In perspective: An update on telomere targeting in cancer

Abstract

Engaging a telomere maintenance mechanism during DNA replication is essential for almost all advanced cancers. The conversion from normal and premalignant somatic cells to advanced malignant cells often results (85%-90%) from the reactivation of the functional ribonucleoprotein holoenzyme complex, referred to as telomerase. Modulation of the human telomerase reverse transcriptase (hTERT) appears to be rate limiting to produce functional telomerase and engage a telomere maintenance mechanism. The remaining 10% to 15% of cancers overcome progressively shortened telomeres by activating an alternative lengthening of telomeres (ALT) maintenance mechanism, through a DNA recombination pathway. Exploration into the specific mechanisms of telomere maintenance in cancer have led to the development of drugs such as Imetelstat (GRN163L), BIBR1532, 6-thio-dG, VE-822, and NVP-BEZ235 being investigated as therapeutic approaches for treating telomerase and ALT tumors. The successful use of 6-thio-dG (a nucleoside preferentially recognized by telomerase) that targets and uncaps telomeres in telomerase positive but not normal telomerase silent cells has recently shown impressive effects on multiple types of cancer. For example, 6-thio-dG overcomes therapy-resistant cancers in a fast-acting mechanism potentially providing an alternative or additional route of treatment for patients with cancer. In this perspective, we provide a synopsis of the current landscape of telomeres and telomerase processing in cancer development and how this new knowledge may improve outcomes for patients with cancer.

Keywords: 6-thio-dG; ALT; BRAF; hTERC; hTERT; melanoma; telomerase.

Figure 1.. Visualization of human telomeres on metaphase chromosomes using digital fluorescence microscopy.

Human cells were treated with colcemid to arrest cells in mitosis and chromosome spreads were made. Samples were prepared for quantitative fluorescence in situ hybridization (Q-FISH) microscopy using labeled peptide nucleic acid probes specific for (TTAGGG)n telomere sequences (red color) and the general DNA dye DAPI (blue color). Fluorescent images were acquired on a digital imaging microscope system to calculate the fluorescence intensity for each telomere. The telomere length is proportional to the number of hybridized probes.

Figure 2.. Description of the linear-to-branched evolutionary process of melanomas.

Melanocytes (blue) with MAPK pathway activation, due to mutations such as BRAFV600E or NRAS, proliferate and one of three things can occur. First, cells experience oncogene-induced premature replicative senescence (red) due to overexpression of the BRAF or NRAS oncogene. Alternatively, cells can engage a DNA recombination mechanism, termed alternative lengthening of telomeres (ALT). These cells continue dividing (green) until telomere based replicative senescence is engaged and then bypassed. Cells then enter a period called crisis. Only a rare cell can emerge from this crisis state, and ALT cells are characterized by having both long and short telomeres, ALT associated PML bodies and extra chromosomal telomere repeats (as identified by the C-circle assay). The final scenario, cells can either spontaneously upregulate telomerase or accumulate telomerase promoter mutations, allowing replication, and partially extending the proliferative life span of the cells until they reach crisis where genomic instability is increased. Then, in combination with other alterations, cells upregulate telomerase further to maintain short telomeres, but in some cases, telomeres may become longer. Once cells progress past this vital barrier and short telomere lengths are maintained, genomic stability is also maintained. However, the immortalized cells have extended time to increase the mutational load, with CDKN2A and PTEN alterations being highly prevalent, branching the evolutionary pathway even further to metastatic disease.