Link:  Pharm/Biotech Resources

Title:  Diagnostic method for a transmissible spongiform encephalopathy or a prion disease

United States Patent:  6,962,787

Issued:  November 8, 2005

Inventors:  Clinton; Michael (Roslin, GB); Miele; Gino (Roslin, GB); Manson; Jean Catherine (Newbury, GB)

Assignee:  Roslin Institute (Edinburgh) (Scotland, GB)

Appl. No.:  999305

Filed:  October 31, 2001

A method is provided for the diagnosis of a transmissible spongiform encephalopathy (TSE) or prion disease in an animal which comprises assaying a sample obtained from said animal to determine the number of hematopoietic cells of the erythroid, megakaryocyte or platelet cell lineages in the sample or an expression product thereof.

The present invention relates to a new method of diagnosis for a transmissible spongiform encephalopathy (TSE) or a prion disease.

A comprehensive overview of TSE biology can be found in published review format (Prusiner, S. B., Proc. Nat'l Acad. Sci. USA 95 13363-13383 (1998)) and/or on the internet (http:/www.mad-cow.org). Transmissible Spongiform Encephalopathies (TSEs) or Prion diseases are a group of invariably fatal disorders of the central nervous system (CNS), which manifest via genetic, infectious or sporadic mechanisms. They include Scrapie in sheep, Bovine Spongiform Encephalopathy (BSE) in cattle and Kuru, Creutzfeldt-Jacob Disease (CJD), new-variant (nv) CJD, Gerstmann-Straussler Sheinker Syndrome (GSS) and Fatal Familial Insomnia in humans. TSE diseases also manifest in other species such as elk, deer, mink, cats (FSE) and exotic zoo species such as Nyala, Arabian Oryx, Cheetah and greater Kudu. Chronic wasting disease (CWD) has also been identified in deer and elk and it has been suggested that this could also be transmitted to humans.

The TSE diseases are characterized by long, asymptomatic incubation periods followed by a relatively rapid clinical course frequently consisting of neurodegeneration, vacuolation, glial cell proliferation and the deposition of protease-resistant Prion protein^Sc (PrP^Sc), an abnormal isoform of a host-encoded glycoprotein, PrP^C. Conversion of normal, proteinase K-sensitive PrP^C into the abnormal, proteinase K-resistant isoform, PrP^Sc, is a frequent characteristic hallmark of the TSE diseases.

Currently, in the United Kingdom, there is considerable interest in TSE diseases as a result of the BSE epidemic and the subsequent emergence of new-variant CJD in humans; a TSE disease thought to have arisen from BSE and to have crossed the species barrier into humans as a result of BSE-infected tissues entering the human food chain. It is probable that the vast majority of UK citizens have been exposed to the BSE infectious agent. In addition to the problem of natural scrapie in sheep, there is great concern that BSE may also be harboring in sheep at a sub-clinical asymptomatic level, and other agricultural species, as a result of BSE infected material having been present in animal feedstuffs.

The extent to which the UK population is harboring nvCJD, and the proportion which will go on to develop the disease is currently unclear. At present, there is no effective pre-mortem diagnostic means of assessing potential TSE infection in humans or animals. Both in order to remove TSE infected agricultural species from the human food chain, and to assess potential TSE infection in humans, suitable diagnostic methods are desirable, and ideally in easily accessible tissues such as blood. At present, the only disease-specific macromolecule identified is PrP^Sc. However, PrP^Sc is only practically detectable in CNS tissue at post-mortem, although in some TSE diseases it may be detected in tonsil biopsies (Coghlan, A., New Scientist, page 5, (15 Jun. 1996)). It has recently been demonstrated that PrP^Sc can be detected in blood of TSE infected animals (Schmerr et al Journal of Chromatography 853 (1-2) 207-214 (1999)). However, this is both a time-consuming and significantly technically demanding procedure, as is the detection of PrP^Sc generally. Additionally, PrP^Sc is often undetectable in CNS tissue of humans and animals clearly affected by TSE diseases. There is therefore no effective pre-mortem diagnostic means of assessing TSE infection. An ideal diagnostic method should therefore be non-PrP^Sc-based, and would allow assessment of potential TSE infection in easily accessible tissues such as blood. Such a diagnostic test would ideally allow assessment of TSE infection in live individuals at an early stage such that therapeutic intervention strategies could be implemented.

