Patent Application
20230127238
Testing for viral infections in many cases requires RNA-based positive controls for testing of instrument suitability, proficiency testing and external quality assurance systems. In a number of cases, the actual virus is used a control which creates numerous occupational health and safety issues and unnecessary exposure of laboratory staff to dangerous human pathogens. Most of these viral controls require specialist shipping and handling procedures and need highly skilled staff and infrastructure, which limits their use. Proficiency testing controls are central to the deployment of any successful diagnostic test and indeed the lack of these remains a significant hurdle to the use of new testing kits as they become available.
The newly emergent severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has resulted in a global pandemic prompting radical measures to contain the spread of the virus. Coronavirus disease 2019 (COVID-19) is the infectious disease caused by SARS-CoV-2. The disease was first identified in December 2019 in Wuhan, China, and has since spread globally, resulting in a global pandemic. Common symptoms of COVID-19 include fever, cough and shortness of breath. Other symptoms may include fatigue, muscle pain, diarrhea, sore throat, loss of smell and abdominal pain. The time from exposure to the onset of symptoms is typically five days but may range from two to fourteen days. The majority of cases result in relatively mild symptoms, however some patients progress to viral pneumonia and multi-organ failure.
The rapid spread of SARS-CoV-2 has created a global pandemic, resulting in a grim forecast for the global economy and public health programs of even the most well-resourced nations. Whilst most countries continue to battle against raging local epidemics, with the precise formula for rapidly achieving control remaining elusive, it has been demonstrated that smart, aggressive public health interventions can reduce the rate of new infections. Countries such as China and South Korea have been able to contain the spread of new infections through rapidly deploying testing for Covid-19 infection and disease. When these diagnostics were coupled with effective public health interventions such as isolation and quarantine, a significant flattening of the curve was achieved. Diagnostic testing can adopt one of two modalities, either directed at detecting the nucleic acid (or genetic content) of the virus, a method commonly referred to as RT-PCR, or through serological testing aimed at detecting antibodies against certain viral proteins. RT-PCR tests have the capacity for detecting the virus early in the infection, in individuals who are asymptomatic, and allows for reduction in spread of new infections. A major challenge in containing the spread of the virus has been identifying asymptomatic infections which are reported to be major drivers of the pandemic. As such the demand for RT-PCR tests has increased exponentially all over the world. However, as these become available the need for proficiency testing tools also becomes an urgent priority. Proficiency testing controls allow for an assessment of whether a test is fit for purpose and provides valuable information on the ability of laboratories to carry out testing in a quality assured manner. These controls usually encompass both positive and negative controls. In many cases the disease-causing agent is used as the positive control, for example the single stranded RNA virus. Using the live virus as a positive control for evaluating test performance in this case is not desirable as the corona virus has a rapid, explosive infection force and it's use in this case creates notable public health concerns.
The inventors have thus employed a biomimicry approach to engineer a positive control organism that mimics the genetic material of the SARS-CoV-2 virus and other single stranded RNA viruses, including human immunodeficiency virus (HIV) and hepatitis C virus (HCV), that is targeted by several RT-PCR based diagnostic kits to create a safe, non-infectious and stable positive control that can be rapidly deployed in any setting. The approach uses a Mycobacterium smegmatis cell, as an encapsulating casing, which contains a plasmid DNA molecule that has the viral diagnostic targets that mimic the diagnostic profile of the clinical pathogen. Because the genetic material that forms the basis of the molecular assay is RNA, the plasmid DNA further includes elements which allow for RNA expression, stabilisation of the RNA and degradation of the remaining DNA, thereby ensuring that the positive control is an effective control that mimics the diagnostic profile of the virus.
