METHOD AND APPARATUS FOR DETERMINING THE EXISTENCE OF A MUTATION

Abstract: In a method for determining the existence of a mutation in a nucleic acid fragment from an electric signal generated by a DNA sequencer and made up of sequence information produced by fluorescent fragment products of different lengths of said nucleic acid fragment, followed by a run-off peak produced by fluorescent full length fragment products of said nucleic acid fragment, any difference between said run-off peak and a reference run-off peak generated by unmutated full length fragment products of said nucleic acid fragment, is determined, a difference indicating the existence of a deletion or insertion mutation.

Description: METHOD AND APPARATUS FOR DETERMINING THE EXISTENCE OF A MUTATIO TECHNICAL FIELD The invention relates to a method and an apparatus for deter¬ mining the existence of a mutation in a nucleic acid fragment from an electric signal generated by a DNA sequencer and made up of sequence information produced by fluorescent fragment products of different lengths of εaid nucleic acid fragment, followed by a run-off peak produced by fluorescent full leng¬ th fragment products of said nucleic acid fragment. BACKGROUND OF THE INVENTION DNA sequencing, i.e. determining the sequence of nucleotides in a gene or in a segment of DNA, commonly involves several sequential steps aimed at: - Isolating genetic material from biological material. - Amplifying the gene of interest using polymerase chain re¬ action (PCR) so that sufficient material of the gene of in¬ terest is available for sequence analysis. - Performing sequencing reactions using the principles of Sanger. This step enzymatically generates a large number of differently elongated complementary copies of the gene. By introduction of base specific elongation terminators, each elongated , copy of the gene will terminate with a specific type of nucleotide. Each reaction corresponds to one specific type of nucleotide, Adenine (A) , Thymine (T) , Guanine (G) or Cytosine (C) , i.e. only one type of elongation terminator, will be incorporated in each reaction. To enable detection of these elongated gene copies, a fluorescently labelled molecu¬ le is introduced in the gene copy during the enzymatic reac¬ tion. Thus, all elongated gene copies will be fluorescently labelled to facilitate their detection. - Separating the mixture of differently elongated complemen¬ tary copies of the gene according to their molecular size using gel electrophoresis. - Sequentially detecting the differently elongated comple en- tary copies of the gene, separated according to molecular size, during gel electrophoresis, using a DNA sequencer sys¬ tem, e.g. the DNA sequencer marketed under the trademark ALF by Pharmacia Biotech AB, Uppsala, Sweden. During the elec¬ trophoresis, any fluorescent molecules passing a perpendicu¬ larly oriented laser beam, will be activated and the fluore¬ scence from each molecule will be detected by light sensitive detectors, each representing one type of nucleotide in the sample. - Determining the nucleotide sequence by superimposing the signals from four detectors representing the four different nucleotides of the sample gene. An example of such signals obtained from four such detectors is shown in Fig. 1 on the appended drawing. As apparent, the diagram in Fig. 1 is divided into a sequence region, contai¬ ning sequence data, and a run-off region, containing the so called "run-off peak" which is described more in detail be- low. Sequence data obtained by processing and sequencing DNA sam¬ ples from tumour tissue in an automatic sequencer, can be used to detect inherited or induced mutations in genes rela- ted to -the occurrence or progression of the tumour. When mu¬ tated, the sample εequence obtained from the sequencer will often consist of a mixture of two superimposed sequence com¬ ponents, namely the wild type component and a mutated compo¬ nent. This could be due either to a mixture of two cell popu- lations in the sample or to a mutation in one of the two co¬ pies of the gene, if both are present in the εample. In cases where the mutated sequence component is the predominant com¬ ponent, insertion and deletion mutations as well as point mutations can be readily detected by aligning the sequence data obtained from the sample with the expected wild type sequence using standard alignment algorithms (see e.g. S. Needleman and C. Wunsch, J. Mol. Biol. 48, 444 (1970)), and W. R. Pearεon and W. Miller, Methods in Enzymology, 210, 575 (1992)). Often, however, the mutated sequence material is mixed up with an equally large amount of non-mutated mate¬ rial. In εome caseε, the non-mutated material will even be predominant. In these cases ordinary alignment algorithms fail to resolve the mutation. BRIEF DESCRIPTION OF THE INVENTION The object of the invention is to bring about a simple and reliable method of determining the existance of a mutation in a nucleic acid fragment. In the method according to the invention for determining the existence of a mutation in a nucleic acid fragment from an electric signal generated by a DNA sequencer and made up of sequence information produced by fluorescent fragment pro- ducts of different lengths of said nucleic acid fragment, followed by a run-off peak produced by fluorescent full leng¬ th fragment products of said nucleic acid fragment, this is attained, mainly, by determining any difference between said run-off peak and a reference run-off peak generated by unmu- tated full length fragment products of the nucleic acid frag¬ ment, a difference indicating the existence of a deletion or insertion mutation. This object