Transcription factor (TF) binding to its DNA target site is a fundamental regulatory interaction. The most common model used to represent TF binding specificities is a position specific scoring matrix (PSSM), which assumes independence between binding positions. However, in many cases, this simplifying assumption does not hold. Here, we present feature motif models (FMMs), a novel probabilistic method for modeling TF–DNA interactions, based on log-linear models. Our approach uses sequence features to represent TF binding specificities, where each feature may span multiple positions. We develop the mathematical formulation of our model and devise an algorithm for learning its structural features from binding site data. We also developed a discriminative motif finder, which discovers de novo FMMs that are enriched in target sets of sequences compared to background sets. We evaluate our approach on synthetic data and on the widely used TF chromatin immunoprecipitation (ChIP) dataset of Harbison et al. We then apply our algorithm to high-throughput TF ChIP data from mouse and human, reveal sequence features that are present in the binding specificities of mouse and human TFs, and show that FMMs explain TF binding significantly better than PSSMs. Our FMM learning and motif finder software are available at http: //genie. weizmann. ac. il/.
Precise control of gene expression lies at the heart of nearly all biological processes. An important layer in such control is the regulation of transcription. This regulation is preformed by a network of interactions between transcription factor proteins (TFs) and the DNA of the genes they regulate. To understand the workings of this network, it is thus crucial to understand the most basic interaction between a TF and its target site on the DNA. Indeed, much effort has been devoted to detecting the TF–DNA binding location and specificities. Experimentally, much of the binding specificity information has been determined using traditional methodologies such as footprinting, gel-shift analysis, Southwestern blotting, or reporter constructs. Recently, a number of high-throughput technologies for identifying TF binding specificities have been developed. These methods can be classified into two major classes, in vitro and in vivo methods. In vitro methods can further be classified to methods that select high-affinity binding sequences for a protein of interest [1], [2] (reviewed in Elnitski et al. [3]), and high-throughput methods that measure the affinities of specific proteins to multiple DNA sequences. Examples of the latter class of methods include protein binding microarrays [4]–[6] and microfluidic platforms [7], which claim to achieve better measurement of transient low affinity interactions. The in vivo methods are mainly based on microarray readout or high throughput sequencing technologies readout of either DNA adenine methyltransferase fusion proteins (DamID) or of chromatin immunoprecipitation DNA-bound proteins (ChIP-chip, ChIP-PET, ChipSeq/Chip-seq) [8]–[16]. The in vivo methods were recently used to characterize the binding specificities of all TFs in the yeast Saccharomyces cerevisiae [8], [9], [17]–[21] and, more recently, to identify genomic targets in mammalian cells [10]–[16], [22]–[26]. However, despite these technological advances, distilling the TF binding specificity from these assays remains a great challenge, since in many cases the in vivo measured targets of a TF do not have common binding sites, and in other cases genes that have the known and experimentally determined site for a TF are not measured as its targets. For these reasons, the problem of identifying transcription factor binding sites (TFBSs) has also been the subject of much computational work (reviewed by Elnitski [3]). The most common approaches start by defining sets of genes that are potentially coregulated, either from clusters of coexpressed genes in microarray data [27], from functional annotations of genes [28], or from TF chromatin immunoprecipitation [17], [29]. They then attempt to identify regulatory elements by searching for common motifs in the promoter regions of the genes in each group [30]–[35]. Recently, Eden et al. [36] developed a discriminative motif finder that is well suited for finding motifs in a ChIP-chip experiment. Their method combines the search for a cutoff that defines the positive set, with the search for a motif that discriminates the positive set from the rest of the chip probes. Other approaches work in the opposite direction, by first reducing the sequence data into some predefined features of the gene (e. g. , presence or absence of all DNA-words of length 6–7), and then identifying the putative binding sites, for example, by keeping the features, or combinations thereof, whose genes are coexpressed [37]–[39]. Recently, this latter approach was extended to comparative genomic methods that filter the initial library of features to those that show high conservation in evolutionarily closely related species [40]–[42]. The experimental and computational approaches above revealed that TFBSs are short, typically 6–20 base pairs, and that some degree of variability in the TFBSs is allowed. For these reasons, the binding site specificities of TFs are described by a sequence motif, which should represent the set of multiple allowed TFBSs for a given TF. The most common representation for sequence motifs is the position specific scoring matrix (PSSM), which specifies a separate probability distribution over nucleotides at each position of the TFBS. The goal of computational approaches is then to identify the PSSM associated with each TF. Despite its successes, the PSSM representation makes the strong assumption that the binding specificities of TFs are position-independent. That is, the PSSM assumes that for any given TF and TFBS, the contribution of a nucleotide at one position of the site to the overall binding affinity of the TF to the site does not depend on the nucleotides that appear in other positions of the site. In theory, it is easy to see where this assumption fails. For example, consider the models described in Figure 1. The TFBS (Figure 1A) data contains only “CG” or “GC” in the center positions. Although the PSSM learned from this data (Figure 1B) assigns high probability to these nucleotide pairs, it also