Methods for degrading nucleic acid

Abstract

Methods for degrading contaminant nucleic acid. The methods use combinations of metal ions and peroxide ions to produce a variety of oxidative species that degrade nucleic acid. Methods of the invention are useful for decontaminating laboratory equipment or solutions. After the equipment or solutions have been decontaminated, the metal ion and peroxide ion solution can be deactivated by raising the temperature to dissociate the peroxide or by binding the metal ions, e.g., with a chelating agent.

Description

FIELD OF THE INVENTION

The invention generally relates to methods for degrading nucleic acid.

BACKGROUND

Bioanalytical techniques commonly require reagents, buffers, and equipment that is free of contaminating DNA. Contaminating DNA can interfere with high-sensitivity methods, such as hybrid assay, and can imperil any process that relies upon DNA amplification prior to subsequent analysis, e.g., sequencing. Contaminating DNA can originate from a researcher directly or it can be introduced by way of contaminated surfaces, reagents, buffers, or air. Accordingly, detailed protocols are available for creating “DNA-free workspaces” and for clearing buffers, etc. from contaminating DNA.

A common protocol for providing a “DNA-free workspace” relies on regular bleach washing of all laboratory surfaces and equipment, the use of absorbent bench pads on all surfaces, and the use of dedicated hood space for certain key procedures (e.g., PCR). Other protocols recommend regularly exposing surfaces to UV light in order to degrade or inactivate contaminating DNA. Maintaining a “DNA-free workspace” also involves judicious use of disposable labware, such as pipette tips and sample vials.

In addition to keeping a clean workspace, analytical reagents, washes, and buffers must be free of contaminant DNA. In many cases, this involves procuring “DNA-free” reagents or buffers from manufacturers who prepare the reagents and buffers under strict conditions and perform post-production analysis to assure that no DNA is present. For example DNA-free buffers are available from MoBio (Carlsbad, Calif.). The cost of using certified DNA-free reagents can be substantial, however. For example, 500 ml of DNA-free PBS buffer may cost approximately $50 with shipping.

In other situations, DNA contamination may be removed from reagents, buffers, and samples using DNase and DNase clean-up kits. DNases are endonucleases that catalyze hydrolytic cleavage of phosphodiester linkages in the DNA backbone. A variety of DNases are known, and they may cleave DNA in different places (e.g., ends, mid-chain, specific sequences), or cleave single-stranded DNA over double-stranded DNA, or vice versa. A DNase treatment of a reagent will typically involve introduction of prepared DNase, such as AMBION DNase I (Life Technologies, Carlsbad, Calif.) along with a buffered solution containing substrates and optimized ionic species. In some protocols, it may be necessary to use high-turnover, recombinant DNase, such as TURBO DNase, also available from Life Technologies. After a reagent or sample is treated with DNase, the DNase may be degraded with heat, alcohol, or EDTA in order to prevent interference by the DNase in subsequent processing.

While good laboratory practices and judicious use of DNase can prevent much DNA contamination, some instances of DNA contamination are harder to control. For example, precision instrument components, such as ports, injectors, and columns are not typically disposable because of the high manufacturing costs. Additionally, it may not be possible to easily decontaminate the components using standard techniques (e.g., bleach cleaning) because the cleaning compounds may damage the components. In other situations where direct decontamination is not possible, it may also not be feasible to use DNase to digest contaminant DNA because of concerns over cross-contamination or an inability to deactivate the DNase afterward with heat or alcohol. DNase degradation products may also become a source of contamination in proteomic measurements.

Additionally, reliance on certified DNA-free reagents and disposable labware is expensive and produces a large amount of solid waste.

SUMMARY

The invention generally provides methods for degrading nucleic acid. Methods of the invention are useful for decontaminating labware, reagents, buffers, and samples. Aspects of the invention are accomplished using reactive oxygen species to degrade nucleic acid. An advantage of the reactive oxygen species is their efficiency of degrading nucleic acid and the ease by which the decontaminating solution can be cleared so that sample nucleic acid is not affected by the decontaminating solution.

