7',5'-alpha-bicyclo-DNA: new chemistry for oligonucleotide exon splicing modulation therapy.

Antisense oligonucleotides are small pieces of modified DNA or RNA, which offer therapeutic potential for many diseases. We report on the synthesis of 7',5'-α-bc-DNA phosphoramidite building blocks, bearing the A, G, T and MeC nucleobases. Solid-phase synthesis was performed to construct five oligodeoxyribonucleotides containing modified thymidine residues, as well as five fully modified oligonucleotides. Incorporations of the modification inside natural duplexes resulted in strong destabilizing effects. However, fully modified strands formed very stable duplexes with parallel RNA complements. In its own series, 7',5'-α-bc-DNA formed duplexes with a surprising high thermal stability. CD spectroscopy and extensive molecular modeling indicated the adoption by the homo-duplex of a ladder-like structure, while hetero-duplexes with DNA or RNA still form helical structure. The biological properties of this new modification were investigated in animal models for Duchenne muscular dystrophy and spinal muscular atrophy, where exon splicing modulation can restore production of functional proteins. It was found that the 7',5'-α-bc-DNA scaffold confers a high biostability and a good exon splicing modulation activity in vitro and in vivo.

NMR solution structure of tricyclo-DNA containing duplexes: insight into enhanced thermal stability and nuclease resistance.

Tc-DNA is a conformationally constrained oligonucleotide analogue which shows significant increase in thermal stability when hybridized with RNA, DNA or tc-DNA. Remarkably, recent studies revealed that tc-DNA antisense oligonucleotides (AO) hold great promise for the treatment of Duchenne muscular dystrophy and spinal muscular atrophy. To date, no high-resolution structural data is available for fully modified tc-DNA duplexes and little is known about the origins of their enhanced thermal stability. Here, we report the structures of a fully modified tc-DNA oligonucleotide paired with either complementary RNA, DNA or tc-DNA. All three investigated duplexes maintain a right-handed helical structure with Watson-Crick base pairing and overall geometry intermediate between A- and B-type, but closer to A-type structures. All sugars of the tc-DNA and RNA residues adopt a North conformation whereas the DNA deoxyribose are found in a South-East-North conformation equilibrium. The conformation of the tc-DNA strand in the three determined structures is nearly identical and despite the different nature and local geometry of the complementary strand, the overall structures of the examined duplexes are very similar suggesting that the tc-DNA strand dominates the duplex structure.

Enzymatic synthesis of biphenyl-DNA oligonucleotides.

The incorporation of nucleotides equipped with C-glycosidic aromatic nucleobases into DNA and RNA is an alluring strategy for a number of practical applications including fluorescent labelling of oligonucleotides, expansion of the genetic alphabet for the generation of aptamers and semi-synthetic organisms, or the modulation of excess electron transfer within DNA. However, the generation of C-nucleoside containing oligonucleotides relies mainly on solid-phase synthesis which is quite labor intensive and restricted to short sequences. Here, we explore the possibility of constructing biphenyl-modified DNA sequences using enzymatic synthesis. The presence of multiple biphenyl-units or biphenyl residues modified with electron donors and acceptors permits the incorporation of a single dBphMP nucleotide. Moreover, templates with multiple abasic sites enable the incorporation of up to two dBphMP nucleotides, while TdT-mediated tailing reactions produce single-stranded DNA oligonucleotides with four biphenyl residues appended at the 3'-end.

Compatibility of 5-ethynyl-2'F-ANA UTP with in vitro selection for the generation of base-modified, nuclease resistant aptamers.

A modified nucleoside triphosphate bearing two modifications based on a 2'-deoxy-2'-fluoro-arabinofuranose sugar and a uracil nucleobase equipped with a C5-ethynyl moiety (5-ethynyl-2'F-ANA UTP) was synthesized. This nucleotide analog could enzymatically be incorporated into DNA oligonucleotides by primer extension and reverse transcribed to unmodified DNA. This nucleotide could be used in SELEX for the identification of high binding affinity and nuclease resistant aptamers.

