Introduction to slalom
26 October 2021
Package
slalom 1.16.0
Contents
This document provides an introduction to the capabilities of slalom. The
package can be used to identify hidden and biological drivers of variation in
single-cell gene expression data using factorial single-cell latent
variable models.
1 Quickstart
Slalom requires:
1. expression data in a SingleCellExperiment object (defined in
SingleCellExperiment package), typically with log transformed gene expression values;
2. Gene set annotations in a GeneSetCollection class (defined in the
GSEABase package). A GeneSetCollection can be read into R from a *.gmt
file as shown below.
Here, we show the minimal steps required to run a slalom analysis on a subset
of a mouse embryonic stem cell (mESC) cell cycle-staged dataset.
First, we load the mesc dataset included with the package. The mesc object
loaded is a SingleCellExperiment object ready for input to slalom
library(slalom)
data("mesc")If we only had a matrix of expression values (assumed to be on the log2-counts
scale), then we can easily construct a SingleCellExperiment object as follows:
exprs_matrix <- SingleCellExperiment::logcounts(mesc)
mesc <- SingleCellExperiment::SingleCellExperiment(
assays = list(logcounts = exprs_matrix)
)We also need to supply slalom with genesets in a GeneSetCollection object.
If we have genesets stored in a *.gmt file (e.g. obtained from
MSigDB or
REACTOME) then it is easy to read these directory into
an appropriate object, as shown below for a subset of REACTOME genesets.
gmtfile <- system.file("extdata", "reactome_subset.gmt", package = "slalom")
genesets <- GSEABase::getGmt(gmtfile)Next we need to create an Rcpp_SlalomModel object containing the input data
and genesets (and subsequent results) for the model. We create the object with
the newSlalomModel function.
We need to define the number of hidden factors to include in the model
(n_hidden argument; 2–5 hidden factors recommended) and the minimum number of
genes required to be present in order to retain a geneset (min_genes argument;
default is 10).
model <- newSlalomModel(mesc, genesets, n_hidden = 5, min_genes = 10)## 14 annotated factors retained; 16 annotated factors dropped.
## 196 genes retained for analysis.Next we need to initialise the model with the init function.
model <- initSlalom(model)With the model prepared, we then train the model with the train function.
model <- trainSlalom(model, nIterations = 10)## pre-training model for faster convergence
## iteration 0
## Model not converged after 50 iterations.
## iteration 0
## Model not converged after 50 iterations.
## iteration 0
## Switched off factor 7
## Switched off factor 18
## Model not converged after 10 iterations.Typically, over 1,000 iterations will be required for the model to converge.
Finally, we can analyse and interpret the output of the model and the sources of variation that it identifies. This process will typically include plots of factor relevance, gene set augmentation and a scatter plots of the most relevant factors.
2 Input data and genesets
As introduced above, slalom requires:
1. expression data in a SingleCellExperiment object (defined in
SingleCellExperiment package), typically with log transformed gene expression values;
2. Gene set annotations in a GeneSetCollection class (defined in the
GSEABase package).
Slalom works best with log-scale expression data that has been QC’d, normalized
and subsetted down to highly-variable genes. Happily, there are Bioconductor
packages available for QC and normalization that also use the
SingleCellExperiment class and can provide appropriate input for slalom.
The combination of scater and
scran is very effective for QC,
normalization and selection of highly-variable genes. A Bioconductor
workflow shows how those
packages can be combined to good effect to prepare data suitable for analysis
with slalom.
Here, to demonstrate slalom we will use simulated data. First, we make a new
SingleCellExperiment object. The code below reads in an expression matrix
from file, creates a SingleCellExperiment object with these expression values
in the logcounts slot.
rdsfile <- system.file("extdata", "sim_N_20_v3.rds", package = "slalom")
sim <- readRDS(rdsfile)
sce <- SingleCellExperiment::SingleCellExperiment(
assays = list(logcounts = sim[["init"]][["Y"]])
)The second crucial input for slalom is the set of genesets or pathways that we
provide to the model to see which are active. The model is capable of handling
hundreds of genesets (pathways) simultaneously.
