The brain controls everything from how we process visual images to our ability to speak to our movements, like swinging a bat or knitting. Millions of Americans suffer with neurological conditions that affect movement, the most prevalent of which are essential tremor (ET), Alzheimer’s disease, epilepsy and Parkinson’s disease.
“These diseases disrupt patients’ lives in different ways, especially when they’re degenerative,” said Duff Rardin, MD, a physician with Mission Neurology.
Genetic testing is advised for some movement disorder patients and their family members. “Knowing you have a higher disease risk can paralyze you emotionally,” said Dr. Rardin. “If no preventive treatment exists for the disorder, testing has dubious value.”
Genetics is Personal
Pursuing genetic testing is a very personal decision, particularly for early onset Alzheimer’s disease (AD). “The knowledge that one is nearly certain to develop AD prior to age 65 may allow for life decisions and prioritization of life goals. On the other hand, learning this information can be overwhelming, isolating and trigger significant anxiety,” said Carolyn Wilson, a genetics counselor with Mission Health’s Fullerton Genetics Center. “It’s important to keep in mind genetic testing is much more involved than a simple blood test.”
Alzheimer’s Disease at an Early Age
Early onset Alzheimer’s disease is defined as onset prior to age 65. While this is the only group that clear genetic testing is available, only a small percent (about 1-5 percent) of Alzheimer’s cases fit into this category.
There are three genes that are well established to cause early onset Alzheimer’s disease: APP (amyloid precursor protein), PSEN1 (presenilin 1) and PSEN2 (presenilin 2). The vast majority of cases of Alzheimer’s occur after the age of 65, however, when a person carries a mutation in one of these genes, they a have close to a 100 percent chance of developing Alzheimer’s, usually before age 65 (but occasionally slightly later). People with a family history who are diagnosed with AD prior to age 65 have a 40-80 percent chance of having a mutation in one of these genes; people with early onset AD and no known family history only have a 6 percent chance of having a mutation in one of these genes. So for those with a personal or family history of early onset AD, genetic testing could be a good option; however, only 1 percent of all people with Alzheimer’s disease will have a mutation in one of these genes.
For people who do have a genetic mutation in one of the three genes, these gene mutations are inherited in a family in a dominant manner, meaning first-degree relatives will have a 50 percent chance to inherit this mutation. These people may be interested in genetic testing to know if they have inherited the family mutation or not, but there is currently no treatment to prevent the development of AD in people with a mutation. Genes do not skip generations, so if a person has an AD mutation and their child did not inherit it, the next generation (the grandchildren) are not at risk.
“From a genetic perspective, the younger the age of a person at Alzheimer’s disease diagnosis and the number of people in the family affected with the condition significantly impact the chance of finding an underlying genetic cause,” said Wilson. “As our understanding of the genetics and causes of AD [and other neurological disorders] improves, the role of genetics in this disease will change.”
“When warranted, genetic testing results alone aren’t enough. They must be interpreted with a counselor to be fully understood and useful,” said Dr. Rardin.
Common Neurological Diseases and Disorders
Essential tremor (ET): Essential tremor, also known as benign familial tremor, affects 7 million people. Hand tremors while performing fine motor skills tasks and head tremors are common. The child of a parent who has ET has a 50 percent chance of inheriting the gene that causes it, though it also occurs in patients who have no family history.
Alzheimer’s disease: The second most common neurodegenerative disorder, with 5.8 million Americans current sufferers. Eighty-one percent are 75 or over. Five to 10 percent of cases are genetic and those are usually early onset (under age 65), which make up only about 200,000 cases; an affected parent has a fifty-fifty chance of passing it on to their child. One in 3 people who live to 85 will develop Alzheimer’s disease. Two-thirds of individuals suffering from Alzheimer’s are women. African Americans and Hispanics have a higher risk of developing Alzheimer’s than Caucasians.
Epilepsy: About 3 million Americans are living with epilepsy, a brain disorder that causes many types of seizures. Childhood onset epilepsy has a genetic component and is far less common than adult onset, which usually has no genetic link.
Parkinson’s disease: Approximately 1 million Americans have Parkinson’s disease, which is degenerative. Prominent symptoms are hand tremors while at rest, slow movement, balance issues and depression. Some genetic mutations have been identified for Parkinson’s disease, but the majority of cases are not genetically linked.
Multiple sclerosis (MS): About 623,000 people per year are diagnosed with MS. Though there is a genetic component that exists if a parent has MS, their child only has about a 2 percent risk of getting the disease. If a sibling has the disease, the risk is raised to 5 percent; if an identical twin has the disease, the other twin’s risk increases to 25 percent.
Huntington’s disease: This is the only movement disorder that is 100 percent genetic. About 30,000 people are diagnosed with Huntington’s each year. Genetic testing, if performed, should not be done until a person is 18 years of age.
Amyotrophic lateral sclerosis (ALS or Lou Gehrig’s disease): About 16,000 cases are diagnosed each year. If another immediate family member has it, genetic testing may be warranted.
The Fullerton Genetics Center offers Alzheimer’s disease genetic testing for people with a personal or family history of early onset Alzheimer’s disease, as well as consultations to discuss the benefits and limitations of genetic testing.
Duff Rardin, MD, is a physician with Mission Neurology.
Carolyn Wilson, MS, CGC, is a genetic counselor at Mission Fullerton Genetics Center.
