Telomeres: The Guardians of the Genome

Telomeres, often referred to as the "caps" of our chromosomes, are crucial structures that play a fundamental role in preserving the integrity and stability of our genetic material. These specialized DNA-protein complexes are located at the ends of linear chromosomes and act as protective buffers, preventing the loss of vital genetic information during cell division. Telomeres are not just molecular bookends; they hold profound implications for the ageing process, cancer development, and overall health. In this article, the fascinating world of telomeres is explored, by discovering their structure, function, and the impact they have on our lives.

The Structure of Telomeres

Telomeres are located at the ends of chromosomes and consist of short, repetitive nucleotide sequences. In humans, the typical telomeric DNA sequence is a six-nucleotide motif, TTAGGG, which is repeated thousands of times in tandem [Figure 1]. This repeated sequence serves as the "code" that forms the telomere. The repetitive nature of telomeric DNA is essential for its function. It allows telomeres to form a loop structure, known as a T-loop, wherein the single-stranded overhang at the 3' end of the telomere can invade and loop back into the double-stranded telomeric DNA (Wright et al., 1997). This looped structure helps protect the chromosome end and prevent it from being recognized as a DNA break.

Figure 1: Telomeres are regions of repetitive sequences at each end of a chromosome (Unique Health and Wellness, n.d.).

In addition to telomeric DNA, telomeres are associated with a specialized protein complex called shelterin. The shelterin complex consists of six core proteins: TRF1 (telomere repeat factor 1), TRF2 (telomere repeat factor 2), POT1 (protection of telomeres 1), TIN2 (TRF1-interacting nuclear factor 2), TPP1 (POT1 and TIN2-interacting protein 1), and RAP1 (repressor/activator protein 1). These proteins work together to protect and maintain the integrity of telomeres (de Lange, 2005) [Figure 2]. TRF1 and TRF2 bind specifically to the double-stranded telomeric DNA, aiding in the formation of the T-loop and protecting the chromosome end (Walker & Zhu, 2012). POT1 binds to the single-stranded telomeric overhang, preventing it from being recognized as a DNA break and thereby protecting the telomere (Zade & Khattar, 2023). TIN2 and TPP1 bridge the interaction between TRF1, TRF2, and POT1, helping to stabilize the shelterin complex (Pike et al., 2019). Finally, RAP1 binds to TRF2 and is involved in regulating telomere length and interactions within the shelterin complex (Cai et al., 2017).

The combination of telomeric DNA and the shelterin complex forms a protective cap at the end of each chromosome. This cap ensures that the natural ends of the chromosome are not mistaken for damaged DNA that needs repair. Telomeric caps are crucial for preventing degradation, fusion, and other detrimental processes that can compromise genomic stability.

Figure 2: Simplified scheme depicting the terminal end of a telomere (Zhu, 2016).

The Role of Telomeres

Telomeres play a vital role in safeguarding our genetic information. During cell division, DNA is replicated to form new cells. However, due to the nature of DNA replication, the ends of linear chromosomes are not fully replicated. This phenomenon can result in the gradual shortening of telomeres with each cell division. Telomere shortening serves as a biological clock, limiting the number of times a cell can divide. When telomeres reach a critically short length, the cell is signalled to stop dividing, entering a state known as replicative senescence, one of the stimuli that leads to cellular senescence (a permanent cell cycle arrest) (Liu et al., 2019). This process prevents cells with damaged or incomplete DNA from proliferating, reducing the risk of genetic mutations and maintaining genomic stability.

Telomere shortening is a hallmark of the ageing process. As cells divide throughout an individual's lifetime, the telomeres at the ends of chromosomes undergo progressive shortening [Figure 3]. At this stage, cells stop dividing and may undergo changes in gene expression and function. The accumulation of these senescent cells in tissues and organs contributes to age-related physiological decline and the onset of various age-related diseases. This shortening is a result of the end replication problem (Cech & Lingner, 1997).

Figure 3: Telomeres shorten as we get older causing aging in our cells (T.A. Sciences, n.d.)

The end replication problem is a consequence of the inability of DNA polymerase to replicate the 5' end of the lagging strand during DNA replication. DNA replication occurs in a semi-conservative manner, where one strand is synthesized continuously (leading strand), while the other strand (lagging strand) is synthesized in short fragments called Okazaki fragments (Balakrishnan & Bambara, 2013). The leading strand is synthesized smoothly in the 5' to 3' direction, but the lagging strand is synthesized in the opposite direction. This means that the RNA primer, which initiates DNA synthesis on the lagging strand, is not replaced with DNA near the very end of the chromosome. Therefore, a small portion of the chromosome's end is not replicated [Figure 4].

