Bardet-Biedl syndrome (BBS) is a rare disease affecting several organs, including the kidneys. BBS is caused by an abnormally functioning cell component called a cilium (or cilia, plural) which is present on many cell types from different organs. Cilia are long, thin, hair-like projections that enable the cell to receive signals from outside and inside the cell. Bardet-Biedl syndrome is therefore categorized as a ‘ciliopathy’.
The aim of this booklet is to explain which organs can be affected by Bardet-Biedl syndrome, and how the prognosis and quality of life of BBS patients can be improved. Early diagnosis is important for quality of life and patients with BBS generally require multi-specialised care.
Kidney problems can cause serious complications of the disease and later in this leaflet we will explain how medication and lifestyle changes can delay the onset of kidney failure.
The disease was first described independently in 1920 by a French doctor, Georges Bardet, and in 1922 by Hungarian-Austrian pathologist, Artur Biedl. Since then, more than 200 cases of BBS have been reported in the world’s medical literature. In 2000, researchers discovered a gene mutation causing Bardet Biedl syndrome. New data indicates that at least 24 genes are involved in the development of BBS. These genes are often called BBS genes and they play a critical role in cilia function.
Laurence-Moon Syndrome ≠ Bardet Biedl Syndrome
In the past, Laurence-Moon-Bardet-Biedl syndrome was described as a single disease. Nowadays, both syndromes (Laurence-Moon syndrome and Bardet-Biedl syndrome) are identified and treated separately, however, both syndromes are categorised as ciliopathies.
Ciliopathies are a group of diseases caused by genetic mutations, which result in cilia dysfunction, affecting many organs of the human body.
Since cilia are located on many body cells, the diseases usually appear more complex and include multiple organs, which is the reason they are called syndromes.
The role of cilia in most tissues remains unknown and how ciliary dysfunction leads to such severe disease is a subject of current research.
|Alström syndrome||Sensenbrenner syndrome||Shortrib-polydactyly syndrome|
|Asphyxiating thoracic dysplasia (ATD)||Jeune syndrome||Ellis-van Creveld syndrome|
|Joubert syndrome||Leber congenital amaurosis||McKusick-Kaufman syndrome|
|Meckel-Gruber syndrome||Nephronophthisis||Orofaciodigital syndrome|
|Polycystic kidney disease||Primary ciliary dyskinesia||Senior-Loken syndrome|
Bardet-Biedl syndrome is caused by the change (mutation) of only one single gene and is therefore called monogenic.
The syndrome is inherited in an autosomal recessive manner.
What does this mean?
For an autosomal recessive disease to occur, a child has to inherit two mutated/faulty copies of the gene, one from each parent. If only one mutated/faulty gene is inherited, the child will not have BBS, but will be a carrier of the syndrome. The child from each pregnancy has a 25% chance of receiving both mutated/faulty genes and thus having BBS. Both sexes are equally affected.
In the example below, you can see the genetic tree of a family with BBS.
For parents who have a child with BBS, the risk to future pregnancies is:
25% that the child will have BBS
50% that the child will not have BBS but will be a carrier of the defective gene
25% that the child will not have BBS and will not be a carrier of the defective gene
To date (2022), mutations in 24 BBS genes have been identified.
The BBS genes are the blueprints for various BBS proteins that are important in cilia function and some cellular transport mechanisms.
In this sense, if one of the BBS proteins is not functioning (due to a mutation in the respective gene) the whole apparatus will not work as it should, and it leads to the same disease → Bardet Biedl syndrome.
The prevalence of this disease in Europe is estimated at 1 of 150000 – 175000 people. In some isolated populations (e.g. among Israeli Bedouins and Puerto Ricans), it is found to be more common: approximately 1 of 100000 people. The presence of specific clinical signs may prompt the genetic testing.
Why does my child have BBS?
In most cases, an individual has BBS because they inherited one copy of the faulty gene from each parent and therefore have two copies of the same, faulty gene.
Carriers generally do not have any signs and symptoms of the disease, because in addition to the faulty gene, they also have a second, normal gene.
Can the disease appear in other family members?
Where one family member has a genetically confirmed diagnosis of BBS, siblings and other relatives can be tested to see if they carry the faulty gene for family planning purposes.
