Telomeres as Therapeutic Targets in Heart Disease

Summary

Telomerase counteracts the process of telomere shortening caused by aging, inflammation, and oxidative stress by maintaining and elongating the telomere length. Patients with atherosclerotic diseases and cardiovascular risk factors have shorter leukocyte telomere length. Following myocardial infarction, telomerase expression and activity in cardiomyocytes and endothelial cells increase significantly, implying that telomerase plays a role in regulating tissue repairs in heart diseases. Although previous studies have focused on the changes of telomeres in heart diseases and the telomere length as a marker for aging cardiovascular systems, recent studies have explored the potential of telomeres and telomerase in the treatment of cardiovascular diseases.

This review discusses the significant advancements of telomere therapeutics in gene therapy, atherosclerosis, anti-inflammation, and immune modulation in patients with cardiovascular diseases. In humans, telomeres are 10 to 15 kb of tandem DNA repeats. There are significant variations of telomere length among individuals, but telomeres invariably shorten with age and cell division.¹ Although there could be asynchrony of telomere length among different tissues,² peripheral leukocyte DNA has been most commonly used in clinical studies to measure leukocyte telomere length.³

Central Illustration

Several methods have been used to measure LTL, including terminal restriction fragment analysis by hybridization with telomere sequence probes, single telomere amplification and blotting, flow cytometry of cells following hybridization with fluorescent peptide nucleic acid probes, quantitative fluorescence in situ hybridization with fluorescent telomere peptide nucleic acid probes, and quantitative polymerase chain reaction assays.⁴ Due to the different methods used in clinical trials and large interbatch coefficients of variations, there is no current gold standard of telomere length measurement, and therefore, comparison of telomere lengths between different clinical trials could be misleading.⁵ Despite the fact that current robust epidemiological and animal study evidence supports the telomere attrition links between age and CVDs, there is no clear route that leads to the development of telomere therapeutics against CVDs in the future due to the following limitations.

First, in adult somatic cells, manipulation of the telomere system bears an oncogenic risk.^{6, 7} Thus, therapeutic techniques based on the overexpression of telomerase and other telomere-related signals should be applied after considering cell-type and tissue interactions. Second, there is still no clear mechanistic insight into the link between telomere and/or telomerase and atherosclerosis development.⁸ Third, the telomere system is complex and regulated by various feedback mechanisms, including circadian rhythm oscillations,⁹ and a direct interruption of 1 target in the telomere pathway can lead to various side effects.

Telomeres and Telomerase

This loss is much larger than the estimation from end-replication mechanisms, indicating that there are other contributing factors for telomere attrition in human cells.¹⁰ Oxidative stress and tissue inflammation have been observed to accelerate telomere shortening and reduced replicative lifespans.¹¹ Telomere shortening is considered a biological molecular clock and is the underlying mechanism proposed to explain the limited lifespan of cells in culture, known as the Hayflick limit.¹² Telomerase is a crucial component in telomere maintenance and regulation.

It consists of an RNA template known as telomerase RNA component and a DNA reverse transcriptase polymerase known as telomerase reverse transcriptase. Telomerase synthesizes new telomeric DNAs to compensate for the loss during cell divisions.¹³ The overexpression of telomerase extends the lifespan of cells in culture and transforms them into cancerous cells.¹⁴ Telomere length is determined by genetic and nongenetic factors.

Although its causal relationships are still unclear, based on epidemiology studies, telomere attrition is considered to be associated with aging and aging-related diseases, such as CVDs, chronic lung disease, metabolic disorders, neurodegenerative diseases, cognitive disorders, and dysregulated immune function.¹⁵ The lifespan of older adults was reported to positively correlate with telomere length.¹⁶ It was observed that the specific telomere syndromes or telomeropathies that affect humans, such as Hoyeraal-Hreidarsson syndrome, dyskeratosis congenita, and aplastic anemia, are caused by germline mutations in telomere maintenance genes. These telomere syndromes present a diverse manifestation but share the features of premature aging, loss of tissue regenerative capacity, increases in inflammation, and prominent organ failure.¹⁷

