METHOD OF NUCLEIC ACID TRANSFER

Abstract: A method of dosaging a nucleic acid species comprises contacting a solid phase member or members (2), each member having a predetermined capacity of binding said nucleic acid species, with a sample containing the nucleic acid species to bind the nucleic acid species thereto, and then releasing the bound nucleic acid species into a desired processing or analytical means (8).

Description: METHOD OF NUCLEIC ACID TRANSFER TECHNICAL AREA OF THE INVENTION The present invention relates to the dosaging of nu- cleic acid fragments, and more particularly to the transfer of a predetermined amount of a nucleic acid species from a sample solution to a processing or analytical means. BACKGROUND OF THE INVENTION The vast majority of DNA in higher organisms is iden- tical in sequence among the chromosomes of different indi¬ viduals. A small fraction of DNA, however, is variable or polymorphic in sequence among individuals, the formal defi¬ nition of polymorphism being that the most frequent variant (or allele) has a population frequency which does not ex- ceed 99%. The analysis of DNA polymorphisms was originally based on the variations in the lengths of DNA fragments produced by restriction enzyme digestion due to sequence variations in one of the recognition sites for the specific enzyme used, hence the name restriction fragment length polymor¬ phisms (RFLP's) . The DNA fragment lengths are determined based on their migration rates on an electrophoretic gel, and a specific set of fragments may be characterized by its fragment pattern. Visualization of the separated fragment bands may be performed by blotting techniques using e.g. radioactively labelled probes. The DNA fragments to be analysed may also be produced by amplification of polymorphic loci, such as by PCR (polymerase chain reaction) with suitable primers. This technique is particularly applicable to the analysis of polymorphic markers represented by repeated sequence mo ¬ tifs, such as di-, tri- or tetranucleotide repeats. Such variations may also be implicated in human disease, as ex ¬ emplified by the fragile X syndrome and Huntington's dis- ease caused by trinucleotide repeats. In the case of ampli ¬ fied fragments, detection of the fragments may easily be provided for by labelling the primer/primers, or one or more of the nucleotides used in the extension reactions, with a fluorophore, e.g. fluorescein. For instance, for use of fragment analysis in forensic medicine, a set of, say, four different primer pairs may be selected which together give a fragment analysis pattern with a 99.9% security in distinguishing between individuals. The primers are de¬ signed to generate products with a moderate variation, which taken together give such a high security. Today, the fragment analysis are often performed in automatic DNA se¬ quencing apparatus using fluorescent label detection. A problem when analysing fragments produced by amplification procedures, such as PCR, or RFLP (restriction fragment length polymorphism) on DNA sequencing apparatus is to load the correct amount of amplified product on the electropho- retic gel. While a PCR reaction normally generates DNA product quantities of the order of picomoles, only a quan¬ tity of the order of attomole to femtomole, i.e. a factor of 1000 less or more, is required for fragment analysis. Many times the signals tend to "hit the ceiling" due to overloading of the fragment product . Of course, the amount of PCR products also vary considerably with the degree of success of the amplification. It is therefore common prac¬ tice to dilute the samples prior to making a test loading on the gel, or to produce a serial dilution series to en¬ sure that a correct sample concentration is obtained for some dilution. SUMMARY OF THE INVENTION The object of the invention is to provide a method for accurate dosaging of nucleic acid species from a sample so¬ lution to a processing or analytical unit, such as a reac- tion vessel or an electrophoretic gel or capillary. In par¬ ticular, the invention seeks to overcome the above dis¬ cussed problem in fragment analysis. Our copending international application PCT/SE93/00929 entitled "A method of processing nucleic acid sample", which is not yet open to public inspection, discloses the use of a manifold with multiple solid phase members, such as a comb structure, for the capturing and transfer of nu¬ cleic acid species from a receptacle or receptacles to an analyzer unit where the nucleic acid species is released. The present invention