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\documentclass[a4paper]{article}


\begin{document}

\title{GxE.scan}
\maketitle

\section*{Comparison with snp.scan.logistic}
Both GxE.scan and snp.scan.logistic can be used for conducting a GWAS scan, however GxE.scan has additional features.
GxE.scan can process a GWAS scan using the snp.logistic, additive.test, score.test or snp.matched functions, 
whereas snp.scan.logistic only calls snp.logistic.  
GxE.scan can process more genotype formats by calling PLINK or GLU to automatically transform the genotype data,
but in snp.scan.logistic the user would have to manually transform the data beforehand.
GxE.scan streams the genotype data by reading in a single SNP at a time so that there will be no memory issues, but 
snp.logistic reads in a set of SNPs all at once so that memory could run out if too many SNPs are read.
GxE.scan also has the capacity for users to run their own customized scan by writing a simple R function; a
capability not possible with snp.scan.logistic.  

\section*{Example of GxE.scan with a TPED genotype file}

For these examples, we will call the snp.logistic function for each SNP in the genotype data.
The genotype data is in a TPED format. Since TPED files do not contain
subject ids, we must also specify the subject.list option which defines the file that lists the order 
of the subjects in the genotype data. We can use the corresponding TFAM file for the TPED subject ids. 
Only the id columns in the TFAM file are used, so that any other column in this file can have all missing values.
This file need not be the TFAM file, just a file containing the ordered genotype subject ids that can
be matched to ids in the phenotype data.
Get the paths to the genotype data and subject file.
<<TPED files>>=
geno.file <- system.file("sampleData", "geno_data.tped.gz", package="CGEN")
subject.file <- system.file("sampleData", "geno_data.tfam", package="CGEN")
geno.file
subject.file
@

<<start>>=
library(CGEN)
@


The subject.list option can be defined either way below. For PLINK genotype formats and when subject.list
is defined in the second statement below, GxE.scan assumes
that the first 2 columns of subject.file are the id columns.
<<TPED subject.list>>=
subject.list <- list(file=subject.file, id.var=1:2, header=0, delimiter=" ")
subject.list <- subject.file
@


Define the snp.list argument. The genotype file corresponds to format="tped" or file.type=12.
<<snp.list>>=
snp.list <- list(file=geno.file, format="tped", subject.list=subject.list)
@


Define the pheno.list argument. This tab-delimited file contains columns "Family" and "Subject" that
correspond to the first 2 columns of the TFAM file. Both these column names must be specified
in the id.var option so that the subjects in the phenotype file can be correctly matched to the genotype data.
The set of subjects used in the analysis will be the subjects in the phenotype file that have matching ids
in the genotype data. 
The response column is called "CaseControl" and the exposure column is called "Exposure".
<<TPED pheno.list>>=
pheno.file <- system.file("sampleData", "pheno.txt", package="CGEN")
pheno.list <- list(file=pheno.file, id.var=c("Family", "Subject"),
              file.type=3, delimiter="\t",
              response.var="CaseControl", strata.var="Study",
              main.vars=c("Gender", "Exposure", "Smoke_CURRENT", "Smoke_FORMER"),
              int.vars="Exposure")
@

Define the output file and options list. 
<<options list>>=
out.file <- paste(getwd(), "/GxE.out", sep="")
op <- list(out.file=out.file)
@

Call the scan function and display the first 5 rows of the output file.
<<TPED scan>>=
GxE.scan(snp.list, pheno.list, op=op)
x <- read.table(out.file, header=1, as.is=TRUE, sep="\t")
x[1:5, ]
@

\section*{User-defined scan}

A customized scan can be run by writing a simple R function that inputs 2 arguments and returns either a named list or
named vector. The first input argument is a data frame containing all the data, and the second argument is a list
containing the GxE.scan options, pheno.list and possibly a list named "scan.func.op". 
The data frame will have a variable called "SNP" (by default) which
contains the SNP and will be coded as 0-1-2 or NA. 
The names in the return list or vector will be column names in the output file. 
When calling GxE.scan for a user-defined scan, the "model" option in the GxE.scan options list must be set to 0 and
the "scan.func" option must be set to the name of the user-defined scan function.