undesirably (and unavoidably) assigns high probability to “CC” and “GG” in the center positions. However, if instead of the PSSM representation, we allow ourselves to assign probabilities to multiple nucleotides at multiple positions, we could use the same number of parameters to specify the desired TF binding specificities (For example, consider the model illustrated in Figure 1C, which uses two parameters that are each associated with two positions to give exact description of the binding specificities over the center positions). This observation lies at the heart of our approach. From the above discussion, it should be clear that the position-independent assumption of PSSMs is rather strong, and that relaxing this assumption may lead to a qualitatively better characterization of TF motifs. Indeed, recent studies revealed specific cases in which dependencies between positions may exist [7], [43]. Several models were developed to capture such dependencies (see [44] for a brief review). These models can be classified into two main classes: Markov chains based models [44], [45] and Bayesian Network based models [46]–[49]. In the first class, the dependencies between neighboring positions are modeled using a Markov model of some order. A recent representative of this class is the permutated variable length Markov model (PVLMM) of Zhao et al. , which incorporates two major improvements: it searches for the best permutation of the motif positions, and it reduces the number of parameters by using a context tree representation for the Markov model representation. Although Markov chain based models may perform well in some datasets, they have a limited ability to model dependencies between more distant positions. Since modeling these dependencies by increasing the order of the Markov model exponentially increases the size of the model representation, it was suggested to search for a permutation of the binding site positions that produces the best model. However, this is only a partial solution, as it poses a limitation on the model learned and easily becomes computational intensive for long motifs (Zhao et al. limit their motifs to length 9 bp). The second class of models was proposed by Barash et al. [46], who developed a Bayesian network approach to represent higher order dependencies between motif positions. They showed that these models predict putative TFBSs in ChIP-chip data with higher accuracy than PSSMs. Ben-Gal et al. [47] extended this approach by using a context dependent representation of the conditional probability distributions, which, to some extent, reduces the representation size (depending on the data). Zhou et al. suggested a simpler Bayesian network model (GWM) where only dependencies between nonoverlapping positions are modeled [49]. Pudimat et al. also extended the Bayesian network framework by adding structural DNA features [48]. However, the Bayesian network representation, due to its acyclicity constraints, imposes nonnatural restrictions on the motif structure, and its conditional probability distributions limit the number of dependencies that can be introduced between positions in practice, due to the exponential increase in the number of parameters introduced with each additional dependency. Although some of these issues may be addressed, e. g. , using sparse conditional probability distribution representations, Bayesian networks do not seem to be the ideal and most intuitive tool for the task. Another class of TF binding specificities models that is complementary to the above two is a mixture of models. In the above mentioned work, Barash et al. also used a mixture of PSSMs to model TFBSs. In this representation, each motif is modeled as a mixture of PSSMs each defining a different mode of binding. This approach was later extended as a part of the LOGOS [50] and MotifBooster [51] motif finding software. However, this approach does not explicitly represent dependencies between binding site positions. Here, we propose a novel approach for modeling TFBS motifs, termed feature motif models (FMMs). Our approach is based on describing the set of sequence properties, or features, that are relevant to the TF–DNA interactions. Intuitively, the binding affinity of a given site to the TF increases as it contains more of the features that are important for the TF in recognizing its target site. In our framework, features may be binary (e. g. , “C at position 2, and G at position 3”) or multivalued (e. g. , “the number of G or C nucleotides at positions 1–4”), and global features are also allowed (e. g. , “the sequence is palindromic”). Each feature is assigned a statistical weight, representing the degree of its importance to the TF–DNA interaction, and the overall strength of a TFBS can then be computed by summing the contribution of all of its constituent features. We argue that this formulation captures the essence of the TF–DNA interaction more explicitly than PSSMs and other previous approaches. It is easy to see that PSSMs are a special case of FMMs, since a PSSM can be described within our framework using four single nucleotide features per position. Our approach can also naturally represent complex and distant dependencies efficiently, thereby overcoming a limitation of other models that have been proposed. The rest of the paper is organized as follows: The Results section starts with a brief overview of our methodology. We then validate our approach by learning FMMs from synthetic and real aligned TFBS data. Next, we devise a novel motif finder algorithm that finds motifs in a set of unaligned sequences, and validate its performance on yeast TF ChIP data [17]. Using this motif finder, we demonstrate the benefits of using FMMs instead of PSSMs as a basic building block of a motif finder, which represents TF binding motifs. Finally, we present insights that we gained from learning FMMs for two human TFs, CTCF and c-MYC. In the Methods section we discuss the details and the mathematical formulation of our FMM approach. The problem of learning an FMM from TFBS data is quite difficult, as it reduces to structure learning in Markov networks, a paradigm that is still poorly developed. In the Methods section we elaborate on our learning strategy and suggest an improved methodology for optimizing the data likelihood, which we define as our objective function.