proposes a development and generali¬ zation of that concept to the extent that it be applied to the dosaging of a predetermined amount of nucleic acid from a sample solution into a processing or analytical unit . This is accomplished by properly adjusting the binding ca¬ pacity of each solid phase member with respect to the nu¬ cleic acid species so that a predetermined quantity of the nucleic acid species will bind to the solid phase members and a controlled amount will then be released into the de¬ sired processing or analytical unit. Thus, the invention generally provides a method for dosaging a nucleic acid species, which method comprises contacting a solid phase member or members, each member having a predetermined capacity of binding said nucleic acid species, with a sample containing the nucleic acid species to bind the nucleic acid species thereto, and then releasing the bound nucleic acid species into a desired processing or analytical means. With the present invention it is possible to dosage a predetermined amount of nucleic acid species, from a high concentrated solution as well as from a solution with a low concentration. In a preferred embodiment the amount of nucleic acid species in the sample is in an amount exceeding the capacity of the solid phase member or members. It is also possible, with the method ac¬ cording to the invention, to concentrate a sample with a low amount of nucleic acid species, an amount below the binding capacity of the solid phase member. The low concen- ' tration can be the result of bad amplification. By contact- ing the solid phase member with the bad amplified sample the predetermined amount will bind to the member. While the invention thus generally relates to accurate dosaging of nucleic acid species, usually DNA, in a pre¬ ferred embodiment of the invention the nucleic acid species are fragments generated by amplification reactions, such as PCR, which fragments are loaded on an electrophoretic gel or capillary in a fragment analysis procedure. The term solid phase members is to be understood in a broad sense and includes e.g. beads, such as magnetic beads, dip-sticks, etc. Preferably, however, the solid phase members are part of a manifold of the type disclosed in our aforementioned PCT application PCT/SE93/00929, as will be described below. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a front view of a comb-like manifold with eight prongs; Fig. 2 is a schematic sectional view of a well strip with eight wells; Fig. 3 is a schematic front view showing the manifold of Fig. 1 aligned with the sample wells of an analytical apparatus; and Fig. 4 is a diagram showing the binding of biotiny- lated PCR product to a streptavidin coated manifold of Fig. 1 in the presence of varying concentrations of biotinylated primer. Fig. 5 present two chromatograms showing the problem with overloading of fragment product. DETAILED DESCRIPTION OF THE INVENTION The comb-like manifold schematically illustrated in Fig. 1 and generally designated by reference numeral 1 is disclosed in our aforementioned international patent appli- cation PCT/SE93/00929 (the disclosure of which is incorpo¬ rated by reference herein) . The manifold 1 has eight prongs or teeth 2. The shape of the teeth 2 and their spacing are adapted to the sample wells of an electrophoretic apparatus as will be described in more detail below. In the figure, the teeth 2 have been derivatized, indicated by shading 3, e.g. coated with avidin-conjugated particles as described in Example 1 below. The manifold 1 is designed to cooperate with corre¬ sponding well set, such as that shown in Fig. 2, which has eight individual wells 6, each well 6 being adapted to re¬ ceive a single manifold tooth 2 and here also shown as par¬ tially filled with sample solution 7. As mentioned above, manifold 1 is adapted to cooperate with the sample wells of an analytical apparatus . This is schematically illustrated in Fig. 3, where manifold 1 is placed above an electrophoretic apparatus, generally desig- nated by reference numeral 8, with the teeth 2 aligned with and capable of being received in respective sample wells 9 of the electrophoretic apparatus. To permit the binding of said nucleic acid species to the comb teeth 2, there is immobilized to each tooth sur- face a molecule or group capable of interacting (i) with the nucleic acid species per se or (ii) with a functional group or molecule incorporated into the nucleic acid spe¬ cies. In the first case (i) , the solid phase surface may support an oligonucleotide, e.g. covalently