DNA ligases and ligase-based technologies

Ultrasensitive Electrochemical Detection of cancer-Related Point Mutations Based on Surface-Initiated Three-Dimensionally Self-Assembled DNA Nanostructures from Only Two Palindromic Probes

Sensitive and selective detection of proto-oncogenes, especially recognition of point mutation, is of great importance in cancer diagnosis. Here, a ligation-mediated technique is demonstrated for the construction of an intertwined three-dimensional DNA nanosheet (3D SDN) on an electrode surface from only two palindromic hairpin probes (HP1 and HP2), creating a powerful electrochemical biosensor (E-biosensor) for the detection of the p53 gene. First, a capturing probe (CP) is immobilized on an electrode surface via Au-S chemistry, forming an electrochemical sensing interface. In the presence of the target p53 (T), the triggering probe is covalently linked to CP by a ligase. Moreover, target hybridization/ligation/dehybridization process is repeated, amplifying the target hybridization event and increasing the content of surface-confined triggering fragments. As a result, HP1 is opened and in turn interacts with HP2, forming intertwined 3D SDN where HP1 and HP2 are alternately arranged in parallel. Common hybridization and interaction between palindromic fragments are responsible for the assembly in the horizontal and vertical directions, respectively. An electrochemical indicator, methylene blue (MB), can be inserted into 3D SDN, generating a strong electrochemical signal. Utilizing the 3D SDN-based E-biosensor, the target DNA is detected down to 3 fM with a linear response range from 10 fM to 10 nM. Single point mutations are reliably identified even in fetal bovine serum and cellular homogenate. Because of the several advantages of simple design, good universality, inexpensive instrumentation, high assay specificity, and sensitivity, the 3D SDN-based E-biosensor is expected to provide a potential platform for screening point mutation required by early clinical diagnostics and medical research.

A high-throughput assay for the comprehensive profiling of DNA ligase fidelity

DNA ligases have broad application in molecular biology, from traditional cloning methods to modern synthetic biology and molecular diagnostics protocols. Ligation-based detection of polynucleotide sequences can be achieved by the ligation of probe oligonucleotides when annealed to a complementary target sequence. In order to achieve a high sensitivity and low background, the ligase must efficiently join correctly base-paired substrates, while discriminating against the ligation of substrates containing even one mismatched base pair. In the current study, we report the use of capillary electrophoresis to rapidly generate mismatch fidelity profiles that interrogate all 256 possible base-pair combinations at a ligation junction in a single experiment. Rapid screening of ligase fidelity in a 96-well plate format has allowed the study of ligase fidelity in unprecedented depth. As an example of this new method, herein we report the ligation fidelity of Thermus thermophilus DNA ligase at a range of temperatures, buffer pH and monovalent cation strength. This screen allows the selection of reaction conditions that maximize fidelity without sacrificing activity, while generating a profile of specific mismatches that ligate detectably under each set of conditions.

Upconversion nanoparticle-based ligase-assisted method for specific and sensitive detection of T790M mutation in epidermal growth factor receptor

In this paper we report a highly specific and sensitive method for the detection of T790M mutation in epidermal growth factor receptor (EGFR). This detection scheme is based on luminescent resonance energy transfer between upconversion nanoparticles and the intercalating dye, SYBR Green I. Target DNA serves as a template for two DNA probes, one of them covalently attached to upconversion nanoparticles, to be joined into a long, hairpin-forming DNA by ligase. The number of the resulting DNA strand, which brings SYBR Green I close to the upconversion nanoparticles, is amplified by thermal cycling. A number of factors affecting the detection specificity and sensitivity, including probe design, ligation temperature, type and amount of ligase, and number of thermal cycles, have been considered and investigated to optimize the performance of the method. The method can easily differentiate the T790M mutation from the wild-type sequence with a mutant-to-wild-type ratio of 1:100. The results show that 0.01pmole of EGFR T790M mutant can be readily detected.

