Patent Application
20060177818
Not Applicable.
Not Applicable.
Not Applicable.
The present invention relates to oligonucleotides useful as primers and probes. The invention also relates to methods of using the primers and probes for detection of classical swine fever virus nucleic acid, preferably in an isolated biological sample, and to reagents and kits therefor. In a particular embodiment, the invention relates to a fully validated, ready-to-use multiplex real-time reverse transcription polymerase chain reaction (“RT-PCR”) assay with an internal control system for the simple and fast diagnosis of classical swine fever.
Classical swine fever (“CSF”) is a highly contagious disease of pigs and wild boar. Although it has been eradicated from many countries, CSF continues to cause serious problems in different parts of the world (Moenning et al., Clinical Signs And Epidemiology Of Classical Swine Fever: A Review Of New Knowledge. Vet. J. 2003, 165, 11-20; Edwards et al., Classical Swine Fever: The Global Situation, Vet. Microbiol. 2000, 73, 103-19). The causative agent, CSF virus (“CSFV”) is a member of the genus Pestivirus within the family Flaviviridae. The other members within the genus Pestivirus are bovine viral diarrhoea virus (“BVDV”) and border disease virus (“BDV”). The natural hosts for BVDV and BDV are cattle and sheep, respectively, but both viruses can naturally infect pigs also. Antibodies against BVD virus and BD virus may cross-react with CSFV in serological assays, and cause diagnostic problems (Terpstra, C., Wensvoort, G., A Congenital Persistent Infection Of Bovine Virus Diarrhoea Virus In Pigs: Clinical, Virological And Immunological Observations. Vet. Q., 1997, 19, 97-101; Paton, D. J., Done, S. H., Congenital Infection Of Pigs With Ruminant-Type Pestiviruses, J. Comp. Pathol. 1994, 111,151-63; Paton et al. Infection Of Pigs And Cattle With Bovine Viral Diarrhoea Virus On A Farm In England, Vet. Rec., 1992, 131, 185-8.). Because only CSFV is classified within the list A diseases of the Office International des Epizooties (“OIE”), it is important to differentiate between CSFV and BVDV or BDV.
Pestiviruses are small enveloped viruses containing a positive sense single stranded RNA of approximately 12.5 kb. Their genomes have a large open reading frame flanked by highly conserved 5′- and 3′-non-translated regions (“NTR”). In the last decade, mainly the 5′NTR served as the template for species and genus overlapping genome amplifications by using the RT-PCR (Vilcek et al., Pestiviruses Isolated From Pigs, Cattle And Sheep Can Be Allocated Into At Least Three Genogroups Using Polymerase Chain Reaction And Restriction Endonuclease Analysis. Arch. Virol. 1994 136, 309-23; Sandvik et al., Detection And Identification Of Ruminant And Porcine Pestiviruses By Nested Amplification Of 5′ Untranslated Cdna Regions. J. Virol. Methods, 1997, 64, 43-56.; Hyndman et al., A Novel Nested Reverse Transcription PCR Detects Bovine Viral Diarrhoea Virus In Fluids From Aborted Bovine Fetuses, J. Virol. Methods, 1998, 71, 69-76.; Paton et al., Classical Swine Fever Virus: A Ring Test To Evaluate RT-PCR Detection Methods, Vet. Microbiol,. 2000, 73, 159-74; Patton et al., Classical Swine Fever Virus: A Second Ring Test To Evaluate RT-PCR Detection Methods, Vet. Microbiol., 2000, 77, 71-81; Barlic-Maganja and Grom, Highly Sensitive One-Tube RT-PCR And Microplate Hybridisation Assay For The Detection And For The Discrimination Of Classical Swine Fever Virus From Other Pestiviruses, J. Virol. Methods, 2001, 95, 101-10). The classical RT-PCR proved to be a sensitive and specific diagnostic tool. However, detection of the amplified PCR products by gel-based systems bears the risk for cross-contaminations, does not allow exact quantification of genome copies in the template, and does not include an additional specificity test. The introduction of fluorogenic probes allows the detection of sequence specific templates achieved in real-time without opening the PCR tubes (Gibson et al., A Novel Method For Real Time Quantitative RT-PCR. Genome Res. 1996, 6, 995-1001 Heid et al., 1996). Since real-time PCR does not require post-PCR sample handling contaminations can be avoided, and the hybridisation of the probe ensures specificity.
For the diagnosis of Pestiviruses, TaqMan-probes proved to be practicable and robust and were used by several authors (McGoldrick et al., A Novel Approach To The Detection Of Classical Swine Fever Virus By RT-PCR With A Fluorogenic Probe (TaqMan). J. Virol. Methods, 1998, 72, 125-35; 1999; Bhudevi and Weinstock, Detection of bovine viral diarrhea virus in formalin fixed paraffin embedded tissue sections by real-time RT-PCR (Taqman). J. Virol. Methods, 2003, 109, 25-30; Bhudevi and Weinstock, Fluorogenic RT-PCR Assay (Taqman) For Detection And Classification Of Bovine Viral Diarrhea Virus. Vet. Microbiol. 2001, 83, 1-10.; Risatti et al., Rapid Detection Of Classical Swine Fever Virus By A Portable Real-Time Reverse Transcriptase PCR Assay, J. Clin. Microbiol. 2003, 41, 500-5). All currently described real-time RT-PCR assays for the detection of pestiviral sequences amplified targets within the 5′NTR gave results with acceptable sensitivity and specificity. However, no internal controls verifying the RNA isolation step as well as the RT-PCR have been used so far. In 2003, a multiplex real-time RT-PCR for the detection of CSFV was described in which a chimeric CSF virus containing the 5′NTR of BVDV as an universal positive control was used (Hofmann, Construction Of An Infectious Chimeric Classical Swine Fever Virus Containing The 5′UTR Of Bovine Viral Diarrhea Virus, And Its Application As A Universal Internal Positive Control In Real-Time RT-PCR. J. Virol. Methods, 2003, 114, 77-90). However, such a complete chimeric virion might be infectious and would therefore not be suitable for routine diagnostic purposes.
The present invention provides a robust ready to use, highly sensitive, and CSFV-specific multiplex real-time RT-PCR assay with two controls for the simple and fast routine diagnosis of CSF. The first control (positive control) proves the efficiency of primers and probes, while the second control (internal control) is designed to check RNA isolation and RT-PCR of each sample tested.
In one embodiment, the invention encompasses oligonucleotides that are useful as primers or probes.
In another embodiment, the invention encompasses methods for detection of classical swine fever virus nucleic acids in a biological sample using a multiplex real-time RT-PCR technique with a heterologous inernal control RNA (i.e., EGFP-RNA).
Another embodiment of the invention encompasses a method of quantifying CSFV viral load in an isolated, biological sample.
Yet another embodiment of the invention encompasses kits for detecting CSFV.
8.1. Definitions
As used herein the term “amplified” or “amplification” refers to the production of many DNA copies from one or a few copies.
As used herein the term “biological sample” includes, but is not limited to, to serum, plasma, semen, urine, or blood.