We first briefly describe the FMM representation, and how it is learned from aligned TFBS sequences. Next, we give a high-level view of our motif finder, that finds motifs in unaligned sequences, and allows their representation as FMMs. All of the algorithms described here are available as downloadable software or as an online web service at our web site: http: //genie. weizmann. ac. il/. See the Methods section for a more elaborate description. We now present an experimental evaluation of our FMM learning approach. First, we used synthetic data to tune the free parameter of the penalty term and to test whether our method can reconstruct sequence features that span multiple positions when these are present. We then compared the ability of our approach to that of PSSMs on learning real binding site specificities of human TFs from two datasets of TFBS [10], [13]. Next, we validated the ability of our motif finder to find TFBS motifs in yeast ChIP-microarry data [17]. We show that its performance is comparable to state-of-the-art motif finders. Finally, we compiled a collection of high throughput human and mouse TF ChIP datasets, and used our motif finder to learn de novo motifs for each of the TFs. We show that our FMM approach learns the binding specificities of these TFs better than the PSSM approach. Before integrating our algorithm for learning FMM from aligned TFBS data into our motif finder algorithm, we separately evaluated it in a controlled setting. As an initial test for our method, we wanted to evaluate the ability of our algorithm to learn sequence features that span multiple positions when such exist, and to avoid learning such features when none exist. For this purpose, we manually created eight sequence models of varying weights and features (which we will refer to as “true” models), and learned both PSSM and FMMs from aligned TFBSs that we sampled from them (Figure 3). Our eight sequence models contained three manually-curated models with features over single and double positions (we denoted these models as Synthetic model I–III). In order to make sure that our model does not learn double position features when none exist, we used as our true model the PSSMs of yeast GCN4 and SWI5 TFs from MacIsaac et al. [29] (denoted GCN4 PSSM and SWI5 PSSM). Even though the relatively small size of the MacIsaac et al. dataset may cause overfitting when used directly for FMM learning, sampling from such an FMM may give a dataset with relatively realistic dependencies, of the type that our model should learn. Thus, we learned also FMMs of yeast GCN4 and SWI5 TFs from the MacIsaac et al. dataset [29], and used them as relatively realistic synthetic models (denoted GCN4 FMM and SWI5 FMM). Finally, in order to test the performance of our approach when strong dependencies are present, we created a model with strong dependencies by adding eight double position features to the GCN4 PSSM model (denoted GCN4 PSSM++). We evaluated the learned models by computing the log-likelihood that the learned models assign to a test set of 10,000 unseen TFBSs sampled from the true model, and by computing the Kullback–Leibler (KL) distance between distributions of the true and learned models. The larger the test set likelihood and the smaller the KL distance, the better the reconstruction. We evaluated two specific aspects of our approach: the dependency of the learning on the penalty term free parameter, α, and the minimum number of samples needed for learning FMMs. We repeated each experiment setting three times. We first tested the effect of the penalty term free parameter, α, on the quality of the model reconstruction. Intuitively, the larger the value of α, the harder it is to learn large weights. Thus, the value of α correlates with our prior belief that the model is simple (close to uniform). Therefore, there is a trade off between setting α to large values that decrease the risk of overfitting but might miss important features, and setting it to small values that might allow overfitting but also increase the ability to learn features. Since larger input datasets have relatively larger values for the gradients, a given value of α allows more weak features to be learned in large sets as compared to smaller datasets. This effect fits well with the notion of lower noise in larger datasets (see Lee et al. [55] and references within for an exact analysis). To this end, we varied α in the range of 10−6 to 100, while using a varied number of 10–1,000 input sequences. The results in the range 10−1≤α≤10 and 50–1,000 input sequences are shown in Figure S2 in terms of test set likelihood. The results show that in the range tested the best overall reconstruction performance is achieved for α≈1. While smaller values tend to allow overfitting, higher values pose harsh constraints on the learned model and learn too few features. Though the effect is stronger for small datasets, α≈1 seems to give good performances also for relatively large datasets. Second, we estimated the minimum number of samples needed for learning FMMs, by sampling different training set sizes in the range of 10–500. In these experiments, we fixed the penalty term free parameter to α = 1. As can be seen in Figure 3, in six out of eight cases, our model reconstructs the true model with high accuracy even with a modest number of ∼100 input TFBSs, and reconstructs the true model nearly perfectly with 200 or more samples. For the more complex GCN4 FMM and SWI5 FMM models, although high accuracy is achieved with ∼200 samples, adding ∼200 more samples further improves the model accuracy. As can be seen from the GCN4 and SWI5 PSSM models, although less than 100 samples might cause overfitting, due to under sampling of the distribution space, when the sample number is sufficient no dependencies are learned. As expected, when the true model includes dependencies between positions, our model significantly outperforms the PSSM, in some cases even when only 20 input sites were