Bradyopsia is a rare condition that affects vision. The term "bradyopsia" is from the Greek words for slow vision. In affected individuals, the eyes adapt more slowly than usual to changing light conditions. For example, people with this condition are blinded for several seconds when going from a dark environment into a bright one, such as when walking out of a darkened movie theater into daylight. Their eyes also have trouble adapting from bright light to dark conditions, such as when driving into a dark tunnel on a sunny day.

Some people with bradyopsia also have difficulty seeing some moving objects, particularly small objects moving against a bright background. As a result, they often have trouble watching or participating in sports with a ball, such as soccer or tennis. People with bradyopsia can have reduced sharpness (acuity) of vision, although acuity may depend on the conditions under which vision is tested. Visual acuity may appear to be severely affected if it is tested under bright lights, but it can be near normal if tested in a dim environment. The ability to see colors and distinguish between them is normal.

The vision problems associated with bradyopsia become apparent in early childhood. They are usually stable, which means they do not worsen over time.

Bradyopsia can be caused by mutations in the RGS9 gene or in the RGS9BP gene (which is also known as R9AP). These genes provide instructions for making proteins that are necessary for normal vision. The proteins are found in light-detecting cells in the eye called photoreceptors. When light enters the eye, it stimulates specialized pigments in these cells. This stimulation triggers a series of chemical reactions that produce an electrical signal, which is interpreted by the brain as vision. Once photoreceptors have been stimulated by light, they must return to their resting state before they can be stimulated again. The RGS9 and RGS9BP proteins play an essential role in returning photoreceptors to their resting state quickly after light exposure.

Mutations in either the RGS9 or RGS9BP gene prevent photoreceptors from recovering quickly after responding to light. Normally they return to their resting state in a fraction of a second, but in people with mutations in one of these genes, it can take ten seconds or longer. During that time, the photoreceptors cannot respond to light. This delay causes temporary blindness in response to changing light conditions and interferes with seeing small objects when they are in motion.

In some people with bradyopsia, no mutations in the RGS9 or RGS9BP gene have been found. The cause of the condition in these individuals is unknown.

Normal Function

The RGS9 gene provides instructions for making two versions (isoforms) of the RGS9 protein, known as RGS9-1 and RGS9-2. They are found in different parts of the nervous system and have very different functions.

RGS9-1 is produced in the retina, which is the specialized tissue at the back of the eye that detects light and color. Within the retina, RGS9-1 is associated with light-detecting cells called photoreceptors. When light enters the eye, it stimulates specialized pigments in these cells. This stimulation triggers a series of chemical reactions that produce an electrical signal, which is interpreted by the brain as vision. (This process is known as phototransduction.) Once photoreceptors have been stimulated by light, they must return to their resting state before they can be stimulated again. RGS9-1 is involved in a chemical reaction that helps return photoreceptors to their resting state quickly after light exposure.

RGS9-2 is found primarily in an area deep within the brain called the striatum. Although its exact role is unknown, RGS9-2 appears to be part of signaling pathways involving a chemical messenger (neurotransmitter) called dopamine. These pathways are important for planning and coordinating movement. Studies suggest that RGS9-2 also plays a role in the brain's response to opioid drugs, such as morphine and cocaine.

Health Conditions Related to Genetic Changes

Bradyopsia

At least two mutations in the RGS9 gene have been found to cause bradyopsia, a rare condition that affects vision. In people with bradyopsia, the eyes adapt more slowly than usual to changing light conditions (for example, walking out of a darkened movie theater into daylight or driving into a dark tunnel on a sunny day). Some affected individuals also have difficulty seeing small moving objects, such as a tennis ball.

The RGS9 gene mutations that cause bradyopsia greatly reduce or eliminate the function of RGS9-1 in photoreceptors. (These mutations do not appear to affect the function of RGS9-2 in the brain.) A loss of RGS9-1 function prevents photoreceptors from recovering quickly after responding to light. Normally they return to their resting state in a fraction of a second, but in people with RGS9 gene mutations, it can take ten seconds or longer. During that time, the photoreceptors cannot respond to light. This delay causes temporary blindness in response to changing light conditions and may interfere with seeing small objects when they are in motion.

Other Names for This Gene

• MGC111763
• MGC26458
• regulator of G-protein signaling 9
• regulator of G-protein signalling 9
• RGS9L

Genomic Location

Normal Function

The RGS9BP gene (which is also known as R9AP) provides instructions for making a protein called RGS9 binding protein. This protein is found in the retina, which is the specialized tissue at the back of the eye that detects light and color. Within the retina, the protein is associated with light-detecting cells called photoreceptors. As its name suggests, RGS9 binding protein interacts with a protein called RGS9 (which is produced from the RGS9 gene). It anchors the RGS9 protein to photoreceptors and stimulates RGS9's activity.

When light enters the eye, it stimulates specialized pigments in photoreceptor cells. This stimulation triggers a series of chemical reactions that produce an electrical signal, which is interpreted by the brain as vision. (This process is known as phototransduction.) Once photoreceptors have been stimulated by light, they must return to their resting state before they can be stimulated again. RGS9 and RGS9 binding protein are involved in a chemical reaction that helps return photoreceptors to their resting state quickly after light exposure.

Health Conditions Related to Genetic Changes

Bradyopsia

At least six mutations in the RGS9BP gene have been found to cause bradyopsia, a rare condition that affects vision. In people with bradyopsia, the eyes adapt more slowly than usual to changing light conditions (for example, walking out of a darkened movie theater into daylight or driving into a dark tunnel on a sunny day). Some affected individuals also have difficulty seeing small moving objects, such as a tennis ball.

The RGS9BP gene mutations that cause bradyopsia prevent RGS9 binding protein from anchoring the RGS9 protein to photoreceptor cells. The resulting loss of RGS9 protein function prevents photoreceptors from recovering quickly after responding to light. Normally they return to their resting state in a fraction of a second, but in people with RGS9BP gene mutations, it can take ten seconds or longer. During that time, the photoreceptors cannot respond to light. This delay causes temporary blindness in response to changing light conditions and may interfere with seeing small objects when they are in motion.

Other Names for This Gene

• FLJ45744
• R9AP
• regulator of G-protein signaling 9 binding protein
• regulator of G-protein signaling 9-binding protein
• RGS9 anchor protein
• RGS9-anchoring protein

Genomic Location

This condition is inherited in an autosomal recessive pattern, which means both copies of the gene in each cell have mutations. The parents of an individual with an autosomal recessive condition each carry one copy of the mutated gene, but they typically do not show signs and symptoms of the condition.

Bradyopsia appears to be rare. Only a few affected individuals worldwide have been described in the medical literature.