Targeting homologous recombination and telomerase in Barrett’s adenocarcinoma: impact on telomere maintenance, genomic instability and tumor growth

Abstract

Homologous recombination (HR), a mechanism to accurately repair DNA in normal cells, is deregulated in cancer. Elevated/deregulated HR is implicated in genomic instability and telomere maintenance, which are critical lifelines of cancer cells. We have previously shown that HR activity is elevated and significantly contributes to genomic instability in BAC. The purpose of this study was to evaluate therapeutic potential of HR inhibition, alone and in combination with telomerase inhibition, in BAC. We demonstrate that telomerase inhibition in BAC cells increases HR activity, RAD51 expression, and association of RAD51 to telomeres. Suppression of HR leads to shorter telomeres as well as markedly reduced genomic instability in BAC cells over time. Combination of HR suppression (whether transgenic or chemical) with telomerase inhibition, causes a significant increase in telomere attrition and apoptotic death in all BAC cell lines tested, relative to either treatment alone. A subset of treated cells also stain positive for β-galactosidase, indicating senescence. The combined treatment is also associated with decline in S-phase and a strong G2/M arrest, indicating massive telomere attrition. In a subcutaneous tumor model, the combined treatment resulted in the smallest tumors, which were even smaller (P=0.001) than those resulted from either treatment alone. Even the tumors removed from these mice had significantly reduced telomeres and evidence of apoptosis. We therefore conclude that although telomeres are elongated by telomerase, elevated RAD51/HR assist in their maintenance/stabilization in BAC cells. Telomerase inhibitor prevents telomere elongation but induces RAD51/HR, which contribute to telomere maintenance/stabilization and prevention of apoptosis, reducing the efficacy of treatment. Combining HR inhibition with telomerase, makes telomeres more vulnerable to degradation and significantly increases/expedites their attrition, leading to apoptosis. We therefore demonstrate that a therapy, targeting HR and telomerase, has potential to prevent both the tumor growth and genomic evolution in BAC.

Telomere biology and translational research

“…If inhibiting telomerase were to select for cells that used the ALT mechanism of telomere maintenance then these cells would be able to proliferate despite telomerase inhibition. This might be a particular problem in certain cancers, like the esophageal cancer discussed in the article in this volume by Pal et al, (78) (79) where both telomerase and homologous recombination based methods of telomere maintenance seem to operate in the same cells. Another problem with manipulating telomeres is the fact that while longer telomeres seem to promote healthy aging telomerase activity promotes tumor growth, and short telomeres, while inhibiting the growth of tumors, can also promote the early steps in tumor formation.…”

Keeping those telomeres short! an innovative intratumoral long-term drug delivery system

“…The ability of the drug to inhibit telomere function was quantified, as described in (Lu et al 2014), by studying the influence of the inhibitor on cell growth under in vitro conditions. For this purpose, the DDS implants were placed in RPMI complete medium and used to release and maintain the continuous exposure of PdTMPyP4 to exponentially growing L428 Hodgkin's lymphoma cells over an 11-and 19-day period.…”

“…In addition to immortalization, telomerase increases the resistance of cancer cells to chemo-or radiotherapy; therefore, confronting this problem requires intensive exploration into methods to prevent activation of this enzyme. The two telomere elongation mechanisms identified thus far are the activation of the telomerase enzyme and a recombination-based alternative lengthening of telomeres (ALT) pathway (Hu et al 2012;Lu et al 2014). Thus, successful cancer therapy necessitates counteracting the cancer cells' own survival strategies, e.g., telomere extension, to increase the cancer cells' sensitivity to conventional treatments and/or its activation of death modes (Wesbuer et al 2010).…”

Emerging therapeutic targets in esophageal adenocarcinoma

“…However, due to the disproportional ratios of telomerase present in normal versus tumor cells, telomerase-targeted therapy can be an effective anticancer therapy while sparing normal tissue [59, 61]. Recently, inhibition of telomerase, and therefore shortening of telomeres, has been considered as a mechanism to target tumor growth in Barrett's induced EAC [62, 63]. …”