There have been several published reports of genes which are differentially expressed in CNS tissue of TSE-infected animals. However, to date, there have been no other reports of differential gene expression in spleen, bone marrow or blood of TSE-infected animals. Although blood from TSE-infected animals has been shown to be infectious and the PrP^C protein has been demonstrated to be expressed in several hematopoietic cells there has been no recognition prior to the present invention of the connection between erythroid hematopoietic cells and TSE pathogenesis. A comprehensive summary of present TSE diagnostic efforts can be found on the internet (http:www.mad-cow.org/00/sci_archive_frame.html).

Current TSE research has so far failed to implicate Erythroid Differentiation Related Factor (EDRF) or erythroid hematopoietic cell biology in TSE pathogenesis, or identify a non-PrP^Sc molecular marker of TSE infection in spleen, blood or bone marrow. Most researchers in the field appear to be focusing on methods aimed at the detection of PrP^Sc.

It has now been discovered that EDRF, a gene normally expressed in erythroid cells of hematopoietic tissues, is differentially expressed in hematopoietic tissues of TSE infected animals. EDRF is expressed in hematopoietic cells of the erythroid and megakaryocyte/platelet lineage, in spleen, bone marrow and blood. It would therefore appear that hematopoiesis, in particular erythropoiesis, is surprisingly affected in individuals affected by TSE infection.

According to a first aspect of the present invention there is provided a method of diagnosis for the presence of a transmissible spongiform encephalopathy (TSE) in an animal, comprising assaying a sample obtained from the animal to determine the relative number of hematopoietic cells of the erythroid, megakaryocyte or platelet lineages in the sample.

Transmissible Spongiform Encephalopathies (TSEs) or Prion diseases include Scrapie in sheep, Bovine Spongiform Encephalopathy (BSE) in cattle and Kuru, Creutzfeldt-Jacob Disease (CJD), new-variant (nv) CJD, Gerstmann-Straussler Sheinker Syndrome (GSS) and Fatal Familial Insomnia in humans. The TSE diseases are characterized by the deposition of Prion protein^Sc (PrP^Sc), an abnormal isoform of a host-encoded glycoprotein, PrP^C. Conversion of normal, proteinase K-sensitive PrP^C into the abnormal, proteinase K-resistant isoform, PrP^Sc is recognised characteristic of a TSE disease.

For the purposes of the present invention the term TSE infection is used equivalently to the term TSE disease condition. The exact means by which a TSE disease is transmitted is still the subject of scientific controversy. Experimental animals can be "infected" with a TSE disease in the laboratory. While the clinical manifestations are clear enough, whether the diseases encountered in practice are "infections" in the normal sense of the word is not yet known.

Animals affected by TSE disease include mammals, such as ovines, bovines, humans, felines, elk, deer, mink, and exotic zoo species such as Nyala, Arabian Oryx, Cheetah and greater Kudu, and avian species, such as poultry, for example chickens, turkey, guinea fowl. Methods in accordance with the present application therefore extend to a method of diagnosis for a TSE in sheep, bovines, deer, elk or humans

The sample to be assayed according to the present invention typically will be a biological sample, for example a blood-based sample or from a source of hematopoietic cells. The sample may be whole or fractionated blood (or partly fractionated blood), plasma, a hematopoietic tissue, such as bone marrow, or from the spleen. The sample may be subject to the addition of further components so as to optimize sample analysis. For example, in samples of blood, it may be convenient to add substances such as heparin, EDTA and/or sodium citrate to prevent coagulation by means of clot formation. Other sample sources include cerebrospinal fluid (CSF), urine, tears, milk, semen, mucous secretions, tissue or organ biopsies, e.g. brain, liver, thymus, pancreas.

