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Chimeric RNA and 2'-O, 4'-C-ethylene-bridged nucleic acids have stronger activity than phosphorothioate oligodeoxynucleotides in induction of exon 19 skipping in dystrophin mRNA.

Antisense phosphorothioate oligodeoxynucleotides against exon 19 of the dystrophin gene have been shown to induce exon 19 skipping and promote the expression of internally deleted dystrophin by correcting the translational reading frame. Because phosphorothioate oligonucleotides are associated with a variety of toxic nonantisense effects, several modifications of nucleic acid have been introduced to alleviate this toxicity. Recently, a 2'-O, 4'-C-ethylene-bridged nucleic acid (ENA trade mark, Sankyo Lifetech Co., Ltd., Tokyo, Japan) was reported to have high affinity to complementary RNA strands and be resistant to nuclease digestion. Here, we examined the ability of this modified nucleic acid to induce exon skipping. Oligonucleotides having the same sequence as the phosphorothioate oligonucleotides but with some stretches of modified backbone (2'-O-methyl RNA with an ENA5-mer at the 5'-end and 3'-end) (RNA/ENA chimera) were transfected into myocytes, and the expressed dystrophin mRNA was analyzed. The RNA/ENA chimera induced exon 19 skipping in a dose-dependent and time-dependent manner. Remarkably, the exon 19-skipping activity of the RNA/ENA chimera was more than 40 times stronger than that of the corresponding conventional phosphorothioate oligodeoxynucleotide. This is the first report of such strong activity of an RNA/ENA chimera in the induction of exon skipping in the dystrophin gene. This new technology will allow the development of less toxic antisense drugs, making long-term therapy possible.