Telomerase reverse transcriptase (TERT) : : promoter mutation and novel function in human cancer

Abstract: Telomerase reverse transcriptase (TERT) is the catalytic component of telomerase, an RNAdependent DNA polymerase responsible for telomere elongation. TERT is transcriptionally repressed in most normal human somatic cells with limited life-span, which contributes to telomerase silence. It is well established that maintenance of telomere length is essential to cellular immortalization and malignant transformation, and predominantly achieved through TERT induction and subsequent telomerase activation. Evidence has also accumulated that, apart from its canonical telomerelengthening function, telomerase or TERT exhibits many other biological activities, thereby promoting cancer progression. Given its critical role in cancer formation and differential expression between normal and malignant cells, TERT/telomerase has been suggested as a novel cancer diagnostic marker and therapeutic target. To rationally develop these telomerase-based clinical applications, it is a demanding task to gain profound insights into the regulatory mechanism underlying cancer-specific TERT expression or telomerase activation, and to delineate various roles of TERT/telomerase in cancer development/progression. My thesis thus focuses on these important issues. Specifically, I sought to determine: (i) the biological role and clinical implication of the somatic TERT promoter mutation, and (ii) the regulatory effect of TERT on the oncogenic factor cyclooxygenase2 (COX2) in human cancer. The somatic TERT promoter mutation, initially identified in malignant melanoma, stimulates TERT gene transcription and activates telomerase. To see whether this genetic event occurs in the pathogenesis of thyroid and gastric cancer, we sequenced the TERT promoter region in tumor specimens derived from patients with premalignant thyroid lesions, thyroid and gastric cancer (papers I, II and III). A moderate to high frequency of TERT promoter mutations was observed in follicular cell-derived but not in parathyroid C cell-derived thyroid cancer. The mutation was correlated with senior age, shorter telomere length, aggressive disease and shorter disease-specific survival in thyroid cancer. Importantly, we found the mutation in a fraction (up to 17%) of premalignant thyroid lesions, coupled with TERT and telomerase expression, which indicates that TERT promoter mutations are early genetic events in thyroid carcinogenesis. In contrast, none of tumors from 200 patients with gastric cancer harbored TERT promoter mutations, suggesting different mechanisms for telomerase activation in gastric cancer. One of non-canonical TERT functions is to serve as a transcription co-factor to regulate gene expression. We thus determined whether the manipulating TERT expression affected the abundance of the oncogenic factor COX2 in cancer cells (paper IV). The inhibition of TERT in cancer cell lines using RNA interference led to a robust increase in COX2 expression at both mRNA and protein levels through a p38 MAPK signal pathway. TERT depletion alone did not affect cell survival, while the COX2 inhibitor together with TERT inhibition synergistically induced apoptosis of cancer cells. TERT depletion combined with the COX2 inhibitor administration significantly inhibited tumor growth in the mouse xenograft model of human gastric cancer. These findings reveal a novel function of TERT and have implications in telomerase-based anti-cancer therapy. In conclusion, the TERT promoter mutation is an early genetic event activating telomerase in the thyroid carcinogenesis, and associated with the aggressive disease and serves as a useful prognostic factor in thyroid cancer. The COX2 induction by TERT inhibition likely protects cancer cells from apoptosis. Thus simultaneously targeting TERT and COX2 may have synergistic killing effects on cancer cells, thereby improving treatment efficacy in a telomerase-based anti-cancer strategy.