bound to the solid phase, which oligonucleotide is complementary with and therefore capable of hybridizing with the nucleic acid species. In case the nucleic acid species is an amplifica¬ tion product, such as obtained by PCR, the hybridizing re- gion thereof should be located internally to prevent the oligonucleotide from hybridizing with non-extended primers in the reaction solution. In the second case (ii) , the functional group incorpo¬ rated into the nucleic acid may be one member of a specific binding pair, the other member being supported by the solid phase. Exemplary of such binding pairs are biotin - avidin, biotin - streptavidin, cystein - thiol groups, antigen - antibody, lectin - sugar. A particularly useful binding pair in the present context is biotin - avidin (or strepta- vidin) . The use of such a specific binding pair for immobi¬ lizing DNA to a solid phase is described in more detail in e.g. WO 89/099282 (the disclosure of which is incorporated by reference herein) . An alternative binding principle (iii) which is advan- tageous in the context of amplification products is the use of a DNA binding protein on the comb teeth surfaces, which protein recognizes sequences added to the 5 ' -end of an am¬ plification primer, which sequences, however, only can be bound in double stranded form, as is per se known in the art . Such an approach would also prevent the capturing of primers by the solid phase. For the purposes of the invention, the comb teeth 2 may advantageously be provided with a substantially ex¬ panded surface area, permitting increased surface loading, by coating the surfaces with porous particles . A suitable coating method involving the attachment of particles to the surface without the use of adhesive is described in our in- ternational patent application PCT/SE93/00928 entitled "A method of surface modification" (the disclosure of which is incorporated by reference herein) . Such particle coating of the solid phase members will readily provide for the achievement of sufficient binding capacities for the in- tended purposes . The immobilization to the solid phase surface of pre¬ determined quantities of the group or molecule capable of binding the nucleic acid species such that a well defined binding capacity is obtained is readily performed by the skilled person with guidance of the above and well known methods in the art. By introducing the comb teeth 2 into a solution con¬ taining the desired nucleic acid species, an amount thereof determined by the binding capacity of the prepared teeth is bound to each tooth surface. This bound nucleic acid mate¬ rial may then be transferred to and released in a desired processing or analytical unit. The necessary environment and conditions for the release or desorption of the nucleic acid material from the comb teeth varies depending on the particular species to be released and the particular proc¬ essing or analytical unit. For example, in the case of an analysis comprising gel electrophoresis, the manifold is designed such that the solid phase members thereof may be conveniently introduced into the sample wells of the elec- trophoretic gel plate, in which wells desorption of the synthesized nucleic acid strands may be effected by means of, for example, a denaturant, such as formamide, heat and/or a denaturing pH, e.g. alkali. An exemplary use of the comb structure in Fig. 1 in fragment analysis will now be described. First, a PCR amplification is performed on chromosomal DNA, utilizing a battery of primer pairs selected to pro- vide for the amplification of specific loci exhibiting some type of polymorphism. One primer in each pair is labelled with a detectable element, such as a fluorophore, say, e.g., fluorescein, and the other primer in the primer-pair is labelled with a separation element, such as one member of a specific binding pair, say, e.g., biotin. When the PCR amplification is completed, the reaction solution contains inter alia double stranded amplification products, wherein one strand carries fluorescein and the other strand has a biotin end, as well as non-extended primers. In the illus- trated case, eight different PCR amplifications may be per¬ formed in the eight wells 6 of the receptacle in Fig. 2. Prior to the PCR amplification, the teeth 2 of a comb 1 in Fig. 1 have been provided with a predetermined amount of avidin, for example by the above described coat- ing procedure. This amount has been selected to provide a defined biotin binding capacity of the teeth 2 which is well below the expected level of