is also attained by the apparatus according to the invention for determining the existence of a mutation in a nucleic acid fragment from an electric signal generated by a DNA sequencer and made up of sequence information produced by fluoreεcent fragment products of different lengths of said nucleic acid fragment, followed by a run-off peak produced by fluorescent full length fragment products of εaid nucleic acid fragment, mainly, in that it comprises means for deter¬ mining any difference between said run-off peak and a refe¬ rence run-off peak generated by unmutated full length frag¬ ment products of said nucleic acid fragment, a difference indicating the existence of a deletion or inεertion mutation. BRIEF DESCRIPTION OF THE DRAWING The invention will be deεcribed more in detail below with reference to the appended drawing on which Fig. 2a εchematically εhows a normal run-off peak, Fig. 2b schematically shows a broadened run-off peak, and Fig. 2c schematically shows a split run-off peak. PREFERRED EMBODIMENTS When εeparating fluorescent DNA fragments of limited size in an electrophoresis gel and exciting these fragments to fluo- resce, e.g. in the caεe of direct εequencing of PCR frag- mentε, full length fragment products exist where the polyme¬ rase have not incorporated any chain terminators. This re¬ sults in the presence of a prominent peak, the so called "run-off peak", in all raw data signal curves from a sequen- cer. This run-off peak is located at the end of the actual sequence information. An example of a normal run-off peak generated by unmutated full length fragment products of a nucleic acid fragment, is schematically εhown in Fig. 2a. The run-off peak in Fig. 2a may correspond to the run-off peak in the run-off region shown in Fig. 1. If two species of DNA fragments are present in the sequencing reaction, e.g. one from a tumour tissue and one from surroun¬ ding normal tisεue, they will generate one run-off peak each. If the DNA fragmentε are of equal εize, theεe two run-off peakε will, of course, coincide. However, in case of a deletion or insertion mutation in one of the fragments, the fragments will differ in size and two separate run-off peakε will be generated. A small deletion or inεertion will result in a broadening of the run-off peak. Such a broadened run-off peak originating from a small inser¬ tion mutation, is schematically shown in Fig. 2b. As apparent from Fig. 2b, the normal "unmutated" run-off peak of Fig. 2a has been broadened, so that the run-off peak of Fig. 2b ends after the run-off peak of Fig. 2a. In case of a large deletion mutation, a εplit run-off peak will be generated aε shown in Fig. 2c. As apparent from Fig. 2c, the normal "unmutated" run-off peak coincides with the normal "unmutated" run-off peak of Fig. 2a, while the run-off peak from the fragments in which a large deletion mutation is present, will appear before the normal "unmutated" run-off peak. The indication of insertion and deletion mutations based on the run-off peak behaviour, is very sensitive and a contribu- tion of less than 5% mutated material can be readily detec¬ ted. The resolution in size difference between the two frag¬ ments is however limited to about ± 2 baεeε. Thiε iε of cour¬ se dependent on the resolution of the electrophoreεiε gel. It should be understood, however, that when using the run-off peak information alone for the mutation assignment, no infor¬ mation about the localization of the mutation along the DNA fragment will be achievable. The run-off peak information is a sensitive means for the mutation detection and may be uεed as a consistency check of the mutation assignment derived from sequence data. In accordance with the invention, any difference between a run-off peak generated by mutated full length fragment pro¬ ducts, such as the run-off peak shown in Fig. 2b, and a nor¬ mal "unmutated" run-off peak or reference run-off peak gene¬ rated by unmutated full length fragment products, such as the run-off peak shown in Fig. 2a, is determined as an indication of the existence of a mutation. In accordance with a first embodiment of the method according to the invention, the difference between the peak width of the run-off peak in Fig. 2b and the peak width of the run-off peak in Fig. 2a, is measured in view of the fact that the size of the peak width difference is directly proportional to the size of the mutation. If, as in Fig. 2b, the wider peak ends after the normal peak of Fig. 2a, this is an indication of an insertion mutation. However, should the wider run-off peak begin before the run¬ off peak of Fig. 2a, this iε an indication of a deletion mu- tation. In accordance with a second embodiment of the method accor¬ ding to the invention, the difference between the run-off peaks in Figs. 2a and 2b may be determined by measuring any difference between the location of the peak centers of theεe run-off peakε. The peak center of the run-off peak of Fig. 2b iε located af¬ ter the center of the run-off peak of Fig. 2a. This is then an indication of an insertion mutation. If, as in Fig. 2c, the center of the "mutated" run-off peak is located in front of the center of the normal or reference run-off peak, this indicates the preεcence of a deletion mu- tation.. The εize of the difference between the peak centerε iε direc¬ tly proportional to the size of the mutation. As should be apparent from the above, the method according to the invention is a simple and reliable method of determining not only the exiεtence of a mutation in a nucleic acid frag¬ ment but alεo whether the mutation is a deletion mutation or an insertion mutation. The apparatus (not shown) according to