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

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-mycand c-mybproto-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-mycand anti-c-mybphosphorothioate 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 degrees C, the physiologically relevant temperature. On the other hand, tetraplex structure formation was observed at 4 and 23 degrees 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.