used. Examining the learned features, we found that for a sample size of 100 or more, only features that appeared in the true model were learned with significant weights. Our results thus show that when we use synthetic data we can successfully learn FMMs, even with a modest setting of 100 input sequences. These numbers are surprisingly small considering the number of parameters examined. Using smaller sets which represent under sampling of the TFBS distribution space or sets that contain noise may lead to overfitting. Therefore, we expect that similar analyses, using a cross validation scheme, on datasets taken from TRANSFAC [56], JASPAR [57] and MacIsaac et al. [29] to be less successful, as most of these datasets are small or contain a considerable amount of noise. Nevertheless, these numbers are far below those for data generated by current genome wide experiments (such as ChIP-chip and ChIP-seq), so our approach is valid for learning TFBS data in realistic settings. Having validated our approach on synthetic data, we next applied it to TFBSs data of human TFs. Our goal was to identify whether FMMs can describe the sequence specificities of human TFs better than PSSMs. To that end, we compared FMMs and PSSMs that were learned from the same sets of aligned TF binding sites. We chose three published sets of aligned binding sites sequences of two important human TFs. The first set contains aligned NRSF binding sites published by Johnson et al. [10]. NRSF binds a DNA element called the neuron-restrictive silencer element (NRSE), canonically described as a 21 bp element. Johnson et al. found 1,655 regions that were enriched for such canonical NRSEs in two independent experiments. The two other sets contain aligned predicted CTCF binding sites published by Kim et al. [13]. They mapped CTCF binding sites through a ChIP-chip experiment, and found a 20 bp motif, defined by a PSSM, that appears in 75% of the binding sites. Using this PSSM motif (and by constraining positions 6,11,14, and 16 to match the consensus) the authors predicted 31,905 CTCF binding sites, 12,799 of which are conserved in at least one more vertebrate. We will refer to these two sets of predicted sites as “CTCF predicted” and “CTCF predicted conserved” sites, respectively. Clearly, our choice of input sets is not biased in favor of the FMM. The canonical NRSE motif is characterized by a highly informative PSSM and the two CTCF sets were extracted from the genome using an initial PSSM representation, thus may contain a bias towards independence of different positions. For each input set we tested whether FMM represents the TFBSs better than PSSM using the following 10-fold cross validation (CV) scheme. Each input set was partitioned into ten subsets. Ten CV groups were created, where in each one a different subset was used as test data, while the other nine were used as training set from which both an FMM and a PSSM were learned. For each CV group, we computed the average likelihood of the test TFBSs according to both the PSSM and FMM, as a measure for the learned sequence model success in representing the binding specificities. The difference between the log average FMM likelihood and the log average PSSM likelihood expresses the improvement of the FMM over the PSSM. The mean and standard deviation for these differences were calculated over the ten CV groups. The results for all three input sets are shown in Figure 4A. The FMM model was found to provide 1. 3–1. 4-fold more likely representations of the binding specificities of the above TFs than the PSSM, with high significance (above 5 standard deviations over the ten CV groups). Figure 4B and 4C show the PSSM and FMM features expectations logo for CTCF predicted conserved BSs. Although four positions were forced to match the consensus, our FMM recognizes several inter-position dependencies. We will discuss the details of the CTCF motif at the end of the Results section, where we use unaligned CTCF bound regions from the same work [13] to derive de novo FMM using our motif finder. Figure 4D and 4E show the PSSM and FMM features expectations logo for NRSF. Notably, the FMM found that “T” at position 11 has strong dependencies to the consensus sequence at positions 12–14 and 16. On the other hand, “C” at position 6 prefers either “C” or “G” at position 11. These results show that the improvement of the FMM over the PSSM is due to the representation of inter-position dependencies by the FMM. As previously described, our motif finder algorithm consists of two steps. The first step results in a collection of K-mer set Motif Models (KMMs). Each KMM is a set of K-mers that defines an enriched motif, and can be used to extract a set of aligned TFBSs from the input positive sequences (see Methods section). These aligned TFBSs are input to the second step, where a motif model is learned from them, be it a FMM or a PSSM. A question arises, then: do KMMs found by the motif finder represent true motifs? Here we show that the KMMs found by our motif finder indeed represent true motifs, and are comparable in quality to motifs found by common motif finding software. In order to evaluate our motif finder' s performance we chose the dataset of Harbison et al. [17]. Although this dataset contains less information for each TF than more recent experiments that used ChIP followed by tilling array or parallel sequencing technologies, it is the most comprehensive study done for TF–DNA interactions and was used by many motif finding software for performance comparison. This data includes 238 sets of sequences of regions which a TF binds under a specific condition (238 TF-condition sets). The 238 datasets represent 146 TFs in various conditions. For 111 TFs (198 datasets), a motif was published by MacIsaac et al. [29]. The MacIsaac et al. motifs were found by two independent motif finding software which use conservation information and were augmented by the authors using literature-known motifs. These motifs were considered by us