Characterization of an Ultra-Conserved Putative cis-Regulatory Module at the Mammalian Telomerase Reverse Transcriptase Gene

Telomeres are regions of repeated DNA sequence that cap the ends of eukaryotic chromosomes. They act as disposable safeguards to prevent the loss of important genetic information during DNA replication due to the inability of DNA polymerase to replicate DNA to the ends of linear chromosomes. The synthesis of new telomeric repeats using an RNA molecule as a template is catalyzed by the enzyme telomerase. In embryonic stem cells, the gene encoding the catalytic protein subunit of the telomerase complex (telomere reverse transcriptase [TERT]) is transcriptionally active and critical for telomere elongation, allowing for continued cellular differentiation during development. The TERT gene is down-regulated as embryogenesis progresses to limit the proliferative capacity of cells. As a result, in normal human adult somatic cells the TERT gene is silenced. However, in over 90% of cancers, the TERT gene is reactivated, allowing cells to bypass senescence and become immortalized. In this study, we explore the molecular mechanisms that regulate transcriptional expression of the TERT gene. Bioinformatic analysis of the noncoding genomic regions around the human TERT gene identified a TERT ultra-conserved (TUC) module located 5 kb upstream of the transcription start site. This 308 bp region is over 75% conserved between distantly related mammalian species and over 91% conserved among primate species. The cis-regulatory potential of the TUC region was tested in cell-based reporter gene assays. Transient transfections into HeLa and lung fibroblast cells demonstrated that the TUC module has transcriptional enhancer activity. Further bioinformatic analysis revealed that the TUC region is highly enriched in putative transcription factor binding sites for proteins involved during hematopoiesis, indicating that the TUC module may be an enhancer for the TERT gene in specific cell lineages.