Telomere shortening is accelerated by factors such as chronic psychological stress, oxidative stress, inflammation, smoking, poor diet, lack of physical activity, and metabolic conditions (Lin & Epel, 2022). These factors can hasten cell turnover and increase the demand for cell division, thereby accelerating telomere shortening. Shortened telomeres are linked to a higher risk of age-related diseases, such as cardiovascular disease, diabetes, and certain cancers. This association has spurred interest in understanding the potential of telomere lengthening as a strategy to mitigate ageing-related health issues.

Figure 4: The end replication problem (Khan Academy, n.d.).

Telomerase: The Telomere Extender

Telomerase is a specialized enzyme that plays a crucial role in the maintenance and elongation of telomeres. The enzyme was discovered by Carol W. Greider and Elizabeth H. Blackburn in the early 1980s, and their research earned them the Nobel Prize in Physiology or Medicine in 2009. Telomerase consists of two main components: the protein component, telomerase reverse transcriptase (TERT) and the RNA component (TERC or TR) [Figure 5]. The protein component is an enzyme that possesses a unique catalytic subunit responsible for the reverse transcription of the RNA template into telomeric DNA. The telomerase RNA component acts as the template for the synthesis of the telomeric DNA (Sandin & Rhodes, 2014). When telomerase is activated, it binds to the telomere overhang and uses the RNA template to extend the telomeric DNA, effectively elongating the telomere and compensating for the natural telomere shortening that occurs during cell division.

The telomerase complex is not only formed by TERT and TERC but also by many other proteins [Figure 5]. Dyskerin is a protein that plays a critical role in stabilizing and guiding the RNA component of telomerase (Angrisani et al., 2014). NHP2 is another protein involved in the assembly and stability of telomerase. It forms a complex with dyskerin and contributes to the proper folding and maturation of the telomerase RNA. NOP10, like NHP2, is essential for the assembly and stability of telomerase. It is part of the complex that binds to the telomerase RNA and helps maintain the integrity of the telomerase complex. GAR1 is yet another protein that is part of this complex and it is essential for the assembly and function of telomerase and contributes to the proper maturation of the telomerase RNA (Shay & Wright, 2019).

Figure 5: Scheme of telomerase and shelterin complex (Nishio, 2010).

Telomerase activity is highly regulated and varies among different cell types. It is typically active during early development, allowing for rapid cell proliferation and ensuring the proper elongation of telomeres. However, in most somatic (non-reproductive) cells of adults, telomerase activity is minimal, leading to gradual telomere shortening with each cell division. In contrast, telomerase is highly active in certain cells, such as stem cells and some immune cells (Choudhary et al., 2012). This activity helps maintain telomere length and supports these cells' ability to divide and regenerate, providing longevity to tissues and organs.

Telomeres and Cancer

The telomere shortening process in normal cells eventually triggers cell cycle arrest and cellular senescence, acting as a barrier to uncontrolled cell proliferation. However, in many cancer cells, the telomere-shortening process is bypassed. Cancer cells often achieve immortality and continuous proliferation by maintaining their telomere length, allowing them to evade this growth barrier. There are two main mechanisms by which cancer cells achieve telomere maintenance: telomerase activation and alternative lengthening of telomeres (ALT) (Shay et al., 2012) [Figure 6]. Telomerase activation or upregulation is the mechanism employed by most cancer cells, ensuring that telomere length is maintained or even extended. This sustained telomere length prevents cells from reaching replicative senescence, contributing to cancer cell immortality and uncontrolled growth. Alternative lengthening of telomeres (ATL) is a telomerase-free mechanism. In ALT-positive cells, telomeres are elongated through homologous recombination and DNA repair processes. This allows cancer cells to achieve telomere maintenance and immortality without telomerase activity.

Figure 6: Most of the cancer cells have sustained telomere length, by activating the telomerase or by alternative lengthening of telomeres (Shay, 2012).

Targeting telomeres and telomerase represents a promising avenue for cancer therapy. Strategies to inhibit telomerase or disrupt telomere maintenance can potentially induce telomere shortening in cancer cells, leading to cell death or a halt in proliferation (Guterres & Villanueva, 2020) [Figure 6]. Small molecule inhibitors that specifically target telomerase are being developed. These inhibitors aim to suppress telomerase activity in cancer cells, ultimately leading to telomere shortening and cessation of cell division. Approaches that target the telomere structure or the shelterin complex are also under investigation. Disrupting the protective shelterin complex can render telomeres vulnerable, making them susceptible to degradation and contributing to cancer cell death. Combining telomerase inhibitors with conventional cancer treatments, such as chemotherapy or radiation therapy, may enhance treatment efficacy by targeting both the tumour and its ability to maintain telomere length.