The reactive oxygen species can be easily produced using reactions between metal ions (Fe²⁺, Fe³⁺) and hydrogen peroxide (H₂O₂). The reactive oxygen species may include peroxides (O₂²⁻), superoxides (O₂⁻), and hydroxyl radicals (OH.), however the exact mixture of the reactive oxygen species may change with time and pH. Using metal ion and peroxide solutions, a buffer or reagent can be quickly and efficiently cleared of nucleic acid. After nucleic acid clearance, the buffer may be heated to dissociate remaining H₂O₂ or exposed to an ion exchange resin or chelating agent to remove or sequester the metal ions. The method is also useful for decontaminating laboratory equipment, separation media, and supports for separation media, such as magnetic beads.

In one instance, the invention is a method for degrading contaminant DNA associated with a separation medium or support. The method includes contacting the separation medium or support with a solution comprising metal ions and peroxide ions. The metal ions are selected from iron, manganese, copper, nickel, and cobalt, however they are typically iron, such as Fe²⁺ ions or Fe³⁺ ions. Typically, the separation medium or support is allowed to contact the solution comprising metal ions and peroxide ions for some time and then the decontamination solution is deactivated. The solution can be deactivated by raising the temperature of the solution to dissociate hydrogen peroxide or the solution can be deactivated by binding the metal ions with an ion-exchange medium or a chelating agent. In a specific embodiment, the separation medium or support is decontaminated with a solution comprising hydrogen peroxide and Fe₂(SO₄)₃.

In another instance, the invention is a method for clearing contaminant DNA from a buffer solution, analytical reagent, or sample. The method includes adding metal ions and peroxide ions to the buffer solution, analytical reagent, or sample and later heating the buffer solution, analytical reagent, or sample to dissociate the peroxide ions. In another embodiment, the method includes adding metal ions and peroxide ions to the buffer solution, analytical reagent, or sample and later binding the metal ions.

Using the methods described herein, it is possible to inexpensively and effectively remove contaminant DNA from labware, reagents, buffers, samples, laboratory equipment, separation media, and supports. Use of the methods of the invention will reduce measurement, amplification, and sequencing errors resulting from contaminant DNA.

DETAILED DESCRIPTION

The invention provides alternative methods for removing contaminant DNA from reagents, labware, and laboratory equipment. Unwanted DNA is a common source of error in bioanalytical analysis, and constant vigilance is needed when using techniques, such as PCR, where minute amounts of DNA can compromise measurements. As discussed above, while DNase may be used to decontaminate some buffers and samples, there are instances where DNase clean-up is not feasible. Additionally, bleach decontamination may not be feasible because of inaccessibility to key instrument components or reactivity with the cleaning products.

The invention uses reactive oxygen species, and reactions that produce reactive oxygen species, to oxidize and degrade DNA present in reagents, buffers, samples, and on laboratory equipment. The methods disclosed involve combining reactive oxygen species, such as hydrogen peroxide, with metal ions, such as Fe²⁺. The reagents are inexpensive, easy to combine, and the remnants of the reaction are easily removed from the reagents, buffers, etc. after degradation of DNA.

