Name: in vivo alkaline comet assay and enzyme-modified alkaline comet assay for measuring dna strand breaks and oxidative dna damage in rat liver
Uploaded: 2016-05-04T14:00:00
Description: Watch this Scientific Journal Video about In Vivo Alkaline Comet Assay and Enzyme-modified Alkaline Comet Assay for Measuring DNA Strand Breaks and Oxidative DNA Damage in Rat Liver at JoVE.com

This work was supported by the US Food and Drug Administration. We acknowledge the original publication of the CPA study by Elsevier B.V.: Ding W, Bishop ME, Peace MG, Davis KJ, White GA, Lyn-Cook LE, Manjanatha MG. Sex-specific dose-response analysis of genotoxicity in cyproterone acetate-treated F344 rats. Mutation Research 774: 1-7, 2014 (PMID: 25440904)

Ethics statement: Procedures involving animals have been approved by the Institutional Animal Care and Use Committee (IACUC) at the US FDA/National Center for Toxicological Research.
NOTE: The study design described here is based on the protocol developed by the Japanese Center for the Validation of Alternative Methods (JaCVAM) for their validation of the in vivo rodent alkaline comet assay18, and further modified based on recommendations in OECD guideline TG48919</sup...

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G., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Genotoxicity+of+doxorubicin+in+F344+rats+by+combining+the+comet+assay,+flow-cytometric+peripheral+blood+micronucleus+test,+and+pathway-focused+gene+expression+profiling.">Genotoxicity of doxorubicin in F344 rats by combining the comet assay, flow-cytometric peripheral blood micronucleus test, and pathway-focused gene expression profiling.</a> Environ Mol Mutagen. 55 (1), 24-34 (2014).</li><li>Smith, C. C., O'Donovan, M. R., Martin, E. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=hOGG1+recognizes+oxidative+damage+using+the+comet+assay+with+greater+specificity+than+FPG+or+ENDOIII.">hOGG1 recognizes oxidative damage using the comet assay with greater specificity than FPG or ENDOIII.</a> Mutagenesis. 21 (3), 185-190 (2006).</li><li>Collins, A. 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This protocol describes the concurrent measurement of both direct and oxidative DNA damage in rat liver at the single cell level. The general protocol is applicable to any tissue from which single cells or nuclei can be isolated with minimal processing-induced DNA damage (i.e., DNA damage induced not by the test agent, but by the handling and processing of the animal tissues). In our research, we have conducted alkaline comet assays on cells from bone marrow7,9, stomach6, kidney9...

The alkaline comet assay measures DNA strand breaks at the single-cell level. Suspensions of single cells are embedded in agarose on a microscope slide and the cells lysed to form nucleoids, which contain supercoiled loops of DNA. Electrophoresis at pH&#62;13 results in the loss of supercoiling in DNA loops containing strand breaks, with the freed strands of DNA migrating toward the anode, creating comet-like structures that can be observed by fluorescence microscopy. Fragmented DNA migrates from the &#34;head&#34; of the comet into the &#34;tail&#34; based on the size of the fragment, and the relative fluorescence of the comet tail compared to the total intensity (head and tail) can be used to quantify DNA breakage1,2. The assay is simple, sensitive, versatile, rapid, and relatively inexpensive1. The detection of fragmented DNA caused by DNA-damaging agents is used an assay for quantifying DNA damage in cells or isolated nuclei from individual tissues of animals treated with potentially genotoxic material(s). Due to its advantages, the in vivo comet assay is recommended as a second in vivo genotoxicity assay (paired with the in vivo micronucleus assay) for conducting product safety evaluations in current International Conference on Harmonisation (ICH)3 and European Food Safety Authority (EFSA)4 regulatory guidelines. In our lab, we have employed the assay for evaluating in vivo DNA damage induced by food ingredients, pharmaceuticals, and nanomaterials5-10. Rat liver will be used as an example in this protocol, but the comet assay can be performed with other tissues/organs of experimental animals, as long as intact single cells can be isolated from the tissue.
Certain types of DNA damage are difficult to detect as DNA strand breaks without modifying the basic alkaline comet assay. In the case of oxidative DNA damage, strand breaks can be created at oxidative lesions in DNA by digesting with repair enzymes such as human 8-oxoguanine-DNA-N-glycosylase 1 (hOGG1, which creates breaks at 8-oxoguanine (8-oxoGua) and methyl-fapy-guanine11. Also, Endonuclease III (Endo III) creates breaks mainly at oxidized pyrimidines1. Thus, the addition of an enzyme-digestion step makes the assay a specific and sensitive method for measuring oxidative DNA damage in vivo12. Utilizing these assays, we have demonstrated toxicant-induced oxidative DNA damage in the liver of rats and mice6-8 and in the heart of rats10.
The alkaline comet assay has many applications in genetic toxicology and human biomonitoring: 1) as a follow-up in vivo assay for genotoxins identified by sensitive in vitro tests3,13, 2) to evaluate mechanisms of xenobiotic-induced DNA damage in multiple tissues14, 3) to investigate if a carcinogen operates using a genotoxic or a non-genotoxic mode of action (MOA)7, 4) to evaluate DNA damage repair15, 5) to investigate human diseases and occupational exposures 16, and 6) as a potential high-throughput screening assay for organ-specific genotoxicity17.

