29 August, 2012. Researchers have discovered a gene that adjusts the course of amyotrophic lateral sclerosis, a fatal and rapidly progressing neurodegenerative disease. When the ephrin receptor EphA4 is mutated or missing, it delays disease onset and slows progression in a variety of model organisms. Scientists led by Wim Robberecht of VIB Leuven, Belgium, also found that two people who had ALS plus an EphA4 mutation lived at least seven years after symptom onset—a long time for this quick-moving neurodegenerative disease. The findings were published in the August 26 Nature Medicine online.
“This is a seminal study showing that EphA4 expression in motor neurons is an important factor in animal models of ALS and, critically, that reduced EphA4 expression correlated with less severe disease in humans,” wrote Andrew Boyd of the Queensland Institute of Medical Research in Herston, Australia, in an e-mail to Alzforum (see full comment below). Boyd was not involved in the study.
As a receptor tyrosine kinase, EphA4 mediates signals that repel axons during brain development. It is also expressed in adult brain, where it appears to regulate synaptic architecture and plasticity (for a review, see Klein, 2009). EphA4 modulates the actin cytoskeleton that controls neuronal growth cones (Shi et al., 2007). It is not the first axon- and actin-regulating protein linked to ALS; profilin, a recently discovered ALS gene, catalyzes actin polymerization and growth cone formation (see ARF related news story on Wu et al., 2012).
EphA4 turned up as the strongest hit that Robberecht and first author Annelies Van Hoecke discovered in a screen for genetic modifiers of motor neuron degeneration. They set out to investigate the variability in age of onset of ALS and in its speed of progression. Some people succumb to ALS in their thirties, while others remain healthy into their eighties before exhibiting symptoms; lifespan ranges from a few months to more than 20 years after disease onset. “Even in families with the same mutation, there is a large variability,” Robberecht said (Penco et al., 2011; Kim et al., 2007).
Van Hoecke used a zebrafish model of ALS based on expression of human mutant superoxide dismutase 1 (mSOD1) . Mutant SOD1 shortens and promotes excess branching of motor neuron axons in these animals (Lemmens et al., 2007). Van Hoecke and colleagues screened 303 different antisense oligonucleotides that block translation of their target genes, and came up with five candidates. The fish Rtk2 gene, which corresponds to human EphA4, was the strongest hit. Shutting down Rtk2 or the related Rtk1, via antisense nucleotides or with a chemical that blocks ephrin receptors (Noberini et al., 2008), rescued mSOD1 axonopathy in the fish.
To see if the same approach would work in mammals, Van Hoecke crossed mSOD1 mice with animals missing one copy of the ephrin receptor gene. In the mSOD1/EphA4 heterozygotes, the 50 percent reduction of EphA4 expression prolonged survival by approximately two weeks compared to mSOD1-only control mice, although the age of symptom onset was the same. The EphA4 heterozygote transgenics also possessed more motor neurons and healthier neuromuscular junctions than did their mSOD1 counterparts.
The story was slightly different in rats. A peptide that blocks EphA4 (Fabes et al., 2007) delayed the onset of symptoms in animals expressing human mSOD1, but did not affect progression. That blocking of EphA4 slowed progression in mice, but delayed onset in rats, could be due to differences between the models or the EphA4 blockers used in each case, the authors suggested.
“The bottom line from the fish, mouse, and rat experiments was that the less EphA4 was expressed, the better,” Robberecht said. Human studies told the same story. Van Hoecke and colleagues analyzed EphA4 mRNA levels in blood samples from 158 Dutch people with ALS, and observed that the less EphA4 people had, the later in life they developed the disease, and the longer they survived with it.
What explains the variability in EphA4 levels in the human population? Genomewide association studies did not uncover any variants near EphA4 that associated with disease severity. However, collaborator Robert Brown, at the University of Massachusetts Medical School in Worchester, sequenced EphA4 in people with ALS and came across two new variants: a stop codon in place of arginine 514, and a substitution of glutamine for a conserved arginine at position 571. A man with the truncation lived for seven years after symptoms started, and a man with a missense mutation has survived 12 years and counting, after diagnosis. ALS normally runs its course in two to three years. The data suggest that the receptor makes an attractive target for pharmaceutical intervention, Robberecht said.
Robberecht and colleagues will investigate the mechanism by which EphA4 enhances ALS. The current study hints that neurons with lots of the receptor are prone to degenerate. Van Hoecke used laser-capture microdissection to isolate individual spinal motor neurons from mice and measure EphA4 mRNA. She discovered that the large neurons, which are most vulnerable to ALS, make more EphA4 than the smaller, resistant neurons. The relationship between EphA4 and disease susceptibility may hold true in other neurodegenerative diseases as well, the authors suggest. They found that blocking EphA4 in fish expressing mutant TDP-43, another ALS gene, or the spinal muscular atrophy gene Smn1, also rescued axonal pathology.—Amber Dance
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