---
title: "Introduction to the `motifcounter` package"
author: "Wolfgang Kopp"
date: "2017-10-06"
output:
prettydoc::html_pretty:
toc: TRUE
number_sections: TRUE
fig_caption: yes
fig_height: 4
fig_width: 6
css: motifcounter.css
vignette: >
%\VignetteIndexEntry{Introduction to the `motifcounter` package}
%\VignetteEngine{knitr::rmarkdown}
%\VignetteEncoding{UTF-8}
#bibliography: literature.bib
---
```{r include=FALSE}
library(knitr)
knit_hooks$set(plot = function(x, options) {
paste('<figure><img src="',
opts_knit$get('base.url'), paste(x, collapse = '.'),
'"><figcaption>', options$fig.cap, '</figcaption></figure>',
sep = '')
})
library(motifcounter)
library(MotifDb)
library(seqLogo)
opts_chunk$set(fig.path="fig/")
```
This software package grew out of the work that I did to obtain my PhD.
If it is of help for your analysis, please cite
```
@Manual{,
title = {motifcounter: R package for analysing TFBSs in DNA sequences},
author = {Wolfgang Kopp},
year = {2017},
doi = {10.18129/B9.bioc.motifcounter}
}
```
Details about the compound Poisson model are available under
```
@article{improvedcompound,
title={An improved compound Poisson model for the number of motif hits in DNA sequences},
author={Kopp, Wolfgang and Vingron, Martin},
journal={Bioinformatics},
pages={btx539},
year={2017},
publisher={Oxford University Press}
}
```
# Usage examples
```{r eval=FALSE}
# Estimate a background model on a set of sequences
bg <- readBackground(sequences, order)
```
```{r eval=FALSE}
# Normalize a given PFM
new_motif <- normalizeMotif(motif)
```
```{r eval=FALSE}
# Evaluate the scores along a given sequence
scores <- scoreSequence(sequence, motif, bg)
```
```{r eval=FALSE}
# Evaluate the motif hits along a given sequence
hits <- motifHits(sequence, motif, bg)
```
```{r eval=FALSE}
# Evaluate the average score profile
average_scores <- scoreProfile(sequences, motif, bg)
```
```{r eval=FALSE}
# Evaluate the average motif hit profile
average_hits <- motifHitProfile(sequences, motif, bg)
```
```{r eval=FALSE}
# Compute the motif hit enrichment
enrichment_result <- motifEnrichment(sequences, motif, bg)
```
# Introduction
## Biological background
Transcription factors (TFs) play a crucial role in gene regulation.
They function by recognizing and binding to specific DNA stretches
that are usually 5-30bp in length
which are referred to as *transcription factor binding sites* (TFBSs).
TF-binding acts on the neighboring
genes by up- or down-regulating their gene expression levels.
The aim of the `motifcounter` package is to provide statistical
tools for studying putative TFBSs in given DNA sequence, including
the presence and location of TFBSs and the enrichment of TFBSs.
## Hallmarks of `motifcounter`
The main ingredients for an analysis with `motifcounter` consist of
1. a position frequency matrix (PFM) (also called TF motif)
2. a background model that serves as a reference for the statistical analysis
3. a set of DNA sequences that is subject to the TFBS analysis
4. a *false positive probability* $\alpha$ for predicting TFBSs
in a random sequence. E.g. $\alpha=0.001$.
A **PFM** represents the affinity of a TF to bind a certain DNA segment. A
large set of known PFMs can be acquired e.g. from
the `MotifDb` package [@motifdb].
On the other hand,
the **background model** defines the properties of unbound DNA sequences.
`motifcounter` implements the background model as an
**order-$d$ Markov model**, where $d$ is prescribed by the user.
The advantage of using higher-order background models is that they
are able to capture higher-order sequence features which is crucial
for studying naturally occurring DNA sequences (e.g. CpGs islands).
Using the PFM and the background model, `motifcounter` computes
the **motif score** for a given DNA sequence,
which is defined to be the log-likelihood ratio
between the PFM and the background.
