Materials and Methods

CRISPRidentify: identification of CRISPR arrays using machine learning approach

MATERIALS AND METHODSThe task of CRISPR array detection consists of the search for repetitive elements in a genome, which might form the repeats of a CRISPR array, followed by the evaluation of the identified putative array. The evaluation criteria can include the length of the repeat consensus sequence, the similarity between the repeats within the array, and features of spacers and nearby cas genes. The existing approaches use manually curated evaluation functions that combine these features into a score. CRISPRDetect, for example, calculates the score for an array by adding up positive and negative score contributions, such as +3 if the repeat, which is required to be longer than 23 bases, contains a known motif at the end, and a metric for identity among the spacers (−3 to +1), or −1.5 for dissimilarity between repeats. Arrays with scores above 4.0 are then classified as positive. Although such manually curated scoring functions can work well for the detection of positive examples, they have several disadvantages. First updating the scoring system to a growing database is a complicated task. Second, there is no built-in mechanism to balance sensitivity and specificity. Our analysis described here shows that, whereas the sensitivity of the existing tools is usually high, these tools do not adequately control for the false positive rate and thus have a relatively low specificity.In order to control for both sensitivity and specificity, we set up the CRISPRidentify pipeline, which replaces the manually curated scoring function with an ML approach. The CRISPRidentify pipeline consists of two major steps, namely, a new approach to detect and generate array candidates, aiming at increased sensitivity, and a versatile, machine-learning based evaluation of the detected candidates to increase the specificity. With regard to the first task, any approach to detect CRISPR arrays faces two main subproblems: (i) accurate identification of the start and end positions for each repeat candidate in the array and (ii) accurate identification of the boundaries of the whole array. Mismatches in the repeat sequences or similarity of some spacers around the start and end positions can lead to incorrect identification of the repeat sequence itself. Incorrect identification of the array boundaries could be caused, for example, by degradation of the repeat sequence at the end of the array which makes it difficult to determine the array end point.The second major task is the evaluation of the candidates. CRISPRidentify learns a classifier for CRISPR arrays from known data to control for the false positive rate, in contrast to the manually curated scoring functions used by the existing tools. In this setting, it is natural to rely on: (i) features that are extracted from the array for estimating the similarity between bona fide CRISPR arrays and array candidates, (ii) a benchmark set of positive and negative array examples and (iii) machine learning to learn the evaluation function from this benchmark. We define new training and test sets consisting of carefully selected positive and negative examples. Using these training sets, we train a classifier for evaluating CRISPR candidates based on 13 array-derived features (see Table S2 in Supplementary), such as the similarity between the repeats, AT-content or stability of the repeat hairpin. The principal advantage of this approach is that we not only increase the sensitivity of CRISPR array detection, but also drastically reduce the false positive rate, i.e. increase the specificity. The problem of false positive predictions has been not properly addressed by the previous approaches. Finally, the pipeline annotates the array with additional information such as its orientation, leader sequence and cas genes.Benchmark datasetsOverall, we prepared six benchmark datasets to test different aspects related to the detection of CRISPR arrays. In particular, we compiled the archaeal and bacterial arrays (29,30,38). The first dataset contained 400 archaeal arrays among which 10 were experimentally validated and the rest were manually verified (Table ​(Table1).1). The second dataset containing 600 bacterial arrays was constructed from published datasets (29,30,38), with the constraint that the corresponding repeat sequences should be at least 85% identical to published experimental CRISPR-repeats. For simplicity, we will refer to these datasets as archaeal and bacterial datasets, respectively. As an independent, high-quality, non-overlapping test set, we additionally selected 550 CRISPR arrays from archaeal and bacterial genomes using the following criteria: 1) the minimal number of repeats in each CRISPR array is three, and 2) the maximum distance between the CRISPR array and the cas gene operon is 500 nucleotides. We will call this the Cas-associated dataset (See distribution of Cas types in Supplementary materials Table S7).Table 1.Generated data sets and their combinations used in the machine learning approachNameNumber comb.NameNumber comb.NameSizeTrain & validate2100Archaeal800Archaeal arrays (positive)400Archaeal shuffled arrays (negative)400Bacterial1200Bacterial arrays (positive)600Bacterial shuffled arrays (negative)600False arrays, train (negative)100Test750Cas-associated (positive)550false arrays, test (negative)200CRISPRCasFinder3263annotated arrays (positive)39 371Open in a separate windowThe first three datasets were partially used by other tools, but the fourth one was designed specifically for this work and is required to control the false positive rate. It contains 300 (100 training and 200 test) false CRISPR arrays and was constructed based on the tandem repeat