Binding Nucleic Acid to a Support

This invention relates to binding nucleic acids (RNA and DNA) to supports, e.g. to carry out DNA hybridization assays.

Such assays have been used to detect specific DNA sequences in samples for several years, and are described in the patent and technical literature, e.g. Falkow et al. U.S. Pat. No. 4,358,535, hereby incorporated by reference. Such assays typically involve spotting a sample, e.g. urine, suspected of containing a particular DNA sequence (in viruses or prokaryotic or eukaryotic cells in the sample) onto a DNA-binding support, e.g. a nitrocellulose membrane, lysing the cells, if necessary, denaturing and neutralizing the DNA, and then affixing the DNA to the support prior to carrying out the hybridization assay. Affixation is typically carried out by air drying followed by drying in a vacuum oven for two hours, as described, e.g., in Gillespie et al. (1965) J. Mol. Biol. 12, 829.

In general, the invention features a method of binding nucleic acid (DNA or RNA) to a nucleic acid-binding support including depositing the nucleic acid on the support and then contacting the nucleic acid and the support with a liquid binding solution which is compatible with the support and which contains an organic solvent capable of binding the nucleic acid to the support, for a period of time sufficient to effect binding.

In preferred embodiments, the nucleic acid is included in a sample to be assayed by hybridization and the binding solution does not alter the DNA in a manner which interferes with hybridization; and the organic solvent contains fewer than 20 carbon atoms, makes up substantially all of the solvent, and is an alcohol, ether, aromatic compound, or ketone, most preferably ethanol, methanol, sec-butyl alcohol, iso-amyl alcohol, isopropyl alcohol, isobutyl alcohol, ethyl ether, or toluene. In other preferred embodiments, the binding solution contains a mixture of more than one such organic solvent and the period of time during which the nucleic acid and the support are contacted with the binding solution is 1 second to 10 minutes, most preferably about 5 minutes.

The method of the invention permits nucleic immobilization in very short times; i.e., in most instances on the order of five minutes or less. Thus the total time required to complete Dot blots, Southern blots, colony lifts, and any other technique requiring denatured nucleic acid immobilization on a support is greatly reduced. Furthermore, the method obviates the use of expensive equipment such as vacuum pumps and vacuum ovens.

Other features and advantages of the invention will be apparent from the following description of the preferred embodiments, and from the claims.

A very wide range of organic solvents, in many different classes of organic compounds, can be used to bind nucleic acid to nucleic acid-binding membranes. The most preferred class of compounds are alcohols, which are generally less toxic and therefore easier to work with than other classes of compounds. Other classes of compounds which will bind nucleic acid, but which are less desirable than alcohols, are ethers, e.g. ethyl ether; aromatic compounds, e.g. toluene; and ketones, e.g. acetone. Still other classes of organic solvents are alkanes, alkenes, alkynes, esters, and hetero-organic compounds such as halogenated alkanes, e.g. chloroform and carbon tetrachloride. For all classes of organic solvents, considerations such as expense dictate a preferred size of fewer than 20 carbon atoms, and most preferably fewer than 10 carbon atoms.

The choice of solvent in a particular instance will be dictated by several factors. First, the binding solution containing the solvent should not alter the nucleic acid in a way which interferes with the intended purpose of the binding procedure, e.g., if the nucleic acid is being immobilized for a hybridization assay, it must be capable of hybridizing after treatment with the solvent.

Secondly, binding solution containing the solvent must be compatible with the support being used; i.e. the solvent should not dissolve the support to an extent which interferes with the purpose of the binding procedure. Some nucleic acid-binding supports are more susceptible to being dissolved by organic solvents than others. Thus, practically any organic solvent can safely be used with the highly solvent-resistant nylon-based supports, while the choice is more limited for more easily dissolved supports such as nitrocellulose. Thus, for example, absolute methanol and acetone are incompatible with nitrocellulose, since they cause a degree of softening of the membrane which is unacceptable in hybridization assays, but are compatible with nylon-based supports. Where the organic solvent and the support are incompatible, the nucleic acid binding solution can often be modified, e.g. by dilution with water or by combination with a milder organic solvent. Thus, for example, 90% methanol is an effective binding solution and is compatible with nitrocellulose, while absolute methanol is not.

Finally, the binding solution should be a liquid at the temperature of use. In many cases this will be room temperature, but in some instances may be higher or lower.

An example of the method, in which Hepatitis B viral DNA is detected in blood serum, is as follows. A 7 .mu.l sample of blood serum is spotted onto a 0.45 .mu.M nitrocellulose membrane and allowed to air-dry. The DNA in the sample is denatured by immersing the membrane in 0.5M NaOH, 1.5M NaCl for 1 min., and then neutralized by immersing the membrane in 1.0M Tris (pH 7.5), 3M NaCl for 1 min. The membrane is then allowed to air-dry and the DNA bound to the membrane by immersing the membrane in anhydrous sec-butyl alcohol for 5 min. The membrane is then removed and air-dried.

The membrane, to which DNA is bound, is placed in a plastic bag, to which is then added hybridization solution of the composition 6X SSCP (0.90M NaCl, 0.090M Na.sub.3 Citrate, 0.12M Phosphate buffer pH 7.0), 2X Denhardt's solution (0.04% bovine serum albumin, 0.04% polyvinylpyrollidine, 0.04% ficoll 500), 40% formamide, 10% Dextran sulphate, 500 .mu.g/ml salmon sperm DNA, 1.6 mg/ml additional bovine serum albumin. Radioactively labelled Hepatitis B DNA probe (specific activity=2-3X 10.sup.8 cpm/.mu.g) is then added, in an amount corresponding to 1.times.10.sup.7 counts per ml hybridization solution.

The plastic bag is sealed and hybridization allowed to proceed for 2-3 hours at 37.degree. C. The membrane is then removed and washed with 3mM tris-base for 20 min. to remove non-specifically bound probe. The washed membrane is placed under X-ray film and autoradiographed for 4-24 hours, to quantitatively determine Hepatitis B DNA in the blood serum sample.

Other embodiments are within the following claims. For example, the nucleic acid binding technique can be used in conjunction with any suitable nucleic acid binding support, e.g., Pall Biodyne nylon membranes; and the sample and nucleic acid can be from any desired source (e.g. bacterial or eukaryotic DNA in urine or sputum samples; viral RNA in blood samples); and the nucleic acid can be purified prior to spotting.