Robust inference of positive selection on regulatory sequences in the human brain

INTRODUCTION

It has long been suggested that changes in gene regulation have played an important role in human evolution and especially in the evolution of the human brain and behavior (1, 2). Many human and chimpanzee divergent traits (3) cannot be explained by protein sequence adaptations. For example, there is little evidence to link protein sequence adaptations to traits related to cognitive abilities (4). Conversely, there is some evidence of brain-specific gene expression divergence in humans (5), which is consistent with a role of regulatory evolution. However, a central question remains open: Which regulatory changes were adaptive, if any? A major limitation in answering this is the lack of a robust model of neutral versus adaptive evolution for regulatory elements.

One approach to detect adaptive evolution on regulatory elements is to detect noncoding regions with lineage-specific accelerated evolutionary rates (68). For example, Gittelman et al. (8) found human accelerated regions close to genes annotated to terms such as brain or neuron development. A major caveat is that this acceleration may result from neutral mechanisms such as biased gene conversion (9) rather than from selection. A second approach is to use an MK (McDonald–Kreitman) test framework (1014). This approach has two limitations. First, an expected neutral divergence to polymorphism ratio needs to be defined, whereas defining neutral sites for regulatory elements is difficult and can bias results (15). Second, it lacks power on individual elements, since many regulatory elements are short and present very few variable sites (14).

We have developed a new method to detect adaptive evolution of transcription factor binding sites (TFBSs) on the basis of predicted binding affinity changes. As a proof of principle, we first applied this method to well-known transcription factors, such as CEBPA and CTCF, in species triples focused on human, mouse, or fly. We validated it with three independent lines of evidence: Our evidence of positive selection is associated to higher empirical binding affinity, higher substitution-to-polymorphism ratio in sequence, and lower variance in expression of neighboring genes. Then, we used this method to detect positive selection of CTCF binding sites in 29 human tissues or cell types. We found the highest positive selection in brain samples, followed by male reproductive system. The same analysis in mouse found the highest positive selection in the lung, with no special signal in the brain. Thus, we provide evidence for adaptive evolution of gene regulation in the human brain.

RESULTS

Detecting positive selection on TFBSs

We propose a computational model to detect positive selection on TFBSs, or any other elements for which we have experimental evidence similar to chromatin immunoprecipitation sequencing (ChIP-seq) (Fig. 1 and Materials and Methods). Briefly, a gapped k-mer support vector machine (gkm-SVM) classifier is trained on ChIP-seq peaks (here, TFBSs). This allows computing SVM weights of all possible 10-mers, which are predictions of their contribution to transcription factor binding affinity (16). We can then predict the binding affinity impact of substitutions by calculating deltaSVM, the difference of sum weights between two homologous sequences. We compare each empirical TFBS to an ancestral sequence inferred from alignment with a sister species and an outgroup.

Fig. 1 Illustration of the procedure for inferring positive selection.

The method includes two parts. Part I (left) is the gapped k-mer support vector machine (gkm-SVM) model training. The gkm-SVM classifier was trained by using TFBSs as a positive training set and randomly sampled sequences from the genome as a negative training set. Then, SVM weights of all possible 10-mers, the contributions of prediction transcription factor binding affinity, were generated from the gkm-SVM. Part II (right) is the positive selection inference. The ancestor sequence was inferred from sequence alignment with a sister species (species B) and an outgroup (species C). Then, the binding affinity change (deltaSVM) of the two substitutions accumulated in the red branch leading to species A was calculated on the basis of the weight list. The significance of the observed deltaSVM was evaluated by comparing it with a null distribution of deltaSVM, constructed by scoring the same number of random substitutions 10,000 times.