Hematopoietic cells of the erythroid lineage, include pluripotent stem cells, myeloid stem cells, CFU-GEMM cells (Colony-Forming Unit Granulocyte/Erythrocyte/Monocyte/Megakaryocyte), BFU-E cells (Blast-Forming Unit—Erythroid), CFU-E cells (Colony-Forming Unit—Erythroid proerythroblasts), reticulocytes and erythrocytes. The hematopoietic cells of the erythroid lineage are shown schematically in FIG. 13 (see Original Patent) .

A unipotential hematopoietic cell is a cell committed to a single cell lineage; a bipotential hematopoietic cell is a cell committed to one of two possible cell lineages, for example the E/Meg or erythrocyte/megakaryocyte cell which is capable of differentiating along either the erythrocyte or megakaryocyte cell lineages; a tripotential hematopoietic cell is a cell committed to one of three possible cell lineages.

The maturation of an erythrocyte from a proerythroblast includes progression through the following recognized cell types, as described by the Junquiera or the Wheater definitions, as follows:

Hematopoietic cells of the mekaryocyte and platelet lineages include the E/Meg precursor cell, the megakaryoblast and the differentiated megakarocyte or platelet cells.

Methods in accordance with the present invention may therefore be directed to an assay for one or more of the above cell types of the hematopoietic erythroid, megakaryocyte, or platelet lineages or expression products associated with one or more the cell types of the hematopoietic erythroid, megakaryocyte, or platelet lineages.

One expression product associated with the cells of hematopoietic erythroid lineage is Erythroid Differentiation Related Factor (EDRF). EDRF is expressed in bipotent cells capable of proceeding through erythroid or megakaryocyte lineages or E/Meg cells, blast-forming unit (BFU-E) cells, colony-forming unit (CPU-E) cells, proerythroblasts (rubriblasts), early normoblasts (basophilic erythroblasts or prorubricytes), intermediate erythroblasts (rubricytes or polychromatophilic erythroblasts), late normoblasts (metarubricytes or orthochromatophilic erythroblasts), including haemoglobin-positive normoblasts and reticulocytes, i.e. cells throughout erythrogenesis, including erythrocytes.

Particular cells of interest which normally express EDRF can be identified using the anti-TER-119 antibody, namely TER-119⁺cells. TER-119 antibody is raised against the immunogen C57BL/6 mouse day-14 fetal liver cells (Ikuta et al Cell 62 863-874 (1990)) and is of the isotype rat (Wistar) IgG_2b′κ (TER-119 antibody available from BD Pharmingen, San Diego). The TER-119 antibody reacts with cells of the erythroid lineage in all erythroid-producing organs and with most of the cells that express EDRF, although not all of such cell types. The TER-119 antigen is specifically expressed on erythroid cells form the early erythroblast through mature erythrocyte stages, but not on cells with CFU-E or BFU-E activities. Cells that can be assayed in this way in methods of the present invention include the proerythroblasts, basophilic erythroblasts, polychromatophilic erythroblasts, orthochromatophilic erythroblasts, reticulocytes (primitive erythrocytes) and erythrocytes. Alternative antibodies useful in characterizing such cells are anti-Glycophorin A, anti-EDRF, anti-CD-61, anti-CD-71, anti-transferrin, anti-ferritin, anti-EDRF and anti-haemoglobin antibodies. Anti-CD-61 antibodies can be used to detect cells of the megakaryocyte/platelet lineages from precursors to mature cells (commercially available from Miltenyibiotec—www.miltenyibiotec.com).

In a preferred embodiment of this aspect of the invention, the method of diagnosis comprises assaying for the number of cells of the hematopoietic erythroid, megakaryocyte or platelet lineages that express EDRF. Such cells can include, but are not limited to, E/Meg, CFU-GEMM cells, BFU-E cells, CFU-E cells, proerythroblasts, basophilic erythroblasts, polychromatophilic erythroblasts, orthochromatophilic erythroblasts, reticulocytes (primitive erythrocytes). The depletion of a cell lineage normally expressing EDRF is indicative of a TSE infection (or TSE disease condition) in a subject. The depletion of a cell lineage is in relation to the numbers of cells of the class assayed for which are present in non-infected individuals, in other words a significant deviation from the normal numbers of particular cell. A normal range can be identified by assaying a statistically relevant population of healthy non-infected individuals.