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			<td height="21" style="height:21px;width:232px;">Coverslips (No. 1, 24 x 50 mm)&#160;</td>
			<td style="width:140px;">Fisher</td>
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			<td height="21" style="height:21px;width:232px;">Microscope Slides</td>
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			<td height="21" style="height:21px;width:232px;">Dimethylsulfoxide (DMSO)&#160;</td>
			<td style="width:140px;">Fisher</td>
			<td style="width:156px;">67-68-5</td>
			<td></td>
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			<td height="21" style="height:21px;width:232px;">EDTA, Disodium</td>
			<td style="width:140px;">Fisher</td>
			<td style="width:156px;">BP120-1</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:232px;">Phosphate buffered saline</td>
			<td style="width:140px;">Fisher</td>
			<td style="width:156px;">ICN1860454</td>
			<td></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:232px;">1X Hanks Balanced Salt Solution (HBSS) (Ca++, Mg++ free)&#160;</td>
			<td style="width:140px;">HyClone</td>
			<td style="width:156px;">SH30588.02</td>
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			<td height="21" style="height:21px;width:232px;">HEPES</td>
			<td style="width:140px;">Fisher</td>
			<td style="width:156px;">BP310-1</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:232px;">Low Melting Point Agarose (LMP)</td>
			<td style="width:140px;">Lonza</td>
			<td align="right" style="width:156px;">50081</td>
			<td>NuSieve GTG Agarose</td>
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		<tr height="21">
			<td height="21" style="height:21px;width:232px;">Normal Melting Agarose (NMA)&#160;</td>
			<td style="width:140px;">Fisher</td>
			<td style="width:156px;">BP1356-100</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:232px;">pH testing paper strips (pH 7.5-14)</td>
			<td style="width:140px;">Fisher</td>
			<td style="width:156px;">M95873</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:232px;">Potassium Cloride</td>
			<td style="width:140px;">Fisher</td>
			<td style="width:156px;">7447-40-7</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:232px;">Potassium Hydroxide</td>
			<td style="width:140px;">Fisher</td>
			<td style="width:156px;">1310-58-3</td>
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		<tr height="21">
			<td height="21" style="height:21px;width:232px;">slide labels, (0.94 x 0.5 in.)</td>
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			<td height="21" style="height:21px;width:232px;">Sodium Chloride (NaCl)&#160;</td>
			<td style="width:140px;">Fisher</td>
			<td style="width:156px;">7647-14-5</td>
			<td></td>
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		<tr height="21">
			<td height="21" style="height:21px;width:232px;">Sodium Hydroxide (NaOH)&#160;</td>
			<td style="width:140px;">Fisher</td>
			<td style="width:156px;">1310-73-2</td>
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			<td height="21" style="height:21px;width:232px;">SYBR&#8482; Gold&#160;</td>
			<td style="width:140px;">Invitrogen</td>
			<td style="width:156px;">S11494</td>
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		<tr height="21">
			<td height="21" style="height:21px;width:232px;">Triton X-100&#160;</td>
			<td style="width:140px;">Fisher</td>
			<td style="width:156px;">9002-93-1</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:232px;">Trizma Base&#160;</td>
			<td style="width:140px;">Fisher</td>
			<td style="width:156px;">77-86-1</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:232px;">2.0 mL microcentrifuge tubes</td>
			<td style="width:140px;">Fisher</td>
			<td style="width:156px;">05-402-6</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:232px;">Cell strainer (40 &#181;m)</td>
			<td style="width:140px;">Fisher</td>
			<td align="right" style="width:156px;">22363547</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:232px;">Endonuclease III (Nth)&#160;</td>
			<td style="width:140px;">New England Biolabs</td>
			<td style="width:156px;">M0268S</td>
			<td>Dilution 1:1000</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:232px;">hOGG1&#160;</td>
			<td style="width:140px;">New England Biolabs</td>
			<td style="width:156px;">M0241S</td>
			<td>Dilution 1:1000</td>
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in vivo alkaline comet assay and enzyme-modified alkaline comet assay for measuring dna strand breaks and oxidative dna damage in rat liver

