Introduction

What’s New?

Workflow from a glance

Quick-Start

Installation
Loading SpliceWiz
The SpliceWiz Graphics User Interface (GUI)

Navigating the GUI

Building the SpliceWiz reference
Process BAM files using SpliceWiz
Collate the experiment
Importing the experiment
Differential analysis

Assigning annotations to samples
Filtering high-confidence events
Performing differential analysis

Visualization

Volcano plots
Scatter plots
Heatmaps

SpliceWiz Coverage Plots

Coverage plots of individual samples
Group-averaged coverage plots
Using the GUI

SessionInfo

Introduction

SpliceWiz is a graphical interface for differential alternative splicing and visualization in R. It differs from other alternative splicing tools as it is designed for users with basic bioinformatic skills to analyze datasets containing up to hundreds of samples! SpliceWiz contains a number of innovations including:

Super-fast handling of alignment BAM files using ompBAM, our developer resource for multi-threaded BAM processing,
Alternative splicing event filters, designed to remove unreliable measurements prior to differential analysis, which improves accuracy of reported results.
Group-averaged coverage plots: publication-ready figures to clearly visualize differential alternative splicing between biological / experimental conditions
Seamless storage and recall of sequencing coverage, using the COV format that stores strand-specific coverage typical of current RNA-seq protocols
Interactive figures, including scatter and volcano plots, heatmaps, and scrollable coverage plots, powered using the shinyDashboard interface

This vignette is a runnable working example of the SpliceWiz workflow. The purpose is to quickly demonstrate the basic functionalities of SpliceWiz.

We provide here a brief outline of the workflow for users to get started as quickly as possible. However, we also provide more details for those wishing to know more. Many sections will contain extra information that can be displayed when clicked on, such as these:

Click on me for more details

In most sections, we offer more details about each step of the workflow, that can be revealed in text segments like this one. Be sure to click on buttons like these, where available.

What’s New?

Note: for all runnable examples, first load the SpliceWiz library:

library(SpliceWiz)

What’s New: Novel splice detection (version 0.99.3+)

In version 0.99.3, SpliceWiz offers detection of novel events in addition to annotated events. How this works:

SpliceWiz compiles counts of all junctions (split reads) compatible with splice junction events (having compatible donor / acceptor splice motifs)
Additionally, it also uses “tandem junction” to identify novel exons (tandem junctions are reads that are split into 3 or more segments, arising from splicing of 2+ consecutive introns from a short-read sequence).

To reduce false positives in novel splicing detection, SpliceWiz provides several filters to reduce the number of novel junctions fed into the analysis:

Novel junctions that are lowly expressed (only in a small number of samples) are removed. The minimum number of samples required to retain a novel junction is set using novelSplicing_minSamples parameter
Alternately, junctions are retained if its expression exceeds a certain threshold (set using novelSplicing_countThreshold) in a smaller number of samples (set using novelSplicing_minSamplesAboveThreshold)
Further, novel junctions can be filtered by requiring at least one end to be an annotated splice site (this is enabled using novelSplicing_requireOneAnnotatedSJ = TRUE)

Novel ASE detection is integrated into the SpliceWiz pipeline at the collation step. After compilation and processing of novel junctions / TJ’s, the novel transcripts are appended to the transcript annotation, which is then used to re-construct the SpliceWiz reference. This reference is contained in the “Reference” subfolder of the output folder of collateData() function.

TL/DR - how to enable novel ASE mode

To enable analysis involving both annotated and novel ASEs, simply set novelSplicing = TRUE when running collateData(). For example:


# Usual pipeline:
ref_path <- file.path(tempdir(), "Reference")
buildRef(
    reference_path = ref_path,
    fasta = chrZ_genome(),
    gtf = chrZ_gtf()
)

pb_path <- file.path(tempdir(), "pb_output")
processBAM(
    bamfiles = bams$path,
    sample_names = bams$sample,
    reference_path = ref_path,
    output_path = pb_path
)

# Modified pipeline - collateData with novel ASE discovery:

nxtse_path <- file.path(tempdir(), "NxtSE_output")
collateData(
    Experiment = expr,
    reference_path = ref_path,
    output_path = nxtse_path,
    
        ## NEW ##
    novelSplicing = TRUE,
        # switches on novel splice detection
    
    novelSplicing_requireOneAnnotatedSJ = TRUE,
        # novel junctions must share one annotated splice site

    novelSplicing_minSamples = 3,
        # retain junctions observed in 3+ samples (of any non-zero expression)
    
    novelSplicing_minSamplesAboveThreshold = 1,
        # only 1 sample required if its junction count exceeds a set threshold
    novelSplicing_countThreshold = 10  
        # threshold for previous parameter
)

What’s New: Visualising junction reads in coverage plots (version 0.99.4+)

In version 0.99.4, SpliceWiz visualises split/junction reads in individual samples and in sample groups

For individual sample coverage plots (i.e. when condition is not set), junction counts for each sample are plotted. Samples with low junction counts (less than 0.01x of the track height) are omitted to reduce clutter.

For group-normalized coverage plots (where coverage of multiple samples in a condition group are combined), junctions are instead labeled by their “provisional PSIs”. These PSIs are calculated per junction (instead of per ASE). This is done by determining the ratio of junction counts as a proportion of all junction reads that share a common exon cluster as the junction being assessed.

TL/DR - how to enable junction plotting

To enable plotting of junctions, set plotJunctions = TRUE from within plotCoverage()

# Retrieve example NxtSE object
se <- SpliceWiz_example_NxtSE()

# Assign annotation of the experimental conditions
colData(se)$treatment <- rep(c("A", "B"), each = 3)

# Return a list of ggplot and plotly objects, also plotting junction counts
p <- plotCoverage(
    se = se,
    Event = "SE:SRSF3-203-exon4;SRSF3-202-int3",
    tracks = colnames(se)[1:4], 
    
        ## NEW ##
    plotJunctions = TRUE
)
#> Warning in geom_line(data = dfJn, aes_string(x = "x", y = "yarc", group = "junction", : Ignoring unknown aesthetics: label
#> Ignoring unknown aesthetics: label
#> Ignoring unknown aesthetics: label
#> Ignoring unknown aesthetics: label

if(interactive()) {
    # Display as plotly object
    p$final_plot
} else {
    # Display as ggplot
    as_ggplot_cov(p)
}

# Plot by condition "treatment", including provisional PSIs
p <- plotCoverage(
    se = se,
    Event = "SE:SRSF3-203-exon4;SRSF3-202-int3",
    tracks = c("A", "B"), condition = "treatment", 

        ## NEW ##
    plotJunctions = TRUE
)
#> Warning in geom_line(data = dfJn, aes_string(x = "x", y = "yarc", group = "junction", : Ignoring unknown aesthetics: label
#> Ignoring unknown aesthetics: label

if(interactive()) {
    # Display as plotly object
    p$final_plot
} else {
    # Display as ggplot
    as_ggplot_cov(p)
}

Workflow from a glance

The basic steps of SpliceWiz are as follows:

Building the SpliceWiz reference
Process BAM files using SpliceWiz
Collate results of individual samples into an experiment
Importing the collated experiment as an NxtSE object
Alternative splicing event filtering
Differential ASE analysis
Visualization

Quick-Start

Installation

To install SpliceWiz, start R (version “4.2.1”) and enter:

if (!requireNamespace("BiocManager", quietly = TRUE))
    install.packages("BiocManager")

BiocManager::install("SpliceWiz")

Enabling OpenMP (multi-threading) for MacOS users (Optional)

For MacOS users, make sure OpenMP libraries are installed correctly. We recommend users follow this guide, but the quickest way to get started is to install libomp via brew:

brew install libomp

Installing statistical package dependencies (Optional)

SpliceWiz uses established statistical tools to perform alternative splicing differential analysis:

limma: models included and excluded counts as log-normal distributions
DESeq2: models included and excluded counts as negative binomial distributions
DoubleExpSeq: models included and excluded counts using beta binomial distributions
satuRn: models included and excluded counts using quasi-binomial distributions.

