Modulating CRISPR-Cas genome editing using guide-complementary DNA oligonucleotides

CRISPR-Cas has revolutionized genome editing and has a great potential for applications, such as correcting human genetic disorders. To increase the safety of genome editing applications, CRISPR-Cas may benefit from strict control over Cas enzyme activity. Previously, anti-CRISPR proteins and designed oligonucleotides have been proposed to modulate CRISPR-Cas activity. Here we report on the potential of guide-complementary DNA oligonucleotides as controlled inhibitors of Cas9 ribonucleoprotein complexes. First, we show that DNA oligonucleotides down-regulate Cas9 activity in human cells, reducing both on and off-target cleavage. We then used in vitro assays to better understand how inhibition is achieved and under which conditions. Two factors were found to be important for robust inhibition: the length of the complementary region, and the presence of a PAM-loop on the inhibitor. We conclude that DNA oligonucleotides can be used to effectively inhibit Cas9 activity both ex vivo and in vitro.

Sequence Effect on the Activity of DNAzyme with Covalently Attached Hemin and Their Potential Bioanalytical Application

In this work we investigated the effect of a DNA oligonucleotide sequence on the activity of a DNAzyme with covalently attached hemin. For this purpose, we synthesized seven DNA-hemin conjugates. All DNA-hemin conjugates as well as DNA/hemin complexes were characterized using circular dichroism, determination of melting temperatures and pKa of hemin. We observed that hemin conjugation in most cases led to the formation of parallel G-quadruplexes in the presence of potassium and increased thermal stability of all studied systems. Although the activity of DNA-hemin conjugates depended on the sequence used, the highest activity was observed for the DNA-hemin conjugate based on a human telomeric sequence. We used this DNAzyme for development of “sandwich” assay for detection of DNA sequence. For this assay, we used electric chip which could conduct electricity after silver deposition catalyzed by DNAzyme. This method was proved to be selective towards DNA oligonucleotides with mismatches and could be used for the detection of the target. To prove the versatility of our DNAzyme probe we also performed experiments with streptavidin-coated microplates. Our research proved that DNAzyme with covalently attached hemin can be used successfully in the development of heterogeneous assays.

Prototype Foamy Virus Integrase Displays Unique Biochemical Activities among Retroviral Integrases

Integrases of different retroviruses assemble as functional complexes with varying multimers of the protein. Retroviral integrases require a divalent metal cation to perform one-step transesterification catalysis. Tetrameric prototype foamy virus (PFV) intasomes assembled from purified integrase and viral DNA oligonucleotides were characterized for their activity in the presence of different cations. While most retroviral integrases are inactive in calcium, PFV intasomes appear to be uniquely capable of catalysis in calcium. The PFV intasomes also contrast with other retroviral integrases by displaying an inverse correlation of activity with increasing manganese beginning at relatively low concentrations. The intasomes were found to be significantly more active in the presence of chloride co-ions compared to acetate. While HIV-1 integrase appears to commit to a target DNA within 20 s, PFV intasomes do not commit to target DNA during their reaction lifetime. Together, these data highlight the unique biochemical activities of PFV integrase compared to other retroviral integrases.

Nanoribbon Biosensor in the Detection of miRNAs Associated with Colorectal Cancer

A nanoribbon biosensor (NRBS) was developed to register synthetic DNAs that simulate and are analogous to miRNA-17-3p associated with colorectal cancer. Using this nanoribbon biosensor, the ability to detect miRNA-17-3p in the blood plasma of a patient diagnosed with colorectal cancer has been demonstrated. The sensing element of the NRBS was a nanochip based on a silicon-on-insulator (SOI) nanostructure. The nanochip included an array of 10 nanoribbons and was designed with the implementation of top-down technology. For biospecific recognition of miRNA-17-3p, the nanochip was modified with DNA probes specific for miRNA-17-3p. The performance of the nanochip was preliminarily tested on model DNA oligonucleotides, which are synthetic analogues of miRNA-17-3p, and a detection limit of ~10−17 M was achieved. The results of this work can be used in the development of serological diagnostic systems for early detection of colorectal cancer.

