Telomerase-Based Therapies: Potential and Cancer Risk

Why Telomerase Is a Therapeutic Target

Telomeres are repetitive DNA-protein structures that protect chromosome ends. They tend to shorten as many somatic cells divide, and critically short or unprotected telomeres can activate DNA-damage responses, senescence, or cell death. Telomerase contains a catalytic reverse-transcriptase subunit, TERT, and an RNA template that together add telomeric repeats. Inherited defects in this maintenance system can produce bone-marrow failure, pulmonary fibrosis, liver disease, and other telomere biology disorders, showing that insufficient telomere reserve can be directly pathogenic. [1]

The therapeutic premise is narrower than simply making every telomere longer. It is to restore enough telomere function in selected cells to reduce persistent damage signalling or support tissue renewal. Whether this is useful depends on the disease, cell population, starting telomere distribution, and delivery method. Average telomere length alone may obscure the small fraction of critically short telomeres most relevant to cellular dysfunction. [1] [2]

Approaches Under Study

What the Animal and Cell Studies Show

In a mouse model with severe, genetically induced telomerase deficiency, reactivating endogenous telomerase reduced DNA-damage signalling and improved degeneration in several proliferative tissues. This demonstrates reversibility of pathology caused by extreme telomere dysfunction, but the animals' telomere state was not equivalent to typical ageing in humans. [2]

A separate study delivered mouse TERT with an AAV9 vector to one- and two-year-old mice. The authors reported improvements in several health measures and increases in median lifespan of 24% and 13%, respectively, without a statistically detected rise in cancer in those cohorts. These findings are proof-of-concept in one species and vector system; they do not establish that systemic TERT delivery is effective or cancer-safe in humans. [3]

Transient delivery offers a different exposure pattern. Modified TERT mRNA produced telomerase activity for roughly one to two days in cultured human fibroblasts and myoblasts, extended telomeres, and allowed additional population doublings; the treated cells eventually stopped proliferating. This supports the feasibility of a non-integrating pulse, while leaving tissue delivery, dosing, durability, and in vivo safety unresolved. [4]

Why Cancer Risk Is Biologically Plausible

Replicative senescence and telomere crisis can restrict the continued expansion of abnormal cells. Cancer cells commonly overcome this barrier by activating telomerase or, less often, an alternative telomere-lengthening pathway. Recurrent mutations in the TERT promoter provide direct evidence that selection for TERT re-expression occurs in multiple human cancers. [5] [6]

The relationship is not a simple equation in which shorter telomeres are safe and longer telomeres are dangerous. Severe telomere dysfunction can generate chromosome fusions and genomic instability, which may help initiate malignant evolution if checkpoint controls fail. Subsequent telomere maintenance can then stabilize a viable cancer genome and support continued growth. Telomerase can therefore be protective against one source of genome instability while also removing a barrier to the expansion of an already altered clone. [5]

Human genetic evidence also points to tradeoffs. A Mendelian-randomization analysis found that variants associated with longer telomeres were linked to higher risk for several cancers but lower risk for some non-neoplastic diseases. Such lifelong inherited differences are not identical to a time-limited therapy, yet they argue against treating telomere length as a universally beneficial target. [8]

What Would Determine Risk in a Therapy

• Exposure duration: A brief mRNA pulse and long-lived vector expression create different opportunities for altered cells to gain proliferative capacity. [3] [4]
• Tissue distribution: Broad delivery may expose both cells with regenerative need and occult premalignant clones; tissue-restricted delivery could change that balance. [3] [6]
• Tumour-suppressor status: Mouse lifespan extension with constitutive TERT expression was demonstrated in animals engineered for enhanced cancer resistance, underscoring the importance of checkpoint context. [9]
• Baseline telomere biology: A person with a defined short-telomere disorder presents a different biological problem from a person with average telomeres seeking a general effect on ageing. [1]
• Follow-up period: Failure to detect excess tumours in a limited animal cohort cannot exclude small, delayed, or tissue-specific risks relevant to longer-lived humans. [3]