Another expression marker may be hemoglobin which also appears to be depleted in subjects suffering from a TSE infection or TSE disease condition.

The measurement of cell numbers in a sample may be carried out using any convenient method. For example, cell numbers can be assayed using fluorescence-activated cell sorting (FACS); manual counting by means of specific cell stains, e.g. Geimsa-Wright histology stain. Alternatively, automated haematology analysers can be used which can be set to count numbers of hematopoietic cells, such as erythrocytes or reticulocytes (Miltenyibiotec—www.miltenyibiotec.com). It is also possible to assess the number of cells in a sample using a hematocrit, for example routinely used for measurement of erythrocyte numbers, packed cell volume etc. Another method to estimate the number of progenitor cells is to culture blood/bone marrow in a semi-solid medium, such as methyl cellulose, and counting colonies.

The identification of hematopoietic cells of interest using an antibody can use a modified version of the antibody, for example an antibody conjugated with biotin, or with an antibody labelled with a fluorescent marker. Cells that are positive for the antibody of interest can be separated using fluorescence-activated cell sorting (FACS), flow-cytometry or magnetic-bead separation method as convenient. For example cells of the erythroid lineage can be identified using an antibody such as the TER-119 antibody which can be coupled to a suitable reporter moiety.

Establishing a control or normal population value is possible using standard techniques of sample collection and analysis, for example haematology and/or cytology. Standard statistical techniques can be used in order to define meaningful average values for a population which may require taking into account factors such as age, sex and/or genotype, and may be based on an uninfected individual or a population of uninfected individuals.

A recent comparison of blood counts in venous, fingertip and arterial blood (Yang, et al, Clin. Lab. Haem. 23 155-159 (2001)) established that normal blood cell population counts in a defined human population were as follows:

In a preferred embodiment of this aspect of the invention, the method of diagnosis comprises assaying for the relative concentration of EDRF in sample including cells of the hematopoietic erythroid, megakaryocyte or platelet lineages that express EDRF. Such cells can include, but are not limited to, E/Meg, GEMM cells, BFU-E cells, CFU-E cells, proerythroblasts, basophilic erythroblasts, polychromatophilic erythroblasts, orthochromatophilic erythroblasts, reticulocytes (primitive erythrocytes), megakaryocytes or platelets. The nucleotide sequences of murine, human and bovine EDRF are shown in FIG. 3  (see Original Patent) and the predicted amino acid sequences are shown in FIG. 4 (see Original Patent) . The relative concentration of EDRF can be assayed directly by measuring the levels of EDRF protein in the sample, or indirectly by using EDRF cDNA to measure levels of mRNA encoding EDRF which is indicative of the level of EDRF expression in cells in the sample. A reduction in levels of EDRF in the sample is indicative of TSE infection (or a TSE disease condition) in a subject. Reduction in expression levels is in relation to the expression levels which are present in non-infected individuals. A normal range can be identified by assaying a statistically relevant population of healthy non-infected individuals.

Further diagnostic assays can involve the use of antibodies to EDRF which can be polyclonal antibodies or monoclonal antibodies. Polyclonal antibodies can be raised by stimulating their production in a suitable animal host (e.g. a mouse, rat, guinea pig, rabbit, sheep, chicken, goat or monkey) when the substance of the present invention is injected into the animal. If necessary an adjuvant may be administered together with the substance of the present invention. The antibodies can then be purified by virtue of their binding to EDRF or as described further below. Monoclonal antibodies can be produced from hybridomas. These can be formed by fusing myeloma cells and spleen cells which produce the desired antibody in order to form an immortal cell line. This is the well known Kohler & Milstein technique (Nature 256 52-55 (1975)).

Techniques for producing monoclonal and polyclonal antibodies which bind to a particular protein are now well developed in the art. They are discussed in standard immunology textbooks, for example in Roitt et al, Immunology second edition (1989), Churchill Livingstone, London.