Unrepaired DNA damage can lead to genetic instability, which in turn may enhance cancer development. Therefore, identifying potential DNA damaging agents is important for protecting public health. The in vivo alkaline comet assay, which detects DNA damage as strand breaks, is especially relevant for assessing the genotoxic hazards of xenobiotics, as its responses reflect the in vivo absorption, tissue distribution, metabolism and excretion (ADME) of chemicals, as well as DNA repair process. Compared to other in vivo DNA damage assays, the assay is rapid, sensitive, visual and inexpensive, and, by converting oxidative DNA damage into strand breaks using specific repair enzymes, the assay can measure oxidative DNA damage in an efficient and relatively artifact-free manner. Measurement of DNA damage with the comet assay can be performed using both acute and subchronic toxicology study designs, and by integrating the comet assay with other toxicological assessments, the assay addresses animal welfare requirements by making maximum use of animal resources. Another major advantage of the assays is that they only require a small amount of cells, and the cells do not have to be derived from proliferating cell populations. The assays also can be performed with a variety of human samples obtained from clinically or occupationally exposed individuals.

The in vivo alkaline comet assay was performed in conjunction with the enzyme-modified comet assay to measure both direct and oxidative DNA damage in the liver of rats treated with cyproterone acetate (CPA)5. CPA is a synthetic hormonal drug that induces rat liver tumors in a sex-specific manner, with five-fold higher doses needed to induce liver tumors in male rats compared to females24. We found that the direct DNA damage produced by CPA in the liver of ma...

Watch this Scientific Journal Video about In Vivo Alkaline Comet Assay and Enzyme-modified Alkaline Comet Assay for Measuring DNA Strand Breaks and Oxidative DNA Damage in Rat Liver at JoVE.com

In Vivo Alkaline Comet Assay and Enzyme-modified Alkaline Comet Assay for Measuring DNA Strand Breaks and Oxidative DNA Damage in Rat Liver

The alkaline comet assay measures DNA strand breaks in eukaryotic cells. By adding an Endonuclease III or human 8-oxoguanine-DNA-N-glycosylase digestion step, the assay can efficiently detect oxidative DNA damage. We describe methods for using these assays to detect DNA damage in rat liver.

In Vivo Alkaline Comet Assay and Enzyme-modified Alkaline Comet Assay for Measuring DNA Strand Breaks and Oxidative DNA Damage in Rat Liver

Unrepaired DNA damage can lead to genetic instability, which in turn may enhance cancer development. Therefore, identifying potential DNA damaging agents ...

