Telomeres and Telomere Length: A General Overview

Abstract

Telomeres are highly conserved tandem nucleotide repeats that include proximal double-stranded and distal single-stranded regions that in complex with shelterin proteins afford protection at chromosomal ends to maintain genomic integrity. Due to the inherent limitations of DNA replication and telomerase suppression in most somatic cells, telomeres undergo age-dependent incremental attrition. Short or dysfunctional telomeres are recognized as DNA double-stranded breaks, triggering cells to undergo replicative senescence. Telomere shortening, therefore, acts as a counting mechanism that drives replicative senescence by limiting the mitotic potential of cells. Telomere length, a complex hereditary trait, is associated with aging and age-related diseases. Epidemiological data, in general, support an association with varying magnitudes between constitutive telomere length and several disorders, including cancers. Telomere attrition is also influenced by oxidative damage and replicative stress caused by genetic, epigenetic, and environmental factors. Several single nucleotide polymorphisms at different loci, identified through genome-wide association studies, influence inter-individual variation in telomere length. In addition to genetic factors, environmental factors also influence telomere length during growth and development. Telomeres hold potential as biomarkers that reflect the genetic predisposition together with the impact of environmental conditions and as targets for anti-cancer therapies.

Keywords: TERT promoter mutations; cancer-risk; end replication; genetic variants; shelterin complex; telomere length heritability; telomere maintenance mechanisms; telomeres.

Figure 1

Schematic representation of (A) telomeres and subtelomeric regions, tandem nucleotide repeats at chromosomal ends that include a double-stranded region and a 50–300 nucleotide single-stranded guanine rich G-tail. Subtelomers (green) represent regions of genes interspersed within repeat elements and interstitial telomeric sequence (red arrow); (B) shelterin complex, the G-tail folds back into the duplex DNA to form the t-loop; (C) G-quadruplex structure, intramolecular G-quadruplex (left) built from G-quartets that are formed through cyclic Hoogsten hydrogen-bonding arrangement of four guanines with each other with G-tetrad structure on the right. Adapted from [23,24].

Figure 6

Schematic representation of genetic mechanisms of telomerase activation. (A) TERT gene amplification leading to an increase in TERT copy number at the 5p15.33 locus; (B) genomic rearrangement in TERT result in inter-chromosomal translocation; (C) Mutations at two hotspots in the TERT promoter, −124 and −146 bp from ATG start site create de novo binding sites for ETS transcription factors (red circles). Various transcriptional binding elements are represented in the core promoter region, E-box (green), ETS (red), and Sp1 (blue). Adapted from [12,97,145].

Figure 2

Schematic representation of chromatin structure and distribution of histone marks on telomeres. The telomeres are tightly packed into nucleosomes, the structural and functional units of chromatin. The euchromatin-associated and heterochromatin-associated histone marks are indicated. The euchromatin-associated marks include H4ac, H4K20me1, H3ac, H3K4me1/2/3, H3K36me2/3, H3K27ac, H3K79me3, and H2BK120ub. The heterochromatin-associated marks include H4K20me3, H3K9me3, and H3K27me3. Adapted from [34].

Figure 3

Representation of shelterin complex, heterotrimeric complex CST, and telomeric repeat containing RNA (TERRA). Shelterin complex comprises of six distinct protein subunits: telomeric-repeat-binding factor 1 and 2 (TRF1 and TRF2), TRF1-interacting nuclear protein 2 (TIN2), protection of telomeres 1 (POT1), POT1 and TIN2-interacting protein (TPP1), and repressor and activator protein 1 (RAP1). TRF1 and TRF2 bind the double-stranded DNA; POT1 binds the single-stranded 3′ G-overhang. TIN2 bridges TRF1 and TRF2 by binding to both the proteins simultaneously through independent domains and recruits TPP1–POT1 complex. RAP1 interacts with TRF2 to localize at the telomeres. CST complex is a heterotrimeric protein consisting of conserved telomere protection component 1 (CTC1), suppressor of cdc13 a (STN1), and telomeric pathway with STN1 (TEN1), which specifically localize to the single-stranded 3′ overhang and protect the telomeres by mediating DNA replication and telomerase regulation, independent of shelterin complex. Telomeric repeat containing RNA (TERRA) transcription initiates within subtelomeres in the direction of telomeres. TERRA is involved in regulating telomere capping and the maintenance of telomeres. Adapted from [13].

Figure 4

Schematic representation of lagging and leading strand replication. DNA polymerase Polα with a single RNA primer initiates synthesis of leading strand, which is subsequently replaced by Polε for further elongation. The lagging strand is copied through discontinuous Okazaki fragments from multiple primers. RNA primers are degraded and the gaps filled by Polδ followed by ligation of discontinuous fragments. The gap at 5′ end remains unfilled, leading to a non-replicated terminal region. Adapted from [7].

Figure 5

Structure of telomerase. Telomerase is a holoenzyme composed of the catalytic subunit, TERT (telomerase reverse transcriptase), and the RNA component, TERC (telomerase RNA component). Dyskerin and other associated proteins, GAR1, NHP2, and NOP10 interact with TERC by binding to the H/ACA box and regulate telomerase biogenesis, subcellular localization, and function. Adapted from [109,110].

Figure 7

Epigenetic regulation of TERT in cancers. (A) Depiction of transcription factors along with binding sites, TERT promoter mutations −124C > T and −146C > T, hypermethylated region upstream of transcription start site (THOR). Binding of transcriptional activators, c-Myc, and repressors, CCCTC-binding factor (CTCF), myeloid zinc finger protein-2 (MZF-2), and Wilms tumor 1 (WT1) to the TERT promoter is controlled by DNA methylation as methylated CpGs prevent the binding to the target sites leading to TERT activation. The black lollipops represent methylated CpG sites. (B) Relative DNA methylation in tumor-derived cell lines with and without TERT promoter mutations. The green box represents a specific region in THOR (−668 to −577 bp relative to ATG) that is shown to be less methylated in cell lines with TERT promoter mutations than in cell lines without mutations. Adapted from [123,146,147].

Figure 8

Alternative lengthening of telomeres (ALT). ALT is a telomerase-independent mechanism that occurs via homologous recombination to maintain telomere length. The inactivation of α-thalassemia/mental retardation syndrome X-linked protein (ATRX) and death domain-associated protein (DAXX) upregulates telomeric repeat containing RNA (TERRA), which activates telomeric recombination and initiation of ALT. Adapted from [97].