as biologically true motifs in order to asses the quality of our motifs. For each of the sets we took the sequences from all of the other 237 sets as a negative set. The total number of sequences in all sets is 6,725, with sets ranging between 10 and 195 sequences. Out of the 238 sets, we discarded those with less than 35 sequences, leaving 123 TF-condition sets, for 78 distinct TFs. The choice of the 35 sequences threshold is discussed in Protocol S1. In order to distinguish between biologically relevant motifs and motifs that can appear by chance, we followed the following procedure. We partitioned the Harbison et al. TF-condition sets into 15 bins according to their sizes. The bins were tagged by the center set sizes, [10,20, …, 100,120, …, 200]. For example, bin “50” contained all TF-condition sets of sizes 45–54. For each bin “X, ” we generated 1,000 sets of X sequences that were randomly picked out of the entire collection of 6,725 Harbison et al. microarray sequences. For each set, all remaining sequences out of the 6,725 sequences were considered as a negative set. We ran our motif finder on all 123 true and 15,000 random sets and computed the best motif MHG p-value. We then assessed for each TF-condition set the percentage of random sets in its bin that got a motif with lower MHG p-value. We considered this percentage as the empirical p-value for getting such MHG p-value for a random set. Hence, we considered this empirical test as an assessment of the percentage of false positive motifs for a given MHG p-value and a given set size. Figure 5A shows the fraction of TF-condition sets that contain a motif with a MHG p-value that is better than the empirical threshold as defined by the x-axis for the set bin. It is clear that the results were distinctively better for TF-condition sets than what would be expected by random. As a threshold for biologically relevant motifs we chose for each bin a MHG p-value that allowed a random motif finding rate of 16%. At this threshold we found motifs for 81% of the TF-condition sets. Thus, our results suggest that we found true motifs for at least 65% of the TF-condition sets. Having chosen biologically relevant motif MHG p-value thresholds, we next compared the KMM motifs found by our motif finder for the TF-condition sets with the motifs published by MacIsaac et al. [29]. For each TF-condition set, we allowed the motif finder to output up to five unique motifs. Only those that passed their sets' bin threshold were considered. We compared our KMM motifs to the MacIsaac et al. PSSMs by learning a PSSM representation of each KMM, and comparing this PSSM with the respective MacIsaac et al. PSSM, relying on a method previously used by Narlikar et al. [58]. (For a complete description of the motif comparison method, see Protocol S1.) A summary of this comparison is shown in Figure 5B. For 66% of the sets we found motifs similar to those found by MacIsaac et al. These results, although they can only be approximately compared with recently published results by Narlikar et al. [58] and Eden et al. [36], show that our motif finder does not fall behind state of the art motif finders, and is at least comparable to other methods. In Protocol S2, we further compare our motif finder to other motif finders, demonstrating that our motif finder has advantages over other motif finders. Thus, we can use the first step of our motif finder to produce TFBSs data for an FMM-PSSM comparison. In a previous section, we compared FMMs and PSSMs learned from aligned TFBS data. Although there are databases that contain sets of aligned TFBS [56], [57], these databases usually contain a relatively small number of TFBS for each TF and contain repeating sequences. Learning FMMs from such datasets, although possible in many cases, might lead to overfitting and is problematic for testing in a cross validation scheme. Recent experiments, however, produced larger sets of TF bound regions. After validating our motif finder on smaller sets, we can now use it to produce aligned TFBS data for a comparison of our FMM approach to the PSSM approach. Table 1 summarizes the collection of datasets that we used. For each dataset, since it contains only positive sequences, we generated a negative set (as described in Protocol S1). We scanned each dataset for de novo motifs in a 5-fold cross validation (CV) scheme. We considered the top motif as the true TF motif. From manual examination of the motifs and comparison of the top motif to the literature, this assumption seems to hold (except for some differences in the Nanog_Boyer and E2F4_Boyer sets motifs). In order to compare the learned FMM to the learned PSSM, we assumed that each sequence in the positive set has at least one TFBS. We computed for each test positive sequence the top motif' s FMM and PSSM binding probabilities over all possible locations on the sequence. Following our assumption, for the FMM, as for the PSSM, the best binding probability was considered as the sequence likelihood to be bound by the TF. Figure 6 shows the improvement of our FMM approach over PSSM in terms of test and train log of the average likelihood. The results clearly show that the likelihood of the maximal-likelihood site is better under the FMM model than under the PSSM, and the results are significant in terms of standard deviation over the CV groups. For more than 50% of the sets, the FMM is at least 2-fold more likely to represent the TFs binding specificities. The entire collection of motifs found for all datasets appears in Protocol S2. We focus next on our results for three important human TFs. For the first two, c-Myc and CTCF, we discuss their best FMM and PSSM motifs, and show how their FMM motifs reveal intriguing insights about their binding specificities, that are missed by the PSSM, and that may be correlated with previously published experimental results. For the third, STAT1, we found several motifs, exhibiting the cooccurrence of STAT1 and other TFs binding sites.
