Friedreich’s ataxia (FA) is an inherited condition affecting the nervous system and causing movement problems. People with FA have diminished muscle coordination (ataxia) that worsens over time. Other symptoms include a continued loss of strength and sensation in the arms and legs, muscle stiffness (spasticity), and impaired speech. Some patients develop diabetes, vision and hearing problems, and scoliosis (curvature of the spine).
FA is caused by mutations in the frataxin (FXN) gene and is inherited in an autosomal recessive manner.
What is autosomal recessive inheritance?
FA is inherited in an autosomal recessive manner, which means that to be affected by the disease, a person must have a mutation in both copies of the gene responsible for the disease. One copy of a gene is inherited from the mother and one copy is inherited from the father, so both parents of a child with FA must be carriers. However, because they have another healthy copy of the gene, they are not affected by the condition themselves.
Two people who are both carriers of the FA-causing mutation have a 25 percent chance of having a child with FA, a 50 percent chance of having children who are carriers, and a 25 percent chance of having children who are neither affected by the disease nor carriers.
What mutation is in the FXN gene?
One region of the FXN gene contains a so-called GAA trinucleotide repeat. This region consists of a series of three DNA building blocks, one guanine and two adenines repeated several times. In healthy people, this segment is repeated five to 33 times. However, in people with FA, the number of repeats ranges between 66 to more than 1,000.
The number of repeats appears to be related to the age at which the symptoms of the disease appear, their severity, and the rate of progression of the disease.
People who have fewer than 300 GAA repeats tend to have later disease onset, usually after age 25.
What does the FXN gene do?
The FXN gene provides information for the synthesis of a protein called frataxin. The role of frataxin is not fully understood, but it is known that it is important for the normal functioning of the mitochondria (the energy-producing centers of the cells), helping assemble clusters of iron and sulfur that are important for the function of many proteins, including those needed for energy production. The highest levels of frataxin are found in the heart, spinal cord, liver, pancreas, and muscles.
The GAA trinucleotide expansion causes no functional frataxin protein to be made leading to the symptoms of the disease.
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