TERT promoter mutations in telomere biology

Abstract

Telomere repeats at chromosomal ends, critical to genome integrity, are maintained through an elaborate network of proteins and pathways. Shelterin complex proteins shield telomeres from induction of DNA damage response to overcome end protection problem. A specialized ribonucleic protein, telomerase, maintains telomere homeostasis through repeat addition to counter intrinsic shortcomings of DNA replication that leads to gradual sequence shortening in successive mitoses. The biogenesis and recruitment of telomerase composed of telomerase reverse transcriptase (TERT) subunit and an RNA component, takes place through the intricate machinery that involves an elaborate number of molecules. The synthesis of telomeres remains a controlled and limited process. Inherited mutations in the molecules involved in the process directly or indirectly cause telomeropathies. Telomerase, while present in stem cells, is deactivated due to epigenetic silencing of the rate-limiting TERT upon differentiation in most of somatic cells with a few exceptions. However, in most of the cancer cells telomerase reactivation remains a ubiquitous process and constitutes one of the major hallmarks. Discovery of mutations within the core promoter of the TERT gene that create de novo binding sites for E-twenty-six (ETS) transcription factors provided a mechanism for cancer-specific telomerase reactivation. The TERT promoter mutations occur mainly in tumors from tissues with low rates of self-renewal. In melanoma, glioma, hepatocellular carcinoma, urothelial carcinoma and others, the promoter mutations have been shown to define subsets of patients with adverse disease outcomes, associate with increased transcription of TERT, telomerase reactivation and affect telomere length; in stem cells the mutations inhibit TERT silencing following differentiation into adult cells. The TERT promoter mutations cause an epigenetic switch on the mutant allele along with recruitment of pol II following the binding of GABPA/B1 complex that leads to mono-allelic expression. Thus, the TERT promoter mutations hold potential as biomarkers as well as future therapeutic targets.

Introduction

The term telomeres, derived from the Greek nouns telos (τέλoζ) ‘end’ and meros (μέρ○ζ, root: μερ-) ‘part’, defines DNA-protein structures at the ends of eukaryotic chromosomes [1], [2]. The ‘capping’ of linear chromosome ends by telomeres is often compared to an aglet which caps a shoelace to protect it from fraying [3]. Telomeres, however, play vital roles in countering two intrinsic biological flaws, end protection and end replication, and set apart natural chromosomal ends from DNA double strand breaks to prevent induction of DNA-damage signaling leading to end-fusions and genome instability (‘end-protection problem’) [4]. The consequential and gradual shortening of DNA at chromosomal ends due to an inherent limitation of DNA replication (‘end replication problem’) is countered by a ribonucleic protein, telomerase [5], [6], [7], [8].

Telomeres are typically comprised of multiple short sequence repeats; the repeat length per telomere varies widely among different species. In humans TTAGGG (G-rich strand 5′–3′) repeats account for telomere lengths ranging 10–15 kb. Telomere sequences, mainly double stranded, end in a single-stranded G-rich 3′ tail of 150–200 nucleotides [6], [7]. Telomerase, which is comprised of the catalytic subunit, encoded by the telomerase reverse transcriptase (TERT) gene, and an RNA component transcribed from Telomerase RNA component (TERC, TR, TER or TRC3), remains active in germ cells and proliferative cells of self-renewing tissues and is mostly silenced in differentiated cells leading to gradual telomere attrition due to incomplete replication and DNA damage [8], [9].

Telomerase activity in most of the somatic cells is limited due to suppression of TERT through repressors and enhancers within the promoter of the gene encoding the catalytic subunit [10], [11], [12]. Non-coding mutations within the core promoter, widespread in many cancers, which increase TERT transcription through binding of E-twenty-six/ternary complex (ETS/TCF) transcription factors in conjunction with native sites, provided a first definitive mechanism of cancer specific telomerase reactivation [13], [14], [15], [16]. TERT upregulation, one of the hallmarks of cancer, constitutes a key step in the process of cellular transformation [17], [18]. Infinite capability of cancer cells to overcome the proliferative barrier through telomere homeostasis is mainly attributed to telomerase reactivation, whereas in a subset of cancers telomere elongation involves homologous recombination [17], [19], [20].

In this review we provide an outline of telomere biology with a short historical perspective and overviews of telomere end protection, end replication, telomerase biogenesis and telomere diseases. Considering the critical role of telomerase in human cancers, we discuss mechanisms of its reactivation with emphasis on cancer specific noncoding mutations in the core promoter of the TERT gene that create de novo binding sites for ETS transcription factors. The TERT promoter mutations marked the first case of driver mutations within the so called dark matter of the genome that act through aberrant expression rather than alteration of a gene product [21]. We have discussed the discovery and distribution of the promoter mutations along with their effect on disease outcomes in different cancers. We also summarized the knowledge, hitherto accrued, through various functional studies on the mechanisms through which the TERT promoter mutations exert influence on cancer specific transcription, telomerase reactivation and telomere length.