Bardet-Biedl syndrome

Description

Bardet-Biedl syndrome is a disorder that affects many parts of the body. The specific features and the course of this condition can vary greatly among affected individuals.

Vision loss is one of the major features of Bardet-Biedl syndrome. Most affected individuals have a condition called
cone-rod dystrophy in which the light-sensing tissue at the back of the eye (the retina) breaks down over time. Cone-rod dystrophy typically begins with problems with night vision during childhood, followed by blind spots that develop in the side (peripheral) vision. Over time, these blind spots enlarge and merge to produce tunnel vision. Most people with Bardet-Biedl syndrome also develop blurred central vision (poor visual acuity) and become legally blind by adolescence or early adulthood.

Obesity is another characteristic feature of Bardet-Biedl syndrome. Abnormal weight gain typically begins during the first months or years after birth and continues to be an issue throughout life. Complications of obesity can include type 2 diabetes, high blood pressure (hypertension), and abnormally high cholesterol levels (hypercholesterolemia).

Other common signs and symptoms of Bardet-Biedl syndrome include the presence of extra fingers or toes (polydactyly), learning problems, and intellectual disabilities. Some people have genital abnormalities, which can include an underdeveloped or abnormally developed penis, uterus, or vagina. Affected individuals may have reduced levels of sex hormones (hypogonadism) and difficulties having biological children. Many people with Bardet-Biedl syndrome also have kidney abnormalities, which can be serious or life-threatening.

Additional features of Bardet-Biedl syndrome can include impaired speech; delayed development of motor skills, such as standing and walking; and behavioral issues, such as anxiety and mood disorders. Distinctive facial features, dental abnormalities, unusually short or fused fingers or toes, and a partial or complete loss of the sense of smell (anosmia) can also occur in people with Bardet-Biedl syndrome. Additionally, this condition can affect the heart, liver, and digestive system.

Frequency

In most of North America and Europe, Bardet-Biedl syndrome affects 1 in 120,000 to 160,000 newborns. The condition is more common on the island of Newfoundland (off the east coast of Canada) and in certain populations of Kuwait. Bardet-Bieldl syndrome is most common on the Faroe Islands in the North Atlantic Ocean, where it affects 1 in 3,700 newborns.

Causes

Genetic changes that cause disease are called pathogenic variants. Pathogenic variants in at least 26 different genes (often called the BBS genes) cause Bardet-Biedl syndrome. These genes are known or suspected to play important roles in the function of certain types of cilia. Cilia are microscopic projections that stick out from the surface of most cells. Primary cilia are a specific type of cilia that transmit signals from the outside of the cell to the inside. They are necessary for the perception of sensory input (such as sight, hearing, and smell), and they play an important role in cell communication. The proteins produced from the BBS genes are involved in the maintenance and function of primary cilia.

Variants in BBS genes lead to problems with the structure or function of primary cilia. Changes in these cell structures likely disrupt important signaling pathways during development, including those that affect cell growth and division (proliferation) and cell specialization (differentiation). Researchers believe that most of the features of Bardet-Biedl syndrome are caused by cilia that do not function properly.

About 23 percent of all cases of Bardet-Biedl syndrome result from variants in the BBS1 gene. Another 15 percent of cases are caused by variants in the BBS10 gene. Variants in the other BBS genes account for only a small percentage of all cases of this condition. In about 20 to 30 percent of people with Bardet-Biedl syndrome, the cause of the disorder is unknown.

The presence of variants in more than one gene may explain the variability in the features and the course of Bardet-Biedl syndrome. These additional genes may be BBS genes or other genes, and variants in these genes may help modify the course of the condition. However, this phenomenon appears to be uncommon, and it has not been found consistently in scientific studies.

Inheritance

Bardet-Biedl syndrome is typically inherited in an autosomal recessive pattern, which means both copies of the gene in each cell must have a pathogenic variant to cause the disorder. The parents of an individual with an autosomal recessive condition each carry one copy of the altered gene, but they typically do not show signs and symptoms of the condition.

Autosomal recessive inheritance. Credit: U.S. National Library of Medicine.

Other Names for This Condition

• BBS

Where this page came from

This page was imported from MedlinePlus Genetics, National Library of Medicine. Courtesy of MedlinePlus from the National Library of Medicine; its genetics summaries and the illustrations credited to the library are in the public domain. Pictures credited to others are not copied.

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