Igor P. Smirnov

High level multiplex genotyping by MALDI-TOF mass spectrometry

A primer extension assay is used to perform highly multiplexed genotyping of single nucleotide polymorphisms (SNPs) present in genomic DNA amplified by a multiplex PCR. The assay uses matrix-assisted laser desorption ionization time-of-flight mass spectrometry to accurately measure the masses of short oligonucleotide primers extended by a single dideoxynucleotide. The multiplexed genotyping assays rely on the natural molecular weight differences of DNA bases. By careful analysis of primer composition complementary to the target, or by judicious addition of one or more noncomplementary 5´ bases to the genotyping primers, mass spectra of interleaved genotyping products can be generated with no ambiguity in allele assignment. Using a model multiplex PCR system, we demonstrate the ability to perform 12-fold multiplex SNP analysis.

Synthesis of oligonucleotides containing 2′-azido- and 2′-amino-2′-deoxyuridine using phosphotriester chemistry☆

Abstract The phosphotriester method of oligonucleotide synthesis is shown to be useful for assembly of oligomers containing multiple internal 2′-azido substitutents. Subsequent reduction of the azido group(s) with trialkylphosphines provides 2′-amino substituted oligonucleotides.

Comparison of the 'chemical' and 'structural' approaches to the optimization of the thrombin-binding aptamer.

Noncanonically structured DNA aptamers to thrombin were examined. Two different approaches were used to improve stability, binding affinity and biological activity of a known thrombin-binding aptamer. These approaches are chemical modification and the addition of a duplex module to the aptamer core structure. Several chemically modified aptamers and the duplex-bearing ones were all studied under the same conditions by a set of widely known and some relatively new methods. A number of the thrombin-binding aptamer analogs have demonstrated improved characteristics. Most importantly, the study allowed us to compare directly the two approaches to aptamer optimization and to analyze their relative advantages and disadvantages as well as their potential in drug design and fundamental studies.

G4 Aptamers: Trends in Structural Design

Many potent DNA aptamers are known to contain a G-quadruplex (G4) core. Structures and applications of the majority of such aptamers have been reviewed previously. The present review focuses on the design and optimization of G4 aptamers. General features of bioactive G4s are analyzed, and the main strategies for construction of aptamers with desired properties and topologies, including modular assembly, control of an aptamer folding and some others, are outlined. Chemical modification as a method for post-SELEX G4 aptamer optimization is also discussed, and the effects of loop and core modifications are compared. Particular attention is paid to the emerging trends, such as the development of genomic G4- inspired aptamers and the combinatorial approaches which aim to find a balance between rational design and selection.

A Universal Base in a Specific Role: Tuning up a Thrombin Aptamer with 5-Nitroindole

In this study we describe new modified analogs of the thrombin binding aptamer (TBA) containing 5-nitroindole residues. It has been shown that all modified TBAs form an anti-parallel G-quadruplex structure and retain the ability to inhibit thrombin. The most advanced TBA variant (TBA-N8) has a substantially increased clotting time and two-fold lower IC50 value compared to the unmodified prototype. Molecular modelling studies suggest that the improved anticoagulant properties of TBA-N8 result from changes in the binding mode of the analog. A modified central loop in TBA-N8 is presumed to participate in the binding of the target protein. Studies of FAM labelled TBA and TBA-N8 showed an improved binding affinity of the modified aptamer and provided evidence of a direct interaction between the modified central loop and thrombin. Our findings have implications for the design of new aptamers with improved binding affinities.

Methidium intercalator inserted into synthetic oligonucleotides.

Abstract A new methidium intercalator phosphoramidite has been synthesized. Methidium incorporation into an oligonucleotide during the synthesis was confirmed by UV and MALDI TOF MS data. UV melting experiments showed enhanced stability of a duplex, containing internal methidium.