Adaptive evolution on TFBSs is expected to push them from a suboptimal toward an optimal binding strength or from an old optimum to a new one (e.g., in response to changing environment). Thus, TFBSs evolving adaptively are expected to accumulate substitutions that consistently change the phenotype to stronger or to weaker binding, whereas TFBSs evolving under purifying selection are expected to accumulate substitutions that increase or diminish binding in approximately equal measure, around a constant optimum. This reasoning follows the principle of a sign test of phenotypes (17, 18), although it uses the actual values and not just the sign. In practice, this should lead to a large absolute value of deltaSVM under adaptive selection. We estimate by randomization a P value specific to each individual TFBS and to its number of substitutions (see Materials and Methods). Thus, our method can infer the action of natural selection pushing a TFBS to a new fitness peak of either higher (positive deltaSVM) or lower (negative deltaSVM) binding affinity than its ancestral state.

Detecting positive selection on liver TFBSs in Mus musculus

We first applied our method to a large set of TFBSs in the liver of three mouse species (M. musculus domesticus C57BL/6J, M. musculus castaneus CAST/EiJ, and M. spretus SPRET/EiJ), identified by ChIP-seq for three liver-specific transcription factors, CEBPA, FOXA1, and HNF4A (19). We inferred positive selection on the lineage leading to C57BL/6J after divergence from CAST/EiJ (Fig. 2A). For the sake of simplicity, we only present the results of CEBPA in the main text; results are consistent for FOXA1 and HNF4A (Supplementary Materials). We first trained a gkm-SVM on 41,945 CEBPA binding sites in C57BL/6J (see Materials and Methods). The gkm-SVM very accurately separates CEBPA binding sites and random sequences (Fig. 2B). On the basis of the experimental ChIP-seq peaks in the three species, using SPRET/EiJ as an outgroup, we identified three categories of CEBPA binding sites: conserved in all three species (“conserved,” 24,280 sites), lineage-specific gain in C57BL/6J (“gain,” 6304 sites), and lineage-specific loss in C57BL/6J (“loss,” 6692 sites). On the basis of whole-genome pairwise alignments of C57BL/6J to CAST/EiJ and to SPRET/EiJ, we derived the substitutions accumulated on the C57BL/6J lineage for each CEBPA binding site (see Materials and Methods). We only kept binding sites with at least two substitutions, leading to 5114, 1445, and 1497 TFBSs for conserved, gain, and loss categories, respectively. For each binding site, we calculated a deltaSVM value and inferred its significance by random in silico mutagenesis (see Materials and Methods).

Fig. 2 Mouse CEBPA binding sites study.

(A) Topological illustration of the phylogenetic relationships between the three mouse species used to detect positive selection on CEBPA binding sites. We want to detect positive selection that occurred on the lineage of C57BL/6J after divergence from CAST/EiJ, as indicated by the red branch. Ma, million years. (B) Receiver operating characteristic (ROC) curve for gkm-SVM classification performance on CEBPA binding sites (fivefold cross-validation). The AUC value represents the area under the ROC curve and provides an overall measure of predictive power. (C) The left-hand graphs are the distributions of deltaSVM. The right-hand graphs are the distributions of deltaSVM P values (test for positive selection). (D) Proportion of CEBPA binding sites with evidence of positive selection. (E to G) The number of binding sites in each category is indicated below each box. The P values from a Wilcoxon test comparing categories are reported above boxes. Positive sites are binding sites with evidence of positive selection (deltaSVM P value <0.01). (E) Conserved binding sites. (F) Lineage-specific gain binding sites. (G) Lineage-specific loss binding sites. We compare the binding intensity from CAST/EiJ, as an approximation for ancestral binding intensity, between positive loss binding sites and nonpositive loss binding sites.