The motif score represents a measure that indicates whether a certain
position in the DNA sequence is bound or unbound by the TF. Intuitively,
the higher the score, the more like does the sequence represent a TFBS.
The motif scores are also used to determine
**motif hits** (e.g. putative TFBSs) in the DNA sequence.
To this end, `motifcounter`
uses a predetermined **score threshold**
and calls putative TFBSs whenever the observed score at a give
position is greater or equal to the score threshold.
`motifcounter` establishes the **score threshold**
automatically based on 1) the
**score distribution** and 2) the user-prescribed false positive level $\alpha$.
To this end, the score distribution is determined by
an efficient dynamic programming algorithm for general order-$d$
background models. Details of the algorithm are described our paper (see above).
Testing for **motif hit enrichment** in `motifcounter` is based
on the **number of motif hits** that are observed in a set of DNA sequences.
In order to be able to judge significance of the observed number of hits
(e.g. 10 predicted TFBSs in the sequence of length 10kb),
the package approximates the **distribution of the number of motif hits**
in random DNA sequences with matched lengths.
Accordingly, `motifcounter` provides two fast and accurate alternatives
for approximating this distribution:
1. A compound Poisson approximation (see Kopp and Vingron (2017) Bioinformatics.)
2. A combinatorial model (manuskript in preparation)
Both of these methods support higher-order background models and
account for the **self-overlapping** structure of the motif.
For example, a repeat-like word, e.g. 'AAAA', likely gives rise to a string of mutually overlapping hits which
are referred to as **clumps** [@reinert, @pape]^[
By contrast, a simple binomial approximation [@rsat1,@rahmann]
does not account for self-overlapping matches.].
`motifcounter` not only account for overlapping motif hits
with respect to a single DNA strand, but also
for overlapping reverse complementary hits,
if both DNA strands are scanned for motif hits.
It is essential
to account for clumping, as that influences the distribution
of the number of motif hits and thereby the motif hit enrichment test.
Ignoring this effect could cause misleading statistical conclusions.
# Getting started
## Preliminary steps
### Acquire a background model
The background model is used to specify the
properties of unbound DNA sequences. That is, it
plays a role as a reference
for identifying putative TFBSs as well as for judging
motif hit enrichment.
`motifcounter` offers the opportunity to use
order-$d$ Markov model with user-defined $d$.
The background model is estimated on a set of user-provided
DNA sequences which are supplied as
`DNAStringSet`-objects from the `Biostrings` Bioconductor package.
The following code fragment illustrates how an order-$1$
background model is estimated from a given set of DNA sequences:
```{r}
order <- 1
file <- system.file("extdata", "seq.fasta", package = "motifcounter")
seqs <- Biostrings::readDNAStringSet(file)
bg <- readBackground(seqs, order)
```
<div class="hint">
**Hint:** Ideally, the DNA sequence for
estimating the background model should be representative (or even the same)
as the sequences that are latter analysed (e.g. for motif hit enrichment).
</div>
<br>
<div class="hint">
**Hint:** For the purpose of motif enrichment testing,
we recommend to use orders $d=1$ or $d=2$. Using a background with
very high order $d$ might be very costly to compute and, more importantly,
due to its increased flexibility, might capture relevant TFBS signals.
In that case, enriched motifs might not be recovered.
</div>
### Acquire a motif
`motifcounter` handles motifs in terms of
*position frequency matrices* (PFMs), which are commonly
used to represent the binding affinity of transcription factors (TFs).
A convenient source of known motifs is the `MotifDb`
Bioconductor package [@motifdb],
which shall be the basis for our tutorial.
For example, we retrieve the motif for the human *Pou5f1* (or *Oct4*)
transcription factor as follows
```{r}
# Extract the Oct4 motif from MotifDb
library(MotifDb)
oct4 <- as.list(query(query(query(MotifDb, "hsapiens"),
"pou5f1"), "jolma2013"))[[1]]
motif <- oct4
# Visualize the motif using seqLogo
library(seqLogo)
seqLogo(motif)
```
<div class="hint">
**Hint:** `motifcounter` requires strictly positive entries for a PFM.
If this is not the case, the package provides
the function `normalizeMotif`, which adds
pseudo-observations and re-normalize the columns:
```{r}
new_motif <- normalizeMotif(motif)
```
</div>
### Optional settings
By default, `motifcounter` identifies TFBS with a
the false positive probability of $\alpha=0.001$.