SNP Genotyping by MALDI-TOF Mass Spectrometry

The Sequazyme Pinpoint assay has successfully typed hundreds of different loci in many different laboratories. Major advantages of the technique are that primer selection is extremely simple and a high percentage of selected primers successfully genotype. The primers need contain no special labels or groups, so they are inexpensive and easily obtainable. Because the PCR and primer extension protocols are solution-based and the extension protocol is universal, the assay does not involve any of the relatively high infrastructure costs of DNA chips. Because MALDI-TOF is largely artefact-free and high in resolution, the base-calling accuracy is extremely high. The ability to detect and quantitate low relative abundance of the minor allele in pooled samples is largely unique to mass spectrometry, due to its high resolution compared to electrophoretic, chromatographic, fluorescent, and isotopic techniques. The ability to process several thousand samples a day, together with the ability to internally multiplex, enables throughputs of about 20,000 alleles a day, suitable for most medium to high throughput genotyping laboratories. Future developments include further miniaturisation to lower reagent costs, faster lasers to support higher throughput MALDI-TOF, and associated robotic systems to provide greater throughput.

Anomeric DNA quadruplexes

Thrombin-binding aptamer (TBA) is a 15-nt DNA oligomer that efficiently inhibits thrombin. It has been shown that TBA folds into an anti-parallel unimolecular G-quadruplex. Its three-dimensional chair-like structure consists of two G-tetrads connected by TT and TGT loops. TBA undergoes fast degradation by nucleases in vivo. To improve the nuclease resistance of TBA, a number of modified analogs have been proposed. Here, we describe anomeric modifications of TBA. Non-natural α anomers were used to replace selected nucleotides in the loops and core. Significant stabilization of the quadruplex was observed for the anomeric modification of TT loops at T4 and T13. Replacement of the core guanines either prevents quadruplex assembly or induces rearrangement in G-tetrads. It was found that the anticoagulant properties of chimeric aptamers could be retained only with intact TT loops. On the contrary, modification of the TGT loop was shown to substantially increase nuclease resistance of the chimeric aptamer without a notable disturbance of its anticoagulant activity.

DNA complexes with Ni nanoparticles: structural and functional properties

Supramolecular complexes of biopolymers based on magnetic nanoparticles play an important role in creation of biosensors, implementation of theragnostic and gene therapeutic methods and biosafety evaluation. We investigated the impact of DNA interactions with nanoparticles of nickel (nNi) on the integrity and functionality of DNA. Data obtained by mass spectrometry, electrophoresis, TEM and AFM microscopy techniques, bacterial transformation, and real-time PCR provide evidence that ssDNA and plasmid DNA (pDNA) efficiently form complexes with nNi. AFM data suggest that the complexes are necklace-type structures, in which nanoparticles are randomly distributed along the DNA chains, rather than highly entangled clot-type structures. After desorption, observed DNA characteristics in bioanalytical and biological systems remain unchanged. Only supercoiled pDNA was nicked, but remained, as well as a plasmid–nNi complex, active in expression vector assays. These results are very important for creation of new methods of DNA immobilization and controlled manipulation.

Mass-spectrometry analysis of modifications at DNA termini induced by DNA polymerases

Non-natural nucleotide substrates are widely used in the enzymatic synthesis of modified DNA. The terminal activity of polymerases in the presence of modified nucleotides is an important, but poorly characterized, aspect of enzymatic DNA synthesis. Here, we studied different types of polymerase activity at sequence ends using extendable and non-extendable synthetic models in the presence of the Cy5-dUTP analog Y. In primer extension reactions with selected exonuclease-deficient polymerases, nucleotide Y appeared to be a preferential substrate for non-templated 3′-tailing, as determined by MALDI mass-spectrometry and gel-electrophoresis. This result was further confirmed by the 3′-tailing of a non-extendable hairpin oligonucleotide model. Additionally, DNA polymerases induce an exchange of the 3′ terminal thymidine for a non-natural nucleotide via pyrophosphorolysis in the presence of inorganic pyrophosphate. In primer extension reactions, the proofreading polymerases Vent, Pfu, and Phusion did not support the synthesis of Y-modified primer strand. Nevertheless, Pfu and Phusion polymerases were shown to initiate terminal nucleotide exchange at the template. Unlike non-proofreading polymerases, these two enzymes recruit 3′–5′ exonuclease functions to cleave the 3′ terminal thymidine in the absence of pyrophosphate.