Development of specific dengue virus 2'-O- and N7-methyltransferase assays for antiviral drug screening

Dengue virus (DENV) protein NS5 carries two mRNA cap methyltransferase (MTase) activities involved in the synthesis of a cap structure, (7Me)GpppA(2'OMe)-RNA, at the 5'-end of the viral mRNA. The methylation of the cap guanine at its N7-position (N7-MTase, (7Me)GpppA-RNA) is essential for viral replication. The development of high throughput methods to identify specific inhibitors of N7-MTase is hampered by technical limitations in the large scale synthesis of long capped RNAs. In this work, we describe an efficient method to generate such capped RNA, GpppA(2'OMe)-RNA₇₄, by ligation of two RNA fragments. Then, we use GpppA(2'OMe)-RNA₇₄ as a substrate to assess DENV N7-MTase activity and to develop a robust and specific activity assay. We applied the same ligation procedure to generate (7Me)GpppA-RNA₇₄ in order to characterize the DENV 2'-O-MTase activity specifically on long capped RNA. We next compared the N7- and 2'-O-MTase inhibition effect of 18 molecules, previously proposed to affect MTase activities. These experiments allow the validation of a rapid and sensitive method easily adaptable for high-throughput inhibitor screening in anti-flaviviral drug development.

Imidazolethyl-phosphoramidate alpha-oligonucleotides

Alpha-ODNs conjugated to imidazole groups via phosphoramidate internucleosidic linkages were synthesized. The presence of the imidazolethyl-phosphoramidate linkage improved the affinity of alpha-ODNs for their nucleic acid targets.

Use of MALDI-TOF mass spectrometry to monitor solid-phase synthesis of oligonucleotides

MALDI-TOF mass spectrometry has been used to characterize solid-supported oligonucleotides containing natural and non-natural and non-nucleoside moieties and a variety of internucleosidic linkages including phosphate and phosphite triesters and H-phosphonate diesters. This technique was used to follow the reactions involved in oligonucleotide synthesis; this enabled direct control of the elongation and optimization of the coupling process.

Step-by-step control by MALDI-TOF MS of an oligonucleotide synthesis on standard CPG solid-support

MALDI-TOF mass spectrometry was used to monitor DNA solid-phase synthesis on long-chain alkylamine controlled-pore glass (LCAA-CPG) with a standard succinyl linker between the solid support and the growing oligonucleotide.

Alpha-oligonucleotides with anionic phosphodiester and cationic phosphoramidate linkages enhanced stability of DNA triple helix

Synthesis of several alpha-oligonucleotides containing both anionic phosphodiester and cationic N-(dimethylaminopropyl)phosphoramidate internucleosidic linkages is described. Their ability to form triple helix with dsDNA was evaluated at various pH by UV melting experiments.

Chiral and steric effects in the efficient binding of alpha-anomeric deoxyoligonucleoside N-alkylphosphoramidates to ssDNA and RNA

We report hybridization properties of new phosphate-modified alpha-oligonucleoside analogs with non-ionic or cationic internucleotide linkages such as methoxy-ethylphosphoramidate (PNHME), phosphoromorpholi-date (PMOR) and dimethylaminopropylphosphor-amidate (PNHDMAP). First we evaluated the chirality effect of the phosphorus atom on the affinity of alpha- or beta-dodecanucleoside phosphodiesters containing one chirally enriched N -alkylphosphoramidate linkage located in the middle of the sequence d(TCTT-AA*CCCACA). As for P-substituted beta-oligonucleo-tides, a difference in binding behavior between the two diastereoisomers (difference in Delta T (m)) exists in the hybridization properties of alpha-analogs when DNA was the target but this effect was not detrimental to duplex stability. This effect was considerably reduced when RNA was the target. Secondly we studied the effect of steric hindrance around phosphorus on the affinity of fully modified beta- and alpha-oligonucleoside N -alkylphosphoramidates for their DNA and RNA targets. This effect was very weak with alpha-analogs whereas it was more pronounced with beta-oligos. PNHME-modified alpha-oligonucleosides formed more stable duplexes with DNA (Delta T (m)+9.6 degrees C) and RNA (Delta T (m)+1.4 degrees C) targets than the 'parent' phosphodiester. Finally, base pairing specificity of these alpha-oligonucleo-side N -alkylphosphoramidates for their targets was found to be as high as for natural oligonucleoside phosphodiesters.

