The ETS Factor ETV5 Activates the Mutant TERT Promoter in Thyroid Cancer.

Abstract

BACKGROUND\nCo-occurrence of TERT 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.\n\n\nMETHODOLOGY\nHere 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 analysed by qRT-PCR, immunoprecipitation (IP), chromatin IP (ChIP) and gene reporter assays.\n\n\nRESULTS\nWe found that ETV5 was abundantly expressed in papillary thyroid cancers (PTCs) from the TCGA dataset, and in thyroid cancer cell-lines TPC1, SW1736 and C643. Furthermore, ETV5 demonstrated specific binding affinity for the -124bp(T) TERTp allele, and stimulated TERT transcription in thyroid cancer cells. In contrast GABPA, which is critical for TERTp activation in several other malignancies, was functionally redundant in our thyroid cancer models. 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.\n\n\nCONCLUSIONS\nETS 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 GABPA.