Spastic paraplegia type 31 is one of a group of genetic disorders known as hereditary spastic paraplegias. These disorders are characterized by progressive muscle stiffness (spasticity) and the development of paralysis of the lower limbs (paraplegia) caused by degeneration of nerve cells that trigger muscle movement (motor neurons). Hereditary spastic paraplegias are divided into two types: pure and complicated. The pure types involve only the lower limbs, while the complicated types also involve the upper limbs and other areas of the body, including the brain. Spastic paraplegia type 31 is usually a pure hereditary spastic paraplegia, although a few complicated cases have been reported.

The first signs and symptoms of spastic paraplegia type 31 usually appear before age 20 or after age 30. An early feature is difficulty walking due to spasticity and weakness, which typically affect both legs equally. People with spastic paraplegia type 31 can also experience progressive muscle wasting (amyotrophy) in the lower limbs, exaggerated reflexes (hyperreflexia), a decreased ability to feel vibrations, reduced bladder control, and high-arched feet (pes cavus). As the condition progresses, some individuals require walking support.

Spastic paraplegia type 31 is caused by mutations in the REEP1 gene. This gene provides instructions for making a protein called receptor expression-enhancing protein 1 (REEP1), which is found in neurons in the brain and spinal cord. The REEP1 protein is located within cell compartments called mitochondria, which are the energy-producing centers in cells, and the endoplasmic reticulum, which helps with protein processing and transport. The REEP1 protein plays a role in regulating the size of the endoplasmic reticulum and determining how many proteins it can process. The function of the REEP1 protein in the mitochondria is unknown.

REEP1 gene mutations that cause spastic paraplegia type 31 result in a short, nonfunctional protein that is usually broken down quickly. As a result, there is a reduction in functional REEP1 protein. It is unclear how REEP1 gene mutations lead to the signs and symptoms of spastic paraplegia type 31. Researchers have shown that mitochondria in cells of affected individuals are less able to produce energy, which may contribute to the death of neurons and lead to the progressive movement problems of spastic paraplegia type 31; however, the exact mechanism that causes this condition is unknown.

Normal Function

The REEP1 gene provides instructions for making a protein called receptor expression-enhancing protein 1 (REEP1), which is found in nerve cells (neurons) in the brain and spinal cord. The REEP1 protein is located within cell compartments called mitochondria, which are the energy-producing centers in cells, and the endoplasmic reticulum, which helps with protein processing and transport.

The REEP1 protein plays a role in forming the network of tubules that make up the structure of the endoplasmic reticulum, regulating its size and determining how many proteins it can process. As part of its role in the endoplasmic reticulum, the REEP1 protein enhances the activity of certain other proteins called G protein-coupled receptors. These receptor proteins are eventually embedded within the outer membrane of cells, where they relay chemical signals from outside the cell to the interior of the cell.

The function of the REEP1 protein in the mitochondria is unknown.

Health Conditions Related to Genetic Changes

Distal hereditary motor neuropathy, type V

MedlinePlus Genetics provides information about Distal hereditary motor neuropathy, type V

Spastic paraplegia type 31

At least 44 mutations in the REEP1 gene have been found to cause spastic paraplegia type 31. This condition is characterized by muscle stiffness (spasticity) and paralysis of the lower limbs (paraplegia) caused by degeneration of nerve cells (neurons) that trigger muscle movement. Most of the REEP1 gene mutations that cause this condition insert or remove small pieces of DNA from the gene or alter the way the gene's instructions are used to make the protein. These mutations often result in a short, nonfunctional protein that is quickly broken down. As a result, there is a reduction in functional REEP1 protein.

It is unclear how REEP1 gene mutations lead to the signs and symptoms of spastic paraplegia type 31. Researchers have shown that mitochondria in cells of affected individuals are less able to produce energy, which may contribute to the death of neurons and lead to the progressive movement problems of spastic paraplegia type 31; however, the exact mechanism that causes this condition is unknown.

Other Names for This Gene

• C2orf23
• FLJ13110
• receptor expression-enhancing protein 1
• REEP1_HUMAN

Genomic Location

This condition is inherited in an autosomal dominant pattern, which means one copy of the altered gene in each cell is sufficient to cause the disorder.

Spastic paraplegia type 31 is one of a subgroup of hereditary spastic paraplegias known as autosomal dominant hereditary spastic paraplegia, which has an estimated prevalence of one to 12 per 100,000 individuals. Spastic paraplegia type 31 accounts for 3 to 9 percent of all autosomal dominant hereditary spastic paraplegia cases.