SciTech

Health Talk: Spinal Muscular Atrophy

Spinal Muscular Atrophy (SMA) is the name given to a group of inherited genetic diseases characterized by the loss of motor neurons, which are the nerve cells that transmit impulses from the brain or spinal cord to muscle tissue.
This inability of the brain and muscle tissue to communicate with each other results in severe muscle weakness and atrophy in the muscles closest to the trunk of the body, which are the muscles necessary for crawling, walking, sitting up, and head control, though other muscles can be affected as well.

SMA is an autosomal recessive genetic disease, which means that for a child to have SMA, both parents must be carriers of the abnormal SMA gene, and both must pass this gene on to their child. This makes the chance of a child inheriting the disease from two patients who are carriers one in four, or 25 percent. The gene that is affected is survival motor neuron 1, or SMN1. This gene produces the survival motor neuron protein, and a deficiency of this protein can have a severe effect on motor neurons. Without the protein, the motor neurons can atrophy, shrink, and eventually die. One in every 40 people, or 7.5 million Americans, is a carrier of the gene that causes SMA, which leads to one in every 6000 children being born with SMA.

SMA is broken up into different types, or categories, depending on the age of onset and the disease’s severity. Type I SMA, called Werdnig-Hoffmann Disease, is the most severe of the different types, and the diagnosis of children with this type is usually made before 6 months of age.

Patients with this form are generally unable to sit unsupported, have trouble eating and swallowing, and have underdeveloped lungs and a weak cough. Type II is usually diagnosed in patients less than 2 years of age and is less severe than Type I. Patients can often sit unsupported, and can get into a standing position with some sort of assistive device. Scoliosis, or the curving of the spine, is very common in Type II patients, as are weak lungs like patients with Type I. Type III SMA is also known as Kugelberg-Welander or Juvenile Spinal Muscular Atrophy and has a much greater range in time of onset.
Patients with Type III can stand on their own and walk, though they may have difficulty walking at some point in their life, usually associated with a growth spurt or illness. These patients often have difficulty walking, and will fall more than normal and have a more difficult time righting themselves.

The last form of SMA, Type IV, is the adult onset form of the disease, and is much rarer than the other types, and has a much slower progression.

In all forms of SMA, patients generally lose function over time, though many patients see a plateau in performance for long periods of time.

Research done in the last few years concerning SMA has led to a great increase in knowledge and understanding about how the disease works. Not only was the SMN1 gene identified as the cause of SMA, and its production of the SMN protein found to affect the motor neurons, but another gene was also found: SMN2.

This gene is believed to be a partially functioning backup gene that makes about 10 percent as much of the SMN protein as a healthy SMN1 gene would produce. The current research is focusing on drugs and treatments that could help enhance SMN2’s production of the SMN protein in order to bring them up to healthy SMN1 levels.