In addition to whole antibodies, the present invention includes derivatives thereof which are capable of binding to EDRF. Thus the present invention includes antibody fragments and synthetic constructs. Examples of antibody fragments and synthetic constructs are given by Dougall et al in Tibtech 12 372-379 (September 1994). Antibody fragments include, for example, Fab, F(ab′)₂ and Fv fragments (see Roitt et al [supra]). Fv fragments can be modified to produce a synthetic construct known as a single chain Fv (scFv) molecule. This includes a peptide linker covalently joining V_h and V_i regions which contribute to the stability of the molecule.

Other synthetic constructs include CDR peptides. These are synthetic peptides comprising antigen binding determinants. Peptide mimetics may also be used. These molecules are usually conformationally restricted organic rings which mimic the structure of a CDR loop and which include antigen-interactive side chains. Synthetic constructs also include chimaeric molecules. Thus, for example, humanised (or primatised) antibodies or derivatives thereof are within the scope of the present invention. An example of a humanised antibody is an antibody having human framework regions, but rodent hypervariable regions. Synthetic constructs also include molecules comprising a covalently linked moiety which provides the molecule with some desirable property in addition to antigen binding. For example the moiety may be a label (e.g. a detectable label, such as a fluorescent or radioactive label) or a pharmaceutically active agent.

The antibodies or derivatives thereof specific for EDRF have a variety of other uses. They can be used in purification and/or identification of EDRF itself or a cell that expresses EDRF. As a result they may be used in a diagnostic method according to the present invention.

After the preparation of a suitable antibody to EDRF, it may be isolated or purified by one of several techniques commonly available (for example, as described in Antibodies: A Laboratory Manual, Harlow and Lane, eds. Cold Spring Harbor Laboratory Press (1988)). Generally suitable techniques include peptide or protein affinity columns, HPLC or RP-HPLC, purification on Protein A or Protein G columns, or combinations of these techniques. Recombinant antibodies to EDRF can be prepared according to standard methods, and assayed for specificity for EDRF using procedures generally available, including ELISA, ABC, dot-blot assays etc.

An example of one method of assaying for EDRF protein expressed in a sample is western blotting. An extract of proteins from the sample can be fractionated through denaturing SDS-polyacrylamide gel electrophoresis. The mixture can then be transferred and immobilized on a solid membrane of nitrocellulose or nylon by electroblotting. The loaded membrane is then incubated with anti-EDRF antibody. The resulting antigen-antibody complex can then be detected by any suitable procedure. For example, a second antibody, raised against the anti-EDRF antibody, to which a reporter moiety has been linked (for example horseradish peroxidase, alkaline phosphatase) can be added. The reaction product generated by enzyme action can then be used to indicate the position of the target protein on the membrane. Measurement of the levels of enzyme reaction is indicative of the levels of target protein present in the sample. The sensitivity of the detection system can be improved by using the biotin-streptavidin system or by chemiluminescent detection.

Alternatively, levels of EDRF protein can be assayed for by the standard techniques of radio-immunoassay (RIA) or Enzyme-linked immunosorbent assay (ELISA). Antibody is linked to a reporter enzyme and then immobilized on a microtitre plate. A lysate of a sample to be measured is then added to allow antibody-antigen complex formation. After washing and provision of substrate, levels of product formation are proportional to the levels of antigen, i.e. target protein, present in the sample. Another method is the DELFIA system based on a time-resolved fluorometric assay. The Delfia Research System (Wallac) measures the fluorescence of metals from the lanthanide series, including europiun, samarium, and terbium, when chelated to molecules that fluoresce. Antibodies are labelled and immobilized on microtitre plates. A lysate from the sample to be measured is added to the plate to allow antibody-antigen complex formation. After washing, signal is determined.

