G-quadruplex DNA (GQ) are non-canonical DNA structures formed by the π−π stacking of multiple G-tetrads held together by Hoogsteen hydrogen-bonding between guanines. Potential GQ-forming DNA sequences are abundant at telomeres and in promoters of oncogenes, making GQs a promising anticancer target. Our research focuses on solving the crystal structures of representative GQ DNA and exploring their interactions with the highly selective and well explored porphyrin ligand, N-methylmesoporphyrin IX (NMM). The sequences we explored are G-rich telomeric sequences found in the promoter region of <i>Tetrahymena thermophila.</i> We studied the sequences named TET4B, Tel14, Tel25, and Tel26. We have performed biophysical characterizations of GQs alone and in complex with NMM using circular dichroism, UV-vis spectroscopy, and PAGE gels. Our data suggest that TET4B forms an anti-parallel structure, whereas Tel14, Tel25, and Tel26 forms hybrid structures in solution. NMM maintains parallel conformations of GQs or converts hybrid GQs into a parallel conformation and stabilizes the structures. Crystal structures for Tel25 and Tel26 were solved to resolutions of 1.56 and 1.99 Å respectively, utilizing molecular replacement. The structure for Tel25 is a four G-tetrad, 3+1 hybrid structure with three lateral loops and one propeller loop. The structure for Tel26 is a four G-tetrad, parallel structure with three propeller loops. TET4B with NMM x-ray data is currently being solved to a resolution of 1.81 Å. Crystallization trials of Tel25 and Tel26 in complex with NMM are underway. Our results will show the crystal structures of novel GQs and explain NMM’s exceptional selectivity toward GQ DNA. Furthering this knowledge will help improve the design of anticancer therapeutics.

Characterization and crystal structures of G-quadruplex DNA and their complexes with N-methylmesoporphyrin IX

ACS Fall 2020 Virtual Meeting

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