Knowledge of the BBS mutation in the family can also form the basis of prenatal screening, should parents wish to find out in early pregnancy, whether the foetus is affected.
Family planning for people affected by BBS: genetic counselling
People with BBS who wish to have children, should receive genetic counselling. The partner of the person who has BBS should be tested for carriers’ status, to determine if they carry a disease-causing mutation in the same gene.
If the partner is a carrier of a mutation in the same gene, each child has a 50% chance of having the syndrome. If both parents have BBS, with mutations in the same genes, then all children will have BBS.
The pathology of BBS: what are cilia & how do they work?
The mechanism that leads to BBS is still unclear, however t he main cause of the disorders that occur in BBS is due to abnormally functioning or structured cilia.
Cilia are thin, hair-like structures that are only visible under a microscope. They occur on the surface of almost all cells of our body. There are different types of cilia performing different functions.
The length of a single cilia is 1-10 micrometers and the width is less than 1 micron.
Cilia play a very important role, even before birth, enabling embryo development, cell migration and organ differentiation.
A main function of cilia is sensory and they play an important role in terms of smell, vision, touch, and temperature sensation.
There are different types of cilia:
Cilia which can move are called motile cilia. They are located on the surface of the lung, respiratory cells, or the middle ear, where they help to remove mucus and infectious agents by way of rhythmic, wavy movements.
These cilia also play an important role in the movement of sperm.
Cilia that are not able to move are called non-motile cilia/primary cilia. These structures receive signals from outside of the cell and transmit them to the inside, allowing neighbouring cells to communicate with each other. For example, in the kidney, cells can receive signals about the level of chemicals in the urine.
In BBS, the functioning of primary cilia is impaired
It has been shown that the genes involved in BBS encode proteins, that are responsible for the function or development of cilia.
A primary cilium consists of a basal body (dark blue) and a tube-like structure (green) build of microtubules. The 24 different BBS proteins have different functions within the cilium.
Eight BBS proteins (BBS1, BBS2, BBS4, BBS5, BBS7, BBS8, BBS9 and BBS18) build a stable complex known as the BBSome, which regulates molecule trafficking to the ciliary membrane. Some others constitute a chaperonin complex that is important for the correct construction of the BBSome. Some other BBS proteins help to locate the BBSome to its final position. The partially overlapping functions of BBS proteins explain why different mutations in different BBS genes result in similar disease signs and symptoms.
BBS affects many organs, and the symptoms can vary significantly between patients, even between individuals within the same family. Classically defined by six features, BBS is usually diagnosed in infancy.
Common symptoms include obesity, visual impairment, additional fingers and/or toes, reduced function of the testes in boys, kidney defects and learning disability.
Although BBS may be diagnosed based on genetic tests revealing mutations within specific genes, the presence of primary and secondary symptoms may be used to indicate which patients are suspected of having BBS and need the genetic examination.
Diagnostics, based on the presence of primary and secondary characteristics, was proposed by Forsythe and Beales in 2003. On the basis of this:
BBS could be diagnosed if the person is known to have at least three primary and two secondary characteristics, or if they have at least four primary characteristics.
The usefulness of these clinical criteria may be limited by the fact that many of these clinical characteristics appear gradually as the child develops and thus the sensitivity of the proposed diagnostic criteria is low in young children. Further, some patients who have a genetic diagnosis, do not always meet the clinical diagnostic criteria and there may also be a variability of symptoms, even for those with the same genetic mutation.
Therefore, it is important that: The presence of these clinical criteria is periodically verified in a child considered for a diagnosis of BBS.
The diagnostics criteria for BBS by Beales et all:
|Primary criteria||Secondary criteria|
|Retinal degeneration (Rod-cone dystrophy)||Squint (strabismus) |
Blurring of lenses (cataracts)
Bending of the cornea (astigmatism)
|Additional fingers/toes (polydactyly)||Shorter fingers/toes (brachydactyly) |
Fused fingers/toes (syndactyly)
|Central obesity||Speech disorders|
|Learning disabilities||Delayed development/ behavioural disorders|
|Kidney malformations||Abnormally large production or output of urine (polyuria) |
Excessive thirst (polydipsia)
|Reduced function of the testes (hypogonadism) (male) / genital abnormalities (female)||Dental crowding/hypodontia /small roots/high arched palate |
Obesity is an excessive accumulation of adipose tissue in the body. The classification of obesity was determined by the WHO (World Health Organization) and is now based on body mass index (BMI), which is calculated from the body weight in kg divided by the height squared (m2).