Oxidative species are recognized as a source of genetic mutation. The reactive oxygen species implicated in metal-ion degradation of DNA include peroxides (O₂²⁻), superoxides (O₂⁻), singlet oxygen (O₂*), and hydroxyl radicals (OH.). The complete mechanism for the formation of the reactive oxygen species in the presence of metal ions has not been elucidated. However, it is time and pH-dependent, and likely includes some or all of the following reactions (M=metal ion, e.g., Fe):O₂+e⁻→O₂⁻  (1)2H₂O₂→2H₂O+O₂  (2)2O₂⁻+2H⁺→O₂+H₂O₂  (3)O₂⁻+M³⁺→M²⁺+O₂  (4)M²⁺+H+H₂O₂→M³⁺+OH.+H₂O  (5)M²⁺+H₂O₂→M³⁺+OH.+OH⁻  (6)M²⁺+H₂O₂→MO₂⁺+H₂O  (7)MO₂⁺+H⁺→MOH³⁺→M³⁺+OH.  (8)M³⁺+H₂O₂→MOOH²⁺+H⁺→M²⁺+2H⁺+O₂⁻  (9)M³⁺+H₂O₂→M²⁺+H⁺+OOH⁻  (10)See Henle and Linn, “Formation, Prevention, and Repair of DNA Damage by Iron/Hydrogen Peroxide,” J. Bio. Chem., vol. 272, 19095-19098 (1997), incorporated by reference herein in its entirety.

A number of different metal ions may react with activated oxygen species through combinations of reactions 1-10 above. The metals ions include iron, manganese, copper, nickel, cobalt, or zinc. The oxidation state of the ions may be +1, +2, +3, +4, or +5, however, +2 and +3 ions are most involved in the creation of reactive oxygen species. Other oxidation states, e.g., +1, +4, +5, are achieved through oxidation or reduction with other species. Because a variety of metal ions may participate in the degradation pathway, DNA protocols often call for the sequestering of metal ions, e.g., with EDTA, to avoid these degradation processes.

Research suggests that DNA degradation in the presence of reactive oxygen species is primarily driven by abstraction of hydrogen atoms from the sugar phosphate backbone. In particular, the hydrogen atom attached to the C5′ carbon of the deoxyribose sugar is most available in double-stranded DNA, and most likely to react with a reactive oxygen species. Accordingly, cleavage of the bond between the C5′ and C6′ atoms is most often observed.

Nonetheless, upon removal of a hydrogen atom, a deoxyribose carbocation may undergo one or more rearrangements prior to cleavage. Upon rearrangement, the molecule may cleave at another point in the deoxyribose molecule, i.e., not between the C5′ and C6′ atoms. Furthermore, the stability of the carbocation (or rearrangement product) influences the equilibrium between reactants (DNA+oxidative species) and products (broken chain+water). In particular, the lack of stable carbocation products for ribose sugars explains why RNA is much less susceptible to oxidative degradation than DNA. See Gates, “The Chemical Reactions of DNA Damage and Degradation,” Reviews of Reactive Intermediate Chemistry, Platz et al., eds., p. 351-356 (2007), incorporated herein by reference.

In addition to abstracting hydrogen from the deoxyribose sugar, oxidative species may also damage DNA bases. In particular, the N7 to C8 bond in the purine moiety (adenine and guanine) and the C5 to C6 bond in the pyrimidine moiety (cytosine and thymine) are susceptible cleavage by oxidative species. Because these cleavages do not break the sugar phosphate chain, the cleavages may not lead to degradation of the DNA chain, and the bases may be repaired in subsequent processing steps. Among bases, cytosine is most likely damaged by oxidative species, followed by thymine, followed by adenine, followed by guanine. See Henle et al., “Oxidative Damage to DNA Constituents by Iron-mediated Fenton Reactions,” J. Bio. Chem., vol. 271, p. 21177-86 (1996), incorporated herein by reference. Other mechanisms, including radical-metal attachment chemistry, have also been implicated in DNA degradation. See Henle and Lin.

