Support vector machines for quality control of DNA sequencing
© Öz and Kaya; licensee Springer 2013
Received: 12 December 2012
Accepted: 15 February 2013
Published: 4 March 2013
Support vector machines, one of the non-parametric controlled classifiers, is a two-class classification method introduced in the context of statistical learning theory and structural risk minimization. Support vector machines are basically divided into two groups as linear support vector machines and nonlinear support vector machines. Nonlinear support vector machines are designed to make classifications by creating a plane in a space by mapping data to that higher dimensional input space. This method basically involves solving a quadratic programming problem. In this study, the support vector machines, which have an increasing rate of use in pattern recognition area, are used in the quality control of DNA sequencing data. Consequently, the classification of quality of all the DNA sequencing data will automatically be made as ‘high quality/low quality’.
The proposed method is tested against a dataset created from public DNA sequences provided by InSNP. We first transformed all DNA chromatograms into feature vectors. An optimal hyperplane is first determined by applying SVM to the training dataset. The instances in the testing dataset are then labeled by using the hyperplane. Finally, the estimated class labels are compared against the true labels by computing a confusion matrix. As the confusion matrix reveals, our method successfully determines the labels of 23 out of 24 chromatograms.
We devised a new method to fulfill the quality screening of DNA chromatograms. It is a composition of feature extraction and support vector machines. It has been tested on a public dataset and it provided quite satisfactory results. We believe that it is a strong solution for DNA sequencing institutions to be used in automatic quality labeling of DNA chromatograms.
Life sciences is one of the most demanding disciplines requiring advanced pattern recognition algorithms like clustering and classification techniques that are frequently used in molecular genetics and bioinformatics studies [1, 2]. With the advent of new DNA sequencing techniques in the last two decades, the amount of data used in classification tasks has grown exponentially . One of the main tasks in DNA sequencing centers is to classify the quality of DNA sequencing data [4–6]. Regarding this problem, a quality of DNA data, which is indeed a four-channel time series, needs to be classified as high or low quality. It is a critically important service especially for large DNA sequencing institutions since they need to know in advance before doing further analysis on data. It is also not a good practice to service low quality data to the customers. If the quality of data is known to be low, data might be reproduced by repeating necessary reactions. Manual labeling is prohibitive or, in most cases, impossible because of large data size. It also requires human intervention which is sometimes error prone. Therefore automatic screening tools need to be devised.
In order to estimate the quality of DNA data, the classification method of support vector machines (SVM), whose aim is to find an optimal hyperplane separating two different classes, can be used. It was first developed by Vapnik in 1995 . It is applied to tremendous number of diverse application fields like finance, telecommunication, life sciences and others [8–13]. Deterministic approach, strong mathematical ground, widely available software implementations and ease of use are the key factors of its success .
Our aim in this article is to verify that SVM can be used as a powerful classification technique to classify the quality of DNA chromatograms. In order to show this, we prepared a dataset consisting of publicly available DNA chromatograms . We then manually determined the quality of each chromatogram in the dataset with the help of a bioinformatics expert. We also divided the dataset into training and testing . We then transformed the chromatograms into a suitable format for SVM by extracting a set of feature vectors . The next thing was the learning phase in which SVM was trained to find an optimal hyperplane. Finally, we classified each data in testing dataset according to the trained SVM classifier. We measured the performance of the method by calculating a confusion matrix.
The next two sections are reserved for explaining SVM and its variant in brief. In the fourth section, we explained the details of our approach in four steps: dataset preparation, feature extraction, training and testing. The article finishes with a conclusion.
Linear support vector machines
Linear support vector machines (LSVM) try to find an optimum hyperplane which has the maximum margin among an infinite number of hyperplanes separating the instances in the input space.
Linearly separable case
for all support vectors .
Linearly nonseparable case
In the previous section, we assumed that the data can be perfectly separable in the sense that the data samples of different class labels resides on different regions. However, in practice, such a hyperplane may not be found because of the specific distribution of instances in the data. This makes linear separability difficult as the basic linear decision boundaries are often not sufficient to classify patterns with high accuracy.
values are guaranteed to be positive by the new Lagrange parameters . The parameter C is used to set the weight between the number of support vectors and the number of nonseparable points. In other words, the instances inside the margin band are penalized together with the misclassified instances to reach a better generalization in testing.
The methods in the separable case can be applied to the nonseparable case. The instances for which hold are not support vectors. The remaining part of the equation that defines the parameters w and b can be determined similarly .
Nonlinear support vector machines
When the data is nonlinearly separable, the linear SVM is not profitable. It is better to use a nonlinear SVM for this situation. In order to transform this input, the nonlinear SVM first uses a nonlinear kernel and then a linear SVM. The nonlinear kernel function, which most probably is a large matrix, makes the nonlinear SVM take a very long time when mapping the input to a higher dimensional space . Here, the purpose is to find the separating hyperplane that has the highest margin in the new dimension, the one where the data are transformed.
The majority of the transformations are not known. However, it is possible to get the dot product of the corresponding space using an input vector function .
The most typical kernel functions in the literature where γ , r , τ and d are kernel parameters
Radial basis function
Inverse multi quadratic
In order to evaluate the performance of the proposed method, a benchmark dataset should be available. To create the dataset, we selected 48 DNA chromatograms from InSNP database. The data in each chromatogram is a four-channel time series with different lengths. Each channel is a series of Gaussian shaped peaks along the time axis which contains information about the nucleotides A, C, G and T, respectively.
We cannot give DNA chromatograms directly as input data to SVM. First of all, they have different lengths and are too long. Using whole length data is not a good strategy. So, we should create a set of features representing the statistical characteristics of each data. The number of features should be low and fixed. Also, the features should be chosen to best represent the quality of the chromatograms . To fulfill these requirements, the following set of features are chosen:
1. Average of all values in the data.
2. Standard deviation of all values in the data.
3. Median of all values in the data.
4. Average of all values created for each peak available in all of the four channels of the data.
5. Standard deviation of all values created for each peak available in all of the four channels of the data.
6. Median of all values created for each peak available in all of the four channels of the data.
7. The number of peaks.
8. Average of all values created for each peak available in the combination of channels.
Having determined the features, we processed each data and obtained a feature vector. Each feature vector is accompanied with a label determined in the previous step. In conclusion, we converted the dataset of DNA chromatograms into an input matrix and output vector. From now on, the data is ready for SVM.
SVM needs to be trained before making classification. So, we reserved some of the data for training by randomly selecting 24 data. We need to give a training set into SVM so that it can create a hyperplane. However, we must first adjust the parameters. The most important parameter is C in Eq. 15. We can set if we are sure that the data is linearly separable. We know that it is generally not perfectly linearly separable. So, we give to make it flexible. SVM is then run for the training samples.
We developed a new quality evaluation technique in which the quality of a DNA chromatogram is classified as low or high. In this sense, it is a two-class classification problem for which SVM is chosen. To apply SVM, some sets of features of the chromatograms are extracted. SVM is trained on a training set to learn the hyperplane; SVM is then run on the testing set, from which a confusion matrix is created. As it clearly shows, the results are quite satisfactory as only one mistake was made. Therefore, our method is a good solution for automatic screening of DNA data, especially for large DNA sequencing facilities.
Dedicated to Professor Hari M Srivastava.
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