About MDD - Subscription Info
June 2002
Vol. 5, No. 6, p 13.
news in brief

ON target

G junction. Hydrogen-bonding structure of a G-quartet.
G junction. Hydrogen-bonding structure of a G-quartet.
Oligonucleotides (ONs) are used extensively as antisense agents to reduce the expression of specific genes. Recent studies illustrate that ONs also hold promise for direct protein targeting through their tertiary structure.

Among the most promising examples are guanosine-rich ONs, which form highly stable G-quartet structures (see figure). G-quartets occur naturally in several biological functions, such as telomere maintenance, transcriptional regulation, and alignment of chromosome pairs.

Previously, Donald Miller’s group at the University of Louisville (KY) identified a G-quartet-forming ON, denoted GRO29A, which exhibits antiproliferative activity in cancer cell lines. This outcome may be due to GRO29A’s ability to bind to nucleolin, a multifunctional protein involved in cell division, nuclear architecture, ribosomal gene regulation, and DNA unwinding. Although direct causation between the GRO29A–nucleolin interaction and cell cycle arrest has yet to be established, GRO29A can block the helicase activity of SV40 T-antigen and may similarly inhibit the unwinding of DNA by nucleolin.

To extend their investigation of GRO29A’s antiproliferative mechanism, Miller and colleagues synthesized phosphodiester derivatives and 2´-O-methyl RNA analogues of their lead compound (Biochemistry 2002, 41 (11), 3676- 3685). “We need to better characterize the mechanism of action,” explains Virna Dapic, Miller lab member and first author on the study. “Our molecules act on numerous targets, some overexpressed in cancer cells.”

As with GRO29A, several of their new ONs inhibited growth of cancer cell lines, but not untransformed fibroblasts, and also bound to nuclear extract proteins in electrophoretic mobility shift assays. By circular dichroism spectroscopy and thermal renaturation, the researchers demonstrated that the most effective inhibitors also formed G-quartets. Molecular models of these active and inactive G-quartets implicated a helical groove, which wraps around the G-quartet core, as critical for interaction with cellular protein targets. Base substitutions confirmed this evidence for binding through the helical quartet core and demonstrated that the loop regions located at the ends were not involved in inhibition of cell growth.

While these studies certainly illustrate the potential of G-quartet-forming ONs as non-sequence-dependent chemotherapeutics, the details of GRO29A’s mechanism remain ambiguous. It is hoped that future experiments will provide structural information about the ON–nucleolin complex and demonstrate inhibition of nucleolin’s helicase activity.


< Previous Article | Return to Top | Table of Contents | MDD Home Page | Next Article >