A biotin-conjugated pyridine-based isatoic anhydride, a selective room temperature RNA-acylating agent for the nucleic acid separation.

Isatoic anhydride derivatives, including a biotin and a disulfide linker were specifically designed for nucleic acid separation. 2'-OH selective RNA acylation, capture of biotinylated RNA adducts by streptavidin-coated magnetic beads and disulfide chemical cleavage led to isolation of highly enriched RNA samples from an initial 9/1 DNA-RNA mixture. Starting from the parent compound N-methylisatoic anhydride A which was used at 65 °C, we improved the extraction process by designing a new generation of isatoic anhydrides that are able to react under smoother conditions. Among them, a pyridine-based isatoic anhydride derivative 15f was found to be reactive at room temperature, leading to enhance the efficiency and selectivity of the extraction process by significantly reducing DNA side extraction. The extracted and purified RNAs can then be detected by RT-PCR.

Stalling of human DNA (cytosine-5) methyltransferase at single-strand conformers from a site of dynamic mutation.

Single-strand conformers (SSCs) from the C-rich strand of the triplet repeat at the FMR-1 locus are rapidly and selectively methylated by the human DNA (cytosine-5) methyltransferase. The apparent affinity of the enzyme for the FMR-1 SSC is about tenfold higher than it is for a control Watson-Crick paired duplex. The de novo methylation rate for the SSC is over 150-fold higher than the de novo rate for the control duplex. Methylation of what is generally called a hemi-methylated duplex occurs with a rate enhancement of over 100-fold, while methylation of what can be viewed as a hemi-methylated FMR-1 SSC is actually slower than the de novo rate. The pronounced inhibition of the methyltransferase by the methylated SSC suggests that the enzyme has a higher affinity for the methylated product of its reaction with the SSC than it has for the unmethylated SSC substrate. Gel retardation studies show that the methyltransferase binds selectively to SSCs from the C-rich strand of the FMR-1 triplet repeat. This suggests a two-step stalling process in which the human methyltransferase first selectively methlyates and subsequently stalls at the C-rich strand SSC. Stalling may reflect the inability of the enzyme to release a DNA product that is fixed in a conformation resembling its transition state by the unusual structure of the substrate. In particular, the data suggest that DNA methyltransferase may physically participate in biological processes that lead to dynamic mutation at FMR-1. In general, the data raise the possibility that a two-step stalling process occurs at secondary structures associated with chromosome instability, chromosome remodelling, viral replication or viral integration and may account for the local hypermethylation and global hypomethylation associated with viral and non-viral tumorigenesis.

Hypermethylation of telomere-like foldbacks at codon 12 of the human c-Ha-ras gene and the trinucleotide repeat of the FMR-1 gene of fragile X.

Runs of G residues on the G-rich strands of 30mers from the region spanning codon 12 of c-Ha-ras appear to be protected against chemical modification by dimethylsulfate. This suggests that the G-rich strand might spontaneously form a Hoogsteen-paired quadruplex, which is characteristic of telomere-like DNA sequences. In this report we show that the predominant species in 1:1 mixtures of complementary 30mers from this region are duplex DNA and a smaller amount of unimolecular foldback formed by the C-rich strand. Foldbacks of this type resemble structures first observed in the C-rich strand of telomeric DNA and also occur at the CCG triplet repeat present in the FMR-1 gene of human fragile X syndrome. Foldbacks from the C-rich strand of c-Ha-ras and the FMR-1 triplet repeat are exceptional substrates for the human methyltransferase in isolation. Substituting inosine for guanosine alters the secondary structure of the folded oligomers and dramatically reduces their ability to serve as substrates for the human methyltransferase, suggesting that secondary structure is required for recognition by the enzyme. These findings suggest that one mechanism by which methyl groups accumulate in the c-Ha-ras region of chromosome 11 during carcinogenesis and at the FMR-1 locus during repeat expansion at fragile X may be structurally induced de novo methylation at sites undergoing local conformational change. Such methylation might serve to mark unusual structures for repair. In the absence of repair, asymmetrically methylated duplexes produced by resolution of the unusual structures would be rapidly converted to symmetrically methylated duplexes through the methyl-directed activity also carried by the human methyltransferase.

The response of M.HpaII to heteroduplexes.

