TRANSKRIPTIONSBASIERENDE DUPLICATION OF DOPPLESTRÄNGIGEN DNS ZIELMOLEKÜLEN
The present invention is directed to a transcription based amplification method for the amplification of DNA targets. Nucleic acid amplification methods are used in the field of molecular biology and recombinant DNA technology. These methods are used to increase the number of copies of a particular nucleic acid sequence, present in small amounts and often in an environment in which a wide variety of other nucleic acid sequences, both RNA and DNA, are also present. In particular, nucleic acid amplification methods are used to facilitate the detection or quantification of nucleic acid and are important for diagnosing for example infectious diseases, inhereted dieseases and various types of cancer. Nucleic acid amplification methods have also found their applications in other fields where samples are investigated in which nucleic acid may be present in minute amounts, such as forensic sciences, archeology or to establish paternity. Several nucleic acid amplification techniques are known based on different mechanisms of action. One method for the amplification of nucleic acid is known as the "Polymerase Chain Reaction" (PCR) is described in European patent applications EP 200362 and EP 201148. PCR is a cyclic process which has double stranded DNA as target. Each cycle in the PCR process starts with the separation of a double stranded DNA target in its two complementary strands. To each strand a primer will anneal and DNA polymerases present will extend the primers along the DNA strand to which it annealed thus forming two new DNA duplexes. When the reaction mixture is heated the strands of the DNA duplexes will be separated again and a new PCR cycle can start. Thus, the PCR process produces multiple DNA copies of a DNA target. If single stranded RNA is the desired target for PCR, it has to be converted to double stranded DNA first by reverse transcriptase. The present invention is concerned with a different class of nucleic acid amplification methods namely the "transcription based amplification techniques". The techniques involve the transcription of multiple RNA copies from a template comprising a promoter recognized by an RNA polymerase. With these methods multiple RNA copies are transcribed from a DNA template that comprises a functional promoter recognized by the RNA polymerase. Said copies are used as a target again from which a new amount of the DNA template is obtained etc. Such methods have been described by Gingeras et al. in WO88/10315 and Burg et al. in W089/1050. Isothermal transcription based amplification techniques have been described by Davey et al. in EP 323822 (relating to the NASBA method), by Gingeras et al. in EP 373960 and by Kacian et al. in EP 408295. Transcription based amplification reactions may also be performed with thermostable enzymes. Transcription based amplifications are usually carried out at a temperature around 41 degrees Celsius. These thermostable enzymes allow the reaction to be carried out at more elevated temperatures. Such a thermostable method is described in EP 682121 filed in the name of Toyo Boseki KK. The methods as described in EP 323822, EP 373960 and EP 408295 are isothermal continuous methods. With these methods four enzyme activities are required to achieve amplification: an RNA dependent DNA polymerase activity, an DNA dependent DNA polymerase activity, an RNase (H) activity and an RNA polymerase activity. Some of these activities can be combined in one enzyme, so usually only 2 or 3 enzymes are necessary. Enzymes having RNA dependent DNA polymerase activities are enzymes that synthesize DNA from an RNA template. A DNA dependent DNA polymerase thus synthesizes DNA from a DNA template. In transcription based amplification reactions a reverse transcriptase such as AMV (Avian Myoblastosis Virus) or MMLV (Moloney Murine Leukemia Virus) reverse transcriptase may be used. Such enzymes have both RNA- and DNA dependent DNA polymerase activity but also an inherent RNase activity. In addition an RNase may be added to the reaction mixture of a transcription based amplification reaction, such as E. coli RNase H. DNA dependent RNA polymerases synthesize multiple RNA copies from a DNA template including a promoter recognized by the RNA polymerase. Examples of RNA polymerases are polymerases from E. coli and bacteriophages T7, T3 and SP6. An example of an RNA polymerase commonly used with transcription based amplification methods is T7 polymerase. Thus the promoter that is incorporated in the template used for transcribing multiple copies of RNA would then be the T7-promoter. Usually the template comprising the promoter has to be created starting from the nucleic acid comprising the target sequence. Said nucleic acid may be present in the starting material that is used as input for the amplification reaction. The nucleic acid present in the starting material will usually contain the target sequence as a part of a much longer sequence. Additional nucleic acid sequences may be present on both the 3'- and the 5'-end of the target sequence. The amplification reaction can be started by bringing together this nucleic acid from the starting material, the appropriate enzymes that together provide the above mentioned activities and at least one, but usually two, oligonucleotide(s). At least one of these oligonucleotides should comprise the sequence of the promoter. Transcription based amplification methods are particularly useful if the input material is single stranded RNA, although single or double stranded DNA can likewise be used as input material. When a