[Interaction of DNA Methyltransferase Dnmt3a with Phosphorus Analogs of S-Adenosylmethionine and S-Adenosylhomocysteine].

Enzymatic methyltransferase reactions are of crucial importance for cell metabolism. S-Adenosyl-L-methionine (AdoMet) is a main donor of the methyl group. DNA, RNA, proteins, and low-molecular-weight compounds are substrates of methyltransferases. In mammals, DNA methyltransferase Dnmt3a de novo methylates the C5 position of cytosine residues in CpG sequences in DNA. The methylation pattern is one of the factors that determine the epigenetic regulation of gene expression. Here, interactions with the catalytic domain of Dnmt3a was for the first time studied for phosphonous and phosphonic analogs of AdoMet and S-adenosyl-L-homocysteine (AdoHcy), in which the carboxyl group was substituted for respective phosphorus-containing group. These AdoMet analogs were shown to be substrates of Dnmt3a, and the methylation efficiency was only halved as compared with that of natural AdoMet. Both phosphorus-containing analogs of AdoHcy, which is a natural methyltransferase inhibitor, showed similar inhibitory activities toward Dnmt3a and were approximately four times less active than AdoHcy. The finding that the phosphonous and phosphonic analogs are similar in activity was quite unexpected because the geometry and charge of their phosphorus-containing groups differ substantially. The phosphorus-containing analogs of AdoMet and AdoHcy are discussed as promising tools for investigation of methyltransferases.

The Effect of Antitumor Antibiotic Olivomycin A and Its New Semi-synthetic Derivative Olivamide on the Activity of Murine DNA Methyltransferase Dnmt3a.

Olivomycin A is a highly active antitumor drug that belongs to the family of aureolic acid antibiotics. The antitumor effect of olivomycin A is related to its ability to bind to the DNA minor groove in GC-rich regions as Mg2+-coordinated complexes. Characterization of cellular targets of olivomycin A and its mechanism of action is crucial for the successful application of this antibiotic in clinical practice and development of semi-synthetic derivatives with improved pharmacological properties. Previously, we have shown that minor groove ligands are able to disrupt the key epigenetic process of DNA methylation. In this paper, we have studied the impact of olivomycin A and its improved semi-synthetic analogue N,N-dimethylaminoethylamide of 1'-des-(2,3-dihydroxy-n-butyroyl)-1'-carboxy-olivomycin A (olivamide) on the functioning of de novo DNA methyltransferase Dnmt3a (enzyme that carries out methylation of cytosine residues in the DNA CG-sites in eukaryotic cells) using an in vitro system consisting of the murine Dnmt3a catalytic domain and a 30-mer DNA duplex containing four consecutive GC pairs. We have shown that olivomycin A and olivamide inhibit Dnmt3a with IC50 of 6 ± 1 and 7.1 ± 0.7 µM, respectively. Neither olivomycin A nor olivamide interfered with the formation of the specific enzyme-substrate complex; however, olivomycin A prevented formation of the covalent DNA-Dnmt3a intermediate that is necessary for the methylation reaction to proceed. The inhibitory effects of olivomycin A and olivamide can be explained by the disruption of the enzyme catalytic loop movement through the DNA minor groove (the reaction stage that precedes the covalent bond formation between DNA and the enzyme). The results of this work indicate the epigenetic contribution to the antitumor effect of aureolic acid group antibiotics.

[Detection of DNA Methylation by Dnmt3a Methyltransferase using Methyl-Dependent Restriction Endonucleases].

DNA methylation at cytosine residues in CpG sites by DNA methyltransferases (MTases) is associated with various cell processes. Eukaryotic MTase Dnmt3a is the key enzyme that establishes the de novo methylation pattern. A new in vitro assay for DNA methylation by murine MTase Dnmt3a was developed using methyl-dependent restriction endonucleases (MD-REs), which specifically cleave methylated DNA. The Dnmt3a catalytic domain (Dnmt3a-CD) was used together with KroI and PcsI MD-REs. The assay consists in consecutive methylation and cleavage of fluorescently labeled DNA substrates, then the reaction products are visualized in polyacrylamide gel to determine the DNA methylation efficiency. Each MD-RE was tested with various substrates, including partly methylated ones. PcsI was identified as an optimal MD-RE. PcsI recognizes two methylated CpG sites located 7 bp apart, the distance roughly corresponding to the distance between the active centers of the Dnmt3a-CD tetramer. An optimal substrate was designed to contain two methylated cytosine residues and two target cytosines in the orientation suitable for methylation by Dnmt3a-CD. The assay is reliable, simple, and inexpensive and, unlike conventional methods, does not require radioactive compounds. The assay may be used to assess the effectiveness of Dnmt3a inhibitors as potential therapeutic agents and to investigate the features of the Dnmt3a-CD function.

