- Research article
- Open Access

# Accurate prediction of protein-lncRNA interactions by diffusion and HeteSim features across heterogeneous network

- Lei Deng
^{1}, - Junqiang Wang
^{1}, - Yun Xiao
^{1}, - Zixiang Wang
^{1}and - Hui Liu
^{2}Email authorView ORCID ID profile

**Received:**5 February 2018**Accepted:**19 September 2018**Published:**11 October 2018

## Abstract

### Background

Identifying the interactions between proteins and long non-coding RNAs (lncRNAs) is of great importance to decipher the functional mechanisms of lncRNAs. However, current experimental techniques for detection of lncRNA-protein interactions are limited and inefficient. Many methods have been proposed to predict protein-lncRNA interactions, but few studies make use of the topological information of heterogenous biological networks associated with the lncRNAs.

### Results

In this work, we propose a novel approach, PLIPCOM, using two groups of network features to detect protein-lncRNA interactions. In particular, diffusion features and HeteSim features are extracted from protein-lncRNA heterogenous network, and then combined to build the prediction model using the Gradient Tree Boosting (GTB) algorithm. Our study highlights that the topological features of the heterogeneous network are crucial for predicting protein-lncRNA interactions. The cross-validation experiments on the benchmark dataset show that PLIPCOM method substantially outperformed previous state-of-the-art approaches in predicting protein-lncRNA interactions. We also prove the robustness of the proposed method on three unbalanced data sets. Moreover, our case studies demonstrate that our method is effective and reliable in predicting the interactions between lncRNAs and proteins.

### Availability

The source code and supporting files are publicly available at: http://denglab.org/PLIPCOM/.

## Keywords

- Protein-lncRNA interaction
- Heterogenous network
- HeteSim score
- Gradient tree boosting

## Background

Long non-coding RNAs (lncRNAs) have been intensively investigated in recent years [1, 2], and show close connection to transcriptional regulation, RNA splicing, cell cycle and disease. At present, a great majority of lncRNAs have been identified, but their functional annotations verified by experiment remains very limited [3, 4]. Recent studies have proved that the function of lncRNAs strikes a chord with the corresponding binding-proteins [5–7]. Therefore, the binding proteins of lncRNAs are urgent to be uncovered for better understand of the biological functions of lncRNAs.

Although high-throughput methods for characterization of protein-RNA interactions have been developed [8, 9], *in silico* methods are appealing for characterization of the lncRNAs that are less experimentally covered due to technical challenge [10]. One common way for computationally predicting lncRNA-binding proteins is based on protein sequence and structural information. For example, Muppirala et al. [11] developed a computational approach to predict lncRNA-protein interactions by using the 3-mer and 4-mer conjoint triad features from amino acid and nucleotide sequences to train a prediction models. Wang et al. [12] used the same data set by Muppirala et al. [11] to develop another predictor based on Naive Bayes (NB) and Extended Naive Bayes (ENB). Recently, Lu et al. [13] presented lncPro, a prediction method for Protein-lncRNA associations using Fisher linear discriminant approach. The features used in lncPro consist of RNA/protein secondary structures, hydrogen-bonding propensities and Van der Waals’ propensities.

In recent years, network-based methods have widely been used to predict lncRNA functions [14, 15]. Many studies have paid attention to integration of heterogeneous data into a single network via data fusion or network-based inference [16–21]. The network propagation algorithms, such as the Katz measure [22], random walk with restart (RWR) [23], LPIHN [24] and PRINCE [25, 26], have been used to investigate the topological features of biomolecular networks in a variety of issues, such as disease-associated gene prioritization, drug repositioning and drug-target interaction prediction. Random Walk with Restart (RWR) [23] is widely used for prioritization of candidate nodes in a weighted network. LPIHN [24] extends the random walk with restart to the heterogeneous network. PRINCE [25, 26] formulates the constraints on prioritization function that relate to its smoothness over the network and usage of prior information. Recently, we developed PLPIHS [27], which uses the HeteSim measure to predict protein-lncRNA interactions in the heterogeneous network.

