Use of base modifications in primers and amplicons to improve nucleic acids detection in the real-time snake polymerase chain reaction.

The addition of relatively short flap sequence at the 5'-end of one of the polymerase chain reaction (PCR) primers considerably improves performance of real-time assays based on 5'-nuclease activity. This new technology, called Snake, was shown to supersede the conventional methods like TaqMan, Molecular Beacons, and Scorpions in the signal productivity and discrimination of target polymorphic variations as small as single nucleotides. The present article describes a number of reaction conditions and methods that allow further improvement of the assay performance. One of the identified approaches is the use of duplex-destabilizing modifications such as deoxyinosine and deoxyuridine in the design of the Snake primers. This approach was shown to solve the most serious problem associated with the antisense amplicon folding and cleavage. As a result, the method permits the use of relatively long-in this study-14-mer flap sequences. Investigation also revealed that only the 5'-segment of the flap requires the deoxyinosine/deoxyuridine destabilization, whereas the 3'-segment is preferably left unmodified or even stabilized using 2-amino deoxyadenosine d(2-amA) and 5-propynyl deoxyuridine d(5-PrU) modifications. The base-modification technique is especially effective when applied in combination with asymmetric three-step PCR. The most valuable discovery of the present study is the effective application of modified deoxynucleoside 5'-triphosphates d(2-amA)TP and d(5-PrU)TP in Snake PCR. This method made possible the use of very short 6-8-mer 5'-flap sequences in Snake primers.