Browsing by Subject "TERT"

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Open Access
Investigating the mechanisms of telomere maintenance in zebrafish tissues and human brain cancer cell lines
(2019-09) Şerifoğlu, Naz
Telomeres are nucleoprotein complexes formed at each end of the chromosomes to protect these ends from deterioration. In each round of cellular division, telomeric sequences shorten due to the end replication problem of DNA polymerase. Progressive telomere shortening results in replicative senescence in healthy somatic cells. To evade replicative senescence, cells need to maintain their telomere length either by activating the telomerase enzyme or through the alternative lengthening of telomeres (ALT). Telomerase is a holoenzyme, which is composed of dyskerin, telomerase RNA subunit (TR or TERC), and telomerase catalytic subunit (TERT). Dyskerin and TR are constitutively expressed in all cells but TERT expression is silenced in adult somatic cells. Thus, telomerase activity is dependent on the expression of TERT. Current studies show that TERT re-activation is a common feature of cancer cells and 85-90% of cancers utilize telomerase enzyme in maintaining telomeres to become immortal. Remaining of cancer cells maintain their telomeres by the alternative lengthening of telomeres (ALT), which is a DNA repair pathway dependent mechanism. Current models suggest that ALT is achieved by homology-directed DNA repair, through the interaction of multiple proteins. DNA methylation is regarded as a key player in epigenetic silencing of transcription. DNA methyltransferase inhibitors are currently being used in cancer treatments. Recent studies show that DNA methyltransferases and their expression levels impact both telomerase- and ALT mediated lengthening of telomeres, and have different outcomes in different tissue types. In this study, we worked on the zebrafish brain and human brain cancer cell lines. In zebrafish brain, we observed differences in methylated regions at Sp1 binding site between young and old that can be associated with telomere shortening. By silencing DNMT1 and DNMT3B in brain cancer cell lines, we investigated the changes in gene expression levels of telomerase and ALT related genes, telomerase activity, population doubling time and replicative senescence status. To further investigate TERT regulation, we introduced mutations to the Sp1 binding sites in the promoter region and measured the promoter activity with luciferase assay. Our results show that Sp1 methylation sites in the telomerase promoter region are critical in brain aging, dependent on their position. We propose a therapeutical option for brain aging and tumorigenesis.
Open Access
Reactivation of telomerase reverse transcriptase gene in liver cancer
(2014-09) Çevik, Dilek
Hepatocellular Carcinoma (HCC) is one of the major causes of cancer related deaths worldwide and its incidence has been increasing drastically, especially in western countries. HCC has a heterogeneous molecular and pathological background with various underlying risk factors and survival rate of HCC patients is very low due to late diagnosis and limited curative therapies. The mechanisms involved in hepatocellular immortality gains critical importance in order to develop preventive and therapeutic options against HCC. Telomerase reactivation is a keystone for HCC cells during transformation process. TERT promoter mutations activating its promoter by creating a novel activating motif were recently identified in different cancer types. In this study; we determined TERT promoter mutation frequency in HCC cell lines and tumors which are 67% (10/15) and 34% (15/44) respectively. High frequency of TERT promoter mutations in HCC indicated a possible functional role during hepatocarcinogenesis. We performed transcriptional factor search to find a candidate TF that could bind to mutant TERT promoter and STAT1 came out of that search. To study the role of STAT1 during reactivation of TERT expression, we activated STAT1 signaling by Interferon alpha (IFN-α) treatment and down regulated STAT1 with RNA interference in several HCC cell lines. We have found that IFN-α was able to upregulate TERT expression in the HCC cell lines carrying a TERT promoter mutation and STAT1 knockdown was enough to eradicate this upregulation. In case of wild type cell lines, IFN-α treatment and STAT1 knock down had no effect on TERT expression. Our data delineates the contributions of TERT promoter mutations to hepatocellular immortality and gives insights into the potential use of TERT as a target for chemoprevention of hepatocarcinogenesis.