As used herein, the term “complementary” refers to Watson-Crick or Hoogsteen base pairing between nucleotides units of a nucleic acid molecule, and the term “binding” means the physical or chemical interaction between two polypeptides or compounds or associated polypeptides or compounds or combinations thereof. Binding includes, but is not limited to, ionic, non-ionic, Van der Waals, hydrophobic interactions. A physical interaction can be either direct or indirect. Indirect interactions may be through or due to the effects of another polypeptide or compound. Direct binding refers to interactions that do not take place through, or due to, the effect of another polypeptide or compound, but instead are without other substantial chemical intermediates.
As used herein the term “conservative amino acid substitution” refers to a substitution in which an amino acid residue is replaced with an amino acid residue having a similar side chain. Families of amino acid residues having similar side chains have been defined in the art. These families include amino acids with basic side chains (e.g., lysine, arginine, histidine), acidic side chains (e.g., aspartic acid, glutamic acid), uncharged polar side chains (e.g., asparagine, glutamine, serine, threonine, tyrosine, cysteine), nonpolar side chains (e.g., glycine, alanine, valine, leucine, isoleucine, proline, phenylalanine, methionine, tryptophan), beta-branched side chains (e.g., threonine, valine, isoleucine) and aromatic side chains (e.g., tyrosine, phenylalanine, tryptophan, histidine).
As used herein, the terms “fragment of a CSFV” or “portion of a CSFV” refer to an amino acid sequence comprising at least about 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 95%, 96%, 97%, 98%, 99% or 100% of a naturally occurring CSFV or mutant thereof.
As used herein, the term “gene” refers to any segment of DNA associated with a biological function. Thus, genes include, but are not limited to, coding sequences and/or the regulatory sequences required for their expression. Genes can also include non-expressed DNA segments that, for example, form recognition sequences for other proteins. Genes can be obtained from a variety of sources, including cloning from a source of interest or synthesizing from known or predicted sequence information, and may include sequences designed to have desired parameters. In addition, the terms “gene” and “recombinant gene” also refer to nucleic acid molecules comprising an open reading frame encoding an CSFV.
As used herein, a “heterologous polynucleotide” or a “heterologous nucleic acid” or a “heterologous gene” or a “heterologous sequence” or an “exogenous DNA segment” refers to a polynucleotide, nucleic acid or DNA segment that originates from a source foreign to the particular host cell, or, if from the same source, is modified from its original form. A heterologous gene in a host cell includes a gene that is endogenous to the particular host cell, but has been modified. Thus, the terms refer to a DNA segment which is foreign or heterologous to the cell, or homologous to the cell but in a position within the host cell nucleic acid in which the element is not ordinarily found. As an example, a signal sequence native to a yeast cell but attached to a human CSFV sequence is heterologous.
A “homologous nucleic acid sequence” or “homologous amino acid sequence,” or variations thereof, refer to sequences characterized by a homology at the nucleotide level or amino acid level. Homologous nucleotide sequences encode those sequences coding for isoforms of peptides. Isoforms can be expressed in different tissues of the same organism as a result of, for example, alternative splicing of RNA. Alternatively, isoforms can be encoded by different genes. In the present invention, homologous nucleotide sequences include nucleotide sequences encoding for a CSFV of species other than humans, including, but not limited to, mammals, and thus can include, for example, mouse, rat, rabbit, dog, cat, pig, cow, horse, and other organisms. Homologous nucleotide sequences also include, but are not limited to, naturally occurring allelic variations and mutations of the nucleotide sequences set forth herein.
As used herein, an “isolated” nucleic acid sequence refers to a nucleic acid sequence which is essentially free of other nucleic acid sequences, e.g., at least about 20% pure, preferably at least about 40% pure, more preferably about 60% pure, even more preferably about 80% pure, most preferably about 90% pure, and even most preferably about 95% pure, as determined by agarose gel electrophoresis. For example, an isolated nucleic acid sequence can be obtained by standard cloning procedures used in genetic engineering to relocate the nucleic acid sequence from its natural location to a different site where it will be reproduced. The cloning procedures may involve excision and isolation of a desired nucleic acid fragment comprising the nucleic acid sequence encoding the polypeptide, insertion of the fragment into a vector molecule, and incorporation of the recombinant vector into a host cell where multiple copies or clones of the nucleic acid sequence will be replicated. The nucleic acid sequence may be of genomic, cDNA, RNA, semi-synthetic, synthetic origin, or any combinations thereof.
As used herein the term “multiplex PCR” refers to PCR, which involves adding more than one set of PCR primers to the reaction in order to target multiple locations throughout the genome; it is useful for DNA typing because, inter alia, the probability of identical alleles in two individuals decreases with an increase in the number of polymorphic loci examined. Furthermore, multiplexing with an IC (e.g., EGFP-RNA) provides internal control of the whole PCR without affecting sensitivity or specificity of the CSFV real-time PCR.
As used herein, the terms “nucleic acid,” “nucleic acid molecule,” or “polynucleotide” refer to deoxyribonucleotides or ribonucleotides and polymers thereof in either single- or double-stranded form. Unless specifically limited, the terms encompass nucleic acids containing analogues of natural nucleotides that have similar binding properties as the reference nucleic acid and are metabolized in a manner similar to naturally occurring nucleotides. Unless otherwise indicated, a particular nucleic acid sequence also implicitly encompasses conservatively modified variants thereof (e.g., degenerate codon substitutions) and complementary sequences as well as the sequence explicitly indicated. Specifically, degenerate codon substitutions may be achieved by generating sequences in which the third position of one or more selected (or all) codons is substituted with mixed-base and/or deoxyinosine residues (Batzer et al. (1991) Nucleic Acid Res. 19:5081; Ohtsuka et al. (1985) J. Biol. Chem. 260:2605-2608; Cassol et al. (1992); Rossolini et al. (1994) Mol. Cell. Probes 8:91-98). The term nucleic acid is used interchangeably with gene, cDNA, and mRNA encoded by a gene. As used herein, the terms “nucleic acid,” “nucleic acid molecule,” or “polynucleotide” are intended to include DNA molecules (e.g., cDNA or genomic DNA), RNA molecules (e.g., mRNA), analogs of the DNA or RNA generated using nucleotide analogs, and derivatives, fragments and homologs thereof.
As used herein the abbreviation “nt” means nucleotide(s).
As used herein, the term “oligonucleotide” refers to a series of linked nucleotide residues, which oligonucleotide has a sufficient number of nucleotide bases to be used in a PCR reaction. A short oligonucleotide sequence may be based on, or designed from, a genomic or cDNA sequence and is used to amplify, confirm, or reveal the presence of an identical, similar or complementary DNA or RNA in a particular cell or tissue. Oligonucleotides comprise portions of a nucleic acid sequence having about 10 nt, 50 nt, or 100 nt in length, preferably about 15 nt to 30 nt in length. Oligonucleotides of the invention may be chemically synthesized and may be used as probes.