Single-molecule sizing through nano-cavity confinement

An approach relying on nano-cavity confinement is developed in this paper for the sizing of nanoscale particles and single biomolecules in solution. The approach, termed nano-cavity diffusional sizing (NDS), measures particle residence times within fluidic nano-cavities to determine their hydrodynamic radii. Using theoretical modelling and simulation, we show that the residence time of particles within nano-cavities above a critical timescale depends on the diffusion coefficient of the particle, which allows estimation of the particle's size. We demonstrate this approach experimentally through measurement of particle residence times within nano-fluidic cavities using single-molecule confocal microscopy. Our data show that the residence times scale linearly with the sizes of nanoscale colloids, protein aggregates and single DNA oligonucleotides. NDS thus constitutes a new single molecule optofluidic approach that allows rapid and quantitative sizing of nanoscale objects for potential application in nanobiotechnology, biophysics, and clinical diagnostics.

Genotyping Arabidopsis T-DNA lines v1

This protocol is used for genotyping Arabidopsis seedlings to test for the presence of a transfer DNA (T-DNA) insertion. By using two primer sets it is possible to determine whether a seedling is homozygous, heterozygous or azygous for an insertion in the predicted genomic location. To identify lines with T-DNA insertions in a gene of interest, you need the Arabidopsis Genome Identifier (AGI) number corresponding to the genomic locus (e.g. RCS1A = AT1G67090), then visit the Salk Institute T-DNA Express site to find all the mapped insertions at your locus of interest. Genotyping primers have been pre-designed for each T-DNA line, these can be retrieved from the Salk Institute T-DNA primer site, and ordered at any supplier of DNA oligonucleotides before starting the protocol. In the US T-DNA lines can be purchased from the Arabidopsis Biological Resource Center (ABRC) and in the UK and EU from the European Arabidopsis Stock Center (NASC). Recommended reading http://signal.salk.edu/tdnaprimers.2.html Setting up the PCR reaction Genotyping is performed with the Phire Direct PCR Mix, this includes the polymerase, nucleotides and salts necessary for amplification. We use the “dilution protocol” which involves taking a small leaf disk and homogenizing it in dilution buffer using a gel tip (see manufacturer’s instructions for more details.)

Genotyping Arabidopsis T-DNA lines v3

This protocol is used for genotyping Arabidopsis seedlings to test for the presence of a transfer DNA (T-DNA) insertion. By using two primer sets it is possible to determine whether a seedling is homozygous, heterozygous or azygous for an insertion in the predicted genomic location. To identify lines with T-DNA insertions in a gene of interest, you need the Arabidopsis Genome Identifier (AGI) number corresponding to the genomic locus (e.g. RCS1A = AT1G67090), then visit the Salk Institute T-DNA Express site to find all the mapped insertions at your locus of interest. Genotyping primers have been pre-designed for each T-DNA line, these can be retrieved from the Salk Institute T-DNA primer site, and ordered at any supplier of DNA oligonucleotides before starting the protocol. In the US T-DNA lines can be purchased from the Arabidopsis Biological Resource Center (ABRC) and in the UK and EU from the European Arabidopsis Stock Center (NASC). Recommended reading http://signal.salk.edu/tdnaprimers.2.html Setting up the PCR reaction Genotyping is performed with the Phire Plant Direct PCR Mix, this includes the polymerase, nucleotides and salts necessary for amplification. We use the “dilution protocol” which involves taking a small leaf disk and homogenizing it in dilution buffer using a gel tip (see manufacturer’s instructions for more details.)

