Abstract:
The invention concerns a method and device for comparing the sequence of test DNA molecule contained in a first solution with the sequence of a reference comprising the steps of extracting ( 11 ) single-stranded DNA molecules from the first solution; mixing ( 14 ) a part of the solution containing the test DNA molecules with reference DNA molecules ( 13 ) attached to a solid support and having a sequence complementary to that of the sequence of reference; filtering ( 15 ) the mixture with a filter having a cut-off size chosen not to retain eventual free test DNA molecules but to retain the solid support and the test DNA molecules attached to it; introducing a solvent ( 16 ) in the filtered solution; illuminating ( 17 ) the final mixture; and measuring ( 18 ) the opacity or turbidity of the illuminated mixture, a non transparency indicating a non identity of the test and reference DNA sequences.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a device and a method for determining whether two DNA sequences are identical.  
         BACKGROUND  
         [0002]    A comparison between two DNA molecules may be useful in many applications. For example, a well-known application is determining whether the DNA of a suspect was present at the scene of a crime by comparing a DNA sequence with another sequence found at the crime location. Indeed, DNA molecules can be found and in any biological substance from a person (for example, a small quantify of saliva or sperm or a single hair). Very small samples contain enough DNA for this type of comparative analysis to be performed.  
           [0003]    Another example of this type of application concerns the authentication of valuable objects (for example, a painting or any other art object). In such an application, synthetic DNA is generally utilized. A particular DNA sequence is included, for example, in the paint. Then, many years later, it is possible to compare the DNA molecule embedded in the paint with a reference molecule to authenticate the painting.  
           [0004]    As human genetic materials differ from each other essentially by single mutations at well defined positions, e.g. (SNPs), the actual implementation of this type of approach in forensic science generates a list of the values of the nucleotides at these positions and these lists are compared (see DNA Technology in Forensic Science by the National Res. Council, ed.) A major drawback is that it is necessary to obtain the list of nucleotides for a test sequence to be compared to the reference list. Such an analysis is long and complex.  
           [0005]    Another known technique uses the Watson-Crick complement strand of a reference sample covalently bound to fluorescent markers and a strand of the sample to be compared. If there is identity of the samples, the strands “stick” or hybridize together to form a double helix. The fluorescent markers serve to identify the presence of the double helix.  
           [0006]    Such a principle is extensively used in the DNA microarrays used for monitoring gene expression levels (see for example: DNA Microarrays: A Practical Approach by M. Schena, ed.)  
           [0007]    A drawback of this technique is the compulsory use of fluorescent markers. Indeed, such markers need precautionnal storage and use to remain efficient. Further, fluorescent markers are very expensive.  
           [0008]    There is thus a tremendous need to develop a simple and inexpensive method and device for determining whether two DNA sequences are identical.  
         SUMMARY OF THE INVENTION  
         [0009]    In order to meet these needs, the present invention is directed to a device and a method for assessing the identity between a reference DNA sequence and the sequence of a test DNA sample. The DNA sequence of the reference DNA is not necessarily known, but it is assumed that the practitioner has access to a sample of single stranded DNA with a sequence complementary to the reference sequence. This sample is referred to herein as the reference sample. The test sample is also single stranded. Both the test and the reference DNA samples are isolated and made singled stranded utilizing standard procedures (see Sambrook, et al. Molecular Cloning: A Laboratory Manual).  
           [0010]    According to the present invention, DNA molecules from the reference sample are attached to a solid support. Then a solution containing the test DNA sample is added. The DNA samples are allowed to hybridize under conditions such that the two strands will hybridize only if they have greater than approximately 95% identity. The test DNA sample hybridizes with the reference DNA sample and then remains attached to the solid support. If the DNA sequences are less than approximately 95 to 100% identical, there is no hybridization, and the test strands remain in the solution.  
           [0011]    Any solid support allowing single stranded DNA molecules to be attached to it can be used. For example, magnetic micro-particles or latex micro-particles can be used. Another example can be microbeads like the Streptavidin MagneSphere Paramagnetic Particles from Promega Corporation, Madison, Wis.  
