Genome-wide screens identify specific drivers of mutant <i>hTERT</i> promoters.

Abstract

Cancer-specific <i>hTERT</i> promoter mutations reported in 19% of cancers result in enhanced telomerase activity. Understanding the distinctions between transcriptional regulation of wild-type (WT) and mutant (Mut) <i>hTERT</i> promoters may open up avenues for development of inhibitors which specially block <i>hTERT</i> expression in cancer cells. To comprehensively identify physiological regulators of WT- or Mut-<i>hTERT</i> promoters, we generated several isogenic reporter cells driven by endogenous <i>hTERT</i> loci. Genome-wide CRISPR-Cas9 and small interfering RNA screens using these isogenic reporter lines identified specific regulators of Mut-<i>hTERT</i> promoters. We validate and characterize one of these hits, namely, MED12, a kinase subunit of mediator complex. We demonstrate that MED12 specifically drives expression of <i>hTERT</i> from the Mut-<i>hTERT</i> promoter by mediating long-range chromatin interaction between the proximal Mut-<i>hTERT</i> promoter and <i>T-INT1</i> distal regulatory region 260 kb upstream. Several hits identified in our screens could serve as potential therapeutic targets, inhibition of which may specifically block Mut-<i>hTERT</i> promoter driven telomerase reactivation in cancers.