New Gene Linked to ALS Severity
EphA4 receptor associated with more aggressive illness.
Dr. Wim Robberecht, a Packard researcher and neuroscientist at the University of Leuven in Belgium.
An international team of researchers, including a Packard scientist, has identified a new gene linked to the severity of ALS. Wim Robberecht, a Packard-funded researcher and neuroscientist at the University of Leuven in Belgium, and colleagues have discovered that blocking the receptor known as EphA4 increases survival in fish and rodent models of ALS. The study, published August 26 in Nature Medicine, also found that human ALS patients with the most EphA4 receptors had the earliest age of onset and the lowest survival times.
"EphA4 helps determine the severity of ALS," Robberecht said. "If someone is going to get ALS, the more of this protein that he or she expresses, the younger they will be at age of onset, and they will have a shorter disease duration."
Most ALS patients are diagnosed in their 50s or 60s, and typically live for an average of 2 to 5 years after diagnosis. These averages, however, conceal a wide variability in both age of onset and duration of illness. Patients have been diagnosed with ALS everywhere from their 20s to their 80s, and can live for anywhere from a few months to a decade or more.
This wide variability in ALS severity made Robberecht wonder what, if any, genetic factors might be involved in determining when the disease began and how long it lasted. The researchers altered the expression of over 300 genes to identify potential genetic modifiers in a zebrafish model of ALS. Zebrafish that have been engineered to express large amounts of a mutant version of SOD1 have shorter, less branched motor neuron axons than healthy zebrafish.
Robberecht and colleagues wanted to find the gene or genes that might alter the effects of the mutant SOD1. To do this, they used a type of small molecule called a morpholino that can bind to a specific mRNA molecule. This binding prevents the gene from being turned into a protein. The researchers screened 303 different genes using 303 morpholinos, each specific to that gene’s mRNA. Robberecht was able to identify 13 genes that, when blocked, returned the axons in the mutant SOD1 zebrafish back to normal.
The gene that was the most protective when disabled was Rtk2, which is the zebrafish version of a human gene known as EphA4 (ephrin type-A receptor 4). The morpholinos rescued motor neurons in a dose-dependent relationship. More morpholinos meant more inhibition of EphA4, which led to more rescuing of motor neurons. Motor neuron rescue also occurred when the researchers used a pharmalogical compound that inhibited EphA4.
In humans, the ephrin proteins interact with rreceptors like EphA4 to regulate the development and growth of neurons in embryos. They help axons branch and form connections with other neurons, and help cells throughout the developing embryo move to the right location. Recently discovered functions in adults include directing the differentiation of stem cells and the formation of blood vessels, as well as enhancing the connection between neurons to help them communicate more efficiently. Researchers still don't know which of these functions are related to ALS.
Robberecht and colleagues then looked at the effects of the EphA4 equivalents in rodents. They found that deleting an EphA4 allele in mice carrying an SOD1 mutation allowed the mice to survive longer after they started showing signs of disease. When Robberecht blocked the action of the EphA4 protein in rats carrying an SOD1 mutation, he found that the animals developed disease significantly later in life.
Looking closer, the researchers found that the large motor neurons had the most EphA4 mRNA (which translates the EphA4 gene into protein), and are the most vulnerable to degeneration in ALS. In contrast, small motor neurons have fewer copies of EphA4 mRNA and their degeneration occurs only at end-stage disease. As well, the presence of EphA4 prevented damaged motor neurons from re-innervating muscle. Together, these results indicate that EphA4 plays an important role in motor neuron degeneration.
Together with Bob Brown, a researcher with P2ALS (a focused collaboration between Packard & Project ALS) at the University of Massachusetts Medical School, Robberecht and colleagues then looked at the potential role of EphA4 in human ALS patients and found that patients with the highest EphA4 expression had the most severe course of disease. They showed symptoms at an earlier age and tended to survive for a shorter period of time after diagnosis. Those ALS patients whose EphA4 proteins didn't function properly survived much longer than patients with functional EphA4.
"EphA4 is a receptor, and a receptor can be blocked to modify the disease course to make it less aggressive," Robberecht said. Finding a molecule that can inhibit the action of EphA4 might be a potential therapeutic target for improving the lifespan of patients diagnosed with ALS, he noted.
–– Carrie Arnold