Interplay between TERT promoter mutations and methylation culminates in chromatin accessibility and TERT expression

Abstract

The telomerase reverse transcriptase (TERT) gene is responsible for telomere maintenance in germline and stem cells, and is re-expressed in 90% of human cancers. CpG methylation in the TERT promoter (TERTp) was correlated with TERT mRNA expression. Furthermore, two hotspot mutations in TERTp, dubbed C228T and C250T, have been revealed to facilitate binding of transcription factor ETS/TCF and subsequent TERT expression. This study aimed to elucidate the combined contribution of epigenetic (promoter methylation and chromatin accessibility) and genetic (promoter mutations) mechanisms in regulating TERT gene expression in healthy skin samples and in melanoma cell lines (n = 61). We unexpectedly observed that the methylation of TERTp was as high in a subset of healthy skin cells, mainly keratinocytes, as in cutaneous melanoma cell lines. In spite of the high promoter methylation fraction in wild-type (WT) samples, TERT mRNA was only expressed in the melanoma cell lines with either high methylation or intermediate methylation in combination with TERT mutations. TERTp methylation was positively correlated with chromatin accessibility and TERT mRNA expression in 8 melanoma cell lines. Cooperation between epigenetic and genetic mechanisms were best observed in heterozygous mutant cell lines as chromosome accessibility preferentially concerned the mutant allele. Combined, these results suggest a complex model in which TERT expression requires either a widely open chromatin state in TERTp-WT samples due to high methylation throughout the promoter or a combination of moderate methylation fraction/chromatin accessibility in the presence of the C228T or C250T mutations.

TERT—Regulation and Roles in Cancer Formation

“…Interestingly, one study showed that TERTp hypermethylation was present in both melanoma and normal skin cells. However, only in melanoma cells with TERTp mutation did this hypermethylation correspond to increased TERT expression and chromatin accessibility ( 77 ). A further study by McKelvey et al.…”

Telomerase Reverse Transcriptase (TERT) Regulation in Thyroid Cancer: A Review

“…The histone mark H3K27me3 maintains heterochromatin and recruitment of the polycomb repressor complex (PRC) for silenced genes ( 43 ). Alternatively, the H3K4me3 mark maintains actively transcribed genes in the open confirmation to enable access for transcriptional activators to bind ( 44 ). Telomerase negative primary cells exhibit high levels of the silencing H3K27me3 mark at the TERT promoter, compared to telomerase positive cancer cell lines ( 45 ), which exhibit the activating H3K4me3 mark ( 46 ).…”

MiR-138-5p Suppresses Cell Growth and Migration in Melanoma by Targeting Telomerase Reverse Transcriptase

“…In addition, we found that hTERT mRNA and protein expressions were increased in pTERTm melanoma cells, compared with nonmutated ones. However, several studies have demonstrated no correlation of hTERT protein expression with mutation status in tumoral tissues, thus suggesting that hTERT protein expression may be regulated by other mechanisms in addition to its promoter mutation [ 9 , 12 , 13 , 14 , 15 , 43 ]. The discrepancy in hTERT expression could be explained by the simplicity of the model used.…”