Methods of the invention include addition of metal ions and oxidative species to reagents, buffers, and samples. In most instances, metal ions of the desired oxidation state are readily available as salts, for example FeCl₃, Fe₂(SO₄)₃, Fe(SO₄), and (NH₄)₂Fe(SO₄)₆. High purity metal salts are available from chemical suppliers such as Sigma-Aldrich (St. Louis, Mo.). In some instances, the metal salts are water soluble. In other instances, the aqueous solubility of the metal salts is increased with the addition of acid, for example hydrochloric acid or sulfuric acids, or with the addition of alcohols. Metal salts that may be used with methods of the invention include copper salts such as Cu₂S, CuS, Cu(CH₃COO)₂, and Cu(SO₄); manganese salts such as Mn(CO₃) and Mn(SO₄); nickel salts such as NiCl₆, (NiCl₄)SO₄, and Ni(SO₄); and cobalt salts such as Co(SO₄). In some embodiments, a final concentration of metal ions is 1 μM or greater, e.g., 10 μM or greater, e.g., 100 μM or greater, e.g., 1 mM or greater, e.g., 10 mM or greater, e.g., 100 mM or greater. Because the metal ions are not consumed during the degradation, the concentration of the metal ions before and after degradation should be approximately equivalent.

The oxidative species are typically initiated by adding H₂O₂ to the reagents, buffers, or samples containing the DNA contaminants. Aqueous H₂O₂ solutions are available from chemical suppliers such as Sigma-Aldrich at a variety of concentrations, e.g., 3%, 10%, 30%. When metal ions are also present in a solution to which hydrogen peroxide is added, a number of oxidative species are created, as outlined above. The oxidative species, in turn degrade the DNA, as described above. In some embodiments, the initial concentration of H₂O₂ in the reagent, buffer, or sample is 1 μM or greater, e.g., 10 μM or greater, e.g., 100 μM or greater, e.g., 1 mM or greater, e.g., 10 mM or greater, e.g., 100 mM or greater. Because the H₂O₂ is consumed during the degradation process, the final concentration of H₂O₂ is typically smaller than the initial concentration of H₂O₂. The initial molar ratio of metal ions to peroxide ions in the solution may be between about 1:1 and about 1:50, e.g., about 1:1 to about 1:25, e.g., about 1:1 to about 1:10, e.g., about 1:5 to about 1:10.

In embodiments where it is feasible to adjust the pH (i.e., not buffers), the conditions of the DNA degradation may be adjusted by modifying the pH of the solution. This may be accomplished with the direct addition of acids (e.g., hydrochloric acid, sulfuric acid, or acetic acid) or bases (e.g., sodium hydroxide, potassium hydroxide). It may also be accomplished with the addition of acid- or base-generating species (e.g., chlorine gas, or sodium hypochlorite). In some embodiments, the pH of a degradation solution or a regent being decontaminated will be greater than or equal to pH=3, e.g., greater than or equal to pH=4, e.g., greater than or equal to pH=5, e.g., greater than or equal to pH=6, e.g., greater than or equal to pH=7, e.g., greater than or equal to pH=8, e.g., greater than or equal to pH=9, e.g., greater than or equal to pH=10, e.g., greater than or equal to pH=11, e.g., pH=12. In some embodiments, the pH of a degradation solution or a regent being decontaminated will be less than or equal to pH=12, e.g., less than or equal to pH=11, e.g., less than or equal to pH=10, e.g., less than or equal to pH=9, e.g., less than or equal to pH=8, e.g., less than or equal to pH=7, e.g., less than or equal to pH=6, e.g., less than or equal to pH=5, e.g., less than or equal to pH=4, e.g., pH=3. The solution may have a pH of from about 3 to about 12, e.g., from about 4 to about 11, e.g., from about 5 to about 10, e.g., from about 6 to about 9, e.g., from about 6 to about 8, e.g., about 7.