To install all of these packages:


install.packages("DoubleExpSeq")

if (!requireNamespace("BiocManager", quietly = TRUE))
    install.packages("BiocManager")

BiocManager::install(c("DESeq2", "limma", "satuRn"))

Loading SpliceWiz

library(SpliceWiz)

Details

The SpliceWiz package loads the NxtIRFdata data package. This data package contains the example “chrZ” genome / annotations and 6 example BAM files that are used in this working example. Also, NxtIRFdata provides pre-generated mappability exclusion annotations for building human and mouse SpliceWiz references

The SpliceWiz Graphics User Interface (GUI)

SpliceWiz offers a graphical user interface (GUI) for interactive users, e.g. in the RStudio environment. To start using SpliceWiz GUI:

if(interactive()) {
    spliceWiz(demo = TRUE)
}

Note that the GUI demo mode is not supported on Bioconductor 3.13 or below.

Navigating the GUI

The SpliceWiz GUI uses the shinyDashboard interface. Use the side menu on the left hand side of the interface to navigate across the various sub-panels in the SpliceWiz GUI.

Menu Side Bar

Building the SpliceWiz reference

Why do we need the SpliceWiz reference?

SpliceWiz first needs to generate a set of reference files. The SpliceWiz reference is used to quantitate alternative splicing in BAM files, as well as in downstream collation, differential analysis and visualisation.

SpliceWiz generates a reference from a user-provided genome FASTA and genome annotation GTF file, and is optimised for Ensembl references but can accept other reference GTF files. Alternatively, SpliceWiz accepts AnnotationHub resources, using the record names of AnnotationHub records as input.

Using the example FASTA and GTF files, use the buildRef() function to build the SpliceWiz reference:

ref_path <- file.path(tempdir(), "Reference")
buildRef(
    reference_path = ref_path,
    fasta = chrZ_genome(),
    gtf = chrZ_gtf()
)

(NEW in version >= 0.99.3) The SpliceWiz reference can be viewed as data frames using various getter functions. For example, to view the annotated alternative splicing events (ASE):

df <- viewASE(ref_path)

See ?View-Reference-methods for a comprehensive list of getter functions

Using the GUI

After starting the SpliceWiz GUI in demo mode, click the Reference tab from the menu side bar. The following interface will be shown:

Reference GUI

The first step to building a SpliceWiz reference is to select a directory in which to create the reference.
SpliceWiz provides an interface to retrieve the genome sequence (FASTA) and transcriptome annotation (GTF) files from the Ensembl FTP server, by first selecting the “Release” and then “Species” from the drop-down boxes.
Alternatively, users can provide their own FASTA and GTF files.
Human (hg38, hg19) and mouse genomes (mm10, mm9) have the option of further refining IR analysis using built-in mappability exclusion annotations, allowing SpliceWiz to ignore intronic regions of low mappability.

For now, to continue with the demo and create the reference using the GUI, click on the Load Demo FASTA/GTF (5), and then click Build Reference (6)

Where did the FASTA and GTF files come from?

The helper functions chrZ_genome() and chrZ_gtf() returns the paths to the example genome (FASTA) and transcriptome (GTF) file included with the NxtIRFdata package that contains the working example used by SpliceWiz:

# Provides the path to the example genome:
chrZ_genome()
#> [1] "/home/biocbuild/bbs-3.16-bioc/R/site-library/NxtIRFdata/extdata/genome.fa"

# Provides the path to the example gene annotation:
chrZ_gtf()
#> [1] "/home/biocbuild/bbs-3.16-bioc/R/site-library/NxtIRFdata/extdata/transcripts.gtf"

What is the chrZ genome?

For the purpose of generating a running example to demonstrate SpliceWiz, we created an artificial genome / gene annotation. This was created using 7 human genes (SRSF1, SRSF2, SRSF3, TRA2A, TRA2B, TP53 and NSUN5). The SRSF and TRA family of genes all contain poison exons flanked by retained introns. Additionally, NSUN5 contains an annotated IR event in its terminal intron. Sequences from these 7 genes were aligned into one sequence to create an artificial chromosome Z (chrZ). The gene annotations were modified to only contain the 7 genes with the modified genomic coordinates.

What is Mappability and why should I care about it?

For the most part, the SpliceWiz reference can be built with just the FASTA and GTF files. This is sufficient for assessment for most forms of alternative splicing events.

For intron retention, accurate assessment of intron depth is important. However, introns contain many repetitive regions that are difficult to map. We refer to these regions as “mappability exclusions”.

We adopt IRFinder’s algorithm to identify these mappability exclusions. This is determined empirically by generating synthetic reads systematically from the genome, then aligning these reads back to the same genome. Regions that contain less than the expected coverage depth of reads define “mappability exclusions”.

See the vignette: SpliceWiz cookbook for details on how to generate “mappability exclusions” for any genome.

How do I use pre-built mappability exclusions to generate human and mouse references?

For human and mouse genomes, SpliceWiz provides pre-built mappability exclusion references that can be used to build the SpliceWiz reference. SpliceWiz provides these annotations via the NxtIRFdata package.

Simply specify the genome in the parameter genome_type in the buildRef() function (which accepts hg38, hg19, mm10 and mm9).

Additionally, a reference for non-polyadenylated transcripts is used. This has a minor role in QC of samples (to assess the adequacy of polyA capture).

For example, assuming your genome file "genome.fa" and a transcript annotation "transcripts.gtf" are in the working directory, a SpliceWiz reference can be built using the built-in hg38 low mappability regions and non-polyadenylated transcripts as follows:

ref_path_hg38 <- "./Reference"
buildRef(
    reference_path = ref_path_hg38,
    fasta = "genome.fa",
    gtf = "transcripts.gtf",
    genome_type = "hg38"
)

Process BAM files using SpliceWiz

The function SpliceWiz_example_bams() retrieves 6 example BAM files from ExperimentHub and places a copy of these in the temporary directory.

bams <- SpliceWiz_example_bams()

What are these example BAM files and how were they generated?

In this vignette, we provide 6 example BAM files. These were generated based on aligned RNA-seq BAMs of 6 samples from the Leucegene AML dataset (GSE67039). Sequences aligned to hg38 were filtered to only include genes aligned to that used to create the chrZ chromosome. These sequences were then re-aligned to the chrZ reference using STAR.

How can I easily locate multiple BAM files?

Often, alignment pipelines process multiple samples. SpliceWiz provides convenience functions to recursively locate all the BAM files in a given folder, and tries to ascertain sample names. Often sample names can be gleaned when: * The BAM files are named by their sample names, e.g. “sample1.bam”, “sample2.bam”. In this case, level = 0 * The BAM files have generic names but are contained inside parent directories labeled by their sample names, e.g. “sample1/Unsorted.bam”, “sample2/Unsorted.bam”. In this case, level = 1

# as BAM file names denote their sample names
bams <- findBAMS(tempdir(), level = 0) 

# In the case where BAM files are labelled using sample names as parent 
# directory names (which oftens happens with the STAR aligner), use level = 1

Process these BAM files using SpliceWiz:

pb_path <- file.path(tempdir(), "pb_output")
processBAM(
    bamfiles = bams$path,
    sample_names = bams$sample,
    reference_path = ref_path,
    output_path = pb_path
)

Using the GUI

After building the demo reference as shown in the previous section, start SpliceWiz GUI in demo mode. Then, click the Experiment tab from the menu side bar. The following interface will be shown:

Experiment GUI

Click on (2) Define Project Folders to bring up the following drop-down box:

Define Project Folders

We need to define the folders that contain our reference, BAM files, as well as processBAM output files and the final compiled experiment (that generates the NxtSE object).

