Laser excited confocal microscope fluorescence scanner and method

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BRIEF DESCRIPTION OF THE INVENTION

This invention relates generally to a fluorescence scanner and more particularly to a laser excited fluorescence gel scanner employing a confocal microscopic detection system.

BACKGROUND OF THE INVENTION

There is a great deal of interest in the development of automated DNA mapping and sequencing methodologies which is particularly important, in view of the recent interest in sequencing the human genome. The successful completion of this ambitious project will require improved automation. Sequence data are presently being added to data banks at a rate of 10.sup.6 bases/year but the human genome contains 3.times.10.sup.9 base pairs.

The detection system presently used by most workers in DNA sequencing or mapping involves using radioisotope labeled DNA. The radioactive slab gels in which the DNA fragments have been separated are placed against an x-ray film for overnight exposure of the film. After the exposure and development of the x-ray film, the sequence or size of the DNA separated fragments are read directly from the images on the film.

The autoradiographic detection method described above is not only slow but also requires handling and disposal of hazardous radioactive materials. The reason autoradiography is still so widely used is because it uniquely provides the necessary sensitivity.

There has been great interest in automating the sequence determination procedures using recent advances in optical, electronic and computer technology. Autoradiographic films can now be digitized by a scanning transmission densitometer or video cameras and the digitized images can be computer processed to determine DNA sequences. These digitizing and automated sequence determination systems use autoradiography as the primary detection method.

In 1986, L. M. Smith, J. Z. Sanders, R. J. Kaiser, P. Hughes, C. Dodd, C. R. Connell, C. Heiner, S. B. H. Kent and L. E. Hood, Nature, vol. 321, pp. 674-679, developed a method for detecting fluorescently labeled DNA on gels which they believe is capable of sequencing .about.15,000 base pairs per day. They state that one of the three areas needing development is "increasing the detection sensitivity of the system thereby allowing less material to be used per reaction which in turn allows the use of thinner gels having higher resolution." An apparatus developed by W. Ansorge, A. Rosenthal, B. Sproat, C. Schwager, J. Stegemann, and H. Voss, Nuc. Acids Res., vol. 16, pp. 2203-2207 (1988), using a slightly different protocol, was able to sequence 500 base pairs in 5 hours with a sensitivity per band of 10.sup.-18 mole or 6.times.10.sup.5 molecules. An analogous approach with similar capabilities was developed by J. M. Prober, G. L. Trainor, R. J. Dam, F. W. Hobbs, C. W. Robertson, R. J. Zagursky, A. J. Cocuzza, M. A. Jensen and K. Baumeister, Science, vol. 238, pp. 336-341 (1987).

The development of a high sensitivity detection system would obviously be very important. If very small amounts of fluorescence labeled DNA can be detected on gels, then less labeled DNA is required and the thickness of the gel can be reduced. A thinner gel will have higher resolution so it will not have to be run out as far to resolve the bands. This could result in a major saving in time. Also, the available fluorescence DNA sequencing systems require a detection system that is dedicated to the electrophoresis system during the entire approximate 10 hour run. The detection system would be more efficiently used if it detected the gels off-line from the electrophoresis. This could also result in a major saving of time and increase in throughput.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of this invention to provide a high sensitivity fluorescence detection system.

It is another object of the invention to provide a detection system with improved light collection.

It is another object of the invention to detect fluorescence emission from nucleic acids, proteins, etc., while rejecting unwanted background emission.

It is another object of the invention to choose the intensity of excitation according to the absorption coefficients and emission life time of the fluorescent molecule to give the optimum fluorescence emission with minimal background emission.

It is a further object of the invention to choose the illumination time so that it is less than the photodestruction time of the fluorescent probe to achieve a high signal-to-noise ratio.

The foregoing and other objects of the invention are achieved by a fluorescence scanner including means for applying excitation light to a medium which carries a fluorescently labeled sample to cause fluorescently labeled sample such as nucleic acids, proteins, etc. to fluoresce and for collecting the fluorescent emission from said labeled sample at a selected volume of said medium while rejecting background and scattered light through confocal spatial filtering of the detected image.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention:

FIG. 1 shows a laser excited fluorescence gel confocal microscope scanner in accordance with one embodiment of the invention.

FIG. 2 is a schematic enlarged view of an objective lens assembly, the gel, gel support, gel objective interface and the volume from which fluorescence is gathered.

FIG. 3 is a schematic enlarged view of another assembly of the type shown in FIG. 2.

FIG. 4 is a block diagram of a suitable data acquisition circuit.

FIG. 5 is a block diagram of another data acquisition circuit.

FIG. 6 shows a typical image formed by the gel scanner of the present invention.

