Dr. István Balogh, Dr. János Kappelmayer, Dr. József Tőzsér (2011)
University of Debrecen
The oligonucleotide ligation assay has also been commercialized in some cases. Figure 15.1 describes the use of oligonucleotide ligation assay (OLA) for the detection of CFTR gene mutations. One mutation site is defined by two oligonucleotide probes. Hybridization of the common probe to the PCR product is independent of the presence of the given mutation. Allele-specific probes, however, hybridize only into the respective allele. After hybridization, ligation using a thermostable ligase is performed and the resulting ligated fluorescent probes are separated using electrophoresis. The method can be multiplexed using different dyes and probes with different length, which makes possible to the analysis of 20-30 different mutations at the same time.
Figure 15.2 shows the basics of the special real-time PCR assay, which is based on the fluorescence resonance energy transfer (FRET). On the top panel, there are two single stranded DNA probes labelled with donor and acceptor dyes together with the DNA sample to be analyzed. If the donor dye is excited with a light of an adequate wavelength, and the two dyes are separated from each other (i.e. the two probes are in solution, not hybridized to the template DNA), it will emit light and gets back to its ground state (middle panel). If both probes are hybridized to the template DNA and light is introduced to the system, the donor dye will not emit at its emission wavelength, rather it will transfer its energy to the acceptor dye which then will be excited (FRET). It means that we can excite the donor dye and detect emission from the acceptor. The method is capable of a real-time following of the PCR reaction and it can be used for the detection of small-scale mutations if one of the probes is designed to cover the mutation site. In this case, after the completion of the PCR, slow heating is applied and the fluorescent signal is continuously monitored. The hybrid that is formed between the wild-type specific detection probe and the template DNA is more stable than in the case of mutation, therefore it will have higher melting temperature. This difference is indicated with a decrease in the fluorescent signal.
Figure 15.2. Figure 15.2. Detecting mutation using hybridization probes - fluorescence resonance energy transfer
Figures 15.3 and 15.4 show the localization of the detection (or sensor) probe and the other, anchor probe on the DNA template.
The detection probe lies on the specific mutation site. In the case described in Figure 15.3., the sample has a mutation. Therefore, the detection probe which is complementer with the wide type sequence has a mismatch (fourth nucleotide position from the right). Fluorescent dyes are labelled with red color (3’ end of the anchor probe) and yellow color (5’ end of the detection probe). One of the dyes serves as donor and the other one as acceptor. Detection is based on the fluorescence resonance energy transfer (FRET). The counterpart of the picture is shown in Figure 15.4, where the detection probe that is complementer with the wild types sequence is hybridizing with wild type genomic DNA, therefore the complementarity is perfect.
The multiplex ligation dependent probe amplification (MPLA) method is a recent development for the detection of small quantitative differences (i.e. gene duplications or deletions) in the genetic material even in heterozygous form which makes it useful for carrier detection in Duchenne/Becker muscular dystrophy (Figure 15.5.). Each probe set contains two specific oligonucleotides, one short and one longer and the assay can be multiplexed. The short probe contains a target-specific sequence (21-30 nucleotides) in its 3' end and a 19 nucleotides long sequence in its 5' end (which is identical to the PCR primer used downstream). The long probe, in addition to the target-specific sequence, contains a stuffer sequence and a second PCR primer sequence. The first step in the assay is a long hybridization period when the probe mix is hybridized to the genomic DNA. With the addition of a ligase enzyme, only the hybridized probes can be ligated, and the amount of the ligated probes is proportional to the amount of genomic DNA copy number. The next step is a PCR reaction with one primer pair. The stuffer sequences in the longer probes differ in a few nucleotides in length so the products of the multiplex PCR reaction can be easily separated using high resolution capillary electrophoresis. The result of the separation is an electropherogram in which the detected fluorescent signal-generated peak area is proportional to the original copy number present in genomic DNA.
The mutation scanning or detection methods described in detail in the previous lectures can be used for investigating the molecular context of a given DNA sample. Although they are very useful, the reference methodology for the analysis of small-scale mutations is DNA sequencing, i.e., the determination of the nucleotide order in a DNA strand. It has been developed by a British scientist, Frederick Sanger, who received one of his Nobel prizes for the DNA sequencing method. Sanger sequencing, which was developed in the late 1970’s, made the Human Genome Project possible. The left-hand side panel of the Figure 15.6. shows the classic Sanger sequencing, which is based on chain termination with dideoxy nucleotides. The method was rather time-consuming and difficult to perform as it required bacterial cloning and pouring of large acrylamide gels was necessary for the electrophoresis. In addition, the detection was based on the incorporation of radioactivity into the DNA. Later it become much easier with the development of some automation and fluorescent detection methods, but it still requires gel (or capillary) electrophoresis for the separation of the sequencing products. The right-hand side panel of the Figure 15.6. shows the basics of a new era of genetic testing, the next generation of sequencing. Electrophoretic separation is not needed anymore in the case of next-generation DNA sequencing methodologies, therefore, they are very high throughput but require highly sophisticated bio-informatics.
It is generally accepted that the next-generation sequencing techniques will become the most important step in the revolution of genetic analysis. It is now clear that they will have an unprecedented impact on molecular diagnostics. It will be soon possible to sequence the entire genome of an individual relatively quickly and at an acceptable cost. The genomic way of investigating human disorders is likely to change our way of thinking of health and disease and the healthcare system for good.