Synthetic oligodeoxyribonucleotide duplexes have been used to study the methylation specificity of M.HpaII, a bacterial DNA methyltransferase. Substrates of four types were compared. A 30-mer containing a Watson-Crick paired CCGG recognition sequence was rapidly methylated at the central cytosine on each strand in the recognition sequence. A 30-mer containing an asymmetrically methylated recognition sequence, of the type transiently produced by DNA replication, was rapidly methylated at the central cytosine on the unmethylated strand. A heteroduplex containing an A.C mispair in the recognition sequence (CCGG/CCAG) was rapidly methylated at the cytosine in the mispair. A heteroduplex containing an A.C and an adjacent C.C mispair in the recognition sequence (CCGG/CCCA) was not methylated at a significant rate. The results show that M.HpaII can tolerate a single mispair at its recognition site in a heteroduplex without loss of activity or specificity.

Aminothiols linked to quinoline and acridine chromophores efficiently decrease 7,8-dihydro-8-oxo-2'-deoxyguanosine formation in gamma-irradiated DNA.

In a search for more active radioprotective compounds, we have prepared and examined a series of model molecules in which the radioprotective beta-aminothiol unit (free or derivatized as acetate or phosphorothioate) is tethered to the DNA-binding chromophores quinoline and acridine through links of variable length. The modifying activity of these 'hybrid' molecules was estimated by measuring the formation of 8-oxo-2'-deoxyguanosine (8-oxodGuo) in double-strand DNA upon exposure to gamma-rays in oxygen-free solution in the presence of the drugs. We show that all hybrid molecules protect the guanine moiety from oxidation more efficiently than the parent beta-aminothiol units. The degree of protection is the highest for the molecules in which the thiol is linked to the strong binding intercalator acridine through a long polyaminochain.

Labeling during cleavage (LDC), a new labeling approach for RNA.

A new and efficient strategy for labeling of RNA sequences prior to their hybridization on high density DNA chip has been developed. Our approach which combines the fragmentation and the labeling is based on the reactivity of the 3'-phosphate of cleaved RNA fragments with a fluorescent molecule bearing aromatic bromomethyl function.

Universal labeling chemistry for nucleic acid detection on DNA-arrays.

We show here a new and efficient aqueous chemistry for labeling of any class of nucleic acids for their detection on DNA chip. The labels contain a diazo function as reactive moiety and biotin as detectable unit. The highly selective reaction of diazo group on the phosphate does not disrupt base pairing recognition and hybridization specificity.

Labeling during cleavage of nucleic acids for their detection on DNA chips.

A new and efficient strategy for labeling of nucleic acids prior to their hybridization on high density DNA chip has been developed. Our approach which combines the fragmentation and the labeling is based on the reactivity of the terminal phosphates of cleaved DNA and RNA fragments with a reporter molecule bearing aryldiazomethane group.

Biotin-conjugated N-methylisatoic anhydride: a chemical tool for nucleic acid separation by selective 2'-hydroxyl acylation of RNA.

An isatoic anhydride derivative conjugated to a biotin and a disulfide linker was specifically designed for the separation of nucleic acids. Starting from a DNA-RNA mixture, a selective 2'-hydroxyl acylation of RNAs followed by capture with streptavidin-coated magnetic beads and cleavage of the disulfide led to elution of RNAs.

Methylation of slipped duplexes, snapbacks and cruciforms by human DNA(cytosine-5)methyltransferase.

When human DNA(cytosine-5)methyltransferase was used to methylate a series of snapback oligodeoxy-nucleotides of differing stem lengths, each containing a centrally located CG dinucleotide recognition site, the enzyme required a minimum of 22 base pairs in the stem for maximum activity. Extrahelical cytosines in slipped duplexes that were 30 base pairs in length acted as effective methyl acceptors and were more rapidly methylated than cytosines that were Watson-Crick paired. Duplexes containing hairpins of CCG repeats in cruciform structures in which the enzyme recognition sequence was disrupted by a C.C mispair were also more rapidly methylated than control Watson-Crick-paired duplexes. Since enzymes have higher affinities for their transition states than for their substrates, the results with extrahelical and mispaired cytosines suggest that these structures can be viewed as analogs of the transition state intermediates produced during catalysis by methyltransferases.

Transition state analogs as affinity labels for human DNA methyltransferases.

