Regulation of Monoallelic TERT Expression in Cancer Cells with Wildtype Promoters

Abstract

During cell division, chromosomal telomere shortening that occurs due to the “end‐replication problem” can be compensated for by telomerase, a ribonucleoprotein enzyme that lengthens telomeres. Telomerase is active in stem cells, enabling maintenance of the telomeres, while it is normally inactive in somatic cells, leading to telomeres progressively shortening until a critical length triggers cell senescence or apoptosis. Importantly, telomerase is pathogenically reactivated in approximately 80–90% of human cancers. The expression of telomerase reverse transcriptase (TERT), the catalytic subunit of telomerase, specifically appears to be crucial to telomerase's tightly regulated activities; the Cech lab and others have shown that an absence of TERT expression is what prevents telomerase activity in human cells. While TERT expression levels are tightly regulated, with only 10–20% decrease in telomerase activity thought to be possibly deleterious for normal stem cell function, it is unclear how TERT expression becomes reactivated in many cancer types. Of particular interest is monoallelic expression (MAE) of TERT, which has been found in the majority of cancerous tissues. While some MAE cancer lines contain activating promoter mutations, other MAE cancer lines have no detectable mutations, raising the intriguing question of why TERT is reactivated in a monoallelic fashion in these wildtype (WT) cancers. To investigate the cause(s) of MAE expression in these WT cancer lines, we are investigating the allelic DNA CpG methylation pattern using bisulfite conversion cloning, histone modification markers using chromatin immunoprecipitation (ChIP), and transcription factor (TF) binding via ChIP, all within the TERT promoter region. Lines being investigated include multiple myeloma, pancreatic, lung, and glioblastoma cancer lines that have a single‐nucleotide polymorphism (SNP) in TERT exon 2 to enable identification of the active allele via reverse transcription PCR. While CpG hypermethylation is typically associated with silent gene expression, the TERT promoter paradoxically contains an apparently hypermethylated promoter in cancers actively expressing TERT. Our group recently found that in MAE cancers with TERT promoter mutations, the active allele is in fact hypomethylated (Stern et al., in press*), raising the question of whether this is similar in the WT TERT MAE lines. We have found that, in at least some of the WT MAE lines investigated, DNA CpG methylation patterns appear to be allelic, with one allele possessing hypermethylation and the other hypomethylation. We are currently investigating how this correlates with allelic expression, histone modification, and TF binding. Improved understanding of how TERT is reactivated in WT MAE cancers could help guide future drug discovery and cancer therapeutic efforts.Support or Funding InformationThis work was funded by National Institutes of Health grant R01 GM099705 to T.R.C.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.