ETS Factor ETV5 Activates the Mutant Telomerase Reverse Transcriptase Promoter in Thyroid Cancer

Background: Co-occurrence of TERT (telomerase reverse transcriptase) promoter (TERTp) mutations with BRAF/RAS mutations is associated with significantly more aggressive thyroid cancer. TERTp mutations are hypothesized to generate de novo binding sites for ETS transcription factors, which are themselves activated by BRAF/RAS-stimulated MEK-ERK activity. To date, a detailed study of this mechanism has been limited to only a few cancer types, and we hypothesized that ETS factors involved in TERTp activation could vary between different cancers. Methodology: Here we sought to identify ETS factor(s) required for TERTp activation in thyroid cancer, using a combination of in silico analyses of TCGA data, and experimentation using in vitro thyroid cell models analyzed by quantitative reverse transcription-PCR, immunoprecipitation (IP), chromatin IP, and gene reporter assays. Results: We found that ETV5 was abundantly expressed in papillary thyroid cancers from the TCGA data set, and in thyroid cancer cell line models. Furthermore, ETV5 was found to preferentially bind to the -124 bp(T) TERTp allele and stimulate TERT transcription in thyroid cancer cells devoid of GA binding protein transcription factor (GABP) activity. We also found that ETV5 functionally cooperates with the transcription factor FOXE1 to further enhance TERTp activity, a mechanism that may at least partially explain why FOXE1 represents a significant genetic determinant of thyroid cancer risk. Conclusions: ETS factors that activate mutant TERTp vary between cancer types, and here we show for the first time that ETV5 demonstrates mutant allele-specific affinity for TERTp in thyroid cancer, a property that has previously only been attributable to GABP.