transcription based amplification method is practiced on a sample with single stranded RNA (of the "plus" sense) with additional sequences on both the 3'-end and the 5' end of the target sequence a pair of oligonucleotides that is conveniently used with the methods as described in the prior art would consist of: The method according to the invention is particularly useful for amplifying small DNA molecules, e.g. plasmid DNA. The method is particularly useful for the detection of small DNA molecules of pathogenic micro-organisms enabling diagnosis. In particular the circular HIV-1 DNA molecules that are formed during the replication of the HIV-1 virus can be detected by this method. Detection of such circular HIV-1 DNA molecules indicates active replication of the virus, which can be correlated with disease progression, i.e. development of AIDS. In another application the method can be used for detection of plasmid DNA molecules naturally present in Chlamydia species. The Chlamydia plasmids can encode certain virulence factors which means that detection of the plasmid not only shows the presence of the Chlamydia infection, but also shows the Chlamydia cells that cause the infection will carry certain virulence factors. In yet other applications the method can be used for amplification of genomic sequences after partial degradation and isolation of the DNA. This has a wide range of applications, of which many can be associated with the detection and identification of mutations in the genomic DNA. These mutation can associated with the diagnosis of cancer, hereditary disease or predisposition for disease. Surprisingly, transcription based amplification can start from double stranded DNA, without the need to treat the DNA with restriction enzymes or, what is more important, to separate the strands before hand by applying heat. All reagents conventionally used with isothermal transcription based amplification reactions can simply be used on starting material containing double stranded DNA, as if it where single stranded RNA. The enzymes used with the method according to the invention may be any enzymes that are known in the art as suitable enzymes for transcription based amplification methods and the reaction conditions are essentially the same as for prior art transcription based amplification methods, commonly used to amplify single stranded RNA. With the present invention it has now been found that essentially the same protocols can be used for isothermal transcription based amplification of double stranded DNA even though, based on the knowledge of the mechanisms according to which transcription based amplification reactions were suspected to operate, the skilled person would not have expected this method to be operable. In a preferred embodiment the DNA is heated to 65 degrees Celsius in the presence of the amplification oligonucleotides, but not in the presence of the amplification enzymes. The enzymes are only added to the reaction after the reaction mixture is cooled to the incubation temperature for the amplification reaction, i.e. 41 degrees Celsius. In another embodiment of the method of the present invention the DNA can be heated to 100 degrees Celsius in the presence of the two amplification oligonucleotides. One skilled in the art would still not expect this method to work due to the fact that after oligonucleotide annealing and extension the newly made DNA strand has to be separated from the original DNA template strand before the second oligonucleotide can anneal and be extended. In this second preferred embodiment with only one heating step the enzymes are only added after the reaction mixture is cooled to the amplification incubation temperature, i.e. 41 degrees Celsius. Preferably two oligonucleotides are used in the method according to the invention; a first oligonucleotide (usually referred to a "promoter-primer") that is capable of hybridizing to a specific sequence in the first strand of the double stranded DNA, which oligonucleotide has the sequence of a promoter (for example the T7 promoter) attached to its 5' end and It is surprising that, by contacting the double stranded DNA with the two oligonucleotides and the appropriate enzymes, a transcription based amplification process can be performed wherein there is no need for several strand separation steps to separate the two strands making up the double stranded DNA. The method will result in multiple linear RNA copies comprising the target sequence that is part of the sequence of the double stranded DNA. The following examples demonstrate the mechanism and utility of the present invention. They are not limiting and should not be considered as such. The enzymes used in the following examples are avian myeloblastosis virus (AMV) reverse transcriptase, T7 RNA polymerase and E. coli RNase H. Other enzymes with similar activities and enzymes from other sources may be used. Other RNA polymerases with different promoter specificity's may also be suitable for use. The NASBA reaction conditions used in the following examples were 40 mM tris, pH 8.5, 42 mM KCI (or in later experiments 70 mM KCI), 12 mM MgCl2, 5 mM DTT, 1 mM each dNTP, 2 mM rATP, 2 mM rCTP, 2 mM rUTP, 1.5 mM rGTP, 0.5 mM ITP, 0.2 µM each oligonucleotide, 375 mM sorbitol, 0.105 g/l BSA, 6.4 units AMV-RT, 32 units T7 RNA polymerase, 0.08 units E. coli RNase H and a specified amount of template in 20 µl volume. The oligonucleotide sequences used are exemplary and are not limiting as other sequences have been employed for these and other target sequences. To demonstrate the feasibility of DNA target amplification with NASBA without high temperature