In most cases of Lou Gehrig's disease, or amyotrophic lateral sclerosis (ALS), it's not known what caused the condition, but a new study finds genes may play a larger role than previously thought.

Only about 5 to 10 percent of people with ALS have family members with the disease, meaning the cases have a known genetic component. The other 90 to 95 percent of ALS cases are "sporadic"; that is, they do not appear to run in families, and the cause of the disease is often a mystery.

But the new study, which analyzed DNA from nearly 400 people with ALS, found that more than a quarter of sporadic cases were linked to new or rare genetic mutations, which could potentially cause the disease.

"These findings shed new light on the genetic origins of ALS, especially in patients who had no prior family history of the disease," said study researcher Dr. Robert Baloh, director of the ALS Program at Cedars-Sinai Medical Center in Los Angeles. [Top 10 Mysterious Diseases]

ALS is a progressive neurological diseases that eventually leads to loss of function of the muscles used in voluntary movements, like walking, speaking and eating. Life expectancy for people with the condition is usually 3 to 5 years. The findings suggest that more than a third of all ALS cases (both inherited and sporadic) could be genetic in origin, the researchers said.

The study does not prove that these new or rare genetic mutations cause ALS, but the mutations are likely suspects, Baloh said.

The study also found that the disease appeared 10 years earlier in people with mutations in two or more of the genes linked with ALS compared to people with defects in only one gene.

More research is needed to identify other genes that could influence the risk of ALS, said study researcher Dr. Matthew B. Harms, an assistant professor of neurology at Washington University in St. Louis.

The study was published online Nov. 27 in the journal Annals of Neurology.

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