The present invention relates to a nucleotide cassette comprising a nucleotide sequence corresponding to at least one target nucleic acid from a single stranded RNA diagnostic target. This nucleotide sequence contains an inducible promoter, a nucleotide sequence that encodes artemin, a molecular switch and a nucleotide sequence that encodes a DNAse enzyme and is under control of the molecular switch, wherein the single stranded RNA diagnostic target is converted to an RNA sequence target that is detected by a molecular diagnostic assay. The invention also relates to an RNA expression product of the nucleotide cassette, a vector comprising the nucleotide cassette, and to a cell carrying the nucleotide cassette, the RNA expression product or the vector. In addition, this invention relates to a diagnostic control composition comprising a non-pathogenic recombinant bacterium having a modified genetic content comprising the nucleotide cassette, as well as to methods of producing these recombinant bacteria. The invention also relates to a kit comprising the cell, diagnostic control composition or recombinant bacterium produced according to the method.
According to a first aspect of the present invention there is provided for a nucleotide cassette having the formula:
X1-X2-X3-X4-X5
wherein, X1 is an inducible promoter; X2 is a nucleotide sequence corresponding to at least one single stranded RNA diagnostic target; X3 is a nucleotide sequence that encodes artemin; X4 is a molecular switch; and X5 is a nucleotide sequence that encodes a DNAse enzyme and is under control of the molecular switch, wherein the single stranded RNA diagnostic target is a sequence detected by a molecular diagnostic assay. In one embodiment, the single stranded RNA diagnostic target is a single stranded RNA virus target, such as a SARS-CoV-2 diagnostic target detected by a molecular diagnostic assay that detects SARS-CoV-2.
In a first embodiment of the nucleotide cassette of the invention, the SARS-CoV-2 target may be selected from the group consisting of genes encoding RdRP1, RdRP2, N protein, E protein, Spike protein, ORF1ab, and combinations thereof. It will be appreciated by those of skill in the art that other regions of the SARS-CoV-2 virus may be contemplated if these are detected in molecular diagnostic tests. For example, the SARS-CoV-2 target may be any sequence in the SARS-CoV-2 genome, which is represented by SEQ ID NOs:18-28. In an alternative embodiment, the target may be one or more other single stranded RNA diagnostic targets that are detected by a molecular diagnostic assay. Non-limiting examples of such targets include single stranded virus targets detected by a diagnostic assay for HCV, HIV, respiratory syncytial virus, influenza virus and Ebola virus. It will be appreciated by those of skill in the art that any single stranded RNA diagnostic target of a molecular diagnostic assay could be used, including the BRCA-ABL human ss-mRNA.
In a second embodiment of the nucleotide cassette of the invention, X2 is selected from the group consisting 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, or a sequence substantially identical or complementary to any of SEQ ID NOs:2-7, and combinations thereof.
According to a third embodiment of the nucleotide cassette of the invention the inducible promoter is a heat inducible promoter, preferably Hsp60.
In a third embodiment of the nucleotide cassette of the invention the molecular switch may be a theophylline riboswitch that is activated via introduction and binding of theophylline, and which induces the production of DNAse enzyme.
In a further embodiment of the invention the nucleotide cassette comprises the sequence of SEQ ID NO:14 or SEQ ID NO:15, or a sequence substantially identical or complementary thereto.
According to a second aspect of the invention there is provided for an RNA expression product produced by the nucleotide cassette of the first aspect.
According to a third aspect of the present invention there is provided for a vector comprising the nucleotide cassette of the first aspect.
According to a fourth aspect of the present invention there is provided for a cell comprising the nucleotide cassette of the first aspect, the RNA expression product of the second aspect, or the vector of the third aspect.
In a second embodiment of the cell of the invention, the cell is a recombinant Mycobacterium smegmatis cell.