We plot the distributions of deltaSVMs and their corresponding P values for each binding site evolutionary category (Fig. 2C). As expected, the distribution of deltaSVMs is symmetric for conserved, has a skew toward positive deltaSVMs for gain, and a skew toward negative deltaSVMs for loss. These results confirm that the gkm-SVM–based approach can accurately predict the effect of substitutions on transcription factor binding affinity change. For the distribution of P values, in all binding site categories, there is a skew of P values near zero, indicating some signal of positive selection. Gain has the most skewed distribution of P values toward zero. Hereafter, we will use 0.01 as a significant threshold to define positive selection, but results are robust to different thresholds (see the “Validation based on ChIP-seq binding intensity” section). This identifies almost 20% of gain having evolved under positive selection (Fig. 2D), relative to 4% of loss and 2% of conserved. Random substitutions tend to decrease the binding affinity rather than increase it (fig. S1), because it is easier to break a function than to improve it. Thus, our method could be biased toward reporting as positive sites with more left-shifted null distributions. However, this is not the case (fig. S2).

In summary, we found widespread positive selection driving the gain of CEBPA binding sites. We also found some evidence of positive selection driving loss or increase in binding affinity in some conserved sites. For the other two transcription factors (FOXA1 and HNF4A), we found very consistent patterns (figs. S3 and S4).

Validation based on ChIP-seq binding intensity

We expect that conserved or gained sites, which evolved under positive selection with positive deltaSVM, should have increased binding affinity. Thus, the positive binding sites (PBSs) should have higher binding affinity than nonpositive selection binding sites (non-PBSs) in the focal species C57BL/6J. This is indeed the case (Fig. 2, E and F). In addition, conserved TFBSs have higher activity than recently evolved ones (“gain”). For loss, however, the PBSs have a strong decrease in binding affinity, so we expect higher binding affinity of PBSs in the ancestor. Using CAST/EiJ as an approximation for ancestor binding affinity, this is indeed the case (Fig. 2G). Results are also consistent with different P value thresholds (fig. S5). We performed the same validations in FOXA1 and HNF4A, with consistent results (fig. S6).

Validating the inference of positive selection with human liver TFBSs

To further validate our method, we took advantage of the abundant population genomics transcriptomics data in humans. We inferred positive selection of CEBPA binding sites in the human lineage after divergence from chimpanzee, with gorilla as outgroup (Fig. 3A). As in mouse, the gkm-SVM trained from 15,806 CEBPA binding sites in human can very accurately separate TFBSs and random sequences (Fig. 3B). The distribution of deltaSVMs is slightly asymmetric, with a higher proportion of positive values (Fig. 3C). This is because these binding sites contain both conserved and gain, but no loss (since we detect only in the focal species). On the basis of the distribution of P values, 7.5% of CEBPA binding sites are predicted to have evolved adaptively in the human lineage.

Fig. 3 Human CEBPA binding site study.

(A) Topological illustration of the phylogenetic relationships between human, chimpanzee, and gorilla. We detected positive selection that occurred on the lineage of human after divergence from chimpanzee, as indicated by the red branch. (B) ROC curve for gkm-SVM classification performance on CEBPA binding sites (fivefold cross-validation). The AUC value represents the area under the ROC curve and provides an overall measure of predictive power. (C) The left graph is the distribution of deltaSVM. The right graph is the distribution of deltaSVM P values (test for positive selection). (D) Ratio between the number of substitutions and the number of polymorphisms [single-nucleotide polymorphisms (SNPs)] for CEBPA binding sites. Positive sites are binding sites with evidence of positive selection (deltaSVM P value <0.01). The P value from Fisher’s exact test is reported above the bars. (E) Comparison of expression variance (adjusted variance) of putative target genes (closest gene to a TFBS) between positive sites and nonpositive sites. The number of binding sites in each category is indicated below each box. The P values from a Wilcoxon test comparing categories are reported above boxes. Positive sites are binding sites with evidence of positive selection (deltaSVM P value <0.01).

Using the MK framework (10), we predict that PBSs should have higher substitution-to-polymorphism ratios than non-PBSs. Note that we do not need to define neutral sites a priori. As expected, we found that the PBSs have a significantly higher ratio of fixed nucleotide changes between human and chimpanzee to polymorphic sites in human than non-PBSs (Fig. 3D). This is an external validation that our method detects positive selection, as the input did not contain any information about polymorphism.