Body mass index: BMI=kg/m²
In adults, being overweight is defined as having a BMI ≥25 and obesity as having a BMI ≥30. In children and adolescents, being overweight is defined as a BMI ≥85 percentile (pc) for age and gender, and obesity as a BMI ≥95 pc.
Obesity in BBS
Leptin is a hormone produced by fat (adipose) cells that helps to regulate energy balance by inhibiting hunger. The main function of leptin is to send a signal to the brain reporting how much fat is stored in the body’s fat cells. Leptin acts on cell receptors in the hypothalamus, consequently mediating eating.In obesity, a decreased sensitivity to leptin occurs (like insulin resistance in type 2 diabetes), resulting in an inability to detect feeling full, despite high energy stores and high levels of leptin. The condition is known as a leptin resistance. Leptin resistance cause irrepressible hunger and reduced the number of burned calories.
BBS proteins are shown to affect the leptin response and the loss of BBS genes results in leptin resistance. This is a reason why many BBS patients are obese.
The eye problems are of central concern in patients with BBS as almost all patients experience progressive vision loss. The first symptom onset is usually that of night-blindness, typically seen around age 8-9 years of age.
What is the reason for this vision loss?
Minor common visual symptoms of BBS:
Visual impairment development in BBS patients:
The first symptom is usually night blindness, which occurs in children as young as 8-9 years old. Field of vision is usually affected from the age of 10. By the age of 17, so-called tunnel vision is usually established. From the second to the third decade of life, visual acuity gradually declines to 10% or less in almost all patients.
For more insight into the biology behind the rod-cone degeneration:
Rods and cones are photoceptor cells of the retina and have a special light-absorbing segment. These segments are modified cilia. The BBS mutation leads to an impaired protein transport between the light-absorbing segment and other parts of the cell. This transport deficiency results in the death of rods and cones and in total retinal degeneration and blindness.
Polydactyly is the presence of additional fingers or toes at birth and is a key indication of Bardet-Biedl syndrome. Since these are usually removed in early childhood, their presence may be forgotten, thus impacting on the diagnostic process.
Polydactyly occurs in approximately 70% of BBS patients, with the presence of an extra toe more common than an extra finger.
Fingers and toes may also be fused (syndactyly) and this is especially common between the second and third toes. Fingers and toes may occasionally be abnormally short in length (brachydactyly), feet may be wide, short in length and have a flat arch.
Syndactyly or brachdactyly are categorised as secondary symptoms of BBS.
In men, a small size and poor function of the testes, is termed “testicular hypogonadism”. This may manifest as a small penis, failure of the testes to descend into the scrotum (“cryptorchidism”) or a delay in the onset of puberty. Undescended testicles are a concern because they are associated with a greater risk for testicular cancer and should not be left unaddressed. Males are almost invariably infertile.
A wide variety of genital malformations have been observed in females, contributing to the low rates of fertility in BBS. This may manifest as an underdeveloped uterus, fallopian tubes, or ovaries. Menstruation cycles are often delayed and may also follow an irregular cycle.
The rate of fertility is low, but individuals from both sexes have been known to have biological children.
Cilia are present in kidney cells which explains why BBS patients are affected by kidney dysfunction and malformation. Kidney abnormalities affect at least 50% of BBS patients.
Recent scientific data show that:
Solitary kidney, (one kidney is missing), or kidney dysplasia (the kidney is not fully developed), may occur. If both kidneys are dysplastic, depending on the severity of the abnormalities, kidney function may be impaired and renal replacement therapy (dialysis or kidney transplantation) may be necessary.
Vesicoureteral reflux – this is where urine flows from the bladder to the kidneys (instead of from the kidneys to the bladder) and can contribute to the development of urinary tract infections.
