Finkel type spinal muscular atrophy (SMA) is an adult-onset disease that was first described by Dr. Richard Finkel in 1962. It usually develops in the late-20s to early-30s and is associated with mutations in the VAPB gene.
Finkel type SMA has an inherited autosomal dominant gene pattern, meaning the patient has only one copy of the altered VAPB gene in each cell that causes the disorder.
The VAPB gene and Finkel type SMA
The VAPB gene contains instructions for the production of a protein that exists in cells throughout the body. There is a limited amount of knowledge about its function.
Research shows that this protein is associated with the membrane that surrounds the endoplasmic reticulum. Among its various functions, the endoplasmic reticulum folds new proteins and prepares them for transport within the cell or to the cell surface. To function correctly, the endoplasmic reticulum relies on a system that detects a buildup of unfolded or misfolded proteins.
This process of prevention or correction of a buildup of abnormal proteins is called the unfolded protein response. It is in this unfolded protein response that the VAPB gene might have a major role.
People with Finkel type SMA have one VAPB gene mutation. The mutation replaces a proline (amino acid) with a serine at position 56 in the VAPB protein. This mutation causes amyothrophic lateral sclerosis (ALS) in some people, and SMA in others, but it’s not yet known how the same mutation causes different conditions.
The abnormal VAPB protein cannot activate the unfolded protein response and as a result, abnormal proteins form aggregates in cells, resulting in cell death. Nerve cells that control the muscle movement appear to be vulnerable to death because of these aggregates. The progressive loss of these nerve cells leads to the signs and symptoms of SMA.
Signs and symptoms of Finkel type SMA
Finkel type SMA affects the proximal muscles and is characterized by muscle cramping of the limbs and abdomen, weakness in the leg muscles, involuntary muscle contractions, tremors, and a protusion of the abdomen, thought to be related to the muscle weakness. Some individuals also experience difficulty swallowing and problems with bladder and bowel function.
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