In this paper, we introduced an computational approach for protein-lncRNA interaction prediction, referred to as PLIPCOM, based on protein-lncRNA heterogeneous network. The heterogeneous network is constructed from three subnetworks, namely protein-protein interaction network, protein-lncRNA association network and lncRNA co-expression network. PLIPCOM incorporates (i) low dimensional diffusion features calculated using random walks with restart (RWR) and a dimension reduction approach (SVD), and (ii) HeteSim features obtained by computing the numbers of different paths from protein to lncRNA in the heterogeneous network. The final prediction model is based on the Gradient Tree Boosting (GTB) algorithm using the two groups of network features. We compared our method to both traditional classifiers and existing prediction methods on multiple datasets, the performance comparison results have shown that our method obtained state-of-the-art performance in predicting protein-lncRNA interactions.

It is worth noting that we have substantially extended and improved our preliminary work published on the BIBM2017 conference proceeding [28]. The improvements include: 1) We presented more detail of the methodology of PLIPCOM, such as the construction of protein-lncRNA heterogenous work, feature extraction and gradient tree boosting algorithm; 2) We have conducted extensive evaluation experiments to demonstrate the performance of the proposed method on multiple data sets with different positive and negative sample ratios, i.e. P:N=1:1,1:2,1:5,1:10, respectively. Particularly, we compared PLIPCOM with our previous method PLPIHS [27] on four independent test datasets, and the experimental results show that PLIPCOM significantly outperform our previous method; 3) To verify the effectiveness of the diffusion and HeteSim features in predicting proteinlncRNA interactions, we evaluated the predictive performance of the two types of features alone and combination of them, on the benchmark dataset; 4) Case studies have been described to show that our method is effective and reliable in predicting the interactions between lncRNAs and proteins; 5) Last but not the least, we have conducted the time complexity analysis of PLIPCOM.

## Methods

### Overview of PLIPCOM

### Data sources

#### Protein-protein interaction

All human lncRNA genes and protein-coding genes were obtained from GENCODE database [29] (Release 24), which includes 15,941 lncRNA genes and 20,284 protein-coding genes. We obtained the human protein-protein interactions (PPIs) from STRING database [30] (V10.0), which collected PPIs from high-throughput experiments, as well as computational predictions and text mining results. A total of 7,866,428 human PPIs are obtained.

#### LncRNA-lncRNA co-expression

We downloaded the expression profiles of lncRNA genes from NONCONDE 2016 database [31], and calculated the lncRNA co-expression similarity between each two lncRNAs using Pearson’s correlation coefficient.

#### Protein-lncRNA association

where *C*(*l*,*p*) is the integrative correlation coefficient between lncRNA *l* and protein-coding gene *p*, *C*_{d}(*l*,*p*) represents the correlation coefficient between *l* and *p* in dataset *d*, and *D* is the number of data sets. In particular, we take into account the gene pairs whose correlation coefficient are positive, and discard those with negative correlation coefficients, as the mutual exclusion relationship indicates that protein is unlikely to interacting with the lncRNA.

An additional paired-end RNA-seq datasest including 19 human normal tissues are obtained from the Human Body Map 2 project (ArrayExpress accession E-MTAB-513) and another study (GEO accession no.GSE30554). Expression levels are calculated using Tophat and cufflinks, and the co-expressions of proteinlncRNA pairs are evaluated using Pearson’s correlation coefficients.

Finally, we built a global heterogenous network by merging the three types of subnetworks (protein-protein interaction network, lncRNA-lncRNA co-expression network, and protein-lncRNA association network). The resulting network has 36,225 nodes (15,941 lncRNAs and 20,284 proteins) and 2,339,152 edges after removal of edges wit similarity scores <0.5.