dataset and cases with identical ‘spacers’ (see Tables S2-S4 in Supplementary for more details). For simplicity, we will now refer to this set as false arrays. To obtain more negative examples, we also created additional distorted CRISPR arrays from the ‘archaeal dataset’ and the ‘bacterial dataset’ by independently shuffling the nucleotides in each repeat and each spacer sequences. We will refer to these as archaeal shuffled arrays and bacterial shuffled arrays, respectively. All generated datasets where split into training and test sets.In addition to the datasets we generated, we used two datasets from literature. The first, most comprehensive one consists of all arrays annotated from the most recent CRISPR classifications (2,3,18,19), and will be called annotated arrays dataset. It consists of 39 371 arrays. The second dataset was downloaded from CRISPRCasFinder (34). It contains 3263 arrays and will be denoted as CRISPRCasFinder dataset.Construction of array candidatesWe use Vmatch version 2.3.0 to scan a genomic region for the occurrences of repeat candidates. Vmatch returns a list of putative repeat pairs. By default, in CRISPRidentify, we set the substring length for Vmatch to be in the range of 21 to 55 nucleotides, and the spacing between the matching pairs has to be in the range between 18 and 78 nucleotides. Both intervals can be changed by the user. To avoid duplicates, we filter out all the repeat candidates except for the unique ones.The next step is to cluster the repeat matches found by Vmatch, and to extract the bona fide repeat sequence. As a proxy, the repeat sequence is the consensus sequence of all repeat matches belonging to a repeat cluster. However, using a consensus sequence to determine possible repeat candidates poses two problems. First, the length of the consensus sequence is uncertain because a threshold has to be defined for conserved columns, and second, we lose control over the number of mismatches to existing repeat candidate sequences in the genome. Thus, instead of using a single consensus sequence, as the first stage, we generate a set F over each Vmatch cluster containing all possible repeat candidates and their extensions. The set F is a subset of all subsequences of a maximal consensus sequence partially ordered by the subsequence relation (see Figure ​Figure1,1, box ‘Extension’, for an example).Open in a separate windowFigure 1.General architecture of the CRISPRidentify approach. The pipeline consists of two major steps. In the first step (Candidate Generation), candidate repeats are identified using Vmatch. Vmatch detects all potential pairwise repeats. This implies that leading and trailing nucleotides have an uncertain assignment as these repeated nucleotides could be part of the repeat, or part of a repetitive part of the spacer. For this reason, we first determine the minimal and maximal forms of the repeat by aligning the repeat candidates, and then consider all possible repeats between the minimal and maximal repeat variants. Mapping these repeat candidates back to the genome generates different array candidates, which are then evaluated using machine learning. For that purpose, in the second step (Candidate Evaluation) we extract features from the array such as repeat similarity, the evenness of spacer lengths, AT-content, minimum free energy of the repeat hairpin and others. We then use different machine learning (ML) approaches, learned on a large data sets of positive and negative examples, to classify them as CRISPR arrays. We use the scoring of the ML approaches to prioritize the different array candidates. Finally, we use different tools to annotate the array. Thus, we determine the strand orientation using CRISPRstrand and annotate the leader sequence using CRISPRleader. In addition, HMM and Prodigal to annotate cas genes.To generate F, we use the alignment for each cluster to construct the maximum element of F as a string that is composed, column-wise, of the nucleotides most frequently found in the alignment. The minimum element of the set F is defined as the most consistent substring, and is a subsequence of the maximal element by definition. After the maximum and minimum elements are calculated, we generate all sequences which fulfil the requirements that they 1) are subsequences of the maximum element and 2) contain the minimal element as a subsequence. These sequences then complete the set F. Mathematically, F is a filter (the dual concept of an ideal) in the partial order defined by the subsequence relation. As the last step, the set of repeat candidates is formed via unification of sets F and initial Vmatch results and filtering of duplicates (see Supplementary Materials, Section 1 ‘Enhancement of Vmatch results’). Lastly, we extend the set of repeat candidates with modifications of each repeat candidate in which up to 3 nucleotides are omitted from both ends, resulting in 15 variations of each repeat. After that, duplicates are filtered out for the last time and the remaining candidates are added to the pre-existing clusters. These clustered repeat candidates are then used to form CRISPR candidates.After the set of potential repeat candidates is extracted, we need to form the bona fide CRISPR array candidates. Unlike the other existing algorithms, where a CRISPR array is formed from a repeat consensus by simply splitting the considered interval into repeats and spacers, our approach utilizes approximate string search to find the locations of the repeat sequences using the python regex library available on PyPI regex. This approach also has an important advantage over simple partitioning of the DNA interval into repeats and spacers because this technique allows incorporating insertions and deletions as editing operations, finding