Geneset annotations must be provided as a GeneSetCollection object as defined
in the GSEABase package.
Genesets are typically distributed as *.gmt files and are available from such
sources as MSigDB or
REACTOME. The gmt format is very simple, so it
is straight-forward to augment established genesets with custom sets tailored to
the data at hand, or indeed to construct custom geneset collections completely
from scratch.
If we have genesets stored in a *.gmt file (e.g. from MSigDB, REACTOME or
elsewhere) then it is easy to read these directory into an appropriate object,
as shown below for a subset of REACTOME genesets.
gmtfile <- system.file("extdata", "reactome_subset.gmt", package = "slalom")
genesets <- GSEABase::getGmt(gmtfile)Geneset names can be very long, so below we trim the REACTOME geneset names to remove the “REACTOME_” string and truncate the names to 30 characters. (This is much more convenient downstream when we want to print relevant terms and create plots that show geneset names.)
We also tweak the row (gene) and column (cell) names so that our example data works nicely.
genesets <- GSEABase::GeneSetCollection(
lapply(genesets, function(x) {
GSEABase::setName(x) <- gsub("REACTOME_", "", GSEABase::setName(x))
GSEABase::setName(x) <- strtrim(GSEABase::setName(x), 30)
x
})
)
rownames(sce) <- unique(unlist(GSEABase::geneIds(genesets[1:20])))[1:500]
colnames(sce) <- 1:ncol(sce)With our input data prepared, we can proceed to creating a new model.
3 Creating a new model
The newSlalomModel function takes the SingleCellExperiment and
GeneSetCollection arguments as input and returns an object of class
Rcpp_SlalomModel: our new object for fitting the slalom model. All of the
computationally intensive model fitting in the package is done in C++, so the
Rcpp_SlalomModel object provides an R interface to an underlying SlalomModel
class in C++.
Here we create a small model object, specifying that we want to include one
hidden factor (n_hidden) and will retain genesets as long as they have at
least one gene present (min_genes) in the SingleCellExperiment object
(default value is 10, which would be a better choice for analyses of
experimental data).
m <- newSlalomModel(sce, genesets[1:23], n_hidden = 1, min_genes = 1)## 20 annotated factors retained; 3 annotated factors dropped.
## 500 genes retained for analysis.Twenty annotated factors are retained here, and three annotated factors are
dropped. 500 genes (all present in the sce object) are retained for analysis.
For more options in creating the slalom model object consult the documentation
(?newSlalomModel).
See documentation (?Rcpp_SlalomModel) for more details about what the class
contains.
4 Initializing the model
Before training (fitting) the model, we first need to establish a sensible
initialisation. Results of variational Bayes methods, in general, can depend on
starting conditions and we have found developed initialisation approaches that
help the slalom model converge to good results.
The initSlalom function initialises the model appropriately. Generally, all
that is required is the call initSlalom(m), but here the genesets we are using
do not correspond to anything meaningful (this is just dummy simulated data), so
we explicitly provide the “Pi” matrix containing the prior probability for each
gene to be active (“on”) for each factor. We also tell the initialisation function
that we are fitting one hidden factor and set a randomisation seed to make
analyses reproducible.
m <- initSlalom(m, pi_prior = sim[["init"]][["Pi"]], n_hidden = 1, seed = 222)See documentation (?initSlalom) for more details.
5 Training the model
With the model initialised, we can proceed to training (fitting) it. Training typically requires one to several thousand iterations, so despite being linear in the nubmer of factors can be computationally expensive for large datasets ( many cells or many factors, or both).
mm <- trainSlalom(m, minIterations = 400, nIterations = 5000, shuffle = TRUE,
pretrain = TRUE, seed = 222)## pre-training model for faster convergence
## iteration 0
## Model not converged after 50 iterations.
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## Model not converged after 50 iterations.