Synthesis, biophysical properties, and RNase H activity of 6'-difluoro[4.3.0]bicyclo-DNA.

Here we present the synthesis, the biophysical properties, and the RNase H profile of 6'-difluorinated [4.3.0]bicyclo-DNA (6'-diF-bc4,3-DNA). The difluorinated thymidine phosphoramidite building block was synthesized starting from an already known gem-difluorinated tricyclic glycal. This tricyclic siloxydifluorocyclopropane was converted into the [4.3.0]bicyclic nucleoside via cyclopropane ring-opening through the addition of an electrophilic iodine during the nucleosidation step followed by reduction. The gem-difluorinated bicyclic nucleoside was then converted into the corresponding phosphoramidite building block which was incorporated into oligonucleotides. Thermal denaturation experiments of these oligonucleotides hybridized to complementary DNA or RNA disclosed a significant destabilization of both duplex types (ΔT m/mod = -1.6 to -5.5 °C). However, in the DNA/RNA hybrid the amount of destabilization could be reduced by multiple insertions of the modified unit. In addition, CD spectroscopy of the oligonucleotides hybridized to RNA showed a similar structure than the natural DNA/RNA duplex. Furthermore, since the structural investigation on the nucleoside level by X-ray crystallography and ab initio calculations pointed to a furanose conformation in the southern region, a RNase H cleavage assay was conducted. This experiment revealed that the oligonucleotide containing five modified units was able to elicit the RNase H-mediated cleavage of the complementary RNA strand.

3CAPS - a structural AP-site analogue as a tool to investigate DNA base excision repair.

Abasic sites (AP-sites) are frequent DNA lesions, arising by spontaneous base hydrolysis or as intermediates of base excision repair (BER). The hemiacetal at the anomeric centre renders them chemically reactive, which presents a challenge to biochemical and structural investigation. Chemically more stable AP-site analogues have been used to avoid spontaneous decay, but these do not fully recapitulate the features of natural AP-sites. With its 3'-phosphate replaced by methylene, the abasic site analogue 3CAPS was suggested to circumvent some of these limitations. Here, we evaluated the properties of 3CAPS in biochemical BER assays with mammalian proteins. 3CAPS-containing DNA substrates were processed by APE1, albeit with comparably poor efficiency. APE1-cleaved 3CAPS can be extended by DNA polymerase ß but repaired only by strand displacement as the 5'-deoxyribophosphate (dRP) cannot be removed. DNA glycosylases physically and functionally interact with 3CAPS substrates, underlining its structural integrity and biochemical reactivity. The AP lyase activity of bifunctional DNA glycosylases (NTH1, NEIL1, FPG), however, was fully inhibited. Notably, 3CAPS-containing DNA also effectively inhibited the activity of bifunctional glycosylases on authentic substrates. Hence, the chemically stable 3CAPS with its preserved hemiacetal functionality is a potent tool for BER research and a potential inhibitor of bifunctional DNA glycosylases.

6'-Fluoro[4.3.0]bicyclo nucleic acid: synthesis, biophysical properties and molecular dynamics simulations.

Here we report on the synthesis, biophysical properties and molecular modeling of oligonucleotides containing unsaturated 6'-fluoro[4.3.0]bicyclo nucleotides (6'F-bc4,3-DNA). Two 6'F-bc4,3 phosphoramidite building blocks (T and C) were synthesized starting from a previously described [3.3.0]bicyclic sugar. The conversion of this sugar to a gem-difluorinated tricyclic intermediate via difluorocarbene addition followed either by a NIS-mediated or Vorbrüggen nucleosidation yielded in both cases the ß-tricyclic nucleoside as major anomer. Subsequent desilylation and cyclopropane ring opening of these tricyclic intermediates afforded the unsaturated 6'F-bc4,3 nucleosides. The successful incorporation of the corresponding phosphoramidite building blocks into oligonucleotides was achieved with tert-butyl hydroperoxide as oxidation agent. Thermal melting experiments of the modified duplexes disclosed a destabilizing effect versus DNA and RNA complements, but with a lesser degree of destabilization versus complementary DNA (ΔT m/mod = -1.5 to -3.7 °C). Molecular dynamics simulation on the nucleoside and oligonucleotide level revealed the preference of the C1'-exo/C2'-endo alignment of the furanose ring. Moreover, the simulation of duplexes with complementary RNA disclosed a DNA/RNA-type duplex structure suggesting that this modification might be a substrate for RNase H.

