Spinal muscular atrophy (SMA) is a rare and heritable disease characterized by the loss of motor neurons, or nerve cells that control voluntary muscle movement. Without these nerve cells, muscles weaken and atrophy.
SMA-LED is caused by mutations in the DYNC1H1 (dynein cytoplasmic 1 heavy chain 1) or the BICD2 (BICD cargo adaptor 2) gene. Both genes provide instructions for making proteins that are part of a protein complex called the dynein-dynactin complex, which is involved in moving materials within cells.
The dynein-dynactin complex binds to various materials within cells, moving them along a system of small tubes called microtubules. The dynein-dynactin complex has very important functions in protein transport, positioning of cell compartments, and the mobility of structures within cells (among other cell processes). In nerve cells, dynein helps neighboring cells to communicate by moving synaptic vesicles that contain chemical messengers.
In SMA-LED, mutations in either DYNC1H1 or BICD2 cause changes in the amino acid sequence (the building blocks of proteins). As a result, the movement of proteins and cellular materials is impaired, resulting in reduced chemical signaling among neurons that control muscle movement. Poor synaptic vesicle transport also affect nerve cell growth, and is thought to contribute to muscle weakness in patients.
SMA-LED is inherited in an autosomal dominant manner, meaning that a single copy of the disease-causing mutation is sufficient to result in disease development. The children of people who are affected by SMA-LED have a 1-in-2 risk of inheriting the gene and developing the disease themselves.
SMA-LED is characterized by weakness in the leg muscles, with symptoms most pronounced in the quadriceps or large muscles of the thighs. Symptoms appear in infancy or childhood, but progress slowly. Typically, they include waddling or an unsteady walk or gait, and difficulties in rising from a seated position and in climbing stairs.
Research is not clear as to why this condition only affects a person’s lower extremities.
No cure currently exists for SMA-LED, but treatments can help to ease its symptoms and improve patients’ quality of life.
Most patients with SMA-LED have a normal lifespan. Some may need support, such as braces or a wheelchair, for mobility later in life.
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