Formation of the i-motif Structures by Human Telomeric c-Rich Sequences d(CCCTAA)n and Its Recognition by Bisbenzylisoquinoline Alkaloids

The human telomeric repeat CCCTAA has been reported to form a higher-order structure called an intercalated motif (i-motif) that plays important roles in telomere function and telomerase activity regulation, and small molecule ligands targeting human telomeric i-motif (hTelo-iM) is a promising therapeutic strategy for cancer treatment, yet the i-motif folding pattern of long CCCTAA repeats and the hTelo-iM ligand screening have not been studied extensively. In this study, we systematically investigated the i-motif structures formed by four and eight telomeric C-rich repeats d(CCCTAA)4 (hTeloC-24mer) and d(CCCTAA)8 (hTeloC-48mer) under varied conditions and found that the long hTeloC-48mer probably forms unstacked tandem i-motif consisting of two hTeloC-24mer i-motif monomers under near physiological conditions. Moreover, natural bisbenzylisoquinoline (BBI) alkaloids, isofangchinoline, fangchinoline, cepharanthine, and tetrandrine, were screened from 33 natural small molecules to effectively disrupt and destabilize the hTelo-iM structures mainly through major groove hydrogen bonding and van der Waals interactions. Further, telomerase repeated amplification protocol (TRAP) assay suggested that the selected BBI alkaloids can inhibit the telomere extension by telomerase. These findings provide a theoretical basis for further telomere structure research as well as a novel class of natural small molecule compounds regulating the hTelo-iM structure and telomerase activity, which may contribute to the anticancer drug design and strategy development taking the hTelo-iM as a target.