Click on (1) Choose Folder (Reference) and select the Reference directory (where the SpliceWiz reference was generated by the previous step. Then,
Click on (2) Choose Folder (BAM files) and select the bams directory (where the demo BAM files have been generated).
Click on (3) Choose Folder (processBAM output) and select the pb_output directory (which should currently be empty).
Finally, click on (4) Choose Folder (NxtSE files) and select the NxtSE directory(which should currently be empty).

After our folders have been defined, on the right hand side, an interactive table should be displayed that looks like the following:

Running processBAM

To run processBAM on the example BAM files, first drag to select the 6 cells containing the BAM file paths (as shown). Next, click the Process BAM files to open the dropdown menu, and then click Run processBAM(). A prompt should then be displayed asking whether you wish to proceed. Click OK to start running processBAM.

What is the processBAM() function

SpliceWiz’s processBAM() function can process one or more BAM files. This function is ultra-fast, relying on an internal native C++ function that uses OpenMP multi-threading (via the ompBAM C++ API).

Input BAM files can be either read-name sorted or coordinate sorted (although SpliceWiz prefers the former). Indexing of coordinate-sorted BAMs are not necessary.

processBAM() loads the SpliceWiz reference. Then, it reads each BAM file in their entirety, and quantifies the following:

Basic QC parameters including number of reads, directionality, etc
Counts of gapped (junction) reads / fragments
Intron coverage depths and other parameters (identical output to IRFinder)
COV files (which are like BigWig files but record strand-specific coverage)
Miscellaneous quants including coverage of chromosomes, intergenic regions, rRNAs, and non-polyadenylated regions

For each BAM file, processBAM() generates two output files. The first is a gzipped text file containing all the quantitation data. The second is a COV file which contains the per-nucleotide RNA-seq coverage of the sample.

More details on the processBAM() function

At minimum, processBAM() requires four parameters:

bamfiles : The paths of the BAM files
sample_names : The sample names corresponding to the given BAM files
reference_path : The directory containing the SpliceWiz reference
output_path : The directory where the output of processBAM() should go

pb_path <- file.path(tempdir(), "pb_output")
processBAM(
    bamfiles = bams$path,
    sample_names = bams$sample,
    reference_path = ref_path,
    output_path = pb_path
)

processBAM() also takes several optional, but useful, parameters:

n_threads : The number of threads for multi-threading
overwrite : Whether existing files in the output directory should be overwritten
run_featureCounts : (Requires the Rsubread package) runs featureCounts to obtain gene counts (which outputs results as an RDS file)

For example, to run processBAM() using 2 threads, disallow overwrite of existing processBAM() outputs, and run featureCounts afterwards, one would run the following:

# NOT RUN

# Re-run IRFinder without overwrite, and run featureCounts
require(Rsubread)

processBAM(
    bamfiles = bams$path,
    sample_names = bams$sample,
    reference_path = ref_path,
    output_path = pb_path,
    n_threads = 2,
    overwrite = FALSE,
    run_featureCounts = TRUE
)

# Load gene counts
gene_counts <- readRDS(file.path(pb_path, "main.FC.Rds"))

# Access gene counts:
gene_counts$counts

Collate the experiment

The helper function findSpliceWizOutput() organises the output files of SpliceWiz’s processBAM() function. It identifies matching "txt.gz" and "cov" files for each sample, and organises these file paths conveniently into a 3-column data frame:

expr <- findSpliceWizOutput(pb_path)

Using this data frame, collate the experiment using collateData(). We name the output directory as NxtSE_output as this folder will contain the data needed to import the NxtSE object:

nxtse_path <- file.path(tempdir(), "NxtSE_output")
collateData(
    Experiment = expr,
    reference_path = ref_path,
    output_path = nxtse_path
)

What is the collateData() function

collateData() combines the processBAM() output files of multiple samples and builds a single database. collateData() creates a number of files in the chosen output directory. These outputs can then be imported into the R session as a NxtSE data object for downstream analysis.

At minimum, collateData() takes the following parameters:

Experiment : The 2- or 3- column data frame. The first column should contain (unique) sample names. The second and (optional) third columns contain the "txt.gz" and "cov" file paths
reference_path : The directory containing the SpliceWiz reference
output_path : The directory where the output of processBAM() should go

collateData() can take some optional parameters:

IRMode : Whether to use SpliceWiz’s SpliceOver method, or IRFinder’s SpliceMax method, to determine total spliced transcript abundance. Briefly, SpliceMax considers junction reads that have either flanking splice site coordinate. SpliceOver considers additional junction reads that splices across exon clusters in common. Exon clusters are groups of mutually-overlapping exons. SpliceOver is the default option.
overwrite : Whether files in the output directory should be overwritten
n_threads : Use multi-threaded operations where possible
lowMemoryMode : Minimise memory usage where possible. Note that most of the collateData pipeline will be single-threaded if this is set to TRUE.

collateData() is a memory-intensive operation when run using multiple threads. We estimate it can use up to 6-7 Gb per thread. lowMemoryMode will minimise RAM usage to ~ 8 Gb, but will be slower and run on a single thread.

Enabling novel splicing detection

Novel splicing detection can be switched on by setting novelSplicing = TRUE from within the collateData() function:

# Modified pipeline - collateData with novel ASE discovery:

nxtse_path <- file.path(tempdir(), "NxtSE_output_novel")
collateData(
    Experiment = expr,
    reference_path = ref_path,
    output_path = nxtse_path,
    novelSplicing = TRUE     ## NEW ##
)

collateData() uses split reads that are not annotated introns to help construct hypothetical minimal transcripts. These are then injected into the original transcriptome annotation (GTF) file, whereby the SpliceWiz reference is rebuilt. The new SpliceWiz reference (which contains these novel transcripts) is then used to collate the samples.

To reduce false positives in novel splicing detection, SpliceWiz provides several filters to reduce the number of novel junctions fed into the analysis:

Novel junctions that are lowly expressed (only in a small number of samples) are removed. The minimum number of samples required to retain a novel junction is set using novelSplicing_minSamples parameter
Alternately, junctions are retained if its expression exceeds a certain threshold (set using novelSplicing_countThreshold) in a smaller number of samples (set using novelSplicing_minSamplesAboveThreshold)
Further, novel junctions can be filtered by requiring at least one end to be an annotated splice site (this is enabled using novelSplicing_requireOneAnnotatedSJ = TRUE).

For example, if one wished to retain novel reads seen in 3 or more samples, or novel spliced reads with 10 or more counts in at least 1 sample, and requiring at least one end of a novel junction being an annotated splice site:

nxtse_path <- file.path(tempdir(), "NxtSE_output_novel")
collateData(
    Experiment = expr,
    reference_path = ref_path,
    output_path = nxtse_path,
    
        ## NEW ##
    novelSplicing = TRUE,
        # switches on novel splice detection
    
    novelSplicing_requireOneAnnotatedSJ = TRUE,
        # novel junctions must share one annotated splice site

    novelSplicing_minSamples = 3,
        # retain junctions observed in 3+ samples (of any non-zero expression)
    
    novelSplicing_minSamplesAboveThreshold = 1,
        # only 1 sample required if its junction count exceeds a set threshold
    novelSplicing_countThreshold = 10  
        # threshold for previous parameter
)

Using the GUI

After running the Reference and processBAM steps as indicated in the previous sections (of the GUI instructions), click Construct Experiment. Your interface should now look like this:

collateData GUI

This drop-down dialog box contains several parameters related to novel splicing detection:

Toggle on/off novel splice detection
Restrict novel junction reads to having one annotated splice site
Filter novel junction counts based on any expression (min number of samples set here)
Filter novel junction counts based on expression threshold (min number of samples set here)
Threshold split read count (expression threshold based novel junction filter)

To collate the experiment, click on (6) Run collateData(). An alert pop-up should be displayed when this process is complete.

