Echo of silence

Decoding of the human genome sequence opened a treasure trove for scientists around the world.1,2⇓ Out of 2.9 gigabases of the human genome, only 1.1 to 1.4% is ultimately translated into protein. In contrast with previous estimates of ∼120,000 genes,3 preliminary analysis of the entire genome sequence revealed ∼31,000 genes: only twice as many as a worm (∼18,000) or a fly (∼13,000). The question then arises: what makes us human? Compared to the number of genes, the full set of proteins (proteome) inside our bodies is complex in both structure and function. A single gene producing a single protein is no longer the answer. A gene is defined as a transcriptional unit that consists of short exons (sequences that are expressed as protein) interrupted by large chunks of intervening sequences, introns. When RNA is transcribed from a gene, introns are spliced out and exons are joined together with remarkable precision. A direct consequence of RNA splicing is a possibility of different combinations of exons spliced together (alternative splicing) producing multiple isoforms of messenger RNA (mRNA) encoding different proteins from the same gene.4 According to current estimates, ∼60% of human genes are alternatively spliced, higher than any other species.1 Indeed, alternative splicing is assumed to be responsible for much of the complexity of human proteome.

Accurate splicing requires signaling sequences within an intron and an exon that are recognized by components of large splicing machinery. Splice signaling sequences are common targets for mutations in genetic …