According to a fifth aspect of the present invention there is provided for a diagnostic control composition comprising a non-pathogenic recombinant bacterium having a modified genetic content comprising a nucleotide cassette having the formula:
X1-X2-X3-X4-X5
wherein, X1 is an inducible promoter; X2 is a nucleotide sequence corresponding to at least one single stranded RNA diagnostic target; X3 is a nucleotide sequence that encodes artemin; X4 is a molecular switch; and X5 is a nucleotide sequence that encodes a DNAse enzyme and is under control of the molecular switch, wherein the single stranded RNA diagnostic target is a sequence detected by a molecular diagnostic assay and wherein the diagnostic control composition mimics the diagnostic profile of the single stranded RNA diagnostic target. In one embodiment, the single stranded RNA diagnostic target is a single stranded RNA virus target, such as a SARS-CoV-2 diagnostic target detected by a molecular diagnostic assay that detects SARS-CoV-2.
According to a first embodiment of the diagnostic control composition of the present invention the SARS-CoV-2 target may be selected from the group consisting of genes encoding RdRP1, RdRP2, N protein, E protein, Spike protein, ORF1ab, and combinations thereof. It will be appreciated by those of skill in the art that other regions of the SARS-CoV-2 virus may be contemplated by the present invention if these are to be detected in molecular diagnostic tests. For example, the SARS-CoV-2 target may be any sequence in the SARS-CoV-2 genome, which is represented by SEQ ID NOs:18-28. In an alternative embodiment, the target may be one or more other single stranded RNA diagnostic targets that are detected by a molecular diagnostic assay. Non-limiting examples of such targets include single stranded virus targets detected by a diagnostic assay for HCV, HIV, respiratory syncytial virus, influenza virus and Ebola virus. It will be appreciated by those of skill in the art that any single stranded RNA diagnostic target of a molecular diagnostic assay could be used, including the BRCA-ABL human ss-mRNA.
In a second embodiment of the diagnostic control composition, X2 is selected from the group consisting 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, or a sequence substantially identical or complementary to any of SEQ ID NOs:2-7, and combinations thereof.
According to a third embodiment of the diagnostic control composition of the invention the inducible promoter is a heat inducible promoter, preferably Hsp60.
In another embodiment of the diagnostic control composition, the molecular switch may be a theophylline riboswitch that is activated via introduction and binding of theophylline, and which induces the production of DNAse enzyme.
In a further embodiment of the diagnostic control composition of the invention the nucleotide cassette may comprise the sequence of SEQ ID NO:14 or SEQ ID NO:15, or a sequence substantially identical or complementary thereto.
In yet another embodiment of the diagnostic control composition of the present invention the non-pathogenic recombinant bacterium may be Mycobacterium smegmatis.
According to a further aspect of the present invention there is provided for a method of producing a recombinant bacterium that mimics the diagnostic profile of a single stranded RNA of interest in a molecular diagnostic assay, the method comprising:
In one embodiment of the method of the invention the non-pathogenic bacterium is Mycobacterium smegmatis.
In a further embodiment of the method of the invention the recombinant bacterium mimics the diagnostic profile of SARS-CoV-2. In an alternative embodiment, the recombinant bacterium mimics one or more other single stranded RNA viruses that are detected by a molecular diagnostic assay. IN yet a further, embodiment, the single stranded RNA of interest may be BRCA-ABL human ss-mRNA.
In another embodiment of the method of the present invention the selection marker may be an antibiotic selection marker.
In yet a further embodiment of the method of the invention the recombinant bacterium may be either stably or transiently transformed with the vector.
According to yet another aspect of the present invention there is provided for a kit comprising a cell of the invention, a diagnostic control composition of the invention, or the recombinant bacterium produced according to the method of the invention, and instructions for use.