[Dimeric bisberzimidazoles: cytotoxicity and effects on DNA methylation in normal and cancer human cells].

Cancer cells are characterized by the hypermethylation of promoter regions of tumor suppressor genes. DNA methyltransferase inhibitors cause re-activation of these genes that allows considering DNA methyltransferases as targets for anticancer therapy. As it was previously shown by us, dimeric bisbenzimidazoles, DB(n), differing in length of the oligomethylene linker between the two bisbenzimidazole fragments (n--number of methylene groups in linker) effectively inhibit the methylation of DNA duplexes by murine methyltransferase Dnmt3a. Here, the cytotoxicity of some of these compounds, their penetration into cells and influence on the methylation of genomic DNA in fetal lung fibroblasts line F-977 and cervical cancer cells HeLa have been studied. In the 0-60 microM concentration range, only the DB(11) displayed a significant toxic effect on the normal cells, whereas the effect of DB(n) investigated on the cancer cells was not significant. Interestingly, the DB(1) and DB(3) to a small extent stimulate the proliferation of HeLa and F-977 cells, respectively. DB(1) and DB(3) display ability to penetrate into the nucleus of HeLa and F-977 cells and accumulate in various parts of the nuclei. DB(11) is not able to penetrate into the nuclei of these cells. The incubation of F-977 cells with 26 microM of DB(1) or DB(3) led to a decrease of the methylation of 18S rRNA gene, which is located in the region of DB(1) and DB(3) accumulation. A similar effect produces the same concentration of DB (3) in the F-977 cells. However, the overall level of genomic DNA methylation was not changed. These data suggest that DB(n) can be directed to act on specific genes demethylation and in the future may selectively inhibit the proliferation of cancer cells.

Dimeric bisbenzimidazoles inhibit the DNA methylation catalyzed by the murine Dnmt3a catalytic domain.

When located in the DNA minor groove, dimeric bisbenzimidazoles DB(n) effectively inhibited in vitro the Dnmt3a catalytic domain (IC50 5-77 µM). The lowest IC50 value was observed for compound DB(11) with an 11-unit methylene linker joining the bisbenzimidazole fragments. Increased time of incubation of DNA with DB(n) as well as the presence of AT-clusters in DNA enhances the inhibitory effect.

[DNA sequence-specific ligands: XIV. Synthesis of fluorescent biologicaly active dimeric bisbenzimidazoles-DB(3, 4, 5, 7, 11)].

Five fluorescent symmetric dimeric bisbenzimidazoles DB(n) have been synthesized containing four 2,6-substited benzimidazole fragments and differ in length of oligomethylene linker (n=3, 4, 5, 7, 11) between the two bisbenzimidazole blocks. The ability of these dimeric bisbenzimidazoles to form complexes with double-stranded DNA (dsDNA) was shown by spectral methods. Upon binding to dsDNA DB(n) are localized in the minor groove. The DNA-methyltransferase Dnmt3a inhibition data are demonstrate the site-specific binding of dimeric bisbenzimidazoles DB(3) and DB(11) with oligonucleotide duplex.

Interaction of murine dnmt3a with DNA containing o6-methylguanine.

O(6)-Methylguanine (O(6)meG) is one of the most toxic, mutagenic, and carcinogenic lesions caused by the interaction of DNA with several catabolism products as well as with environmental methylating agents. Carcinogenic impact of O(6)meG can be conditioned not only by its mutagenic properties but also by alteration in enzymatic methylation of the C5 carbon atom of cytosine residue in CpG sequences. In this study, the effect of O(6)meG on DNA methylation by the catalytic domain of murine DNA methyltransferase (MTase) Dnmt3a (Dnmt3a-CD) is assessed. Damaged DNA duplexes cooperatively bind with Dnmt3a-CD, and O(6)meG changes the stability of enzyme-substrate complexes. Kinetic analysis of the methylation reaction revealed that O(6)meG varies the ratio of productive and nonproductive enzyme-substrate complexes and, depending on localization in substrate, causes decrease or increase in DNA methylation. Dnmt3a-CD is less sensitive to the presence of O(6)meG in DNA substrate than procaryotic MTase SssI recognizing CpG.

