dream {variancePartition} | R Documentation

## Differential expression with linear mixed model

### 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"), REML = TRUE, useWeights = TRUE,
      weightsMatrix = NULL, control = lme4::lmerControl(calc.derivs =
      FALSE, check.rankX = "stop.deficient"), ...)

### 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 exproximation for most datasets.
`REML` | use restricted maximum likelihood to fit linear mixed model. default is TRUE. Strongly discourage against changing this option
`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()
`...` | 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 to 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 foreach/dopar to run loops 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

### Value

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

### Examples

    # load library
    # library(variancePartition)

    # optional step to run analysis in parallel on multicore machines
    # Here, we used 4 threads
    library(doParallel)
    cl <- makeCluster(4)
    registerDoParallel(cl)
    # or by using the doSNOW package

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

    # get contrast matrix testing if the coefficient for Batch2 is zero
    # The variable of interest must be a fixed effect
    form <- ~ Batch + (1|Individual) + (1|Tissue)
    L = getContrast( geneExpr, form, info, "Batch3")

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

    # Run empirical Bayes post processing from limma
    fitEB = eBayes( fit )

    # view top genes
    topTable( fitEB )

    # stop cluster
    stopCluster(cl)

[Package variancePartition version 1.10.4 Index]