Mutation detection using ENDO1: Application to disease diagnostics in humans and TILLING and Eco-TILLING in plants
© Triques et al; licensee BioMed Central Ltd. 2008
Received: 11 October 2007
Accepted: 23 April 2008
Published: 23 April 2008
Most enzymatic mutation detection methods are based on the cleavage of heteroduplex DNA by a mismatch-specific endonuclease at mismatch sites and the analysis of the digestion product on a DNA sequencer. Important limitations of these methods are the availability of a mismatch-specific endonuclease, their sensitivity in detecting one allele in pool of DNA, the cost of the analysis and the ease by which the technique could be implemented in a standard molecular biology laboratory.
The co-agroinfiltration of ENDO1 and p19 constructs into N. benthamiana leaves allowed high level of transient expression of a mismatch-specific and sensitive endonuclease, ENDO1 from Arabidopsis thaliana. We demonstrate the broad range of uses of the produced enzyme in detection of mutations. In human, we report the diagnosis of the G1691A mutation in Leiden factor-V gene associated with venous thrombosis and the fingerprinting of HIV-1 quasispecies in patients subjected to antiretroviral treatments. In plants, we report the use of ENDO1 system for detection of mutant alleles of Retinoblastoma-related gene by TILLING in Pisum sativum and discovery of natural sequence variations by Eco-TILLING in Arabidopsis thaliana.
We introduce a cost-effective tool based on a simplified purification protocol of a mismatch-specific and sensitive endonuclease, ENDO1. Especially, we report the successful applications of ENDO1 in mutation diagnostics in humans, fingerprinting of complex population of viruses, and in TILLING and Eco-TILLING in plants.
Scanning DNA sequences for mutations and polymorphisms is an analytic tool in a broad range of disciplines. However, identification of such mutations and polymorphisms in long stretches of DNA and in large numbers of samples by direct sequencing is not a trivial exercise. Several mutation detection techniques based on the physical properties of single stranded DNA or heteroduplex DNA [1–5] have been described. Among such methods are conformation sensitive gel electrophoresis (CSGE) , denaturing gradient gel electrophoresis (DGGE) , constant denaturing capillary electrophoresis, (CDCE) , Temperature Gradient Capillary Electrophoresis (TGCE) , single strand conformation polymorphism (SSCP)  and denaturing high-performance liquid chromatography (DHPLC) . Other methods exploit chemicals like groove binders or chemicals that cleave single strand DNA at the mismatch site in heteroduplex DNA .
Single strand specific nucleases have also been used to cleave heteroduplex DNA at the mismatch site [10–13]. Among them, CEL I is a mismatch specific endonuclease  that is widely used for reverse genetics in plants, animals and bacteria [15–22] and for disease diagnostic in human such as cancers related to BRCA1 [23, 24]. Enzymatic mutation detection is advantageous over other popular mismatch detection systems, like TGCE and DHPLC [8, 25–27] because mismatch specific endonucleases allow to screen large stretches of DNA without reducing diagnostic sensitivity or specificity, while at the same time providing information about the location of the mutation. However, many mismatch specific cleavage enzymes were reported to cleave preferentially certain types of mismatches, display low sensitivity or lead to a high background [10, 11, 20, 28, 29].
Previously, we reported the biochemical analysis of five S1 type nucleases from Arabidopsis thaliana. We demonstrated that one of them, ENDO1, is a mismatch specific endonuclease, which cleaves with a high efficiency all types of mismatches and has a high sensitivity, detecting one allele in pool of sixty . Here, we report a very simple protocol for the expression and the preparation of ENDO1. Especially, we report the use of ENDO1 in combination with universal labelled primers as a cost-effective tool for mutation diagnostics in humans, plants and viruses.
Rapid preparation of ENDO1
Ten grams of agroinfiltrated leaves were homogenized and cleared by centrifugation in the absence and in the presence of 30% ammonium sulphate. Ammonium sulphate was then added to 80% final concentration in the supernatant and the pelleted proteins were resuspended in buffer containing 50% glycerol, dialysed against the same buffer and stored at -80°C.
To test whether ENDO1 obtained from this simplified purification protocol is active we tested the protein extract on heteroduplex DNA created from two clones that differ by a single base insertion (Figure 1b). In this experiment we predicted that if the ENDO1 preparation contains a mismatch specific endonuclease, the heteroduplex DNA will be cleaved at the mismatch site releasing two bands of 208 bp and 436 bp (Figure 1b). Duplex DNAs were incubated with dilutions of ENDO1 or GFP protein extracts. ENDO1 led to the predicted digestion product and as expected the GFP control showed undetectable mismatch specific cleavage activity (Figure 1b, lower picture). As shown for His-tag purified ENDO1  and CEL I , at high concentration (50 to 250 fold dilution, Figure 1b), ENDO1 presented double strand DNAse activity and digested the entire fragments. From this biochemical analysis we concluded that ENDO1 simplified purification led to an active enzyme and the test mutation was detected at dilutions ranging from 500 to 10 000 fold (Figure 1b, upper picture). Based on this analysis and duplicates of this experiment (data not shown), we estimate that the amount of enzyme produced from 10 grams of agroinfiltrated leaf material, and used at 10 000 fold dilution, is enough to carry more than a million mismatch detection tests.
