Development of an Automated, Non-Enzymatic Nucleic Acid Amplification Test

Among nucleic acid diagnostic strategies, non-enzymatic tests are the most promising for application at the point of care in low-resource settings. They remain relatively under-utilized, however, due to inadequate sensitivity. Inspired by a recent demonstration of a highly-sensitive dumbbell DNA amplification strategy, we developed an automated, self-contained assay for detection of target DNA. In this new diagnostic platform, called the automated Pi-powered looping oligonucleotide transporter, magnetic beads capture the target DNA and are then loaded into a microfluidic reaction cassette along with the other reaction solutions. A stepper motor controls the motion of the cassette relative to an external magnetic field, which moves the magnetic beads through the reaction solutions automatically. Real-time fluorescence is used to measure the accumulation of dumbbells on the magnetic bead surface. Left-handed DNA dumbbells produce a distinct signal which reflects the level of non-specific amplification, acting as an internal control. The autoPiLOT assay detected as little as 5 fM target DNA, and was also successfully applied to the detection of S. mansoni DNA. The autoPiLOT design is a novel step forward in the development of a sensitive, user-friendly, low-resource, non-enzymatic diagnostic test.

Addition of mirror-image L-DNA elements to DNA amplification circuits to distinguish leakage from target signal

DNA amplification circuits that rely on thermodynamically-driven hybridization events triggered by a target nucleic acid are becoming increasingly utilized due to their relative simplicity. A drawback of these circuits is that non-specific amplification, or circuit leakage, must be estimated using a separate "no-target" control reaction to eliminate false positives. Aside from requiring an additional reaction, the problem with this approach is the difficulty of creating a no-target control for biological specimens. To overcome this limitation, we propose a strategy that combines both reactions into the same tube using naturally-occurring right-handed D-DNA circuit elements for the target detection reaction and identical synthetic mirror-image left-handed L-DNA circuit elements for the no-target control reaction. We illustrate this approach using catalyzed hairpin assembly (CHA), one of the most studied DNA amplification circuits. In a dual-chirality CHA design, the right-handed circuit signal is produced by target-specific amplification and circuit leakage, whereas the left-handed circuit signal is produced only by circuit leakage. The target-specific amplification is calculated as the difference between the two signals. The limit of detection of this dual-chirality CHA reaction was found to be similar to that of traditional CHA (81 vs 92 pM, respectively). Furthermore, the left-handed no-target signal matched the right-handed leakage across a wide range of sample conditions including background DNA, increased salt concentration, increased temperature, and urine. These results demonstrate the robustness of a dual-chirality design and the potential utility of left-handed DNA in the development of new DNA amplification circuits better-suited for target detection applications in biological samples.

Sequencing Mirror-Image DNA Chemically

The development of mirror-image biology systems faces a crucial barrier of lacking an L-DNA sequencing technique. Here, we developed a practical method for sequencing mirror-image DNA by adopting the Maxam-Gilbert sequencing approach, through which specific nucleobases in an end-labeled L-DNA are cleaved by achiral chemicals. This technique may facilitate the therapeutic application of nuclease-resistant L-aptamer drugs, and bring the vision of building an alternative, mirror-image self-replicating system closer to reality.

Mirror-image polymerase chain reaction

The construction of mirror-image biological systems may open the next frontier for biomedical technology development and discovery. Here we have designed and chemically synthesized a mutant version of the thermostable Sulfolobus solfataricus P2 DNA polymerase IV (Dpo4) consisting of d-amino acids. With a total peptide length of 358 amino acid residues, it is the largest chemically synthesized d-amino acid protein reported to date. We show that the d-polymerase is able to amplify a 120-bp l-DNA sequence coding for the Escherichia coli 5S ribosomal RNA gene rrfB by mirror-image polymerase chain reaction, and that both the natural and mirror-image systems operate with strict chiral specificity. The development of efficient miPCR systems may lead to many practical applications, such as mirror-image systematic evolution of ligands by exponential enrichment for the selection of therapeutically promising nuclease-resistant l-nucleic acid aptamers.