Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disease, and inheritance is thought to play an essential role in its less common form, called familial ALS (FALS).

A genetic test is usually only performed on individuals with a known family history of ALS.

Familial versus sporadic ALS 

The two most common forms of ALS are sporadic ALS, or SALS, and familial ALS or FALS.

FALS refers to ALS cases where the individual has one or more family members with ALS, and it is known that he or she inherited the condition from a parent.

SALS refers to ALS cases without a family history — those that appear at random with no apparent risk factors other than possibly age (55 or older) and race/ethnicity (Caucasian or non-Hispanic). Most cases of ALS — about 90 percent — are currently thought to be sporadic.

The causes of SALS are not entirely understood, but with advances in genetic testing, it is emerging that many SALS cases may be inherited and classified as FALS. Relatives of SALS patients are seen to have an increased risk of developing ALS, and mutations linked to FALS are being found in people with sporadic ALS. Such findings suggest that genetic factors and heredity  also have an impact on the development of SALS.

Inheritance of ALS

Most ALS-causing mutations are inherited in an autosomal dominant manner, meaning that a mutation in one of the two gene copies inherited — one copy from each parent — is sufficient for the disease to manifest. People with autosomal dominant mutations have a 50 percent chance to pass them to a child.

If a condition is inherited in an autosomal recessive manner, both copies of the gene need to be mutated for the disease to manifest. A person with an autosomal recessive mutation in one gene also has a 50 percent chance of passing it to the next generation. But a child inheriting the mutated gene will only develop the disease if the second copy of the same gene is also mutated.

ALS can be inherited in an autosomal recessive manner, but this is very rare.

ALS can also be inherited in an X-linked dominant manner. In this case, the mutated gene is located on the X-chromosome. Because women have two X-chromosomes, they have a 50 percent chance of passing a mutated gene to their children. Men with a mutated gene on the X-chromosome can only pass this mutation to a daughter.

Genetic tests for ALS

A genetic test for ALS is done using a blood or saliva sample. Because the genetics of ALS are complex, and new disease-causing mutations are continually being discovered, it is difficult to clearly diagnose ALS with a genetic test.

There are different kinds of genetic tests. The most common used detects alterations in the C9orf72 gene, which is the most frequently mutated gene in ALS. It is mutated in 40 percent of FALS, and 7 percent of SALS cases. There are also panels available that allow the testing of up to 36 genes at a time.

People with clinical ALS symptoms may test negative if their disease-causing mutation that has not yet been identified and linked to ALS. At the same time, an asymptomatic individual with a family history of ALS may receive positive results on a genetic test, but still has a chance that he or she may never develop disease symptoms.

Asymptomatic individuals with ALS in a family member may be advised, or decide, to undergo genetic testing. Family planning is a frequent reason for this.

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