As used herein, a DNA segment is referred to as “operably linked” or “operatively linked” when it is placed into a functional relationship with another DNA segment. Generally, DNA sequences that are operably linked are contiguous, and in the case of a signal sequence or fusion protein both contiguous and in reading phase. However, enhancers need not be contiguous with the coding sequences whose transcription they control. Linking, in this context, is accomplished by ligation at convenient restriction sites or at adapters or linkers inserted in lieu thereof.
As used herein, “PCR” generally refers to a method for amplifying a DNA or RNA base sequence using a heat-stable polymerase and two oligonucleotide primers, one complementary to the (+)-strand at one end of the sequence to be amplified and the other complementary to the (−)-strand at the other end. Because the newly synthesized DNA or cDNA strands can subsequently serve as additional templates for the same primer sequences, successive rounds of primer annealing, strand elongation, and dissociation produce rapid and highly specific amplification of the desired sequence.
As used herein, the term “probes” refer to nucleic acid sequences of variable length, preferably between at least about 10 nt or about 100 nt depending on use. Probes are used in the detection of identical, similar, or complementary nucleic acid sequences. Longer length probes are usually obtained from a natural or recombinant source, are highly specific and much slower to hybridize than oligomers. Probes may be single- or double-stranded and designed to have specificity in PCR, membrane-based hybridization technologies, or ELISA-like technologies, preferably PCR, more preferably RT-PCR, and even more preferably in real-time RT-PCR.
As used herein, the term “primer” refers to a short, artificial oligonucleotide strands usually not more than fifty, preferably 18-25 bp nucleotides (since DNA is usually double-stranded, its length is measured in base pairs; the length of single-stranded DNA is measured in bases or nucleotides) that exactly match the beginning and end of the genomic fragment to be amplified. Primers anneal (adhere) to the DNA template at the starting and ending points, where the DNA-Polymerase binds and begins the synthesis of the new DNA strand. The choice of the length of the primers and their melting temperature (Tm) depends on a number of considerations. The melting temperature of a primer—not to be confused with the melting temperature of the DNA in the first step of the PCR process—is defined as the temperature below which the primer will anneal to the DNA template and above which the primer will dissociate (break apart) from the DNA template. The melting temperature increases with the length of the primer. Primers that are too short would anneal at several positions on a long DNA template, which would result in non-specific copies. On the other hand, the length of a primer is limited by the temperature required to melt it. Melting temperatures that are too high, (i.e., above 80° C.), can also cause problems since the DNA-Polymerase is less active at such temperatures. The optimum melting temperature is between 60° C. and 75° C. A forward sequencing primer anneals 5′ with respect to the reverse primer, and the reverse sequencing primer that anneals 3′ with respect to the forward primer. The relationship between the primers and the reference sequence depends on the coordinate system that is used. The forward primer's annealing positions will usually be less than the annealing positions of the reverse primer since the forward primer should fall to the logical left of the reverse primer in the coordinate system.
As used herein, the term “promoter” refers to a region of DNA involved in binding RNA polymerase to initiate transcription.
As used herein, the term “recombinant” refers to a cell, tissue or organism that has undergone transformation with a new combination of genes or DNA.
As used herein, the phrase “stringent hybridization conditions” refers to conditions under which a probe, primer or oligonucleotide will hybridize to its target sequence, but to no other sequences. Stringent conditions are sequence-dependent and will be different in different circumstances. Longer sequences hybridize specifically at higher temperatures than shorter sequences. Generally, stringent conditions are selected to be about 5° C. lower than the thermal melting point (Tm) for the specific sequence at a defined ionic strength and pH. The Tm is the temperature (under defined ionic strength, pH and nucleic acid concentration) at which 50% of the probes complementary to the target sequence hybridize to the target sequence at equilibrium. Since the target sequences are generally present at excess, at Tm, 50% of the probes are occupied at equilibrium. Typically, stringent conditions will be those in which the salt concentration is less than about 1.0 M sodium ion, typically about 0.01 to 1.0 M sodium ion (or other salts) at pH 7.0 to 8.3 and the temperature is at least about 30° C. for short probes, primers or oligonucleotides (e.g., 10 nt to 50 nt) and at least about 60° C. for longer probes, primers and oligonucleotides. Stringent conditions may also be achieved with the addition of destabilizing agents, such as formamide. Stringent conditions are known to those skilled in the art and can be found in Ausubel et al., (eds.), Current Protocols in Molecular Biology, John Wiley & Sons, N.Y. (1989), 6.3.1-6.3.6. Preferably, the conditions are such that sequences at least about 65%, 70%, 75%, 85%, 90%, 95%, 98%, or 99% homologous to each other typically remain hybridized to each other. A non-limiting example of stringent hybridization conditions are hybridization in a high salt buffer comprising 6×SSC, 50 mM Tris-HCl (pH 7.5), 1 mM EDTA, 0.02% PVP, 0.02% Ficoll, 0.02% BSA, and 500 mg/ml denatured salmon sperm DNA at 65° C., followed by one or more washes in 0.2×SSC, 0.01% BSA at 50° C.
As used herein, the term “subject” can be a human, or an animal, preferably a pig, cow, sheep, or boar.
As used herein, the term “TaqMan” generally refers to the probe used to detect specific sequences in PCR products by employing the 5′—>3′ exonuclease activity of Taq DNA polymerase. The TaqMan probe (about 20-30 bp), disabled from extension at the 3′ end, consists of a site-specific sequence labeled with a fluorescent reporter dye and a fluorescent quencher dye. During PCR the TaqMan probe hybridizes to its complementary single strand DNA sequence within the PCR target. When amplification occurs the TaqMan probe is degraded due to the 5′—>3′ exonuclease activity of Taq DNA polymerase, thereby separating the quencher from the reporter during extension. Due to the release of the quenching effect on the reporter, the fluorescence intensity of the reporter dye increases. During the entire amplification process this light emission increases exponentially, the final level being measured by spectrophotometry after termination of the PCR. Because increase of the fluorescence intensity of the reporter dye is only achieved when probe hybridization and amplification of the target sequence has occurred, the TaqMan assay offers a sensitive method to determine the presence or absence of specific sequences. Therefore, this technique is particularly useful in diagnostic applications, such as the screening of samples for the presence or incorporation of favorable traits and the detection of pathogens and diseases. The TaqMan assay allows high sample throughput because no gel-electrophoresis is required for detection. TaqMan probes depend on the 5′-nuclease activity of the DNA polymerase used for PCR to hydrolyze an oligonucleotide that is hybridized to the target amplicon. In particular, TaqMan probes are oligonucleotides that have a fluorescent reporter dye attached to the 5′ end and a quencher moeity coupled to the 3′ end. These probes are designed to hybridize to an internal region of a PCR product. In the unhybridized state, the proximity of the fluorescent reporter and the quench molecules prevents the detection of fluorescent signal from the probe. During PCR, when the polymerase replicates a template on which a TaqMan probe is bound, the 5′-nuclease activity of the polymerase cleaves the probe. This decouples the fluorescent and quenching dyes and the Fluorescence Resonance Energy Transfer (FRET) no longer occurs. Thus, fluorescence increases in each cycle, proportional to the amount of probe cleavage.