In this paper we present feature motif models (FMMs), a novel probabilistic method for modeling the binding specificities of TFs. The current dominant model used is the position specific scoring matrix (PSSM), which assumes independence between binding motif positions. The richer FMM formulation may be viewed as a generalization of the PSSM model, enabling complex position dependencies to be captured, as well as other high-level features (e. g. , palindromes). In this work, we used FMM models that extend the PSSM by capturing dinucleotide dependencies. To show that FMM models describe TF binding motifs better than PSSM models, we compared their likelihoods over held-out synthetic and real data. The real biological data included both aligned TFBS data and unaligned TF bound regions data. We showed that for all types of data, the FMM representation of motifs outperforms the PSSM representation. Importantly, FMMs can be presented using a clear and easy to understand logo, where important position dependencies are plainly visible. Examining our FMM results for two human TFs, c-Myc and CTCF, we found intriguing dinucleotide features that may be important for their binding. Some of those features are well-correlated with previously published results, while others may provide hypotheses on the binding specificities of these TFs. These hypotheses can be further studied experimentally to gain better understanding of how the TF recognizes its binding sites. Notably, in those examples, the FMMs hint at the importance of positions that are regarded uninformative by the PSSMs. In order to allow de novo FMM motif finding, we developed a novel motif finder. Our motif finder finds motifs that are discriminatively enriched in a positive set of unaligned sequences over a negative set of unaligned sequences. For each motif, it learns either an FMM or a PSSM representation. An important property of our motif finding algorithm is that it extracts enriched sets of K-mers from the data, thus maintaining dependencies between positions, if such exist. We show that our motif finder performs well and use it to demonstrate how FMMs can easily be integrated as a basic building block of a motif finding software which represents TF binding specificities. As a future direction, we suggest to integrate the FMM into both common and novel state-of-the-art motif finding algorithms. We demonstrated the benefits of using log-linear models (a representation of Markov networks) for representing important features of TF binding specificities, and suggested a methodology to learn such features from both aligned and unaligned input sequences. In the Methods section we also contribute to the general problem of learning log-linear models by suggesting a methodology for optimizing the objective function, which may give better performance under settings that require approximation. There are several directions for refining and extending our FMM approach. First, our rich framework can model many other types of features. Examples of features that can be added are: to what extent is the sequence a palindrome and the structural curvature of the sequence. Another direction is to add to our learning process the ability to learn binding energies associated with a given set of sequences. Finally, using our models as an improved basic building block, we can integrate it into higher level regulatory models (e. g. , [65]) and obtain a much better quantitative understanding of the underlying principles of transcriptional regulatory networks.
We now present our approach for representing TF binding specificities. Much like in the PSSM representation, our goal is to represent commonalities among the different TFBSs that a given TF can recognize, and assign a different strength to each potential site, corresponding to the affinity that the TF has for it. The key difference between our approach and the PSSM is that we want to represent more expressive types of motif commonalities compared to the PSSM representation, in which motif commonalities can only be represented separately for each position of the motif. Intuitively, we think of a TF–DNA interaction as one that can be described by a set of sequence features, such as pairs or triplets of nucleotides at key positions, which are important for the interaction to take place: the more important features a specific site has, the higher affinity it will have for the TF. One way to achieve the above task is to represent a probability distribution over the set of all sequences of the length recognized by the given TF. That is, for a motif of length L, we represent a probability distribution over all 4L possible L-mer sequences. Formally, we wish to represent a joint probability distribution P (X1, …, XL), where Xi is a random variable with domain {A, C, G, T} corresponding to the nucleotide at the ith position of the sequence. However, rather than representing this distribution using the prohibitively large number of 4L−1 independent parameters, our goal is to represent this joint distribution more compactly in a way that requires many fewer parameters but still captures the essence of TF–DNA interactions. The PSSM does exactly this, but it forces the form of the joint distribution to be decomposable by positions. Barash et al. [46] presented alternative representations to the PSSM, using Bayesian networks, that allow for dependencies to exist across the motif positions. However, as discussed above, the use of Bayesian networks imposes unnecessary restrictions and is not natural in this context. A more natural approach that can easily capture our above desiderata is the framework of undirected graphical models, such as log-linear representation of Markov networks (log-linear model), which have been used successfully in an increasingly large number of settings. As it is more intuitive for our setting, we focus our presentation on log-linear models. Let X = {X1, …, XL} be a set of discrete-valued random variables. A log-linear model is a compact representation of a probability distribution over assignments to X. The model is defined in terms of a set of feature functions fk (Xk), each of which is a function that defines a numerical value for each assignment xk to some subset Xk ⊂ X. Given a set of feature functions F = {Fk}, the parameters of the log-linear model are weights θ = {θk: fk ∈ F}. The overall joint distribution is then defined as: (1) is the partition function that ensures that the distribution P is properly normalized (i. e. , ∑x∈X P (x) = 1), and xk is the assignment to Xk in x. Although we chose the log-linear model representation, we note that it is in fact equivalent to the Markov network representation, and the mapping between the two is straightforward. We now demonstrate how we can use this log-linear model representation in our setting, to represent feature-based motifs. We start by showing how PSSMs can be represented within this framework. In the previous section, we presented our feature-based model for representing motifs. Given a collection of features F, our method uses the log-linear model to integrate them, as in Equation 1. As we showed, the standard PSSM model can be represented in our framework. However, our motivation in defining the model was to allow for integration of other features, which may span multiple positions. A key question is how to select the set of features for a given model. In this section, we address this problem. Since log-linear models are equivalent to Markov networks, our problem essentially reduces to structure learning in Markov networks. This problem is quite difficult, since even the simpler problem of estimating the parameters of a fixed model does not have an analytical closed form solution. Thus, the solutions proposed for this problem have been various heuristic searches, which incrementally modify the model by adding and deleting features to it in some predefined scheme [55], [66]. We now present our algorithm for learning a feature-based model from TFBSs data. Our approach follows the Markov network structure learning method of Lee et al. [55]. It incrementally introduces (or selects) features using the grafting method of Perkins et al. [54]. We first present the simpler task of estimating the parameters of a given model, as this is a sub-problem that we need to solve when searching over the space of possible network structures. Above, we developed our approach for estimating the feature parameters for a fixed model in which the feature set F is defined. We now turn to the more complex problem of automatically learning the set of features from aligned TFBSs data. This problem is an instance of the more general problem of learning the structure of Markov networks from data. However, quite surprisingly, although Markov networks are used in a wide variety of applications, there are very few effective algorithms for learning Markov network structure from data. In this paper we followed the Markov network structure learning approach suggested by Lee et al. [55]. This approach extends the learning approach of Perkins et al. [54] to learning the structure of log-linear representation of Markov networks using the L1-Regularization over the model parameters. To incorporate the L1-Regularization into our model we need to introduce a Laplacian parameter prior over each feature, leading to the modified objective function: (6) where and log P (D|θ, M) is the data likelihood function as in Equation 2. Taking the log of this parameter prior and eliminating constant terms, we arrive at the final form of our objective function: (7) It is easy to see that this modified objective function is also concave (as it is an addition of a concave function and a linear function) in the feature parameters θ and we can thus optimize it using the same conjugate gradient procedure described above. We then follow the grafting approach of adding features in a stepwise manner. In each step, the algorithm first optimizes the objective function relative to the current set of active features F, and then adds the inactive feature fi ∉ F with the maximal gradient at θi = 0. Using an L1-Regularized concave function provides a stopping criteria to the algorithm that leads to the global optimum [54]. The L1-Regularization has yet another desirable quality for our purpose, as it has a preference for learning sparse models with a limited number of features [55]. It has long been known to have a tendency towards learning sparse models, in which many of the parameters have weight zero [69] and theoretical results show that it is useful in selecting the features that are most relevant to the learning task [70]. Since the grafting feature addition method is a heuristic, it seems reasonable that features that were added at an early stage may become irrelevant at later stages, and hence get a zero weight. We thus introduce an important difference from the method of Lee et al. , by allowing the removal of features that become irrelevant. In the previous sections we described how to learn an FMM model from aligned TFBS data. We now turn to the more complex problem of finding de novo FMM elements that are enriched in a target set of relatively long and unaligned sequences compared with a background set. Recent years have shown a development of several high throughput methods reviewed in the introduction. The most dominant methods include chromatin immunoprecipitation (ChIP) of DNA-bound proteins followed by either DNA chip (ChIP-chip) [12], [13], [16] or high throughput sequencing (ChIP-PET, ChIPSeq/ChIP-seq) [10], [11]. The common analysis of these two methods usually includes a step of peak finding (or fitting) [10], [21] which results in a set of unaligned DNA sequences that are bounded by the TF with a measure of intensity. A common practice is then to define an intensity cutoff. This cutoff defines a target set of sequences bound by the TF (positive set), while the sequences below some cutoffs are defined as background set (negative set). The problem of optimally determining such a cutoff was previously addressed by Eden et al. [48]. We, however, do not address it, and assume that the cutoff is given. Having these two sets we can search for a TF binding motif that is enriched in the positive set compared to some negative model, or compared to the negative sequences set. Here we developed a novel motif finder that searches the positive set for motif elements that are enriched compared to the negative set. Our motif finder is unique in its ability to output either PSSM or an FMM. We use the motif finder both to produce data for an FMM - PSSM comparison, and as a proof of concept of integrating FMM into a motif finding algorithm. To properly describe our motif finding algorithm, we introduce the notion of a K-mer set motif model (KMM). A KMM consists of a set of short aligned (not necessarily overlapping) sequences (see Figure S1 for a KMM example). The KMM can be described as having an “OR” term between all of its sequences. Following