the locations that minimize the number of editing operations. This feature allows our approach to effectively identify potential CRISPR candidates including cases with abnormalities, such as truncated repeat (39–41) sequences or complete spacer deletion (see section seven spacer deletion and section eight degenerated repeat in the Supplementary materials).Feature vector associated with an array candidateHaving obtained suitable CRISPR candidates, we generate a feature vector for each candidate to use at the evaluation stage of our pipeline. Users may specify the set of desired features by choosing one or several of the three classifier models we developed. Each of these models is defined by feature selection and encompasses a different combination of the 13 features we devised for classifying CRISPR arrays. When designing these features, we had three conceptual categories in mind, depending on whether we expect their values to be positively or negatively correlated with the likelihood of a true CRISPR array, or whether they provide supporting information. These are, however, only conceptual categories that are not further used during the machine learning approach. Among the factors, which we expect to augment the evidence for a true CRISPR array with their growing value, are number of repeats, length of the repeat sequence and repeat similarity because the more repeats are found, and the longer and more similar they are, the stronger the evidence that the result did not occur by chance.On top of these features, we compute the RNA minimum free energy (MFE) of the consensus repeat sequence. The negative MFE is called the MFE score, and lower MFE scores are positively correlated with the likelihood of a true CRISPR array (22,27,28,42–44) in some systems. The reason is that, in many cases, for instance, in the vast majority of type I cases, the crRNA tends to form stable stem-loop structures, due to the palindromic organization of the repeat sequences. Therefore, a low MFE score can be used to identify stem-loop structures in these systems, thus buttressing the evidence for true crRNA and, by extension, CRISPR array. The MFE score was computed using RNAfold (45). Another feature positively correlated with the likelihood of a true CRISPR array is high similarity between the CRISPR candidate repeat sequence and a verified repeat sequence in the database. The similarity between a given sequence and the sequences in the database is computed using BLAST (46), which is provided as executable with our pipeline. In our approach, two BLAST similarity scores for each CRISPR candidate were computed. The first score reflects the highest similarity between a candidate and all experimentally validated repeat sequences. The second score measures the similarity to a modified version of the same dataset to which one or two mismatches were added, thus extending the range of similarity considered by the first BLAST score. These two scores were treated as two separate features.In contrast to the first group of features, the values of the following features are negatively correlated with the likelihood that a candidate is a true CRISPR array. The first such feature is the number of mismatches because repeats are usually highly conserved. Furthermore, spacers of the CRISPR array candidate were assessed in terms of length and similarity. Because in bona fide CRISPR arrays, the spacer length is typically uniform, a high variance in spacer length negatively correlates with the likelihood of a CRISPR array. With respect to similarity, spacers differ from repeats in that they are generally not significantly similar to each other given their origin from different virus or plasmid genomes. Thus, high level of similarity between spacers (see Supplementary Figure S2 in Supplementary) is negatively correlated with the likelihood of a candidate being a CRISPR array. Another feature that is included in our pipeline is whether a candidate region contains a protein-coding sequence. Because CRISPR arrays do not code for proteins, the presence of coding sequences is negatively correlated with the likelihood of an interval being a CRISPR array. Our pipeline identifies Open Reading Frames (ORFs) using Prodigal (47), again provided as executable with our pipeline. If ORFs were found, the result with the highest confidence value was taken as the feature value. To account for protein coding-regions even more thoroughly, our tool also looks for tandem repeat proteins. To this end, the Prodigal output is fed into a Hidden Markov Model (48), which searches for similarity with the PFAM database of tandem repeat proteins. The resulting tandem protein similarity score serves as a feature negatively correlated with the likelihood of the interval being a CRISPR array.In the third category, our approach computes two features whose growing value is not directly correlated with the likelihood of the interval being a CRISPR array but could potentially guide the likelihood in different ways. The first such feature is the AT-content of the identified repeat sequences within a given CRISPR array candidate. Because bacteria and archaea differ in AT-content, this feature can help to define implicit subclasses that allow the classifier to model different properties for bacterial and archaeal arrays. The other additional feature is average spacer length (see Supplementary Figure S1 in the Supplementary Materials). This feature is important because, in contrast to repeats, spacers in the spacer array can have different lengths. Overly short or long average spacer length, however, might signal a false array, which is why such outliers should be considered in the scoring of the candidates. After all the features determined by the classification model were calculated