Non-limiting embodiments of the invention will now be described by way of example only and with reference to the following figures:
The nucleic acid sequences listed in the accompanying sequence listing are shown using standard letter abbreviations for nucleotide bases. It will be understood by those of skill in the art that only one strand of each nucleic acid sequence is shown, but that the complementary strand is included by any reference to the displayed strand. In the accompanying sequence listing:
SEQ ID NO:1—Nucleotide sequence of the Hsp60 promoter
SEQ ID NO:2—Nucleotide sequence of SARS CoV-2 target RdRp1
SEQ ID NO:3—Nucleotide sequence of SARS CoV-2 target RdRp2
SEQ ID NO:4—Nucleotide sequence of SARS CoV-2 target N protein
SEQ ID NO:5—Nucleotide sequence of SARS CoV-2 target E protein
SEQ ID NO:6—Nucleotide sequence of SARS CoV-2 target Spike protein
SEQ ID NO:7—Nucleotide sequence of SARS CoV-2 target ORF1ab 5′ region
SEQ ID NO:8—Nucleotide sequence of artemin
SEQ ID NO:9—Nucleotide sequence of DNAse
SEQ ID NO:10—Nucleotide sequence of plasmid backbone
SEQ ID NO:11—Nucleotide sequence of kanamycin resistance cassette
SEQ ID NO:12—Nucleotide sequence of mycobacterial origin of replication
SEQ ID NO:13—Nucleotide sequence of theophylline riboswitch
SEQ ID NO:14—Nucleotide sequence of the expression cassette for targets RdRp1, RdRp2, N protein and E protein
SEQ ID NO:15—Nucleotide sequence of the expression cassette for targets Spike protein, ORF1ab 5′ region, RdRp1, RdRp2, N protein and E protein
SEQ ID NO:16—Nucleotide sequence of RNA expression product for targets RdRp1, RdRp2, N protein and E protein
SEQ ID NO:17—Nucleotide sequence of RNA expression product for targets Spike protein, ORF1ab 5′ region, RdRp1, RdRp2, N protein and E protein
SEQ ID NO:18—Nucleotide Sequence of fragment 1 of SARS-CoV-2 genome
SEQ ID NO:19—Nucleotide Sequence of fragment 2 of SARS-CoV-2 genome
SEQ ID NO:20—Nucleotide Sequence of fragment 3 of SARS-CoV-2 genome
SEQ ID NO:21—Nucleotide Sequence of fragment 4 of SARS-CoV-2 genome
SEQ ID NO:22—Nucleotide Sequence of fragment 5 of SARS-CoV-2 genome
SEQ ID NO:23—Nucleotide Sequence of fragment 6 of SARS-CoV-2 genome
SEQ ID NO:24—Nucleotide Sequence of fragment 7 of SARS-CoV-2 genome
SEQ ID NO:25—Nucleotide Sequence of fragment 8 of SARS-CoV-2 genome
SEQ ID NO:26—Nucleotide Sequence of fragment 9 of SARS-CoV-2 genome
SEQ ID NO:27—Nucleotide Sequence of fragment 10 of SARS-CoV-2 genome
SEQ ID NO:28—Nucleotide Sequence of fragment 11 of SARS-CoV-2 genome.
The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all embodiments of the invention are shown.
The invention as described should not be limited to the specific embodiments disclosed and modifications and other embodiments are intended to be included within the scope of the invention. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.
As used throughout this specification and in the claims which follow, the singular forms “a”, “an” and “the” include the plural form, unless the context clearly indicates otherwise.
The terminology and phraseology used herein is for the purpose of description and should not be regarded as limiting. The use of the terms “comprising”, “containing”, “having” and “including” and variations thereof used herein, are meant to encompass the items listed thereafter and equivalents thereof as well as additional items.