           [0012]    The non-solubility of DNA in a set of given solvents is utilized to determine whether the test and reference sequences are at least approximately 95 to 100% identical. Having filtered the mixture of the preceding step with a filter having a characteristic size adapted to retain the solid support but to permit single stranded DNA molecules to pass through the filter, a solvent (for example, ethanol, acetone) is added to the filtered solution. If DNA is present, the DNA will precipitate upon addition of the solvent. This occurs when the test and reference sequences are different. If DNA is not present in the filtered solution, the solvent mixture will remain clear (transparent) because there is no DNA to precipitate upon addition of the solvent. This occurs when the test and reference sequences are approximately 95 to 100% identical (the DNA molecules of the test sample have hybridized with the molecules from the reference sample). In sum, if the test and reference DNA samples are identical, the test sample remains bound to the reference sample and cannot pass through the filter. If DNA does not pass through the filter, there is no DNA to precipitate upon addition of the solvent. If the test DNA sequence is different, it does not hybridize to the reference DNA, but instead passes through the filter where it can be precipitated with solvent and detected.  
           [0013]    According to the present invention, to detect DNA it is then sufficient to test the light scattering or turbidity of the resulting filtered solution. By detecting the turbidity of the filtered mixture, one can determine whether or not the sample of DNA introduced in an analyzing chamber already containing a reference sample is 95-100% identical to the reference sample.  
           [0014]    Preferably, the turbidity of the filtered solution added to the solvent is compared to the turbidity of a control sample made of a solution containing only the DNA molecules to be tested and the solvent. Then, the control sample should have a high turbidity resulting from the presence of aggregates. It must be different from the turbidity of the solution mixture if the DNA sequence to be compared is identical to the reference sequence. This format alleviates a false detection in the case the sample to be tested does not contain any DNA.  
           [0015]    The present invention is thus directed to a method for determining whether or not the sequence of a test DNA molecule and the sequence of a reference DNA molecule are 95 to 100% identical.  
           [0016]    The method of the invention includes the steps of: a) preparing a first solution of single stranded test DNA molecules; b) attaching a reference DNA molecule to a solid support to form a solid support-reference DNA complex; c) hybridizing the single stranded test DNA molecules with the solid support-reference DNA complex to form a solid support-reference DNA-test DNA complex wherein the solid support-reference DNA-test DNA complex is formed when the reference DNA and the test DNA sequences are 95 to 100% identical; d) filtering the solid support-reference DNA-test DNA complex through a filter under conditions such that said test DNA molecules passes through said filter into a filtered solution if the test DNA has not hybridized to said reference DNA; e) adding a solvent to the filtered solution to form solvent treated filtered solution; f) detecting the presence or absence of the test DNA in said solvent treated filtered solution by measuring the opacity or turbidity or the solvent treated filtered solution wherein the presence of DNA is the filtered solution indicates that the sequences of the test and reference DNA samples were less than 95% identical and wherein the absence of DNA is the filtered solution indicates that the sequences of the test and reference DNA samples were more than 95% identical.  
           [0017]    In the method of the invention the solvent may be selected ethanol or acetone. In one format, the opacity or turbidity is measured by shining light through said solvent treated filtered solution. The light may be a laser light. As discussed above, in the method of the invention, the solid support may be a suspension of streptavidin-coated microbeads, and the reference DNA molecules may include a biotin group.  
           [0018]    The present invention is further directed to a device ( 20 ) for comparing a test DNA sequence with a reference DNA sequence.  
           [0019]    The device may include an injection chamber ( 21 ) to receive a test solution containing single stranded test DNA molecules; a hybridization chamber ( 23 ) for containing reference DNA molecules attached to microbeads, an output of the injection chamber being connected to an input of the hybridization chamber; and a detection chamber ( 26 ) for receiving the content of the hybridization chamber after filtering with a filter ( 25 ) having a cut-off size chosen to prevent the flow of the microbeads but allowing the flow of free DNA solution.  
           [0020]    The device may also include a light source ( 30 ), preferably a laser source, to illuminate the indicator solution contained in the detection chamber ( 26 ). The device may further include a photodiode unit ( 31 ,  32 ) to record the light intensity scattered by the indicator solution in detection chamber ( 26 ). In the device, the detection chamber may include a first compartment ( 26 ) to contain the indicator solution outputted from the hybridization chamber ( 23 ), and a second compartment ( 22 ) to contain a control solution of the DNA solution to be tested. 
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0021]    The foregoing and other objects, features, aspects and advantages of the invention will become apparent from the following detailed description of embodiments, given by way of illustration and not of limitation with reference to the accompanying drawings.  
         [0022]    [0022]FIG. 1 is a schematic flowchart of a known DNA authentication process;  
         [0023]    [0023]FIG. 2 is a flowchart of an embodiment of the DNA authentication process of the present invention; and  
         [0024]    [0024]FIG. 3 represents, very schematically and function ally, an embodiment of a DNA authentication device according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]    In order to more fully understand the invention, the following definitions are provided:  
         [0026]    As used herein a “reference sample” refers to a sample containing a reference DNA sequence.  
