Temperature and salt dependence of higher order structure formation by antisense c-myc and c-myb phosphorothioate oligodeoxyribonucleotides containing tetraguanylate tracts

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Abstract
The use of complementary RNA or DNA sequences to selectively interfere with the utilization of mRNA of a target gene is an attractive therapeutic strategy. Two well-studied targets for oligonucleotide therapy are the c-myc and c-myb proto-oncogenes. It has been reported that sequences which contain four contiguous Gs can elicit a non-antisense response due to the formation of a homotetrameric G quartet structure. Therefore it was of interest to determine whether anti-c-myc and anti-c-myb phosphorothioate DNAs including tetraguanylate form higher order structures under physiologically relevant salt conditions and temperature. First the identity of the higher order structure was established and was found to be a tetraplex. Employing intracellular (high K+) extracellular (low K+) and normal saline (no K+) salt mixtures native gel electrophoresis revealed no tetraplex formation at 37°C the physiologically relevant temperature. On the other hand tetraplex structure formation was observed at 4 and 23°C. Hence the potential for these sequences to form tetraplex structures at lower temperatures may not be relevant for their activity in cells and animals at physiological temperature.