R/TSPM.R

In DEsubs: DEsubs: an R package for flexible identification of differentially expressed subpathways using RNA-seq expression experiments

##-------------------------------------------------------------------
##     Name: TSPM.R
##      R code for the paper by Paul L. Auer and R.W. Doerge:
##     "A Two-Stage Poisson Model for Testing RNA-Seq Data"
##     Date: February 2011
##     Contact: Paul Auer         plivermo@fhcrc.org
##         R.W. Doerge         doerge@purdue.edu


##     Example:
##     counts <- matrix(0, nrow=1000, ncol=10)
##     for(i in 1:1000){
##        lambda <- rpois(n=1, lambda=10)
##        counts[i,] <- rpois(n=10, lambda=lambda)
##     }
##     x1 <- gl(n=2, k=5, labels=c("T", "C"))
##     x0 <- rep(1, times=10)
##     lib.size <- apply(counts,2,sum)
##    result <- TSPM(counts, x1, x0, lib.size)
##---------------------------------------------------------------------


#######################################################################
###### The TSPM function ##############################################
#######################################################################


TSPM <- function(counts, x1, x0, lib.size, alpha.wh=0.05){

## Input:
#counts:   a matrix of RNA-Seq gene counts (genes are rows, samples are 
#          columns)
#x1:       a vector of treatment group factors (under the alternative 
#          hypothesis)
#x0:       a vector of treatment group factors (under the null hypothesis)
#lib.size: a vector of RNA-Seq library sizes. This could simply be obtained
#          by specifying lib.size <- apply(counts,2,sum). It may also be any 
#          other appropriate scaling factor.
#alpha.wh: the significance threshold to use for deciding whether a gene is 
#          overdispersed.
#          Defaults to 0.05.


## Output:
#log.fold.change:     a vector containing the estimated log fold changes for 
#                     each gene
#pvalues:             a vector containing the raw p-values testing differential 
#                     expression for each gene.
#index.over.disp:     a vector of integer values containing the indices of the 
#                     over-dispersed genes.
#index.not.over.disp: a vector of integer values containing the indices of the 
#                     non-over-dispersed genes.
#padj:                a vector containing the p-values after adjusting for 
#                     multiple testing using the 
#                     method of Benjamini-Hochberg


######## The main loop that fits the GLMs to each gene #####################

### Initializing model parameters ####
n <- dim(counts)[1]
per.gene.disp <- NULL
LRT <- NULL
score.test <- NULL
LFC <- NULL

###### Fitting the GLMs for each gene #################
    for(i in 1:n){
        ### Fit full and reduced models ###
        model.1 <- glm(as.numeric(counts[i,]) ~ x1, offset=log(lib.size), 
                    family=poisson)
        model.0 <- glm(as.numeric(counts[i,]) ~ x0, offset=log(lib.size), 
                    family=poisson)

        ### Obtain diagonals of Hat matrix from the full model fit ###
        hats <- hatvalues(model.1)

        ### Obtain Pearson overdispersion estimate ####
        per.gene.disp[i] <- sum(residuals(model.1, 
                            type="pearson")^2)/model.1$df.residual

        ### Obtain Likelihood ratio statistic ####
        LRT[i] <- deviance(model.0)-deviance(model.1)

        ### Obtain score test statistic ####
        score.test[i] <- 1/(2*length(counts[i,])) * sum(residuals(model.1, 
            type="pearson")^2 - ((counts[i,] - 
            hats*model.1$fitted.values)/model.1$fitted.values))^2

        ### Obtain the estimated log fold change ###
        LFC[i] <- -model.1$coef[2]
    }

## Initialize parameters for Working-Hotelling bands around the score TSs ###
qchi <- qchisq(df=1, (1:n-0.5)/n)
MSE <- 2
UL <- NULL

#### Obtain the upper boundary of the WH bands ############################
xbar <- mean(qchi)
bottom <- sum((qchi-xbar)^2)
top <- (qchi-xbar)^2
s <- sqrt(MSE*(1/n) + (top/bottom))
W <- sqrt(2*qf(df1=1, df2=n-1, p=1-(alpha.wh/n)))
UL <- pmax(qchi + W*s,1)

###### Obtain the indices of the over-dispersed and not-over-dispersed genes, 
# respectively ##########

cutoff <- min(which(sort(score.test)-UL > 0))
temp <- cutoff-1 + seq(cutoff:length(score.test))
over.disp <- which(score.test %in% sort(score.test)[temp])
not.over.disp <- setdiff(1:length(score.test), over.disp)

###### Compute p-values ####################################
p.f <- pf(LRT[over.disp]/per.gene.disp[over.disp], df1=1, 
        df2=model.1$df.residual, lower.tail=FALSE)
p.chi <- pchisq(LRT[not.over.disp], df=1, lower.tail=FALSE)
p <- NULL
p[over.disp] <- p.f
p[not.over.disp] <- p.chi

##### Adjust the p-values using the B-H method ####################
p.bh.f <- p.adjust(p.f, method="BH")
p.bh.chi <- p.adjust(p.chi, method="BH")
final.p.bh.tagwise <- NULL
final.p.bh.tagwise[over.disp] <- p.bh.f
final.p.bh.tagwise[not.over.disp] <- p.bh.chi

### Output ###
list(log.fold.change=LFC, pvalues=p, index.over.disp=over.disp, 
        index.not.over.disp=not.over.disp, padj=final.p.bh.tagwise)
}