Methylation of adenosine in strongly alkaline medium: Preparation and Properties of o'-methyl derivatives of adenosine and N6-methyladenosine

A practical and scalable synthesis of several base modified 3'-O-methyl ribonucleosides

An easy and efficient large-scale synthesis of 1, 2,-di-O-acetyl-5-O-benzoyl-3-O-methyl-d-ribofuranose (8) was accomplished from commercial 1,2:5,6-di-O-isopropylidene-α-d-allofuranose in 7-steps and 30 % overall yield. The utility of protected 8 was demonstrated via synthesis of 9-(3'-O-methyl-β-d-ribofuranosyl)-6-chloropurine (21) and six other nucleoside analogues in good yields. A library of five novel base modified nucleosides were generated starting from purine nucleoside 21 via functional group manipulations. The 3'-O-modified nucleosides are known to act as chain terminator exerting antiviral activity. The synthesis strategy described herein offers direct access to 3'-O-alkylated nucleosides with wide range of applications, including cap analogues for mRNA vaccine production. This protocol provides a route to exclusive synthesis of 3'-O-alkylated nucleosides, devoid of isomeric 2'-O-alkylated products essential for both therapeutic and biological research.

Elevated Adenosine Induces Placental DNA Hypomethylation Independent of A2B Receptor Signaling in Preeclampsia

Preeclampsia is a prevalent pregnancy hypertensive disease with both maternal and fetal morbidity and mortality. Emerging evidence indicates that global placental DNA hypomethylation is observed in patients with preeclampsia and is linked to altered gene expression and disease development. However, the molecular basis underlying placental epigenetic changes in preeclampsia remains unclear. Using 2 independent experimental models of preeclampsia, adenosine deaminase-deficient mice and a pathogenic autoantibody-induced mouse model of preeclampsia, we demonstrate that elevated placental adenosine not only induces hallmark features of preeclampsia but also causes placental DNA hypomethylation. The use of genetic approaches to express an adenosine deaminase minigene specifically in placentas, or adenosine deaminase enzyme replacement therapy, restored placental adenosine to normal levels, attenuated preeclampsia features, and abolished placental DNA hypomethylation in adenosine deaminase-deficient mice. Genetic deletion of CD73 (an ectonucleotidase that converts AMP to adenosine) prevented the elevation of placental adenosine in the autoantibody-induced preeclampsia mouse model and ameliorated preeclampsia features and placental DNA hypomethylation. Immunohistochemical studies revealed that elevated placental adenosine-mediated DNA hypomethylation predominantly occurs in spongiotrophoblasts and labyrinthine trophoblasts and that this effect is independent of A2B adenosine receptor activation in both preeclampsia models. Extending our mouse findings to humans, we used cultured human trophoblasts to demonstrate that adenosine functions intracellularly and induces DNA hypomethylation without A2B adenosine receptor activation. Altogether, both mouse and human studies reveal novel mechanisms underlying placental DNA hypomethylation and potential therapeutic approaches for preeclampsia.

Substrate/inhibitor properties of human deoxycytidine kinase (dCK) and thymidine kinases (TK1 and TK2) towards the sugar moiety of nucleosides, including O'-alkyl analogues

Nucleoside analogues with modified sugar moieties have been examined for their substrate/inhibitor specificities towards highly purified deoxycytidine kinase (dCK) and thymidine kinases (tetrameric high-affinity form of TK1, and TK2) from human leukemic spleen. In particular, the analogues included the mono- and di-O'-methyl derivatives of dC, dU and dA, syntheses of which are described. In general, purine nucleosides with modified sugar rings were feebler substrates than the corresponding cytosine analogues. Sugar-modified analogues of dU were also relatively poor substrates of TK1 and TK2, but were reasonably good inhibitors, with generally lower Ki values vs TK2 than TK1. An excellent discriminator between TK1 and TK2 was 3'-hexanoylamino-2',3'-dideoxythymidine, with a Ki of approximately 600 microM for TK1 and approximately 0.1 microM for TK2. 3'-OMe-dC was a superior inhibitor of dCK to its 5'-O-methyl congener, consistent with possible participation of the oxygen of the (3')-OH or (3')-OMe as proton acceptor in hydrogen bonding with the enzyme. Surprisingly alpha-dT was a good substrate of both TK1 and TK2, with Ki values of 120 and 30 microM for TK1 and TK2, respectively; and a 3'-branched alpha-L-deoxycytidine analogue proved to be as good a substrate as its alpha-D-counterpart. Several 5'-substituted analogues of dC were good non-substrate inhibitors of dCK and, to a lesser extent, of TK2. Finally, some ribonucleosides are substrates of the foregoing enzymes; in particular C is a good substrate of dCK, and 2'-OMe-C is an even better substrate than dC.