To clear all the setting on the Experiment page, click (7).

Importing the experiment

Before differential analysis can be performed, the collated experiment must be imported into the R session as an NxtSE data object.

After running collateData(), import the experiment using the makeSE() function:

se <- makeSE(nxtse_path)

Using the GUI

After running the steps in the previous GUI sections, navigate to Analysis and then click the Load Experiment on the menu bar. The display should look like this:

Loading the NxtSE GUI

First, click the Open Folder containing NxtSE to open the dropdown, then click Choose Folder (NxtSE). Select the NxtSE directory. The interface should now look like this:

Loading the NxtSE GUI

To load the NxtSE object, click the Load NxtSE object to open the dropdown, then click Load NxtSE object to load the NxtSE object into the current session.

What is the makeSE() function

The makeSE() function imports the compiled data generated by the collateData() function. Data is imported as an NxtSE object. Downstream analysis, including differential analysis and visualization, is performed using the NxtSE object.

More details about the makeSE() function

By default, makeSE() uses delayed operations to avoid consuming memory until the data is actually needed. This is advantageous in analysis of hundreds of samples on a computer with limited resources. However, it will be slower. To load all the data into memory, we need to “realize” the NxtSE object, as follows:

se <- realize_NxtSE(se)

Alternatively, makeSE() can realize the NxtSE object at construction:

se <- makeSE(nxtse_path, realize = TRUE)

By default, makeSE() constructs the NxtSE object using all the samples in the collated data. It is possible (and particularly useful in large data sets) to read only a subset of samples. In this case, construct a data frame object with the first column containing the desired sample names and parse this into the colData parameter as shown:

subset_samples <- colnames(se)[1:4]
df <- data.frame(sample = subset_samples)
se_small <- makeSE(nxtse_path, colData = df, RemoveOverlapping = TRUE)

In complex transcriptomes including those of human and mouse, alternative splicing implies that introns are often overlapping. Thus, algorithms run the risk of over-calling intron retention where overlapping introns are assessed. SpliceWiz removes overlapping introns by considering only introns belonging to the major splice isoforms. It estimates a list of introns of major isoforms by assessing the compatible splice junctions of each isoform, and removes overlapping introns belonging to minor isoforms. To disable this functionality, set RemoveOverlapping = FALSE.

Differential analysis

Assigning annotations to samples

colData(se)$condition <- rep(c("A", "B"), each = 3)
colData(se)$batch <- rep(c("K", "L", "M"), 2)

Using the GUI

After following the previous GUI sections, with the NxtSE loaded, click on Review Annotations. Then, click Edit Interactively. Your interface should now look like this:

NxtSE Annotations GUI

Now, lets add an extra column. In the Annotation Columns box which has just appeared, type in a name condition, then click Add. Then, click batch and again click Add. Then, click on each empty cell and fill in the fields as shown:

NxtSE Annotations GUI

Finally, the annotated NxtSE needs to be reloaded into memory. Click Load NxtSE object and then Load NxtSE object to reload the NxtSE object with the new annotations.

NB: Annotations can be assigned prior to loading the NxtSE object, but can only be done after the NxtSE folder is selected.

Alternatively, users can provide a file that contains the annotations in tabular format (e.g. a comma-separated values (CSV) or tab-separated values (TSV) file). Click on Review Annotations and then Import Data Frame from File. Then, select the demo_annotations.csv file that has been pre-generated by SpliceWiz’s demo mode.

Saving and reloading the NxtSE as an RDS file (GUI)

Once an NxtSE object has been loaded into memory, you can save it as an RDS object so it can be reloaded in a later session. To do this, click the Save NxtSE to/from RDS file, then click Save NxtSE as RDS. Choose a file name and location and press OK. This RDS file can be loaded as an NxtSE object in a later GUI session by clicking the Load NxtSE from RDS button.

What is the NxtSE object

NxtSE is a data object which contains all the required data for downstream analysis after all the BAM alignment files have been process and the experiment is collated.

se
#> class: NxtSE 
#> dim: 168 6 
#> metadata(11): Up_Inc Down_Inc ... ref BuildVersion
#> assays(5): Included Excluded Depth Coverage minDepth
#> rownames(168): TRA2B/ENST00000453386_Intron8/clean
#>   TRA2B/ENST00000453386_Intron7/clean ...
#>   RI:SRSF2-203-exon2;SRSF2-202-intron2
#>   RI:SRSF2-203-exon2;SRSF2-206-intron1
#> rowData names(18): EventName EventType ... is_annotated_IR
#>   NMD_direction
#> colnames(6): 02H003 02H025 ... 02H043 02H046
#> colData names(2): condition batch

The NxtSE object inherits the SummarizedExperiment object. This means that the functions for SummarizedExperiment can be used on the NxtSE object. These include row and column annotations using the rowData() and colData() accessors.

Rows in the NxtSE object contain information about each alternate splicing event. For example:

head(rowData(se))
#> DataFrame with 6 rows and 18 columns
#>                                                  EventName   EventType
#>                                                <character> <character>
#> TRA2B/ENST00000453386_Intron8/clean TRA2B/ENST0000045338..          IR
#> TRA2B/ENST00000453386_Intron7/clean TRA2B/ENST0000045338..          IR
#> TRA2B/ENST00000453386_Intron6/clean TRA2B/ENST0000045338..          IR
#> TRA2B/ENST00000453386_Intron5/clean TRA2B/ENST0000045338..          IR
#> TRA2B/ENST00000453386_Intron4/clean TRA2B/ENST0000045338..          IR
#> TRA2B/ENST00000453386_Intron3/clean TRA2B/ENST0000045338..          IR
#>                                          EventRegion              intron_id
#>                                          <character>            <character>
#> TRA2B/ENST00000453386_Intron8/clean chrZ:1921-2559/- ENST00000453386_Intr..
#> TRA2B/ENST00000453386_Intron7/clean chrZ:2634-3631/- ENST00000453386_Intr..
#> TRA2B/ENST00000453386_Intron6/clean chrZ:3692-5298/- ENST00000453386_Intr..
#> TRA2B/ENST00000453386_Intron5/clean chrZ:5383-6205/- ENST00000453386_Intr..
#> TRA2B/ENST00000453386_Intron4/clean chrZ:6322-7990/- ENST00000453386_Intr..
#> TRA2B/ENST00000453386_Intron3/clean chrZ:8180-9658/- ENST00000453386_Intr..
#>                                     Inc_Is_Protein_Coding Exc_Is_Protein_Coding
#>                                                 <logical>             <logical>
#> TRA2B/ENST00000453386_Intron8/clean                  TRUE                  TRUE
#> TRA2B/ENST00000453386_Intron7/clean                  TRUE                  TRUE
#> TRA2B/ENST00000453386_Intron6/clean                  TRUE                  TRUE
#> TRA2B/ENST00000453386_Intron5/clean                  TRUE                  TRUE
#> TRA2B/ENST00000453386_Intron4/clean                  TRUE                  TRUE
#> TRA2B/ENST00000453386_Intron3/clean                  TRUE                  TRUE
#>                                     Exc_Is_NMD Inc_Is_NMD     Inc_TSL
#>                                      <logical>  <logical> <character>
#> TRA2B/ENST00000453386_Intron8/clean      FALSE       TRUE           1
#> TRA2B/ENST00000453386_Intron7/clean      FALSE       TRUE           1
#> TRA2B/ENST00000453386_Intron6/clean      FALSE       TRUE           1
#> TRA2B/ENST00000453386_Intron5/clean      FALSE       TRUE           1
#> TRA2B/ENST00000453386_Intron4/clean      FALSE       TRUE           1
#> TRA2B/ENST00000453386_Intron3/clean      FALSE       TRUE           1
#>                                         Exc_TSL          Event1a     Event2a
#>                                     <character>      <character> <character>
#> TRA2B/ENST00000453386_Intron8/clean           1 chrZ:1921-2559/-          NA
#> TRA2B/ENST00000453386_Intron7/clean           1 chrZ:2634-3631/-          NA
#> TRA2B/ENST00000453386_Intron6/clean           1 chrZ:3692-5298/-          NA
#> TRA2B/ENST00000453386_Intron5/clean           1 chrZ:5383-6205/-          NA
#> TRA2B/ENST00000453386_Intron4/clean           1 chrZ:6322-7990/-          NA
#> TRA2B/ENST00000453386_Intron3/clean           1 chrZ:8180-9658/-          NA
#>                                         Event1b     Event2b
#>                                     <character> <character>
#> TRA2B/ENST00000453386_Intron8/clean          NA          NA
#> TRA2B/ENST00000453386_Intron7/clean          NA          NA
#> TRA2B/ENST00000453386_Intron6/clean          NA          NA
#> TRA2B/ENST00000453386_Intron5/clean          NA          NA
#> TRA2B/ENST00000453386_Intron4/clean          NA          NA
#> TRA2B/ENST00000453386_Intron3/clean          NA          NA
#>                                     is_always_first_intron
#>                                                  <logical>
#> TRA2B/ENST00000453386_Intron8/clean                  FALSE
#> TRA2B/ENST00000453386_Intron7/clean                  FALSE
#> TRA2B/ENST00000453386_Intron6/clean                  FALSE
#> TRA2B/ENST00000453386_Intron5/clean                  FALSE
#> TRA2B/ENST00000453386_Intron4/clean                  FALSE
#> TRA2B/ENST00000453386_Intron3/clean                  FALSE
#>                                     is_always_last_intron is_annotated_IR
#>                                                 <logical>       <logical>
#> TRA2B/ENST00000453386_Intron8/clean                    NA           FALSE
#> TRA2B/ENST00000453386_Intron7/clean                 FALSE           FALSE
#> TRA2B/ENST00000453386_Intron6/clean                 FALSE           FALSE
#> TRA2B/ENST00000453386_Intron5/clean                 FALSE           FALSE
#> TRA2B/ENST00000453386_Intron4/clean                 FALSE            TRUE
#> TRA2B/ENST00000453386_Intron3/clean                 FALSE           FALSE
#>                                     NMD_direction
#>                                         <numeric>
#> TRA2B/ENST00000453386_Intron8/clean             1
#> TRA2B/ENST00000453386_Intron7/clean             1
#> TRA2B/ENST00000453386_Intron6/clean             1
#> TRA2B/ENST00000453386_Intron5/clean             1
#> TRA2B/ENST00000453386_Intron4/clean             1
#> TRA2B/ENST00000453386_Intron3/clean             1

Columns contain information about each sample. By default, no annotations are assigned to each sample. These can be assigned as shown above.

Also, NxtSE objects can be subsetted by rows (ASEs) or columns (samples). This is useful if one wishes to perform analysis on a subset of the dataset, or only on a subset of ASEs (say for example, only skipped exon events). Subsetting is performed just like for SummarizedExperiment objects:

# Subset by columns: select the first 2 samples
se_sample_subset <- se[,1:2]

# Subset by rows: select the first 10 ASE events
se_ASE_subset <- se[1:10,]

Filtering high-confidence events

SpliceWiz offers default filters to identify and remove low confidence alternative splice events (ASEs). Run the default filter using the following:

se.filtered <- se[applyFilters(se),]
#> Running Depth filter
#> Running Participation filter
#> Running Participation filter
#> Running Consistency filter
#> Running Terminus filter
#> Running ExclusiveMXE filter

Using the GUI

After following the GUI tutorials in the prior sections, click on Analysis and then Filters from the menu bar. It should look like this:

Filters - GUI

To load SpliceWiz’s default filters, click the top right button Load Default Filters. Then to apply these filters to the NxtSE, click Apply Filters. After the filters have been run, your session should now look like this:

SpliceWiz default filters- GUI

Why do we need to filter alternative splicing events?

Often, the gene annotations contain isoforms for all discovered splicing events. Most annotated transcripts are not expressed, and their inclusion in differential analysis complicates results including adjusting for multiple testing. It is prudent to filter these out using various approaches, akin to removing genes with low gene counts in differential gene analysis. We suggest using the default filters which work well for small experiments with sequencing depths at 100-million paired-end reads.

To learn more about filters, consult the documentation via ?ASEFilters

Performing differential analysis

Using the limma wrapper ASE_limma(), perform differential ASE analysis between conditions “A” and “B”:

# Requires limma to be installed:
require("limma")
res_limma <- ASE_limma(
    se = se.filtered,
    test_factor = "condition",
    test_nom = "B",
    test_denom = "A",
)

Using the GUI

After running the previous sections (of the GUI instructions), click Analysis and then Differential Expression Analysis on the menu side bar. It should look something like this (but with empty fields):

Differential Expression GUI

To perform limma-based differential analysis, first ensure Method is set to limma. Using the Variable drop-down box, select condition. Then, select the Nominator and Denominator fields to A and B, respectively. Leave the batch factor fields as (none). Then, click Perform DE.

Once differential expression analysis has finished, your session should look like below. The output is a DT-based data table equivalent to the ASE_limma() function.

Differential Expression (limma) GUI

NB: The interface allows users to choose to sort the results either by nominal or (multiple-testing) adjusted P values

NB2: There are 3 different ways Intron Retention events can be quantified and analysed - see “What are the different ways intron retention is measured?” below for further details.

What are the options for differential ASE analysis?

SpliceWiz provides wrappers to three established algorithms:

ASE_limma uses limma to model isoform counts as log-normal distributions. Limma is probably the fastest method and is ideal for large datasets
ASE_DESeq uses DESeq2 to model isoform counts as negative binomial distribution. This method is the most computationally expensive, but gives robust results. Time series analysis is also available for this mode
ASE_DoubleExpSeq uses the lesser-known CRAN package DoubleExpSeq. This package uses the beta-binomial distribution to model isoform counts. The method is at least as fast as limma, but for now it is restricted to analysis between two groups (i.e. batch correction is not implemented)
(New in 0.99.4+) ASE_satuRn uses the new satuRn package which models counts using the quasi-binomial distribution.

# Requires limma to be installed:
require("limma")
res_limma <- ASE_limma(
    se = se.filtered,
    test_factor = "condition",
    test_nom = "B",
    test_denom = "A"
)

# Requires DESeq2 to be installed:
require("DESeq2")
res_deseq <- ASE_DESeq(
    se = se.filtered,
    test_factor = "condition",
    test_nom = "B",
    test_denom = "A",
    n_threads = 1
)

# Requires DoubleExpSeq to be installed:
require("DoubleExpSeq")
res_DES <- ASE_DoubleExpSeq(
    se = se.filtered,
    test_factor = "condition",
    test_nom = "B",
    test_denom = "A"
)

# Requires satuRn to be installed:
require("satuRn")
res_saturn <- ASE_satuRn(
    se = se.filtered,
    test_factor = "condition",
    test_nom = "B",
    test_denom = "A",
    n_threads = 1
)

What are the different ways intron retention is measured?

Intron retention can be measured via two approaches.