Probing the Backbone Topology of DNA: Synthesis and Properties of 7',5'-Bicyclo-DNA.

We describe the synthesis and pairing properties of the novel DNA analogue 7',5'-bicyclo(bc)-DNA. In this analogue, the point of attachment of the connecting phosphodiester group is switched from the 3' to the 7' position of the underlying bicyclic sugar unit and is thus in a topological position that is inaccessible in natural DNA. The corresponding phosphoramidite building blocks carrying all natural nucleobases were synthesized and incorporated into oligonucleotides. From Tm experiments of duplexes with complementary DNA and RNA we find that single modifications are generally well tolerated with some variability as to the nature of the nucleobase. Fully modified oligonucleotides show low affinity for RNA and DNA complements. However, they form antiparallel homo-duplexes with similar thermal stability as DNA. CD spectra of the homo-duplexes show distinct changes in the helix conformation compared to natural DNA. A conformational analysis at the ab initio level of the mononucleosides revealed two minimal energy structures which primarily deviate in the conformation of the cyclopentane ring. Molecular dynamics simulation of a 7',5'-bc-DNA homo-duplex revealed a right-handed structure with a smaller helical rise and a significantly wider minor groove compared to DNA. Interestingly, this duplex is characterized by an atypical, alternating 6'-endo/6'-exo conformational pattern of consecutive nucleotides which seems to be responsible for the poor binding to natural nucleic acids.

Modulation of Excess Electron Transfer through LUMO Gradients in DNA Containing Phenanthrenyl Base Surrogates.

The modulation of excess electron transfer (EET) within DNA containing a dimethylaminopyrene (C-AP) as an electron donor and 5-bromouracil (Br dU) as an electron acceptor through phenanthrenyl pairs (phen-R) could be achieved by modifying the phenanthrenyl base surrogates with electron withdrawing and donating groups. Arranging the phenanthrenyl units to form a descending LUMO gradient increased the EET efficiency compared to the electron transfer through uniform LUMOs or an ascending LUMO gradient.

2'ß-Fluoro-Tricyclo Nucleic Acids (2'F-tc-ANA): Thermal Duplex Stability, Structural Studies, and RNase†H Activation.

We describe the synthesis, thermal stability, structural and RNase H activation properties of 2'ß-fluoro-tricyclo nucleic acids (2'F-tc-ANA). Three 2'F-tc-ANA nucleosides (T, 5Me C and A) were synthesized starting from a previously described fluorinated tricyclo sugar intermediate. NMR analysis and quantum mechanical calculations indicate that 2'F-tc-ANA nucleosides prefer sugar conformations in the East and South regions of the pseudorotational cycle. UV-melting experiments revealed that non-consecutive insertions of 2'F-tc-ANA units in DNA reduce the affinity to DNA and RNA complements. However, an oligonucleotide with five contiguous 2'F-tc-ANA-T insertions exhibits increased affinity to complementary RNA. Moreover, a fully modified 10-mer 2'F-tc-ANA oligonucleotide paired to both DNA (+1.6 °C/mod) and RNA (+2.5 °C/mod) with significantly higher affinity compared to corresponding unmodified DNA, and similar affinity compared to corresponding tc-DNA. In addition, CD spectroscopy and molecular dynamics simulations indicate that the conformation of the 2'F-tc-ANA/RNA duplex is similar to that of a DNA/RNA duplex. Moreover, in some sequence contexts, 2'F-tc-ANA promotes RNase H-mediated cleavage of a complementary RNA strand. Taken together, 2'F-tc-ANA represents a nucleic acid analogue that offers the advantage of high RNA affinity while maintaining the ability to activate RNase H, and can be considered a prospective candidate for gene silencing applications.

