Osteoporosis-pseudoglioma syndrome is a rare condition characterized by severe thinning of the bones (osteoporosis) and eye abnormalities that lead to vision loss. In people with this condition, osteoporosis is usually recognized in early childhood. It is caused by a shortage of minerals, such as calcium, in bones (decreased bone mineral density), which makes the bones brittle and prone to fracture. Affected individuals often have multiple bone fractures, including in the bones that form the spine (vertebrae). Multiple fractures can cause collapse of the affected vertebrae (compressed vertebrae), abnormal side-to-side curvature of the spine (scoliosis), short stature, and limb deformities. Decreased bone mineral density can also cause softening or thinning of the skull (craniotabes).

Most affected individuals have impaired vision at birth or by early infancy and are blind by young adulthood. Vision problems are usually caused by one of several eye conditions, grouped together as pseudoglioma, that affect the light-sensitive tissue at the back of the eye (the retina), although other eye conditions have been identified in affected individuals. Pseudogliomas are so named because, on examination, the conditions resemble an eye tumor known as a retinal glioma.

Rarely, people with osteoporosis-pseudoglioma syndrome have additional signs or symptoms such as mild intellectual disability, weak muscle tone (hypotonia), abnormally flexible joints, or seizures.

Osteoporosis-pseudoglioma syndrome is caused by mutations in the LRP5 gene. This gene provides instructions for making a protein that participates in a chemical signaling pathway that affects the way cells and tissues develop. In particular, the LRP5 protein helps regulate bone mineral density and plays a critical role in development of the retina.

LRP5 gene mutations that cause osteoporosis-pseudoglioma syndrome prevent cells from making any LRP5 protein or lead to a protein that cannot function. Loss of this protein's function disrupts the chemical signaling pathways that are needed for the formation of bone and for normal retinal development, leading to the bone and eye abnormalities characteristic of osteoporosis-pseudoglioma syndrome.

Normal Function

The LRP5 gene provides instructions for making a protein that is embedded in the outer membrane of many types of cells. It is known as a co-receptor because it works with another receptor protein, frizzled-4 (produced from the FZD4 gene), to transmit chemical signals from outside the cell to the cell's nucleus. Frizzled-4 and the LRP5 protein participate in the Wnt signaling pathway, a series of steps that affect the way cells and tissues develop. Wnt signaling is important for cell division (proliferation), attachment of cells to one another (adhesion), cell movement (migration), and many other cellular activities.

The LRP5 protein plays an important role in the development and maintenance of several tissues. During early development, it helps guide the specialization of cells in the retina, which is the light-sensitive tissue at the back of the eye. The LRP5 protein is also involved in establishing a blood supply to the retina and the inner ear. Additionally, this protein helps regulate bone mineral density, which is a measure of the amount of calcium and other minerals in bones. The minerals give the bones strength, making them less likely to break.

Health Conditions Related to Genetic Changes

Familial exudative vitreoretinopathy

More than 15 mutations in the LRP5 gene have been identified in people with the eye disease familial exudative vitreoretinopathy. Some of these mutations change single protein building blocks (amino acids) in the LRP5 protein, while others insert or delete genetic material in the gene. Most of these mutations reduce the amount of functional LRP5 protein that is produced within cells.

A reduction in the amount of LRP5 protein disrupts chemical signaling in the developing eye, which interferes with the formation of blood vessels at the edges of the retina. The resulting abnormal blood supply to this tissue can lead to retinal damage and vision loss. Because the LRP5 protein plays a role in bone formation, LRP5 gene mutations also cause reduced bone mineral density in some people with familial exudative vitreoretinopathy.

Juvenile primary osteoporosis

At least five LRP5 gene mutations have been found in people with juvenile primary osteoporosis. Individuals with this condition have low bone mineral density and thinning of the bones (osteoporosis) beginning in childhood. Osteoporosis causes the bones to be brittle and to break easily, which leads to multiple bone fractures. The LRP5 gene mutations that cause this condition result in an LRP5 protein that is unable to transmit chemical signals along the Wnt signaling pathway. The resulting reduction in signaling disrupts regulation of bone mineral density, leading to osteoporosis at a young age.

Osteoporosis-pseudoglioma syndrome

More than 40 LRP5 gene mutations that cause osteoporosis-pseudoglioma syndrome have been identified. Beginning in childhood, people with this condition have extremely low bone mineral density and osteoporosis, which leads to multiple bone fractures. Affected individuals also have eye abnormalities that cause vision impairment from birth or early infancy. Many LRP5 gene mutations that cause osteoporosis-pseudoglioma syndrome prevent cells from making any LRP5 protein. Other mutations change single amino acids in the LRP5 protein. These abnormal proteins cannot insert into the outer membrane of the cell, which makes them unable to perform their function. Loss of LRP5 protein function disrupts the chemical signaling pathways that are needed for the formation of bone and for normal retinal development, leading to the bone and eye abnormalities characteristic of osteoporosis-pseudoglioma syndrome. It is unclear why some LRP5 gene mutations affect eye development and others do not.

Other disorders

Studies suggest that changes in the LRP5 gene may influence the risk of developing osteoporosis in adulthood. Other genetic and environmental factors likely contribute to this common disorder.

Other LRP5 gene mutations cause disorders associated with an increase in bone mineral density. These include autosomal dominant osteopetrosis type 1 and autosomal dominant osteosclerosis. In some cases, these conditions can cause abnormal bone growth and related skeletal abnormalities. Rarely, affected individuals have hearing loss or circulation problems in the brain. Other people with increased bone mineral density do not have any associated health problems. The mutations responsible for increased bone mineral density syndromes overactivate the LRP5 protein, which increases Wnt signaling within cells and enhances bone formation.

Other Names for This Gene

• BMND1
• EVR1
• EVR4
• HBM
• low density lipoprotein receptor-related protein 5
• low density lipoprotein receptor-related protein 7
• LR3
• LRP5_HUMAN
• LRP7
• OPS
• OPTA1
• VBCH2

Genomic Location

Osteoporosis-pseudoglioma syndrome is inherited in an autosomal recessive pattern, which means both copies of the LRP5 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. However, some carriers may have decreased bone mineral density.

Osteoporosis-pseudoglioma syndrome is a rare disorder that occurs in approximately 1 in 2 million people.