this description, a “hit” of a KMM is defined as an appearance (“hit”) of at least one of its sequences in an input sequence. Our motif finding algorithm consists of two main steps. In the first step we extract KMMs that are enriched in the positive set compared to the negative set. In the second step we use the hits of each of the top scored KMM in the positive set to generate aligned sequences (these are putative TFBSs) from which a FMM or a PSMM is learned. In the first step we start with extracting the hits of all sequences of length K (referred as “K-mers”) that appear in positive or negative input sequences (Figure 2 (1) ). In order to evaluate the enrichment of a K-mer (and later a KMM) in the positive set compared to the negative set, we use its hits count as input for the multidimensional hypergeometric p-value (MHG p-value) test, introduced by Eden et al. [36]. Preferring the MHG p-value over a “simple” hypergeometric p-value has the benefit of quantifying the significance of a multiple motif occurrence in the same sequence in a data driven manner [36]. As Eden et al. , we too restricted ourselves to three dimensions, considering cases of 0,1, and ≥2 hits per input sequence. Let n be the number of positive sequences, and N be the total number of sequences (positive and negative). For a certain KMM, suppose first that there is a single KMM hit in K1 of the N sequences and in k1 of the n positive sequences, and second, that there are two or more KMM hits in K2 of the N and in k2 of the n positive. The multidimensional hypergeometric probability for that event is given by: and the multidimensional hypergeometric p-value is given by: Next we rank the K-mers based on their MHG p-value (Figure 2 (2) ). We use a p-value threshold MHGthreshold, controlled by FDR (in this work we used MHGthreshold = 10−3), to filter the K-mers (Figure 2 (3) ). Due to computational resources constraints, we limit the maximum number of K-mers that pass the filter to T = 200 top scored K-mers. We use the filtered K-mers to construct a “K-mer graph” (Figure 2 (4) ). Each K-mer forms a singleton KMM and is a vertex of the graph. Two K-mers (vertices) are connected by an undirected edge if and only if one of two conditions hold: either the Hamming distance between the K-mers does not exceed a threshold HDistance (in this work we used HDistance = 1), or the two K-mers can be perfectly aligned when one is shifted up to MShift base pairs with respect to the other (in this work we usedMShift = 1). The node with the best MHG p-value is then chosen as a “seed KMM”, and a greedy algorithm performs a series of steps along the edges in which the seed neighbors are joined into the seed KMM vertex, growing its KMM (Figure 2 (5) ). For a detailed description, see Protocol S1. In brief, this step either adds the neighbor K-mer to the seed KMM or uses the neighbor K-mer to extend a subset of the seed KMM sequences (this step allows the KMM to grow in length). Each time, the step that best improves the KMM seed' s MHG p-value is chosen. When no such step exists, the seed node is removed from the graph and a new seed node is chosen, repeating the process of growing the seed KMM. When there are no more nodes left in the graph, the KMMs are ranked by their MHG p-value, and the best M are picked (Figure 2 (5) ). The above process is repeated for every Kmin≤K≤Kmax (in this work we used Kmin = 5 and Kmax = 8). At the end of the first step the best M of all picked KMMs are chosen as input for the next step. In the second step, the motif finder produces either a PSSM or a FMM for each KMM. For each KMM, the algorithm uses all of its hits in the positive set to generate aligned TFBS data (Figure 2 (6) ), with the length of these TFBSs (which will be the motif length) induced by the KMM alignment (for an elaborate illustration of this process see Figure S1). It then learns the requested model (FMM or PSSM) that describes the KMM hits. As a last step similar KMM motifs are removed (those with larger MHG p-value) and unique motifs are outputted (the similarity measure is described in Protocol S1). A special case that our motif finder recognizes and handles is that of dimer motifs. KMMs may represent dimers by holding two different alignment offsets per single K-mer sequence. For a detailed description of how dimer motifs are recognized and produced, see Protocol S1. The main novelty in our motif finder is in its ability to produce FMMs instead of PSSMs. Producing FMMs requires the motif finding algorithm to preserve inter-position dependencies, if they exists in the data. Our KMM methodology of producing motifs from K-mers, and of properly extracting TFBSs that contain these K-mers from the data, ensures that we learn a motif model from TFBSs in which inter-position dependencies are indeed preserved. Finally, the performance of our motif finder with respect to memory and running time is discussed in Protocol S1.
Transcription factor (TF) protein binding to its DNA target sequences is a fundamental physical interaction underlying gene regulation. Characterizing the binding specificities of TFs is essential for deducing which genes are regulated by which TFs. Recently, several high-throughput methods that measure sequences enriched for TF targets genomewide were developed. Since TFs recognize relatively short sequences, much effort has been directed at developing computational methods that identify enriched subsequences (motifs) from these sequences. However, little effort has been directed towards improving the representation of motifs. Practically, available motif finding software use the position specific scoring matrix (PSSM) model, which assumes independence between different motif positions. We present an alternative, richer model, called the feature motif model (FMM), that enables the representation of a variety of sequence features and captures dependencies that exist between binding site positions. We show how FMMs explain TF binding data better than PSSMs on both synthetic and real data. We also present a motif finder algorithm that learns FMM motifs from unaligned promoter sequences and show how de novo FMMs, learned from binding data of the human TFs c-Myc and CTCF, reveal intriguing insights about their binding specificities.
cell biology/gene expression
computational biology/sequence motif analysis
genetics and genomics/gene expression
computational biology
genetics and genomics/bioinformatics
A Feature-Based Approach to Modeling Protein–DNA Interactions