The first (and preferred) approach is using IR-ratio. We presume that every intron is potentially retained (thus ignoring annotation). Given this results in many overlapping introns, SpliceWiz adjusts for this via the following:

Where there are mutually-overlapping introns, the less abundant intron is removed from the analysis. Abundance is estimated across the entire dataset, and less-abundant overlapping introns are removed at the makeSE() step.
IR-ratio measures included (intronic) abundance using an identical approach to IRFinder, i.e., it calculates the trimmed mean of sequencing depth across the intron, excluding annotated outliers (other exons, intronic elements). Given we cannot assume whether the exact intron corresponds to the major isoform, we estimate splicing abundance by summing junction reads that share either exon cluster as the intron of interest (SpliceOver method in SpliceWiz). Alternately, users can choose to use IRFinder’s SpliceMax method, summing junction reads that share either splice junction with the intron of interest. This choice is also made by the user at the makeSE() step.

At the differential analysis step, users can choose the following:

IRmode = "all" - all introns are potentially retained, use IR-ratio to quantify IR (EventType = "IR")
IRmode = "annotated" - only annotated retained intron events are considered, but use IR-ratio to quantify IR (EventType = "IR")
IRmode = "annotated_binary" - only annotated retained intron events are considered, use PSI to quantify IR - which considers the IR-transcript and the transcript isoform with the exactly-spliced intron as binary alternatives. Splicing of overlapping introns are not considered in PSI quantitation.

res_limma_allIntrons <- ASE_limma(
    se = se.filtered,
    test_factor = "condition",
    test_nom = "B",
    test_denom = "A",
    IRmode = "all"
)

res_limma_annotatedIR <- ASE_limma(
    se = se.filtered,
    test_factor = "condition",
    test_nom = "B",
    test_denom = "A",
    IRmode = "annotated"
)

res_limma_annotated_binaryIR <- ASE_limma(
    se = se.filtered,
    test_factor = "condition",
    test_nom = "B",
    test_denom = "A",
    IRmode = "annotated_binary"
)

Can I account for batch factors?

ASE_limma, ASE_DESeq and ASE_satuRn can accept up to 2 categories of batches from which to normalize. For example, to normalize the analysis by the batch category, one would run:

require("limma")
res_limma_batchnorm <- ASE_limma(
    se = se.filtered,
    test_factor = "condition",
    test_nom = "B",
    test_denom = "A",
    batch1 = "batch"
)

Can I do time series analysis?

Time series analysis can be performed only using DESeq2 (for now). To do this, simply do not specify the test_nom and test_denom parameters. As long as the test_factor contains numeric values, ASE_DESeq will treat it as a continuous variable. See the following example:

colData(se.filtered)$timevar <- rep(c(0,1,2), 2)

require("DESeq2")
res_deseq_cont <- ASE_DESeq(
    se = se.filtered,
    test_factor = "timevar"
)

Visualization

Volcano plots

Volcano plots show changes in PSI levels (log fold change, x axis) against statistical significance (-log10 p values, y axis):

library(ggplot2)

ggplot(res_limma,
        aes(x = logFC, y = -log10(adj.P.Val))) + 
    geom_point() +
    labs(title = "Differential analysis - B vs A",
         x = "Log2-fold change", y = "BH-adjusted P values (-log10)")

Can I visualize significant events for each modality of alternative splicing events?

Yes. We can use ggplot2’s facet_wrap function to separately plot volcanos for each modality of ASE. The type of ASE is contained in the EventType column of the differential results data frame.

ggplot(res_limma,
        aes(x = logFC, y = -log10(adj.P.Val))) + 
    geom_point() + facet_wrap(vars(EventType)) +
    labs(title = "Differential analysis - B vs A",
         x = "Log2-fold change", y = "BH-adjusted P values (-log10)")

Using the GUI

After following the previous sections including differential analysis, navigate to Display and then Volcano Plot. The default volcano plot (top 1000 ASEs) will be automatically generated as a plotly object:

Volcano Plot - GUI

You can customize this volcano plot using the following controls:

1. Adjust the number of top events. Differential ASEs are ranked either by adjusted or nominal p-values depending on the Sort by Adjusted P Values in the differential analysis interface.
1. Select one or more Splice Types. The volcano plot will be filtered by the selected ASE modalities
1. You can facet the volcano plot to show separate volcano plots for each modality
1. Instead of nominal p values, Benjamini Hochberg adjusted p values are shown if this is ON.
1. If NMD Mode is ON, the horizontal axis represents whether splicing is shifted towards (positive values) or away from (negative values) a NMD substrate transcript
1. Clear settings

Scatter plots

Scatter plots are useful for showing splicing levels (percent-spliced-in, PSI) between two conditions. The results from differential analysis contains these values and can be plotted:

library(ggplot2)

ggplot(res_limma, aes(x = 100 * AvgPSI_B, y = 100 * AvgPSI_A)) + 
    geom_point() + xlim(0, 100) + ylim(0, 100) +
    labs(title = "PSI values across conditions",
         x = "PSI of condition B", y = "PSI of condition A")

Using the GUI

After following the previous sections including differential analysis, navigate to Display and then Scatter Plot.

You can customize the scatter plot using the following controls:

1. Adjust the number of top events. Differential ASEs are ranked either by adjusted or nominal p-values depending on the Sort by Adjusted P Values in the differential analysis interface.
1. Select one or more Splice Types. The scatter plot will be filtered by the selected ASE modalities
1. Variable refers to the annotation category
1. and (5) The X-axis and Y-axis dropdowns refers to the experimental conditions for which to contrast the average PSI values.
1. If NMD Mode is ON, the PSI values are altered such that they represent the inclusion values of the NMD substrate (instead of the “included” isoform)
1. Clear settings

For now, change the Variable to condition, X-axis condition to A and Y-axis condition to B. A scatter plot should be automatically generated as follows:

Scatter plot - GUI

Selecting ASEs of interest using the interactive plots (GUI)

SpliceWiz GUI generates plotly interactive figures. For volcano and scatter plots, points of interest can be selected using the lasso or box select tools. For example, we can select the top hits from the faceted volcano plot as shown:

Volcano plot - selecting ASEs of interest via the GUI

These ASEs of interest will then be highlighted in other plots, for example scatter plot:

Scatter plot - highlighted ASE events

How do I generate average PSI values for many conditions?

SpliceWiz provides the makeMeanPSI() function that can generate mean PSI values for each condition of a condition category. For example, the below code will calculate the mean PSIs of each “batch” of this example experiment:

meanPSIs <- makeMeanPSI(
    se = se,
    condition = "batch",
    conditionList = list("K", "L", "M")
)

Heatmaps

Heatmaps are useful for visualizing differential expression of individual samples, as well as potential patterns of expression.

First, obtain a matrix of PSI values:

# Create a matrix of values of the top 10 differentially expressed events:
mat <- makeMatrix(
    se.filtered,
    event_list = res_limma$EventName[1:10],
    method = "PSI"
)

How does makeMatrix() work?

makeMatrix() provides a matrix of PSI values from the given NxtSE object. The parameters event_list and sample_list allows subsetting for ASEs and/or samples, respectively.

The parameter method accepts 3 options:

"PSI" : outputs raw PSI values
"logit" : outputs logit PSI values
"Z-score" : outputs Z-score transformed PSI values

Also, makeMatrix() facilitates exclusion of low confidence PSI values. These can occur when counts of both isoforms are too low. Setting the depth_threshold (default 10) will set samples with total isoform count below this value to be converted to NA.

Splicing events (ASEs) with too many NA values are filtered out. Setting the parameter na.percent.max (default 0.1) means any ASE with the proportion of NA above this threshold will be removed from the final matrix.