The invention relates to a recombinant bacterium that mimics the diagnostic profile of one or more single stranded RNA diagnostic targets of interest that are detected by a molecular diagnostic assay, in particular a single stranded RNA virus, including SARS-CoV-2. The recombinant bacterium of the invention is based on a non-pathogenic bacterium that has a modified genetic content containing a target nucleic acid from a single stranded RNA diagnostic target, such as an ssRNA virus like SARS-CoV-2, wherein the target nucleic acid is detected by a molecular diagnostic assay. The approach of the inventors is to first culture the biomimetic Mycobacterium smegmatum comprising the DNA cassette to increase the biomass, then transfer to a high temperature to increase RNA expression of the single stranded RNA targets, and the artemin and DNAse genes. Once the artemin is expressed, it binds and stabilizes the RNA of the targets. Thereafter, theophylline is added to induce production of the DNAse, and the temperature is raised to activate the DNase. Once the DNAse is expressed it degrades the DNA, leaving only RNA in an encapsulated bacterial cell. This serves as a safe, non-infectious control organism for proficiency testing of molecular diagnostic assays for the single stranded RNA diagnostic targets, for example SARS-CoV-2 diagnostics. The process is outlined in
The recombinant bacterium of the present invention may be used as both an internal and external control for single stranded RNA diagnostic assays. Specifically, the recombinant Mycobacterium smegmatis of the present invention is used as an internal control for measuring efficacy of RNA extraction and to detect the presence of inhibitors of PCR, however, it is most importantly a positive control for the binding of the specific probes employed in the diagnostic assays, i.e. whether the test is fit for purpose, as well as of the handling of the sample by laboratory staff. In this way, the present invention represents an external control used to determine quality assurance of the testing program. Further, according to the present invention, the clinical sample is not spiked with the recombinant bacterium of the invention, rather the two samples are tested side-by-side.
The controls described herein focus on those required for test validation and proficiency testing and include validation controls, proficiency testing controls and external quality assessment (EQA) controls. Validation controls are required for two main reasons (i) when a test is conducted for the first time in a reference laboratory or a network of reference laboratories, positive and negative controls are required to determine if the test is fit-for-purpose and produces the correct result and (ii) as new testing kits emerge in the market, their utility can only be assessed by using controls that best mimic clinical specimens. Proficiency testing controls are useful as Covid-19 testing is rolled out to different reference labs, it becomes important to assess the proficiency of the lab testing system, including staff and infrastructure, in executing test procedures. For this, controlled specimens, where the expected outcome of the test result is known, are necessary. Finally, EQA controls are required in order to determine if laboratories are handling specimens correctly and executing test procedures appropriately. Again, it is important to develop controls that mimic clinical specimens.
In the case of viruses, a clinical specimen for the molecular diagnostic assay would comprise intact virus, for example SARS-CoV-2. As this poses numerous safety concerns, laboratory products that best mimic these specimens are more appropriate. For genetic testing, the viral RNA (inactivate) can serve as an appropriate control. Alternatively, plasmid DNA (naked or encapsulated) can serve as controls. In the case of serological tests, live viral particles, or suspensions that most closely mimic this are required.
The term “recombinant” means that something has been recombined. When used with reference to a nucleic acid construct, the term refers to a molecule that comprises nucleic acid sequences that are joined together or produced by means of molecular biological techniques. Recombinant nucleic acid constructs may include a nucleotide sequence which is ligated to, or is manipulated to become ligated to, a nucleic acid sequence to which it is not ligated in nature, or to which it is ligated at a different location in nature. Accordingly, a recombinant nucleic acid construct indicates that the nucleic acid molecule has been manipulated using genetic engineering, i.e. by human intervention. Recombinant nucleic acid constructs may be introduced into a host cell by transformation and preferably by transformation with a vector.
The term “vector” refers to a means by which polynucleotides or gene sequences can be introduced into a cell. There are various types of vectors known in the art including plasmids, viruses, bacteriophages and cosmids. Generally, polynucleotides or gene sequences are introduced into a vector by means of recombinant DNA technology.
The recombinant bacterium of the invention is preferably a recombinant Mycobacterium smegmatis bacterium. Mycobacterium smegmatis is preferred as this bacterium is non-pathogenic and does not require high levels of biosafety for its propagation. It will be appreciated that the recombinant bacterium of the invention preferably has a modified genetic content containing a target nucleic acid which is detected by a diagnostic assay. However, due to the incorporation of the target nucleic acid sequence of the single stranded RNA diagnostic target that is detected by the diagnostic assay, the recombinant Mycobacterium smegmatis mimics the diagnostic profile of the single stranded RNA diagnostic target, for example a virus, including SARS-CoV-2 pathogen.