The overall goal of the in vivo alkaline comet assay and the enzyme-modified alkaline comet assay, is to concurrently measure direct and oxidative DNA damage at the single-cell level. This method was first proposed as a regulatory tool for safety analysis for FDA regulated products. It can help answer key questions in fields ranging from regulatory safety evaluation, to molecular epidemiology, to basic cancer research.This technique is sensitive and versatile, and also it is relatively inexpensive. It is capable of detecting DNA damage in virtually any animal tissue. Generally, individuals knew this method will struggle to quickly prepare single cell suspensions from various tissues, and it can be a challenge.Visual demonstration of this method is critical because it requires a certain amount of art, as well as science. After euthanizing rats with carbon dioxide gas, according to the text protocol, palpate to confirm that the rat has no heartbeat, and pinch the toes to confirm that the rat does not withdraw its limb. Use ethanol to disinfect the skin, then use surgical scissors to open the abdominal cavity, before removing the intact liver.With scissors, separate the left lateral lobe from the rest of the organ, and place it on a cutting board. Next, using a disposable single-edged razor blade, cut two adjacent 3 to 4 millimeter thick transverse sections. Place one transverse section on filter paper, then, into 10%neutral buffered formalin, or NBF.Place the second liver section into five milliliters of ice-cold mincing buffer in a six centimeter petri dish, and rinse three times to remove the residual blood. Then, transfer the liver section to a new two milliliter centrifuge tube, containing one milliliter of ice-cold mincing buffer. With fine scissors, mince the section to release the cells, then with a 40 micrometer cell strainer, strain the cell suspension to remove any tissue lumps.Place the single cell suspension on ice, and with a hemocytometer, verify that the concentration is approximately two times 10 to the sixth cells per milliliter. To prepare comet slides, mix one volume of the single cell suspension with 10 volumes of molten 37 degree Celsius 0.5%low-melting agarose solution. Immediately pipette 100 microliters onto a previously prepared agarose-coated slide, and use a cover slip to cover the agarose cell mixture.Lay the slides flat, and cool at four degrees Celsius for 30 minutes. After the gel has solidified, gently remove the cover slips and immerse the slides in pre-chilled lysis buffer. Incubate in the dark at four degrees Celsius for one hour to overnight.Following the incubation, remove six of the eight slides prepared for each tissue sample from the lysis buffer, and use room temperature enzyme buffer to rinse them three times for five minutes each. Then, use tissues to blot excess solution. Next, use enzyme buffer to dilute hOGG1 and Endo III enzyme stocks, to one to one thousand, and place on ice.Place the slides into petri plates lined with moisten paper towels, then apply 200 microliters of the following solutions to the slides in each treatment group. Enzyme buffer alone to two slides, hOGG1 solution to two slides, and Endo III solution to two slides, and use Parafilm to cover the slides. Then place the slides at 37 degrees Celsius, incubating the Endo III treated and reference slides for 45 minutes, and the hOGG1 treated slides for 30 minutes.Following the incubation, drain the slides and for the reference slides, use cold neutralization buffer to remove residual detergent and salts. Then use distilled water to rinse the enzyme treated slides. Randomly place the slides in a horizontal gel electrophoresis tank, avoiding any spaces.Then add freshly made electrophoresis buffer to the tank, while avoiding bubbles, until it just covers the slides. Set the power supply to 0.7 volts per centimeter, and incubate the slides in the alkaline buffer for 20 minutes, to allow unwinding of the DNA and expression of the Alkali-liable damage. Record the slide placement, the temperature of the buffer, and the current.After the incubation for unwinding, place the electrophoresis tank at four degrees Celsius. Then press the Run button on the power supply, and if needed, adjust the current to 300 milliamps by raising or lowering the buffer level, keeping it just above the slides. Run the samples for 20 minutes, before again recording the slide placement, the temperature of the buffer, and the current.Following electrophoresis, gently lift the slides from the tank, and immerse a neutralization buffer to neutralize the excess Alkali. Then drain the slides and repeat the neutralization two more times. With cold 100%ethanol, fix the slides for five minutes.Allow the slides to air dry and store in a dry environment. To stain the DNA, place the slides on a cardboard or metal tray, and add 200 microliters of nucleic acid fluorescent stain working solution to each slide, before covering with a cover slip. Protect the slides from light and incubate for at least 30 minutes.Then to read the slides, gently blot away the excess staining solution, taking care not to disturb the cover glass. Next, place the comet slide on the stage of a fluorescence microscope equipped with a video camera, and randomly choose a field containing cells. After opening the comet assay software, using the mouse, select a cell, then left-click the center of the comet head of the cell.The software will calculate parameters for the cell, and add to a data file. Once all the cells have been scored in the current field, move to another field and repeat the scoring of any suitable comet cell that appears without bias. Score 10 random fields and at least 75 comets per slide.Finally, save the data into a spreadsheet for later analysis. As previously reported, CPA is a synthetic hormonal drug that induces rat liver tumors in a sex-specific manner, with five-fold higher doses needed to induce liver tumors in male rats, compared to females. As shown here, the in vivo alkaline comet assay, and the enzyme-modified comet assay, demonstrated that a five-fold higher dose of CPA was needed, to induce a significant increase in DNA damage in male livers, compared to females.Hydroxysteroid sulfotransferase, or HST, is expressed at 15-fold higher levels in adult female rats, compared to adult males. HST metabolism is a rate-limiting step in the activation of CPA to DNA-binding metabolites. As seen here, CPA-induced oxidative DNA damage was generally greater in male compared to female rat livers, and therefore, less likely to be rate-limiting in tumor formation in male rats.As reported in these tables, histopathology evaluation of livers from CPA-treated rats, showed no evidence of agent-induced apoptosis or necrosis, confirming previous conclusions that the positive comet assay results were not a secondary effect of cytotoxicity. While doing this procedure, it is important to collect single cell suspension, and prepare comet assay slides as quickly as possible. After its development, this technique paved the way for researchers in the field of genetic toxicology to explore DNA damage, and repair quantitatively in virtually any animal tissue and in human samples, as well.After watching this video, we hope you can have a better understanding of how to use the in vivo alkaline comet assay together with the enzyme-modified comet assay, to concurrently measure posts of the rat and oxidative DNA damage, at the single cell level in animal tissues. Please remember that working with DNA damage reagent can be extremely hazardous. Please always wear the personal protective equipment when you perform these procedures.

