Potential Protein Target for Antibody Therapy Identified in Early Study
Antibodies that target proteins wrongly expressed in the brain and driven by mutations in C9ORF72, a cause of amyotrophic lateral sclerosis (ALS), eased symptoms and prolonged survival in a mouse disease model, a new study shows.
Its researchers suggest that such antibodies could be used to treat people with ALS.
The study, “Antibody Therapy Targeting RAN Proteins Rescues C9 ALS/FTD Phenotypes in C9orf72 Mouse Model,” was published in Neuron.
Mutations in the C9ORF72 gene account for about 7% of sporadic ALS cases and up to 40% of familial cases, making them the most common known genetic cause of ALS. This gene is also implicated in frontotemporal dementia (FTD); some people with C9ORF72 mutations develop ALS, some FTD, and others have symptoms of both conditions.
How mutations in C9ORF72 cause ALS or FTD is not fully understood; likely, there are multiple concurrent mechanisms of action. What is known is that these mutations involve the expansion of six DNA elements in terminal portion of C9ORF72, and are associated with the buildup of certain repeat proteins in the brain, called repeat associated non-ATG (RAN) proteins.
“These proteins, which are unexpectedly produced without the normal signals for protein production, accumulate in the brains of affected individuals,” Laura Ranum, PhD, a professor at the University of Florida and study co-author, said in a press release.
Six RAN proteins are known to accumulate in the brain of ALS patients, each made of repeat sequences of amino acids (organic compounds that are the building blocks of proteins). The researchers used B-cells from healthy elderly donors to create human antibodies against two of these RAN proteins: poly(Gly-Ala), or GA, and poly(Gly-Pro), or GP.
After confirming that these antibodies bound to their target proteins as expected, the team tested them in several models using cells in dishes. Broadly, these antibodies reduced the accumulation of RAN proteins in the cells and limited toxicity to the cells.
Researchers then tested murine versions of the antibodies in female C9-BAC mice, a mouse model of ALS/FTD with mutated C9ORF72. These antibodies were able to cross the blood-brain barrier and bind to their target proteins in the brain.
The antibodies also lessened RAN protein aggregation in the mice. For example, in an area of the brain called the retrosplenial cortex, 10 weeks of once-weekly injections of a GA-targeting antibody reduced GA aggregates to about 52% of the amount found in untreated mice. After 37 weeks, GA aggregates dropped to 45% of the amount seen in untreated mice.
Interestingly, this GA-targeting antibody didn’t just affect GA aggregates. Levels of GP also fell to 61% and 59% of untreated mice after 10 and 37 weeks of treatment, respectively, as did levels of poly(Gly-Arg) (GR), another RAN protein.
“By targeting one mutant protein, there is a collateral beneficial effect that results in the reduction of multiple related mutant proteins,” Ranum said.
This may be because GA aggregates themselves can interfere with the proteasome — the machinery in cells that helps to break down proteins. It’s possible that, by breaking up the GA aggregates, the proteasome becomes more active, allowing for better clearance of aberrant proteins.
“In other words,” Ranum said, “the treatment antibody leads to the activation of a garbage disposal-like system in cells that gets rid of multiple types of proteins produced by the disease mutation.”
In addition to changes at the protein level, treatment with GA-targeting antibodies significantly improved behavioral symptoms in the mice, with those treated showing more normal gait patterns and fewer anxiety-like behaviors. Treatment also significantly increased survival rates; nearly 80% of treated mice were alive after 37 weeks, compared to about 40% of untreated mice.
The one tested antibody targeting GP did not have significant effects on behavior or on survival, though it did significantly lower GP aggregates (by about 35% after 37 weeks of treatment). This antibody also did not significantly affect GA aggregates.
“This could be because further reduction [of GP] would be needed to be efficacious or because GP does not play a critical role in disease pathology,” the researchers wrote.
They added that future studies could investigate how targeting one RAN protein affects other RAN proteins, potentially providing broader clues into which RAN proteins specifically affect ALS progression and how.
Overall, this study is a proof-of-concept for using antibodies that target RAN proteins as a treatment strategy for ALS with C9ORF72 mutations.
“This can be developed further to create a drug candidate for human clinical trials,” said Lien Nguyen, PhD, a study co-author and professor at the University of Florida. “A similar antibody-based approach may be applicable to other neurologic diseases.”