The interaction with DNA of a peptide nucleic acid (PNA) oligomer (16nt) conjugated with a nuclear localization signal (NLS) peptide, which was previously found to be able to inhibit tumor cell proliferation through block of transcription of the MYCN oncogene, was studied by UV and CD spectroscopy. While data obtained by UV were not conclusive, the use of circular dichroism gave clear-cut evidence of the formation of a PNA:DNA duplex of exceptionally high stability (Tm >or= 90 degrees C). Using the same approach, the effect of mutations on DNA:PNA stability was evaluated, and was found in accordance with that expected for a Watson-Crick interaction. The role of the NLS peptide was evaluated by using a PNA lacking of this part, which gave rise to less stable PNA:DNA duplexes. Finally, a competition experiment carried out with a 26mer dsDNA, containing the target 16mer sequence in its middle region, in the presence of PNA-NLS gave evidence for the formation of a ternary complex at 25 degrees , while at higher temperature, the PNA:DNA duplex and the displaced homologous DNA strand were detected. The present results support the possibility of an analogous mechanism of action of this antitumor PNA in vivo.
(c) 2007 Wiley-Liss, Inc.