GENO-07A MECHANISM OF MUTANT TERT PROMOTER ACTIVATION SHARED ACROSS CANCER TYPES

Abstract

The human telomerase is an enzyme critical for maintaining telomere length and chromosomal stability in stem cells. The transcriptional regulation of the telomerase reverse transcriptase (TERT) gene, encoding the catalytic subunit of telomerase, is a rate-limiting step in modulating telomerase activity. Although normally silenced in somatic cells, TERT is aberrantly expressed in 90% of aggressive cancers, highlighting this event as a hallmark of tumorigenesis. Reactivating telomerase helps cells with finite lifespan to achieve limitless proliferative potential and bypass cellular senescence induced by DNA replication-associated telomere shortening. Recently discovered non-coding mutations in the TERT promoter are among the most common genetic alterations observed across multiple cancer types. Specifically, one of two positions, G228A or G250A, is mutated in 20% of medulloblastomas, 44% of hepatocellular carcinomas (HCC), 66% of urothelial carcinomas of the bladder, 71% of melanomas, 78% of oligodendrogliomas, and 83% of primary glioblastomas (GBM). The common mutation sites may create de-novo ETS family transcription factor binding sites, but the precise mechanism of how these mutations confer increased TERT expression has remained elusive. Here, a focused siRNA screen of the many ETS transcription factors expressed in GBM identifies GABPA as the single ETS factor to selectively regulate the mutant but not the wild type TERT promoter. Single molecule binding assays and ChIP-qPCR analysis reveal that GABPA is exclusively recruited to the mutant allele in vitro and in vivo respectively. Furthermore, this allelic recruitment is consistent across four tested cancer types, highlighting a shared mechanism underlying mutant TERT promoter activation. Tandem flanking native ETS motifs critically cooperate with these mutations to activate TERT, likely by facilitating GABP heterotetramer binding. GABP thus directly links TERT promoter mutations to aberrant expression across multiple cancers.