As used herein, the term “thermostable polymerase enzyme” refers to an enzyme, which is stable to heat and is heat resistant and catalyzes (facilitates) combination of the nucleotides in the proper manner to form the primer extension products that are complementary to each nucleic acid strand. Generally, the synthesis will be initiated at the 3′ end of primer and will proceed in the 5′ direction along the template strand, until synthesis terminates, producing molecules of different lengths. There may be a thermostable enzyme, however, which initiates synthesis at the 5′ end and proceeds in the other direction, using the same process as described above. The preferred thermostable enzyme herein is a DNA polymerase isolated from Thermus aquaticus. Various strains thereof are available from the Americal Type Culture Collection, Rockville, Md., and are described by T. D. Brock, J. Bact. (1969) 98:289-297, and by T. Oshima, Arch. Mircobiol. (1978)117:189-196. One of these preferred strains is strain YT-1.
As used herein, the term “transformation” refers to the transfer of nucleic acid (i.e., a nucleotide polymer) into a cell. As used herein, the term “genetic transformation” refers to the transfer and incorporation of DNA, especially recombinant DNA, into a cell.
“Variants or variant” refers to a polynucleotide or nucleic acid differing from a reference nucleic acid or polypeptide, but retaining essential properties thereof. Generally, variants are overall closely similar, and, in many regions, identical to the reference nucleic acid or polypeptide.
As used herein, the term “vector” refers broadly to any plasmid, phagemid or virus encoding an exogenous nucleic acid. The term is also be construed to include non-plasmid, non-phagemid and non-viral compounds which facilitate the transfer of nucleic acid into virions or cells, such as, for example, polylysine compounds and the like. The vector may be a viral vector that is suitable as a delivery vehicle for delivery of the nucleic acid, or mutant thereof, to a cell, or the vector may be a non-viral vector which is suitable for the same purpose. Examples of viral and non-viral vectors for delivery of DNA to cells and tissues are well known in the art and are described, for example, in Ma et al. (1997, Proc. Natl. Acad. Sci. U.S.A. 94:12744-12746). Examples of viral vectors include, but are not limited to, a recombinant vaccinia virus, a recombinant adenovirus, a recombinant retrovirus, a recombinant adeno-associated virus, a recombinant avian pox virus, and the like (Cranage et al., 1986, EMBO J. 5:3057-3063; International Patent Application No. WO 94/17810, published Aug. 18, 1994; International Patent Application No. WO 94/23744, published Oct. 27, 1994). Examples of non-viral vectors include, but are not limited to, liposomes, polyamine derivatives of DNA, and the like.
As used herein, the term “wild type” refers to a polynucleotide or polypeptide sequence that is naturally occurring.
8.2. Description of the Embodiments of the Invention
8.2.1. Oligonucleotides of the Invention
The invention encompasses oligonucleotides that are useful as primers or probes. When used as a probe, it is preferable that the oligonucleotides contain a label. In an illustrative embodiment, the oligonucleotides of the invention encompass an oligonucleotide having the nucleotide sequence of SEQ ID NO: 2, SEQ ID NO: 3, SEQ ID NO: 4, SEQ ID NO: 5, SEQ ID NO. 6, SEQ ID NO:7, SEQ ID NO. 8, or SEQ ID NO. 9. It is preferred that the oligonucleotides of the invention are useful as primers or probes. A particular embodiment of the invention encompasses a set of primers of oligonucleotides of SEQ ID NO: 2, SEQ ID NO: 4, SEQ ID NO: 5, and SEQ ID NO: 7. Another particular embodiment of the invention encompasses a set of probes of oligonucleotides of SEQ ID NO: 3, SEQ ID NO: 6, SEQ ID NO. 8, or SEQ ID NO. 9. In another embodiment, the probes of the invention further comprise a label. In a particular embodiment, the label is a fluorescent group, digoxigenin, or biotin.
Illustrative oligonucleotides of the invention have the following nucleotide sequences:
8.2.1.1. Oligonucleotide Synthesis
Oligonucleotide synthesis has become routine. For a detailed description of nucleic acid synthesis see Gait, M. J., Oligonucleotide Synthesis: a Practical Approach. IRL Press, Oxford England. Preferably, the oligonucleotides of the invention are synthesized on supports in what is known as solid phase synthesis. Alternatively, they are synthesized in solution. Those of ordinary skill in the art will recognize that both labeled, unlabeled and/or modified oligonucleotides (DNA, RNA and synthetic analogues thereof) are readily available. They can be synthesized using commercially available instrumentation and reagents or they can be purchased from commercial vendors of custom manufactured oligonucleotides. Patents that discuss various compositions, supports and methodologies for the synthesis and labeling of nucleic acids include: U.S. Pat. Nos. 5,476,925, 5,453,496, 5,446,137, 5,419,966, 5,391,723, 5,391,667, 5,380,833, 5,348,868, 5,281,701, 5,278,302, 5,262,530, 5,243,038, 5,218,103, 5,204,456, 5,204,455, 5,198,527, 5,175,209, 5,164,491, 5,112,962, 5,071,974, 5,047,524, 4,980,460, 4,923,901, 4,786,724, 4,725,677, 4,659,774, 4,500,707, 4,458,066, and 4,415,732, each of which is incorporated by reference.
8.2.2. Methods of the Invention
The invention also encompasses methods for detection of CSFV nucleic acids in an isolated biological sample of a subject comprising reverse transcribing RNA encoding one or more CSFV genes to obtain complementary CSFV (“CSFV cDNA”); amplifying said CSFV cDNA utilizing two or more primers under conditions to produce a PCR Product, wherein at least one forward primer hybridizes to a target site corresponding to a first group of nucleotides of the SEQ ID NO: 1 or a complementary strand thereof and at least one reverse primer hybridizes to a target site corresponding to second group of nucleotides of SEQ ID NO: 1 or a complementary strand thereof; contacting said PCR Product with a nucleic acid probe such that the probe hybridizes to the PCR Product or a complementary strand thereof to provide a hybridized probe, and detecting the presence of the hybridized probe.
In an illustrative embodiment, the invention encompasses a method for detecting CSFV nucleic acids in a biological sample comprising:
In another embodiment, the invention encompasses further adding an internal control of the EGFP-Sequence of plasmid vector pEGFP-1 (i.e., SEQ ID NO. 10).
In another embodiment, the invention further comprises adding at least one second forward primer and at least one second reverse primer idize to said additional primer.
In another illustrative embodiment, a second forward primer hybridizes to a target site corresponding to nucleotides 625 and 675, preferably nucleotides 630 to 665, and most preferably nucleotides 637 to 656 of SEQ. ID NO. 10 or a complementary strand thereof, and the second reverse primer hybridizes to a target site corresponding to nucleotides 740 to 780, preferably nucleotides 745 to 775, and most preferably nucleotides 750 to 768 of SEQ ID NO. 10 or a complementary strand thereof.
