Spinal muscular atrophy (SMA) is a heritable disease that affects motor neurons, the nerve cells that control muscle movement. The loss of these nerve cells causes muscle weakness and atrophy.
This gene contains the instructions for the production of a protein that exists in cells throughout the body but whose function is not fully understood. The protein is located in the membrane that surrounds a cellular structure called the endoplasmic reticulum.
Among its various duties, the endoplasmic reticulum folds new proteins and prepares them for transport within a cell or to the cell surface. To do its work correctly, the endoplasmic reticulum relies on a system that detects a buildup of unfolded or misfolded proteins. This process of preventing or correcting a collection of abnormal proteins is called an unfolded protein response. Scientists believe it is here, in this response process, that the protein encoded by the VAPB gene may play a major role.
A mutated or abnormal VAPB protein cannot activate the unfolded protein response and, as a result, abnormal proteins collect and aggregate in cells, resulting in cell death.
Nerve cells that control muscle movement appear to be particularly vulnerable to these aggregates. The progressive loss of these nerve cells leads to FSMA and disease symptoms.
Inheritance and FSMA
FSMA is a disease inherited in an autosomal dominant manner, meaning that a single copy of the mutated VAPB gene is sufficient to cause the disease.
Patients with FSMA have a one-in-two, or 50 percent, risk of passing the mutated gene to their children. Should both parents have a copy of the disease-causing gene, their children would have a three-in-four risk of inheriting one or more copies of the disease-causing gene and developing the disease.
FSMA is usually first evident when a person is in their late 20s or early 30s. Symptoms are often mild to moderate, and include muscle weakness, tremors, and twitching.
FSMA affects the proximal muscles, or those closest to the body’s middle, and is characterized by muscle cramping of the limbs and abdomen, weakness in the leg muscles, involuntary muscle contractions, and tremors.
No cure now exists for any type of SMA. However, available treatments may help to alleviate disease symptoms and improve patients’ quality of life.
FSMA patients generally have a normal lifespan, although they made need support or assistance with such daily activities as walking as they age and certain muscles weaken.
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