It's important to emphasize that the products of alternative splicing must be functional because we know that splicing is error-prone and that mispliced, nonfunctional, RNAs will be quite common. Every gene will produce a bunch of these aberrantly spliced variants but that doesn't mean that every primary transcript is alternatively spliced.
It's important to distinguish between real functional alternative splicing and junk RNAs that arise from splicing errors. One of the ways to do this is to report on the concentrations of the various transcripts but that's rarely done in papers that promote alternative splicing [see: The most important rule for publishing a paper on alternative splicing].
The importance of alternative splicing is related to the debate over the importance of pervasive transcription and junk DNA since advocates of alternative splicing are often the same people who object to junk DNA [see: Vertebrate Complexity Is Explained by the Evolution of Long-Range Interactions that Regulate Transcription?]. I call this The Deflated Ego Problem because these scientists are usually looking for way to "explain" the complexity of humans in light of the fact that we seem to have the same number of genes as many other species.
If it's true that most human genes are alternatively spliced then let's see the evidence. That means actually demonstrating that different proteins with different functions are produced from the same gene. We've known for 35 years that this is possible but that's not the point. The point is whether all, or most, human RNAs are alternatively spliced. I've issued a simple challenge to those who use the alternative splice databases [A Challenge to Fans of Alternative Splicing]. So far, nobody has stepped up to the plate.
Some of the examples that are promoted in those databases make no sense whatsoever [Two Examples of "Alternative Splicing"] [The Frequency of Alternative Splicing ].
Someone raised this issue in the comments to another post and send me a link to a paper published in 2010. Here's the paper and part of the introduction.
Keren, H., Lev-Maor, G. and Ast, G. (2010) Alternative splicing and evolution: diversification, exon definition and function. Nature Reviews Genetics 11:345-355 [doi: 10.1038/nrg2776].
Splicing of precursor mRNA (pre-mRNA) is a crucial regulatory stage in the pathway of gene expression: introns are removed and exons are ligated to form mRNA. The inclusion of different exons in mRNA — alternative splicing (AS) — results in the generation of different isoforms from a single gene and is the basis for the discrepancy between the estimated 24,000 protein-coding genes in the human genome and the 100,000 different proteins that are postulated to be synthesized.I'd like you to answer two questions.
... Comparing species to see what has changed and what is conserved is proving valuable in addressing these issues and has recently yielded substantial progress. For example, new high-throughput sequencing technology has revealed that >90% of human genes undergo AS — a much higher percentage than anticipated. Such technological progress is providing more comprehensive studies of splicing and genomic architecture in an increasing number of species, and these studies have extended our evolutionary understanding.
- Do you believe that there are about four (4) different, functional, proteins produced on average from every human protein-encoding gene?
- Do you believe that more than 90% of human genes produce a transcript that can be alternatively spliced, where alternative splicing is restricted to producing different functional RNAs and not just noise?