The “Immortality Enzyme”: Human Telomerase Holoenzyme as a potential anti-cancer therapeutic target

By Alice Barocco

Human Telomerase Holoenzyme, most known as Telomerase and often regarded as the “Immortality enzyme”, is a cellular reverse transcriptase comprised of protein subunits and RNA.1 The enzyme can be pictured as a molecular motor: its role is to add new stretches of DNA, more specifically a “TTAGGG” sequence, onto the ends of chromosomes known as telomeres. This telomeric DNA addition counteracts the natural telomere shrinking process in eukaryotic cells which is one of the main factors contributing to the gradual aging of the human body.2

Recently scientists have been able to tap into telomerase’s fascinating potential as an anti-cancer therapeutic target.3 However, before being able to understand telomerase’s potential significance in the cancer biology domain, it is crucial to discuss its relation to telomeres. 

Telomeres are nucleoprotein structures comprised of long tandem repeats of 5’-TTAGGG-3’ nucleotides, a terminal 3’ G-rich overhang and a telomere-binding protein complex termed the shelterin complex.1 The main function of telomeres is to protect chromosomes’ ends against deterioration and loss of important genetic information following each mitotic cycle. This function is mainly attributed to the shelterin complex, which, by forming special T-loop like structures at the end of chromosomes, protects them from end-to-end fusion or degradation.1 Additionally, the shelterin complex prevents the chromosomes ends from being mistaken as single and/or double stranded DNA breaks which would require immediate DNA repair action to avoid serious genomic rearrangements consequences.3-4 

Following each cell division cycle, telomere ends shorten mainly due to the “end replication problem” afflicting all cells.1 This refers to the incapability of DNA polymerase to fully replicate the end of a DNA duplex during mitosis.5 Over time, this eventually leads to critically shortened telomeres and subsequently induces a growth arrest in the affected cells.1-3 This growth arrest state triggers the activation of cellular senescence (i.e., a prolonged cellular quiescent state) which is strongly regarded as an anticancer protection mechanism adopted by long-lived species to prevent the accumulation of multiple oncogenic mutations.3,6 Therefore, with each cell division telomeres do shorten leading to the gradual ageing of cells and consequently of the human body, but also ensure that, once they have reached a critically shortened length, a preventive mechanism against the development of cancer is put in place.

The only available mechanism to counteract this telomere shortening phenomenon is the Human Telomerase Holoenzyme. During human development, its expression is strictly regulated to ensure that an effective balance is established between the proliferative demands of specific cellular functions and the preservation of proliferative barriers against tumorigenesis.6 As a result, in somatic cells its activity is supressed to a minimum, making its levels in tissues undetectable. However, its expression is significantly upregulated in self-renewing cells including germ line cells and embryonic stem cells, but most importantly in 85-90% of all human cancer cells1,7. As a result of the highly expressed telomerase, cancers have unlimited replicative potential and are thus able to acquire immortality and overcome cellular senescence caused by telomere shortening.8 Ergo, telomerase represents an attractive target for highly selective cancer therapeutics. 

In the past, development of such selective cancer therapeutics had proven to be extremely challenging mainly due to the lack of high-resolution imaging of the enzyme.9

However, in 2018, Dr. Kelly Nguyen and her laboratory team at the Medical Research Council Laboratory of Molecular Biology in Cambridge used cryo-electron microscopy to determine the structure of the telomerase enzyme bound to telomeric DNA at a sub-4 Å resolution.10 The visualisation of the enzyme at such a resolution provided pioneering insight into the organisation of its active site.11 To simplify its complex structure, telomerase entails a telomerase reverse transcriptase (TERT) and a telomerase RNA (hTR in humans). These components work together to allow effective telomeric synthesis: the hTR provides the RNA template sequences that is copied into telomeric DNA by the associated TERT.12 

Thanks to the now available detailed structural composition of the enzyme and recent advances in telomerase biology, the development of telomerase-targeted anti-cancer therapeutics has gained popularity once again within the scientific community. Such approaches range from immunotherapies that recognise TERT associated antigens, to telomerase inhibitors that directly bind the enzyme and prevent telomere extension, to more indirect approaches of disrupting telomerase activity such as using G-quadruplex (G4) stabilizers.9

One of the most recent approaches targets TERT gene expression driven by TERT promoter mutations (TPMs).9 TERT expression is regulated by a promoter that houses various binding sites for multiple transcription factor (TF) families including the Erythroblast Transformation Specific (ETS) TF family. In non-cancerous cells, epigenetic modifications, and repressive chromatin remodelling silence TERT gene expression. Conversely,  approximately 25% of cancers can reactivate telomerase via mutations in the TERT promoter that generate de novo binding sites for ETS TFs.9 Genome editing studies via CRISPR have revealed that merely reversing the TPMs in cancer cell lines does not eliminate TERT expression and telomerase activity.9 Thus, since TMPs are suspected to be necessary to sustain cancers’ replicative immortality, researchers have started to target the TFs which bind to these new ETS binding sites as a therapeutic strategy. 

A key theoretical benefit of RPM-based approaches is their ability to discriminate between normal and telomerase positive cells, making this potential therapy extremely precise at targeting and destroying cancerous cells. However, one of the biggest challenges this approach faces is the fact that the dependence of TERT expression on individual ETS TFs following the acquisition of TPMs appears to differ between cancer types.9 For example, in glioblastoma cell lines the ETS transcription factor GABP has been evidenced to directly activate the mutant TERT promoter whilst, in melanoma cells, it is the ETS1 transcription factor which reactivates TERT expression.13-14 This adversity makes the development of a universal approach to a TERT gene targeted anti-cancer therapy extremely challenging. 

As seen from the vast array of collaborative research on telomerase anti-cancer therapeutic approaches, scientists worldwide are now more than ever working together in the hopes of being able to develop telomerase targeted effective anti-cancer therapies. As it is now clear that tumours’ replicative immortality relies on the reactivation of telomerase, the immortality enzyme is without a doubt an essential player that needs to be exploited in the future development of targeted anti-cancer therapeutics. Hopefully, especially after having imaged telomerase at a sub-4 Å resolution, new possibilities for the identification of effective forms of cancer treatments are only a few steps ahead of us. 


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