Inhibition of murine DNA methyltransferase Dnmt3a by DNA duplexes containing pyrimidine-2(1H)-one.

Here we studied the inhibition of the catalytic domain of Dnmt3a methyltransferase (Dnmt3a-CD) by DNA duplexes containing the mechanism-based inhibitor pyrimidine-2(1H)-one (P) instead of the target cytosine. It has been shown that conjugates of Dnmt3a-CD with P-DNA (DNA containing pyrimidine-2(1H)-one) are not stable to heating at 65°C in 0.1% SDS. The yield of covalent intermediate increases in the presence of the regulatory factor Dnmt3L. The importance of the DNA minor groove for covalent intermediate formation during the methylation reaction catalyzed by Dnmt3a-CD has been revealed. P-DNA was shown to inhibit Dnmt3a-CD; the IC(50) is 830 nM. The competitive mechanism of inhibition of Dnmt3a-CD by P-DNA has been elucidated. It is suggested that therapeutic effect of zebularine could be achieved by inhibition of not only Dnmt1 but also Dnmt3a.

Inhibition of C5-cytosine-DNA-methyltransferases.

Changes in the methylation pattern of genomic DNA, particularly hypermethylation of tumor suppressor genes, occur at early stages of tumor development. Errors in DNA methylation contribute to both initiation and progression of various cancers. This stimulates significant interest in searching for inhibitors of C5-DNA-methyltransferases (MTases). Here we review the known nucleoside mechanism-based reversible and irreversible inhibitors of the MTases, as well as non-nucleoside ones, and discuss their inhibitory mechanisms and application for MTase investigations and cancer therapy.

[Probing of contacts between EcoRII DNA methyltransferase and DNA using substrate analogs and molecular modeling].

To determine the molecular mechanism of DNA recognition and catalysis by EcoRII DNA-methyltransferase (M.EcoRII) binding and methylation by the enzyme of 14-mer substrate analogs containing 2-aminopurine or 1',2'-dideoxy-D-ribofuranose in the M.EcoRII recognition site have been studied. Efficiencies of methylation and DNA binding affinities depend on the location of modified nucleoside residues within the M.EcoRII recognition site. A structural model of M.EcoRII in complex with substrate DNA and cofactor analog S-adenosyl-L-homocysteine (AdoHcy) was built using the previously solved structures of Hhal and HaeIII DNA-methyltransferases as templates. The model was constructed according to the recently developed "Frankenstein's monster" approach. Based on the model, amino acid residues taking part in interactions with DNA were predicted. Besides, based on both theoretical and experimental data obtained the groups of atoms of the heterocyclic bases within the M.EcoRII recognition site presumably involved in interaction with the enzyme were proposed.

[Purification and site-directed mutagenesis of DNA methyltransferase SssI].

Prokaryotic DNA methyltransferase SssI (M.SssI) methylates C5 position of cytosine residue in CpG sequences. To obtain functionally active M.SssI and its mutants as His6-tagged proteins, bacterial strains have been produced. To test a possible role of Ser300 in recognition of CpG site by this enzyme, M.SssI mutants containing Ser300 replacements with Gly or Pro have been obtained. These replacements have practically no effect on DNA binding and methylation by M.SssI except small disturbance of DNA binding affinity in the case of S300P mutant. It indicates that there are no interactions of both the side chain and, probably, the main chain of Ser300 with DNA. A replacement of highly conserved Va1188 residue with Ala has been performed. Vall88 may participate in the stabilization of the flipped target cytosine during reaction. The replacement results in a 5-fold decrease of dissociation constant of the enzyme-substrate complex and a 2-fold decrease of initial velocity of DNA methylation. Though there are no noticeable differences in the functioning of the mutant in comparison with the wild-type enzyme, the formation of contact between Val 188 and cytosine could not be excluded. In the case of V 188A mutant the contact may be probably formed between Ala and cytosine residue.

Oligonucleotide cleavage by restriction endonucleases MvaI and EcoRII: a comprehensive study on the influence of structural parameters on the enzyme-substrate interaction.