Another illustrative embodiment of the invention encompasses adding a second nucleic acid probe, wherein the probe hybridizes to SEQ. ID NO. 10 or a complementary strand thereof.
In another illustrative embodiment, the second nucleic acid probe hybridizes to a target site corresponding to nucleotides 690 to 735, preferably nucleotides 695 to 730, and most preferably nucleotides 703 to 724 of SEQ ID NO. 10 or a complementary strand thereof.
Another illustrative embodiment of the invention encompasses a method for detection of CSFV nucleic acids in a biological sample comprising:
In another particular embodiment of the methods of the invention, the probe can be independently detected in a closed tube format in either real-time or at the end-point of the assay. In yet another particular embodiment of the methods of the invention, at least two or more independently detectable PCR Products are present in a single multiplex assay that is used to simultaneously detect, identify or quantitate two or more target molecules of interest (e.g., CSFV nucleic acids) in the same sample and in the same assay.
In another embodiment the invention encompasses methods for multiplex real-time RT-PCR detection of CSFV nucleic acids in a biological sample comprising reverse transcribing RNA encoding CSFV genes to obtain a CSFV cDNA and amplifying said CSFV cDNA with at least one forward primer (i.e., a first forward primer) and at least one reverse primer (i.e., a first reverse primer) under conditions such that said primers hybridize to said cDNA to form a PCR Product, wherein at least one forward primer hybridizes to a target site corresponding to a first group of nucleotides of the SEQ ID NO. 1 and at least one reverse primer hybridizes to a target site corresponding to second group of nucleotides of SEQ ID NO. 1; contacting the PCR Product with a first nucleic acid probe wherein the probe hybridizes to the PCR Product or a complementary strand thereof to form a hybridization probe, and detecting the presence of the hybridization probe. The invention further comprises the presence of an internal control of SEQ ID NO. 10 and two or more additional forward primers (i.e., a second forward primer) and two or more additional reverse primers (i.e., a second reverse primer), wherein at least one additional forward primer hybridizes to a target site corresponding to a first group of nucleotides of the SEQ ID NO. 10 or a complementary strand thereof and at least one additional reverse primer hybridizes to a target site corresponding to second group of nucleotides of SEQ ID NO. 10 or a complementary strand thereof and further comprises addition of a second nucleic acid probe, wherein the second probe hybridizes to a target site corresponding to group of nucleotides of SEQ ID 10 or a complementary strand thereof.
In an illustrative embodiment, the first forward primer hybridizes to a target site corresponding to nucleotides 75 to 150, preferably nucleotides 90 to 130, and most preferably nucleotides 100 to 120 of SEQ ID NO. 1 or a complementary strand thereof. In another illustrative embodiment, the first reverse primer hybridizes to a target site corresponding to nucleotides 155 to 205, preferably nucleotides 165 to 195, and most preferably nucleotides 172 to 192 of SEQ ID NO. 1 or a complementary strand thereof. In another illustrative embodiment, the first probe hybridizes to a target site corresponding to nucleotides 130 to 175, preferably nucleotides 135 to 170, and most preferably nucleotides 141 to 166 of SEQ ID NO. 1 or a complementary strand thereof.
In another illustrative embodiment, the second forward primer hybridizes to a target site corresponding to nucleotides 625 to 675, preferably nucleotides 630 to 665, and most preferably nucleotides 637 to 656 of SEQ ID NO. 10 or a complementary strand thereof. In another illustrative embodiment, the second reverse primer hybridizes to a target site corresponding to nucleotides 740 to 780 preferably nucleotides 745 to 775, and most preferably nucleotides 750 to 768 of SEQ ID NO 10 or a complementary strand thereof. In another illustrative embodiment, the second probe hybridizes to a target site corresponding to nucleotides 690 to 735, preferably nucleotides 695 to 730, and most preferably nucleotides 703 to 724 of SEQ ID NO 10 or a complementary strand thereof.
In a particular embodiment, the invention encompasses a standardized, sensitive CSFV-specific multiplex real-time RT-PCR for the detection of CSFV genomes including a heterologous internal control (“IC”) in a one-tube protocol. According to an embodiment of the invention, both primers (e.g., the forward and reverse primer) and a TaqMan-probe were selected using a 5′-NTR consensus sequence of 78 different CSFV strains that illustrated 100% matching oligonucleotides were chosen, and only a small group of CSFV isolates had primer binding sites with maximal one nucleotide exchange compared to the published CSFV sequences. The CSFV primers and TaqMan-probe of a CSF-System 1 enabled the detection of genome sequences with up to 2 mismatches in the probe region. The “CSF-System 1” consists of the primer pair CSF100-F/CSF192-R, and the FAM-labelled probe CSF-Probe 1 (
The IC is used to avoid false negative results due to RNA degradation or inhibitory effects. (Hofmann, Construction Of An Infectious Chimeric Classical Swine Fever Virus Containing The 5′UTR Of Bovine Viral Diarrhea Virus, And Its Application As A Universal Internal Positive Control In Real-Time RT-PCR; J. Virol. Methods, 2003, 114, 77-90.). In vitro-transcribed EGFP RNA, which was detected using an independent primer/probe system, can be used as the internal control. This IC can be also used in multiplex PCR assays for other pathogens. Alternatively, a housekeeping gene or an internal control which mimics the targeted CSFV sequence can be used (Gorzelnik et al., Validation Of Endogenous Controls For Gene Expression Studies In Human Adipocytes And Preadipocytes. Horm. Metab. Res., 2001, 33, 625-627; Korimbocus et al., Improved Detection Of Sugarcane Yellow Leaf Virus Using A Real-Time Fluorescent (Taqman) RT-PCR Assay. J. Virol. Methods, 2002, 103, 109-120;). However, housekeeping genes have the disadvantage that their concentrations are unknown, especially in cell-free samples like plasma or serum. In contrast to a recently published CSFV multiplex PCR (e.g., Hofmann, 2003), the use of infectious, genetically engineered CSFV is avoided, so the IC system of the present invention can also be used in standard, non bio-safety level 2 laboratories. Using heterologous in vitro-transcribed RNA as IC does not completely simulate RNA extraction from packaged RNAse-protected viral RNA (Drosten et al, 2001. Taqman 5′Nuclease Human Immunodeficiency Virus Type 1 PCR Assay With Phage-Packaged Competitive Internal Control For High-Throughput Blood Donor Screening. J. Clin. Microbiol., 2001, 39, 4302-4308). Neither IC RNA degradation nor release of RNA from virions during the extraction step was crucial for CSFV RNA extraction. Therefore, the present invention offers a universal, non-infectious IC RNA, which was demonstrated to be stable even after 40 freeze/thaw cycles.