Anomeric inversion (from beta to alpha) in methylphosphonate oligonucleosides enhances their affinity for DNA and RNA

Here we report that the poor binding of methylphosphonate oligodeoxynucleosides (MP-ODNs) to their nucleic acid targets can be improved by additional inversion of the anomeric configuration (from beta to alpha) in the sugar moieties to give a new class of analogs, MP alpha-oligonucleosides. MP alpha-dT12and MP 5' alpha-d(TCTTAACCCACA) 3' were synthesized and their ability to form hybrids with complementary single stranded (ss)DNA and ssRNA, as well as with double stranded (ds)DNA, was evaluated. The thermal stability of hybrids formed with MP alpha-analogs was compared with the affinity of phosphodiester (PO) and phosphorothioate (PS) beta- and alpha-oligomers for their targets. Non-ionic MP alpha-oligonucleosides bound to their complementary DNA and RNA strands more tightly than their homologues with natural beta-anomeric configuration did. With DNA target, MP alpha-oligomers formed duplexes more stable than the corresponding natural PO beta-oligomer did. MP alpha-heteropolymer hybridized to RNA target better than PS beta-oligonucleotide did but the hybrid was less stable (DeltaTm-0.5 degrees C per mod.) than the hybrid formed with the natural PO beta-oligomer. Only MP alpha-dT12 bound to dsDNA target at low salt concentration (0.1 M NaCl).

Esterase-triggered fluorescence of fluorogenic oligonucleotides

In the prooligonucleotide approach, a step of activation by cellular esterases is necessary for the removal of internucleoside phosphate masking groups and subsequent intracellular delivery of active antisense oligonucleotides. The efficacy of this approach implies that prooligonucleotides, once they are taken up by cells, are demasked by esterases during their course to their nucleic acid targets. In this regard, a method for labeling oligomers with esterase-activable fluorogenic tag was designed. The two phenolic functions of carboxyfluorescein were protected by pivaloyl groups, yielding a nonfluorescent lactone which was further activated as a N-hydroxysuccinimide ester. Two nuclease-resistant phosphorothioate 18-mer and methylphosphonate 19-mer oligodeoxynucleosides were attached to this biprotected fluorescein derivative via an amino linker at the 5'-end of the oligomers. The two conjugates were assayed for their carboxyesterase substrate ability in different biological media. In the presence of purified esterases or when incubated in serum or cell extracts, both oligonucleotide conjugates became fluorescent. In addition, the phosphorothioate oligoconjugate was microinjected into the cytoplasm of human fibroblasts, and a fast cytoplasmic release of fluorescence was observed with a rapid translocation of the fluorescent oligomer into the nucleus.

Chimeric alpha-beta oligonucleotides as antisense inhibitors of reverse transcription

Alpha-beta chimeric 17-mer oligodeoxyribonucleotides containing either 5, 10 or 15 beta nucleotides were synthesized. The stability of the RNA/chimera hybrids was only slightly affected by the alpha stretch and by the alpha-beta link, as was the affinity of the Moloney Murine Leukemia Virus reverse transcriptase for the duplexes. All chimeras inhibited in vitro cDNA synthesis in a cell-free system to various extent, via the degradation of the RNA target by RNase H.

Synthesis and base-pairing properties of the nuclease-resistant alpha-anomeric dodecaribonucleotide alpha-[r(UCUUAACCCACA)]

The non natural oligoribonucleotide alpha-[r(UCUUAACCCACA)] consisting exclusively of alpha-anomeric ribonucleoside units was synthesized according to the phosphoramidite methodology and the solid support technology. For this purpose, the base-protected alpha-ribonucleosides were synthesized and converted into their O-methylphosphoramidites. Assembling was carried out on a DNA synthesizer with an average efficiency of 97% per step. Base composition of this nuclease-resistant alpha-RNA strand was ascertained after chemical and enzymatic hydrolysis and HPLC analysis of the hydrolysate. Whereas no spectroscopic evidence of base pairing was found above 0 degrees C between alpha-[r(UCUUAACCCACA)] and beta-[d(TGTGGGTTAAGA)], a clear UV absorbance transition (Tm 25.5 degrees C) was observed during the hybridization of the same alpha-RNA strand with beta-[d(AGAATTGGGTGT)]. In this latter case, the mixing curve titration suggests formation at low temperature of a triplex involving two alpha-RNA and one beta-DNA strands. Moreover, this alpha-decaribonucleotide complementary in parallel orientation of the splice receptor of HIV-1 tat mRNA was found to inhibit (10 microM less than ED50 less than 20 microM), with apparent lack of sequence specificity, the de novo HIV-1 infection in cultured cells.

Synthesis, DNA binding, and biological evaluation of synthetic precursors and novel analogues of netropsin

A series of oligopeptides have been synthesized that are structurally related to the natural agent netropsin. The binding constants to double-stranded polynucleotides as well as the cytostatic activity against both murine human tumor cell lines and the in vitro activity against a range of DNA and RNA viruses have been determined for these novel compounds and some of their synthetic precursors. 1-Methyl-5-nitropyrrole-2-carboxylic acid methyl ester (4), N-[[1-methyl-4-(1-methyl-4-nitropyrrole-2-carboxamido)pyrrol-2- yl]carbonyl]-L-alanine tert-butyl ester (28), and N-[[1-methyl-4-(1-methyl-4-nitropyrrole-2-carboxamido)pyrrol-2- yl]carbonyl]-L-alanyl-L-alanine tert-butyl ester (29) showed modest inhibitory effect on tumor cell proliferation (CD50 = 26-85 micrograms/mL). Of all the compounds that were evaluated, 28 proved the most potent antiviral agent. It was inhibitory to parainfluenza-3 virus and Coxsackie virus B4 in Vero cells at a concentration of 20 micrograms/mL.

