%\VignetteIndexEntry{Tutorial on How to Use the  Functions in the \texttt{attract} Package}
%\VignetteKeywords{analysis, attractor, microarray}
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\author{Jessica Mar} 
\begin{document}
\title{Using the \texttt{attract} Package To Find the Gene Expression Modules That Represent The Drivers of Kauffman's Attractor Landscape}

\maketitle

\section{Introduction} 

A mammalian organism is made up of over 200 types of specialized cells. Each cell type carries out a specific 
task integral to maintaining homeostasis of the organism. Cell types can vary by morphology, structure, lifespan, 
functional ability and much more. Despite such remarkable diversity, all cells within an organism are derived from 
an original precursor cell, and in most cases, share the same genome. Diversity comes about largely through differential 
expression programs where cells regulate the abundance of certain gene transcripts and their downstream molecules such as 
proteins and microRNAs. While epigenetic regulation also plays a role, finding the gene expression signatures represents the 
key to understanding the mechanisms underlying the unique properties that cells acquire. The attractor hypothesis proposed by 
Stuart Kauffman [1] describes how cell fate transitions between cell types occur through coordinated changes in genome-wide gene 
expression. In our approach [2], we show how by identifying the biological processes that are differentially activated between 
cell types, we can find the drivers of Kauffman's attractor landscape. 

\section{Experimental Data Set}

To illustrate our approach, we demonstrate the functions in the \texttt{attract} package on a gene expression data set 
published by M\"{u}ller et al. [3] (NCBI GEO accession number GSE11508). The primary goal of this study was to elucidate the 
transcription profiles characterizing different stem cell lines and their progenitors. For our purposes, we have selected a 
subset of four cell lines - embryonic stem cells, neural stem cells, neural progenitors and teratoma-differentiated cells. 
These lines span a spectrum of pluripotent abilities and have also been derived from a range of different tissue sources. 

\begin{flushleft}
The M\"{u}ller expression data set is stored as a matrix object \texttt{exprs.dat}, and the corresponding 
cell line information is in the data.frame \texttt{samp.info}. \\
\end{flushleft}

<<loadlib, echo=TRUE>>=
library(attract)
data(exprs.dat) 
data(samp.info)
@

\begin{flushleft}
The functions in our package operate off Bioconductor \texttt{ExpressionSet} objects.  \\
\end{flushleft}

<<makeESet, echo=TRUE>>=
loring.eset <- new("ExpressionSet")
loring.eset@assayData <- new.env()
assign("exprs", exprs.dat, loring.eset@assayData)
p.eset <- new("AnnotatedDataFrame", data=samp.info)
loring.eset@phenoData <- p.eset
@

\begin{flushleft}
The first step of the analysis is to find the core pathway modules that show the most differential expression changes between the 
four different cell types. These core pathways represent the drivers of the attractor landscape. \\
\end{flushleft}

\section{Finding Core Attractor State Pathway Modules} 

The pathway modules as defined by KEGG and identified using the \texttt{GSEAlm} algorithm. These modules 
represent the pathways with the expression profiles that discriminate between the different celltypes or 
experimental groups of interest.

<<findAttractors, echo=TRUE>>=
attractor.states <- findAttractors(loring.eset, "celltype", nperm=10, annotation="illuminaHumanv1.db")
@

\begin{flushleft}
The output of the \texttt{findAttractors} object is is an S4 class \texttt{AttractorModuleSet} object. \\
\end{flushleft}

\begin{flushleft}
It contains the following slots: \\
\end{flushleft}

<<showSlots, echo=TRUE>>= 
class(attractor.states) 
slotNames(attractor.states) 
@

where: 
\begin{itemize}
	\item	\texttt{eSet} - is the \texttt{ExpressionSet} object that was supplied as input to the \texttt{findAttractors} function.
	\item	\texttt{cellTypeTag} - is the character string denoting which variable in the \texttt{pData(eSet)} object stores the cell type or 
						experimental group of interest info. Note that \texttt{cellTypeTag} must be one of \texttt{colnames(pData(eSet))}. 
	\item	\texttt{incidenceMatrix} - is incidence matrix where rows correspond to KEGG pathways, columns correspond to genes. An entry of 1 at location 
							(X,Y) in the matrix indicates membership of gene Y in pathway X (0 indicates non-membership). 
	\item	\texttt{rankedPathways} - is a \texttt{data.frame} object that lists the KEGG pathway modules, ranked from most to least significant. 
							The permutation P-values represent over-enrichment for each KEGG pathway from the \texttt{GSEAlm} package.
\end{itemize}

\section{Removing Flat or Uninformative Genes}

We next remove genes that show no significant expression changes across the four cell types. 

<<removeFlats, echo=TRUE>>=
remove.these.genes <- removeFlatGenes(loring.eset, "celltype", contrasts=NULL, limma.cutoff=0.05)
@

\begin{flushleft}
This step runs a LIMMA model which tests for differences in expression between any of the four cell types. More specific differences 
can be tested instead by inputing these as a set of contrasts and supplying this object to the \texttt{contrasts} argument. 
See \texttt{?removeFlatGenes} for more info. \\
\end{flushleft}

\section{Finding the Synexpression Groups} 

Different cell or tissue types acquire their diversity by driving differentially coordinated expression patterns through interacting 
gene networks that are broadly captured by the pathways listed in the KEGG database. We next focus on elucidating what this repertoire 
of transcriptionally-coherent expression patterns being sustained within a pathway are, in other words the synexpression groups. 

\begin{flushleft}
A synexpression group contains genes that share similar expression profiles across the four groups. \\
\end{flushleft}

\begin{flushleft}
As an example, we use the MAPK signaling pathway module: \\
\end{flushleft}