denaturation steps the two following oligonucleotides were used in combination with the above described NASBA reaction ingredients: The T7 promoter part of the P1 is given in italics. These primers target part of the gag region of the HIV-1 genome for amplification. As input for the amplification plasmid DNA pUCp24 encompassing the gag region of the HIV-1 genome was used in different input amounts. The protocol that was used consisted of mixing the target plasmid DNA with the ingredients described above in the preface except for the enzymes, heating to 65°C, cooling to 41°C, addition of enzymes and incubation at 41 ° for 90 minutes. The amplified material was electrophoresed in an agarose gel, blotted onto a nylon filter and hybridized with the32P-labeled HIV-1 gag probe 5' GAA TGG GAT AGA GTG CAT CCA GTG CAT G 3'. A positive result could be obtained with a sensitivity of 105 molecules input of plasmid DNA in the amplification. The same results could be obtained with the same protocol without the 65°C incubation. To demonstrate the feasibility of DNA target amplification with NASBA without high temperature denaturation steps the two following oligonucleotides were used in combination with the above described NASBA reaction ingredients: The T7 promoter part of the P1 is given in italics. These primers target part of the HPV16 genome for amplification. As input for the amplification plasmid DNA pHPV16 containing a full length HPV16 genome was used in different input amounts. The protocol that was used consisted of mixing the target plasmid DNA with the ingredients described above in the preface except for the enzymes, heating to 65°C, cooling to 41°C, addition of enzymes and incubation at 41° for 90 minutes. The amplified material was electrophoresed in an agarose gel, blotted onto a nylon filter and hybridized with the32P-labeled HPV16 probe 5' AGT ACA AAT ATG TCA TTA TGT GC 3'. A positive result could be obtained with a sensitivity of 1 pg input of plasmid DNA in the amplification. To demonstrate the feasibility of DNA target amplification with NASBA without high temperature denaturation steps the two following oligonucleotides were used in combination with the above described NASBA reaction ingredients: The T7 promoter part of the P1 is given in italics. These primers target part of the Chlamydia trachomatis natural plasmid for amplification. As input for the amplification plasmid preparations from Chlamydia trachomatis were used in different input amounts relating to the amount of Chlamydia trachomatis Inclusion Forming Units (IFU's). The protocol that was used consisted of mixing the target plasmid DNA with the ingredients described above in the preface except for the enzymes, heating to 65°C, cooling to 41°C, addition of enzymes and incubation at 41 ° for 90 minutes. The amplified material was electrophoresed in an agarose gel, blotted onto a nylon filter and hybridized with the32P-tabeted Chlamydia trachomatis natural plasmid probe 5' CGT GCG GGG TTA TCT TAA AAG GGA T 3'. A positive result could be obtained with an amount of plasmid DNA in the amplification corresponding to 0.01 IFU. To demonstrate the feasibility of DNA target amplification with NASBA without high temperature denaturation steps the two following oligonucleotides were used in combination with the above described NASBA reaction ingredients: The T7 promoter part of the P1 is given in italics. These primers target part of the human Tissue Factor gene for amplification. As input for the amplification plasmid DNA pUC13-TF containing part of the Tissue Factor gene was used in different input amounts. The protocol that was used consisted of mixing the target plasmid DNA with the ingredients described above in the preface except for the enzymes, heating to 65°C, cooling to 41°C, addition of enzymes and incubation at 41° for 90 minutes. The amplified material was electrophoresed in an agarose gel, blotted onto a nylon filter and hybridized with the32P-labeled Tissue Factor probe 5' GTT CAG GAA AGA AAA CAG CCA 3'. A positive result could be obtained with a sensitivity of 103 molecules of input of plasmid DNA in the amplification. To demonstrate the feasibility of DNA target amplification with NASBA without high temperature denaturation steps the two following oligonucleotides were used in combination with the above described NASBA reaction ingredients: The T7 promoter part of the P1 is given in italics. These primers target part of the human CD14 gene for amplification. As input for the amplification plasmid DNA pπH3M containing part of the CD14 gene was used in different input amounts. The protocol that was used consisted of mixing the target plasmid DNA with the ingredients described above in the preface except for the enzymes, heating to 65°C, cooling to 41 °C, addition of enzymes and incubation at 41 ° for 90 minutes. The amplified material was electrophoresed in an agarose gel, blotted onto a nylon filter and hybridized with the32P-labeled CD14 probe 5' CCA TGG AGC GCG CGT CCT 3'. A positive result could be obtained with a sensitivity of 103 molecules of input of plasmid DNA in the amplification. To demonstrate the feasibility of DNA target amplification with NASBA without high temperature denaturation steps the two following oligonucleotides were used in combination with the above described NASBA reaction ingredients: The T7 promoter part of the P1 is given in italics. Nucleotides between brackets denote "degenerated" position were any of the nucleotides between the brackets can occur. These primers target part of the human Actin gene for amplification. As input for the amplification total human