What is Bardet Biedl Syndrome (BBS)?

Bardet Biedl Syndrome is a rare genetic disorder with highly variable symptoms which may include retinal degeneration, obesity, reduced kidney function, polydactyly (extra digits of the hands or feet) among many other features. While there are more than 20 genes associated with BBS, the underlying cause regardless of gene is malfunction of primary cilia, a key component of cellular communication. BBS is thus categorized as a ciliopathy, or a disease of the cilia.

Why do I sometimes hear about Lawrence Moon Bardet Biedl Syndrome or LMBBS?

Before the genetic and molecular basis of BBS was discovered, it could only be recognized as a set of symptoms. But because of the wide range of symptoms associated with BBS, there is substantial overlap of symptoms with other diseases. It was once believed that a syndrome described by Lawrence and Moon in 1866 was the same as one described by Georges Bardet and Artur Biedl in the early 1920s, but these syndromes are now recognized as distinct. There are several other syndromes and diseases caused by cilia defects that share some symptoms with BBS including Joubert Syndrome, Meckel-Gruber Syndrome, Senior-Loken Syndrome, polycystic kidney disease, and nephronophthisis.

What are cilia?

Cilia are hair-like structures that are parts of almost every cell in the body. There are two kinds of cilia, motile cilia (which are present in a small number of specialized cells) and primary cilia. The function of primary cilia is to pass information from one cell to another. You can think of them as radio antennas broadcasting and receiving signals. It’s by sending and receiving these signals that different cells in the body know what to do or are able to tell other cells to do something, which is important especially during both early fetal and childhood development. Sometimes they “sense” data like determining chemical levels in urine in the kidneys, molecules that are responsible for taste and smell, or the sense of balance.

In BBS, the cilia do not function correctly and therefore the cellular messaging does not occur as it should. Different BBS genes code for different proteins involved in cilia formation or function, but all have a similar effect in terms of disrupting cellular messaging.

What are the symptoms?

Dr. Philip Beales, a doctor and researcher based in the UK, compiled a set of primary (or more common) and secondary symptoms associated with BBS.

These symptoms are used to diagnose BBS in an individual before genetic testing has been completed or if the genetic testing does not reveal a mutation in a gene known to cause BBS. According to Dr. Beales’ diagnostic criteria, a person should be diagnosed with BBS if they have four of the primary characteristics (see below) or if the person had three primary characteristics and at least two secondary characteristics. Note that most people with BBS do not have all of the characteristics listed below.

The primary characteristics are:

-Visual impairment caused by retinal abnormalities
-Obesity, typically apparent by age one
-Polydactyly (extra fingers or toes)
-Hypogonadism
-Renal anomalies (kidney malformations and/or malfunctions)
-Learning disabilities

Secondary characteristics include:

-Developmental delays
-Behavioral problems
-Neurological problems
-Hypertension (high blood pressure)
-Speech disorders
-Dental anomalies (small teeth, small lower jaw, short teeth)
-Lack of a sense of smell (anosmia)
-Flat, wide feet; no arches
-Thyroid problems
-Strabismus (“lazy eye” -- one eye focuses and the other wanders)
-Short stature relative to parents’ height
-Toe and finger variations: short (brachydactyly); curved (clinodactyly), especially the outer fingers or toes; mild webbing (syndactyly) especially between the 2nd and 3rd toes

As genetic testing for BBS has improved it is becoming clearer that the symptoms and severity of symptoms are even more diverse than originally thought.

For example, while some people with BBS lose most of their vision by their mid-teens, others have maintained enough vision to drive into their 30s. While some people with BBS have significant learning disabilities and cognitive impairment, others do not and have IQ scores well above normal.

A diagnosis of BBS does not necessarily indicate any particular symptom or severity of symptoms.

How many BBS genes are there?

There are currently 21 genes known to cause BBS. However, these 21 genes account for about 75% of the people diagnosed with BBS, so there are likely several additional genes that have not yet been discovered or confirmed.

How many people have BBS?

