The Physicochemical Properties of DNA-RNA Duplexes Containing Pure 7′R-Me- or 7′S-Me-Carba-LNA Derivatives of A, G, 5-MeC or T in the DNA Strand: Diastereomer Specific Comparison of The 3′-Exonuclease Stability and RNase H Elicitation
(English)In: Article in journal (Refereed) Submitted
Recently, the intramolecular 5-exo-5-hexenyl free-radical cyclization gave access to 2′, 4′-locked carba-LNAs with different nucleobase moieties, i.e. 7′S- and 7′R-Me-cLNA-A, -G, -MeC and -T nucleosides (J. Am. Chem. Soc. 2007, 129, 8362-8379; J. Org. Chem. 2011, 76, 4408-4431). In these studies, diastereomeric mixtures of 7′S/R-Me-cLNA-MeC and -T and diastereomerically pure 7′R-Me-cLNA-A and -G have been incorporated into antisense oligonucleotides (AONs) for biological evaluations. These cLNA modified oligos have shown to have comparable RNA affinity and highly improved nuclease and blood serum stabilities relative to that of their LNA modified counterparts. In order to fully understand the spatial effect of diastereomeric orientation of 7′-methyl group in cLNA-A/G/MeC/T on the RNA affinity, nuclease stability and RNase H elicitation efficiency, we have synthesized and preparatively HPLC separated and tested each of the 7′S- (minor) and 7′R- (major) pure diastereomer of 7′S/R-Me-cLNA-A/G/MeC/T nucleosides. Incorporation into oligos of each pure diastereomer of cLNA led to higher RNA affinity (1-4°C/mod). Tm increase was found to be dependent both on the modification site in the AON as well as whether it is 7′S or 7′R modified cLNA is incorporated. RNA selectivity (DDTm) was found to be in the range of 3.1-6.7°C compared to DDTm of 2.7°C for the native counterpart. The Tm variations modulated by 7′S- and 7′R-Me-cLNAs in the AON have been found to be sequence and position-dependent. Molecular dynamics (MD) simulations of DNA-RNA duplexes with AON2 and AON5, with pure diastereomer incorporated at the 7th position of the AON strand from 3′-end, revealed that both 7′S- and 7′R-Me-cLNA-A modifications have only small local effect on stacking and hydrogen bonding within the duplexes, with Watson-Crick base-pairing remained intact during 98-100% duration of the MD simulations. It has been however found that the Tm of each of the modified heteroduplex is dictated by the individual solvation energy (CPCM) of the 7′S- or 7′R-Me-cLNA diastereomer of A, G, MeC or T nucleobase. This demonstrates that the major factor behind variation in the thermal stabilities of the 7′R- or 7′S-Me-cLNA modified AON-RNA duplexes lies in the intrinsic hydrophobicity, hence its relative solvation energy, inherent in the 7′R- vis-a-vis 7′S-Me-cLNA modified monomer blocks, compared to those of the native and LNA counterparts. We have also found that AONs containing 7′S- and 7′R-Me-cLNA-MeC modifications exhibited unprecedented nuclease stabilities: the most stable AON is the one that contains 7′S-Me-cLNA-MeC, which is ~40 times more stable towards 3′-exonuclease (SVPDE) than 7′S- and 7′R-Me-cLNA-T modified AONs, which was in turn much more stable than 7′S- and 7′R-Me-cLNA-A and G modified counterparts. It is noteworthy that 7′S-methyl group of cLNAs endows the AON strand with more nuclease stability than that of 7′R configured counterpart when compared within the same nucleobase. Thus the carba-LNA modified AONs show nucleobase-dependent activity in the following order: MeC > T > A > G, regardless of 7′S- or 7′R-configurations in the carba-LNA. All of the cLNA and LNA modified AON/RNA hybrids can elicit RNase H activity with similar or even more enhanced rates of digestion by E. coli RNase H1 compared to that of the native AON/RNA. The cleavage rates and patterns of modified AON/RNA hybrids by E. coli RNase H1 are only correlated with the modification site in AON sequence of AON/RNA hybrids, but irrelevant to the structural features of incorporated modifications.
Research subject Chemistry
IdentifiersURN: urn:nbn:se:uu:diva-179066OAI: oai:DiVA.org:uu-179066DiVA: diva2:543253