the invention for de¬ termining the existence of a mutation in a nucleic acid frag¬ ment from an electric signal generated by a DNA sequencer and made up of sequence information produced by fluoreεcent frag- ment products of different lengths of said nucleic acid frag ment, followed by a run-off peak produced by fluorescent ful length fragment productε of said nucleic acid fragment, comp riseε meanε (not εhown) for determining any difference bet- ween εaid run-off peak and a reference run-off peak generate by unmutated full length fragment productε of said nucleic acid fragment, a difference indicating the existence of a deletion or insertion mutation. In a first embodiment of the apparatus according to the in¬ vention, the means (not shown) for determining any differenc between said run-off peak and said reference run-off peak, is adapted to measure any difference in peak width between these run-off peakε, a wider run-off peak ending after said refe- rence run-off peak, indicating an insertion mutation, while a wider run-off peak beginning before said reference run-off peak indicating a deletion mutation, the size of the peak width difference being directly proportional to the size of the mutation. In a εecond embodiment of the apparatuε according to the in¬ vention, the meanε (not εhown) for determining any difference between said run-off peak and said reference run-off peak, is adapted tp measure any difference between the location of the centres of these run-off peaks, a location of the centre of said run-off peak in front of the centre of said reference run-off peak indicating a deletion mutation, a location of the centre of said run-off peak after the centre of said re¬ ference run-off peak indicating an insertion mutation, the εize of said difference being directly proportional to the size of the mutation. The apparatuε according to the invention iε preferably imple¬ mented in computer εoftware.

Claims: CLAIMS 1. Method for determining the existence of a mutation in a nucleic acid fragment from an electric signal generated by a DNA sequencer and made up of sequence information produced by fluorescent fragment products of different lengths of said nucleic acid fragment, followed by a run-off peak produced by fluorescent full length fragment products of said nucleic acid fragment, characterized by determining any difference between said run-off peak and a reference run-off peak gene¬ rated by unmutated full length fragment products of said nuc¬ leic acid fragment, a difference indicating the existence of a deletion or insertion mutation. 2. Method according to claim l, characterized in that said difference between said run-off peak and said reference run¬ off peak is determined by measuring any difference in peak width between these run-off peaks, a wider run-off peak en¬ ding after said reference run-off peak, indicating an inser- tion mutation, while a wider run-off peak beginning before said reference run-off peak indicating a deletion mutation, the size of the peak width difference being directly propor¬ tional to the size of the mutation. 3. Method according to claim 1, characterized in that said difference between said run-off peak and said reference run¬ off peak is determined by meaεuring any difference between the location of the centreε of these run-off peaks, a loca¬ tion of the centre of εaid run-off peak in front of the cen- tre of εaid reference run-off peak indicating a deletion mu¬ tation, a location of the centre of said run-off peak after the centre of said reference run-off peak indicating an in¬ sertion mutation, the εize of εaid difference being directly proportional to the εize of the mutation. 4. Apparatus for determining the existence of a mutation in a nucleic acid fragment from an electric signal generated by a DNA sequencer and made up of sequence information produced by fluorescent fragment products of different lengths of εaid nucleic acid fragment, followed by a run-off peak produced b fluorescent full length fragment products of said nucleic acid fragment, characterized in that it comprises means for determining any difference between the run-off peak and a reference run-off peak generated by unmutated full length fragment products of said nucleic acid fragment, a difference indicating the existence of a deletion or insertion mutation. 5. Apparatus according to claim 4, characterized in that said means for determining any difference between εaid run-off peak and said reference run-off peak, is adapted to measure any difference in peak width between theεe run-off peaks, a wider run-off peak ending after said reference run-off peak, indicating an insertion mutation, while a wider run-off peak beginning before said reference run-off peak indicating a deletion mutation, the size of the peak width difference be¬ ing directly proportional to the εize of the mutation. 6. Apparatus according to claim 4, characterized in that said meanε for determining any difference between said run-off peak and said reference run-off peak, iε adapted to measure any difference between the location of the centreε of these run-off peaks, a location of the centre of said run-off peak in front of the centre of said reference run-off peak indica- ting a . deletion mutation, a location of the centre of said run-off peak after the centre of said reference run-off peak indicating an insertion mutation, the εize of εaid difference being directly proportional to the εize of the mutation.

Inventors: NORBERG TORBJOERN, NORBERG, TORBJOERN
Applicants: NORBERG TORBJOERN, PHARMACIA BIOTECH AB, NORBERG, TORBJOERN
Application year: 1996
Main classification: C12Q1
IPC codes: C12N15, C12Q1, G01N33