Besides a higher substitution-to-polymorphism ratio, we also expect that the expression of PBS putative target genes (see Materials and Methods) should be more conserved among human populations. If the expression of PBS target genes is an adaptive trait in humans, further changes in expression will reduce fitness. Moreover, recent adaptive sweeps are expected to have reduced variability for the regulation of these genes. As expected, we found that PBS target genes have significantly lower expression variance (adjusted variance, controlling for the dependency between mean and variance; see Materials and Methods) across human populations than non-PBS target genes (Fig. 3E).

Thus, results from different sources of information support the expectations of our PBS predictions. We performed the same analyses in HNF4A, and results are consistent (fig. S7). These results strongly suggest that our method is detecting real adaptive evolution signals.

Detecting positive selection of TFBSs in Drosophila melanogaster

By using an MK test framework (10), Ni et al. (20) detected signatures of adaptive evolution on CTCF binding sites in D. melanogaster. They reported that positive selection has shaped CTCF binding evolution and that newly gained binding sites show a stronger signal of positive selection than conserved sites. We applied our method to the same data as used in Ni et al. (20). We detected positive selection in the D. melanogaster lineage after divergence from Drosophila simulans (fig. S8, A and B). Consistent with the findings of Ni et al. (20), we observed widespread positive selection for both conserved and gain (fig. S8C). In addition, the gain has a higher proportion of positive selection than conserved (fig. S8D). As Ni et al. (20) did not report specific sites, we cannot compare results more precisely. For lineage-specific loss binding sites, however, we did not detect any signal of positive selection (fig. S8C). The proportion of positive selection in D. melanogaster is much higher than in M. musculus. For example, we find almost 40% of gain under positive selection in D. melanogaster, twice the proportion in M. musculus. It should be noted that different transcription factors and tissues were used, which complicates direct comparison.

Adaptive evolution of CTCF binding sites across tissues in human

To test whether there is stronger adaptive evolution of gene regulation in some human tissues, we applied our method to 80,074 CTCF binding sites across 29 adult tissues or primary cell types (hereafter “cell types”; see table S2). We chose CTCF because it was the factor with the largest number of tissues or primary cell types studied in a consistent manner by the ENCODE (Encyclopedia of DNA Elements) consortium (21, 22). CTCF is well known as a transcriptional repressor, but it is also involved in transcriptional insulation and chromatin architecture remodeling (23). The gkm-SVM model trained from one cell type can accurately predict the binding sites in another cell type, and the model trained with all CTCF binding sites has better performance than the model trained with cell type–specific binding sites (fig. S9). Thus, we used a general gkm-SVM rather than different models for different cell types.

We detected 3.52% of PBSs for adaptation on the human lineage (fig. S10A). We found that PBSs have higher substitution-to-polymorphism ratio than non-PBSs (fig. S11). In addition, PBSs are associated with a lower number of active cell types (fig. S12A) than non-PBSs, consistent with the prediction that pleiotropy can limit adaptive evolution (24). We ranked cell types according to the proportion of binding sites that exhibit statistically significant evidence of positive selection. Brain-related cell types have a higher proportion of positive selection than other cell types (Fig. 4A). This pattern is consistent if we only use tissue-specific CTCF binding sites (fig. S13A). Choroid plexus epithelial cell, brain microvascular endothelial cell, and retinal pigment epithelial cell have notably high PBS frequencies. Non–brain-related nervous system cell types do not share this high positive selection, nor does in vitro differentiated neural cell, which may reflect that they do not preserve the signal of specific in vivo differentiated cells. Notably, these brain-related cell types also have a higher fraction of substitutions fixed by positive selection (see Materials and Methods) than other cell types, except lower leg skin (fig. S14).

Fig. 4 Proportion of positive CTCF binding sites in different tissues or cell types.