FIG. 7 shows another laser excited fluorescence gel scanner in accordance with the present invention.

FIG. 8 shows still another embodiment of a fluorescence gel scanner in accordance with another embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. While the invention will be described in conjunction with the preferred embodiments, it will be understood that they are not intended to limit the invention to those embodiments. On the contrary, the invention is intended to cover alternatives, modifications and equivalents, which may be included within the invention as defined by the appended claims.

Increasing the fluorescence emission rate from a sample above that from the background depends on the ability to focus the excitation light to a very small spot or volume, and to gather the light from the spot or volume while rejecting background and scattered light. In fluorescence spectroscopy, it is difficult to illuminate a large area uniformly or to focus the light to a small spot with conventional light sources such as arc lamps. When mapping nucleic acids or their derivatives they are either separated by polyacrylamide or agarose gel or other gel like separation matrices with thickness from 100 .mu.m to several millimeters. To detect nucleic acids in the gel directly by fluorescence emission, one must get rid of or reduce the scattered light from the surface of the gel, scattered light from the substrates, and background emission from the substrates.

In the present invention, a confocal microscope forms an illuminated volume in the gel. The light comprises a polarized laser beam oriented so that the background scattering can be minimized as much as possible by the polarization characteristics of the scattered light. An oil immersion objective may be used to match the refractive index of the gel and the objective lens of the optical microscope so as to reduce the scattering. It is to be understood that an air gapped objective lens may also be employed. A confocal optical arrangement is used to reject stray scattering and emission from unwanted regions of the gel or sample carrier. Dichroic beam splitters may be used in the optical system to reject the scattered light by its spectral characteristics. A spatial filter or beam stop can be used to filter out background and scattered light.

Referring to FIG. 1, a laser excited confocal microscope scanner is illustrated. A laser (not shown) is used as the excitation source. The laser beam 11 is first collimated and focused onto an spatial filter 12 by an 160.about.mm lens (not shown). After passing through spatial filter 12, the laser beam is reflected by dichroic beam splitter 13 through a 100.times., N.A. 1.3 oil immersion objective 14 such as a Rolyn Optics 80.3610 10.times., NA 1.3. The dichroic beam splitter reflects light at the wavelength of the excitation laser beam, but transmits fluorescence light, which is Stoke's shifted to longer wavelengths by the fluorescent sample. The objective 14 serves both as focusing lens and collection lens with very high collection efficiency. The position of the probe, volume 15, in the gel 16 supported by plate 17, FIG. 2, can be scanned either by translating the objective and the spatial filter 18 or by translating the sample holder as shown by the arrows 19. The depth of the probe column is adjusted by moving the same elements up and down. The fluorescence emission with Stoke's shifted wavelength is collected by the objective 14 and passed through the dichroic beam splitter 13 which passes Stoke's shifted emission. This reduces scattering because it will not pass scattered light at the laser wavelength.

The light is reflected to spatial filter or image stop 18 by a second dichroic beam splitter 20. The spatial filter 18 spatially filters the emitted light and passes it to the phototube detector 21 while rejecting stray, background and scattered light from the various surfaces. The second dichroic beam splitter 20 reflects only the Stoke's shifted fluorescence and lets the scattered light pass through thus providing a second rejection of the background light. It also rejects scattered or reflected light because its polarization direction is such as to pass the reflected and scattered polarized light. The emitted light is detected by a phototransducer such as a phototube 21.

The use of 160 mm focusing lens, illumination stop or spatial filter 12, and image stop or spatial filter 18 at 160 mm away from the objective lens gives this detection system a confocal arrangement which allows depth profiling of the gel. The depth of view of the detection system with 100.times. objective is estimated to be on the order of micrometers. By using this confocal arrangement, one can selectively probe a DNA sample volume 15 in the gel. The scattering from the surfaces of the gel is rejected by the image stop or pinhole 18. Also, the scattering and fluorescence background from the substrates will be rejected by the spatial filter. The fluorescence photons can be either directly detected by the transducer 21 or be passed through a bandpass filter 22 to further reject background emissions before detection by the detector 21.

The sample is preferably separated by electrophoresis on a slab gel. For an immersion objective the slab gel is covered with nonfluorescent immersion oil 23, FIG. 2 right after electrophoresis to prevent it from drying and cracking. The gel is then placed on a computer controlled DC servo motor driven XY translation stage 24 to translate or scan the sample gel past the focused laser volume 15.

FIG. 3 shows a non-immersion objective lens assembly in which the gel is covered by a thin glass plate 25 which prevents drying out of the gel.