### Low-dimensional network diffusion features

*G*denote the adjacency matrix for the global network, and

*T*represent the transition probability matrix. Each entry

*T*

_{ij}holding the transition probability from node

*i*to node

*j*is computed as below

*G*

_{ij}is equal to 1 if node

*i*is connected to node

*j*in the network, and 0 otherwise. The RWR process can be written as follows:

where *α* is the restart probability leveraging the importance of local and global topological information; *P*_{t} is a probability distribution whose *i*-th element represents the probability of node *i* being visited at step *t*. After enough number of iterations, RWR will converge so that *P*_{t} holds the stable diffusion distribution. If two nodes have similar diffusion states, they locate in similar situation within the global network with respect to other nodes. Since there are 36,225 nodes (15,941 lncRNA nodes and 20,284 protein nodes) in the network, each node has a 36,225-dimensional diffusion state.

*P*is factorized into the form as below:

*Σ*are the singular values of

*P*, and the columns of

*U*and

*V*are the left-singular vectors and right-singular vectors of

*P*, respectively. For a given number

*n*of output dimensions, we assign the top

*n*columns of

*Σ*

^{1/2}

*V*to

*x*

_{i}, namely,

where *X* is the derived low-dimensional feature matrix from the high-dimensional diffusion features. In this work we set *n*=500 according to previous study [38].

### HeteSim score-based features

*L*and

*P*two kinds of nodes in a heterogenous network, (

*A*

_{LP})

_{n∗m}is an adjacent matrix, the normalization matrix of

*A*

_{LP}with respect to the row vector is defined as

where \(\mathcal {P}=(P_{1}P_{2}\cdots P_{n+1})\) represents the set of paths of length *n*, and *P*_{i} belongs to any nodes in the heterogenous network.

14 different paths from a protein to a lncRNA with length less than 6 in the heterogenous network

ID | name | path |
---|---|---|

1 | PLL | protein-lncRNA-lncRNA |

2 | PPL | protein-protein-lncRNA |

3 | PPLL | protein-protein-lncRNA-lncRNA |

4 | PLPL | protein-lncRNA-protein-lncRNA |

5 | PLLL | protein-lncRNA-lncRNA-lncRNA |

6 | PPPL | protein-protein-protein-lncRNA |

7 | PPPPL | protein-protein-protein-protein-lncRNA |

8 | PLPPL | protein-lncRNA-protein-protein-lncRNA |

9 | PPLPL | protein-protein-lncRNA-protein-lncRNA |

10 | PLLPL | protein-lncRNA-lncRNA-protein-lncRNA |

11 | PPPLL | protein-protein-protein-lncRNA-lncRNA |

12 | PLPLL | protein-lncRNA-protein-lncRNA-lncRNA |

13 | PPLLL | protein-protein-lncRNA-lncRNA-lncRNA |

14 | PLLLL | protein-lncRNA-lncRNA-lncRNA-lncRNA |

### The gradient tree boosting classifier

Based on the derived diffusion and HeteSim features, we build a classifier using the gradient tree boosting (GTB) [40] algorithm to predict protein-lncRNA interactions. Gradient tree boosting algorithm is an effective machine learning-based method that has been successfully applied for both classification and regression problems [41–43].

*N*is the number of protein-lncRNA pairs in the training dataset. The gradient tree boosting algorithm repeatedly constructs

*m*different classification trees

*h*(

*χ*,

*α*

_{1}),

*h*(

*χ*,

*α*

_{2}),...,

*h*(

*χ*,

*α*

_{m}), each of which is trained based on a subset of randomly extracted samples, and then constructs the following additive function

*Θ*

_{m}(

*x*):

*β*

_{m}and

*α*

_{m}are the weight and parameter vector of the

*m*-th classification tree

*h*(

*χ*,

*α*

_{m}). The loss function

*L*(

*y*,

*Θ*

_{m}(

*χ*)) is defined as:

*y*is the real class label and

*Θ*(

*χ*) is the decision function. Both

*β*

_{m}and

*α*

_{m}are iteratively optimized by grid search so that the loss function

*L*(

*y*,

*Θ*

_{m}(

*χ*)) is minimized. Accordingly, we obtain the gradient tree boosting model \(\tilde {\Theta }({\chi })\) as follows:

We use grid search strategy to select the optimal parameters of GTB with 10-fold cross-validation on the benchmark dataset. The optimal number of trees of the GTB is 600, and the selected depth of the trees is 13. The rest parameters are set to default values.