To elucidate the mechanism of action of the restriction endonucleases--isoschizomers EcoRII and MvaI--a study was made of their interaction with a set of synthetic oligonucleotide duplexes containing a single 5'-d(CCA/TGG)-3' EcoRII (MvaI) recognition site. The substrates had varying length and structure of the nucleotide sequences flanking the recognition site. The structure of the flanking sequence is important for the cleavage by EcoRII and MvaI enzymes; there is a structure which was found to speed up the EcoRII and MvaI action. The cleavage of oligonucleotide duplexes by EcoRII enzyme does not go to completion. EcoRII endonuclease cleaved extended substrates less efficiently than short ones. Extension of the flanking sequences, with the same nucleotide surrounding of the recognition site, substantially altered the whole kinetic pattern of MvaI hydrolysis. This was not observed with EcoRII enzyme. The restriction endonuclease MvaI distinguished between dA and dT residues in the recognition site, which was reflected in the higher rate of hydrolysis of the dA-containing strand of the quasi-palindromic DNA duplex.

Chemical cross-linking of MvaI and EcoRII enzymes to DNA duplexes containing monosubstituted pyrophosphate internucleotide bond.

DNA duplexes containing a monosubstituted pyrophosphate internucleotide group, instead of a phosphodiester bond, were used as cross-linking reagent for the affinity modification of the restriction endonucleases EcoRII and MvaI (R.EcoRII and R.MvaI). An active group was introduced into the enzyme's recognition site or between the recognition site and flanking sequence. The substrate properties of such DNA duplexes were determined. Cross-linking specificity was demonstrated by competition experiments with unmodified substrate, as well as by the absence of cross-linking to an active duplex lacking a recognition site. It was shown that the nucleophilicity of the buffer solution and the presence of the enzyme cofactor Mg2+ dramatically affected the cross-linking yield.

Interaction of the MvaI and SsoII methyltransferases with DNAs altered at the central base pair of the recognition sequence.

The interaction of the MvaI and SsoII DNA methyltransferases (MTases; M.MVaI and M.SsoII, respectively) with a set of synthetic DNA duplexes, containing a M.MvaI and M.SsoII recognition site (CCWGG), was investigated. In these DNA duplexes dA or dT of the recognition site was replaced by nucleoside analogs with modified sugar moieties and heterocyclic bases (2'-deoxy-2'-fluorouridine (flU), 1-(beta-D-2'-deoxy-threo-pentofuranosyl)thymine (xT), 1-(beta-D-3'-deoxy-threo-pentofuranosyl)uracil (tU)), or by 1,3-propanediol (Prd). A new approach for monitoring methylation of each strand of DNA duplexes by MTases was developed. It allowed the determination of the influence of the modification in one DNA strand on the methylation of the other. In most cases, for both M.MvaI and M.SsoII, sugar analog-containing duplexes showed inhibition of methylation of only the modified strand. Prd-containing DNA duplexes were not substrates for M.MvaI. M.SsoII did not methylate DNA duplexes in which the dT residue was replaced by Prd.

DNA duplexes containing methylated bases or non-nucleotide inserts in the recognition site are cleaved by restriction endonuclease R.EcoRII in presence of canonical substrate.

DNA duplexes containing the natural methylated bases N6-methyladenine (m6Ade), N4-methylcytosine (m4Cyt) or C5-methylcytosine (m5Cyt) in one strand of the recognition sequence are resistant to EcoRII restriction endonuclease (R.EcoRII). Hydrolysis of these modified duplexes was observed in the presence of the canonical substrate. Incorporation of m4Cyt or m5Cyt into both strands of the recognition sequence precludes such activation by a canonical substrate. R.EcoRII also fails to cleave substrate analogs in which one of the nucleosides in the recognition site is replaced by the 1,2-dideoxyribose (D) or by 1,3-propanediol (Prd) (modeling DNA with an abasic site). The hydrolysis of DNA duplexes with non-nucleotide inserts is also activated in the presence of canonical substrate. Thus, the two-substrate mechanism of EcoRII-DNA interaction allows hydrolysis of apurinic/apyrimidinic and hemimethylated DNA.

Modified substrates as probes for studying uracil-DNA glycosylase.