As used herein the terms “nucleic acid”, “nucleic acid molecule” or “polynucleotide” encompass both ribonucleotides (RNA) and deoxyribonucleotides (DNA), including cDNA, genomic DNA, and synthetic DNA. A nucleic acid may be double-stranded or single-stranded. Where the nucleic acid is single-stranded, the nucleic acid may be the sense strand or the antisense strand. A nucleic acid molecule may be any chain of two or more covalently bonded nucleotides, including naturally occurring or non-naturally occurring nucleotides, or nucleotide analogs or derivatives. The term “DNA” refers to a sequence of two or more covalently bonded, naturally occurring or modified deoxyribonucleotides.
The recombinant Mycobacterium smegmatis as described herein may be used as a control for the detection of in a diagnostic assay for a single stranded RNA virus, such as SARS-CoV-2. Further, the recombinant bacterium may be used as a control for the detection of other RNA targets in a molecular diagnostic assay based on the detection using probes or amplification of single stranded RNA, such as the BRCA-ABL human ss-mRNA
Particularly, the recombinant Mycobacterium smegmatis may be used for the calibration of diagnostic devices used to diagnose a single stranded RNA virus, such as SARS-CoV-2, in a subject and for active surveillance monitoring of single stranded RNA viral infections, including SARS-CoV-2 infections. For example, the recombinant Mycobacterium smegmatis may be used as a control in the GeneXpert® system, Thermo Fisher, Cobas SARS-CoV-2, Abbot Realtime SARS-CoV-2, BGI, CDC, SeeGene, Tib Mol Bio and Ultragene ABL assays.
In one embodiment of the invention the recombinant Mycobacterium smegmatis is used for the purposes of external quality assessment (EQA) of the GeneXpert® modular cartridge system. The GeneXpert® system relies on the use of complementary nucleic acid probes to detect the presence or absence of the target single stranded RNA, for example a SARS-CoV-2 nucleic acid or even the BRCA-ABL human ss-mRNA. The present invention includes the same target nucleic acids as the GeneXpert® system using the same complementary nucleic acid probes.
As used herein, the term “mimics the diagnostic profile” means that the recombinant bacterium has the same diagnostic profile as the clinical sample containing the single stranded RNA, indistinguishably, and with the same specificity in an assay, when using the same detection probes or primers for both the clinical sample and the recombinant bacterium. This diagnostic profile that is mimicked may be, for example, a GeneXpert® assay readout.
As used herein the term “single stranded RNA diagnostic target” refers to a target region of single stranded RNA that is detected by a molecular diagnostic assay, in particular a nucleic acid assay that directly detects the presence of the RNA using either a probe or primer that specifically binds the region of single stranded RNA. Preferably the assay is an RT-PCR assay.
The term “complementary” refers to two nucleic acids molecules, e.g., DNA or RNA, which are capable of forming Watson-Crick base pairs to produce a region of double-strandedness between the two nucleic acid molecules. It will be appreciated by those of skill in the art that each nucleotide in a nucleic acid molecule need not form a matched Watson-Crick base pair with a nucleotide in an opposing complementary strand to form a duplex. One nucleic acid molecule is thus “complementary” to a second nucleic acid molecule if it hybridizes, under conditions of high stringency, with the second nucleic acid molecule. A nucleic acid molecule according to the invention includes both complementary molecules.