Detection of G1691A mutation in factor V
Primers used to amplify target loci
Fingerprinting of HIV-1 quasispecies
Human immunodeficiency virus type 1 (HIV-1) present in infected individuals has been described as quasispecies of related but distinct viruses [35–38]. When the selective pressure of antiretroviral therapy is exerted on such a population, drug-resistant mutants may emerge and consequently lead to treatment failure [35, 37]. The objective of this work was to assess the HIV-1 quasispecies evolution during different treatments. Direct sequencing of PCR amplified HIV-1 DNA fragment from an infected individual will detect only predominant mutations. Detailed analysis of the HIV quasispecies using sequencing would require the cloning of PCR products and systematic sequencing of individual clones. A list of alternative technologies have been also tested to identify minor HIV drug-resistant populations [39–43].
Previously we demonstrated that ENDO1 can detect rare alleles in pools of DNA . Thus, we tested the use of ENDO1 system as a fingerprinting protocol to assess the HIV-1 quasispecies evolution during different treatments. We focused the analysis on the sequence variation in a DNA fragment of 843 bp coding for the reverse transcriptase gene of HIV-1. Genomic DNAs were extracted from peripheral blood mononuclear cells of an HIV-1 infected patient before (Figure 2d, lane 1) and after 48 weeks of antiretroviral therapy (Figure 2d, lane 2). The reverse transcriptase gene was PCR amplified and analysed using ENDO1. Two different patterns of bands, representing fingerprints of HIV-1 quasispecies at each time, were observed (Figure 2d). Comparing the overall patterns and changes in intensity of particular fragments allowed the identification of mutations that either appeared or were lost upon treatment (Figure 2d, empty arrows). Bands of invariant intensity indicate stable mutations (Figure 2d, filled arrows).
Exploitation of ENDO1 in TILLING
TILLING (Targeting Induced Local Lesions IN Genomes) method combines the induction of a high number of random mutations with mutagens like Ethyl Methane Sulfonate (EMS) and mutational screening systems to discover induced mutations in sequence DNA targets . This reverse genetic strategy encompasses all types of organisms as plants, animals, bacteria, without being subjected to the genome size [15–19, 45, 46].
To evaluate the robustness of ENDO1 purified using the simplified protocol, in combination with universal primers for mutation detection, RBR TILLING was carried out with conventional method based on Ni-Column-purified ENDO1 and gene specific primers. A similar number of mutants were identified (data not shown). Based on the TILLING of RBR and other targets (data not shown) we concluded that the ENDO1 simplified protocol is suitable for high throughput TILLING screen.
Exploitation of ENDO1 in Eco-TILLING
EcoTILLING is a means to determine the extent of natural sequence variation across many germplasms, enabling both SNP discovery and haplotyping . This technique is now applied to rice, maize, lotus, poplar , Brassica, zebrafish, Drosophila, Caenorhabditis and human , indicating its broad applicability.
Mutation detection is often an expensive and time-consuming obstacle to many molecular genetic applications including reverse genetics and clinical diagnostics. In this work we describe an enzymatic mutation detection technique based on ENDO1. We focused our work on the main limitation of any enzymatic mutation detection system, the production of a mismatch specific endonuclease. CEL I is the most commonly used endonuclease in TILLING and EcoTILLING in plants and animals [15–19, 21, 22, 45]. Three protocols are reported for the purification of CEL I from celery stalks. The highly purified enzyme is prepared from 105 kg of celery stalks and a more simplified protocol use 7 kg. Both protocols use a series of chromatography purifications steps requiring special equipments and skills in biochemistry. A more simplified protocol was reported  that use 0.5 kg of store-bought celery stalks. However, the minimum information on the name of the celery variety and the age of the plants to be used in the purification are missing, which renders the quality of produced enzyme uneven.
In this work, the co-agroinfiltration of ENDO1 and p19 constructs into N. benthamiana leaves allowed high level of transient expression. ENDO1 represented at least 6% of total proteins (data not shown) and from 10 grams of plant material, we routinely obtain enough enzyme to carry out more than a millions mismatch detection tests. The crude purified ENDO1 was found to be stable, allowing at least 4 months storage at -20°C. For longer period storage we recommend to purify His-Tagged ENDO1 on Ni-NTA agarose beads as described previously . The His-tag purified ENDO1 was found to be very stable, allowing at least two years storage at -80°C, one year storage at -20°C and more than four months storage at 4°C.
