METHOD AND DEVICE FOR IMAGING AND ANALYSIS OF BIOPOLYMER ARRAYS

Abstract: Total internal reflection fluorescence is used for imaging biopolymer arrays. A beam of light (2) is guided into the solid support (1) at a certain angle, evoking total internal reflection in the solid support. A certain portion of light will not reflect from the inner glass surface but penetrate out of the glass as an evanescent wave. It will excite fluorophores incorporated in the biopolymer molecules, contiguously attached to the surface of the support. Fluorescence thus evoked will be guided to a light sensitive element (7), providing data on the fluorescent molecules attached to the surface of the support. The said detection of fluorescence signals is rapid, taking about 10 seconds per fluorescence channel.

Description: Method and device for imaging and analysis of biopolymer arrays TECHNICAL FIELD The present invention falls in the fields of molecular biology, molecular diagnostics and laser optics. It more specifically concerns a method of parallel detection and analysis of fluorescently labelled biopolymer molecules on a two-dimensional array, using total internal reflection fluorescence and a device for fluorescence detection. BACKGROUND ART On chemical binding of pre-fabricated short biopolymers (nucleic acids, peptides etc) on a solid transparent support (glass) in a two-dimensional structure, the said array structure can be used for diagnostic purposes, for example, when adding (hybridizing) a sample target nucleic acid to the array of bound nucleotide polymers. The target sample may be labelled with fluorophores prior to hybridization, but the incorporation of the label may take place immediately prior to scanning. For imaging and analyzing nucleic acid arrays, apparatuses based on different physical principles may be used, mainly falling in two categories: firstly, detectors and secondly, scanners based on confocal microscopy. The closest model to the device described in this invention, a fluorescence detector based on total internal reflection, is an instrument based on CCD camera called GenoSensor™, manufactured by Nysis, Inc. (Downers Grove, IL, USA). The CCD camera-based instrument GenoSensor™ functions as follows. The fluorescently labelled target molecules, hybridized to DΝA probes bound on a glass support, are detected following excitation by light traversing the DΝA array (fig. 1). The light is generated by a xenon bulb and passed through a filter to select for the necessary spectral band. The light emitted by fluorophores will be filtered through an emission filter and guided through an optical system onto the high-resolution cooled CCD camera. The signals obtained will be processed in a computer. A nucleic acid array is a high-density, two-dimensional structure of nucleic acid molecules bound to a solid transparent support. Assessing the results of a biological reaction made on the array is problematic due to the low sensitivity and selectivity (using traversing light for excitation of fluorochromes is inefficient as a band for excitation has to be filtered out of the full spectrum) and the speed of detection (scanning is usually time-consuming). As in the case of arrayed primer extension reaction (APEX), four different dideoxynucleotides or terminators, each carrying a different fluorophore, are used simultaneously in the reaction, a detector functioning in four spectral areas is needed for imaging the nucleic acid array. The said detector must be capable of maximal excitation and signal capture of each fluorophore, enabling an assembly of composite reaction result. DISCLOSURE OF INVENTION As a solution to the problems mentioned above, this invention suggests a method comprising total internal reflection fluorescence (fig. 2) for imaging biopolymer arrays (like determining sequences of target DNA) and a device of fluorescence detector enabling a quick and precise estimation of reaction results on a nucleic acid array in four different spectral bands. The purpose of this invention is to provide a device and method for the analysis of biological reactions made on a biopolymer array or "chip". The arrays of nucleic acids can mainly be used in two ways. Firstly, in the case of assays based on hybridization only, the fluorescently labelled target to be analyzed will be hybridized to genetic material immobilized on the array. The label has been incorporated in the course of amplification. As the energy of hydrogen bonds formed between nitrogen bases in the double strand nucleic acid is limited, this type of hybridization-based reaction mechanism is not particularly selective and does not permit a clear distinction between signal and noise. Another possibility lies in the addition of an enzymatic reaction to the hybridization of immobilized oligonucleotide probes and the target nucleic acid. An example would be polymerase-mediated extension, where every immobilized oligonucleotide becomes elongated by one, fluorescently labelled nucleotide. The DNA polymerase adds a nucleotide with respect to the genetic information of the target DNA. The arrayed DNA extension reaction uses as a substrate for DNA polymerase, dideoxynucleotides labelled with fluorophores. Four differently labelled dideoxynucleotides are used simultaneously, but only one will be incorporated in a probe, that corresponding to the primary structure of the target nucleic acid. The said enzymatic mechanism possesses advantages over the reaction based solely on hybridization: 1. If the hybridization between the immobilized probe and the target is not perfect, the polymerase will neither recognize the structure nor carry out a reaction. 2. If the hybridization is perfect, the enzyme will incorporate a dideoxynucleotide in the probe, synthesizing a stable covalent bond. This enables a wash of the array after the reaction and getting rid of non-specifically bound biological material. Thus a substantially improved signal/noise ratio will be achieved, enabling this system to detect heterozygous mutations. The method described in this invention is used for analysis of biopolymer arrays. A beam of light of known wavelength (a laser beam) is directed into the edge of the support (glass) under an angle that will evoke total internal reflection of the beam. The support becomes a waveguide (fig. 2). A certain part of light will not reflect from the inner surface of glass but will penetrate out of the glass as an evanescent wave. Its intensity will fall exponentially but will remain sufficient for a distance of V* of the wavelength. This distance is sufficient to excite the fluorophores incorporated in the probes bound immediately to the glass. As in the case of polymerase extension there are four different nucleotides/labels, laser beams of four different wavelengths are used to achieve maximal excitation of fluorescent labels. The light emitted will be gathered through respective emission filters to discard the background light and focused through an optical system (objective) into a CCD camera. As the camera used is cooled, the imaging time is short, taking about 10 seconds for each nucleotide/fluorescence channel. BRIEF DESCRIPTION OF DRAWINGS The invention is described with the help of figures, where: fig. 1 is the excitation of fluorochromes on the surface of a biopolymer array by a traversing light; fig. 2 is the said excitation by total internal reflection fluorescence according to the method in this invention; fig. 3 is an application of the method in this invention where the laser beam to evoke total internal reflection is focused through a cylindrical lens thus that its diameter is less than the thickness of the support; fig. 4 is an application of the method in this invention where a prism is used to direct the laser beam into the support. Between the prism and the support there is transparent liquid, refractive index of which is close to those of the prism and the support; fig. 5 is the principal design of the device in this invention, the fluorescence detector. Figures 2 to 4 illustrate the method in this invention for imaging biopolymer arrays. The fluorescently labelled biopolymer molecules are bound to a thin transparent support with parallel walls (1). For maximal excitation of the fluorescent molecules a laser beam (2) is used. The beam is focused through a cylindrical lens (3) in a shape of a fan thinner than the edge (1), wherein the beam is directed into the edge (1) under such an angle that the support becomes a waveguide of total internal reflection. The fluorescence obtained by laser beam is optically directed to a light-sensitive element which renders images of the fluorescent molecules bound on the support (1). The fluorescents obtained by previously prescribed mechanism is projected to the light- sensitive element, which enables to obtain to the support (1) the images of the bound and excited by laser beam fluorescent molecules as shown in the fig. 5. The laser beam is directed to the support through the side surface. A digitally controlled CCD camera is used as the light-sensitive element. The laser beam is scattered through a transparent slab or hexahedron (4) and modulated in the same time with an optical wedge (5). These elements combined modify the angle of the laser beam coming from the mirror (6) into the support To direct the beam into the support (1) trough the edge surface the prism (8) is used as shown in the fig. 4. To reduce the reflection loss in transitions of the light from the prism (8) to support (1) is used the transparent liquid (9), for example the immersion oil used in the microscopy, which refractive index is close to the refractive index of the prism (8) and the support (1). Though the invention is prescribed in connection with the practical preferred embodiment it is understandable, that the invention is not limited with the prescribed subject matter but in opposite, the invention is intended to include different modification and equivalent devices which comprises in the idea and scope of the added patent claim.

