Bardet-Biedl syndrome (BBS)
Bardet-Biedl (pronounced BAR-day BEED-el) syndrome, also known as BBS, is part of a larger group of disorders known as rare genetic disorders of obesity.
What causes BBS?
BBS is caused by variants variant A change in the standard “spelling” of a gene. Genetic variants may result in serious health problems or be “silent” with no negative health effects. , or changes, in several genes genes Our body’s instruction manual that informs how our bodies grow, develop, and function. It is through our genes that traits are passed down from parent to offspring. that play a role in the melanocortin-4 receptor (MC4R) pathway MC4R pathway The MC4R pathway is one of the pathways that regulates weight and appetite in the brain. Click here to learn more about the MC4R pathway. and affect hunger signals signals Chemicals and proteins that the body and brain use to communicate and direct processes in the body. in the brain. The MC4R pathway is an important pathway in the brain that helps tell your body when you are hungry and when you are full. These genes also play an important role in the maintenance and function of microscopic cilia, which are tiny finger-like protrusions that allow our cells to move from one place to another and help with senses like sight, sound, and smell. Because the variants affect the function of cilia, you might hear people refer to BBS as a ciliopathy, or disease of the cilia.
What are the key clinical features of BBS?
Signs and symptoms of Bardet-Biedl syndrome include:
- Severe, early-onset obesity
- Insatiable hunger
- Progressive vision impairment/loss
- Extra toes or fingers (polydactyly polydactyly The presence of extra toes and/or fingers. )
- Kidney problems
- Delayed puberty in adolescence
How is BBS inherited?
BBS deficiency is inherited in an autosomal recessive autosomal recessive Autosomal recessive (AR) conditions occur when a person inherits two non-working copies of a disease-causing (pathogenic) gene variant from their parents. manner. This means that someone with BBS will have two copies of the genetic variant genetic variant Changes (also known as variants) in the standard “spelling” of a gene. Gene variants can result in serious health problems or be “silent” with no negative health effects. that causes the disorder. You can only inherit an autosomal recessive disorder if both of your parents are “carriers carrier In an autosomal recessive inheritance pattern, if an individual inherits one normal copy of a gene and one copy of a disease-causing variant, they are called a carrier and will generally not have any symptoms of the disease that variant causes. ” for that disorder, meaning that they both carry one copy with the genetic variant and one copy without it.
In very rare cases, someone who is a carrier for BBS (meaning they have just one disease-causing variant) may develop some of the symptoms of the disorder. This is known as BBS heterozygous deficiency.
How is BBS diagnosed?
BBS is diagnosed using the Beales Diagnostic Criteria, a set of guidelines developed by Dr. Philip Beales. According to Dr. Beales, a person should be diagnosed with BBS if they have four of the primary characteristics listed below, or three primary characteristics and at least two secondary characteristics.
- Visual impairment caused by retinal abnormalities
- Obesity, typically apparent by age 1
- Polydactyly (extra fingers or toes)
- Hypogonadism (lack of hormones hormone Chemical messengers made in the endocrine glands that control body functions. that normally stimulate the ovaries or testes)
- Kidney malformations and/or malfunctions
- Learning disabilities
- Developmental delays
- Behavioral problems
- Neurological problems
- Hypertension (high blood pressure)
- Speech disorders
- Dental differences (small teeth, small lower jaw, short teeth)
- Lack of a sense of smell (anosmia)
- Thyroid problems
- Lazy eye (when one eye focuses and the other wanders)
- Short stature relative to parents’ height
- Flat, wide feet without arches
- Toe and finger variations (such as short or curved outer fingers or toes or mild webbing, especially between the 2nd and 3rd toes)