PBSs are binding sites with evidence of positive selection (deltaSVM P value <0.01). Colors correspond to broad anatomical systems. (A) CTCF binding sites in 29 human tissues or cell types. (B) CTCF binding sites in 11 mouse tissues.

To check whether our test could be too liberal or conservative for some sites, we first analyzed the substitution rate of all possible substitutions and their corresponding affinity change (deltaSVM) in human CTCF binding sites. We split all substitutions into two categories: substitutions on CpG and substitutions not on CpG. Within each category, we found, as expected, that the transition rate is much higher than the transversion rate, but we did not find a trend for specific substitution types to strengthen or weaken binding affinity (figs. S15A and S16). Between categories, we found that there is generally higher substitution rate on CpG, again as expected. Substitutions on CpG tend to weaken binding affinity (figs. S15A and S16), indicating that our test could be conservative for sites with CpG substitutions. Second, we checked whether neighboring substitutions (dinucleotide substitutions) have a general tendency to change affinity in the same direction. Indeed, this is the case (fig. S15B), suggesting that our test could be too liberal or too conservative for dinucleotide substitutions, depending on the direction of affinity change.

To check whether these biases (substitutions on CpG and dinucleotide substitutions) affect the pattern we found, first, we split all CTCF binding sites into two categories: sites with neither CpG substitutions nor dinucleotide substitutions and sites with either CpG substitutions or dinucleotide substitutions. For both categories, the proportion of positive selection binding sites (PBSs) detected is highly correlated with the original pattern (fig. S17, A and B). In addition, as expected, there is a higher proportion of PBSs for sites without substitutions on CpG, cofirming that our test is conservative for sites with CpG substitutions. Second, we both excluded all CpG sequences and dinucleotide substitution sequences from all binding sites, and we integrated the transition and transversion rate (4:1, estimated from fig. S15A) into our null model. Patterns of results were very robust to these changes (fig. S17C).

To test whether the high regulatory adaptive evolution in brain is general to mammals, we performed the same analysis on CTCF binding data from 11 mouse adult tissues (table S2 and fig. S18). We investigated adaptive evolution in the M. musculus branch after divergence from M. spretus, a similar evolutionary divergence to that between human and chimpanzee (5). Similarly to human, we detected 3.54% binding sites that evolved under positive selection (fig. S10B) and found PBSs associated with a lower number of active cell types (fig. S12B). However, no tissue type had especially high adaptive evolution, and brain-related tissues were among the lowest (Fig. 4B). When restricting to tissue-specific CTCF binding sites, lung has notably high adaptive evolution (fig. S13B).

DISCUSSION

A robust test for positive selection on regulatory elements

Detecting positive selection on regulatory sequences has long been a difficult problem (15). Nearby noncoding regions are often used as a neutral reference (13, 14, 25), but this neutrality is difficult to establish. Our approach does not require defining a priori neutral sites but instead considers the effects of variation on activity (2628). Moreover, positive selection on a background of negative selection might not elevate the evolutionary rate above the neutral expectation, yet consistent changes in binding affinity can still be detectable. Indeed, the TFBSs of cell types detected under selection do not necessarily evolve faster (fig. S19). In principle, our method can also be applied to other genomic regions for which experimental peaks are available, such as open chromatin regions or histone modification regions.

Because positive selection on regulatory sequences is difficult to determine, it is important to validate our predictions with independent evidence. The most important validation is that predictions made independently of population data verify the expectations of higher substitution-to-polymorphism ratio (Fig. 3D). Both this and the lower expression variance of neighboring genes (Fig. 3E) are consistent with the prediction that positive selection will increase divergence but remove polymorphism (10) and that recently selected phenotypes will be under stronger purifying selection. Moreover, binding affinity change occurs in the direction predicted by our model (Fig. 2, E to G), and we can verify the prediction that pleiotropy limits adaptation (fig. S12) (24).