in-vivo-alkaline-comet-assay-enzyme-modified-alkaline-comet-assay-for

Research

JoVE Journal

Biology

Imaging Mismatch Repair and Cellular Responses to DNA Damage in Bacillus subtilis

Both prokaryotes and eukaryotes respond to DNA damage through a complex set of physiological changes. Alterations in gene expression, the redistribution of existing proteins, and the assembly of new protein complexes can be stimulated by a variety of DNA lesions and mismatched DNA base pairs. Fluorescence microscopy has been used as a powerful experimental tool for visualizing and quantifying these and other responses to DNA lesions and to monitor DNA replication status within the complex subcellular architecture of a living cell. Translational fusions between fluorescent reporter proteins and components of the DNA replication and repair machinery have been used to determine the cues that target DNA repair proteins to their cognate lesions in vivo and to understand how these proteins are organized within bacterial cells. In addition, transcriptional and translational fusions linked to DNA damage inducible promoters have revealed which cells within a population have activated genotoxic stress responses. In this review, we provide a detailed protocol for using fluorescence microscopy to image the assembly of DNA repair and DNA replication complexes in single bacterial cells. In particular, this work focuses on imaging mismatch repair proteins, homologous recombination, DNA replication and an SOS-inducible protein in Bacillus subtilis. All of the procedures described here are easily amenable for imaging protein complexes in a variety of bacterial species.

Imaging Mismatch Repair and Cellular Responses to DNA Damage in Bacillus subtilis

A detailed protocol is described for imaging the real time formation of DNA repair complexes in Bacillus subtilis cells.

Both prokaryotes and eukaryotes respond to DNA damage through a complex set of physiological changes. Alterations in gene expression, the redistribution ...

Analysis of DNA Double-strand Break (DSB) Repair in Mammalian Cells

DNA double-strand breaks are the most dangerous DNA lesions that may lead to massive loss of genetic information and cell death. Cells repair DSBs using two major pathways: nonhomologous end joining (NHEJ) and homologous recombination (HR). Perturbations of NHEJ and HR are often associated with premature aging and tumorigenesis, hence it is important to have a quantitative way of measuring each DSB repair pathway. Our laboratory has developed fluorescent reporter constructs that allow sensitive and quantitative measurement of NHEJ and HR. The constructs are based on an engineered GFP gene containing recognition sites for a rare-cutting I-SceI endonuclease for induction of DSBs. The starting constructs are GFP negative as the GFP gene is inactivated by an additional exon, or by mutations. Successful repair of the I-SceI-induced breaks by NHEJ or HR restores the functional GFP gene. The number of GFP positive cells counted by flow cytometry provides quantitative measure of NHEJ or HR efficiency.

Analysis of DNA Double-strand Break DSB Repair in Mammalian Cells

This article describes GFP-based fluorescence in vivo assays that separately quantify homologous recombination and nonhomologous end joining in mammalian cells.

DNA double-strand breaks are the most dangerous DNA lesions that may lead to massive loss of genetic information and cell death.  Cells repair DSBs using ...

Real-time Imaging of Single Engineered RNA Transcripts in Living Cells Using Ratiometric Bimolecular Beacons

The growing realization that both the temporal and spatial regulation of gene expression can have important consequences on cell function has led to the development of diverse techniques to visualize individual RNA transcripts in single living cells. One promising technique that has recently been described utilizes an oligonucleotide-based optical probe, ratiometric bimolecular beacon (RBMB), to detect RNA transcripts that were engineered to contain at least four tandem repeats of the RBMB target sequence in the 3&#8217;-untranslated region. RBMBs are specifically designed to emit a bright fluorescent signal upon hybridization to complementary RNA, but otherwise remain quenched. The use of a synthetic probe in this approach allows photostable, red-shifted, and highly emissive organic dyes to be used for imaging. Binding of multiple RBMBs to the engineered RNA transcripts results in discrete fluorescence spots when viewed under a wide-field fluorescent microscope. Consequently, the movement of individual RNA transcripts can be readily visualized in real-time by taking a time series of fluorescent images. Here we describe the preparation and purification of RBMBs, delivery into cells by microporation and live-cell imaging of single RNA transcripts.

