August 24, 2016
Usain Bolt sprinting is a thing of beauty. It is just amazing how he can kick in the afterburners at the end of a race and just dominate the thing. I am sure Justin Gatlin of the U.S. would love for Bolt to lose this extra burst of speed so Gatlin could beat him at the Olympics.
Turns out that transcription elongation has an afterburner a bit like Bolt’s too. It goes by the name SPT4 in yeast and SUPT4H1 in you and me. The protein from this gene is needed to push through long transcripts.
A new study in Science by Kramer and coworkers suggests that like Gatlin, some people would like to see their cells lose the burst of speed that SUPT4H1 gives their polymerases. But instead of helping these folks win a race, this loss might help them deal with their amyotrophic lateral sclerosis (ALS) or frontotemporal dementia (FTD).
ALS is a progressive neurodegenerative disease that is always fatal. It was first made famous by Lou Gehrig and later with the bucket challenge. After Alzheimer’s, FTD is the second most common form of dementia.
Back in 2011 two groups found that a significant number of cases of FTD and ALS were associated with a gene called C9orf72 (chromosome 9 open reading frame 72). These people had hundreds or even thousands of copies of the hexanucleotide repeat GGGGCC in the first intron of their gene instead of the 30 or so that is more typical.
Later studies showed that these repeats caused two very specific problems in cells. First, the RNA (and antisense RNA) from this allele tended to build up in small bundles called foci. Some researchers think that these foci trap some of the important RNA binding proteins that the cell needs.
The second phenotype is a strange one. These RNAs get translated by a process called repeat-associated non-ATG, or RAN, which causes a buildup of dipeptide repeat proteins. Apparently there is something about the secondary structure of the RNA that allows it to get translated without a typical AUG start codon.
The idea is that these foci and weird dipeptide proteins are at least part of the reason why these folks have their ALS symptoms. Ideally you’d want to get at all three issues (the sense and antisense RNA-laden foci, and those newly translated proteins) with a single approach.
Kramer and coworkers reasoned that they might get such a result if they could get the cell to make a whole lot less of SUPT4H1 (or Spt4p in yeast). They reasoned correctly.
Previous research had shown that its deletion didn’t affect too many genes except for those involved in diseases like Huntington’s – those with long CAG repeats. Perhaps, then, deleting it might also just affect the copies of the C9orf72 gene with those hexanucleotide repeats without affecting too many other genes.
When they forced yeast, nematode, fruit fly and human cells to make less Spt4p or SUPT4H1, the number of RNA foci went down or even disappeared in all of these different cells. There was also much less of those dipeptide repeat proteins lurking about the cell as well.
They first set out to do some experiments in everyone’s favorite workhorse, Saccharomyces cerevisiae. They found that expressing either the sense or antisense RNA with the 66 hexanucleotide repeats caused both the RNA foci and the dipeptide repeat proteins seen in the cells of ALS patients to form in yeast too. Neither happened with the sense or antisense 2 repeat constructs.
Next they showed that deleting SPT4 greatly reduced the level of 66 repeat RNA but had little effect on the 2 repeat RNA. These researchers also saw no RNA foci and much less dipeptide repeat proteins in the deletion strain expressing the 66 hexanucleotide repeats. All without much affecting any other genes.
This yeast work suggests that targeting SUPT4H1 might reduce the effects of the ALS version of the C9orf72 gene without affecting the more typical version. Now Kramer and coworkers were ready to see what happens in bigger beasts.
When they expressed the 66 repeat in Caenorhabditis elegans neurons, these nematodes lived for a shorter time and their neurons had RNA foci and the dipeptide repeat proteins. Expressing human SUPT4H1 in these worms’ neurons worsened their condition while feeding them RNAi against nematode SPT4 helped.
The RNAi let these worms live longer and it decreased the number of RNAi foci and the amount of dipeptide repeat proteins. They saw similar results with a Drosophila system.
Finally they moved to the main stage—human cells from ALS patients who had the C9orf72 protein with too many hexanucleotide repeats. RNAi against either SUPT4H1 or its partner in crime, SUPT5H, reduced the number of RNA foci and reduced the amount of dipeptide repeat proteins with no “overt toxicity.” RNA-seq showed that only a small subset of genes was affected with the RNAi treatment.
So it looks like targeting SUPT4H1 may be a good strategy for dealing with ALS if the RNA foci and dipeptide repeat proteins are a big part of the problem. This is a big if.
But if it all does work out, we can thank yeast yet again (#APOYG!) for showing us the way to a new treatment for a devastating disease. Of course, though, yeast can’t do everything. It is unlikely to show sprinters the best way to beat Usain Bolt in a race!
by Barry Starr, Ph.D., Director of Outreach Activities, Stanford Genetics
Categories: Research Spotlight Yeast and Human Disease