Normalization and Gene p-Value Estimation: Issues in Microarray Data Processing

• Published in: Bioinformatics and biology insights 2008-01, Vol.2008 (2), p.291-305
• Main Authors: Fundel, Katrin, Küffner, Robert, Aigner, Thomas, Zimmer, Ralf
• Format: Article
• Language: English
• Subjects: Bioinformatics; Original Research
• ISSN: 1177-9322; 1177-9322
• DOI: 10.4137/BBI.S441

Katrin Fundel1, Robert Küffner1, Thomas Aigner2 and Ralf Zimmer1 1Institut für Informatik, Ludwig-Maximilians-Universität München, Amalienstrasse 17, 80333 München, Germany. 2Institut für Pathologie, Universitätsklinikum Leipzig, Liebigstr. 26, 04103 Leipzig, Germany. Abstract Introduction: Numerous methods exist for basic processing, e.g. normalization, of microarray gene expression data. These methods have an important effect on the final analysis outcome. Therefore, it is crucial to select methods appropriate for a given dataset in order to assure the validity and reliability of expression data analysis. Furthermore, biological interpretation requires expression values for genes, which are often represented by several spots or probe sets on a microarray. How to best integrate spot/probe set values into gene values has so far been a somewhat neglected problem. Results: We present a case study comparing different between-array normalization methods with respect to the identification of differentially expressed genes. Our results show that it is feasible and necessary to use prior knowledge on gene expression measurements to select an adequate normalization method for the given data. Furthermore, we provide evidence that combining spot/probe set p-values into gene p-values for detecting differentially expressed genes has advantages com- pared to combining expression values for spots/probe sets into gene expression values. The comparison of different methods suggests to use Stouffer's method for this purpose. The study has been conducted on gene expression experiments investigating human joint cartilage samples of Osteoarthritis related groups: a cDNA microarray (83 samples, four groups) and an Affymetrix (26 samples, two groups) data set. Conclusion: The apparently straight forward steps of gene expression data analysis, e.g. between-array normalization and detection of differentially regulated genes, can be accomplished by numerous different methods. We analyzed multiple methods and the possible effects and thereby demonstrate the importance of the single decisions taken during data processing. We give guidelines for evaluating normalization outcomes. An overview of these effects via appropriate measures and plots compared to prior knowledge is essential for the biological interpretation of gene expression measurements.