The effect of RNA base lesions on mRNA translation.

The biological effect of oxidatively damaged RNA, unlike oxidatively damaged DNA, has rarely been investigated, although it poses a threat to any living cell. Here we report on the effect of the commonly known RNA base-lesions 8-oxo-rG, 8-oxo-rA, ε-rC, ε-rA, 5-HO-rC, 5-HO-rU and the RNA abasic site (rAS) on ribosomal translation. To this end we have developed an in vitro translation assay based on the mRNA display methodology. A short synthetic mRNA construct containing the base lesion in a predefined position of the open reading frame was (32)P-labeled at the 5'-end and equipped with a puromycin unit at the 3'-end. Upon in vitro translation in rabbit reticulocyte lysates, the encoded peptide chain is transferred to the puromycin unit and the products analyzed by gel electrophoresis. Alternatively, the unlabeled mRNA construct was used and incubated with (35)S-methionine to prove peptide elongation of the message. We find that all base-lesions interfere substantially with ribosomal translation. We identified two classes, the first containing modifications at the base coding edge (ε-rC, ε-rA and rAS) which completely abolish peptide synthesis at the site of modification, and the second consisting of 8-oxo-rG, 8-oxo-rA, 5-HO-rC and 5-HO-rU that significantly retard full-length peptide synthesis, leading to some abortive peptides at the site of modification.

Synthesis and properties of 6'-fluoro-tricyclo-DNA.

The synthesis of the two fluorinated tricyclic nucleosides 6'-F-tc-T and 6'-F-tc-5(Me)C, as well as the corresponding building blocks for oligonucleotide assembly, was accomplished. An X-ray analysis of N(4)-benzoylated 6'-F-tc-(5Me)C reavealed a 2'-exo (north) conformation of the furanose ring, characterizing it as an RNA mimic. In contrast to observations in the bicyclo-DNA series, no short contact between the fluorine atom and the H6 of the base, reminiscent of a nonclassical F···H hydrogen bond, could be observed. Tm measurements of modified oligodeoxynucleotides with complementary RNA showed slightly sequence-dependent duplex stabilization profiles with maximum ΔTm/mod values of +4.5 °C for 6'-F-tc-(5Me)C and +1 °C for 6'-F-tc-T. In comparison with parent tc-modified oligonucleotides, no relevant changes in Tm were detected, attributing the fluorine substituent a neutral role in RNA affinity. A structural analysis of duplexes with DNA and RNA by CD-spectroscopy revealed a shift from B- to A-type conformation induced by the 6'-F-tc-nucleosides. This is not a specific "fluorine effect", as the same is also observed for the parent tc-modifications. The two fluorinated tc-nucleosides were also incorporated into a pure tricyclo-DNA backbone and showed no discrimination in Tm with complementary RNA, demonstrating that 6'-F substitution is also compatible within fully modified tc-oligonucleotides.

Synthesis and biochemical characterization of tricyclothymidine triphosphate (tc-TTP).

Tricyclo-DNA (tc-DNA) is a conformationally restricted oligonucleotide analogue that exhibits promising properties as a robust antisense agent. Here we report on the synthesis and biochemical characterization of tc-TTP, the triphosphate of a tc-DNA nucleoside containing the base thymine. Tc-TTP turned out to be a substrate for the Vent (exo(-) ) DNA polymerase, a polymerase that allows for multiple incorporations of tc-T nucleotides under primer extension reaction conditions. However, the substrate acceptance is rather low, as also observed for other sugar-modified analogues. Tc-TTP and tc-nucleotide-containing templates do not sustain enzymatic polymerization under physiological conditions; this indicates that tc-DNA-based antisense agents will not enter natural metabolic pathways that lead to long-term toxicity.

A 6'-fluoro-substituent in bicyclo-DNA increases affinity to complementary RNA presumably by CF­HC pseudohydrogen bonds.