Several new techniques of mutation scanning based on microchips and/or sequencing or re-sequencing of a given genome are currently being developed ; however, these technologies yet have in common the major drawback of their cost. Such tools, despite their outstanding potential for mutation scanning, are now likely to be restricted for such applications to big laboratories specialised in sequencing or genotyping. On the other hand, using ENDO1 system, we demonstrated its universality as a low-cost strategy for easy high-throughput diagnostic of genetic mutations in different genomes, including those incompletely sequenced like pea. In humans, the ENDO1 system permitted the diagnosis of the well-known genetic mutation, G1691A in Factor V gene, which is often associated with activated protein C resistance that is a common risk factor for venous thrombosis . In viruses, we described a fingerprinting protocol to assess the HIV-1 quasispecies evolution during different treatments. We showed that the detection of quasispecies using universal primers with two PCR-rounds or using specific primers in one PCR-round gave the same results (data not shown). The natural progression of viral species within HIV-1 infected patients and/or the correlation of the appearance of quasispecies with the resistance to certain antiretroviral could be essential for clinical survey. In plants, we demonstrated the use of ENDO1 in combination with universal primers to decrease further the cost of the screening in TILLING and Eco-TILLING, two strategies that require high throughput screening protocols.
The Agrobacterium transient-expression assay could be used not only to produce (at low cost) large amounts of ENDO1, but also to over-express other nucleases for which the expression in bacteria is toxic or require post-translational modifications. Initial attempts to express ENDO1 and CEL I in E. coli were unsuccessful and when we succeeded, the expressed proteins showed no DNase activity (data not shown).
The ENDO1 preparation protocol described in this work does not require any particular equipment or skills and could be implemented in any standard molecular biology research laboratory. The produced ENDO1 was successfully used in mutations diagnostics in different genomes. In humans, the ENDO1 system permitted the diagnosis of the well-known genetic mutation in Factor V gene, which is often associated with venous thrombosis. In viruses, we described a fingerprinting protocol to assess the HIV-1 quasispecies evolution during different treatments. In plants, we demonstrated the use of ENDO1 in combination with universal primers to decrease further the cost of TILLING and Eco-TILLING.
Overexpression of ENDO1
pBIN35S-ENDOI construct was described previously . Agrobacterium transient overexpression in Nicotiana benthamiana was performed as described previously  except that the cells were co-infiltrated into N. benthamiana leaves in presence of Agrobacterium harbouring pBIN61-p19 construct . Agroinfiltrated plants were incubated for at least 60 to 96 hours in the green house before protein extraction.
Purification of ENDO1
Ten grams of agroinfiltrated leaves were homogenized in 10 ml buffer containing 0.1 M Tris-HCl pH 8, 200 μM phenylmethylsulphonyl fluoride (PMSF), 0.125 mM β mercaptoethanol and 10% of glycerol and cleared by centrifugation at 3,000 × g for 30 min. Ammonium sulphate was added to the supernatant to the final concentration of 30% and the samples incubated on ice for 1 hour were centrifuged at 30,000 × g for 30 min. Ammonium sulphate was added to the supernatant to 80% final concentration and the proteins were precipitated by centrifugation as above. The pellet was resuspended in 500 μl of dilution buffer (50 mM Tris-HCl pH 8, Glycerol 50%, 100 μM PMSF), dialysed against the same buffer and stored at -80°C.
PCR amplification and mutation detection
In the TILLING screen, target loci were PCR-amplified using nested-PCR and universal primers. The first PCR amplification is a standard PCR reaction using target-specific primers . One microlitre of the first PCR served as a template for the second nested PCR amplification, using a mix of gene-specific inner primers carrying a universal M13 tail (Table 1), in combination with M13 universal primers, M13F700 (CACGACGTTGTAAAACGAC) and M13R800 (GGATAACAATTTCACACAGG), labelled at the 5'end with infra-red dyes IRD700 and IRD800 (LI-COR®, Lincoln, Nebraska, USA), respectively. This PCR was carried out with each primers at 0.1 μM, using the following 2 steps cycling program: 94°C for 2 min, 10 cycles at 94°C for 15 sec, primers specific annealing temperature for 30 sec and 72°C for 1 min, followed by 25 cycles at 94°C for 15 sec, 50°C for 30 sec and 72°C for 1 min, then a final extension of 5 min at 72°C. The PCR amplification of factor V, HIV-1 reverse transcriptase and Arabidopsis BAC 15K9 DNA fragment was carried out using only the second PCR conditions described above using 30 ng of genomic DNA as template. Mutation detection, in non-purified PCR products, was carried out as described previously  except that the crude extracted enzyme was used at 10 000 fold dilution and 0.6 μl of ENDO1-digestion products were loaded on the gel using disposable paper membrane combs (The Gel Company, San Francisco, USA).
This work was supported by INRA TRANSFERT, Génoplante, the French consortium for plant genomics, GENOPOLE of EVRY and the Grain Legumes Integrated Project (FOOD-CT-2004-506223). Authors would like to thank Pascal Audigier and Julien Martinet for their technical help during this work.
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