In order to study the mechanism of action of uracil-DNA glycosylase (UDG) from human placenta, single-stranded (ss) and double-stranded (ds) oligodeoxyribonucleotides (oligos), containing deoxyuridine (dU) and a wide variety of their analogs were used. It was shown that UDG has a twofold preference for ss oligos over ds oligos and a twofold preference for intermolecular duplexes over similar hairpin-like duplexes. The replacement of dU with 1-(beta-D-2'-deoxy-threo-pentofuranosil)uracil (xU) or 1-(beta-D-3'-deoxy-threo-pentofuranosil)uracil (tU), which results in a change in sugar hydroxyl configuration, has no influence on UDG binding to such substrates, but inhibits uracil removal. A oligo containing 2'-deoxy-2'-fluorouridine (flU), with a 3'-endo conformation of modified sugar is recognized by UDG 100-200-fold less efficiently than the natural ones. F or Br atoms or a methyl group were introduced at position 5 of a dU residue in an oligo. It was shown that the nature of a substituent at this position is essential for UDG function.

EcoRII endonuclease has two identical DNA-binding sites and cleaves one of two co-ordinated recognition sites in one catalytic event.

EcoRII is a typical restriction enzyme that cleaves DNA using a two-site mechanism. EcoRII endonuclease is unable to cleave DNA which contains a small number of EcoRII recognition sites but the enzyme activity can be stimulated in the presence of DNA with a high frequency of EcoRII sites. To investigate the mechanism of activation, the kinetics of stimulated EcoRII cleavage has been studied. A 14 bp substrate activated the cleavage of the 71 bp substrate, containing one EcoRII recognition site (trans-activation) by a competitive mechanism: the activator increased substrate binding but not catalysis. The activation increased if the substrate concentration decreased and if the activator had a lower affinity for the enzyme than the substrate. The introduction of the second recognition site into the 71 bp duplex also enabled cleavage of this substrate (cis-activation). Pyrophosphate bonds were incorporated into one of two recognition sites to switch off the cleavage of the phosphodiester bonds. Analysis of cleavage products of these modified substrates showed that EcoRII cuts one of two coordinated recognition sites in one catalytic event.

Cross-linking of SsoII restriction endonuclease to cognate and non-cognate DNAs.

Specific and non-specific interactions of SsoII restriction endonuclease (R.SsoII) were probed by the method of covalent attachment to modified DNA containing an active monosubstituted pyrophosphate internucleotide bond instead of a phosphodiester one. R.SsoII with six N-terminal His residues was shown to be cross-linked to duplexes with this type of modification, either containing or not the recognition sequence. Competition experiments with covalent attachment of R.SsoII to activated DNAs demonstrated the similar affinity of the enzyme to cognate and non-cognate DNAs in the absence of cofactor, Mg2+ ions.

The Ecl18kI restriction-modification system: cloning, expression, properties of the purified enzymes.

Ecl18kI is a type II restriction-modification system isolated from Enterobacter cloaceae 18kI strain. Genes encoding Ecl18kI methyltransferase (M.Ecl18kI) and Ecl18kI restriction endonuclease (R.Ecl18kI) have been cloned and expressed in Escherichia coli. These enzymes recognize the 5'.../CCNGG...3' sequence in DNA; M.Ecl18kI methylates the C5 carbon atom of the inner dC residue and R.Ecl18kI cuts DNA as shown by the arrow. The restriction endonuclease and the methyltransferase were purified from E. coli B834 [p18Ap1] cells to near homogeneity. The restriction endonuclease is present in the solution as a tetramer, while the methyltransferase is a monomer. The interactions of M.Ecl18kI and R.Ecl18kI with 1,2-dideoxy-D-ribofuranose containing DNA duplexes were investigated. The target base flipping-out mechanism is applicable in the case of M.Ecl18kI. Correct cleavage of the abasic substrates by R.Ecl18kI is accompanied by non-canonical hydrolysis of the modified strand.

Interaction of EcoRII restriction and modification enzymes with synthetic DNA fragments. EcoRII endonuclease cleavage of substrates with repeated natural and modified recognition sites.

Interaction of EcoRII restriction endonuclease with a set of synthetic concatemer DNA duplexes with natural and modified sites for this enzyme has been studied. DNA duplexes with repeated natural sites are cleaved by EcoRII. Substitution of central AT-pair in the recognition site for a non-complementary TT-or AA-pair reduces the rate of cleavage, this effect being much more pronounced in the last case. Absence of site flanking in one strand from the 5'-terminus also results in very slow cleavage. The results obtained testify to the interaction of EcoRII with both strands of the substrate.