The synthesis of a novel bicyclic thymidine analogue carrying a ß-fluoro substituent at C6' (6'F-bcT) has been achieved. Key steps of the synthesis were an electrophilic fluorination/stereospecific hydrogenation sequence of a bicyclo sugar intermediate, followed by an N-iodo-succinimide-induced stereoselective nucleosidation. A corresponding phosphoramidite building block was then prepared and used for oligonucleotide synthesis. Tm measurements of oligonucleotides with single and double incorporations showed a remarkable stabilization of duplex formation particularly with RNA as complement without compromising pairing selectivity. Increases in Tm were in the range of +1­2 °C compared to thymidine and +1­3 °C compared to a standard bc-T residue. Structural investigations of the 6'F-bcT nucleoside by X-ray crystallography showed an in-line arrangement of the fluorine substituent with H6 of thymine, however, with a distance that is relatively long for a nonclassical CF­HC hydrogen bond. In contrast, structural investigations in solution by 1H and 13C NMR clearly showed scalar coupling of fluorine with H6 and C6 of the nucleobase, indicating the existence of at least weak electrostatic interactions. On the basis of these results, we put forward the hypothesis that these weak CF­HC6 electrostatic interactions increase duplex stability by orienting and partially freezing torsion angle χ of the 6'F-bcT nucleoside.

TricycloDNA-modified oligo-2'-deoxyribonucleotides reduce scavenger receptor B1 mRNA in hepatic and extra-hepatic tissues--a comparative study of oligonucleotide length, design and chemistry.

We report the evaluation of 20-, 18-, 16- and 14-mer phosphorothioate (PS)-modified tricycloDNA (tcDNA) gapmer antisense oligonucleotides (ASOs) in T(m), cell culture and animal experiments and compare them to their gap-matched 20-mer 2'-O-methoxyethyl (MOE) and 14-mer 2',4'-constrained ethyl (cEt) counterparts. The sequence-matched 20-mer tcDNA and MOE ASOs showed similar T(m) and activity in cell culture under free-uptake and cationic lipid-mediated transfection conditions, while the 18-, 16- and 14-mer tcDNA ASOs were moderate to significantly less active. These observations were recapitulated in the animal experiments where the 20-mer tcDNA ASO formulated in saline showed excellent activity (ED(50) 3.9 mg/kg) for reducing SR-B1 mRNA in liver. The tcDNA 20-mer ASO also showed better activity than the MOE 20-mer in several extra-hepatic tissues such as kidney, heart, diaphragm, lung, fat, gastrocnemius and quadriceps. Interestingly, the 14-mer cEt ASO showed the best activity in the animal experiments despite significantly lower T(m) and 5-fold reduced activity in cell culture relative to the 20-mer tcDNA and MOE-modified ASOs. Our experiments establish tcDNA as a useful modification for antisense therapeutics and highlight the role of chemical modifications in influencing ASO pharmacology and pharmacokinetic properties in animals.

Structure/affinity studies in the bicyclo-DNA series: Synthesis and properties of oligonucleotides containing bc(en)-T and iso-tricyclo-T nucleosides.

We present the synthesis of the two novel nucleosides iso-tc-T and bc(en)-T, belonging to the bicyclo-/tricyclo-DNA molecular platform. In both modifications the torsion around C6'-C7' within the carbocyclic ring is planarized by either the presence of a C6'-C7' double bond or a cyclopropane ring. Structural analysis of these two nucleosides by X-ray analysis reveals a clear preference of torsion angle γ for the gauche orientation with the furanose ring in a near perfect 2'-endo conformation. Both modifications were incorporated into oligodeoxynucleotides and their thermal melting behavior with DNA and RNA as complements was assessed. We found that the iso-tc-T modification was significantly more destabilizing in duplex formation compared to the bc(en)-T modification. In addition, duplexes with complementary RNA were less stable as compared to duplexes with DNA as complement. A structure/affinity analysis, including the already known bc-T and tc-T modifications, does not lead to a clear correlation of the orientation of torsion angle γ with DNA or RNA affinity. There is, however, some correlation between furanose conformation (N- or S-type) and affinity in the sense that a preference for a 3'-endo like conformation is associated with a preference for RNA as complement. As a general rule it appears that T m data of single modifications with nucleosides of the bicyclo-/tricyclo-DNA platform within deoxyoligonucleotides are not predictive for the stability of fully modified oligonucleotides.