Multiplex ligase-based genotyping methods combined with CE

In this genomic era, the ability to assay multiple genomic hot spots that have strong clinical implications is greatly desired. Conventional PCR-based methods suffer from frequent false-positive detections, particularly when a multiplex analysis is desirable. As an alternative to the error-prone conventional methods, multiplex ligase-based genotyping methods combined with CE have a strong potential. In this review, both previously developed methods and emerging methods are described to reveal the specificity, sensitivity, and simplicity of the ligase-based methods. For each step (ligation, amplification, and separation), the principles of several alternative methods are discussed along with their applications to explore the future development of ligase-based diagnostic methods.

Profiling the selectivity of DNA ligases in an array format with mass spectrometry

This article describes a method for the global profiling of the substrate specificities of DNA ligases and illustrates examples using the Taq and T4 DNA ligases. The method combines oligonucleotide arrays, which offer the benefits of high throughput and multiplexed assays, with mass spectrometry to permit label-free assays of ligase activity. Arrays were prepared by immobilizing ternary biotin-tagged DNA substrates to a self-assembled monolayer presenting a layer of streptavidin protein. The array represented complexes having all possible matched and mismatched base pairs at the 3′ side of the nick site and also included a number of deletions and insertions at this site. The arrays were treated with ligases and adenosine triphosphate or analogs of the nucleotide triphosphate and then analyzed by matrix-assisted laser desorption-ionization mass spectrometry to determine the yields for both adenylation of the 5′-probe strand and joining of the two probe strands. The resulting activity profiles reveal the basis for specificity of the ligases and also point to strategies that use ATP analogs to improve specificity. This work introduces a method that can be applied to profile a broad range of enzymes that operate on nucleic acid substrates.

PCR/LDR/capillary electrophoresis for detection of single-nucleotide differences between fetal and maternal DNA in maternal plasma

BACKGROUND

The discovery of fetal DNA in maternal plasma has opened up an approach for noninvasive diagnosis. We have now assessed the possibility of detecting single-nucleotide differences between fetal and maternal DNA in maternal plasma by polymerase chain reaction (PCR)/ligase detection reaction((LDR)/capillary electrophoresis.

METHODS

PCR/LDR/capillary electrophoresis was applied to detect the genotype of c.454-397T>gene (ESR1) from experimental DNA models of maternal plasma at different sensitivity levels and 13 maternal plasma samples.alphaC in estrogen receptor.

RESULTS

(1) Our results demonstrated that the technique could discriminate low abundance single-nucleotide mutation with a mutant/normal allele ratio up to 1:10 000. (2) Examination of ESR1 c.454-397T>C genotypes by using the method of restriction fragment length analysis was performed in 25 pregnant women, of whom 13 pregnant women had homozygous genotypes. The c.454-397T>C genotypes of paternally inherited fetal DNA in maternal plasma of these 13 women were detected by PCR/LDR/capillary electrophoresis, which were accordant with the results of umbilical cord blood.

CONCLUSIONS

PCR/LDR/capillary electrophoresis has very high sensitivity to distinguish low abundance single nucleotide differences and can discriminate point mutations and single-nucleotide polymorphisms(SNPs) of paternally inherited fetal DNA in maternal plasma.

Template-independent ligation of single-stranded DNA by T4 DNA ligase

T4 DNA ligase is one of the workhorses of molecular biology and used in various biotechnological applications. Here we report that this ligase, unlike Escherichia coli DNA ligase, Taq DNA ligase and Ampligase, is able to join the ends of single-stranded DNA in the absence of any duplex DNA structure at the ligation site. Such nontemplated ligation of DNA oligomers catalyzed by T4 DNA ligase occurs with a very low yield, as assessed by quantitative competitive PCR, between 10(-6) and 10(-4) at oligonucleotide concentrations in the range 0.1-10 nm, and thus is insignificant in many molecular biological applications of T4 DNA ligase. However, this side reaction may be of paramount importance for diagnostic detection methods that rely on template-dependent or target-dependent DNA probe ligation in combination with amplification techniques, such as PCR or rolling-circle amplification, because it can lead to nonspecific background signals or false positives. Comparison of ligation yields obtained with substrates differing in their strandedness at the terminal segments involved in ligation shows that an acceptor duplex DNA segment bearing a 3'-hydroxy end, but lacking a 5'-phosphate end, is sufficient to play a role as a cofactor in blunt-end ligation.