Gene Therapy With Telomere and Telomerase for CVDs

In mice models, short telomeres and associated pathologies were treated and halted by telomerase re-expression.¹⁸ These findings provided the concept for therapeutic strategies to delay age-associated pathologies by transiently increasing telomerase expression. Telomerase gene therapy was first achieved by delivering mouse TERT with an adeno-associated virus into young and old mice. Applications of AAV-TERT gene therapy in specific telomere syndromes also showed expected therapeutic effects in preclinical mice models, such as aplastic anemia and pulmonary fibrosis.^{19, 20}

Apart from direct TERT delivery by nonintegrative AAV vectors, new gene therapy methods using modified mRNA for in vitro encoding of TERT in human fibroblasts can transiently increase telomerase activity, rapidly extend telomeres, and increase proliferative capacity without the risks of insertional mutagenesis and off-target effects.²¹ In addition to proof-of-concept experimental data in mice, the development of safe strategies for transient and controllable telomerase activation in humans can be a subject of future studies.

Pharmaceutical Interventions for Telomeres and Telomerase Activity

The low potency telomerase activator TA-65, a bioactive molecule extracted from Astragalus membranaceus, has been historically used in Chinese traditional medicine as an antiaging drug and has been shown to have effects on telomere lengthening in mice. TA-65 treatment induces telomerase-dependent elongation of short telomeres and reverses DNA damages in fibroblasts²² and human T cells.²³ A recent study showed that the upregulated telomerase activity is responsible for the effectiveness of the androgen treatment effect in aplastic anemia. In mice with aplastic anemia induced by short telomeres, testosterone therapy halted telomere attrition and prevented subsequent death, by enhancing telomerase expression and lengthening telomeres.²⁴

The off-target effects of these telomerase activating or telomere-lengthening compounds, including those in mitogen signaling and oncogenesis, should be considered before clinical usage.

Inflammation, Atherosclerosis, and Clonal Hematopoiesis of Indeterminate Potential

The risk factors of atherosclerotic diseases, such as aging, smoking, obesity, sedentary lifestyle, and unhealthy diet, have been reported to be associated with telomere shortening based on observational epidemiological studies.²⁵ As observed in 1 of these studies, each kilobase pair shortening of telomeres in peripheral blood cells was estimated to result in 2.8- to 3.2-fold higher risk of myocardial infarction and stroke.²⁶ Telomeres in coronary endothelial cells are shorter in patients with atherosclerosis than in healthy individuals.²⁷ Telomere shortening of endothelial cells might play a role in atherogenesis by increasing proinflammatory reactions and promoting high-risk unstable atherosclerotic plaques.²⁸

In human abdominal aorta analysis, shorter telomere and higher attrition were observed in aged vessels with increased shear wall stress.²⁹ As these blood cells gain a competitive expansion advantage, they give rise to some expanded clones of leukocytes that circulate in the peripheral blood, which is termed clonal hematopoiesis of indeterminate potential (CHIP).³⁰ Mice with CHIP mutations in hematopoietic cells also exhibited aggravated the development of heart failure.³¹ Both telomere attrition and CHIP increase with age.

Accordingly, progressive leukocyte telomere attrition can lead to genomic instability, which later results in CHIP.³² Therefore, it is assumed that the manipulation of the telomere system would be a possible treatment target of CHIP-related CVDs. In a whole-genome sequencing study, telomere lengths were observed to be significantly shortened in individuals with CHIP.³³ Dyskeratosis congenita is characterized by short telomeres with poor telomere maintenance, mainly caused by some abnormal mutations in ribosome and telomerase RNA components.³⁴

Clonal expansion of hematopoietic cells bearing nonsynonymous coding somatic mutations is a common feature that occurs in one-half of patients with dyskeratosis congenita.³⁵ The telomerase complex controls hematopoietic cell differentiation and senescence in the induced pluripotent stem cell model.³⁶ The telomere-CHIP-atherosclerosis axis may provide several possible therapeutic targets, including modulating telomerase activity, rescuing senescent or mutated clonal cells, and inhibiting the inflammation from CHIP.