<<findSynE, echo=TRUE>>=
mapk.syn <- findSynexprs("04010", attractor.states, remove.these.genes)
@

\begin{flushleft}
The output of \texttt{findSynexpress} is an S4 class \texttt{SynExpressionSet} object \\
\end{flushleft}

<<showSynSlots, echo=TRUE>>=
class(mapk.syn)
slotNames(mapk.syn)
@

\begin{flushleft}
where: \\
\end{flushleft}

\begin{itemize}
	\item \texttt{groups} - is a \texttt{list} object containing the genes in each synexpression group (each component corresponds to a different synexpression group). 
	\item \texttt{profiles} - is a \texttt{matrix} object that stores the average expression profiles for each synexpression group. The rows of the matrix
			correspond to the synexpression groups, the columns correspond to the samples in the data set. 
\end{itemize} 

<<howMany, echo=TRUE>>=
length(mapk.syn@groups) 
sapply(mapk.syn@groups, length) 
@

\begin{flushleft}
The number of synexpression groups is determined by an informativeness metric [4]. There are 3 synexpression groups for the MAPK pathway module. \\
\end{flushleft}

\begin{flushleft}
Using the same \texttt{findSynexprs} function, we can find the synexpression groups for the top 5 pathway modules: \\
\end{flushleft}

<<findMultiSynE, echo=TRUE>>=
top5.syn <- findSynexprs(attractor.states@rankedPathways[1:5,1], attractor.states, removeGenes=remove.these.genes)
@

\begin{flushleft}
Note in this case, the output object of \texttt{findSynexprs} is an \texttt{environment} variable. \\
\end{flushleft}

\begin{flushleft}
The keys are given as: \\
\end{flushleft}

<<demoEnv, echo=TRUE>>=
ls(top5.syn) 
@ 

\begin{flushleft}
Each of the values is stored as an individual \texttt{SynExpressionSet} object: \\
\end{flushleft}

<<demoClass, echo=TRUE>>=
class(get(ls(top5.syn)[1], top5.syn))
@

\begin{figure}
\section{Visualizing the Synexpression Groups for a Core Attractor Pathway}

We can visualize the average expression profiles of the synexpression groups using base R's \texttt{plot} functions or alternatively using \texttt{plotsynexprs}.

	\begin{center}
<<plotSyn, fig=TRUE, echo=TRUE>>=
par(mfrow=c(2,2)) 
pretty.col <- rainbow(3) 
for( i in 1:3 ){
	plotsynexprs(mapk.syn, tickMarks=c(6, 28, 47, 60), tickLabels=c("ESC", "PRO", "NSC", "TER"), vertLines=c(12.5, 43.5, 51.5), index=i, 
			main=paste("Synexpression Group ", i, sep=""), col=pretty.col[i])
 }
@
	\caption{Average Expression Profiles of the Synexpression Groups}
  \end{center}
\end{figure}

\section{Finding Correlated Partners of the Synexpression Groups} 

\begin{flushleft}
In our approach so far, the attractor pathway modules and their following synexpression groups have been 
inferred from information restricted to well-curated sources like KEGG. As a result, these inferences are 
of high quality and we have strong confidence in their applicability to the system under study. These benefits 
in accuracy clearly come at the expense of low coverage because we are ultimately only describing a small proportion of the genome. \\
\end{flushleft}

\begin{flushleft}
However by using the synexpression groups to pick up genes with highly correlated expression profiles, we can then extrapolate out to 
make inferences about the entire set of genes in the genome. Genes with highly correlated profiles to the synexpression groups 
(e.g. R > 0.85) are also likely to be integral in maintaining cell type-specific differences, however due to their lack of inclusion in 
resources like KEGG, would not have been picked up by the first GSEA step. \\
\end{flushleft}

\begin{flushleft}
For the MAPK synexpression groups, we can find out what other genes on the chip share similar expression profiles. \\
\end{flushleft}

<<findCorrP, echo=TRUE>>=
mapk.cor <- findCorrPartners(mapk.syn, loring.eset, remove.these.genes)
@

\begin{flushleft}
The output of \texttt{findCorrPartners} is a list which stores vectors of genes that are highly correlated with at least one gene in the 
synexpression expression group. More or less stringent criterion can be applied by changing the \texttt{cor.cutoff} argument (default setting is 0.85). \\
\end{flushleft}

<<lookatCorr, echo=TRUE>>=
sapply(mapk.cor@groups, length)
@

\section{Functional Enrichment Analysis of the Synexpression Groups} 

For each of these correlated sets, we look for functional enrichment using GO terms to learn about any trends in common roles that these 
genes may potentially share. 

<<funcE, echo=TRUE>>=
mapk.func <- calcFuncSynexprs(mapk.syn, attractor.states, "CC", annotation="illuminaHumanv1.db") 
@

\section{References}

\begin{flushleft}

[1] Kauffman S. 2004. A proposal for using the ensemble approach to understand genetic regulatory networks. J Theor Biol. 230:581. \\
\end{flushleft}

\begin{flushleft}
[2] Mar JC, Wells CA, Quackenbush J. 2010. Identifying Gene Expression Modules that Represent the Drivers of Kauffman's 
Attractor Landscape. To Appear.\\
\end{flushleft}

\begin{flushleft}
[3] M\"{u}ller F et al. 2008. Regulatory networks define phenotypic classes of human stem cell lines. Nature. 455(7211): 401. \\
\end{flushleft}

\begin{flushleft}
[4] Mar JC, Wells CA, Quackenbush J. 2010. Defining an Informativeness Metric for Clustering Gene Expression Data. To Appear.\\
\end{flushleft}

\section{Session Information}

<<SessionInfo, echo=FALSE>>=
sessionInfo()
@

\end{document}



