Globs of protein clustered in the neurons that control muscles have long been the hallmark of amyotrophic lateral sclerosis (ALS), the fatal neurodegenerative disease also commonly known as Lou Gehrig's disease. Now, a study of the most commonly found mutant gene in people with ALS reveals an unexpected origin of some of those sticky masses, a finding that may offer drug developers a new target for treatments.
Located on the ninth chromosome, which explains part of its unwieldy name, the C9orf72 gene has a bit of a stutter. A typical version in healthy people contains a stretch of DNA where a string of six genetic letters—GGGGCC—repeats up to 25 times. Scientists have recently found that in a sizable share of people with ALS and frontotemporal dementia (FTD), a less common neurological disease characterized by language, memory, and emotional problems, this repeat occurs many more times; some people have thousands of copies.
Since these C9orf72 mutations were discovered in 2011, some researchers have speculated that the repeats interrupt production of the gene's normal protein, which serves some as-yet unknown, but vital function in motor neurons or other brain cells. Others have hypothesized that the mutation spawns a large, misshapen strand of RNA that grabs on to proteins such as TDP-43, which normally help process RNA, creating protein tangles that starve the cell of the machinery it needs to function.
Molecular biologists at the Ludwig Maximilians University Munich in Germany and the University of Antwerp in Belgium, however, wondered whether the genetic stutters themselves coded for proteins that became tangled in the cell. Few scientists had considered this because the stutters don't contain the "start signal" that allows proteins to be made. Still, in a few other diseases caused by genetic repeats, the cell manages to produce proteins from the abnormal gene despite lacking this signal. Sometimes these proteins are toxic and ultimately kill the cell.
Based on the DNA sequence of the GGGGCC-laden C9orf72 seen in ALS and FTD patients, the European team determined that if translated, the gene would produce various proteins containing strings of repeat amino acids. Dubbed dipeptide repeat (DPR) proteins, these molecules don't normally appear in humans and should be prone to clumping, the scientists concluded. Indeed, when they began to search for DPR protein clusters in actual human brain tissues, they found them in tissue from FTD and ALS patients with the C9orf72 mutation. No such lumps showed up in the brain tissue of healthy controls or ALS and FTD patients without the C9orf72 mutation, increasing the likelihood that the mutation produced them, Dieter Edbauer, a molecular biologist at Ludwig Maximilians, and his co-authors report online today in Science.
So what about TDP-43? The protein previously suspected of forming the cellular tangles is present in most of the abnormal protein deposits in the brains of people with ALS and FTD; however, people with the C9orf mutation have distinctive star and dot-shaped protein clusters lacking it. Edbauer's team found that those unusual clusters contain DPR proteins instead. An antibody to the proteins bound successfully to the stars and dots, and also to small round cores of protein within circular deposits of TDP-43, suggesting that DPR could be a catalyst for clump formation, Edbauer says.
The new study is a "gem," says Bryan Traynor, a neurologist at the National Institute on Aging in Bethesda, Maryland, who first discovered the GGGGCC mutation in ALS patients. "I really do think this represents a step forward," he says, adding that the findings could eventually lead to therapies that use a molecule to overcome the effect of the mutant gene.
Further verification of the results is needed, however, due to the small number of ALS and FTD patients whose brain tissue was studied, cautions Robert Bowser, a neurobiologist at the Barrow Neurological Institute in Phoenix. Traynor agrees, and says that there are many mysteries still to be solved: For example, researchers now need to determine which of the two proteins, DPR or TDP-43, drives the neurodegenerative process, and how they're doing it. "Do you need both? Is one protective and one detrimental?" he says. "Like all good science, this leads us to more questions."