genomic DNA , (commercially obtained human placental DNA) treated with RNase A was used at 400 ng input amount. The protocol that was used consisted of mixing the target human placental genomic DNA with the ingredients described above in the preface except for the enzymes, heating to 65°C, cooling to 41°C, addition of enzymes and incubation at 41 ° for 90 minutes. The amplified material was electrophoresed in an agarose gel, blotted onto a nylon filter and hybridized with the32P-labeled Actin probe 5' CTG TCC ACC TTC CAG CAG ATG TGG A 3'. A positive result could be shown using the human genomic DNA as input for the amplification. To demonstrate the feasibility of DNA target amplification with NASBA without high temperature denaturation steps the two following oligonucleotides were used in combination with the above described NASBA reaction ingredients: The T7 promoter part of the P1 is given in italics. These primers target part of the CMV genoom for amplification. As input for the amplification total DNA of CMV infected HEL cells, treated with RNase A was used at an input amount equivalent to 0.1 cells. The protocol that was used consisted of mixing the DNA with the ingredients described above in the preface except for the enzymes, heating to 65°C, cooling to 41 °C, addition of enzymes and incubation at 41 ° for 90 minutes. The amplified material was electrophoresed in an agarose gel, blotted onto a nylon filter and hybridized with the32P-labeled CMV probe 5' CTG CTA TGT CTT AGA GGA GA 3'. A positive result could be shown using the DNA of 0.1 cell equivalent as input for the amplification. The present invention is directed to a transcription based amplification method for the amplification of DNA targets. With the method of the present invention an isothermal transcription based amplification method is provided for the amplification of double stranded DNA. The method of the present invention for the amplification of double stranded DNA comprises the steps of contacting said double stranded DNA with at least one oligonucleotide comprising a sequence complementary to a part of the first of the two DNA strands comprised in the double stranded DNA, said oligonucleotide further comprising the sequence of a promoter recognized by a RNA polymerase; a further olignucleotide comprising a sequence complementary to a part of the second strand comprised in the double stranded DNA; an enzyme having RNA dependent DNA polymerase activity; an enzyme having DNA dependent DNA polymerase activity; an enzyme having RNase H activity; an enzyme having RNA polymerase activity, and maintaining the thus created reaction mixture under the appropriate conditions for a sufficient amount of time for the amplification to take place. The method according to the invention is particularly useful for amplifying small DNA molecules, e.g. plasmid DNA. Surprisingly, transcription based amplification can start from double stranded DNA, without the need to treat the DNA with restriction enzymes or, what is more important, to separate the strands before hand by applying heat. All reagents conventionally used with isothermal transcription based amplification reactions can simply be used on starting material containing double stranded DNA, as if it where single stranded RNA. With the present invention it has now been found that essentially the same protocols can be used for isothermal transcription based amplification of double stranded DNA. The method is particularly useful for the detection of small DNA molecules of pathogenic microorganisms enabling diagnosis. In particular the circular HIV-1 DNA molecules that are formed during the replication of the HIV-1 virus can be detected by this method. Detection of such circular HIV-1 DNA molecules indicates active replication of the virus, which can be correlated with disease progression, i.e. development of AIDS. Method for the amplification of double stranded DNA with a transcription based amplification method, wherein the double stranded DNA is contacted with:
- at least one oligonucleotide comprising a sequence complementary to a part of the first of the two DNA strands comprised in the double stranded DNA, said oligonucleotide further comprising the sequence of a promoter recognized by a RNA polymerase; - a further oligonucleotide comprising a sequence complementary to a part of the second strand comprised in the double stranded DNA; - an enzyme having RNA dependent DNA polymerase activity; - an enzyme having DNA dependent DNA polymerase activity; - an enzyme having RNase H activity; - an enzyme having RNA polymerase activity; Method according to claim 1, wherein the DNA is heated once in the presence of the oligonucleotides, but before the enzymes are added. Method according to claim 2, wherein the DNA is heated to 65 degrees Celsius. Method according to claim 2, wherein the mixture is heated to 100 degrees Celsius. Method according to claim 1, wherein the double stranded DNA is a plasmid DNA or genomic DNA. Method according to claim 5, wherein the DNA is genomic DNA which has been partially degraded. Method according to any of the preceding claims wherein a reverse transcriptase is used. Method according to claim 7, wherein the reverse transcriptase is AMV reverse transcriptase. Method according to any of the preceding claims wherein the promoter sequence is the T7 promoter sequence and the RNA polymerase used is T7 RNA polymerase. Method according to any of the preceding claims wherein a separate RNase H enzyme is used.DESCRIPTION OF THE FIGURES:
EXAMPLES:
Preface
Example 1
Example 2
Example 3
Example 4
Example 5
Example 6
Example 7
SEQUENCE LISTING