Structure and conformation of the duplex consensus 5'-splice site d[(CpApGpGpTpApApGpT).(ApCpTpTpApCpCpTpG)] deduced from high field 1H-NMR of the non-exchangeable and imino protons

The complementary consensus donor exon intron junction d(ApCpTpTpApCpCpTpG) has been synthesized by a solid phase procedure. The non-exchangeable proton assignments were obtained using one- and two-dimensional NMR techniques including NOE, COSY, NOESY and 1H-1H-INADEQUATE. The non self-complementary nonamer exists as a random coil form in aqueous buffer at 21 degrees C as evidenced by the temperature variable 1H-NMR and NOE measurements. The nonamer was annealed to the primary consensus donor junction d(CpApGpGpTpApApGpT) and confirmation of complete annealing was obtained by detection and assignment of base pair imino protons in D2O/H2O mixtures. Application of one- and two-dimensional NMR techniques permitted the complete assignment of all the non-exchangeable protons in the duplex nonamer. These data, together with determination of vicinal coupling constants in the individual deoxyribose moieties, permits the following conclusions on the structure and conformation of the consensus donor junction: (i) it exists in aqueous solution in a conformation that belongs to the B family (ii) the sugar-base orientations are anti (iii) the deoxyribose units exist predominantly in the S conformation (2'-endo-3'-exo) (iv) the contiguous A.T base pairs d[T(5)-A(6)-A(7)].d[T(12)-T(13)-A(14)], two positions removed downstream from the splice site (5'-CAG decreases GTAAGT-3'), are uniquely propeller twisted. The propeller twisting occurs in the region in which there is partial complementarity with the branch site splice signal TACTAAC. The cross-correlation rates were used to derive the interproton distances between adjacent AH2 protons of 4.00 A in the T(5)-A(6).T(13)-A(14) step and of 3.87 A in the A(6)-A(7).T(12)-T(13) step. This structural and conformational feature if carried over into the primary RNA transcripts may serve as a recognition signal for this critical site in the genome.

Structure and conformation of the duplex consensus acceptor exon:intron junction d[(CpTpApCpApGpGpT). (ApCpCpTpGpTpApG)] deduced from high-field 1H-NMR of non-exchangeable and imino protons

The complementary consensus acceptor exon:intron junction d(ApCpCpTpGpTpApG) has been synthesized by a modified phosphotriester method. The non self-complementary octamer exists in the random coil form in aqueous buffer at 20 degrees C as evidenced by temperature variable 1H-NMR and NOE measurements. The non-exchangeable proton assignments were secured using a combination of techniques including two-dimensional COSY, NOESY and 1H-1H-INADEQUATE. The octamer was annealed with the primary consensus sequence d(CpTpApCpApGpGpT). Confirmation of complete duplex formation was confirmed by detection and assignment of imino protons in D2O:H2O mixtures. Assignment of the non-exchangeable proton signals in the duplex consensus junction was then secured by a combination of two-dimensional COSY correlations, NOESY and NOE experiments. Determination of individual vicinal coupling constants in the component deoxyribose moieties permitted deduction of the population of S conformations in this sequence. It is concluded that the consensus acceptor junction exists in solution in a conformation belonging to the B family, and that the bases are oriented anti. In addition the deoxyribose moieties in the 5' regions exist predominantly in the S form (2'endo-3'exo) whereas those residues on or adjacent to the junction on the primary strand show more N character (2'exo-3'endo). The contiguous bases A5-G6 (adjacent to the junction) and A15-G16 are stacked more closely than the other neighbor bases in this duplex sequence. These subtle structural and conformational differences in the exon:intron junction may serve as recognition signals for these critical sites in the genome.

1H and 31P-NMR assignments of the non-exchangeable protons of the consensus acceptor exon:intron junction d(CpTpApCpApGpGpT)

The consensus acceptor exon:intron junction d(CpTpApCpApGpGpT) has been synthesized by a modified phosphotriester method. The non-self complementary octamer exists in the single strand form in aqueous buffer at 20 degrees C as evidenced by temperature variable 1H-NMR and NOE measurements. The non-exchangeable proton assignments were secured using a combination of techniques including two-dimensional COSY, NOESY and the double quantum technique 1H-1H-INADEQUATE as well as inversion recovery T1 experiments. The new technique of 31P-1H shift correlation is particularly valuable in removing certain ambiguities in the sugar proton assignments. Characteristic chemical shifts for the base protons which are determined by their immediate molecular environments are also useful in assignments. The consensus acceptor exon:intron junction adopts a random coil conformation in solution under the experimental conditions employed.