biotin-labelled PCR prod¬ ucts in the reaction solution. The comb teeth 2 are then inserted into the corresponding wells 6 to capture the de- sired quantity of biotin-labelled double stranded amplified DNA products corresponding to the polymorphic loci of the chromosomal DNA. After washing of the comb teeth 2 with bound DNA frag¬ ments, the comb 1 is now transferred to and aligned with the electrophoretic apparatus 8 as shown in Fig. 3, and the manifold teeth 2 are introduced into the sample wells 9 of the electrophoretic apparatus 8. This apparatus may, for instance, be an automatic DNA sequencer, for example an A.L.F. Sequencer™ (supplied by Pharmacia LKB Biotechnology AB, Sweden) which has 40 electrophoresis lanes, laser light excitation and photodiode detectors for detection of fluo¬ rescence, one detector for each lane. The fluroescein- labelled strands of each bound DNA fragment are then melted off, e.g. by the sample wells 8 containing a suitable agent solution, e.g. formamide, for effecting elution of ' the strands. After elution for a few minutes, the comb 1 is lifted out from the wells 9, and the electrophoretic proc- ess is started. Hereby a predetermined well defined amount of labelled DNA fragments have been applied to the electro¬ phoretic gel, insuring that an appropriate signal within the dynamic range of the analyzer is obtained. From the mi¬ gration patterns obtained in each lane, polymorphic charac- teristics are determined in conventional manner. Experiments have indicated that the binding of double stranded PCR products are not affected to any great extent by even a large excess of biotinylated primers, as shown in Example 2 below. In the following, the preparation of a multipronged manifold with coated prongs as well as binding tests with a biotin-coated manifold are illustrated by two non-limiting examples. EXAMPLE 1 A. Con-iuσation of avidin to Sepharose® particles Sepharose® particles (HiTrap®, NHS-activated Se¬ pharose® HP, Pharmacia LKB Biotechnology AB, Uppsala Swe¬ den) corresponding to 6.0 ml of sedimented material, were carefully washed with ice cold 1 mM HC1 (3 x 10 ml) on a sintered funnel, making sure that the Sepharose® surface did not at any time become dry. The particles were quickly washed with a solution of 1.0 M NaCl and 0.4 M NaHCθ3, buffered to pH 8.3, and transferred to a final volume of 5 ml of the above buffer, containing 10 g of avidin. The suspension was incubated rotating end-over-end for one hour, filtered, and the particles were blocked in 0.1 M ethanolamine buffer, pH 8.3, for 15 minutes. The avidin- conjugated Sepharose® particles were then washed with 0.1 M acetate buffer, pH 4.0, and used immmediately or stored in 0.05 M Tris-buffer, pH 7.3, with 0.02% (w/v) sodium az- ide. B. Attachment of particles to a polystyrene solid support Avidin-conjugated particles as prepared above were filtered, washed with distilled water, dried with methanol (3 x 5 ml) , and then equilibrated with triethylamine (Et3N; 3 x 5 ml) . The solid was quickly transferred to a suitable vessel and Et 3 N was added to obtain a slurry of about 75% (v/v) particles. A polystyrene support, configured as a mi- crotiter plate lid with 8 rows of 12 pin-and-ball exten- sions adapted to project into individual microtiter wells of a corresponding microtiter plate (F.A.S.T. system, Fal¬ con, Oxnard, California, U.S.A.) was washed with ethanol for 20 minutes in an ultrasonic bath and the particles were then grafted onto the projections from the polystyrene sup- port by two submersions in the slurry, each for 2 seconds, followed by immediate evaporation of the residual Et N in air. After washing in deionized water, detached particles were collected and reused. Loosely bound particles were re¬ moved by a 10 minutes incubation shaking in water. The manifold was stored until use in buffer (1 M NaCl, 100 mM Tris-HCl, pH 7.5, and 0.1% (v/v) Triton X 100) , with the addition of 0.5% (w/v) fat-free dry milk and 0.02% sodium azide. Testing of the binding capacity of the avidin-coated support by means of a 32 P-labelled oligonucleotide, 5'- modified with biotin, indicated that each prong of the sup¬ port could bind in the order of 20 pmol of biotinylated oligonucleotides. By the guidance of the above, the person skilled in the art may readily prepare solid phase surfaces of any de¬ sired type with a desired binding capacity for each special application. EX MP E 2 Study of the influence of residual primer concentration on the binding of PCT products 20 μl PCR amplification reactions were carried out