The present invention allows the use of a marked low concentration of the IC, a strictly limited amount of IC primers, and an IC amplicon, which is selected to be significantly longer than the CSFV-specific amplicon to enable on one hand, a high sensitivity of CSF System 1 and, on the other hand, to allow a relatively constant amplification of the IC in the multiplex assay. As a consequence, identical analytical sensitivities for the single and the multplex CSF assay can be achieved. Using log2-dilutions of an in vitro-transcribed positive RNA, a mean detection limit of 8 copies per RT-PCR reaction is attained in combination with a wide dynamic range of 9 log10 steps for the single and the multiplex systems. Nevertheless, an inhibition of the IC amplification could be observed using very high amounts of target RNA. This partial or complete inhibition of IC amplification is tolerable, since the IC is used to exclude false negative results, which are irrelevant in the case of high amounts of CSFV target RNA (Hofmann, 2003, see supra). Furthermore, the sensitivity of the CSF-System 1 multiplex RT-PCR is comparable to that of virus isolation, which is regarded as a ‘gold standard’ in CSF diagnosis. Furthermore, the sensitivity of the cell culture system used as a ‘gold standard’ is crucial for the calculation and results can therefore be different for cell culture-adapted strains like the CSFV Alfort 187 in comparison to CSFV field isolates.
The analysis of 36 CSFV isolates of various genotypes with the multiplex version of CSF-System 1 showed similar detection characteristics for all CSFV. Even in the case of the genotype 3.4 CSFV strain “Kanagawa,” no differences concerning the fluorescence signals and threshold cycles were observed. In contrast to our CSF-System 1, a published CSFV specific real-time RT-PCR assay, based on sequences located more downstream within the 5′NTR, generated only shallow fluorescence curves with the “Kanagawa” strain (McGoldrick et al., 1998).
For an efficient control of the CSFV primers and the TaqMan-probe, a plasmid-based, non-infectious positive control is constructed. Dilution series of the in vitro-transcribed positive control RNA are used as a standard to quantify the viral RNA contained in CSFV positive samples. Therefore both controls (IC and PC) in the multiplex CSF-System 1 of the invention are in-vitro-transcribed, non-infectious RNAs.
The present invention further encompasses a two-color multiplex real-time RT-PCR system encompassing: (a) “one tube” RT-PCR reaction, (b) use of a validated and highly efficient RT-PCR-kit, (c) integration of a stable and non-infectious IC, (d) use of a quantifiable, non-infectious PC, and (e) validation of the complete system as a highly sensitive and CSFV-specific two-colour multiplex RT-PCR reaction.
Particularly, the real time RT-PCR method of the present invention allows transiently-infected pigs with no clinical signs of CSFV to be detected earlier, more often, and for a longer time period than with cell culture isolation methods. The standardized PCR system can be used as a robust tool for the highly sensitive and specific detection of CSFV in eradication campaigns or in case of emergencies.
8.2.2.1. Multiplex Analysis
In a preferred embodiment of this invention, a multiplex hybridization assay is performed. Multiplex analysis relies on the ability to sort sample components or the data associated therewith, during or after the assay is completed. In preferred embodiments of the invention, distinct independently detectable moieties are used to label component of two or more different complexes. The ability to differentiate between and/or quantitate each of the independently detectable moieties provides the means to multiplex a hybridization assay because the data which correlates with the hybridization of each of the distinctly (independently) labeled complexes to a target sequence can be correlated with the presence, absence or quantity of each target sequence or target molecule sought to be detected in a sample.
Consequently, the multiplex assays of this invention may be used to simultaneously detect the presence, absence or quantity of two or more target sequence or target molecule in the same sample and in the same assay. Because the complexes are self-indicating, and can be designed to be independently detectable, the multiplex assays of this invention can be performed in a closed tube format to provide data for simultaneous real-time and end-point analysis of a sample for two or more target sequences or target molecules of interest in the same assay. Additionally, the assays can be further multiplexed by the incorporation of unimolecular probes to thereby confirm assay performance or be used to identify a specific feature of a target sequence or target molecule of interest.
8.2.2.2. Multiplex Applications
As illustrated by the examples that follow, the oligonucleotides of the invention are particularly useful for applications involving multiple oligonucleotides sets wherein each oligonucleotide contains at least one independently detectable moiety. Preferably, the independently detectable moieties are independently detectable fluorophores. For example, a mixture of one or more different oligonucleotides may be used to detect each of four different target sequences, wherein one or more oligonucleotides comprises one or more independently detectable fluorophores. For this example, detection of the presence, absence or quantity of the different target sequences is made possible by the detection and/or quantitation of each of the different independently detectable fluorophores after the mixture has been incubated with the sample of interest. As previously discussed, the oligonucleotides may also be used in assays wherein the independently detectable moieties are used to distinguish the operation of the same or different processes occurring in the same assay. Such multiplex assays are possible whether the oligonucleotides are used as probes or as primers.
8.2.3. Probes of the Invention
Another embodiment of the invention encompasses a first probe wherein the first probe hybridizes to a target region of SEQ. ID NO. 1 or a complementary strand thereof, and provides a detectable signal. Yet another embodiment of the invention encompasses a second probe, wherein the second probe hybridizes to a target region of SEQ. ID NO. 10 or a complementary strand thereof, and provides a detectable signal.
In a more particular embodiment, the first probe hybridizes to a target site corresponding to nucleotides 130 to 175 of SEQ. ID. NO. 1 or a complementary strand thereof. In another particular embodiment, the first probe hybridizes to a target site corresponding to nucleotides 135 to 170 of SEQ. ID. NO. 1 or a complementary strand thereof. In yet another particular embodiment, the first probe hybridizes to a target site corresponding to nucleotides 141 to 166 of SEQ. ID. NO. 1 or a complementary strand thereof.
In another particular embodiment, a second probe hybridizes to a target site corresponding to nucleotides 690 to 735 of SEQ ID NO 10 or a complementary strand thereof. In another particular embodiment, the second probe hybridizes to a target site corresponding to nucleotides 695 to 730 of SEQ. ID. NO. 10 or a complementary strand thereof. In yet another particular embodiment, the second probe hybridizes to a target site corresponding to nucleotides 703 to 724 of SEQ. ID. NO. 10 or a complementary strand thereof.
In another particular embodiment of the invention, the probes of the invention are oligonucleotide probes. In a more particular embodiment the probes comprise up to 50 nucleotides, preferably the probe is about 10-30 nucleotides long, and more preferably oligonucleotide probe is about 15-25 nucleotides long. In an even more particular embodiment the probe is of sequence SEQ ID. NO. 3 or sequence SEQ ID. NO. 6. In another particular embodiment, the probe is fluorescently labeled.
8.2.3.1. Labels
The labels attached to the probes of this invention comprise a set of energy or electron transfer moieties comprising at least one donor and at least one acceptor moiety. The label can be any type of differentiating label (e.g., a nucleic acid sequence that is not CSF-specific), a detectable molecule (e.g., a fluorescent group that can be inserted by known methods using, for example, fluorescein isothiocyanate), or digoxigenin, or a molecule that can be immobilized, such as biotin (by means of which the oligonucleotide can be bound to a streptavidin-coated surface, for instance).