Base pairing and miscoding properties of 1,N(6)-ethenoadenine- and 3,N(4)-ethenocytosine-containing RNA oligonucleotides.

Two RNA phosphoramidites containing the bases 1,N(6)-ethenoadenine (εA) and 3,N(4)-ethenocytosine (εC) were synthesized. These building blocks were incorporated into two 12-mer oligoribonucleotides for evaluation of the base pairing properties of these base lesions by UV melting curve (Tm) and circular dichroism measurements. The Tm data of the resulting duplexes with the etheno modifications opposing all natural bases showed a substantial destabilization compared to the corresponding natural duplexes, confirming their inability to form base pairs. The coding properties of these lesions were further investigated by introducing them into 31-mer oligonucleotides and assessing their ability to serve as templates in primer extension reactions with HIV, AMV, and MMLV reverse transcriptases (RT). Primer extension reactions showed complete arrest of the incorporation process using MMLV RT and AMV RT, while HIV RT preferentially incorporates dAMP opposite εA and dAMP as well as dTMP opposite εC. The properties of these RNA lesions are discussed in the context of its putative biological role.

Synthesis and properties of isobicyclo-DNA.

We present the synthesis of the isobicyclo-DNA building blocks with the nucleobases A, C, G and T, as well as biophysical and biological properties of oligonucleotides derived thereof. The synthesis of the sugar part was achieved in 5 steps starting from a known intermediate of the tricyclo-DNA synthesis. Dodecamers containing single isobicyclo-thymidine incorporations, fully modified A- and T-containing sequences, and fully modified oligonucleotides containing all four bases were synthesized and characterized. Isobicyclo-DNA forms stable duplexes with natural nucleic acids with a pronounced preference for DNA over RNA as complements. The most stable duplexes, however, arise by self-pairing. Isobicyclo-DNA forms preferentially B-DNA-like duplexes with DNA and A-like duplexes with complementary RNA as determined by circular dichroism (CD) spectroscopy. Self-paired duplexes show a yet unknown structure, as judged from CD spectroscopy. Biochemical tests revealed that isobicyclo-DNA is stable in fetal bovine serum and does not elicit RNaseH activity.

Enhancement of excess electron transfer efficiency in DNA containing a phenothiazine donor and multiple stable phenanthrenyl base pairs.

EET grown ohm: Excess electron transfer (EET) was observed within a DNA duplex containing π-stacked phenothiazine as an electron donor, phenanthrenes as electron carriers and 5-bromouracil as an electron trap. Increasing the number of phenanthrenyl base pairs increased EET efficiency.

Synthesis, base pairing properties and trans-lesion synthesis by reverse transcriptases of oligoribonucleotides containing the oxidatively damaged base 5-hydroxycytidine.

The synthesis of a caged RNA phosphoramidite building block containing the oxidatively damaged base 5-hydroxycytidine (5-HOrC) has been accomplished. To determine the effect of this highly mutagenic lesion on complementary base recognition and coding properties, this building block was incorporated into a 12-mer oligoribonucleotide for T(m) and CD measurements and a 31-mer template strand for primer extension experiments with HIV-, AMV- and MMLV-reverse transcriptase (RT). In UV-melting experiments, we find an unusual biphasic transition with two distinct T(m)'s when 5-HOrC is paired against a DNA or RNA complement with the base guanine in opposing position. The higher T(m) closely matches that of a C-G base pair while the lower is close to that of a C-A mismatch. In single nucleotide extension reactions, we find substantial misincorporation of dAMP and to a lesser extent dTMP, with dAMP almost equaling that of the parent dGMP in the case of HIV-RT. A working hypothesis for the biphasic melting transition does not invoke tautomeric variability of 5-HOrC but rather local structural perturbations of the base pair at low temperature induced by interactions of the 5-HO group with the phosphate backbone. The properties of this RNA damage is discussed in the context of its putative biological function.