March 23, 2016
Diagnosing why something has gone wrong in a complicated system can be difficult. There are so many bells and whistles that you can easily get lost.
That’s why it can sometimes help to turn to a simple system and then apply what you have learned to the more complicated one. This will, of course, sound familiar to any scientists studying that marvel of a model organism, Saccharomyces cerevisiae.
For example, it is amazing what you can glean from this yeast about human brain and blood diseases. Even though, of course, baker’s yeast has neither blood nor a brain!
This becomes very clear in a study out in PLOS Genetics by Fernandez-Murray and coworkers. In this study they use yeast to help figure out why mutations in the SLC25A38 gene in people leads to something called congenital sideroblastic anemia. And even better, their work hints at a possible treatment.
People with sideroblastic anemia make too little hemoglobin in their red blood cells and have too much iron in the mitochondria close to the nucleus (perinuclear mitochondria). The current treatment for this condition is not ideal, involving lots of transfusions and iron chelation.
Sometimes people are born with this anemia and sometimes people get it later in life. One subset of the inherited version happens because a gene with an unknown function, SLC25A38, isn’t working correctly. This group of patients is the focus of this study.
Fernandez-Murray and coworkers started out by using yeast to figure out what the yeast homolog, HEM25, does in a yeast cell. When the gene was deleted, the cells made about 50% less heme than wild type yeast and adding back the human gene, SLC25A38, to this deletion strain restored heme levels. Looks like they had made a yeast model of this inherited anemia.
Previous work had suggested that SLC25A38 might be a glycine or serine transporter and the next set of experiments confirmed it as a glycine transporter in a couple of ways. In both, they took advantage of cases in which yeast can use glycine as their sole nitrogen source if the glycine can make it into the mitochondria.
In the first case, they showed that yeast cells deleted for HEM25 grew poorly on plates where glycine was available as the only nitrogen source. In the second case, they showed that cells deleted for both SER1 and HEM25 grew poorly on plates where again glycine was the only nitrogen source. This last result confirms HEM25 as a glycine transporter since yeast deleted for the SER1 phosphoserine aminotransferase can only grow in the absence of serine if they can get glycine into their mitochondria. (There isn’t space to go into it here, but they also showed that HEM25 was not a serine transporter.)
OK so now they had created a yeast cell that mimicked the effect of sideroblastic anemia and figured out why people with a mutated SLC25A38 gene had the condition. Now it was time to find a treatment.
The researchers came up with three possibilities. The first treatment was just to give the yeast extra glycine, the second was to drive glycine synthesis within the cell by adding a lot of serine, and the third was to add a downstream precursor of heme synthesis, 5-aminolevulinic acid (5-Ala).
They tested each scenario on yeast cells deleted for HEM25 and found that both glycine and 5-Ala worked to restore heme synthesis, but that added serine had no effect. Both glycine and 5-Ala returned heme levels to that seen in wild type.
Of course we aren’t yeast, so they next tested their treatment on something a bit more complicated — zebrafish. By using morpholino technology to knock down both copies of the zebrafish SLC25A38 homolog, SLC25A38a and SLC25A38b, the researchers managed to lower a zebrafish’s heme levels to about 50% of normal.
When they gave these zebrafish extra glycine or 5-Ala, their heme levels did not improve. They were still anemic!
After a bit of thought, the researchers realized that folate might be what the zebrafish were missing. In work that we didn’t have time to go over before, the researchers had shown that a folate dependent pathway was critical for getting heme levels up to normal.
Yeast could get away without added folate in these experiments because they make their own. However, zebrafish, like people, do not.
So the final step was to try to add both glycine and folate to these fish. Now the zebrafish’s heme levels returned to about 80% of normal.
These results suggest a better treatment for some people with sideroblastic anemia — added folate and glycine. And it all came from studying the problem in the simpler, bloodless S. cerevisiae. Nice work again yeast.
by Barry Starr, Ph.D., Director of Outreach Activities, Stanford Genetics
Categories: Research Spotlight