Typically, the label will include a single donor moiety and a single acceptor moiety. Nevertheless, a label may contain more than one donor moiety and/or more than one acceptor moiety. For example, a set could comprise three moieties. Moiety one may be a donor fluorophore which, when exited and located in close proximity to moiety two, can then transfer energy to moiety two of the label. Thereafter, moiety two, which when excited and located in close proximity to moiety three, can transfer energy to moiety three of the label. Consequently, energy is transferred between all three moieties. In this set, moiety two is both an acceptor of energy from moiety one and a donor of energy to moiety three.
The donor and acceptor moieties operate such that one or more acceptor moieties accepts energy transferred from the one or more donor moieties or otherwise quench signal from the donor moiety or moieties. Transfer of energy may occur through collision of the closely associated moieties of a label (non-FRET) or through a nonradiative process such as fluorescence resonance energy transfer (FRET). For FRET to occur, transfer of energy between donor and acceptor moieties requires that the moieties be close in space and that the emission spectrum of a donor have substantial overlap with the absorption spectrum of the acceptor (See: Yaron et al. Analytical Biochemistry, 95, 228-235 (1979) and particularly page 232, col. 1 through page 234, col. 1). Alternatively, non-FRET energy transfer may occur between very closely associated donor and acceptor moieties whether or not the emission spectrum of a donor moiety has a substantial overlap with the absorption spectrum of the acceptor (See: Yaron et al. Analytical Biochemistry, 95, 228-235 (1979) and particularly page 229, col. 1 through page 232, col. 1). This process is referred to as intramolecular collision since it is believed that quenching is caused by the direct contact of the donor and acceptor moieties
Preferred donor and acceptor moieties are fluorophore and quencher combinations, respectively. Numerous amine reactive labeling reagents are commercially available (as for example from Molecular Probes, Eugene, Oreg.). Preferred labeling reagents will be supplied as carboxylic acids or as the N-hydroxysuccinidyl esters of carboxylic acids. Preferred fluorochromes (fluorophores) include 5(6)-carboxyfluorescein (Flu), 6-((7-amino-4-methylcoumarin-3-acetyl)amino)hexanoic acid (Cou), 5(and 6)-carboxy-X-rhodamine (Rox), Cyanine 2 (Cy2) Dye, Cyanine 3 (Cy3) Dye, Cyanine 3.5 (Cy3.5) Dye, Cyanine 5 (Cy5) Dye, Cyanine 5.5 (Cy5.5) Dye Cyanine 7 (Cy7) Dye, Cyanine 9 (Cy9) Dye (Cyanine 2, 3, 3.5, 5 and 5.5 are available as NHS esters from Amersham, Arlington Heights, Ill.) or the Alexa dye series (Molecular Probes, Eugene, Oreg.). The most preferred fluorophores are the derivatives of fluorescein and particularly 5 and 6-carboxyfluorescein. The acceptor moiety may be a second fluorophore but preferably the acceptor moiety is a quencher moiety. A quencher moiety is a moiety which can quench detectable signal from a donor moiety such as a fluorophore. Most preferably, the quencher moiety is an aromatic or heteroaromatic moiety which is substituted with one or more azo or nitro groups. The most preferred quencher moiety is 4-((−4-(dimethylamino)phenyl)azo)benzoic acid (dabcyl).
8.2.4. Kits of the Invention
The invention also encompasses kits for detecting CSFV comprising primers according to the invention and separately packaged reagents for performing PCR. Such a kit preferably comprises at least one labelled oligonucleotide, wherein that labelled oligonucleotide detects CSFV in the region amplified by the primers. The oligonucleotide primers of the kit may be SEQ ID NO. 2, SEQ ID NO. 4, SEQ ID NO. 6 and SEQ ID NO. 7. The labelled oligonucleotide probes of the kit may be SEQ ID NO. 3, SEQ ID NO. 6, SEQ ID NO. 8, or SEQ ID NO. 9, wherein SEQ ID NO. 3 and SEQ ID NO. 6 would be labelled as desired.
Preferred kits of this invention comprise all the reagents to perform a PCR reaction wherein each of the labeled probes of the kit are used to monitor a sample for the presence, absence or quantity of CSFV. In preferred embodiments, one or more of the oligonucleotides of the kit perform as the primers in the PCR reaction.
A typical kit will contain at least two primers, at least one probe, nucleotide triphosphates, polymerase enzyme (preferably thermostable polymerase) and a buffer solution (with controlled ionic strength, controlled magnesium content and pH modulator).
Viruses used in this study (9×BDV, 22× BVDV 1, 19× BVDV 2, 36× CSFV, atypical Pestivirus Giraffe (Avalos et al., 2001), atypical pestivirus D32/00 “HoBi”[Schirrmeier et al., 2004]) are listed within the virus bank of the National Reference Laboratory for CSF located at the Friedrich-Loeffler-Institut, Insel Riems (Germany). Some of the isolates were obtained from the Community Reference Laboratory for CSF (TiHo Hannover, Germany). All CSFV strains were cultured using porcine kidney cells (PK-15), whereas the BDV and BVDV were propagated using bovine kidney cells (MDBK), or sheep thymus cells (SFT-R). The cell lines were provided by the Collection of Cell Lines in Veterinary medicine (CCLV), Insel Riems (Germany).
Cell cultures were inoculated with different dilutions of the virus strains, and incubated at 37° C. After 4 days, the monolayers of the cells were heat-fixed, and stained with the Pestivirus specific monoclonal antibody C16 (Peters et al., 1986). All virus isolations were performed in duplicate. CSFV titers were determined using log10 dilution steps in quadruplicate. Virus titers were calculated as tissue culture infectious dosage 50% (TCID50) per ml.
Viral RNA was extracted from cell culture using the QIAamp viral RNA kit (Qiagen) according to the manufacturers instructions, modified by addition of the internal control (IC) RNA. Briefly, 140 μl cell culture supernatant was added to 560 μl lysis buffer, vortexed and incubated at room temperature for 5 min. Subsequently, 5 μl in vitro transcribed IC RNA (2×105 copies/μl) was added. After 5 min, 560 μl of ethanol was added and the solution was centrifuged through a QIAamp spin column. After washing the column twice with the appropriate buffer, the RNA was eluted using 50 μl elution buffer, and stored at −20° C.
The Alignment of the different CSFV sequences was performed using the Genetics Computer Group software package (GCG Wisconsin). Alignment-based primer and probe selection was supported by the software package Beacon Designer 2.06 (PremierBiosoft). All oligonucleotides were synthesized by the MWG Biotech AG (Ebersberg, Germany) and stored at −20° C. until use. Table 1 shows the primers and probes used for the real-time RT-PCR assays of this report.
In order to minimize the risk of cross contaminations, a one-step RT-PCR protocol was chosen using the commercially available QuantiTect™ Probe RT-PCR kit (Qiagen). The real-time RT-PCR assay was optimised using a total volume of 25 μl. Briefly, for a single well 3.25 μl RNase-free water, 12.51 μl 2×QuantiTect Probe RT-PCR Master Mix, 0.25 μl QuantiTect Probe RT Mix, 2 μl CSF-specific primer-probe-mix (0.6 μM CSF-specific primers +0.1 μM CSF-specific probes) and 2 μl IC-specific primer-probe-mix (0.2 μM EGFP-specific primers +0.1 μM EGFP1-HEX probe) were pooled as a master mix, and finally 5 μl RNA template was added.