Among retroviral integrases prototype foamy virus integrase displays unique biochemical activities

Integrase enzymes of different retroviruses assemble as functional complexes with varying multimers of the protein. Retroviral integrases require a divalent metal cation to perform one-step transesterification catalysis. Tetrameric prototype foamy virus (PFV) intasomes assembled from purified integrase and viral DNA oligonucleotides were characterized for their activity in the presence of different cations. While most retroviral integrases are inactive in calcium, PFV intasomes appear to be uniquely capable of catalysis in calcium. The PFV intasomes also contrast other retroviral integrases by displaying an inverse correlation of activity with increasing manganese beginning at relatively low concentrations. The intasomes were found to be significantly more active in the presence of chloride co-ions compared to acetate. While HIV-1 integrase appears to commit to a target DNA within 20 seconds, PFV intasomes do not commit to target DNA during their reaction lifetime. Together these data highlight the unique biochemical activities of PFV integrase compared to other retroviral integrases.

A CRISPR/Cas9 Method Facilitates Efficient Oligo-Mediated Gene Editing in Debaryomyces Hansenii

The halophilic and osmotolerant yeast Debaryomyces hansenii has a high potential for cell factory applications due to its resistance to harsh environmental factors and compatibility with a wide substrate range. However, currently available genetic techniques does not allow the full potential of D. hansenii as a cell factory to be harnessed. Moreover, most of the currently available tools rely on the use of auxotrophic markers that are not suitable in wild-type prototrophic strains. In addition, the preferred non-homologous end-joining (NHEJ) DNA damage repair mechanism pose further challenges when precise gene targeting is required. In this study, we present a novel plasmid based CRISPRCUG/Cas9 method for easy and efficient gene editing of the prototrophic strains of D. hansenii. Our toolset design is based on a dominant marker and facilitates quick assembly of the vectors expressing Cas9 and single or multiple sgRNAs that provides possibility for multiplex gene engineering even in prototrophic strains. Moreover, we have constructed an NHEJ deficient D. hansenii that enable our CRISPRCUG/Cas9 tools to support highly efficient introduction of point mutations and single/double gene deletions. Importantly, we also demonstrate that 90-nt single stranded DNA oligonucleotides are sufficient to direct repair of DNA breaks induced by sgRNA-Cas9 resulting in precise edits reaching 100% efficiencies. In conclusion, tools developed in this study will greatly advance basic and applied research in D. hansenii. In addition, we envision that our tools can be rapidly adapted for gene editing of other non-conventional yeast species including the ones belonging to the CUG clade.

Hg2+ Detection with Rational Design of DNA-Templated Fluorescent Silver Nanoclusters

Atomically precise silver nanoclusters (AgNCs) are small nanostructures consisting of only a few atoms of silver. The combination of AgNCs with cytosine-rich single-stranded oligonucleotides results in DNA-templated silver nanoclusters (DNA-AgNCs). DNA-AgNCs are highly luminescent and can be engineered with reproducible and unique fluorescent properties. Furthermore, using nucleic acids as templates for the synthesis of AgNCs provides additional practical benefits by expanding optical activity beyond the visible spectral range and creating the possibility for color tunability. In this study, we explore DNA oligonucleotides designed to fold into hairpin-loop (HL) structures which modulate optical properties of AgNCs based on the size of the loop containing different number of cytosines (HL-CN). Depending on the size of the loop, AgNCs can be manufactured to have either single or multiple emissive states. Such hairpin-loop structures provide an additional stability for AgNCs and further control over the base composition of the loop, allowing for the rational design of AgNCs’ optical properties. We demonstrate the potential of AgNCs in detecting Hg2+ by utilizing the HL-C13 design and its variants HL-T2C11, HL-T4C9, and HL-T6C7. The replacement of cytosines with thymines in the loop was intended to serve as an additional sink for mercury ions extending the detectable range of Hg2+. While AgNC@HL-T0C13 exhibits an interpretable quenching curve, AgNC@HL-T6C7 provides the largest detectable range of Hg2+. The results presented herein suggest that it is possible to use a rational design of DNA-AgNCs based on the composition of loop sequence in HL structures for creating biosensors to detect heavy metals, particularly Hg2+.