Forster resonance energy transfer for studying nucleic acids denaturation: A chemical and biological sciences laboratory experiment

The Förster resonance energy transfer (FRET) melting assay intends to evaluate the unfolding, denaturation process of DNA secondary structures, and its stabilization using compounds known as DNA binders, some of which are highly specific for G-quadruplex DNAs versus duplex DNAs. First, students determined the melting temperature (T_m ) of DNA sequences double labeled with 5'-FAM (fluorescein) and 3'-TAMRA (tetramethylrhodamine) in the absence of DNA binders. Second, they determined the melting temperature of the DNAs in the presence of DNA binders by monitoring fluorescence. After completing this experiment, students understood that this method allows a semiquantitative analysis to test a variety of DNA binders against DNA secondary structures, and it can be used to rapidly identify the most promising drug candidates in the drug development stages at the basic research level.

Specific stabilization of promoter G-Quadruplex DNA by 2,6-disubstituted amidoanthracene-9,10-dione based dimeric distamycin analogues and their selective cancer cell cytotoxicity

We have designed and synthesized anthraquinone containing compounds which have oligopyrrole side chains of varying lengths. These compounds stabilized the G-quadruplex DNA formed in the promoter regions of c-MYC oncogenes selectively over the duplex DNA. These observations were recorded using UV-vis spectroscopic titrations, fluorescence measurements and circular dichroism (CD) spectral titrations. The potency of the compounds to stabilize the G4 DNA has been shown from the thermal denaturation experiments. The compound interacts with c-MYC G-quadruplex DNA through stacking mode as obtained from ethidium bromide displacement assay, cyclic voltammetric titration, and docking experiments. Molecular modeling studies suggested that the stacking of the anthraquinone moiety over the G-tetrad of the G4 structures are responsible for the stability of such quadruplex secondary structure. Furthermore, polymerase stop assay also supported the formation of stable G4 structures in the presence of the above-mentioned compounds. The compounds have shown selective cancer cell (HeLa and HEK293T) cytotoxicity over normal cells (NIH3T3 and HDFa) under in vitro conditions as determined from MTT based cell viability assay. Apoptosis was found to be the mechanistic pathway underlying the cancer cell cytotoxicity as obtained from Annexin V-FITC and PI dual staining assay which was further substantiated by nuclear morphological changes as observed by AO/EB dual staining assay. Cellular morphological changes, as well as nuclear condensation and fragmentation upon treatment with these compounds, were observed under bright field and confocal microscopy.