Evidence Quality and Interpretation

The strongest mechanistic evidence shows that restoring telomerase can rescue cells or animals whose pathology is driven by inadequate telomere maintenance. Evidence for treating ordinary organismal ageing is less direct: positive studies rely on mice, while human-cell experiments measure telomere length and proliferative capacity rather than health outcomes. Species differences in telomere biology further limit simple extrapolation from mice to humans. [2] [3] [4] [10]

Cancer safety cannot be inferred from telomere elongation alone or from a single negative tumour count. A credible evaluation would need to establish where and for how long TERT is expressed, whether clonal cell populations expand, whether genome integrity changes, and whether risk remains acceptable over prolonged follow-up. The appropriate comparison is specific to the disease and delivery platform, not to “telomerase therapy” as one uniform intervention. [3] [5] [6]

What This Does Not Mean

• Telomere shortening is not the sole cause of ageing; it is one interacting mechanism whose importance differs by tissue and disease context. [10]
• Finding telomerase activity in a cancer does not show that telomerase initiated that cancer; it often supports continued proliferation after other oncogenic changes. [5] [6]
• A mouse study reporting no increase in cancer does not prove zero cancer risk in humans. [3]
• Longer average telomeres do not necessarily indicate better health or lower risk across all outcomes. [8]

Summary

Telomerase-based therapy has a coherent regenerative rationale where critically short telomeres limit cell function, and experiments in deficient mice, normally ageing mice, and cultured human cells show that telomere function can be modified. The same capacity to extend cellular proliferation creates a cancer-relevant constraint, especially if exposure reaches premalignant cells or persists without effective tumour-suppressor control. The field's central question is therefore not whether telomerase is simply beneficial or harmful, but whether a defined exposure can produce tissue-specific benefit with an adequately characterized long-term risk. [1] [3] [5]

References

1. Savage, S. A., & Bertuch, A. A. (2010). The genetics and clinical manifestations of telomere biology disorders. Genetics in Medicine. https://pmc.ncbi.nlm.nih.gov/articles/PMC3825100/
2. Jaskelioff, M., et al. (2011). Telomerase reactivation reverses tissue degeneration in aged telomerase-deficient mice. Nature. https://doi.org/10.1038/nature09603
3. Bernardes de Jesus, B., et al. (2012). Telomerase gene therapy in adult and old mice delays aging and increases longevity without increasing cancer. EMBO Molecular Medicine. https://doi.org/10.1002/emmm.201200245
4. Ramunas, J., et al. (2015). Transient delivery of modified mRNA encoding TERT rapidly extends telomeres in human cells. FASEB Journal. https://doi.org/10.1096/fj.14-259531
5. Chang, S., et al. (2003). Telomere-based crisis: functional differences between telomerase activation and ALT in tumor progression. Genes & Development. https://pmc.ncbi.nlm.nih.gov/articles/PMC195968/
6. Stern, J. L., et al. (2015). TERT promoter mutations and monoallelic activation of TERT in cancer. Oncogenesis. https://doi.org/10.1038/oncsis.2015.39
7. Jäger, K., & Walter, M. (2016). Therapeutic targeting of telomerase. Genes. https://doi.org/10.3390/genes7070039
8. Telomeres Mendelian Randomization Collaboration, et al. (2017). Association between telomere length and risk of cancer and non-neoplastic diseases: a Mendelian randomization study. JAMA Oncology. https://doi.org/10.1001/jamaoncol.2016.5945
9. Tomás-Loba, A., et al. (2008). Telomerase reverse transcriptase delays aging in cancer-resistant mice. Cell. https://doi.org/10.1016/j.cell.2008.09.034
10. Jones-Weinert, C., Mainz, L., & Karlseder, J. (2025). Telomere function and regulation from mouse models to human ageing and disease. Nature Reviews Molecular Cell Biology. https://doi.org/10.1038/s41580-024-00800-5