Spinal muscular atrophy - Causes of spinal muscular atrophy

Spinal muscular atrophy (SMA) is caused by faulty genes, usually passed on to a child by their parents.

The most common types of spinal musclar atrophy – types I, II and III – are caused by a problem with a gene called SMN1.

The SMN1 gene is responsible for the production of a protein needed by the nerves (motor neurones) that connect the brain and spinal cord to the muscles. If there is a problem with the gene, this protein is only produced in very low levels, causing the motor neurone cells in the spinal cord to deteriorate, meaning that the connection between the nerves and the muscles disappears and the muscles cannot work effectively.

The muscles will eventually start to shrink due to a lack of use. This is known as muscular atrophy.

How it is inherited

People who have the defective gene that causes SMA but who don't have SMA themselves are known as carriers. Both parents must be carriers of the defective gene to pass it on to their child.

If two people who are SMA carriers have a child, there is a:

50% chance the child will be a carrier of SMA
25% chance the child will develop SMA
25% chance the child will be completely unaffected by SMA

Approximately one in every 40-60 people is a carrier of a defective SMN1 gene.

Other types of SMA

The causes of some rarer types of SMA can be different to those of types I, II and III.

Spinal muscular atrophy with respiratory distress

A type of SMA called spinal muscular atrophy with respiratory distress (SMARD) is inherited in the same way as types I, II and III, but it's not related to a problem with the SMN1 gene.

Instead, a problem with a gene called IGHMBP2 is responsible for the condition.

Adult-onset SMA

SMA that develops in adults (type IV) is linked to the SMN1 gene in some cases, although not all cases are thought to be inherited. In cases where adult SMA is passed on, the way it's inherited can be different from the types I, II and III. For example, it's sometimes possible for someone to develop adult-onset SMA if only one parent has the defective gene.

In Kennedy's syndrome, the condition is passed on by the mother and only affects male children, although female children can become carriers.