The real-time RT-PCR was carried out in an iCycler IQ™ Real-Time Detection System (BioRad) using the following temperature profile: 30 min at 50° C. (reverse transcription), 15 min at 95° C. (inactivation reverse transcriptase/activation Taq polymerase), followed by 42 cycles of 15 sec at 94° C. (denaturation), 30 sec at 57° C. (annealing) and 30 sec at 68° C. (elongation). The identical temperature profile was used for all real-time RT-PCR runs and fluorescence values were collected during the annealing step.
Linearised and gel purified plasmid DNAs were in vitro transcribed using the Riboprobe® System-SP6/T7 (Promega) according to the manufacturers instructions. The T7 transcribed positive control and the SP6 transcribed internal control were digested with the supplied DNase and purified using the RNeasy kit (Qiagen). The correct length of the transcribed RNAs was confirmed by formaldehyde agarose gel electrophoreses, and the concentration was determined by spectrophotometry. The exact number of RNA molecules was calculated using the formula: (X g/μl RNA/[transcript length in nucleotides×340])×6.022×1023=Y molecules/μl.
Usually 1012 to 1013 RNA molecules were obtained in a 50 μl in vitro transcription procedure, enough for several thousand RT-PCR. The stock solutions of the in vitro-transcribed RNA was stored at −70° C., and the diluted working solutions were stored at −20° C.
In a first step, 78 sequences of the 5′NTR of different CSFV strains were aligned and a consensus sequence was calculated (
Both primers and the probe of “CSF-System 1” were titrated in chequerboard assays to identify the most suitable concentrations, which were defined as the maximal fluorescence values combined with the earliest threshold cycle. For the “CSF-System 1” 0.6 μM/reaction of the primers and 0.1 μM/reaction of the probe were identified as an optimum for the detection of CSFV genomes.
Using the primers CSF100-F and CSF383-R (Table 1), a 284 bp fragment of the 5′-NTR of CSFV Alfort/187 (Accession No. X87939) was amplified by RT-PCR, purified and inserted into the standard cloning vector pGEM-Teasy (Promega). The obtained plasmid pGEM-PC3alf (
The analytical sensitivity of “CSF-System 1” was determined with a dilution series of the in vitro-transcribed PC-RNA. In a first 10-fold dilution series of the in vitro-transcribed PC-RNA it was ascertained that the “CSF-System 1” amplified the PC in a linear manner from 101 copies/well up to 109 copies/well with a PCR Efficiency of 99.8% (
For the internal control of each sample or sample pool concerning both RNA isolation and RT-PCR, a heterologous IC-RNA real-time RT-PCR assay was designed. The in vitro transcribed IC could be added to each sample either before RNA isolation or before RT-PCR, and was amplified with a EGFP-specific real-time RT-PCR system, which was named “IC-System 1.” The 132 bp amplicon of primer pair EGFP 1-F/EGFP 2-R was detected using the HEX-labelled probe EGFP 1-HEX (Table 1). In chequerboard assays the concentrations for both primers and the probe were titrated and the analytical sensitivity of the “IC-System 1” was analysed using dilution series of the in vitro-transcribed IC-RNA. Finally, a sensitivity limit of the IC assay between 10 and 100 copies/well was determined (data not shown).
For a maximal sensitivity of the CSFV-specific real-time RT-PCR system it was necessary to limit the primers of “IC-System 1”. The minimal concentration of primers which amplified the EGFP fragment without a significant increase of the threshold cycle were determined for the fine-tuning of multiplex real-time RT-PCR assays. Finally, 0.2 μM/reaction of the EGFP forward and reverse primers and 0.1 μM/reaction of the probe were used for the IC detection in multiplex assays.
After determination of the primer and probe concentrations in an optimal single CSF-specific real-time RT-PCR assay, and identification of limited concentrations for the IC-specific primers, an optimised multiplex CSF-specific real-time RT-PCR protocol was established. Therefore, the analytical sensitivities of the single and the multiplex real-time RT-PCR assays were compared. Firstly, batches of a 2 fold dilution series of the PC-RNA were amplified with or without co-amplification of the IC-RNA. Starting with 1000 copies PC-RNA per RT-PCR well, an identical sensitivity limit of 8 copies per RT-PCR reaction for the single as well as for the multiplex assay could be detected (Table 4). Secondly, dilution series of extracted CSFV-RNA were amplified with or without co-amplification of the IC. In these experiments, similar results were generated for both systems. As an example, fluorescence signals following amplification of a dilution series of RNA of CSFV strain “Paderborn” (genotype 2.1.) are shown in
For a comparative study of “CSF-System 1” with the “gold standard” virus isolation, 10 fold dilution series of CSFV positive cell culture supernatants were prepared (Table 2). One aliquot of each dilution series was used for virus isolation, a second aliquot was used for RNA isolation and multiplex real-time RT-PCR analysis. Several strains of different CSFV genotypes were included in the investigation, and the results clearly demonstrated the high sensitivity of the multiplex real-time RT-PCR assay. In most cases, the sensitivity of the real-time RT-PCR assay was at least identical (CSFV-Uelzen, genotype 2.3) or even higher (CSFV-Kozlov, genotype 1.2; CSFV-Paderborn, genotype 2.1) than virus isolation (Table 3). However, in the case of the cell culture adapted strain Alfort 187 (genotype 1.1), sensitivity of the real-time RT-PCR was approximately 10-fold reduced in comparison to the virus isolation (Table 3).
In order to estimate the ability of the multiplex assay to distinguish CSFV from BVDV, BDV, or atypical pestiviruses a panel of 90 different pestivirus strains was tested (Table 3). All 36 different CSFV strains, members of all known CSFV genotypes, were correctly detected (Table 3), and furthermore, a scarped curve progression of the fluorescence signals was observed in all cases (data not shown). Even in the case of the CSFV strain “Kanagawa”, a genotype 3.4 CSFV, no marked differences of the TC values and fluorescence signals were observed (data not shown). The specificity of the real-time RT-PCR assay was also proven with serum samples (n=100) from a defined negative pig population. RNA was isolated from the samples and tested in the multiplex CSFV-specific real-time RT-PCR. All samples scored negative and only background fluorescence levels were observe).
The present invention is not to be limited in scope by the specific embodiments disclosed in the examples which are intended as illustrations of a few aspects of the invention and any embodiments that are functionally equivalent are within the scope of this invention. Indeed, various modifications of the invention in addition to those shown and described herein will become apparent to those skilled in the art and are intended to fall within the scope of the appended claims.
All cited patents, patent applications and publications referred to in this application are herein incorporated by reference in their entirety.
(a) Genome position according to CSFV Alfort/187 (accession number: X87939) and standard cloning vector pEGFP-1 (BD Bioscience Clontech, accession number: U55761), respectively.
n.d. = not dedicated