In this next example, we will define our own scan function called "myscan". Note in the "myscan"
function below that the second argument "lists" is not used, but this function still must have
two arguments. The function "myscan" below calls
snp.logistic and then computes the CML SNP by exposure interaction odds-ratio and confidence interval. The return
object is a list with names "Odds.Ratio" and "OR.95.CI" which will be column names in the output file.
<<myscan>>=
myscan <- function(data, lists) {

 # Call snp.logistic
 fit <- snp.logistic(data, "CaseControl", "SNP", main.vars=~Gender + Exposure, 
                     int.vars=~Exposure, strata.var="Study")

 # Extract the CML log-odds ratio and standard error
 temp <- fit$CML
 beta <- temp$parms["SNP:Exposure"]
 se   <- temp$cov["SNP:Exposure", "SNP:Exposure"] 

 # Compute the odds-ratio and 95 percent confidence interval
 or   <- exp(beta)
 l    <- round(exp(beta - 1.96*se), digits=4)
 u    <- round(exp(beta + 1.96*se), digits=4)
 ci   <- paste("(", l, ", ", u, ")", sep="")

 # Return list. The names "Odds.Ratio" and "OR.95.CI" will be column names in the output file.
 list(Odds.Ratio=or, OR.95.CI=ci)

} 
@

Define subject.list, snp.list and pheno.list using data from the TPED example.
<<Data lists>>=
geno.file <- system.file("sampleData", "geno_data.tped.gz", package="CGEN")
subject.file <- system.file("sampleData", "geno_data.tfam", package="CGEN")
subject.list <- subject.file
snp.list <- list(file=geno.file, format="tped", subject.list=subject.list)
pheno.file <- system.file("sampleData", "pheno.txt", package="CGEN")
pheno.list <- list(file=pheno.file, id.var=c("Family", "Subject"),
              file.type=3, delimiter="\t",
              response.var="CaseControl", strata.var="Study",
              main.vars=c("Gender", "Exposure", "Smoke_CURRENT", "Smoke_FORMER"),
              int.vars="Exposure")
@

Define the options list.
Note that model must be set to 0 and scan.func to "myscan". Setting geno.counts to 0 will cause
the genotype frequency counts not to be output.
<<set options>>=
op <- list(out.file=out.file, model=0, scan.func="myscan", geno.counts=0)
@

Run the customized scan and display the first 5 rows of the output file.
<<user-defined scan>>=
GxE.scan(snp.list, pheno.list, op=op)
x <- read.table(out.file, header=1, as.is=TRUE, sep="\t")
x[1:5, ]
@

\section*{Advanced user-defined scan}

Here we will use other features of a user-defined scan: scan.setup.func and scan.func.op.
The function scan.setup.func is a function that is called once after the phenotype file is loaded, but
before the genotype data is processed. This function can be used to modify the phenotype data, 
fit a base model without any SNP, or create and save objects to be used in the scan function.
The object scan.func.op is a list of objects to be used in the scan function scan.func.

In the function mysetup below, new variables are added to the phenotype data, a base model is fit,
and values needed for likelihood ratio tests are saved in the scan.func.op list. This function can return NULL or
a list containing any of the names "data", "pheno.list", or "scan.func.op". Since the input data frame,
pheno.list and scan.func.op are modified in this function, the return list should contain all 3 of
these objects.
<<scan.setup.func>>=
mysetup <- function(data, lists) {

  # data      Data frame containing all the data except the genotype data.
  # lists     A list containing pheno.list and possibly scan.func.op

  pheno.list <- lists$pheno.list
  op.list    <- lists[["scan.func.op", exact=TRUE]]
  if (is.null(op.list)) op.list <- list()
  svar       <- pheno.list$strata.var
  svec       <- data[, svar]

  # Add indicator variables for study to data 
  data[, "Study_A"] <- as.integer(svec %in% "A")
  data[, "Study_B"] <- as.integer(svec %in% "B")

  # Include one of the indeicators as a main effect
  mvars <- pheno.list$main.vars
  pheno.list$main.vars <- c(mvars, "Study_A")

  # Create a formula to fit a NULL model
  str  <- paste(pheno.list$main.vars, collapse=" + ", sep="")
  str  <- paste(pheno.list$response.var, " ~ ", str, sep="")
  form <- as.formula(str)

  # Fit the base model
  fit <- glm(form, data=data, family=binomial()) 
  print(summary(fit))

  # Save the formula string. This will be used in the scan function.
  op.list$form.str <- str

  # Save the log-likelihood and rank for LRT tests
  op.list$rank.base    <- fit$rank
  op.list$loglike.base <- (2*fit$rank - fit$aic)/2
  