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## Model converged after 4550 iterations.We apply the shuffle (shuffling the update order of factors at each iteration
of the model) and pretrain (burn 100 iterations of the model determine the
best intial update order of factors) options as these generally aid the
convergence of the model. See documentation (?trainSlalom) for more details
and further options.
Here the model converges in under 2000 iterations. This takes seconds for 21 factors, 20 cells and 500 genes. The model is, broadly speaking, very scalable, but could still require hours for many thousands of cells and/or hundreds of factors.
6 Interpretation of results
With the model trained we can move on to the interpretation of results.
6.1 Top terms
The topTerms function provides a convenient means to identify the most
“relevant” (i.e. important) factors identified by the model.
topTerms(m)## term relevance type n_prior n_gain n_loss
## 1 APOPTOTIC_CLEAVAGE_OF_CELLULAR 1.1862945226 annotated 46 0 0
## 2 NEF_MEDIATED_DOWNREGULATION_OF 0.9533517719 annotated 27 0 0
## 3 CELL_CELL_COMMUNICATION 0.8834821681 annotated 76 0 0
## 4 NEF_MEDIATES_DOWN_MODULATION_O 0.8194190105 annotated 49 0 0
## 5 CELL_CYCLE 0.5370374523 annotated 466 0 0
## 6 NEUROTRANSMITTER_RECEPTOR_BIND 0.0008010153 annotated 33 0 0
## 7 CELL_CYCLE_MITOTIC 0.0007054114 annotated 70 0 0
## 8 CELL_SURFACE_INTERACTIONS_AT_T 0.0006495766 annotated 101 0 0
## 9 ACTIVATION_OF_NF_KAPPAB_IN_B_C 0.0005060579 annotated 104 0 0
## 10 INTEGRIN_CELL_SURFACE_INTERACT 0.0004000095 annotated 179 0 0
## 11 ANTIGEN_ACTIVATES_B_CELL_RECEP 0.0003983484 annotated 107 0 0
## 12 DOWNSTREAM_SIGNALING_EVENTS_OF 0.0003473931 annotated 205 0 0
## 13 NOTCH1_INTRACELLULAR_DOMAIN_RE 0.0003460340 annotated 325 0 0We can see the name of the term (factor/pathway), its relevance and type
(annotated or unannotated (i.e. hidden)), the number genes initially in the
gene set (n_prior), the number of genes the model thinks should be added as
active genes to the term (n_gain) and the number that should be dropped
from the set (n_loss).
6.2 Plotting results
The plotRelevance, plotTerms and plotLoadings functions enable us to
visualise the slalom results.
The plotRelevance function displays the most relevant terms (factors/pathways)
ranked by relevance, showing gene set size and the number of genes gained/lost
as active in the pathway as learnt by the model.
plotRelevance(m)## Warning: `guides(<scale> = FALSE)` is deprecated. Please use `guides(<scale> =
## "none")` instead.
The plotTerms function shows the relevance of all terms in the model, enabling
the identification of the most important pathways in the context of all that
were included in the model.
plotTerms(m)
Once we have identified terms (factors/pathways) of interest we can look specifically at the loadings (weights) of genes for that term to see which genes are particularly active or influential in that pathway.
plotLoadings(m, "CELL_CYCLE")
See the appropriate documentation for more options for these plotting functions.
7 Using results for further analyses
Having obtained slalom model results we would like to use them in downstream
analyses. We can add the results to a SingleCellExperiment object, which
allows to plug into other tools, particularly the scater package which
provides useful further plotting methods and ways to regress out unwanted
hidden factors or biologically uninteresting pathways (like cell cycle, in some
circumstances).
7.1 Adding results to a SingleCellExperiment object
The addResultsToSingleCellExperiment function allows us to conveniently add
factor states (cell-level states) to the reducedDim slot of the
SingleCellExperiment object and the gene loadings to the rowData of the
object.
It typically makes most sense to add the slalom results to the
SingleCellExperiment object we started with, which is what we do here.
sce <- addResultsToSingleCellExperiment(sce, m)8 Session Info
sessionInfo()## R version 4.1.1 (2021-08-10)
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