Ratiometric bimolecular beacons (RBMBs) can be used to image single engineered RNA transcripts in living cells. Here, we describe the preparation and purification of RBMBs, delivery of RBMBs into cells by microporation and fluorescent imaging of single RNA transcripts in real-time.

The growing realization that both the temporal and spatial regulation of gene expression can have important consequences on cell function has led to the ...

Stimulation of Cytoplasmic DNA Sensing Pathways In Vitro and In Vivo

In order to efficiently stimulate an innate immune response, DNA must be of sufficient length and purity. We present a method where double stranded DNA (dsDNA) which has the requisite characteristics to stimulate the cytoplasmic DNA sensing pathways can be generated cheaply and with ease. By the concatemerization of short, synthetic oligonucleotides (which lack CpG motifs), dsDNA can be generated to be of sufficient length to activate the cytosolic DNA sensing pathway. This protocol involves blunt end ligation of the oligonucleotides in the presence of polyethylene glycol (PEG), which provides an environment for efficient ligation to occur. The dsDNA concatemers can be used, following purification by phenol/chloroform extraction, to simulate the innate immune response in vitro by standard transfection protocols. This DNA can also be used to stimulate innate immunity in vivo by intradermal injection into the ear pinna of a mouse, for example. By standardizing the concatemerization process and the subsequent stimulation protocols, a reliable and reproducible activation of the innate immune system can be produced.

Stimulation of Cytoplasmic DNA Sensing Pathways In Vitro and In Vivo

The aim of the protocol is to use optimal methods for stimulating the cytoplasmic DNA sensing pathways in cells and in vivo. This is achieved by improving the generation of long, double-stranded DNA during blunt-end ligation. Cells or mice are then transfected using a lipid transfection reagent.

In order to efficiently stimulate an innate immune response, DNA must be of sufficient length and purity. We present a method where double stranded DNA ...

Comet Assay as an Indirect Measure of Systemic Oxidative Stress

Higher eukaryotic organisms cannot live without oxygen; yet, paradoxically, oxygen can be harmful to them. The oxygen molecule is chemically relatively inert because it has two unpaired electrons located in different pi * anti-bonding orbitals. These two electrons have parallel spins, meaning they rotate in the same direction about their own axes. This is why the oxygen molecule is not very reactive. Activation of oxygen may occur by two different mechanisms; either through reduction via one electron at a time (monovalent reduction), or through the absorption of sufficient energy to reverse the spin of one of the unpaired electrons. This results in the production of reactive oxidative species (ROS). There are a number of ways in which the human body eliminates ROS in its physiological state. If ROS production exceeds the repair capacity, oxidative stress results and damages different molecules. There are many different methods by which oxidative stress can be measured. This manuscript focuses on one of the methods named cell gel electrophoresis, also known as &#8220;comet assay&#8221; which allows measurement of DNA breaks. If all factors known to cause DNA damage, other than oxidative stress are kept constant, the amount of DNA damage measured by comet assay is a good parameter of oxidative stress. The principle is simple and relies upon the fact that DNA molecules are negatively charged. An intact DNA molecule has such a large size that it does not migrate during electrophoresis. DNA breaks, however, if present result in smaller fragments which move in the electrical field towards the anode. Smaller fragments migrate faster. As the fragments have different sizes the final result of the electrophoresis is not a distinct line but rather a continuum with the shape of a comet. The system allows a quantification of the resulting &#8220;comet&#8221; and thus of the DNA breaks in the cell.

Comet assay measures DNA breaks, induced by different factors. If all factors (except oxidative stress) causing DNA damage are kept constant, the amount of DNA damage is a good indirect parameter of oxidative stress. The goal of this protocol is to use comet assay for indirect measurement of oxidative stress.

Higher eukaryotic organisms cannot live without oxygen; yet, paradoxically, oxygen can be harmful to them. The oxygen molecule is chemically relatively ...

