Risk Factors for Amyotrophic Lateral Sclerosis
ALS risk factors include an inherited genetic defect, which accounts for 5–10% of cases of familial ALS (FALS) in the United States. FALS is linked to a genetic defect on chromosome 21. This gene codes for an enzyme called superoxide dismutase (SOD), an antioxidant that protects motor neurons from free radical damage (i.e., molecules introduced to the body, or produced by body processes that interact and cause cellular damage). More than 60 different mutations that cause SOD to lose its antioxidant properties have been found. However, only 40% of familial ALS cases are linked to SOD mutations, so there may be other unknown genetic defects involved.
In the United States, 90–95% of ALS cases are sporadic. Sporadic ALS appears to be increasing worldwide. The causes are not clear, yet some evidence suggests that the immune system may be involved. Excessive levels of glutamate can overstimulate motor neurons and cause them to die. Glutamate is one of the most important neurotransmitters for healthy brain function. Neurotransmitters are chemicals that transmit signals from one nerve to another.
In Guamanian ALS, a dietary neurotoxin is the risk factor. The suspected neurotoxin is an amino acid (BMAA) found in the seed of the cycad Cyas cirinalis, a tropical plant found in Guam, which was used to make flour and was a major dietary component during the 1950s and the early 1960s, when this type of ALS had an exceptionally high incidence.
Causes of ALS
The cause of ALS is not completely understood. Researchers and physicians suspect viruses, neurotoxins (especially in Guamanian ALS), heavy metals, DNA defects (especially in familial ALS), immune system abnormalities, and enzyme abnormalities.