Here's the problem. The explosion of EST data in the 1990's resulted in detection of many sequences suggesting that alternative splicing was much more widespread that previously suspected. The vast majority of these claims have not been verified and many of them have been removed from the annotated genomes published in the past few years.
Now we have a whole new set of claims based on high throughput analysis of transcripts from a variety of organisms and tissues. Many workers believe that the majority of human genes are alternatively spliced and some even publish articles stating that 95% of humans genes exhibit alternative splicing. One of my colleagues who makes such a claim says that just because a gene is alternatively spliced doesn't mean that the various isoforms of RNA are functional but I think that's disingenuous. If it's going to be a meaningful term then "alternative splicing" has to imply that that at least two different versions of RNA have some biological function.
I've asked repeatedly for evidence that some particular genes are alternatively spliced to give rise to two or more functional products. It should be possible to get this information from the databases used by these researchers—the ones that support their claim of widespread alternative splicing. Unfortunately, this has proven to be difficult. Whenever I search common alternative splicing databases I'm told that those databases aren't very good and the results aren't reliable.
Here's the challenge to all researchers who believe that a majority of human genes are alternatively spliced (in a biologically relevant manner). Show me your data. Pick one of the following sets of genes and demonstrate that most of them have functional alternatively spliced transcripts. If none of the genes in the set qualifies then explain why you reject the presumed alternative transcripts shown in popular databases. This shouldn't be much of a challenge if your claim is correct.
Note that this is a two part challenge. You have to first present evidence that there are functional alternative splicing events and then you also have to present the reasons why you reject some of the data from sequenced RNAs.
Here's an example from the human gene for triose phosphate isomerase (TPI1). The Entrez Gene entry is Gene ID: 7167. The primary entry shows one alternatively spliced transcript that removes the N-terminal coding region of the protein and creates an new larger N-terminal sequence. What is the evidence that this is biologically relevant? Now check out the known transcripts that have been detected according to UCSC Genome Browser, AceView, and Model Maker. These show additional variants affecting the splice junction sequences around exons 2, 4, and 6. Are these also examples of alternative splicing? Are they functional? If you reject these variants then what's the rationale for accepting some possible transcripts as real but rejecting others? Are some of them artifacts?
The three gene sets are ...
- Human genes for the enzymes of glycolysis.
- Human genes for the subunits of RNA polymerase with an emphasis on the large conserved subunits [Two Examples of Alternative Splicing]
- Human genes for ribosomal proteins.
I selected these examples because we know the structures of the proteins so we can evaluate the possibility that an alternatively spliced message might produce a novel polypeptide chain. Of course there might be other reasons (regulation?) for producing alternatively spliced transcripts. Feel free to present the evidence.
Now it's possible that I've accidentally chosen sets of genes that do not exhibit alternative splicing. If that's the case then pick any other set of genes with a common function where the structure of the protein product is known. Meanwhile, you can explain why you reject all the putative splice variants for these genes.