Measuring DNA Damage and Repair in Mouse Splenocytes After Chronic In Vivo Exposure to Very Low Doses of Beta- and Gamma-Radiation

Low dose radiation exposure may produce a variety of biological effects that are different in quantity and quality from the effects produced by high radiation doses. Addressing questions related to environmental, occupational and public health safety in a proper and scientifically justified manner heavily relies on the ability to accurately measure the biological effects of low dose pollutants, such as ionizing radiation and chemical substances. DNA damage and repair are the most important early indicators of health risks due to their potential long term consequences, such as cancer. Here we describe a protocol to study the effect of chronic in vivo exposure to low doses of &#947;- and &#946;-radiation on DNA damage and repair in mouse spleen cells. Using a commonly accepted marker of DNA double-strand breaks, phosphorylated histone H2AX called &#947;H2AX, we demonstrate how it can be used to evaluate not only the levels of DNA damage, but also changes in the DNA repair capacity potentially produced by low dose in vivo exposures. Flow cytometry allows fast, accurate and reliable measurement of immunofluorescently labeled &#947;H2AX in a large number of samples. DNA double-strand break repair can be evaluated by exposing extracted splenocytes to a challenging dose of 2 Gy to produce a sufficient number of DNA breaks to trigger repair and by measuring the induced (1 hr post-irradiation) and residual DNA damage (24 hrs post-irradiation). Residual DNA damage would be indicative of incomplete repair and the risk of long-term genomic instability and cancer. Combined with other assays and end-points that can easily be measured in such in vivo studies (e.g., chromosomal aberrations, micronuclei frequencies in bone marrow reticulocytes, gene expression, etc.), this approach allows an accurate and contextual evaluation of the biological effects of low level stressors.

Measuring DNA Damage and Repair in Mouse Splenocytes After Chronic In Vivo Exposure to Very Low Doses of Beta- and Gamma-Radiation

A protocol to evaluate changes in DNA damage levels and DNA repair capacity that may be induced by chronic in vivo low dose irradiation in mouse spleen lymphocytes, by measuring phosphorylated histone H2AX, a marker of DNA double-strand breaks, using flow cytometry is presented.

Low dose radiation exposure may produce a variety of biological effects that are different in quantity and quality from the effects produced by high ...

Chromatin Immunoprecipitation Assay for the Identification of Arabidopsis Protein-DNA Interactions In Vivo

Intricate gene regulatory networks orchestrate biological processes and developmental transitions in plants. Selective transcriptional activation and silencing of genes mediate the response of plants to environmental signals and developmental cues. Therefore, insights into the mechanisms that control plant gene expression are essential to gain a deep understanding of how biological processes are regulated in plants. The chromatin immunoprecipitation (ChIP) technique described here is a procedure to identify the DNA-binding sites of proteins in genes or genomic regions of the model species Arabidopsis thaliana. The interactions with DNA of proteins of interest such as transcription factors, chromatin proteins or posttranslationally modified versions of histones can be efficiently analyzed with the ChIP protocol. This method is based on the fixation of protein-DNA interactions in vivo, random fragmentation of chromatin, immunoprecipitation of protein-DNA complexes with specific antibodies, and quantification of the DNA associated with the protein of interest by PCR techniques. The use of this methodology in Arabidopsis has contributed significantly to unveil transcriptional regulatory mechanisms that control a variety of plant biological processes. This approach allowed the identification of the binding sites of the Arabidopsis chromatin protein EBS to regulatory regions of the master gene of flowering FT. The impact of this protein in the accumulation of particular histone marks in the genomic region of FT was also revealed through ChIP analysis.

Chromatin Immunoprecipitation Assay for the Identification of Arabidopsis Protein-DNA Interactions In Vivo

Chromatin immunoprecipitation is a powerful technique for the identification of DNA binding sites of Arabidopsis proteins in vivo. This procedure includes chromatin cross-linking and fragmentation, immunoprecipitation with selective antibodies against the protein of interest, and qPCR analysis of bound DNA. We describe a simple ChIP assay optimized for Arabidopsis plants.

Intricate gene regulatory networks orchestrate biological processes and developmental transitions in plants. Selective transcriptional activation and ...