The user might want to change
the stringency level of $\alpha$, which is facilitated by `motifcounterOptions`:
```{r eval=FALSE}
alpha <- 0.01
motifcounterOptions(alpha)
```
For other options consult `?motifcounterOptions`.
## Retrieve position- and strand-specific scores and hits
For the following example, we explore the DNA sequences of
a set of *Oct4*-ChIP-seq peaks that were obtained in human *hESC* by
the ENCODE project [@encode2012]. The peak regions were trimmed to 200 bps centered around the midpoint.
```{r}
file <- system.file("extdata", "oct4_chipseq.fa", package = "motifcounter")
oct4peaks <- Biostrings::readDNAStringSet(file)
```
### Analysis of individual DNA sequences
The `motifcounter` package provides functions for exploring
position- and strand-specific putative TFBSs in individual DNA sequences.
One way to explore a given DNA sequence for TFBSs is by
utilizing `scoreSequence`. This function returns the per position and
strand scores for a given `Biostring::DNAString`-object
(left panel below).
To put the observed scores into perspective, the right panel shows
the theoretical score distribution in random sequences, which
is obtained by `scoreDist`^[The score distribution is computed using an
efficient dynamic programming algorithm.]. Scores at the tail of
the distribution occur very rarely by chance.
Those are also the ones that give rise to TFBS predictions:
```{r fig.show=TRUE, fig.cap="Per-position and strand scores"}
# Determine the per-position and per-strand scores
scores <- scoreSequence(oct4peaks[[1]], motif, bg)
# As a comparison, compute the theoretical score distribution
sd <- scoreDist(motif, bg)
par(mfrow = c(1, 2))
# Plot the observed scores, per position and per strand
plot(1:length(scores$fscores), scores$fscores, type = "l",
col = "blue", xlab = "position", ylab = "score",
ylim = c(min(sd$score), max(sd$score)), xlim = c(1, 250))
points(scores$rscores, col = "red", type = "l")
legend("topright", c("forw.", "rev."), col = c("blue", "red"), lty = c(1, 1))
# Plot the theoretical score distribution for the comparison
plot(sd$dist, sd$scores, type = "l", xlab = "probability", ylab = "")
```
To obtain the predicted TFBSs positions and strands, `motifcounter` provides
the function `motifHits`. This function calls motif hits
if the observed score exceeds a pre-determined score
threshold^[The threshold is determined for a user-defined false
positive level $\alpha$ (e.g. $\alpha=0.001$) based on
the theoretical score distribution.].
```{r}
# Call putative TFBSs
mhits <- motifHits(oct4peaks[[1]], motif, bg)
# Inspect the result
fhitpos <- which(mhits$fhits == 1)
rhitpos <- which(mhits$rhits == 1)
fhitpos
rhitpos
```
In the example sequence, we obtain no motif hit on the forward strand and
one motif hit on the reverse strand at position 94.
The underlying DNA sequence at this hit can be retrieved by
```{r}
oct4peaks[[1]][rhitpos:(rhitpos + ncol(motif) - 1)]
```
Next, we illustrate how a relaxed stringency level influences
the number of motif hits. Using `motifcounterOptions`, we prescribe a
false positive probability of $\alpha=0.01$ (the default was $\alpha=0.001$).
This will increase the tendency of producing motif hits
```{r}
# Prescribe a new false positive level for calling TFBSs
motifcounterOptions(alpha = 0.01)
# Determine TFBSs
mhits <- motifHits(oct4peaks[[1]], motif, bg)
fhitpos <- which(mhits$fhits == 1)
rhitpos <- which(mhits$rhits == 1)
fhitpos
rhitpos
```
Now we obtain four hits on each strand.
