dream-method: Differential expression with linear mixed model

In variancePartition: Quantify and interpret divers of variation in multilevel gene expression experiments

Description

Fit linear mixed model for differential expression and preform hypothesis test on fixed effects as specified in the contrast matrix L

Usage

dream(
  exprObj,
  formula,
  data,
  L,
  ddf = c("Satterthwaite", "Kenward-Roger"),
  useWeights = TRUE,
  weightsMatrix = NULL,
  control = lme4::lmerControl(calc.derivs = FALSE, check.rankX = "stop.deficient"),
  suppressWarnings = FALSE,
  quiet = FALSE,
  BPPARAM = bpparam(),
  computeResiduals = TRUE,
  REML = TRUE,
  ...
)

Arguments

exprObj	matrix of expression data (g genes x n samples), or ExpressionSet, or EList returned by voom() from the limma package
formula	specifies variables for the linear (mixed) model. Must only specify covariates, since the rows of exprObj are automatically used a a response. e.g.: ~ a + b + (1|c) Formulas with only fixed effects also work, and lmFit() followed by contrasts.fit() are run.
data	data.frame with columns corresponding to formula
L	contrast matrix specifying a linear combination of fixed effects to test
ddf	Specifiy "Satterthwaite" or "Kenward-Roger" method to estimate effective degress of freedom for hypothesis testing in the linear mixed model. Note that Kenward-Roger is more accurate, but is *much* slower. Satterthwaite is a good enough approximation for most datasets.
useWeights	if TRUE, analysis uses heteroskedastic error estimates from voom(). Value is ignored unless exprObj is an EList() from voom() or weightsMatrix is specified
weightsMatrix	matrix the same dimension as exprObj with observation-level weights from voom(). Used only if useWeights is TRUE
control	control settings for lmer()
suppressWarnings	if TRUE, do not stop because of warnings or errors in model fit
quiet	suppress message, default FALSE
BPPARAM	parameters for parallel evaluation
computeResiduals	if TRUE, compute residuals and extract with residuals(fit). Setting to FALSE saves memory
REML	use restricted maximum likelihood to fit linear mixed model. default is TRUE. See Details.
...	Additional arguments for lmer() or lm()

Details

A linear (mixed) model is fit for each gene in exprObj, using formula to specify variables in the regression. If categorical variables are modeled as random effects (as is recommended), then a linear mixed model us used. For example if formula is ~ a + b + (1|c), then the model is

fit <- lmer( exprObj[j,] ~ a + b + (1|c), data=data)

useWeights=TRUE causes weightsMatrix[j,] to be included as weights in the regression model.

Note: Fitting the model for 20,000 genes can be computationally intensive. To accelerate computation, models can be fit in parallel using BiocParallel to run code in parallel. Parallel processing must be enabled before calling this function. See below.

The regression model is fit for each gene separately. Samples with missing values in either gene expression or metadata are omitted by the underlying call to lmer.

Hypothesis tests and degrees of freedom are producted by lmerTest and pbkrtest pacakges

While REML=TRUE is required by lmerTest when ddf='Kenward-Roger', ddf='Satterthwaite' can be used with REML as TRUE or FALSE. Since the Kenward-Roger method gave the best power with an accurate control of false positive rate in our simulations, and since the Satterthwaite method with REML=TRUE gives p-values that are slightly closer to the Kenward-Roger p-values, REML=TRUE is the default. See Vignette "3) Theory and practice of random effects and REML"

Value

MArrayLM2 object (just like MArrayLM from limma), and the directly estimated p-value (without eBayes)

Examples

# load library
# library(variancePartition)
library(BiocParallel)

# Intialize parallel backend with 4 cores
library(BiocParallel)
register(SnowParam(4))

# load simulated data:
# geneExpr: matrix of gene expression values
# info: information/metadata about each sample
data(varPartData)

form <- ~ Batch + (1|Individual) + (1|Tissue) 

# Fit linear mixed model for each gene
# run on just 10 genes for time
fit = dream( geneExpr[1:10,], form, info)

# view top genes
topTable( fit )

# get contrast matrix testing if the coefficient for Batch2 is 
# different from coefficient for Batch3
# The variable of interest must be a fixed effect
L = getContrast( geneExpr, form, info, c("Batch2", "Batch3"))

# plot contrasts
plotContrasts( L )

# Fit linear mixed model for each gene
# run on just 10 genes for time
# Note that that dream() is not compatible with eBayes()
fit2 = dream( geneExpr[1:10,], form, info, L)

# view top genes
topTable( fit2 )

# Parallel processing using multiple cores with reduced memory usage
param = SnowParam(4, "SOCK", progressbar=TRUE)
fit3 = dream( geneExpr[1:10,], form, info, L, BPPARAM = param)

# Fit fixed effect model for each gene
# Use lmFit in the backend
# Need to run eBayes afterward
form <- ~ Batch 
fit4 = dream( geneExpr[1:10,], form, info)
fit4 = eBayes( fit4 )

# view top genes
topTable( fit4 )

# Compute residuals using dream
fit5 = dream( geneExpr[1:10,], form, info, L, BPPARAM = param, computeResiduals=TRUE)

# Return residuals
residuals(fit5)

variancePartition documentation built on Nov. 8, 2020, 5:18 p.m.