Scaling down SAGE: from miniSAGE to microSAGE

Since Serial Analysis of Gene Expression (SAGE) was introduced more than a decade ago, it has been widely applied to characterise gene expression profiles in various tissues, cell types and cell lines of diverse origin including human, mouse, rat, yeast, plant and parasites. Throughout the past years many modifications to the original SAGE protocol have been developed, which address several aspects of SAGE, including an increase in sequencing efficiency (deepSAGE), improved tag-to-transcript mapping of SAGE tags (LongSAGE) and a reduction of the amount of required input RNA (microSAGE). Furthermore, the applications of SAGE have expanded from exclusively transcriptome analysis to now also include genome analysis, identifying genome signature tags that pinpoint transcription factor binding sites throughout the genome (Serial Analysis of Chromatin Occupancy or SACO). The review gives an overview of the main modifications to the SAGE technology that have been developed in the last decade, with a particular focus on the large reduction in the amount of required input RNA that has been achieved in the many SAGE modifications for downscaling or miniaturisation of SAGE (including microSAGE, PCR-SAGE and small amplified RNA-SAGE). The available methods for downscaling or miniaturisation of SAGE and their specific features will be discussed, illustrated by some examples of their application. This reduction in required quantity of input RNA has greatly expanded the possible applications of SAGE, allowing characterisation of global gene expression in material obtained from needle biopsies, small anatomical structures and specific cell types isolated by fluorescence activated cell sorting or laser microdissection.