         [0027]    A “test sample” refers to a sample to which the reference sample is compared.  
         [0028]    “Reference DNA” refers to a DNA sequence in the reference sample that is compared to the test sequence in the test sample.  
         [0029]    “Test DNA” refers to a sequence in the test sample that is compared to the reference DNA sequence in the reference sample.  
         [0030]    “DNA Sequence Identify” refers to DNA sequences having at least approximately 95% identity between the component nucleotides.  
         [0031]    “DNA extraction” refers to the operation whereby the DNA molecules to be tested are purified and isolated (and possibly amplified).  
         [0032]    Taking into account these definitions, the present invention is directed a device and a method for determining if two DNA sequences are identical.  
         [0033]    A DNA molecule is a linear assembly of nucleotides (Adenine, Cytosine, Guanine or Thymine). The order of these nucleotides can be arbitrary in the linear assembly. In nature, a DNA molecule constitutes a biologic identifiant. In synthetic form, a DNA molecule can be used to store information. Nucleotides A, C, G and T are pair-wise complementary (C with G and A with T).  
         [0034]    DNA molecules can be single stranded or double stranded. The conditions under which 2 single strands of DNA can combine to form double stranded DNA under a process known as hybridization are very well known (see Sambrook, et al. Molecular Cloning: A Laboratory Manual) DNA hybridization is at a maximum whenever the sequences of the 2 DNA strands are pair-wise complementary along 100% of the sequence. Whenever mismatches occur between the 2 sequences, the amount of hybridization is reduced. Temperature and ionic strength and the length of the DNA molecules are all known to play a role: it is possible to limit hybridization to pair of molecules with a very small number of sequence mismatches (e.g. 25% by increasing temperature and ionic strength). Also, the longer the DNA molecules, the more restrictive are the hybridization conditions. For DNA molecule A with a given nucleotide sequence, it is possible to determine chemical conditions hybridization conditions under which the probability that another randomly chosen DNA molecule B will hybridize to A is extremely small, for example less than 5%. Unless otherwise stated, we will assume in the practice of the present invention hybridization reactions are performed under these hybridization conditions whenever dealing with the hybridization of DNA molecules. This means that, under such conditions, the 95 to 100% identity of two sequences SEQA and SEQB can be inferred by monitoring the hybridization of DNA molecules with sequence SEQA with DNA molecules whose sequence is complementary to SEQB. If the DNA molecules hybridize, then SEQA is 95 to 100% identical to SEQB, if not, they are different or less than 95% identical. This defines the notion of “identical sequences” for the present invention.  
         [0035]    In this invention, we make use of the fact that single stranded-DNA molecules can be attached to solid supports. By attached, we mean that it is possible to create a permanent bond between the DNA molecule and the solid surface. Once attached to a solid support the lifetime of the attached DNA will be much greater than the timescale on which the present invention is intended to be used. This permanent bond can be a chemical bond if the end of the DNA molecule is functionalized in order to perform a chemical reaction with reactive groups present on the surface of the solid support. It can also be any sort of bond that will have the property that the DNA molecules won&#39;t be free to leave the molecular vicinity of the surface. For example, DNA molecules can be modified to provide a biotin group at end of the DNA molecule and the surface of the solid support can be coated with streptavidin groups. Because of the strong biotin/streptavidin interaction, under appropriate conditions, the DNA molecule will also be attached to the surface. It is essential to note that single stranded DNA molecules attached to a solid support in this manner are free to hybridize with single stranded, complementary, DNA molecules. If such hybridization occurs, both DNA strands will be attached to the solid surface, in the meaning that has been defined previously.  
         [0036]    [0036]FIG. 1 is a schematic flowchart of a conventional DNA authentication (comparison) process.  
         [0037]    In a first step (block  1 , DNA-EXTRACT) DNA molecules, and more precisely, strands of the DNA molecule to be compared are extracted in order to obtain a solution (SOLA) containing the DNA sequence.  
         [0038]    The second step (block  2 , DNA-ANALYSIS) includes in analyzing the DNA strand in order to obtain its DNA sequence (SEQA). At the end of this step, the list of the nucleotides codes (A, C, G, T) of the DNA sequence to be compared is known.  
         [0039]    A reference DNA sequence (SEQB) is then extracted (block  3 , DNA-REF), for instance, from a memory.  
         [0040]    To determine the identity of the sequences SEQA and SEQB, the nucleotides lists are compared to each other (block  4 , SEQA=SEQB?).  
