## 4 Discussion  
In this study, we comprehensively assessed the ability of the ETS family of transcription factors to control _TERT_ transcription in thyroid cancer cells with and without TPMs. We also evaluated the _TERT_ promoter genotype-dependent role of MAPK signaling in _TERT_ expression in these same specimens. Our findings suggest a multi-layered pattern of regulation, likely more complex than that described in glioblastoma cells carrying TPMs. In our cell systems, multiple ETS factors regulated _TERT_ expression in a manner that is not exclusive of the presence of TPMs. We also observed that TPMs likely render thyroid cancer cells less dependent to MAPK-mediated control of _TERT_ transcription, whereas pharmacological inhibition of MAPK signaling effectively suppressed _TERT_ mRNA levels in the absence of TPMs. The discovery of hotspot mutations in the _TERT_ core promoter, first in metastatic melanomas and subsequently in aggressive tumors of multiple lineages, revitalized the interest in understanding the mechanisms of telomerase reactivation in cancer. The fact that the two non-overlapping c.-124C\(>\)T and c.-146C\(>\)T mutations create de novo consensus binding sites for the ETS family of transcription factors made them excellent candidates to be controlling TPM-mediated TERT re-expression (2-4). The former prompted to the search for which ETS factors bind mutant _TERT_ promoter as a first step towards exploring the idea of targeting _TERT_ mutant transcription. Several studies, typically using siRNA silencing, reporter assays, and ChIP approaches, on glioblastoma cells carrying TPMs demonstrated that the ETS factor GABPA controls _TERT_ mutant promoter expression. [PERSON] and colleagues sequentially knocked-down thirteen ETS transcription factors in two glioblastoma lines and showed that GABPA selectively reduces _TERT_ transcription in the presence of the c.-124C \(>\) T mutation. They extended their observations to three other TPM-carrying tumor types: melanomas, hepatocellular, and urothelial carcinomas [13]. This same group elegantly showed that GABPA acts as a tetramer with their beta-isoform (GABPB) and that GABPB1L disruption reverses replicative immortality in glioblastoma cells with TPMs [15]. Interestingly, another group showed that glioblastoma cells carrying the alternative hotspot mutation (_TERT_ c.-146C\(>\)T) are bound by ETS factors ETS1/2, which cooperate with non-canonical NF-kB signaling via p52 to reactivate mutant _TERT_[26]. They subsequently showed that GABPA is also able to upregulate mutant _TERT_ in glioblastoma and melanoma cells, but it does so, at least partially, via long-range chromatin interactions [14]. Overall, GABPA seems a solid candidate to explore TPM-specific epigenetic blockade of _TERT_ re-expression in glioblastoma, but its role in thyroid cancers remains uncertain. In this regard, the capacity of GABPA to bind to mutant _TERT_ promoters in thyroid cancer was reported through ChIP assays on the K1 papillary thyroid carcinoma cell line [16]. The authors suggested that FOS cooperates with GABPA in _TERT_ mutant control. Our results show that GABPA stable silencing does not universally reduce TERT transcription in _TERT_-mutant thyroid cancer cells and that it also controls _TERT_ wildtype promoter (e.g., in Cal62 cells). In addition, GABP seems an unattractive target for inhibition in thyroid cancers due to its recently reported tumor suppressor role [19]. Our study shows that multiple ETS factors are able to control _TERT_ mutant transcription, suggesting that no clear (GABPA-like) candidate exists in this lineage and that compensatory mechanisms across ETS proteins likely operate. Our work confirms some of the observations and further expands the findings reported by other thyroid cancer groups. Of note, [PERSON] and colleagues nicely showed that both ELK1 and ETV5 are able to bind _TERT_ promoter in discrete thyroid cell lines and that they do so in cooperation with thyroid transcription factor FOXE1 [17; 18].