Human telomeric G‐quadruplex

Abstract

The telomeric ends of chromosomes terminate in a single-stranded DNA overhang of a few kilobases, most of which are guanines. With a -TTAGGG- motif repeated in tandem, this G-rich overhang has the potential to fold back into a four-stranded secondary structure called the G-quadruplex. Most often, this single-stranded overhang terminates in a T-loop which is bound by DNA-binding proteins into a telomeric shelterin complex, thus protecting the cell from chromosomal fusions and DNA damage responses. The shelterin protein complex is an assembly of multiple different proteins such as TRF1, TRF2, TIN1, TIN2 and hPOT1. The complex holds the telomeric ends stable, whereas dismantling the assembly uncaps the telomeric ends; these are sensed as a DNA damage signal, which causes the cell to undergo apoptosis. In addition, telomerase, a ribonucleoprotein complex conspicuous in 80–90% of cancer cells, uses its hTERT component, which is responsible for adding more bases to the receding telomeric tail, to maintain telomere length and integrity. This is required to evade cellular senescence and death. Cancerous cells have shorter telomeric lengths and hence are liable to require more active telomerase function for their survival; thus targeting telomerase function is a good anticancer strategy. The folding of DNA into a quadruplex at the telomeric end prevents telomerase from adding further bases to the tail. Many telomerase inhibitors are ligands of quadruplexes and applying them to check telomerase function has resulted in apoptosis in cellular assays, suggesting their possible use as a therapeutic agents. Moreover, it has recently been reported that telomeric ends are also actively transcribed and the product finds a role in chromatin organization at the telomere. In light of this, stabilization of the human telomeric quadruplex is expected to halt such transcription, which will in turn perturb chromatin organization leading to alteration in telomeric function. All in all, telomere biology is attracting a lot of attention as knowledge of secondary structures adopted by telomeric ends, their folding and unfolding kinetics, interactions with small molecules and proteins could provide a platform for drug design. This minireview series deals with different aspects of telomeric quadruplexes. The opening minireview, contributed by Jonathan B. Chaires, takes us through the thermodynamic and kinetics of quadruplex folding and stability. The biophysical methods to study human telomeric quadruplex along with its polymorphic forms, salt dependence and stability are explained. The energetics and multistage interconversion involved also receives due attention. The next minireview by Anh Tuân Phan is devoted to the diverse topologies of telomeric quadruplexs. Short and long telomeric sequences are addressed separately and evidence for quadruplex in telomeric RNA is also presented. Stephen Neidle focuses on the mechanistic explanations of quadruplex ligands. The probable mechanisms of action of quadruplex ligands, their applications to modulate telomere function and the challenges in the development of functional quadruplex-specific drugs are discussed. Finally, Maiti and co-workers describe the various categories of quadruplex ligands. The challenges and future outlook for small molecule–quadruplex interaction are reviewed. This minireview series attempts to elucidate the advances made in the study of the telomeric quadruplex from biophysical, structural, biological and pharmacological standpoints. The vast amount of information gained over decades in this field makes it a timely topic.