Chemical Synthesis of a 5'-Terminal TMG-Capped Triribonucleotide m(3)(2,2,7)G(5)(')pppAmpUmpA of U1 RNA

The 5'-terminal TMG-capped triribonucleotide, m(3)(2,2,7)G(5)(')pppAmpUmpA, has been synthesized by condensation of an appropriately protected triribonucleotide derivative of ppAmpUmpA with a new TMG-capping reagent. During this total synthesis, it was found that the regioselective 2'-O-methylation of 3',5'-O-(1,1,3,3-tetraisopropyldisiloxane-1,3-diyl)-N-(4-monomethoxytrityl)adenosine was achieved by use of MeI/Ag(2)O without affecting the base moiety. A new route to 2-N,2-N-dimethylguanosine from guanosine via a three-step reaction has also been developed by reductive methylation using paraformaldehyde and sodium cyanoborohydride. These key intermediates were used as starting materials for the construction of a fully protected derivative of pAmpUmpA and a TMG-capping reagent of Im-pm(3)(2,2,7)G. The target TMG-capped tetramer, m(3)(2,2,7)G(5)(')pppAmpUmpA, was synthesized by condensation of a partially protected triribonucleotide 5'-terminal diphosphate species, ppA(MMTr)mpUmpA, with Im-pm(3)(2,2,7)G followed by treatment with 80% acetic acid. The structure of m(3)(2,2,7)G(5)(')pppAmpUmpA was characterized by (1)H and (31)P NMR spectroscopy as well as enzymatic assay using snake venom phosphodiesterase, calf intestinal phosphatase, and nuclease P1.

Tautomerism and conformation of the promutagenic analogue N6-methoxy-2',3',5'-tri-O-methyladenosine

N6-Methoxy-2',3',5'-tri-O-methyladenosine crystallizes in space group P2(1)2(1)2(1) with cell dimensions a = 4.693, b = 11.412, c = 31.741 A. Least-squares refinement of diffractometer data converged at R = 0.038. The location of a hydrogen atom at N1 and the observed bond lengths and bond angles indicate unequivocally the imino tautomer of the adenine moiety. The N6-methoxy group is oriented syn to N1 and the glycosidic torsion angle XCN is -3.6 degrees, i.e. in the anti range. The furanose ring has a C2'-exo/C3'- endo pucker (P = 0.9 degrees) and is unusually flattened (tau m = 30.0 degrees). The conformations of the O-methyl groups of the ribose ring are compared with those of monomethylated nucleosides, including the biologically important 2'-O-methyl nucleosides. Evidence is presented for the existence of C-H ... N intermolecular hydrogen bonds between adenine moieties. Bearing in mind that N6-methoxyadenosine is a promutagenic analogue, the results are compared with those for the corresponding promutagenic N4-methoxycytidine. They are also discussed in relation to the tautomerism, the conformation of the N6-methoxy group, and the associated base-pairing abilities in the absence and presence of polymerases.

Structure of the planar complex of N4-methoxycytosine with adenine, and its relevance to the mechanism of hydroxylamine mutagenesis

Infrared, and 1H- and 13C-NMR spectroscopy has been applied to a study of the planar interaction in apolar media between 1-substituted N4-methoxycytosine (and the corresponding 5-methyl analogue) and 9-substituted adenines. In both chloroform and carbon tetrachloride solutions, the exocyclic N4-methoxy group of N4-methoxycytosine, and of 5-methyl N4-methoxycytosine (which is in the oxime form under these conditions), is so oriented that it is predominantly syn to the ring N(3), and neither compound forms planar auto-associates. In chloroform solution, both the N(3)-H and the C(2) = O interact weakly with the solvent, the interaction being of the nature of non-typical hydrogen bonding. The 13C-NMR chemical shift of the C(2) of N4-methoxycytosine is modified during formation of hetero-associates with 9-substituted adenine, in accordance with the C(2) = O of the former being the acceptor of an adenine amino proton. The resulting planar hetero-associate is a non-Watson-Crick type of base-pair. This was further substantiated by infrared absorption studies of the carbonyl frequency during complex formation. The results are examined in the light of the mechanism of hydroxylamine (and methoxyamine) mutagenesis.

Hydroxylamine mutagenesis: observation of inverted Watson-Crick base-pairing between N4-methoxycytosine and adenine with the aid of natural-abundance high-resolution 15N NMR spectroscopy

Natural-abundance high-resolution 15N NMR spectra have been recorded for 1-methyl-N4-methoxycytosine (the product of reaction of 1-methylcytosine with the mutagen methoxyamine), 2',3',5'-tri-O-methyladenosine, and a 1:1 mixture of the two, all in chloroform solution. The spectra were obtained by application of a sequence of impulses of the type referred to as insensitive nuclear enhancement by polarization transfer (INEPT). Assignments of all the nitrogen resonances were based on nitrogen-proton coupling constants and literature data. Analysis of the 15N chemical shifts demonstrated the formation of base pairs with hydrogen bonds involving the ring N(1) and an N6-H of adenine, and the N(3)-H and C(2) = O of N4-methoxycytosine, viz. N(1) . H-N(3) and N6-H . O2 = C. The hydrogen-bond-stabilized complex formed thus involves an inverse type of Watson-Crick base-pairing. The data excluded formation of Hoogsteen-type base-pairing. The results are considered in relation to hydroxylamine and methoxyamine mutagenesis, to types of base-pairing at the monomer and polymer levels, and to interpretations of 15N NMR spectra of heterocyclic rings in general.