TERT upstream promoter methylation regulates TERT expression and acts as a therapeutic target in TERT promoter mutation-negative thyroid cancer.

[BACKGROUND] DNA hypermethylation and hotspot mutations were frequently observed in the upstream and core promoter of telomerase reverse transcriptase (TERT), respectively, and they were associated with increased TERT expression and adverse clinical outcomes in thyroid cancer. In TERT promoter mutant cancer cells, the hypomethylated TERT mutant allele was active and the hypermethylated TERT wild-type allele was silenced. However, whether and how the upstream promoter methylation regulates TERT expression in TERT mutation-negative cells were largely unknown.

[METHODS] DNA demethylating agents 5-azacytidine and decitabine and a genomic locus-specific demethylation system based on dCas9-TET1 were used to assess the effects of TERT upstream promoter methylation on TERT expression, cell growth and apoptosis of thyroid cancer cells. Regulatory proteins binding to TERT promoter were identified by CRISPR affinity purification in situ of regulatory elements (CAPTURE) combined with mass spectrometry. The enrichments of selected regulatory proteins and histone modifications were evaluated by chromatin immunoprecipitation.

[RESULTS] The level of DNA methylation at TERT upstream promoter and expression of TERT were significantly decreased after treatment with 5-azacytidine or decitabine in TERT promoter wild-type thyroid cancer cells. Genomic locus-specific demethylation of TERT upstream promoter induced TERT downregulation, along with cell apoptosis and growth inhibition. Consistently, demethylating agents sharply inhibited the growth of thyroid cancer cells harboring hypermethylated TERT but had little effect on cells with TERT hypomethylation. Moreover, we identified that the chromatin remodeling protein CHD4 binds to methylated TERT upstream promoter and promotes its transcription by suppressing the enrichment of H3K9me3 and H3K27me3 at TERT promoter.

[CONCLUSIONS] This study uncovered the mechanism of promoter methylation mediated TERT activation in TERT promoter mutation-negative thyroid cancer cells and indicated TERT upstream promoter methylation as a therapeutic target for thyroid cancer.