A microcomputer with an analog-to-digital board or a frame grabber board is used to digitize data from the data acquisition circuit. The data acquisition circuit 24, FIG. 4 consists of a preamplifier 26 to amplify signal from photomultiplier tube, a discriminator 27 to reject thermal noises and other low amplitude electronic noises introduced by the photomultiplier and the preamplifier. A frequency to voltage converter 28 converts counts to analog signal for processing with analog-to-digital converter 29. This photon counting and analog signal conversion method has the fast and relatively easy processing advantage of analog circuits while it also has the digital photon counting sensitivity that DC measurements lack. A more inexpensive version would simply use a photomultiplier tube, preamplifier and analog-to-digital converter as in FIG. 5.

The data can also be processed digitally, in which mode, the counts from the photodetector 21 are processed by a timer/counter 31, FIG. 1, or frequency counter or multichannel scaler directly without the frequency to voltage converter and the digital-to-analog converter. Data acquisition circuit 32 provides data to a computer 33. The computer controls the XY translation stage 24 and displays acquired image in real time.

FIG. 6 shows an example of a DNA sequencing gel image obtained by this invention. Each band in the image represents one DNA fragment of a particular length. The averaged quantity per band in this image is about 10.sup.5 molecules. The fluorescence images are contrast-stretched with a histogram equalization method to enhance the images. The direct imaging of DNA or RNA in the gel by fluorescence emission has much better linearity than densitometric scanning of autoradiographic films which usually has logarithmic response.

FIGS. 7 and 8 show other embodiments of fluorescence gel scanners incorporating the present invention. Reference numbers have been applied to parts previously described. In each instance a collimated laser beam is directly applied through the lens assembly to a small volume of the gel. The fluorescence from the sample at the small volume is collected by the objective and focused through a spatial filter by a lens (40) to a detector (photomultiplier tube). In other respects, the scanner operates as described above. The location of the sensed volume within the gel may be selected by moving the gel, or by moving the lens assembly and spatial filter.

It is important to point out that these configurations are fundamentally better than an imaging charge coupled device (CCD) or charge injection device (CID) coupled to a fluorescence microscope. A fluorescence microscope is designed to image a very tiny area; however, the entire field is illuminated so that the incident intensity in any given small area is low. Furthermore, to image a large area like a electrophoresis slab gel, one has to use a lens that can project the whole image onto the camera at one time. This means using a large F-number lens with low collection efficiency. Despite the high quantum efficiency of charge coupled devices, the detection system as a whole is not as sensitive as a photon counting photomultiplier tube in this kind of application.

It is also worth pointing out that this invention has better sensitivity over the fluorescence detection system described by Smith et al., Nature, (1986) simply due to the better collection efficiency offered by the microscope objective lens and the background rejection methods intrinsic to the confocal spatial filtering of the detected light.

In a preliminary experiment the detection limits of this invention for DNA sequencing gel reached about 1.times.10.sup.5 molecules per band on the average with a 250 .mu.m thick gel and 3 mm wide sample well. Although the on-line detection method described by Smith, et al. (1986) has advantages, the off-line detection method taught in this invention doesn't require a laser and detection system dedicated to every electrophoresis system. This allows several gels being run simultaneously to have higher throughput and is more economical.

In summary, the laser excited confocal fluorescence scanning system taught herein is a very sensitive method for detecting DNA and RNA in gels. This invention employs a confocal microscope with a high numerical aperture objective to achieve the highest collection efficiency possible. The invention teaches polarization, spectral filtering, spatial filtering and refractive index matching principles to effectively reduce background in order to obtain optimum detection limits. The electronic gating of noises by photon counting with frequency to voltage conversion provides further improvement in signal-to-noise. The translation of the gel across the laser beam provides a means to image a large electrophoresis gel with high sensitivity and spatial resolution. The ultimate spatial resolution in this fluorescence imaging system is determined by the spot size of the laser beam which can be as small as 1-2 .mu.m in diameter using the optical elements described in this invention. The combination of high spatial resolution and low detection limits of this fluorescence imaging system means that very low detection limits can be easily achieved which approach the limits of autoradiography.

As an improved instrument for fluorescence detection and imaging of electrophoresis gels, this invention should be of interest to biological or biochemical researchers, particularly those working in DNA sequencing and mapping. This invention can detect DNA samples that cannot be detected by conventional fluorescence detection methods, and it provides a faster and safer way to do DNA sequencing without handling radioactive materials. It is apparent that the invention may be used to detect and image fluorescent labeled molecules, proteins, virus and bacteria, etc., which are electrophoretically or otherwise separated on a variety of carriers such as membranes, filter paper, petrie dishes, glass substrates, etc.

The foregoing descriptions of specific embodiments of this invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, to thereby enable others skilled in the art to best use the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents.

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