Telomere shortening at chromosomal ends due to the constraints of the DNA replication process acts as a tumor suppressor by restricting the replicative potential in primary cells. Cancers evade that limitation primarily through rejuvenation of telomerase via different mechanisms. Mutations within the promoter of the telomerase reverse transcriptase (TERT) gene define a definite method for the ribonucleic enzyme regeneration predominantly in cancers that arise from tissues with low rates of self-renewal. The promoter mutations cause a moderate surge in TERT transcription and telomerase rejuvenation to the levels sufficient to delay replicative senescence but not prevent bulk telomere shortening and genomic instability. Since the discovery, a staggering number of studies and publications have resolved the discrete aspects, effects, and clinical relevance of the TERT promoter mutations. Those noncoding alterations link the TERT transcription with oncogenic pathways, associate with markers of poor outcome, and define patients with reduced survivals in several cancers. In this review, we discuss the occurrence and impact of the promoter mutations and highlight the mechanism of TERT activation. We further deliberate on the foundational question of the abundance of the TERT promoter mutations and a general dearth of functional mutations within noncoding sequences as evident from pan-cancer analysis of the whole-genomes. We posit that the favorable genomic constellation within the TERT promoter may be less than a common occurrence in other noncoding functional elements and the evolutionary constraints limit the functional fraction within the human genome, hence the lack of abundant mutations outside the coding sequences.
Replicative senescence of hematopoietic stem cells during serial transplantation: does telomere shortening play a role?