## Results

### Training data sets

We randomly select 2,000 protein-lncRNA interactions from the experimentally validated protein-lncRNA associations as positive examples, and randomly generated 2,000, 4,000, 10,000, 20,000 negative samples that are not included in all known associations. As a result, we build a standard training set with 2,000 positive and 2,000 negative samples, and other three unbalanced data sets with more negative samples than positive ones. The ratios of positive and negative samples are 1:1, 1:2, 1:5 and 1:10 in the four training sets, respectively.

### Test data sets

For objective performance evaluation, an independent test set is built by randomly selecting 2,000 protein-lncRNA associations from the experimentally validated ones, plus 2,000 randomly generated negative samples. To be more realistic, we accordingly construct other three unbalanced test data sets with positive *vs* negative ratio 1:2, 1:5 and 1:10, respectively. Note that all the positive and negative samples in these test sets are independently chosen and excluded from the training set.

### Performance measures

*TP*and

*FP*represent the numbers of correctly predicted positive and negative samples,

*FP*and

*FN*represent the numbers of wrong predicted positive and negative samples, respectively. The AUC score is computed by varying the cutoff of the predicted scores from the smallest to the greatest value.

### Predictive power of topological features

### Benefit from gradient tree boosting algorithm

Since our method is based on the gradient tree boosting algorithm, we compared our method to several widely used classifiers, including k-nearest neighbors algorithm (kNN) [44], random forest (RF) [45] and support vector machine (SVM) [46], on our build standard training set using 10-fold cross validation. The counterpart classifiers are obtained from the python toolkits scikit-learn [47], and trained using the 1,014-dimensional combined features. For kNN classifier, we use 15 nearest neighbors and leaf size of 30 points. RF builds a number of decision tree classifiers trained on a set of randomly selected samples of the benchmark to improve the performance. A total number of 600 tree classifiers are built in this study. For SVM, we use radial basis function (RBF) as the kernel, and the penalty *c* and gamma *g* parameters are optimized to 512 and 0.00195, respectively. The number of trees used in the gradient tree boosting of PLIPCOM is set to 600, and the maximum tree depth is set to 13.

Performance comparison of GTB with other machine learning algorithms(k-NN, RF and SVM)

AUC | ACC | SEN | SPE | F1-Score | MCC | |
---|---|---|---|---|---|---|

KNN | 0.916 | 0.860 | 0.871 | 0.849 | 0.862 | 0.721 |

RF | 0.969 | 0.918 | 0.868 | 0.966 | 0.913 | 0.839 |

SVM | 0.973 | 0.931 | 0.921 | 0.940 | 0.930 | 0.862 |

PLIPCOM | 0.982 | 0.947 | 0.931 | 0.963 | 0.946 | 0.895 |

### Performance comparison with existing methods

*α*=0.9,

*c*=-15,

*d*=log(9999) and the iteration number is set to 10. The parameters of LPIHN are set to their default values, i.e.

*γ*=0.5,

*β*=0.5 and

*δ*=0.3. For RWR, the restart probability

*r*is set to 0.5. The ROC curves are drawn using the true positive rate (TPR)

*vs*. false positive rate (FPR) upon different thresholds of these prediction results. As shown in Fig. 3, PLIPCOM obtain the best performance among these protein-lncRNA interaction prediction methods, its AUC values achieved on four data sets are both more than 0.98. Particularly, the performance of PLIPCOM keeps stable on severely unbalanced data sets, while the performance of other methods is significantly influenced. For instance, on the ratio of 1:10 dataset, PLIPCOM achieved an AUC score of 0.990, and remarkably outperform PLPIHS (0.929), PRINCE (0.854), LPIHN (0.849) and RWR (0.556).