  # Return modified lists and data
  list(data=data, pheno.list=pheno.list, scan.func.op=op.list)
}
@

Define the scan function "myscan2". In this function, we will compute a likelihood ratio test using the UML
estimates and a Wald test using the EB estimates. This function has the option called MAF.cutoff which
is defined in the scan.func.op list and is used to run either an additive or dominant genetic model 
depending on the MAF of the SNP. Notice that the values "loglike.base" and "rank.base" which were
created in the function "mysetup" are used for efficiency: no need to re-run the base model for
SNPs with no missing values.
The returned object from this function should be a named vector or named list.
<<myscan2>>=
myscan2 <- function(data, lists) {

 # data      Data frame containing all the data except the genotype data.
 # lists     A list containing pheno.list and possibly scan.func.op

 plist    <- lists$pheno.list
 op       <- lists$scan.func.op

 # By default, the SNP variable snp.var is called "SNP"
 snp.var  <- plist$snp.var 
 int.vars <- plist$int.vars
 
 # Change genetic.model depending on the MAF
 snp     <- data[, snp.var]
 missing <- is.na(snp)
 nmiss   <- sum(missing)
 MAF     <- 0.5*mean(snp, na.rm=TRUE)
 if (MAF < op$MAF.cutoff) {
   gmodel <- 1
   str    <- "Dominant"
 } else {
   gmodel <- 0
   str    <- "Additive"
 }
 
 # Fit full model  
 fit <- snp.logistic(data, plist$response.var, snp.var, 
         main.vars=plist$main.vars, int.vars=int.vars, 
         strata.var=plist$strata.var, 
         op=list(genetic.model=gmodel))

 # Get the log-likelihood and rank for likelihood ratio tests.
 loglike.full <- fit$UML$loglike
 rank.full    <- length(fit$UML$parms) 

 # If the SNP had missing values, refit the base model.
 if (nmiss) {
   subset <- !missing
   form   <- as.formula(op$form.str)
   fit0   <- glm(form, data=data, family=binomial(), subset=subset)
   rank.base    <- fit0$rank
   loglike.base <- (2*fit0$rank - fit0$aic)/2
 } else {
   # No missing values, use saved values from scan.setup.func
   rank.base    <- op$rank.base
   loglike.base <- op$loglike.base
 }

 # Compute the likelihood reatio test and p-value
 LRT   <- 2*(loglike.full - loglike.base) 
 LRT.p <- pchisq(LRT, rank.full-rank.base, lower.tail=FALSE)

 # Compute a Wald test of the main effect of SNP and interaction using the EB estimates.
 vars  <- c(snp.var, paste(snp.var, ":", int.vars, sep=""))
 temp  <- getWaldTest(fit, vars, method="EB")
 EB.p  <- temp$p
 EB.df <- temp$df
 EB.t  <- temp$test

 # Define the return vector. 
 ret <- c(str, LRT, LRT.p, EB.t, EB.p, EB.df)
 names(ret) <- c("Genetic.Model", "UML.LRT", "UML.LRT.Pvalue",
                 "EB.Wald.Test", "EB.Wald.Pvalue", "DF")

 ret
}
@

Define the list of options for the user-defined scan function.
<<scan.func.op>>=
myoptions <- list(MAF.cutoff=0.05)
@

Define the list of options for the GxE.scan function. Here we set the option scan.setup.func to
"mysetup" and the option scan.func.op to myoptions. Also by setting geno.counts, geno.MAF and
geno.missRate to 0, the MAF, missing rate and genotype frequency counts will not appear in the 
output data set.
<<GxE.scan.op>>=
op <- list(out.file=out.file, model=0, scan.func="myscan2", scan.setup.func="mysetup",
        scan.func.op=myoptions, geno.counts=0, geno.MAF=0, geno.missRate=0)
@

Run the scan. The base model summary from function "mysetup" gets printed.
<<user-defined scan 2>>=
GxE.scan(snp.list, pheno.list, op=op)
@

Read in the output file and display the first 5 rows.
<<output for scan 2>>=
x <- read.table(out.file, header=1, as.is=TRUE, sep="\t")
x[1:5, ]
@

\section*{Running GxE.scan on a cluster}
Although the GxE.scan function can process an entire scan, it would certainly take a long time if 
running on a single processor.
For users with access to a computing cluster, processing an entire scan can be performed much quicker.
The steps to accomplish this are: \newline
1. Call the function GxE.scan.partition to create the job files. \newline
2. Submit the jobs. \newline
3. Call GxE.scan.combine to combine all output files after all the jobs have finished. \newline
After creating the job files by calling GxE.scan.partition, it is usually a good idea to see if the 
jobs will produce reasonable output by running one of the R program files or by submitting one of the 
job files before submitting all the jobs in step 2.


\section*{Session Information}
<<sessionInfo>>=
sessionInfo()
@ 

\end{document}


