Episode notes
The early 1990s marked the era of expressed sequence tag (EST) sequencing, an ingenious strategy aimed at identifying human genes without waiting for the complete human genome sequence. Instead of sequencing the entire genome, researchers sequenced short portions of cDNA derived from expressed transcripts. These ESTs provided sequence tags for expressed genes and rapidly expanded the catalogue of human genes.
The success of the EST approach laid the foundation for microarray technology, which enabled genome-wide measurement of gene expression across different tissues, developmental stages, disease states, and experimental conditions. Microarrays became one of the dominant technologies for transcriptome profiling for more than a decade.
RNA sequencing (RNA-seq) has largely replaced microarrays for transcriptome analysis because sequencing provides a more direct and comprehensive measurement of RNA abundance. Microarrays are fundamentally hybridization-based detection technologies: a transcript can be detected only if a corresponding probe is already represented on the array. Consequently, transcripts that are novel, poorly annotated, highly divergent, or expressed as previously unknown isoforms may be missed.
Microarrays also have limitations in their quantitative range. Fluorescence intensity is used as a proxy for transcript abundance, but the relationship between transcript concentration and measured fluorescence is not perfectly linear over the entire dynamic range. At high transcript concentrations, probe spots can become saturated, placing an upper limit on the measurable signal. At the other end of the spectrum, weak signals can be difficult to distinguish from background fluorescence. Because genes can differ by several orders of magnitude in expression level, capturing both very highly and very weakly expressed transcripts accurately in a single hybridization experiment is challenging.
RNA-seq addresses many of these limitations by counting sequenced reads derived from RNA molecules rather than measuring hybridization intensity. It does not require a predefined probe for every transcript and can therefore detect novel transcripts, alternative splice isoforms, allele-specific expression, and previously unannotated genes, provided sufficient sequencing depth and appropriate analysis methods are used. Its digital nature also provides a substantially broader dynamic range than microarray fluorescence measurements.
Thus, the progression from EST sequencing → microarrays → RNA-seq represents a broader evolution in transcriptomics: from identifying individual expressed sequences, to measuring predefined transcripts simultaneously, and finally to directly sampling and quantifying the transcriptome through high-throughput sequencing.