         [0041]    The comparison step  4  gives the result (block  5 , RESULT) of the comparison.  
         [0042]    [0042]FIG. 2 is a flowchart illustrating an embodiment of the process according to the present invention.  
         [0043]    The first step includes, as in a conventional process, extracting from a biological sample (hair, saliva, sperm, etc.) or from a synthetic sample, the DNA strand in a solution (block  11 , DNA-EXTRACT). The DNA solution (SOLA) containing the test sample is to be compared to a reference sample of DNA.  
         [0044]    According to the present invention, both DNA sequences may be compared in a soluble form.  
         [0045]    A reference solution (SOLNB) containing the reference sample attached to a solid support such as silica or latex microbeads is then (or in parallel) prepared (DNA-REF, block  13 ). In one format, the microbeads are streptavidin-coated beads. In this format, the reference DNA strands have a biotin group at their end, allowing them to be attached to the streptavidin groups present on the surface of the solid surface.  
         [0046]    The solutions SOLA and SOLNB are mixed (block  14 , SOL-MIX). If the sequence of the test sample is 95 to 100% identical to the reference sequence, the test and reference strands will hybridize. This will ensure that the test molecules will in turn be attached to the solid surface. If not, the test DNA strands will remain free in the solution.  
         [0047]    The mixture is then filtered (block  15 , FILTER) to retain the solid surfaces (and the DNA strands attached thereto). The filtered solution contains DNA molecules from the test sample only if the test and reference sequences are not identical and hybridization did not take place.  
         [0048]    A solvent (block  16 , SOLVENT), for example, ethanol, acetone or any solvent in which DNA molecules are not soluble (a poor solvent), is added to the filtered solution. According to the invention, the solvent is used as a way to reveal the presence of DNA molecules in the filtered solution. If DNA molecules are present, they will aggregate to form large (i.e. size &gt;1 micron) aggregates.  
         [0049]    Such aggregates modify the turbidity of the solution. Then, according to a preferred embodiment of the present invention, one will illuminate (block  17 , LIGHT) a transparent container containing the solution to directly obtain the result (block  18 , RESULT) of the comparison of the two DNA samples.  
         [0050]    Preferably, the turbidity of the solution obtained after the filtration step is compared to the turbidity of a control sample made of solution A and only the solvent. Then, the control sample comprises aggregates and has a turbidity that is different from the turbidity of the filtered solution if the current DNA is identical to the reference DNA.  
         [0051]    Such a preferred embodiment alleviates a false detection in case the sample to be tested does not contain any DNA.  
         [0052]    An advantage of the present invention is that the comparison of two DNA samples is very easy. In particular, according to the present invention, it is not necessary to analyze both sequences in order to obtain the complete list of nucleotides as in the conventional example of FIG. 1.  
         [0053]    Compared to the use of fluorescent markers, the present invention has the advantage of eliminating the needs of such fluorescent markers.  
         [0054]    Another advantage of the present invention is that the results are obtained very quickly compared to methods requiring DNA sequence analysis.  
         [0055]    Another advantage of the present invention is that measuring the turbidity makes detection easier and less expensive. In particular, the sensor to detect the modification of turbidity does not need to be as sensitive as would be the case for a detection based on the measurement of the opacity.  
         [0056]    Another advantage of the present invention is that its implementation is compatible with a miniaturization required to constitute a portable device. Further more, the invention could also be implemented as a microfluidic device.  
         [0057]    This advantage will be better understood in connection with the description of an embodiment of a device according to the present invention made in connection with FIG. 3.  
         [0058]    [0058]FIG. 3 represents, schematically and functionally, an exemplary embodiment of a device  20  for comparing two DNA samples according to the present invention. First, DNA strands are extracted from any conventional source. Next, DNA solutions SOLA and SOLNB to be compared are introduced in the device  20  according to the present invention. In the example of FIG. 3, the DNA solution A is introduced in an injection chamber  21 . The solution in chamber  21  is preferably divided into two parts. One part goes directly to a first compartment  22  of a turbidity detection chamber, which constitutes the control sample compartment according to a preferred embodiment. The other part goes through a hybridization chamber  23 . The hybridization chamber  23  contains a solid support such as microbeads grafted with DNA molecules with a sequence complementary to the reference sequence B. Such grafted microbeads are obtained in a conventional way by using the appropriate chemical procedure to graft the DNA molecules to the microbeads, for example, such as described above for biotin/streptavidin. The actual chemical reaction to be carried depends on the active functional groups formed on the surface of the solid surface.  