A new class of affinity labels has been developed for human DNA (cytosine-5) methyltransferases. These oligodeoxynucleotides contain 5-fluorodeoxycytidine at a mispair within the recognition motif of the human enzyme. They were not effectively recognized by bacterial methyltransferases. They can be viewed as analogs of the intermediates transiently produced by methyltransferases during catalysis. Affinity labelling patterns suggest that both the structurally induced activity and the methyl-directed activity of the human enzymes operate by the same mechanism and reside on the same polypeptide chains.

DNA adduct 8-hydroxyl-2'-deoxyguanosine (8-hydroxyguanine) affects function of human DNA methyltransferase.

8-Hydroxyl-2'-deoxyguanosine (also referred to as 8-hydroxyguanine [8-OH-dG] or 7,8-dihydro-8-oxoguanine), a common DNA adduct resulting from injury to DNA via reactive oxygen species, affects the in vitro methylation of nearby cytosine moieties by the human DNA methyltransferase. The exact position of 8-OH-deoxyguanosine relative to a CpG dinucleotide appears important to this effect. Our data indicate that 8-OH-deoxyguanosine diminishes the ability of the methyltransferase to methylate a target cytosine when the 8-OH-deoxyguanosine is one or two nucleotides 3' from the cytosine, on the same strand. On the other hand 8-OH-deoxyguanosine does not diminish the ability of the enzyme to respond to a methyl director (5-methylcytosine) when the 8-OH-deoxyguanosine is on the same strand but one or two nucleotides 3' from the methyl director. Differences in methylation rates as great as 13-fold have been detected using various 8-OH-deoxyguanosine-containing oligonucleotides as substrates in methylation assays. Our findings suggest that oxidative damage of parental strand guanines would permit normal copying of methylation patterns through maintenance methylation, while oxidative damage of guanines in the nascent strand DNA would inhibit such methylation.

Hairpins are formed by the single DNA strands of the fragile X triplet repeats: structure and biological implications.

Inordinate expansion and hypermethylation of the fragile X DNA triplet repeat, (GGC)n.(GCC)n, are correlated with the ability of the individual G- and C-rich single strands to form hairpin structures. Two-dimensional NMR and gel electrophoresis studies show that both the G- and C-rich single strands form hairpins under physiological conditions. This propensity of hairpin formation is more pronounced for the C-rich strand than for the G-rich strand. This observation suggests that the C-rich strand is more likely to form hairpin or "slippage" structure and show asymmetric strand expansion during replication. NMR data also show that the hairpins formed by the C-rich strands fold in such a way that the cytosine at the CpG step of the stem is C.C paired. The presence of a C.C mismatch at the CpG site generates local flexibility, thereby providing analogs of the transition to the methyltransferase. In other words, the hairpins of the C-rich strand act as better substrates for the human methyltransferase than the Watson-Crick duplex or the G-rich strand. Therefore, hairpin formation could account for the specific methylation of the CpG island in the fragile X repeat that occurs during inactivation of the FMR1 gene during the onset of the disease.

Design of molecules that specifically recognize and cleave apurinic sites in DNA.

We have prepared a series of tailor-made molecules that recognize and cleave DNA at apurinic sites in vitro. These molecules incorporate in their structure different units designed for specific function: an intercalator for DNA binding, a nucleic base for abasic site recognition and a linking chain of variable length and nature (including amino and/or amido functions). The cleavage efficiency of the molecules can be modulated by varying successively the nature of the intercalating agent, the nucleic base and the chain. All molecules bind to native calf thymus DNA with binding constants ranging from 10(4) to 10(6) M-1. Their cleavage activity was determined on plasmid DNA (pBR 322) containing 1.8 AP-sites per DNA-molecule. The minimum requirements for cleavage are the presence of the three units, the intercalator, the nucleic base and at least one amino function in the chain. The most efficient molecules cleave plasmid DNA at nanomolar concentrations. Enzymatic experiments on the termini generated after cleavage of AP-DNA suggest a strand break induced by a beta-elimination reaction. In order to get insight into the mode of action (efficiency, selectivity, interaction), we have used synthetic oligonucleotides containing either a true abasic site at a determined position to analyse the cleavage parameters of the synthetic molecules by HPLC or a chemically stable analog (tetrahydrofuran) of the abasic site for high field 1H NMR spectrometry and footprinting experiments. All results are consistent with a beta-elimination mechanism in which each constituent of the molecule exerts a specific function as indicated in the scheme: DNA targeting, abasic site recognition, phosphate binding and beta-elimination catalysis.(ABSTRACT TRUNCATED AT 250 WORDS)