Plot this matrix of values in a heatmap:

library(pheatmap)

anno_col_df <- as.data.frame(colData(se.filtered))
anno_col_df <- anno_col_df[, 1, drop=FALSE]

pheatmap(mat, annotation_col = anno_col_df)

Using the GUI

Navigate to Display, then Heatmap in the menu side bar. A heatmap will be automatically generated:

Heatmap - GUI

The heatmap can be customized as follows:

1. Select the top hits from either Top All Results - unfiltered results, Top Filtered Results - results filtered using the Differential Expression interactive DT data table, or Top Selected Results - results selected using the interactive box or lasso select tools in the interactive volcano and/or scatter plots.
1. Highlight annotation categories. Samples will be highlighted based on the selected annotation categories
1. Number of top events to display
1. Mode: Either PSI, Logit-PSI, or Z-score transformed values
1. Color palette

SpliceWiz Coverage Plots

Coverage plots visualize RNA-seq coverage in individual samples. SpliceWiz uses its coverage normalization algorithm to visualize group differences in PSIs.

What are SpliceWiz coverage plots and how are they generated?

SpliceWiz produces RNA-seq coverage plots of analysed samples. Coverage data is compiled simultaneous to the IR and junction quantitation performed by processBAM(). This data is saved in “COV” files, which is a BGZF compressed and indexed file. COV files show compression and performance gains over BigWig files.

Additionally, SpliceWiz visualizes plots group-averaged coverages, based on user-defined experimental conditions. This is a powerful tool to illustrate group-specific differential splicing or IR. SpliceWiz does this by normalising the coverage depths of each sample based on transcript depth at the splice junction / intron of interest. By doing so, the coverage depths of constitutively expressed flanking exons are normalised to unity. As a result, the intron depths reflect the fraction of transcripts with retained introns and can be compared across samples.

Coverage plots of individual samples

First, lets obtain a list of differential events with delta PSI > 5%:

res_limma.filtered <- subset(res_limma, abs(deltaPSI) > 0.05)

Plot up to 4 individual samples using plotCoverage():

p <- plotCoverage(
    se = se,
    Event = res_limma.filtered$EventName[1],
    tracks = colnames(se)[c(1,2,4,5)],
)

if(interactive()) {
    # Display as plotly object
    p$final_plot
} else {
    # Display as ggplot
    as_ggplot_cov(p)
}

Plotting junction counts (new in 0.99.4+)

Sashimi plots, first popularised by MISO, are coverage plots with arcs that denote split reads. They are each labelled by a number denoting the number of split reads spanning the corresponding junctions.

In SpliceWiz (>=0.99.4) junctions can be plotted by setting plotJunctions = TRUE from within plotCoverage()

p <- plotCoverage(
    se = se,
    Event = res_limma.filtered$EventName[1],
    tracks = colnames(se)[c(1,2,4,5)],
    plotJunctions = TRUE
)
#> Warning in geom_line(data = dfJn, aes_string(x = "x", y = "yarc", group = "junction", : Ignoring unknown aesthetics: label
#> Ignoring unknown aesthetics: label
#> Ignoring unknown aesthetics: label
#> Ignoring unknown aesthetics: label

if(interactive()) {
    # Display as plotly object
    p$final_plot
} else {
    # Display as ggplot
    as_ggplot_cov(p)
}

The genome track is too cluttered! How can I customize it?

We can set the selected_transcripts parameter of the plotCoverage() function to only display the specified transcripts. Selected transcripts can be supplied by name or ID (transcript_name and transcript_id field in the GTF file, respectively)

p <- plotCoverage(
    se = se,
    Event = res_limma.filtered$EventName[1],
    tracks = colnames(se)[c(1,2,4,5)],
    selected_transcripts = c("NSUN5-201", "NSUN5-203", "NSUN5-206")
)

if(interactive()) {
    # Display as plotly object
    p$final_plot
} else {
    # Display as ggplot
    as_ggplot_cov(p)
}

Group-averaged coverage plots

We plot average coverages of groups “A” and “B” of the “condition” category, as follows:

p <- plotCoverage(
    se = se,
    Event = res_limma.filtered$EventName[1],
    tracks = c("A", "B"),
    condition = "condition",
    plotJunctions = TRUE,
)
#> Warning in geom_line(data = dfJn, aes_string(x = "x", y = "yarc", group = "junction", : Ignoring unknown aesthetics: label
#> Ignoring unknown aesthetics: label

if(interactive()) {
    # Display as plotly object
    p$final_plot
} else {
    # Display as ggplot
    as_ggplot_cov(p)
}

Note that junctions are labelled not by their counts (which are different between replicates of a group, and are dependent on gene expression and sequencing depth). Instead, SpliceWiz labels junction arcs by the mean +/- sd of “provisional PSI”. These are PSI estimates calculated for each sample, based on the ratio of the junction read count as a proportion of total spliced read counts that share either exon cluster with that of the assessed junction.

How do I stack conditions together to better visualize their coverage differences?

Differences in group-normalized coverage are best visualized when the traces are stacked together in the same track. To do this, set stack_tracks = TRUE from within plotCoverage(). Note this is only enabled when plotting group-normalized coverage plots (i.e., when condition is set).

p <- plotCoverage(
    se = se,
    Event = res_limma.filtered$EventName[1],
    tracks = c("A", "B"),
    condition = "condition",
    stack_tracks = TRUE,
)

if(interactive()) {
    # Display as plotly object
    p$final_plot
} else {
    # Display as ggplot
    as_ggplot_cov(p)
}

What are the shaded ribbons in this plot?

The shaded ribbons represent the uncertainty of the coverage. This is denoted by the ribbon_mode parameter of the plotCoverage() command. The default is standard deviation sd. Alternatively, users can choose the 95% confidence interval ci, or the standard error of the mean sem.

This feature is important for two reasons:

The uncertainties around the regions expected to be invariable (e.g. constitutively expressed flanking exons) are expected to be low if SpliceWiz’s estimate of exonic abundance is accurate,
The confidences of the differentially expressed alternatively-spliced exons (or retained introns) represents the certainty of the differential expression of these regions.

Using the GUI

Click on Display and then Coverage plot from the menu side bar. It should look like this:

Coverage Plots - GUI

Coverage plots can be customized as follows:

1. Genes: display whole genes
1. Events: display individual ASE events
1. Coordinates: display by genomic coordinates
1. Zoom in or out (each step is a factor of 3)
1. Strand: select unstranded (*), positive (+) or negative (-). NB most RNA-seq protocols are reverse-stranded; this means the strand is opposite to that of the expressed RNA
1. Select events ranked by p values from All results, Filtered results (using the interactive DT data table), or Selected results using lasso / box select tools on the volcano or scatter plots
1. Graph mode. You can either pan (across the genomic axis), zoom tool (drag to zoom in) or movable labels (labels can be moved by dragging)
1. Number of top events to display in the Events dropdown menu
1. Refresh plot. The plot will be automatically refreshed after a few seconds when selections are changed. However, for some controls (such as genes, events), the plot needs to be manually refreshed
1. View either individual samples or group-averaged by condition
1. Normalize RNA-seq coverage values by the junction coverage reads of the selected ASE event. This ensures that exon depths are normalized to unity. Only applicable to View by condition
1. Condition: select which category of condition to contrast
1. Tracks: select either individual samples, or experimental conditions
1. Plot Junctions: Plot split read counts when viewing by individual samples, or provisional PSI values (calculated based on split reads across exon clusters) when viewing by conditions. Not plotted when stack traces is ON.
1. Stack traces: if on, all traces are output onto the same track (view by condition only)
1. Pairwise t-test (only applicable to view by condition, 2 tracks highlighted only). A pairwise t-test of normalized coverages will be performed, and per-window t-test results (-log10 transformed) will be displayed. Higher values indicate higher significance
1. Display Key Isoforms: Only plots transcripts that contain at least one of the two alternate isoforms being plotted.
1. Condensed tracks: whether the individual transcripts OFF or collapsed combined transcripts ON is displayed