As used herein a “substantially identical” sequence is an amino acid or nucleotide sequence that differs from a reference sequence only by one or more conservative substitutions, or by one or more non-conservative substitutions, deletions, or insertions located at positions of the sequence that do not destroy or substantially reduce the binding of one or more of the probes for the targets in the molecular diagnostic assay. Alignment for purposes of determining percent sequence identity can be achieved in various ways that are within the knowledge of those with skill in the art. These include using, for instance, computer software such as ALIGN, Megalign (DNASTAR), CLUSTALW or BLAST software. Those skilled in the art can readily determine appropriate parameters for measuring alignment, including any algorithms needed to achieve maximal alignment over the full length of the sequences being compared. In one embodiment of the invention there is provided for a polynucleotide sequence that has at least about 80% sequence identity, at least about 90% sequence identity, or even greater sequence identity, such as about 95%, about 96%, about 97%, about 98% or about 99% sequence identity to the sequences described herein.
Alternatively, or additionally, two nucleic acid sequences may be “substantially identical” if they hybridize under high stringency conditions. The “stringency” of a hybridisation reaction is readily determinable by one of ordinary skill in the art, and generally is an empirical calculation which depends upon probe length, washing temperature, and salt concentration. In general, longer probes required higher temperatures for proper annealing, while shorter probes require lower temperatures. Hybridisation generally depends on the ability of denatured DNA to re-anneal when complementary strands are present in an environment below their melting temperature. A typical example of such “stringent” hybridisation conditions would be hybridisation carried out for 18 hours at 65° C. with gentle shaking, a first wash for 12 min at 65° C. in Wash Buffer A (0.5% SDS; 2×SSC), and a second wash for 10 min at 65° C. in Wash Buffer B (0.1% SDS; 0.5% SSC).
Herein, the inventors have generated strains of a non-pathogenic Mycobacterium smegmatis comprising a plasmid that includes target nucleic acids of diagnostic RT-PCR based tests inserted into the plasmid.
The following examples are offered by way of illustration and not by way of limitation.
Bacterial Strains and Culture Conditions
All cloning was performed in Escherichia coli strain DH5a. Experiments were performed in Mycobacterium smegmatis strain mc2155. Nucleic acid sequence inserts used in the constructs are listed in Table 1. The plasmid backbone and the kanamycin resistance cassette were purchased as synthetic DNA fragments from GenScript™.
E. coli strains were grown at 37° C. in standard Luria Bertani (LB) or 2YT liquid medium or on solid medium (LA) supplemented with 50 μg/ml kanamycin (kan). Mycobacterium smegmatis strains were grown at 37° C. shaking in Middlebrook 7H9 liquid medium (Difco) supplemented with 0.085% NaCl, 0.2% glucose, 0.2% glycerol and 0.05% Tween80, or on Middlebrook 7H10 solid medium (Difco) supplemented with 0.085% NaCl, 0.2% glucose and 0.5% glycerol. Kanamycin was used at a concentration of 50 μg/mL kanamycin.
Cloning of Shuttle Plasmids for Integration into Mycobacterium smegmatis
The following elements were cloned and prepared in E. coli to obtain the plasmid shown in
The resulting plasmid was electroporated into M. smegmatis mc2155 by standard laboratory methods. Briefly, electrocompetent mc2155 were prepared as follows: Cells were grown to log phase (OD600 0.5-0.9) and harvested by centrifugation (3 500 rpm, 10 min, 4° C.). The pelleted cells were then washed three times by gentle resuspension in 10 ml ice-cold 10% glycerol and cells pelleted by centrifugation between washes (3 500 rpm, 10 min, 4° C.). The cells were resuspended in an appropriate volume of ice-cold 10% glycerol and used immediately. For transformation, 400 μl of electro-competent cells were transferred to pre-chilled 0.2 cm electroporation cuvettes (Bio-Rad), together with plasmid DNA. The Gene PulserX cell (Bio-Rad) was used for electroporation set at 2.5 kV, 25 μF and 1000Ω. Cells were immediately rescued with 1 ml of 2×TY for 3 hours or overnight at 37° C. with shaking at 100 rpm. The rescued cells were plated on Middlebrooks Medium 7H9 supplemented with glucose, NaCl and kanamycin (25 μg/mL) for maintenance of the episomal shuttle plasmid.
