Synthesis of 2′-<i>O</i>-[2-(<i>N</i>-Methylcarbamoyl)ethyl]ribonucleosides Using Oxa-Michael Reaction and Chemical and Biological Properties of Oligonucleotide Derivatives Incorporating These Modified Ribonucleosides

To develop oligonucleotides containing new 2′-<i>O</i>-modified ribonucleosides as nucleic acid drugs, we synthesized three types of ribonucleoside derivatives modified at the 2′-hydroxyl group with 2-(methoxycarbonyl)ethyl (MOCE), 2-(<i>N</i>-methylcarbamoyl)ethyl (MCE), and 2-(<i>N</i>,<i>N</i>-dimethylcarbamoyl)ethyl (DMCE) groups, as key intermediates, via the oxa-Michael reaction of the appropriately protected ribonucleoside (U, C, A, and G) derivatives. Among them, the 2′-<i>O</i>-MCE ribonucleosides were found to be the most stable under basic conditions. To study the effects of the 2′-<i>O</i>-modification on the nuclease resistance of oligonucleotides incorporating the 2′-<i>O</i>-modified ribonucleosides and their hybridization affinities for the complementary RNA and DNA strands, 2′-<i>O</i>-MCE-ribonucleoside phosphoramidite derivatives were successfully synthesized and subjected to the synthesis of 2′-<i>O</i>-MCE-oligonucleotides and 2′-<i>O</i>-methyl-oligonucleotides incorporating 2′-<i>O</i>-MCE-ribonucleosides. The 2′-<i>O</i>-MCE-oligonucleotides and chimeric oligomers with 2′-<i>O</i>-MCE and 2′-<i>O</i>-methyl groups thus obtained demonstrated complementary RNA strands and much higher nuclease resistances than the corresponding 2′-<i>O</i>-methylated species. Finally, we incorporated the 2′-<i>O</i>-MCE-ribonucleosides into antisense 2′-<i>O</i>-methyl-oligoribonucleotides to examine their exon-skipping activities in splicing reactions related to pre-mRNA of mouse dystrophin. The exon-skipping assay of these 2′-<i>O</i>-methyl-oligonucleotide incorporating 2′-<i>O</i>-MCE-uridines showed better efficacies than the corresponding 2′-<i>O</i>-methylated oligoribonucleotide phosphorothioate derivatives

Mutation- and age-related expression of dystrophin-positive revertant fibers in TA and GC muscles from <i>mdx</i> and <i>mdx52</i> mice.

<p>(A) The number of RFs in one TA or GC section. (B) The number of RF clusters containing 2 or more positive fibers. (C) The maximum number of RFs in a single cluster. <i>Mdx</i> mice have a significantly higher number of RFs in all criteria than <i>mdx52</i> mice except for 2<b> </b>months of age in maximum number of RFs per cluster. The number of RFs in all criteria increases with age. Values are mean ± S.D. (<i>n</i> = 3–6 mice per each group). *<i>P</i><0.05, **<i>P</i><0.01 between <i>mdx</i> and <i>mdx52</i> mice; †<i>P</i><0.05, ††<i>P</i><0.05 compared to 2<b> </b>months old. M: months.</p

No expression of eMHC in RFs and attenuation of ongoing muscle regeneration in aged <i>mdx</i> and <i>mdx52</i> mice.

<p>(A) Triple staining of <i>mdx</i> and <i>mdx52</i> mice for RF (green), eMHC (red), and nucleus (blue). Revertant dystrophin is not co-localized with newly regenerated eMHC-positive fibers in TA and GC muscles from <i>mdx</i> and <i>mdx52</i> mice at any age. The pictures are representative GC muscles from <i>mdx</i> and <i>mdx52</i> mice at each age. 20x objective lens, scale bar  = 100<b> </b>μm. (B) The number of eMHC-positive fibers. Values are mean ± S.D. (<i>n</i> = 3–6 mice per group). A significant decrease in the number of eMHC-positive fibers is found only at 18<b> </b>months old in <i>mdx</i> mice (**<i>P</i><0.01 compared to 2<b> </b>months old, †<i>P</i><0.05 compared to 6 and 12<b> </b>months old). Symbol colors are accordant with the color of mice (red; <i>mdx</i>, blue; <i>mdx52</i>).</p

Distinct changes in the percentage of centrally nucleated fibers by mutations and age in <i>mdx</i> and <i>mdx52</i> mice.

<p>(A) Representative images of TA muscles from <i>mdx</i> and <i>mdx52</i> mice at ages 2, 6, 12 and 18<b> </b>months with hematoxylin and eosin staining. Wild-type C57BL/6 muscle at 2<b> </b>months of age is displayed as a control. M: months. Scale bar  = 100<b> </b>μm. (B) The percentage of centrally nucleated fibers in TA and GC muscles from <i>mdx</i> and <i>mdx52</i> mice. Three hundred to one thousand myofibers were counted in left and right muscles and the percentage of CNFs was averaged between the two muscles per mouse. Values are mean ± S.D. (<i>n</i> = 3–6 mice per group). *<i>P</i><0.05, **<i>P</i><0.01 between <i>mdx</i> and <i>mdx52</i> mice; †<i>P</i><0.05, ††<i>P</i><0.01 compared to 2<b> </b>months old; ‡<i>P</i><0.05, ‡‡<i>P</i><0.01 compared to 6<b> </b>months old. Symbol colors are accordant with the color of mice (red; <i>mdx</i>, blue; <i>mdx52</i>).</p

Dystrophin-positive revertant fibers with central nuclei at ages of 2, 6, 12, and 18 months in <i>mdx</i> and <i>mdx52</i> mice.

<p>Representative immunohistochemical images of maximum clusters of RFs in TA muscles are shown in each group. <i>Mdx</i> shows a higher maximum number of RFs than <i>mdx52</i> in all age groups. Wild-type C57BL/6 muscle at 2<b> </b>months old is displayed as a control. An anti-dystrophin C-terminal antibody (green) and DAPI staining (blue) were used. M: months. 20x objective lens, scale bar  = 100<b> </b>μm.</p