Prior to using the newly-prepared DNA-free reagents, buffers, or samples, it will often be necessary to remove excess oxidative species and/or metal ions. Typically, any excess H₂O₂ can be removed by heating the solution, thereby increasing the rate at which H₂O₂ dissociates into oxygen gas and water (see equation 2 above). With the removal of excess H₂O₂, there are few sources for additional oxidative species, and the degradation reactions will quickly terminate once the other oxidative species (e.g., superoxides, hydroxyl radicals) have reacted. In one embodiment, a reagent, buffer, or sample will be heated to at least about 35° C., e.g., at least about 40° C., e.g., at least about 45° C., e.g., at least about 50° C., e.g., at least about 55° C., e.g., at least about 60° C., e.g., at least about 65° C., e.g., at least about 70° C., e.g., at least about 75° C., e.g., at least about 80° C., e.g., at least about 85° C. The reagent, buffer, or sample will be heated for at least about 5 minutes, e.g., at least about 10 minutes, e.g., at least about 15 minutes, e.g., at least about 20 minutes, e.g., at least about 30 minutes, e.g., at least about 60 minutes. When samples containing proteins or RNA are being decontaminated, it is important to monitor the temperature of the solution during the decontamination so that the proteins or RNA are not damaged. For example, the solution may be kept at a temperature between 35 and 45° C. for a period of time to assure that all H₂O₂ is dissociated, but that the proteins or RNA remain, for the most part, intact.

In some heat-sensitive applications, it may be necessary to use enzymes that facilitate dissociation of oxidative species, such as superoxides, to deactivate reactive oxygen species. Enzymes such as superoxide dismutases (SODs) may be used to “turn off” the reactions once the degradation is complete. SOD is commercially available from Sigma-Aldrich. SOD may also sequester free metal ions, which are cofactors to the dissociate reactions, however this is not the dominant mechanism for reducing oxidative degradation. While SOD administration is an option for terminating oxidative degradation reactions, it is expensive and results in many of the same complications seen with the use of DNase (discussed above).

In addition to removing the oxidative species, e.g., H₂O₂, it may be beneficial to remove metal ions from the reagents, buffers, or samples after the degradation process is complete. Free metal ions can be removed using a number of known techniques, and some techniques will be better than others depending upon the intended use of the reagents, buffers, or samples. In one embodiment, the metal ions may be removed using an ion-exchange column, such as available from GE Healthcare Biosciences (Pittsburgh, Pa.). Using ion-exchange chromatography the reagents, buffers, or samples are put in contact with a stationary phase having ionic functional groups that bind ions of opposite charge, e.g., M²⁺. The reagents, buffers, or samples either pass through the stationary phase, as in a column, or the reagents, buffers, or samples are agitated with the stationary phase and then the stationary phase removed, decanted, etc.

In another embodiment, the free metal ions can be removed using chelating agents such as EDTA, citric acid, or phosphonates. Chelating agents are widely available from chemical suppliers such as Sigma-Aldrich. In some instances, the chelating agents do not remove the metal ions from the reagents, buffers, or samples, but rather deactivate them by forming complexes which remain in the solution. In other instances, the chelating agents cause the metal ions to precipitate from solution. The precipitate may be removed with filtering, for example.

The methods of the invention may also be used to decontaminate laboratory equipment, including labware, scientific instruments, and portions thereof. For example, an aqueous degradation solution comprising Fe₂(SO₄)₃ and H₂O₂ may be prepared and the laboratory equipment allowed to soak (incubate) in the solution for some time, e.g., 1 hour or longer, 2 hours or longer, 4 hours or longer, 8 hours or longer, 12 hours or longer, or 24 hours or longer. After soaking, the laboratory equipment can be rinsed (soaked, incubated) with DNA-free water, and then allowed to dry in a DNA-free hood or baked in an oven.

In some instances, an aqueous solution comprising Fe₂(SO₄)₃ and H₂O₂ can be used to decontaminate separation media, such as polymer beads or magnetic beads. In other embodiments, an aqueous solution comprising Fe₂(SO₄)₃ and H₂O₂ can be used to decontaminate a support, i.e., the precursor to the separation medium. After preparation of a degradation solution, the separation media can be allowed to soak in the solution for some time, e.g., 1 hour or longer, 2 hours or longer, 4 hours or longer, 8 hours or longer, 12 hours or longer, or 24 hours or longer. In instances where the separation media is packed, e.g., in a column, it may be sufficient to add the degradation solution directly to the column and allow the separation media to soak in the presence of the degradation solution for some time, e.g., 1 hour or longer, 2 hours or longer, 4 hours or longer, 8 hours or longer, 12 hours or longer, or 24 hours or longer. The separation media can be rinsed with DNA-free water, and then allowed to dry in a DNA-free hood or baked in an oven. In some instances the separation media will be decontaminated prior to activation or binding of ligands e.g., antibodies. In other instances, the separation media will be decontaminated after the separation media has been activated or ligands bound thereto.