Cultures were grown overnight at 37° C. and once sufficiently dense (OD600>2), the culture was transferred to 42° C. to induce expression of the SARS-CoV-2 targets, artemin and DNAse cassette (SEQ ID NO:16) and grown shaking for an additional 30 minutes. Theophylline was then added at a concentration of 2 mM for 2 hours at 37° C. The temperature was then raised to 60° C. for 30 min to induce the DNAse. Cultures were split into 22.5 ml aliquots, in 50 ml conical bottom centrifuge, and incubated at 80° C. for 70 minutes.
The cells were harvested by centrifugation at 3080×g for 10 minutes. Thereafter, the pellets were resuspended in 15 ml of 1×PBS solution with 0.05% Tween-80 and 32 ml SR buffer supplied by Cepheid® with GeneXpert® MTB/Ultra. Cells were incubated for 60 minutes at room temperature and the cells were harvested by centrifugation at 3080×g for 10 minutes. The pellets were resuspended in 45 ml of 1×PBS solution with 0.05% Tween-80. The cells were harvested by centrifugation at 3080×g for 10 minutes.
The pellets were resuspended in 15 ml of 1×PBS solution with 0.05% Tween-80. Clones were stored at 4° C. and used for analysis in standard GeneXpert® diagnostics in parallel with clinical samples.
Analysis of Strains by Standard GeneXpert® Laboratory Diagnostics for SARS-CoV-2 Assay
Single colonies of modified Mycobacterium smegmatis were picked and resuspended in 500 μl of PBS and heated for 20 min at 60° C. Then, resulting liquid was added to the Xpert® Xpress SARS-CoV-2 cartridge. Three different colonies were picked and processed this way.
The digital output of the Xpert®-MTB/RIF tests was analysed and the results are shown in Table 2. All samples gave a test result of “SARS-CoV-2 POSITIVE”, indicating that the non-infectious biomimetic control cells mimic the diagnostic profile of SARS-CoV-2.
Development of SARS-CoV-2 Genome Library for Use as Nucleic Acid Sequence Inserts in the Diagnostic Control Constructs
In order to ensure that SARS-CoV-2 targets could span the entire SARS-CoV-2 genome, a library of the whole SARS-CoV-2 genome was constructed. The constructed genome includes 11 fragments as shown in Table 3 below.
Briefly, the genomic RNA was obtained by culture in a BSLIII facility, by tissue culture of the virus in the Chimpanzee Vero cell line. Eleven 3 kbp fragments were obtained by reverse transcription to generate cDNA product. PCR of the cDNA was performed to obtain dsDNA product, which was captured into E. coli replicating plasmid. To cover the entire 30 kbp genome, 11 fragments were designed to allow for overlap between the fragments, thereby ensuring coverage in case the region of interest should lie between fragments.
Any target of interest may be cloned from the library into M. smegmatis using the method described in Example 1 above.
Cloning of Shuttle Plasmids with HCV and HIV Nucleic Acid Targets and Integration into Mycobacterium smegmatis
Nucleic acid targets for both HCV and HIV were cloned and a construct was prepared in E. coli as described in Example 1, with the nucleic acid targets for HCV and HIV replacing the SARS-CoV-2 targets.
The nucleic acid fragments were derived from publicly available sequences for HIV (NCBI Accession Number NC_001802.1) and HCV (NCBI Accession Number NC_004102.1).
The resulting plasmid was electroporated into M. smegmatis mc2155 by standard laboratory methods, as described in Example 1.
The recombinant M. smegmatis containing the HIV and HCV targets was added to the Xpert® HIV-1 Viral Load and Xpert® HCV Viral Load cartridges. The digital output of the Xpert® HIV-1 Viral Load and Xpert® HCV Viral Load tests was analysed and a positive result was obtained using the recombinant M. smegmatis containing the HIV and HCV genome fragments.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2005985.3 | Apr 2020 | GB | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IB2021/053369 | 4/23/2021 | WO |