### Analysis of a set of DNA sequences
While, `scoreSequence` and `motifHits` can be applied to study
TFBSs in a single DNA sequence (given by a `DNAString`-object),
one might also be interested in the
average score or motif hit profiles across multiple sequences of equal length.
This might reveal positional constraints of the motif occurrences
with respect to e.g. the TSS, or the summit of ChIP-seq peaks.
On the one hand,
`motifcounter` provides the method `scoreProfile`
which can be applied for `Biostrings::DNAStringSet`-objects.
```{r fig.show=TRUE, fig.cap="Average score profile"}
# Determine the average score profile across all Oct4 binding sites
scores <- scoreProfile(oct4peaks, motif, bg)
plot(1:length(scores$fscores), scores$fscores, type = "l",
col = "blue", xlab = "position", ylab = "score")
points(scores$rscores, col = "red", type = "l")
legend("bottomleft", legend = c("forward", "reverse"),
col = c("blue", "red"), lty = c(1, 1))
```
On the other hand, `motifHitProfile` constructs a similar profile by computing
the position and strand specific mean motif hit frequency
```{r fig.show=TRUE, fig.cap="Average motif hit profile"}
motifcounterOptions() # let's use the default alpha=0.001 again
# Determine the average motif hit profile
mhits <- motifHitProfile(oct4peaks, motif, bg)
plot(1:length(mhits$fhits), mhits$fhits, type = "l",
col = "blue", xlab = "position", ylab = "score")
points(mhits$rhits, col = "red", type = "l")
legend("bottomleft", legend = c("forward", "reverse"),
col = c("blue", "red"), lty = c(1, 1))
```
## Test for motif hit enrichment
A central feature of `motifcounter` represents a sophisticated
novel approach for identifying motif hit enrichment in DNA sequences.
To this end, the package contains the method `motifEnrichment`,
which evaluates the *P-value* associated with the number of motif hits
that are found in the observed sequence, compared to the background model.
```{r}
# Enrichment of Oct4 in Oct4-ChIP-seq peaks
result <- motifEnrichment(oct4peaks[1:10], motif, bg)
result
```
The method returns a list that contains
`pvalue` as well as `fold`. While, the `pvalue` represents
the probability that more or equally many hits are produced in
random DNA sequences, `fold` represents the fold-enrichment of
motif hits relative the
expected number of hits in random DNA sequences.
That is, it represents a measure of the effect size.
<div class="hint">
**Hint:** In case, very long or many DNA sequences are scanned for
TFBSs, the distribution of the number of motif hits becomes very narrow.
In that case, the tiniest differences between the observed and the
expected number of hits give rise to very small *P-values*.
In this case, the fold-enrichment should be consulted to reveal
if the effect size is of biological relevance.
</div>
<br>
<div class="hint">
**Hint:** By default, `motifEnrichment` scans both DNA strands
for motif hits and draws its statistical conclusions based
on the compound Poisson model. However, motif enrichment can
also be performed with respect to scanning
single strands (e.g. when analyzing RNA sequences).
Please consult `?motifEnrichment` for the single strand option.
</div>
<br>
<div class="hint">
**Hint:** `motifEnrichment` may optionally invoke two alternative approaches for
approximating the *P-value*, 1) by a **compound Poisson approxmiation**
and 2) by a **combinatorial approximation** (see `?motifEnrichment`).
As a rule of thumb, we recommend the use
compound Poisson model for studying long (or many ) DNA sequences with
a fairly stringent $\alpha$ (e.g. 0.001 or smaller).
On the other hand, if a relaxed $\alpha$ is
desired for your analysis (e.g $\alpha\geq 0.01$),
the **combinatorial approximation** is likely to give more accurate results.
</div>
<div class="hint">
**Hint:** We recommend against using too relaxed choices for $\alpha$ (e.g $\alpha\geq 0.05$), as this violates some of the assumptions on which
the models are based on. The consequence might be significant biases of the results.
</div>
# Session Info
```{r sessionInfo}
sessionInfo()
```