INCORPORATION BY REFERENCE

References and citations to other documents, such as patents, patent applications, patent publications, journals, books, papers, web contents, have been made throughout this disclosure. All such documents are hereby incorporated herein by reference in their entirety for all purposes.

EQUIVALENTS

The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The foregoing embodiments are therefore to be considered in all respects illustrative rather than limiting on the invention described herein. Scope of the invention is thus indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

Claims

1. A method for degrading nucleic acid, the method comprising: contacting nucleic acid with a solution comprising adding metal ions and peroxide ions to degrade the nucleic acid, such that the nucleic acid is not available for amplification; andbinding the metal ions subsequent to degradation of the nucleic acid.

2. The method of claim 1, wherein the metal ion to peroxide ion molar ratio is from about 1:1 to about 1:25.

3. The method of claim 1, wherein the metal ion to peroxide ion molar ratio is from about 1:5 to about 1:10.

4. The method of claim 1, wherein the nucleic acid is associated with a separation medium or a solid support.

5. The method of claim 4, wherein the solid support is a bead.

6. The method of claim 5, wherein the bead is a type selected from the group consisting of magnetic, paramagnetic, ferromagnetic, and a combination thereof.

7. The method of claim 4, wherein the solid support is a planar substrate.

8. The method of claim 4, further comprising removing the separation media or support from the solution and subsequently rinsing the separation media or support with an aqueous solution.

9. The method of claim 1, wherein the metal ions are selected from the group consisting of iron, manganese, copper, nickel, and cobalt.

10. The method of claim 9, wherein the metal ions are Fe2+ ions or Fe3+ ions or a combination thereof.

11. The method of claim 1, wherein the solution comprises hydrogen peroxide.

12. The method of claim 1, wherein the solution has a pH of about 3 to about 12.

13. The method of claim 12, wherein the solution has a pH of about 3 to about 6.

14. The method of claim 1, wherein the solution has a concentration of about 1 mM or greater of metal ions.

15. The method of claim 1, wherein contacting lasts for at least about 10 minutes.

16. The method of claim 15, wherein contacting lasts for at least about 60 minutes.

17. The method of claim 1, further comprising heating the solution to dissociate peroxide ions.

18. The method of claim 17, wherein the temperature of the solution is raised to at least about 40° C.

19. The method of claim 1, wherein binding comprises contacting the solution with an ion-exchange medium or chelating agent.

20. The method of claim 1, wherein the solution is an aqueous solution comprising hydrogen peroxide and Fe2(SO4)3.

21. The method of claim 1, wherein binding comprises contacting the solution with an ion-exchange medium or chelating agent.

22. A method for degrading nucleic acid, such that the nucleic acid is unavailable for amplification, the method comprising: contacting nucleic acid with a solution comprising metal ions and peroxide ions; andheating the solution to dissociate the peroxide ions for at least about 15 minutes; andbinding the metal ions subsequent to degradation of the nucleic acid.

23. The method of claim 22, wherein a temperature of the solution is raised to at least about 40° C.

24. The method of claim 22, wherein binding comprises contacting the solution with an ion-exchange medium or chelating agent.

25. The method of claim 22, wherein the metal ions are Fe2+ ions or Fe3+ ions or a combination thereof.

26. The method of claim 22, further comprising adjusting a pH of the solution to between about 3 and 12.