Tuning single nucleotide discrimination in polymerase chain reactions (PCRs): synthesis of primer probes bearing polar 4'-C-modifications and their application in allele-specific PCR

Covalent modification of primers improves PCR amplification specificity and yield

We report a method for covalent modification of primers that enhances the specificity of PCR and increases the yield of specific amplification products at the end of PCR. The introduction of thermally stable covalent modifications, such as alkyl groups to the exocyclic amines of deoxyadenosine or cytosine residues at the 3'-ends of primers results in enhanced specificity of reactions. This higher specificity can result in greater sensitivity of detection by reducing competition with non-productive reactions. The reduction in the amplification of unintended byproducts is most apparent when both primers are modified at their respective 3'-ends. The T Ms of such modified primers are only slightly affected by the inclusion of these modifiers. The principal mode of action is believed to be driven by the poor enzyme extension of substrates with closely juxtaposed bulky alkyl groups, such as would result from the replication of primer dimer artifact.

Synthesis and duplex-forming ability of oligonucleotides containing 4'-carboxythymidine analogs

Oligonucleotides containing 4'-carboxy-, 4'-methoxycarbonyl-, 4'-carbamoyl-, and 4'-methylcarbamoyl-thymidines, and their 2'-methoxy, 2'-amino or 2'-acetamido analogs were prepared. Their duplex-forming ability with DNA and RNA complements was evaluated by UV melting experiments. Interestingly, 4'-carboxythymidine existing in the S-type sugar conformation was found to lead to an increase in the stability of the duplex formed with RNA complements compared to natural thymidine.

Interactions of non-polar and "Click-able" nucleotides in the confines of a DNA polymerase active site

Modified nucleotides play a paramount role in many cutting-edge biomolecular techniques. The present structural study highlights the plasticity and flexibility of the active site of a DNA polymerase while incorporating non-polar "Click-able" nucleotide analogs and emphasizes new insights into rational design guidelines for modified nucleotides.

FIT probes: peptide nucleic acid probes with a fluorescent base surrogate enable real-time DNA quantification and single nucleotide polymorphism discovery

The ability to accurately quantify specific nucleic acid molecules in complex biomolecule solutions in real time is important in diagnostic and basic research. Here we describe a DNA-PNA (peptide nucleic acid) hybridization assay that allows sensitive quantification of specific nucleic acids in solution and concomitant detection of select single base mutations in resulting DNA-PNA duplexes. The technique employs so-called FIT (forced intercalation) probes in which one base is replaced by a thiazole orange (TO) dye molecule. If a DNA molecule that is complementary to the FIT-PNA molecule (except at the site of the dye) hybridizes to the probe, the TO dye exhibits intense fluorescence because stacking in the duplexes enforces a coplanar arrangement even in the excited state. However, a base mismatch at either position immediately adjacent to the TO dye dramatically decreases fluorescence, presumably because the TO dye has room to undergo torsional motions that lead to rapid depletion of the excited state. Of note, we found that the use of d-ornithine rather than aminoethylglycine as the PNA backbone increases the intensity of fluorescence emitted by matched probe-target duplexes while specificity of fluorescence signaling under nonstringent conditions is also increased. The usefulness of the ornithine-containing FIT probes was demonstrated in the real-time PCR analysis providing a linear measurement range over at least seven orders of magnitude. The analysis of two important single nucleotide polymorphisms (SNPs) in the CFTR gene confirmed the ability of FIT probes to facilitate unambiguous SNP calls for genomic DNA by quantitative PCR.

Modified oligonucleotides as tools for allele-specific amplification

Allele-specific polymerase chain reaction (PCR), a method that reports nucleotide variations through either the presence or the absence of a DNA product obtained through PCR amplification, holds the promise to combine target amplification and analysis in one single step. Recently, it has been reported that the selectivity of allele-specific PCR can be significantly increased through the employment of chemically modified primer probes. Here, we report on significant developments of primer probe design and synthesis along this line.

Increased single-nucleotide discrimination in allele-specific polymerase chain reactions through primer probes bearing nucleobase and 2'-deoxyribose modifications

The diagnosis of genetic dissimilarities between individuals is becoming increasingly important due to the discovery that these variations are related to complex phenotypes like the predisposition to certain diseases or compatibility with drugs. The most common among these sequence variations are single-nucleotide polymorphisms (SNPs). The availability of reliable and efficient methods for the interrogation of the respective genotypes is the basis for any progress along these lines. Many methods for the detection of nucleotide variations in genes exist, in which amplification of the target gene is required before analysis can take place. The allele-specific polymerase chain reaction (asPCR) combines target amplification and analysis in a single step. The principle of asPCR is based on the formation of matched or mismatched primer-target complexes. The most important parameter in asPCR is the discrimination of these matched or mismatched pairs. In recent publications we have shown that the reliability of SNP detection through asPCR is increased by employing chemically modified primer probes. In particular, primer probes that bear a polar 4'-C-methoxymethylene residue at the 3' end have superior properties in discriminating single-nucleotide variations by PCR. Here we describe the synthesis of several primer probes that bear nucleobase modifications in addition to the 4'-C-methoxymethylenated 2'-deoxyriboses. We studied the effects of these alterations on single-nucleotide discrimination in allele-specific PCR promoted by a DNA polymerase and completed these results with single-nucleotide-incorporation kinetic studies. Moreover, we investigated thermal denaturing of the primer-probe-template complexes and recorded circular dichroism (CD) spectra for inspecting the thermodynamic and photophysical duplex behaviour of the introduced modifications. In short, we found that primer probes bearing a 4'-C-methoxymethylene residue at the 2'-deoxyribose moiety in combination with a thiolated thymidine moiety have synergistic effects and display significantly increased discrimination properties in asPCR.

A new photoactive building block for investigation of DNA backbone interactions: photoaffinity labeling of human DNA polymerase beta

The cross-linking of target proteins or nucleic acids to light-activatable ligands is an important tool for elucidating molecular interactions. Through the use of photoaffinity-labeling reagents, several new insights into nucleic acid interactions have been obtained, for example in DNA replication and repair. In most known photoprobes, the applied light-sensitive functionalities are placed directly at the nucleobase or are attached via linkers to either the nucleobase or the phosphate backbone. Here we describe the first photoprobe that bears a light-sensitive aryl(trifluoromethyl)diazirine at the sugar moiety of a DNA oligonucleotide. We devised a route for the synthesis of the modified nucleoside and its incorporation into an oligonucleotide. The photoactive species was proven to be stable under the conditions employed in routine automated DNA synthesis. The modified oligonucleotide was shown by subsequent photolabeling studies of human DNA polymerase beta to form a covalent complex to the enzyme upon irradiation with near-UV light.

Increased single nucleotide discrimination in arrayed primer elongation by 4'C-modified primer probes

Herein we describe the beneficial impact of immobilized 4'C-modified primer probes on detecting single nucleotide variations in arrayed primer extension by a DNA polymerase.

Genotyping—From Genomic DNA to Genotype in a Single Tube

Nucleotide variations in the human genome, such as single-nucleotide polymorphisms, have been researched more intensively since it became apparent that these deviations are linked to various diseases and also several side effects of drugs. The investigation of genomic DNA in the laboratory requires routine methods that are time-, labour-, and cost-effective. These criteria are fulfilled by so-called closed-tube methods, which are applied directly to isolated genomic DNA without any preamplification.

Fluorescence based strategies for genetic analysis

Synthetic chemistry has been central to the design of modern methods of genetic analysis. In this article, we discuss the underlying chemistry and biophysical principles that have been used in the development of robust methods for the analysis of DNA in the diagnostic laboratory.