Hi all,

I'm struggling with the meta-analysis of microarray data. In the user guide of the bioconductor package "MAMA" there is an exemple of meta-analysis in cancer microarray data concerning the comparison of expression profiles in MSI (microsatelite instable) and MSS (microsatelite stable) colon cancer. Data are gathered from three microarray experiments from public databases 

The original data sets have been preprocessed and subsampled (same set of 500 genes). All data sets have been normalized and are in log2-scale. The corresponding sample sizes for the three datasets, named denmark, australia  and japan, are 77 (39 MSI and 38 MSS), 36 (5 MSI and 31 MSS) and 41 (16 MSI and 25 MSS), respectively. Each of the datasets is stored as an ExpressionSet object.

I would like to perform a meta-analysis combining the three ExpressionSets into one and using the limma package with a convenient design matrix to model the study effect.

I have two questions:

1) I tried using the function "combine" in order to combine the three datasets (two by two, too) but I obtain the following error message:

library(MAMA)

library(affy)

load(url("http://math.muni.cz/~xihnatov/DataColonHalf.RData"))

ls() 

data<-combine(australia,denmark,japan)

Error in data.frame(<S4 object of class "ExpressionSet">, <S4 object of class "ExpressionSet">, : arguments imply differing number of rows: 36, 77, 41. 

I checked the function instructions:

combine(eSet,eSet): Combine two eSet objects. To be combined, eSets must have identical numbers of featureNames, distinct sampleNames, and identical annotation

and the three datasets meet these conditions.

So, can someone help me? Is there a problem in the datasets I downloaded? 

2) How can I access the code of the function "model.matrix" (package : stats)? Typing the name of the function as I usuallly do seems not to work in this case. Is it a "protected" function?

Thank you for your help!

Manuela

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Manuela Di Russo, Ph.D. Student
Department of Experimental Pathology, MBIE
University of Pisa
Pisa, Italy
e-mail: manuela.dirusso@for.unipi.it
tel: +39050993538
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