The presence of RNA transcripts encoding EDRF ready for expression can be assayed for using northern blotting (Thomas, P. S., Proc. Nat'l Acad. Sci. USA 77 5201-5205 (1980)). Briefly, denatured RNA from sample cells is transferred onto a nitrocellulose or nylon filter for subsequent use in a hybridisation assay. The RNA is electrophoresed in a denaturing agarose gel before being transferred onto a membrane either by capillary action or under the action of an electrical field. A radioactively labelled DNA or RNA probe specific for EDRF RNA is hybridised to the filter-bound RNA to enable detection. Alternatively, RNA levels can be measured using "taqman™" which is a real-time quantitative RT-PCR procedure whose specificity derives from the use of a fluorescence energy transfer (FRET) probe, or by the "invader" technology based on the discovery of a unique class of structure specific endonuclease enzymes (cleavases). Invader and signal probes are designed to hybridise to overlapping sites on the target DNA/RNA such that the invader probe displaces a portion of the signal probe. This forms a structure that a cleavase enzyme will recognise and cut, thus creating detectable products.

This aspect of the invention can alternatively be defined as a method of diagnosis for the presence of a transmissible spongiform encephalopathy (TSE) in an animal, comprising assaying a sample obtained from the animal to determine the number of haematopoietic cells of the erythroid, megakaryocyte or platelet lineages in the sample relative to a control or normal population value.

According to a second aspect of the present invention there is provided a method of diagnosis for the presence of a transmissible spongiform encephalopathy (TSE) in an animal, comprising assaying a sample obtained from the animal to determine the relative amount in the sample of an expression product of a hematopoietic cell of erythroid, megakaryocyte or platelet lineages. The expression product of a hematopoietic cell of erythroid, megakaryocyte or platelet lineages to be assayed can be as described above. Methods for sample analysis and detection of expression product can be as described above.

The presence of a TSE in an individual can be established by means of this assay without reference to the actual number of hematopoietic cells in the sample, in which the amount of expression product is compared to a normal range identified by assaying a statistically relevant population of healthy non-infected individuals.

According to a third aspect of the present invention there is provided a kit for the diagnosis for the presence of a transmissible spongiform encephalopathy (TSE) in an animal, where the kit comprises an antibody to a cell of the hematopoietic erythroid, megakaryocyte or platelet lineages or an antibody to EDRF. In alternative embodiments of this aspect of the invention, the kit may comprise antibodies to both a cell of the hematopoietic erythroid, megakaryocyte or platelet lineages and to EDRF. Such dual antibody kits may allow for greater sensitivity of the diagnostic method.

Antibodies specific to cells of the hematopoietic erythroid, megakaryocyte or platelet lineages are as described above. Antibodies to EDRF can be made as described above.

Assays for the binding of antibody to target can be performed using standard techniques commonly available such as ELISA, ABC, dot-blot assays. Additional detection labels such as fluorescent or radioactive markers may also be used.

As discussed above, the tissue sources from which samples may be assayed can include blood, or fractions thereof, or other tissues and/or organs. Thus a method of the present application may find utility in pre-screening of blood, (or a fraction thereof), tissue, and/or organs prior to transfusion or transplantation into a recipient. The source of blood, or fractions thereof, or other tissues and/or organs may be allogenic with respect to the recipient, or may be xenogeneic. Where a sample shows a positive result for the presence of a TSE infection using a method according to the first aspect of the invention, this will enable a decision to be taken to avoid using donated blood, (or a fraction thereof), tissue, and/or organs from the individual whose sample was analysed in subsequent transfusion or transplantation into a recipient. By such means, a potential route of TSE infection into the general population may be avoided.

Alternatively, the screening may carried out on samples of blood, (or a fraction thereof), tissue, and/or organs prior to the preparation of a blood or tissue or organ-based product. Such products may include blood plasma or concentrates of blood clotting factors, such as for example, Factor IX, Factor XI, Factor VII, Factor XII, Factor V, Factor XIII, Factor VIIIC, Factor VIIIvWFAg, or hormones such as growth hormone, erthropoietin, thyroxin or insulin prepared from whole blood or another tissue source.

According to a fourth aspect of the invention there is therefore provided a method for the preparation of a blood product, the method comprising assaying a sample of blood for the presence of a transmissible spongiform encephalopathy (TSE) in the animal from which the sample was obtained, in which the assay comprises determining the relative number of hematopoietic cells of the erythroid, megakaryocyte or platelet lineages in the sample.

In a preferred embodiment of the present invention, the method of diagnosing for the presence of a transmissible spongiform encephalopathy (TSE) in the animal, the method may be performed as follows.