on the DQB-gene in separate wells (DQB is a transplantation antigen) using biotinylated and radioactively labelled primers, respectively. After completed amplification, the PCR products were purified from the primers. Different amounts of biotinylated primer were then added to each well. In successive experiments, the teeth of a polystyrene comb of the type shown in Fig. 1, the tips of which had been coated with streptavidin-Sepharose® by the procedure described in Example 1, were then inserted and incubated in each well to bind the amplified DQB-gene product thereto. The total radioactivity as well as the radioactivity of the comb after removal thereof from the well and washing were then measured. The results are shown in Fig. 4, where the fraction of bound PCR products is plotted against the con¬ centration of biotinylated primer (pmol oligo) . As can be seen from the diagram, substantially the same fraction is bound independently of the concentration of biotinylated primers up to a high primer concentration. EXAMPLE 3 In a further experiment the teeth of a polystyrene comb of the type shown in Fig. 1, the tips of which had been coated w th streptavidin-Sepharose by the procedure described in Example 1, were inserted and incubated in each well to bind the amplified product thereto. The result is shown in Fig. 5. In the chromatograms in Fig. 5 the main peaks refer to samples and the small peaks to internal ref¬ erences. In the first chromatogram 1 μl of the original sample has been loaded directly onto the instrument. As can be seen from the figure the main peak has "hit the ceiling" due to saturation of the detector. In the second chroma- togram the solid phase member, with a predetermined binding capacity, has been contacted with the original sample for 5 minutes, whereafter the solid phase member has been placed on the instrument, whereby the sample is eluated and de¬ tected. The obtained sample peak is within the upper limit of the instrument. The invention is, of course, not restricted to the em¬ bodiments specifically described above and shown in the drawings, but many changes and modifications may be made without departing from the scope of the general inventive concept as defined in the following claims.

Claims: CLAIMS 1. A method of dosaging a nucleic acid species, which method comprises contacting a solid phase member or members (2) , each member having a predetermined capacity of binding said nucleic acid species, with a sample containing the nu¬ cleic acid species to bind said nucleic acid species thereto, and then releasing the bound nucleic acid species into a desired processing or analytical means (8) . 2. A method of claim 1, characterized in that the amount of nucleic acid species n the sample is in an amount ex¬ ceeding the binding capacity of the solid phase member or members . 3. A method of claim 1 characterized in that the amount of nucleic acid species in the sample is in an amount below the binding capacity of the solid phase member or members. 4. The method of claim 1, 2 or 3, characterized in that said nucleic acid species are products obtained in amplifi¬ cation reactions. 5. The method of any one of the claims 1 to 4, character¬ ized in that it comprises binding double stranded DNA via one strand thereof to the solid phase member or members, and then releasing the other DNA strand to said processing or analytical means (8) . 6. The method of any one of the claims 1 to 5, charac¬ terized in that said release of tne nucleic acid species from the solid phase is effected by treatment with a dena- turant, for example selected from formamide, heat and a de¬ naturing pH, such as alkali, and combinations thereof. 7. The method of any one of claims 1 to 6, characterized in that said solid phase members are part of a manifold, preferably a comb-like element (1) having a number of teeth (2) , e.g. four or eight, forming said solid phase members. 8. The method of any one of claims 1 to 7, characterized in that said analytical means comprises an electrophoretic separation gel or capillary (8) and has sample wells (9) adapted to receive said manifold teeth (2) . 9. The method of any one of claims 1 to 8, characterized in that it is part of a fragment analysis procedure.

Inventors: ASP ALLAN, CARSTENIUS PEDER, LANDEGREN ULF, ASP, ALLAN, CARSTENIUS, PEDER, LANDEGREN, ULF
Applicants: ASP ALLAN, CARSTENIUS PEDER, LANDEGREN ULF, PHARMACIA BIOTECH AB, ASP, ALLAN, CARSTENIUS, PEDER, LANDEGREN, ULF
Application year: 1995
Main classification: C12Q1
IPC codes: C12N15, C12Q1, C40B40, C40B60