Telomerase Inhibition as Novel Cancer Therapeutic Method

Presented By : Aayushree Kharel (0323131)Bryan Chong Boon Hooi (0323527)Cynthia Teo Yu Ai (0322550) Hardeep Kaur (0323338)Vincensa Nicko Widjaja (0322042)

Introduction

• Human telomere region consists of repetitive TTAGGG (Shay 2001a).• Telomerase holoenzyme consists of 2 subunits : a reverse

transcriptase (hTERT), the catalytic subunit, and a RNA subunit (hTR) (Tahtouh et al. 2015).

• Works by elongating the 3’ end of a telomere.

• Cancer is a condition in which a cell loses its cell division regulation.

• Many factor contributes to cancer, however in this presentation, its only limited to telomerase activity.

• Tracking to genomic level, it is believed that telomerase expression has been increased to maximum rate. Thus, leading to uncontrollable cell division (Shay 2001a).

Figure 1: Location of telomere region (Dreamstime 2011).

Figure 2: Telomerase function (Leslet and Walker, 2014) shown on upper side ; A functional telomerase structure (Carreno 2015) shown on lower side.

Discussion

• The RNA component of telomerase has been a popular and effective target for inhibiting telomerase activity in cancer cells. In the case of TERT transcript knockdown, antisense oligonucleotides against the human TR template can be employed to reduce or eliminate telomerase activity.

• Result of this approach is that single-stranded templates, such as the TR component, are specifically cleaved. The anticancer utility of this approach has been proven in vitro and in vivo.

• Hammerhead ribozymes and RNAi can be directed to the RNA component of telomerase.

• Effect is immediate growth inhibition of cancer cells both in vitro and in vivo independent of telomere length of the target cancer cell.

• Advantage of this technique is that it greatly reduces the lag period that is often encountered in approaches that are dependent upon the shortening of telomeres to inhibit cancer cell growth.

(Andrews & Tollefsbol 2008)

TR Inhibition as an Anticancer Approach

Discussion

• Approaches to telomerase inhibition have been developed that do not directly inhibit the TERT or TR components of telomerase but rather inhibit target proteins that are associated with telomerase activity.

• Signaling pathways such as those carried out by mitogen-activated protein (MAP) kinase can result in stimulation of the TERT gene

• Inhibition of this pathway could be a novel approach to reducing TERT expression and telomerase activity.

Targeting proteins associated with telomerase activity

(Andrews & Tollefsbol 2008)

Discussion

• Recent studies show that linoleic acid is able to lower the activity of hTERT on the transcriptional level.

• Linoleic acid downregulates the production of as c-Myc and SP-1 which serve as transcription factors for the expression of hTERT.

• Only works on a limited range of cancer cells, which excludes bladder and breast cancer cells (Choi 2014).

Blocking telomerase activity on the transcriptional level

Discussion

• Aside from the gene silencing methods, two chemical compounds have been tested to have a direct effect on the activity of the ribonucleoprotein telomerase. The compounds, β-rubromycin and oleic acid, act by competitively binding to the active site of telomerase, thereby competing with the binding of primers and deoxynucleotide substrates, two components crucial to the synthesis of telomeric DNA in the chromosome (Mizushina et al. 2012).

• Outside of inhibition:Another novel approach to cancer treatment would be using the human TERT as a target in immunotherapy. TERT is found to be expressed by large number of malignant cells, including melanoma cells, and it was found to also be a viable target antigen for CD8+ CTL immune response. Upon antigen detection, CTLs will perform standard protocol of cytokine secretion followed by lysis of target cell. This would pave the way for potential vaccines against certain cancers in the future (Vonderheide 2002).

Chemical regulator perspective

Figure 3: Cellular consequences of telomerase and telosome targeting. Telomerase inhibitors may induce either a delayed anti-proliferative effect through a “slow” pathway mainly based on telomere erosion (BIBR1532, hTR inhibitors) or a quick anti-proliferative effect through a “fast” pathway mainly based on telomere uncapping (hTERT inhibitors). Inhibitors of telosome proteins and G4 ligands mainly induce telomere uncapping. (Folini et al. 2009)

Current Development• Anti-telomerase therapeutics aims to selectively

induce apoptosis and cell death in cancer cells while minimizing the effects on normal cells (Buseman et al. 2012).

• Vaccines, antisense oligonucleotides, and small-molecule inhibitors targeting hTERT has been developed (Jafri et al. 2016).

• Oligonucleotide imetelstat is the only anti-telomerase compound that has been extensively evaluated in clinical trials (Jafri et al. 2016).

• A novel telomerase inhibitor, BIBR1532, has been reported but this compound has not yet progressed to clinical trial (Jafri et al. 2016).

Figure 4: Structure (a) and action of imetelstat (b). Jafri et al. 2016

Challenges Treatment needs to be coordinated in conjunction with other treatments like

chemotherapeutics and radiation therapy (Cunningham et al. 2006).

Intestinal crypt, hematopoietic stem cells and cells lining the endometrium express telomerase (Cunningham et al. 2006). Hence, these cells should be less affected than tumor cells and telomere shortening should not occur in absence of cell division (Shay et al. 2001).

Telomerase inhibition should not affect the rate of cell growth until progressive telomere shortening (Shay et al. 2001b).

The response for telomerase inhibition varies. These varying results are due to the extra-telomeric functions of telomerase which includes cell protection, anti-apoptotic function and support of the tumorigenic phenotype (Chung et al. 2005).

Challenges Some cells might undergo telomerase independent mechanism- alternative

lengthening of telomeres (ALT) pathway (Henson et al. 2002). Inhibiting the telomerase activity may promote the development of ALT pathway.  

TERT are intracellular components, and are dependent on MHCs (major histocompatibility complexes) particularly MHC1 to be presented on the cell surface for CTL detection. Some cancer cells are shown to possess the ability to downregulate MHC1 in efforts to avoid detection (Wang et al. 2007).

ConclusionTelomerase inhibition is one potential treatment for cancer

This might be done either : • TERT inhibition by complement oligonucleotides• Inhibition of telomerase-associated proteins (MAP kinase)• Further until transcriptional level (linoleic acid)

Currently developed : Imetelstat

Challenges to overcome : • Involvement of other treatments (financial problem).• Increase the specificity of telomerase inhibitors until transformed cell level.• Ability to produce one result – the telomerase inhibition itself. (not to

produce other effects)

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