Epigenetic plasticity of hTERT gene promoter determines retinoid capacity to repress telomerase in maturation-resistant acute promyelocytic leukemia cells

Abstract

The expression of hTERT gene, encoding the catalytic subunit of telomerase, is a feature of most cancer cells. Changes in the chromatin environment of its promoter and binding of transcriptional factors have been reported in differentiating cells when its transcription is repressed. However, it is not clear whether these changes are directly involved in this repression or only linked to differentiation. In a maturation-resistant acute promyelocytic leukemia (APL) cell line (NB4-LR1), we have previously identified a new pathway of retinoid-induced hTERT repression independent of differentiation. Using a variant of this cell line (NB4-LR1 SFD ), which resists to this repression, we show that although distinct patterns of histone modifications and transcription factor binding at the proximal domain of hTERT gene promoter could concur to modulate its expression, this region is not sufficient to the on/off switch of hTERT by retinoids. DNA methylation analysis of the hTERT promoter led to the identification of two distinct functional domains, a proximal one, fully unmethylated in both cell lines, and a distal one, significantly methylated in NB4-LR1 SFD cells, whose methylation was further re-enforced by retinoid treatment. Interestingly, we showed that the binding to this distal domain of a known hTERT repressor, WT1, was defective only in NB4-LR1 SFD cells. We propose that epigenetic modifications targeting this distal region could modulate the binding of hTERT repressors and account either for hTERT reactivation and resistance to retinoid-induced hTERT downregulation.

A preclinical mouse model of glioma with an alternative mechanism of telomere maintenance (ALT)

“…One of these mechanisms could be mediated by WT1, a transcriptional repressor of hTERT. In TG20 cells, we showed that the WT1 binding site, which is present in the distal region of the tert promoter, was unmethylated, a condition required for WT1 repression of htert transcription 32,33…”

Understanding TERT Promoter Mutations: A Common Path to Immortality

“…In contrast, transformation of TERT -expressing stem cells such as hematopoetic stem cells may not require promoter mutation to maintain TERT expression through tumorigenesis. As an alternative to mutation, TERT promoter activation may occur through an epigenetic switch(53). Stern et al 2015 has additionally suggested that TERT promoter mutations can convert the silent TERT promoter into an active chromatin state(54).…”

Telomerase: The Devil Inside

“…In the same line, aberrant hTERT promoter hypermethylation correlates with elevated hTERT gene expression in the majority of non-infant Sonic-Hedgehog subgroup medulloblastoma tumors [46] whereas hTERT promoter methylation was shown to correlate with reduced hTERT gene expression in B-cell lymphocytic leukemia and childhood acute lymphoblastic leukemia (ALL) [47,48]. Alterations in the epigenetic pattern of hTERT promoter, e.g., by point mutations, may also lead to a deregulation of promoter activity [49,50]. In this line, recurrent genomic rearrangements in the chromosomal region at 5p15.33 proximal of the hTERT gene induced massive chromatin remodeling and DNA methylation leading to the transcriptional upregulation of hTERT expression and telomerase activity in high-risk neuroblastomas [51].…”