         [0059]    In FIG. 3, the retention of the DNA reference molecules attached to the microbeads introduced in hybridization chamber  23  is illustrated in the form of a preparation chamber  24  in which are introduced to the solid surface and the DNA molecules (globally designated by SOLNB). Alternatively solution SOLNB may be prepared well in advance and stored in the hybridization chamber  23  or in a container connected to this chamber.  
         [0060]    A microscopic filter  25  is inserted between the hybridization chamber  23  and a second compartment  26  of the detection chamber. For example, the microscopic filter  25  will have a cut-off size of approximately 1 micrometer to prevent the solid surfaces from moving inside the detection chamber, while free DNA molecules pass through the filter.  
         [0061]    A solvent is preferably introduced in the detection chamber in both compartments. Alternatively, the solvent can be introduced in the injection chamber if the solvent does not affect both the attachment and hybridization chemistry.  
         [0062]    The detection chamber is provided with a light source  30  to illuminate the solution contained in both compartments  22  and  26 . The light source  30  may be laser light source. In order to facilitate the detection, walls of compartments  22  and  26  should be transparent if the light source  30  is disposed outside the chamber. The detection chamber detects the presence of DNA. If DNA is present in both compartments, that means that the two sequences of the DNA samples which have been compared are not 95 to 100% identical. If DNA is detected in compartment  22  containing the check sample and not in compartment  26 , that means that the DNA strands introduced in the hybridization chamber  23  have hybridized to the complementary strand of the reference DNA sample, i.e. the two DNA samples to be compared are identical.  
         [0063]    The detection of light scattering or turbidity may be automatic. The device  20  then comprises light sensors  31  and  32  to detect and convert the light intensity into an electric signal. For example, photodiodes are disposed to detect the light intensity in the detection compartments. Preferably, the light intensity is sensed in a direction perpendicular to the incoming ray of light from the light source. This format facilitates the measurement of turbidity and not the opacity. Alternatively, the opacity of the obtained solution can be measured with a sensor disposed in the direct beam of the light source. In this format, a more sensitive sensor is required. In addition, measurements are correlated with eventual variations of the intensity of the light source.  
         [0064]    Device  20  is controlled by a central unit  33  (CTRL) that controls not only the light source  30  and sensors  31  and  32 , but also valves  34 ,  35 ,  36  and  37  inter posed in the links between the different chambers/compartments. The control of the different valves is well within the ability of one with an ordinary skill in the art, on the basis of the functional description above.  
         [0065]    The invention will be better understood by reference to the following non-limiting example.  
       EXAMPLE  
       [0066]    In this example, we used two different DNA solutions (one solution of herring sperm DNA and one solution of random oligonucleotides) 40 microliters of these solutions were added to an equal amount of acetone. After one minute, a laser beam (from a laser module 280-460 from the Farnel Company powered by a standard 9-volts battery) was shined through the glass test tube containing the solution. A photodiode unit (327-646 from the Farnel Company) powered at 20 volts/0.03 ampere was used to record the intensity scattered at 90 degrees. Using an oscilloscope to visualize the output from the photodiode unit, the output voltage went from 17.5 millivolts in the absence of laser beam to 23.5 millivolts when the beam was turned on. This corresponds to a variation of 33% of the output voltage. For a test tube with no DNA, the output voltage did not show any measurable change when the laser was switched on. This demonstrates the possibility to detect the presence of DNA inside the test tube using laser light scattering. In the best case (i.e. with herring sperm DNA), the scattered light was strong enough to be observed with naked eye. Such results have been obtained without any specific precaution to protect the photodiode from the ambient light.  
         [0067]    As shown above, the invention is compatible with a small size device. In particular, this is due to the fact that a very small amount of DNA sample is sufficient to be compared to a reference sample. Further, the small element needed to implement the invention participates to obtain such a result.  
         [0068]    The amplitude of the variations of the output voltage of the photodiode(s) is large enough, so that standard electronic control equipment can be used to interface with the device according to the present invention. For example, an electronic module turns on a LED to indicate that the DNA solution contains the right type of DNA. Alternatively, the device of the present invention can be connected to a computer in order to record the details of the output signal and to take a decision. To flow the DNA solutions between the different chambers, one can use for example an externally applied pressure (either manually or using a stepping electrical motor). It should be noted that the embodiment illustrated in FIG. 3 is a functional one.  
         [0069]    Having thus described at least one illustrative embodiment of the invention, various alterations, modifications and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be within the spirit and scope of the invention as claimed. Accordingly, the fore going description is by way of example only and is not intended to be limiting.