### Evaluation on independent test sets

### Case studies

To further illustrate the effectiveness of the proposed method, We present three lncRNAs for case studies, including HOTAIRM1 (ensemble ID: ENSG00000233429), XIST (ensemble ID:ENSG00000229807) and HOTAIR (ensemble ID:ENSG00000228630). The HOTAIRM1 is a long non-coding RNA that plays a critical role in regulating alternative splicing of endogenous target genes, and is also a myeloid lineage-specific ncRNA in myelopoiesis [48]. HOTAIRM1 locates between the human HOXA1 and HOXA2 genes. A multitude of evidence indicates that HOTAIRM1 play vital role in neural differentiation and is a potential diagnostic biomarkers of colorectal cancer [49]. The XIST encodes an RNA molecule that plays key roles in the choice of which X chromosome remains active, and in the initial spread and establishment of silencing on the inactive X chromosome [50]. HOTAIR is a long intervening non-coding RNA (lincRNA) whose expression is increased in pancreatic tumors compared to non-tumor tissue. Knockdown of HOTAIR (siHOTAIR) by RNA interference shows that HOTAIR plays an important role in pancreatic cancer cell invasion [51].

*α*-subunit of the stimulatory G protein [52]. Indeed, GNAS has been shown to underlie some important quantitative traits in muscle mass and domestic mammals [53]. In addition, HOTAIRM1 can interact with SFPQ in colorectal cancer (CRC) tissues that release PTBP2 from the SFPQ or PTBP2 complex. The interaction between HOTAIRM1 and SFPQ is a promising diagnostic biomarker of colorectal cancer [54]. NFKB1 is a transcriptional factor that plays crucial role in the regulation of viral and cellular gene expressions [55], and its association with HOTAIRM1 is helpful to uncover the function of HOTAIRM1. Take HOTAIR for another example, EZH2 is the catalytic subunit of the polycomb repressive complex 2 (PRC2) and is involved in repressing gene expression through methylation of histone H3 on lysine 27 (H3K27) [56], EZH2 (predominant PRC2 complex component) inhibition blocked cell cycle progression in glioma cells, which is consistent with the effects elicited by HOTAIR siRNA. Through the study of EZH2, we can understand the biological function of HOTAIR more deeply [57]. These cases demonstrate that PLIPCOM is effective and reliable in predicting the interactions between lncRNAs and proteins.