for each CRISPR array candidate, the resulting up to 13-dimensional feature vector was subject to machine learning classification as described below.Feature selectionAfter the first successful results using all thirteen features described above, we went on to evaluate which of the features are actually necessary. This can be determined by feature subset selection (FSS), which generates a subset of features best suited for classification. For this purpose, we applied a wrapper approach (49), which works as follows. We generated all subsets of the full sets of 13 features. We then trained for each of the 8192 subsets a model using only the features from the respective subset, and assessed the prediction quality. The results of these experiments showed that a subset of the 13-dimensional feature vector is sufficient to reach the 99.8% accuracy that is achieved with the complete feature set. The same accuracy levels could be achieved with three of these models. These models included 8, 9 and 10 features, respectively, with each feature appearing in at least one of the models. Although the feature combinations overlapped, they were not subsets of one another (see Table S1 in Supplementary for the models and their feature subsets).Classification of CRISPR arrays using machine learningThe last step of our CRISPR identification pipeline consists of the evaluation of the obtained CRISPR candidates. For this purpose, we integrated the implementation of the Extra trees classifier from the python Scikit-learn package (50) into our pipeline. We provide the user with an opportunity to pick either of the three pretrained classifiers that we describe in the Feature Selection and Importance Analysis subsection or use all of them simultaneously. It is important to notice that we do not use the class prediction directly but take the label pseudo-probability and treat it as a certainty score. In case of using many classifiers, the score is computed as the average of scores. Based on the distribution of the output CRISPR array scores, we introduced two decisive score values around which we structured the labels: 0.75 and 0.4. CRISPR arrays with scores greater than or equal to 0.75 were labeled to ‘candidate’, meaning that one of them likely corresponds to a true CRISPR array. CRISPR arrays with scores between 0.75 and 0.4 were assigned to the group ‘potential candidate’, meaning that there is a smaller probability for one of them to be a true CRISPR array. The CRISPR candidates with scores lower than 0.4 were assigned to the group ‘low score candidates’, indicating that they are probably not valid CRISPR arrays. After performing the grouping, the algorithm picks the ‘bona fide candidate’ as the candidate with the highest score among the candidates in the ‘possible candidate’ group. Then all the candidates are reported in files corresponding to their groups. Hence, the user may not only inspect the bona fide candidates but also all the alternatives with their respective statistics.Repeat and spacer databasesOur tool can also be used to build a user-specific CRISPR array databases. In order to illustrate the potential of this feature, we used data from archaea and bacteria genomes. The archaeal dataset consisted of 251 complete and 736 incomplete genomes, while the bacterial dataset included 1693 complete genomes and 25335 incomplete ones. Having identified CRISPR arrays in each dataset with our tool, we extracted the complete set of spacers as well as the consensus repeat sequence for each CRISPR array. Using these results, we built two databases, one for the identified repeats and the other one for spacers. The output of the repeat database lists, for each consensus repeat sequence, all genomic locations, the organisms, as well as the number of locations and organisms where an array with this consensus repeat can be found (see Table S6 and Figure S6 in Supplementary).

Article TitleCRISPRidentify: identification of CRISPR arrays using machine learning approach

Abstract

CRISPR–Cas are adaptive immune systems that degrade foreign genetic elements in archaea and bacteria. In carrying out their immune functions, CRISPR–Cas systems heavily rely on RNA components. These CRISPR (cr) RNAs are repeat-spacer units that are produced by processing of pre-crRNA, the transcript of CRISPR arrays, and guide Cas protein(s) to the cognate invading nucleic acids, enabling their destruction. Several bioinformatics tools have been developed to detect CRISPR arrays based solely on DNA sequences, but all these tools employ the same strategy of looking for repetitive patterns, which might correspond to CRISPR array repeats. The identified patterns are evaluated using a fixed, built-in scoring function, and arrays exceeding a cut-off value are reported. Here, we instead introduce a data-driven approach that uses machine learning to detect and differentiate true CRISPR arrays from false ones based on several features. Our CRISPR detection tool, CRISPRidentify, performs three steps: detection, feature extraction and classification based on manually curated sets of positive and negative examples of CRISPR arrays. The identified CRISPR arrays are then reported to the user accompanied by detailed annotation. We demonstrate that our approach identifies not only previously detected CRISPR arrays, but also CRISPR array candidates not detected by other tools. Compared to other methods, our tool has a drastically reduced false positive rate. In contrast to the existing tools, our approach not only provides the user with the basic statistics on the identified CRISPR arrays but also produces a certainty score as a practical measure of the likelihood that a given genomic region is a CRISPR array.


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