Investigations of the Telomerase Template Antagonist GRN163L and Implications for Augmenting Breast Cancer Therapy

Abstract

Breast cancer is the second most common cancer among women in the US after skin cancer. While early detection and improved therapy has led to an overall decline in breast cancer mortality, metastatic disease remains largely incurable, indicating a need for improved therapeutic options for patients. Telomeres are repetitive (TTAGGG)n DNA sequences found at the end of chromosomes that protect the ends from recombination, end to end fusions, and recognition as damaged DNA. The enzyme telomerase acts to stabilize short telomeres, preventing apoptosis or senescence due to genomic instability. Telomerase is active in 85-90% of cancers, and inactive in most normal cells, making telomerase an attractive target for cancer therapy. Use of the telomerase-specific, lipidated oligonucleotide GRN163L can antagonize telomerase activity and telomere maintenance in cancer cells by preventing telomerase from binding to telomeres. GRN163L has been shown by our laboratory to inhibit breast cancer cell growth and metastasis in animal models. However, the mechanisms of cancer cell growth and metastatic inhibition via GRN163L are not completely understood. The overall goal of this research project was to further elucidate the role of telomerase in breast cancer cell survival by: 1) determining the effects of combining telomere dysfunction induced by GRN163L with a DNA damage inducer (irradiation); 2) elucidating the mechanisms underlying the cellular response to GRN163L and the effect of combination therapy with the mitotic inhibitor paclitaxel; and 3) testing the hypothesis that a telomerase inhibitor can augment the effects of trastuzumab in breast cancer cells with HER2 amplification. Results support the central hypothesis that the telomere dysfunction, structural and proliferative changes in breast cancer cells induced by GRN163L can synergize with irradiation, paclitaxel, and trastuzumab to inhibit the tumorigenicity of breast cancer cells both in vitro and in vivo. Furthermore, GRN163L can restore sensitivity of therapeutically resistant breast cancer cells to trastuzumab. These results provide insight into the role of telomerase in cancer cell growth. Additionally, implications of this research support GRN163L as an important part of therapeutic regimens for primary tumors, recurrence, and metastatic disease.