BBS is officially a “rare disease” affecting approximately 1 in 250,000 people around the world. Based on general population trends, there are likely about 3000 people in the United States and Canada living with BBS. BBS is more common in some areas of the world including Newfoundland (Canada) and in the Middle East (among Bedouin populations).

What does the gene number tell me?

As yet there are no definitive correlations between BBS genes and symptoms or severity of symptoms.

The severity and even the appearance of certain symptoms is more likely due to variations in ciliary genes that are in addition to the causal BBS gene that makes an individual more or less able to compensate for the partial loss of cilia function. There are many families with two or more children who have the exact same causal BBS gene but have different severity of symptoms. It is possible that as we gather more and more detailed clinical and genetic data about individuals with BBS, researchers will find that the specific BBS gene may have some impact on symptoms.

Where the specific BBS gene does matter is that it will affect future treatment via gene therapy or stem cell therapy. It is possible that a gene-based therapy for BBS6 will not be useful to treat a person with BBS12, for instance. The treatments will likely have to be customized to the specific gene that the individual has as a cause of BBS. There are other approaches to BBS treatment that involve making an individual better able to cope with the inability of cells to effectively talk to each other. These therapies are much more likely to be useful regardless of what causal BBS gene a person has.

My child was just diagnosed with BBS. What should I do now?

First, take a deep breath. Having a child diagnosed with BBS can be quite frightening. Happily, very few children with BBS have immediately life-threatening issues. While it’s not currently possible to cure BBS, there are many symptoms that can be alleviated with therapy and some can be improved with surgery. You don’t have to do everything right away.

1. Of course, deal with immediate medical problems in consultation with your doctors. It may be helpful to go over, with your doctors, this comprehensive list of recommended evaluations from the National Institutes of Health.

2. Join the BBS Family Association if you are in the Western Hemisphere, or one of the associations in other parts of the world (see Resources). This will help you connect to the latest research and information about BBS, and likely find other families in your area affected by BBS.

3. Join the private Facebook group Families of Bardet Biedl Syndrome. This is an international group of families who can help answer questions and point you to local resources. But it’s also a place to develop a network of caring people who understand your situation.

4. Have your child evaluated for developmental delays. In some areas, a diagnosis of BBS or of developmental delays can help you access appropriate services very early in life which can help improve the developmental delays. For example, speech therapy, occupational therapy, physical therapy, and vision therapy may be helpful.

5. Look at longer term concerns: removal of extra fingers or toes, appropriate shoes for flat feet and weak ankles, possible orthodontia for some dental anomalies.

6. Contact the Marshfield Clinic to join CRIBBS, the BBS clinical registry, a vital part of research efforts on improving the care and treatment for individuals with BBS.

Does everyone with BBS lose vision?

Everyone with BBS loses some vision because cilia are necessary to maintain healthy retinal cells. Most people with BBS are categorized as legally blind by middle age at the latest, some as early as the early teens. Rarely do people with BBS lose all light sensitivity.

While the rate and specifics of vision loss vary widely--as with all BBS symptoms--there are some general tendencies. For most people with BBS, retinal degeneration begins at the edges of the retina and progress toward the center. Both dim light vision and color vision of the retina is concentrated on the outside edges of the retina, so most people with BBS lose their ability to see in dim light at a relatively young age and color vision is affected as well. Degeneration of the outside edges of the retina also causes a loss of field, or loss of peripheral vision.

Is there a cure for BBS?

There is currently no cure for BBS, but that does not mean that there is nothing that can be done to help people with BBS.

Children with BBS benefit greatly from therapies like physical therapy, occupational therapy, speech therapy and vision services. Everyone with BBS benefits from exercise and careful attention to diet to limit weight gain.

Research is ongoing into understanding the basic mechanisms at the cellular level that ultimately cause BBS. This research will provide clues to develop future treatments.

Treatment research general falls into three categories:

Stem Cell Therapy: Delivering new cells without the defective BBS gene that can multiply and replace defective cells within the body.

Gene Therapy: rather than replacing cells, replacing the defective gene inside of existing cells.

Suppressor Therapy: rather than changing DNA with new cells or in existing cells, overcoming the limitations of those cells by boosting their function through the inactivation of other genes or cellular pathways.