Top 10 ranked proteins for lncRNA HOTAIRM1, XIST and HOTAIR

lncRNA | Protein | Ensemble ID | Score |
---|---|---|---|

HOTAIRM1 | GNAS | ENSG00000087460 | 0.978906 |

NFKB1 | ENSG00000109320 | 0.962423 | |

SFPQ | ENSG00000116560 | 0.956276 | |

PLEKHG2 | ENSG00000090924 | 0.948234 | |

MMP14 | ENSG00000157227 | 0.942456 | |

WDR73 | ENSG00000177082 | 0.939295 | |

HNRNPC | ENSG00000092199 | 0.938295 | |

RPS24 | ENSG00000138326 | 0.937062 | |

CPSF7 | ENSG00000149532 | 0.936224 | |

SRSF11 | ENSG00000116754 | 0.935515 | |

XIST | GDF15 | ENSG00000130513 | 0.98304 |

NME4 | ENSG00000103202 | 0.965669 | |

MOV10 | ENSG00000155363 | 0.962258 | |

SFPQ | ENSG00000116560 | 0.961144 | |

QKI | ENSG00000112531 | 0.958775 | |

WDR73 | ENSG00000177082 | 0.95635 | |

CASKIN2 | ENSG00000177303 | 0.950001 | |

WDR33 | ENSG00000136709 | 0.943944 | |

DPF2 | ENSG00000133884 | 0.941258 | |

AKT1 | ENSG00000142208 | 0.940658 | |

HOTAIR | EZH2 | ENSG00000106462 | 0.994214 |

PUM2 | ENSG00000055917 | 0.993374 | |

IGF2BP2 | ENSG00000073792 | 0.970273 | |

UPF1 | ENSG00000005007 | 0.965562 | |

PCBP1 | ENSG00000169564 | 0.959887 | |

WDR33 | ENSG00000136709 | 0.947819 | |

RTCB | ENSG00000100220 | 0.946163 | |

HNRNPA2B1 | ENSG00000122566 | 0.945789 | |

SNIP1 | ENSG00000163877 | 0.942754 | |

HOXD8 | ENSG00000175879 | 0.93755 |

## Discussion and conclusion

Identification of the associations between long non-coding RNAs (lncRNAs) and protein-coding genes is essential for understanding the functional mechanism of lncRNAs. In this work, we introduced a machine learning method, PLIPCOM, to predict protein-lncRNA interactions. The major idea of PLIPCOM is to take full advantage of the topological feature of lncRNA-protein heterogenous network. We first build a protein-lncRNA heterogeneous network by integrating a variety of biological networks including lncRNA-lncRNA co-expression network, protein-protein interaction network, and protein-lncRNA association network. Two categories of features, including diffusion features and HeteSim features, are extracted from the global heterogeneous network. Subsequently, we apply the gradient tree boosting (GTB) algorithm to train the protein-lncRNA interaction prediction model using the diffusion and HeteSim features. Cross validations and independent tests are conducted to evaluate the performance of our method in comparison with other state-of-the-art approaches. Experimental results show that PLIPCOM gains superior performance compared to other state-of-the-art methods.

From our perspective, the superior performance of PLIPCOM benefits from at least three aspects: (i) diffusion features calculated using random walks with restart (RWR) on the protein-lncRNA heterogenous network, and the feature dimension is further reduced by applying singular value decomposition (SVD); (ii) HeteSim features obtained by computing the numbers of different paths from protein to lncRNA in the heterogenous network; and (iii) effective prediction model built by using the gradient tree boosting (GTB) algorithm. As far as our knowledge, we are the first to apply both diffusion and HeteSim features to predict protein-lncRNA interactions, although these two types features are regularly used in characterizing biological networks in previous works. As shown in our experimental results, diffusion and HeteSim features are complementary and their combination can further improve the predictive power. Moreover, compared to other classifiers, such as SVM and kNN, GTB used by PLIPCOM can not only achieve high prediction accuracy, but also select the feature of importance for identifying lncRNA-protein interactions.

The time complexity of our method depends mainly on the feature extraction procedure and GTB algorithm. The diffusion feature is calculated using RWR and its time complexity can be inferred from the equation *P*=(*E*−(1−*α*)*T*)^{−1}(*α**E*)=*α**Q*^{−1}*E*, in which *E* is unit matrix, *T* is the transition probability matrix, *α* is the restart probability and *Q* is an *n*∗*n* sparse matrix (*n* is number of nodes in the network). The time complexity of calculating inverse matrix *Q*^{−1} is *O*(*n*^{3}), and can be optimized by using Cholesky algorithm. From our previous work, we know that the time complexity of calculating HeteSim feature is *O*(*k**n*), where *k* is the number of samples and *n* is the number of nodes. Note that these two network features can be calculated in parallel. Moreover, we use the truncated SVD to reduce the diffusion feature dimension so that the time of GTB training process is greatly reduced. As a result, the time complexity of the methodology of PLIPCOM is moderate, and can be scaled to large networks.

Although PLIPCOM show effectiveness and promising predictive power, we think its performance can be further improved by adding protein sequence and structural information. In the near future, we will integrate sequence and structural features to promote the prediction of potential lncRNA-protein interactions.

## Declarations

### Acknowledgements

This work was supported by National Natural Science Foundation of China under grants No. 61672541 and No. 61672113, and Natural Science Foundation of Hunan Province under grant No. 2017JJ3287.

### Availability of data and materials

The source code and data are available at http://denglab.org/PLIPCOM/.

### Authors’ contributions

LD, JW and HL conceived this work and designed the experiments. JW, YX and ZW carried out the experiments. LD, JW and HL collected the data and analyzed the results. LD, JW, YX, ZW and HL wrote, revised, and approved the manuscript.

### Ethics approval and consent to participate

Not applicable.

### Consent for publication

Not applicable.

### Competing interests

The authors declare that they have no competing interests.

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## Authors’ Affiliations

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