Claims: Claims 1. A method of imaging and analysis of a biopolymer array characterized in that a beam of light of certain wavelength is directed into the array support under such an angle that total internal reflection is evoked in the support, the support becomes a waveguide and part of the light penetrates out of the surface of the support as an evanescent wave and excites fluorophores incorporated in molecules immediately attached to the surface. 2. A method according to claim 1, characterized in that the beam of light is a laser beam. 3. A fluorescence detector for applying the methods in claim 1 and 2 comprising a digitally controlled CCD camera (7), band-pass filters and a scattering cylindrical lens (3) for broadening the zone of illumination. 4. A fluorescence detector according to claim 3, characterized in that the broadening of the zone in the biopolymer array support is accomplished by a scanning movement of the laser beam, wherein the said scanning accomplished by a transparent hexahedron or slab (4), revolving of which changes the laser beam's angle of entrance into the support (1). 5. A fluorescence detector according to claim 3 or 4, characterized in that a modulating element is used for a more even distribution of light in the support (1). 6. A fluorescence detector according to claim 5, characterized in that the modulating element is an optical wedge (5) revolving around its axis, wherein the axis of the optical wedge lies close to the optical axis of the laser beam. 7. A fluorescence detector according to any claim from 3 to 6, characterized in that the edge or side surface of entry of the laser beam into the support (1), is frosted for dispersion of light for a more even distribution of the laser beam in the support. 8. A fluorescence detector according to any claim from 3 to 7, characterized in that the edge or side surface of entry of the laser beam into the support (1), is polished for a greater intensity of the laser beam in the support. 9. A fluorescence detector according to any claim from 3 to 8, characterized in that a diffraction grating applied on the support (1) is used to direct the laser beam into the support (1). 10. A fluorescence detector according to any claim from 3 to 9, characterized in that the laser beam is directed into the support (1) through its side surface using the optical prism (8). 11. A fluorescence detector according to any claim from 3 to 10, characterized in that between the support (1) and the prism (8) is placed a transparent liquid (9), for example, immersion oil, refractive index of which is close to those of the prism (8) and the support (1).

Inventors: BERIK JEVGENI, KURG ANTS, METSPALU ANDRES, BERIK, JEVGENI, KURG, ANTS, METSPALU, ANDRES
Applicants: ASPER OUE, BERIK JEVGENI, KURG ANTS, METSPALU ANDRES, BERIK, JEVGENI, KURG, ANTS, METSPALU, ANDRES
Application year: 2000
Main classification: G01N21
IPC codes: C12M1, C12N15, G01N21, G01N33, G01N37