Inside the brains of patients with amyotrophic lateral sclerosis, a debilitating neurodegenerative disease, is a telltale sign that marks almost every case: clumps of toxic proteins.

Now, researchers have pinpointed a key gene behind the formation of one type of these neuron-damaging aggregates. They've also shown how inhibiting the gene's function curbs production of the harmful protein.

The gene, RPS25, codes for a piece of cellular machinery necessary for creating the protein-based gunk that amasses in some forms of ALS and damages healthy neurons. When the gene's activity was experimentally depleted -- in yeast, in neurons derived from patients with ALS and in fruit flies -- the team saw levels of the lethal protein drop by about 50 percent across the board.

The team also tested the function of RPS25 in human cells modeling Huntington's disease and spinocerebellar ataxia, two other neurodegenerative illnesses that have protein-aggregate "hallmarks" similar to ALS, said a graduate student. There, too, inhibiting the gene helped tamp down the levels of bad protein.

A paper detailing the results of the research is published in Nature Neuroscience.

In ALS, a string of DNA that erroneously repeats itself.  It's these DNA repeats that are transformed into the harmful proteins that build up in the brain. As the proteins amass, they interfere with healthy neurons, blocking the cells' ability to function normally.

The protein aggregates aren't made like other proteins found in the body, the lead author said. "These repeats actually shouldn't be made into proteins at all; they come from DNA that isn't supposed to code for anything, and yet somehow the proteins come to be anyway."

The translation is initiated by a code in the mRNA that shows the ribosome where to start translating. The ALS-associated DNA repeats don't have that start code, unlike normal mRNA.

"So regular translation doesn't work with the repeats," the author said. But it turns out there's a molecular workaround: an unconventional translation process called repeat-associated non-AUG translation, or RAN translation, that turns the ALS repeats into destructive protein bodies.

The exact mechanism of RAN translation and its role in human biology is not clear, but scientists do know that it still depends on the ribosome. To better understand the process, the authors turned to yeast, a simple organism that still has the major proteins and pathways of human cells. One at a time, the researchers decreased the function of individual yeast genes and monitored the fungus' RAN function. When subdued, several genes swayed RAN function, but one in particular, RPS25, stood out. With the gene hindered, production of the toxic protein fell by 50 percent.

The researchers also saw a 50 percent dip in the toxic protein when they tested how neurons derived from patients with ALS fared without RPS25.

"Through genomic analyses, we could see that the ALS-associated repeats were still there; the sequences hadn't changed," the author said. "What was changing was the output of the ribosome; the repeats weren't being made into toxic proteins nearly as often."

Slashing a part of the cell's protein-making machine might sound risky, but it turns out a defunct RPS25 gene doesn't spoil normal protein production. Yet the researchers also showed that an inactive RPS25 gene affects more than just the ALS repeats; the dysfunctional gene similarly stunted erroneous protein production in cellular models of Huntington's disease and spinocerebellar ataxia, two neurodegenerative illnesses that have hallmark protein aggregates similar to ALS.

Finally, the researchers turned to fruit fly models of ALS to investigate how depleting RPS25 affected the insect overall. Not only did they see a similar decrease in toxic protein levels, they also saw an increased life span in the flies that lacked fully functional RPS25. Flies that harbored both the ALS mutation and a working RPS25 gene died by day 29, on average, while those that had the ALS mutation and lower amounts of RPS25 lived on average for 38 days. A healthy fruit fly lives about 50 days on average.

The findings are intriguing, but before the scientists can begin to pursue RPS25 as a drug target, the team has a couple boxes to tick off. The team now is investigating how a more complex animal model -- like a mouse -- would fair without RPS25.

Furthermore, the authors are still after a clearer picture of RAN translation in humans, overall. "Does it only occur under neurogenerative conditions? Or is there a broader role for it in healthy individuals?" the author said. "We don't know the answer to those questions yet, and it will be crucial to figure out before pursuing RPS25 as a therapeutic target."

http://med.stanford.edu/news/all-news/2019/07/key-gene-behind-hallmark-of-lou-gehrigs-disease-identified.html

https://www.nature.com/articles/s41593-019-0455-7