Amplification, Next-generation Sequencing, and Genomic DNA Mapping of Retroviral Integration Sites

Retroviruses exhibit signature integration preferences on both the local and global scales. Here, we present a detailed protocol for (1) generation of diverse libraries of retroviral integration sites using ligation-mediated PCR (LM-PCR) amplification and next-generation sequencing (NGS), (2) mapping the genomic location of each virus-host junction using BEDTools, and (3) analyzing the data for statistical relevance. Genomic DNA extracted from infected cells is fragmented by digestion with restriction enzymes or by sonication. After suitable DNA end-repair, double-stranded linkers are ligated onto the DNA ends, and semi-nested PCR is conducted using primers complementary to both the long terminal repeat (LTR) end of the virus and the ligated linker DNA. The PCR primers carry sequences required for DNA clustering during NGS, negating the requirement for separate adapter ligation. Quality control (QC) is conducted to assess DNA fragment size distribution and adapter DNA incorporation prior to NGS. Sequence output files are filtered for LTR-containing reads, and the sequences defining the LTR and the linker are cropped away. Trimmed host cell sequences are mapped to a reference genome using BLAT and are filtered for minimally 97% identity to a unique point in the reference genome. Unique integration sites are scrutinized for adjacent nucleotide (nt) sequence and distribution relative to various genomic features. Using this protocol, integration site libraries of high complexity can be constructed from genomic DNA in three days. The entire protocol that encompasses exogenous viral infection of susceptible tissue culture cells to integration site analysis can therefore be conducted in approximately one to two weeks. Recent applications of this technology pertain to longitudinal analysis of integration sites from HIV-infected patients.

We describe a protocol for amplifying retroviral integration sites from the genomic DNA of infected cells, sequencing the amplified virus-host junctions, and then mapping these sequences to a reference genome. We also describe techniques to quantify the distribution of integration sites relative to various genomic annotations using BEDTools.

Retroviruses exhibit signature integration preferences on both the local and global scales. Here, we present a detailed protocol for (1) generation of ...

Filtration Isolation of Nucleic Acids:  A Simple and Rapid DNA Extraction Method

FINA, filtration isolation of nucleic acids, is a novel extraction method which utilizes vertical filtration via a separation membrane and absorbent pad to extract cellular DNA from whole blood in less than 2 min. The blood specimen is treated with detergent, mixed briefly and applied by pipet to the separation membrane. The lysate wicks into the blotting pad due to capillary action, capturing the genomic DNA on the surface of the separation membrane. The extracted DNA is retained on the membrane during a simple wash step wherein PCR inhibitors are wicked into the absorbent blotting pad. The membrane containing the entrapped DNA is then added to the PCR reaction without further purification. This simple method does not require laboratory equipment and can be easily implemented with inexpensive laboratory supplies. Here we describe a protocol for highly sensitive detection and quantitation of HIV-1 proviral DNA from 100 &#181;l whole blood as a model for early infant diagnosis of HIV that could readily be adapted to other genetic targets.

We describe here a simple and rapid paper-based DNA extraction method of HIV proviral DNA from whole blood detected by quantitative PCR. This protocol can be extended for use in detecting other genetic markers or using alternative amplification methods.

FINA, filtration isolation of nucleic acids, is a novel extraction method which utilizes vertical filtration via a separation membrane and absorbent pad ...

Parallel High Throughput Single Molecule Kinetic Assay for Site-Specific DNA Cleavage

Site-specific DNA cleavage (SSDC) is a key step in many cellular processes, and it is crucial to gene editing. This work describes a kinetic assay capable of measuring SSDC in many single DNA molecules simultaneously. Bead-tethered substrate DNAs, each containing a single copy of the target sequence, are prepared in a microfluidic flow channel. An external magnet applies a weak force to the paramagnetic beads. The integrity of up to 1,000 individual DNAs can be monitored by visualizing the microbeads under darkfield imaging using a wide-field, low magnification objective. Injecting of a restriction endonuclease, NdeI, initiates the cleavage reaction. Video microscopy is used to record the exact moment of each DNA cleavage by observing the frame in which the associated bead moves up and out of the focal plane of the objective. Frame-by-frame bead counting quantifies the reaction, and an exponential fit determines the reaction rate. This method allows collection of quantitative and statistically significant data on single molecule SSDC reactions in a single experiment.

A highly parallel method for measuring the site-specific cleavage of DNA at the single molecule level is described. This protocol demonstrates the technique using the restriction endonuclease NdeI. The method can easily be modified to study any process that results in site-specific DNA cleavage.

Site-specific DNA cleavage (SSDC) is a key step in many cellular processes, and it is crucial to gene editing. This work describes a kinetic assay capable ...