Despite its advantages, our method can still be improved. For example, in the null model of sequence evolution, we assume independent mutation patterns at each base pair (bp) site and a uniform mutation rate over all sites. However, both mutation rate and pattern can depend on neighboring nucleotides (29). These limitations of our null model might explain why the observed P values do not quite follow the expected uniform distribution for high values.

Importance of regulatory adaptation on human brain evolution

Our results support the long proposed importance of adaptive regulatory changes in human brain evolution (1). They are consistent with accelerated gene expression evolution in the human brain, but neither in human blood or liver nor in rodents, from Enard et al. (5). Previous studies on human regulatory sequence evolution reported acceleration in brain-related functions but could not demonstrate adaptive evolution nor direct activity in the brain (58, 25). The reported link between human accelerated regions and function was very indirect, depending both on the attribution of a region to the closest gene and on the functional annotation of that gene.

The brain-related cell types for which we detect a high proportion of positive selection are functionally related with cognitive abilities. For example, for astrocyte, abnormal astrocytic signaling can cause synaptic and network imbalances, leading to cognitive impairment (30). In addition, for choroid plexus epithelial cell, its atrophy has been reported to be related with Alzheimer’s disease (31).

While we did not find a similar pattern by applying the same analysis to mouse, it is not possible yet to conclude to a human- or primate-specific pattern. Indeed, the mouse analyses have two potential caveats. First, for the olfactory bulb and cortical plate in the mouse analyses, there are no corresponding anatomical structures in the human analyses. It is an open question whether the human olfactory bulb and cortical plate also have high adaptation. Second, the human analyses were based on ChIP-seq at cell type level, but the mouse analyses were based on ChIP-seq at tissue level. In mouse, the astrocyte in cerebellum may also have high adaptation like the astrocyte in human, but the signal might be diluted by other cell types in cerebellum.

Regulatory adaptation differs between tissues

Outside of brain cell types, we found that male reproduction system (prostate and foreskin) has higher adaptive regulatory evolution than female reproduction system (ovary, uterus, and vagina). This is consistent with the observation of high adaptive sequence evolution in human male reproduction (32, 33) and probably caused by sexual selection–related selective pressures, such as sperm competition. However, testis has a relatively low proportion of adaptive evolution, similar to ovary. This suggests that the high expression divergence in testis (34) is mainly caused by relaxed purifying selection, maybe due to the role of transcription in testis for “transcriptional scanning” (35). Outside of the brain, the top adaptive regulatory evolution systems seem to be the same as found for adaptive protein evolution, i.e., male reproduction, immune, and endocrine systems (32, 3638). The high fraction of substitutions fixed by positive selection in the skin is interesting (fig. S14), since the skin is both involved in defense against pathogens and has evolved specifically in the human branch with loss of fur (39). The lack of adaptive protein sequence evolution despite high adaptive regulatory evolution might be related to selective pressure on proteins in the brain (40, 41).

Acknowledgments: We thank J. Goudet, G. Wagner, D. Garfield, L. Duret, and members of the Robinson-Rechavi lab for helpful discussions. Part of the computations was performed at the Vital-IT (www.vital-it.ch) Center for high-performance computing of the SIB Swiss Institute of Bioinformatics. Funding: This work was supported by the Swiss National Science Foundation grants 31003A_153341/1 and 31003A_173048. Author contributions: J.L. designed the work, with input from M.R.-R. J.L. performed data collection and computational analyses. J.L. and M.R.-R. interpreted the results. J.L. wrote the first draft of the paper. J.L. and M.R.-R. finalized the paper. Competing interests: The authors declare that they have no competing interests. Data and materials availability: All data needed to evaluate the conclusions in the paper are present in the paper and/or the Supplementary Materials. Data files and analysis scripts are available on GitHub: https://github.com/ljljolinq1010/A-robust-method-for-detecting-positive-selection-on-regulatory-sequences. Additional data related to this paper may be requested from the authors.

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