Telomere length and telomerase activity have been also explored as potential diagnostic and prognostic markers for various types of cancer. Abnormal telomere length or telomerase activity is often associated with malignancy. Researchers have found that shorter telomeres in some cancer types may correlate with a worse prognosis and higher disease aggressiveness. Additionally, telomerase activity detection can be utilized as a diagnostic tool. Elevated telomerase levels in tumour samples can indicate the presence of cancer or predict the aggressiveness of the disease (Yuan et al., 2020).

Telomeres and Other Diseases

Shortened telomeres have been associated with many different diseases including cardiovascular diseases (CVD), diabetes, neurodegenerative diseases and osteoporosis. In the case of CVD, telomere length is inversely correlated with the incidence of heart attacks, strokes, atherosclerosis, and overall cardiovascular health. Telomere shortening can contribute to the ageing of endothelial cells, which line blood vessels, leading to impaired blood vessel function and increased risk of CVD (Zhan & Hägg, 2019) [Figure 7]. In the case of diabetes telomere shortening can affect insulin sensitivity, glucose metabolism, and inflammation, all of which are key factors in diabetes development and progression (Cheng et al., 2021). Telomere shortening has been linked to various neurodegenerative conditions, including Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis (ALS) (Rodríguez-Fernández et al., 2022). Shortened telomeres can affect neuronal health and increase susceptibility to neurodegenerative processes. Telomere length is related to bone density, and telomere shortening can impair bone-forming cells, contributing to osteoporosis (Fragkiadaki et al., 2020).

Figure 7: Schematic overview of telomere length and cardiovascular diseases (Yeh, 2016).

Telomere shortening is also associated with the decline of the immune system, a process known as immunosenescence (de Punder et al., 2019). Telomeres play a critical role in the lifespan and function of immune cells, such as T cells and B cells, which are vital components of the immune system. When immune cells divide to combat infections or diseases, telomeres undergo shortening due to the end replication problem. Over time, this repetitive shortening results in reduced telomere length in immune cells. As immune cell telomeres reach a critically short length, the cells may enter a state of replicative senescence or undergo cell death, impairing their ability to effectively combat pathogens. This decline in immune cell function is associated with increased susceptibility to infections, reduced vaccine efficacy, and diminished immune responses, which are common features of immunosenescence.

Conclusion

Telomeres are pivotal structures in the realm of genetics, acting as guardians of cellular youth and health. Their role in cellular division and ageing, as well as their involvement in various diseases, makes them an exciting area of study in contemporary biology. Understanding telomere dynamics and exploring strategies to preserve telomere length can potentially revolutionize healthcare, offering avenues to promote longevity and improve the quality of life for individuals around the world.

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Visual Sources

Cover Image. (Li, 2020) Telomere length. [Image] Nebula Genomics. Retrieved on 19th October, 2023 from https://nebula.org/blog/genetics-of-telomere-length-li-2020/

Figure 1: [Telomeres are regions of repetitive sequences at each end of a chromosome]. [Image] Unique Health and Wellness. Retrieved on 19th October, 2023 from https://uniquehealthandwellness.com.au/lengthen-telomeres-lengthen-life/

Figure 2: (Zhu, 2016). [Simplified scheme depicting the terminal end of a telomere]. [Image] Inflammation, Advancing Age and Nutrition. Retrieved on 19th October, 2023 from https://www.sciencedirect.com/topics/neuroscience/terf1

Figure 3: [Telomeres shorten as we get older causing ageing in our cells]. [Image] T.A. Sciences. Retrieved on 19th October, 2023 from https://www.tasciences.com/telomeres-and-cellular-aging.html

Figure 4: The end replication problem. [Image] Khan Academy. Retrieved on 19th October, 2023 from https://www.khanacademy.org/science/biology/dna-as-the-genetic-material/dna-replication/a/telomeres-telomerase

Figure 5: (Nishio, 2010) [Scheme of telomerase and shelterin complex]. [Image] International Journal of Hematology. Retrieved on 19th October, 2023 from https://www.researchgate.net/figure/Schema-of-telomerase-and-shelterin-complex-Telomerase-complex-consists-of-the-enzyme_fig1_46821562

Figure 6: (Shay, 2012). [Most of the cancer cells have sustained telomere length, by activating the telomerase or by alternative lengthening of telomeres]. [Image] Science. Retrieved on 19th October, 2023 from https://www.science.org/doi/10.1126/science.1222394

Figure 7: (Yeh, 2016) [Schematic overview of telomere length and cardiovascular diseases]. [Image] Genes. Retrieved on 19th October, 2023 fromhttps://www.mdpi.com/2073-4425/7/9/58