January 25, 2008

Unexpected Discovery
Provides New Lead in ALS

Investigators at the University of Iowa led by molecular biologist John Engelhardt say they’ve uncovered an unexpected protein-protein interaction that appears to open a new avenue in the study of amyotrophic lateral sclerosis. Their results were published online yesterday in the Journal of Clinical Investigation.

Engelhardt and colleagues found that a protein called SOD1 normally interacts with the protein Rac1. The SOD1-Rac1 complex then triggers a third protein, Nox2, to produce potentially dangerous molecules called reactive oxygen species, or ROS. (The connection between Nox2 and ROS production was already known.)

ROS are highly reactive molecules that are needed for normal functioning of cells. However, in excess, they can cause a type of cell damage known as oxidative stress, which is known to occur in ALS, and inflammation, which has also been implicated in this disease.

“ALS is a devastating disease that paralyzes and kills people in the prime of their lives,” said Valerie Cwik, MDA’s medical director and vice president of Research. “MDA is dedicated to leaving no stone unturned in the search for effective treatments, and we plan to follow up on this lead immediately.”

A mutation in the gene for SOD1 is the cause of an inherited form of ALS in some 1 percent to 3 percent of people with the disease. Most ALS cases are not directly inherited (although genetic factors contribute to disease susceptibility), and their cause is so far unknown. Mice with mutated SOD1 genes have been used for several years to study ALS.

“When SOD1 binds to Rac1, the Nox2 protein produces ROS,” Engelhardt said. “However, [normally] as soon as there is an excess of ROS (hydrogen peroxide), SOD1 separates from Rac1 and the Nox2 complex stops producing ROS. In essence, SOD1 acts like a thermostat, which senses ROS and tells the Nox2 complex when to stop producing ROS.”

In contrast, when the SOD1 gene is mutated, it leads to production of an abnormal SOD1 protein, which is apparently not sufficiently sensitive to ROS levels and doesn’t disengage from Rac1 as they rise.
“With the thermostat broken, mutant SOD1 keeps the ROS-producing furnace burning in the cell,” Engelhardt said.

The new finding is the first to directly connect mutated SOD1 genes with the overproduction of toxic levels of ROS molecules and, therefore, with oxidative stress and inflammation.

In another set of experiments, the scientists treated mice bred with a specific SOD1 gene mutation with apocynin, a small chemical found in certain plants that interferes with production of ROS molecules by “unplugging the molecular furnace,” Engelhardt said.

Mice carrying this SOD1 mutation that received a high dose of apocynin (300 milligrams per kilogram per day) in their drinking water starting at 2 weeks of age lived significantly longer than untreated mice and also had significantly more motor neurons (cells that die in ALS) in their spinal cord samples.

Apocynin also delayed disease progression when given to 80-day-old mice with ALS, but the therapeutic effect was much reduced.

Engelhardt noted that they used pure apocynin and discourages patients from moving too quickly to self-medicate with herbal formulations that contain complex mixtures of apocynin with other chemicals and can have toxic side effects from long-term use.

“As frustrating as it might be, it is important that patients wait for the completion of important preclinical toxicology studies prior to anticipating clinical trials,” Engelhardt said.

He said he doesn’t know how relevant these findings are to sporadic (non-inherited) ALS, which is the type that affects 90 percent of patients. However, he said, his group is encouraged by studies from another research team showing that Nox2 levels are higher than average in spinal cord cells from people with the sporadic form of the disease.