SessionInfo

sessionInfo()
#> R version 4.2.3 (2023-03-15)
#> Platform: x86_64-pc-linux-gnu (64-bit)
#> Running under: Ubuntu 20.04.6 LTS
#> 
#> Matrix products: default
#> BLAS:   /home/biocbuild/bbs-3.16-bioc/R/lib/libRblas.so
#> LAPACK: /home/biocbuild/bbs-3.16-bioc/R/lib/libRlapack.so
#> 
#> locale:
#>  [1] LC_CTYPE=en_US.UTF-8       LC_NUMERIC=C              
#>  [3] LC_TIME=en_GB              LC_COLLATE=C              
#>  [5] LC_MONETARY=en_US.UTF-8    LC_MESSAGES=en_US.UTF-8   
#>  [7] LC_PAPER=en_US.UTF-8       LC_NAME=C                 
#>  [9] LC_ADDRESS=C               LC_TELEPHONE=C            
#> [11] LC_MEASUREMENT=en_US.UTF-8 LC_IDENTIFICATION=C       
#> 
#> attached base packages:
#> [1] stats4    stats     graphics  grDevices utils     datasets  methods  
#> [8] base     
#> 
#> other attached packages:
#>  [1] pheatmap_1.0.12             ggplot2_3.4.1              
#>  [3] satuRn_1.6.0                DoubleExpSeq_1.1           
#>  [5] DESeq2_1.38.3               SummarizedExperiment_1.28.0
#>  [7] Biobase_2.58.0              MatrixGenerics_1.10.0      
#>  [9] matrixStats_0.63.0          GenomicRanges_1.50.2       
#> [11] GenomeInfoDb_1.34.9         IRanges_2.32.0             
#> [13] S4Vectors_0.36.2            limma_3.54.2               
#> [15] fstcore_0.9.14              AnnotationHub_3.6.0        
#> [17] BiocFileCache_2.6.1         dbplyr_2.3.2               
#> [19] BiocGenerics_0.44.0         SpliceWiz_1.0.4            
#> [21] NxtIRFdata_1.4.0           
#> 
#> loaded via a namespace (and not attached):
#>   [1] lazyeval_0.2.2                shinydashboard_0.7.2         
#>   [3] splines_4.2.3                 crosstalk_1.2.0              
#>   [5] BiocParallel_1.32.6           digest_0.6.31                
#>   [7] foreach_1.5.2                 ca_0.71.1                    
#>   [9] htmltools_0.5.5               viridis_0.6.2                
#>  [11] fansi_1.0.4                   magrittr_2.0.3               
#>  [13] memoise_2.0.1                 BSgenome_1.66.3              
#>  [15] shinyFiles_0.9.3              Biostrings_2.66.0            
#>  [17] annotate_1.76.0               R.utils_2.12.2               
#>  [19] prettyunits_1.1.1             colorspace_2.1-0             
#>  [21] blob_1.2.4                    rappdirs_0.3.3               
#>  [23] xfun_0.38                     dplyr_1.1.1                  
#>  [25] crayon_1.5.2                  RCurl_1.98-1.12              
#>  [27] jsonlite_1.8.4                genefilter_1.80.3            
#>  [29] survival_3.5-5                iterators_1.0.14             
#>  [31] glue_1.6.2                    registry_0.5-1               
#>  [33] gtable_0.3.3                  zlibbioc_1.44.0              
#>  [35] XVector_0.38.0                webshot_0.5.4                
#>  [37] DelayedArray_0.24.0           Rhdf5lib_1.20.0              
#>  [39] HDF5Array_1.26.0              scales_1.2.1                 
#>  [41] DBI_1.1.3                     Rcpp_1.0.10                  
#>  [43] progress_1.2.2                viridisLite_0.4.1            
#>  [45] xtable_1.8-4                  bit_4.0.5                    
#>  [47] DT_0.27                       htmlwidgets_1.6.2            
#>  [49] httr_1.4.5                    RColorBrewer_1.1-3           
#>  [51] ellipsis_0.3.2                farver_2.1.1                 
#>  [53] pkgconfig_2.0.3               XML_3.99-0.14                
#>  [55] R.methodsS3_1.8.2             sass_0.4.5                   
#>  [57] locfit_1.5-9.7                utf8_1.2.3                   
#>  [59] labeling_0.4.2                tidyselect_1.2.0             
#>  [61] rlang_1.1.0                   later_1.3.0                  
#>  [63] AnnotationDbi_1.60.2          munsell_0.5.0                
#>  [65] BiocVersion_3.16.0            tools_4.2.3                  
#>  [67] cachem_1.0.7                  cli_3.6.1                    
#>  [69] generics_0.1.3                RSQLite_2.3.0                
#>  [71] evaluate_0.20                 fastmap_1.1.1                
#>  [73] heatmaply_1.4.2               yaml_2.3.7                   
#>  [75] ompBAM_1.2.0                  fs_1.6.1                     
#>  [77] knitr_1.42                    bit64_4.0.5                  
#>  [79] purrr_1.0.1                   KEGGREST_1.38.0              
#>  [81] dendextend_1.17.1             egg_0.4.5                    
#>  [83] pbapply_1.7-0                 sparseMatrixStats_1.10.0     
#>  [85] mime_0.12                     rhandsontable_0.3.8          
#>  [87] R.oo_1.25.0                   compiler_4.2.3               
#>  [89] plotly_4.10.1                 filelock_1.0.2               
#>  [91] curl_5.0.0                    png_0.1-8                    
#>  [93] interactiveDisplayBase_1.36.0 geneplotter_1.76.0           
#>  [95] tibble_3.2.1                  bslib_0.4.2                  
#>  [97] highr_0.10                    lattice_0.20-45              
#>  [99] Matrix_1.5-3                  vctrs_0.6.1                  
#> [101] pillar_1.9.0                  lifecycle_1.0.3              
#> [103] rhdf5filters_1.10.1           BiocManager_1.30.20          
#> [105] jquerylib_0.1.4               data.table_1.14.8            
#> [107] bitops_1.0-7                  seriation_1.4.2              
#> [109] httpuv_1.6.9                  rtracklayer_1.58.0           
#> [111] R6_2.5.1                      BiocIO_1.8.0                 
#> [113] promises_1.2.0.1              TSP_1.2-3                    
#> [115] gridExtra_2.3                 codetools_0.2-19             
#> [117] boot_1.3-28.1                 assertthat_0.2.1             
#> [119] rhdf5_2.42.0                  rjson_0.2.21                 
#> [121] withr_2.5.0                   shinyWidgets_0.7.6           
#> [123] GenomicAlignments_1.34.1      Rsamtools_2.14.0             
#> [125] locfdr_1.1-8                  GenomeInfoDbData_1.2.9       
#> [127] hms_1.1.3                     parallel_4.2.3               
#> [129] fst_0.9.8                     grid_4.2.3                   
#> [131] tidyr_1.3.0                   rmarkdown_2.21               
#> [133] DelayedMatrixStats_1.20.0     numDeriv_2016.8-1.1          
#> [135] shiny_1.7.4                   restfulr_0.0.15

SpliceWiz: Quick Start

Alex CH Wong

03/29/2023

Introduction

What’s New?

Workflow from a glance

Quick-Start

Installation

Loading SpliceWiz

The SpliceWiz Graphics User Interface (GUI)

Navigating the GUI

Building the SpliceWiz reference

Process BAM files using SpliceWiz

Collate the experiment

Importing the experiment

Differential analysis

Assigning annotations to samples

Filtering high-confidence events

Performing differential analysis

Visualization

Volcano plots

Scatter plots

Heatmaps

SpliceWiz Coverage Plots

Coverage plots of individual samples

Group-averaged coverage plots

Using the GUI

SessionInfo