ALS leaves people unable to walk, talk or feed themselves but does not usually affect their intellect and other senses. Doctors diagnose about 120,000 new cases each year, according to the International Alliance of ALS.
Shaw and colleagues isolated a mutation in a gene called TARDBP in people with a rare, inherited form of ALS. They found that people with this variation produced a mutant and toxic protein called TDP-43.
Previous research had suggested the protein might have existed as sort of cellular junk generated as a harmless by-product of the disease. But by injecting these mutated proteins into the spines of chicks in eggs, Shaw's team showed they actually killed motor neurons.
While only about 1 percent of people have this form of ALS, the findings have wider implications because most people with the disease have these proteins accumulating in the wrong place within the cell, Shaw said.
"It also means we develop new and better disease models, which will bring us close to developing more effective therapies," he said.
In 1993 a group of U.S. researches identified a gene called SOD1 that caused a form of ALS affecting about 5 percent of people with the disease -- the only people with the condition who do not accumulate TDP-43 proteins, Shaw said.
That finding triggered a flood of new research into the disease, and the discovery of a second gene could draw even more people into the field, said Brian Dickie, director of Research Development at Britain's Motor Neuron Disease Association.
"The discovery of a new cause of the disease is of international importance, allowing researchers around the world to rapidly generate more pieces of the complex puzzle that is motor neuron disease," he said.
(Reporting by Michael Kahn, Editing by Maggie Fox and Keith Weir)