum of TERT pro-malignant activities, both telomeric and non-telomeric, which may explain the prevalence of its upregulation in cancer. Keywords: TERT; TERT interacting proteins; TERT non-telomeric functions 1. Introduction Telomeres are terminal regions of linear chromosomes, each being a tract of tandem TTAGGG repeats bound by the hexameric protein complex shelterin [1]. The presence of shelterin proteins, consisting of telomeric repeat-binding factor 1 (TRF1, also known as TERF1), telomeric repeat-binding factor 2 (TRF2, also known as TERF2), protection of telomeres protein 1 (POT1), tripeptidyl-peptidase 1 (TPP1, also known as ACD), TRF1-interacting nuclear factor 2 (TIN2, also known as TINF2), and repressor/activator protein 1 homolog (RAP1, also known as TERF2IP), enables the formation of telomere loop (T-loop)—higher-order structure that results from telomeric DNA folding back onto itself—which in turn distinguishes chromosomal ends from DNA breaks that are susceptible to degradation and end-to-end fusion [2–4]. Every time a cell divides, telomeric DNA loses 50–100 bp both from its extreme end where the primer of DNA polymerase latches on, and from the resolution of T-loop that allows the replication machinery to pass through [5,6]. When telomeres are eroded to a critical length, either senescence or apoptosis is triggered depending on the cell type [7]. Thus, sufficiently long telomeres are the prerequisite for both genomic integrity and replicative capacity of a cell. The canonical means of telomere length maintenance is through telomerase—a ribonucleoprotein (RNP) holoenzyme that synthesizes telomeric hexanucleotide repeats from an internal template onto telomeres [8,9]. Telomerase activity is present in embryonic but absent in most adult somatic cells, with the exception of certain populations of tissue stem cells and hematopoietic progenitor cells; thus, telomeres shorten with age by default [10,11]. Age-dependent telomere shortening restricts tissue renewal capacity and represents an important cellular aging mechanism [12]. Accordingly, intrinsically Cancers 2020, 12, 1679; doi:10.3390/cancers12061679 www.mdpi.com/journal/cancers Cancers 2020, 12, 1679 2 of 17 short telomeres secondary to telomerase dysfunction underlie a number of accelerated aging disorders that are collectively referred to as the short telomere syndromes [12,13]. On the other hand, since telomere length dictates cell division potential, overriding the endogenous telomere attrition process is crucial for tumorigenesis [1]. While telomerase-independent strategies are available, the majority of human cancers (85–90%) employ telomerase to maintain the length of their telomeres above the Cancers 2020, 12, x 2 of 18 senescence-inducing threshold [14]. The[12]. catalytic Accordingly, core of intrinsically telomerase short is made telomeres up of telomerasesecondary to RNA telomerase component dysfunction (TERC) underlie and telomerase a reversenumber transcriptase of accelerated (TERT) aging [15]. Thedisorders 5’ end that of TERCare collectively bears the referred telomeric to as repeat the short template, telomere whereas the 3’ endsyndromes contains [12,13]. a hairpin-hinge-hairpin-ACA On the other hand, since telomere (H/ACA) length motif dictat thates mediatescell division TERC potential, interaction with twooverriding sets of the the H endogenous/ACA-RNP telomere complex, attrition each of pr whichocess is consists crucial offor dyskerin tumorigenesis (also known[1]. While as DKC1, telomerase-independent strategies are available, the majority of human cancers (85–90%) employ NAP57, or Cbf5p), NHP2, NOP10, and GAR1 [16,17]. The H/ACA-RNP complexes protect TERC telomerase to maintain the length of their telomeres above the senescence-inducing threshold [14]. from exonucleolyticThe catalytic degradation, core of telomerase and they is made remain up boundof telomerase to TERC RNA as component part of the (TERC) fully assembledand telomerasetelomerase holoenzyme reverse transcriptase [15]. On the (T otherERT) [15]. hand, The the5’ end full-length of TERC bears TERT the proteintelomeric (FLrepeat TERT) template, consists of four mainwhereas functional the 3’ regions end contains (Figure a 1hairpin-hinge-ha). The N-terminusirpin-ACA comprises (H/ACA) the telomerasemotif that mediates essential TERC N-terminal (TEN) domain,interaction which with two tethers sets of the to telomeresH/ACA-RNP during complex, catalysis, each of which and consists the RNA of dyskerin binding (also domain known (RBD) that directlyas DKC1, associates NAP57, withor Cbf5p), TERC NHP2, [16]. NOP10, Between and theGAR1 two [16,17]. termini The isH/ACA-RNP the reverse complexes transcriptase protect domain TERC from exonucleolytic degradation, and they remain bound to TERC as part of the fully (RTD)—theassembled catalytic telomerase site of holoenzyme telomerase [15]. [18]. On The the C-terminusother hand, the of full TERT-length (CTE) TERT does protein not (FL have TERT) a precisely definedconsists function; of however,four main mutationsfunctional regions within (Fig thisure region 1). The have N-terminus been shown comprises to impair the telomerase both subcellular localizationessential and N-terminal activity of (TEN) telomerase domain, [16 which]. tethers to telomeres during catalysis, and the RNA Humanbinding TERC domain and (RBD) TERT that are directly encoded associates by hTERC with TERCon [16]. chromosome Between the 3 two and terminihTERT is theon reverse chromosome 5, respectivelytranscriptase [16]. domain While (RTD)—thehTERC expression catalytic site is of ubiquitous, telomerase [18].hTERT The C-terminexpressionus of TERT is largely (CTE) suppresseddoes not have a precisely defined function; however, mutations within this region have been shown to in adult somatic cells; thus, with TERT being the rate-limiting determinant of telomerase activity, its impair both subcellular localization and activity of telomerase [16]. transcriptionalHuman upregulation TERC and TERT is an are expected encoded hallmarkby hTERC on of chromosome telomerase-positive 3 and hTERT tumors on chromosome [19]. However, given that5, respectively the amount [16]. of While telomerase hTERC expression activity a is cell ubiquitous, needs to hTERT overcome expression senescence is largely corresponds suppressed to as low as 1%in adult of TERT somatic expression cells; thus, level with inTERT cancer being [20 the,21 rate-limiting], TERT likely determinant has pro-malignant of telomerasefunctions activity, its that are independenttranscriptional from its upregulation canonical roleis an in expected telomere hallmark extension. of telomerase-positive Indeed, evidence tumors of TERT[19]. However, non-telomeric activities—regulationgiven that the amount of apoptosis, of telomerase DNA activity damage a cell response, needs to overcome and transcription senescence corresponds among others—in to as the low as 1% of TERT expression level in cancer [20,21], TERT likely has pro-malignant functions that context of cancer has amassed since the late 1990s [21]. Furthermore, given that expression of certain are independent from its canonical role in telomere extension. Indeed, evidence of TERT non- TERT alternativetelomeric activities—regulation splice variants whose of productsapoptosis, areDNA catalytically damage response, inactive and proteins transcription can enhance among survival and promoteothers—in cell the proliferation context of cancer post-DNA has amassed damage, sinc TERTe the non-telomericlate 1990s [21]. Furthermore, functions can given be uncoupledthat from theexpression canonical of telomerase certain TERT activity alternative [22, 23splice]. variants whose products are catalytically inactive In vitroproteinsprotein-protein can enhance survival interaction and promote (PPI) cell assays proliferation including post-DNA yeast two-hybrid damage, TERT assay, non-telomeric affinity and gel functions can be uncoupled from the canonical telomerase activity [22,23]. filtration purification have discovered numerous species that directly interact with TERT at different In vitro protein-protein interaction (PPI) assays including yeast
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