Monitoring the change in expression patterns over time provides the distinct possibility of unraveling the mechanistic drivers characterizing cellular responses. Gene arrays measuring the level of mRNA expression of thousands of genes simultaneously provide a method of highthroughput data collection necessary for obtaining the scope of data required for understanding the complexities of living organisms. Unraveling the coherent complex structures of transcriptional dynamics is the goal of a large family of computational methods aiming at upgrading the information content of time-course gene expression data. In this review, we summarize the qualitative characteristics of these approaches, discuss the main challenges that this type of complex data present, and, finally, explore the opportunities in the context of developing mechanistic models of cellular response. Keywords microarrays; bioinformatics; regulation; clustering; pharmacogenomics
TEMPORAL GENE EXPRESSION ANALYSISAt any given time a cell will only express a small fraction of the thousands of genes in the organism's genome. Expressed genes reflect the structure and functional capacities of the cell as well as the ability of the cell to respond to external stimuli. In a complex organism, external stimuli to a great extent take the form of chemical messages whose purpose is to coordinate the function of the complex society of cells (1). Gene arrays, which measure the level of mRNA expression of thousands of genes simultaneously, provide a method of highthroughput data collection necessary for obtaining the scope of data required for understanding the complexities of living organisms. Monitoring the change in expression patterns over time using gene arrays provides an approach for capturing the multidimensional dynamics of complex biological systems. By using gene arrays in a time series paradigm, we are able to observe the emergence of coherent temporal responses of many interacting components. The data should provide the basis for understanding evolving Copyright © 2007 Global gene expression analysis has been celebrated as a major revolution in modern biology (2). The ability to monitor simultaneously the expression of the genes composing the entire genome has generated unimaginable possibilities (3-5). Despite some criticism regarding the cross-platform reproducibility of expression experiments (6, 7), more recent evidence (8, 9) supports the informative nature of the experiment and the importance of the approach (10). Microarray analysis has found widespread applications from characterizing terminal states, i.e., benign versus malignant tumors (11), to attempts to decipher the evolution of complex diseases and cell fates (12-16). Hence, the nature of the data broadly defines the nature of the problems to be addressed.Boundary problems use the expression measurements as feature vectors that characterize static points in multidimensional spaces. Therefore, multiple samples, for example, from the same tissue of different patients (diseased/nondi...