Characterization of human telomerase reverse transcriptase promoter methylation and transcription factor binding in differentiated thyroid cancer cell lines

Telomerase reverse transcriptase (TERT) activation plays an important role in cancer development by enabling the immortalization of cells. TERT regulation is multifaceted, and its promoter methylation has been implicated in controlling expression through alteration in transcription factor binding. We have characterized TERT promoter methylation, transcription factor binding, and TERT expression levels in five differentiated thyroid cancer (DTC) cell lines and six normal thyroid tissue samples by targeted bisulfite sequencing, ChIP-qPCR, and qRT-PCR. DTC cell lines express varying levels of TERT and exhibit TERT promoter methylation patterns similar to patterns seen in other telomerase positive cancer cell lines. The minimal promoter immediately surrounding the transcription start site is hypomethylated, while further upstream portions show dense methylation. In contrast, the TERT promoter in normal thyroid tissue is largely unmethylated throughout and expresses TERT minimally. Transcription factor binding is also affected by TERT mutation status. The E-twenty-six (ETS) factor GABPA exhibits TERT binding in the TERT mutant DTC cells only, and allele-specific methylation patterns at the minimal promoter were observed as well, which may indicate allele-specific factor recruitment at the minimal promoter. Furthermore, we identified binding sites for activators MYC and GSC in the hypermethylated upstream region, pointing to its possible importance in TERT regulation. Overall, TERT expression and telomerase activity depend on the interplay of multiple regulatory mechanisms including TERT promoter methylation, mutation status, and recruitment of transcription factors. This work explores of the interplay between these regulatory mechanisms and offers insight into cellular control of active telomerase in human cancer.