Serial Analysis of Gene Expression

Abstract

Serial Analysis of Gene Expression (SAGE) is a method designed to measure the abundance of a large number of mRNAs in tissues and cells to characterize their transcriptome. By the end of the twentieth century, when determining the entire genomic sequence of higher organisms began to be perceived as a realistic and attainable goal, the interest for genome-wide analysis of gene expression rapidly increased. One of the first strategies was to generate expressed sequence tags (ESTs), that is, partial sequences of cDNA clones randomly selected in libraries prepared from various tissue samples. The EST approach was highly valuable for gene discovery. Moreover, as long as the abundance of a given mRNA could be correlated with the frequency of its EST, it might also provide quantitative information. However, this approach evaluated only a limited number of genes per library. Obviously, getting larger quantitative expression profiles required to increase the rate of acquisition of experimental data. SAGE rests on the sequencing of short diagnostic tags, and as such increased by more than 1 order of magnitude the number of transcripts analyzed by current sequencing methods. Sets of 100 000 SAGE tags are now routinely sequenced, making possible the quantification of low copy number mRNAs.