Horseshoe kidney is where the two kidneys join (fuse) together at the bottom during pregnancy, to form a ‘U’ shape which gives it the name “horseshoe”. Kidney drainage can be affected, resulting in increased frequency of kidney stones and urinary tract infections. Horseshoe kidney can occur alone or with other disorders.
Crossed fused ectopia occurs when both kidneys develop on the same side of the body. In many cases, the two kidneys could also be fused together retaining their own vessels and ureters.
Renal/ kidney dysplasia also known as multicystic dysplastic kidney – this is where the internal structures of one or both kidneys fail to develop normally during pregnancy. Babies with severe kidney dysplasia affecting both kidneys generally do not survive birth. Children with dysplasia in only one kidney will have normal kidney function if the other kidney is unaffected. Those with mild dysplasia of both kidneys may need renal support sooner or later.
Neurogenic bladder – this is caused by a malfunction in the nerves that control bladder function and the bladder may not fill or empty in the right way. The muscles of the bladder may become overactive and contract abnormally, even before the bladder is full, or the muscles may become too loose leading to incontinence. In other cases, the muscles become underactive and even if the bladder is full the muscles do not contract, and the person may not feel a sensation or an urge to go to the toilet.
Have you heard of this before?
As recently shown BBS patients have also a higher prevalence of certain autoimmune diseases:
Altered red blood cell and platelet compartments, as well as elevated white blood cell levels have been found in BBS patients. Some study reveals a connection between a ciliopathy and dysregulated immune and hematopoietic systems and immunity. Some of these alterations are associated with BBS-induced obesity which leads to elevated concentration of white blood cells in BBS patients. Obesity can induce the state of low-grade metabolic inflammation and one of the major players in obesity-associated inflammation is leptin, an adipocyte-derived hormone which acts as a pro-inflammatory cytokine. It has been shown that leptin signalling in the central nervous system regulates immune responses. Thus, it is possible that defective leptin signalling in the nervous system directly contributes to high prevalence of autoimmunity in BBS patients.
BBS is such a rare condition that many paediatricians, throughout their careers, have never met a patient with the syndrome. Early diagnosis gives you a better chance of providing your child with the right medical care.
To make a diagnosis, knowledge of the patient’s medical history, symptoms, and physical and intellectual development are needed, as well as the results of laboratory tests. Due to the high variability in the occurrence of clinical manifestations and the different times of their appearance, the patient suspected of BBS requires regular assessment. Ultimately, a genetic test confirms a clear diagnosis, although new mutations responsible for BBS are still to be identified.
Family history: BBS is an autosomal recessive disease, which means that two abnormal copies of a particular gene are required for it to occur. Most often, parents are healthy carriers and show no symptoms, because each of them has only one abnormal copy. However, the gene mutation may also be non-hereticate, i.e., the formation of an abnormal copy of the gene occurred spontaneously during embryo development. This is called “de novo mutation”.
General Check of body height, body weight, body mass index (BMI) and blood pressure measurement as part of the routine monitoring for patients with suspected BBS should take place at each consultation. It is recommended to also monitor and
preferably record these measurements regularly at home.
Radiological, ultrasound examinations: to detect the presence of cysts or other abnormalities of the urinary system, testes and liver.
Kidney ultrasound: shows the presence of kidney cysts or other malformations of the urinary or reproductive system.
Cardiac ultrasound: to diagnose heart defects that feature with heart hypertrophy.
Abdomen magnetic resonance imaging (MRI): is more accurate in assessing the number, place and size of cysts andis used primarily to monitor the progression of the disease.
Complete eye examination: to detect the degeneration of the retina e.g. the presence of pigmentary retinopathy.
Pigmentary retinopathy can be detected by an examination of the eyeball, using a special lamp, after the pupil has been dilated.
Electroretinogram (ERG): is an eye test used to diagnose retinopathy and can be done at any age. It involves recording the electrical reaction of the retina to the stimulation of light and may show early changes within the first two years of life, although significant changes are rarely visible before the age of five.
Field of vision measurement: usually carried out after the age of about 7 years, because it requires active cooperation with the patient.
Blood and urine tests: Blood and urine tests are carried out regularly, mainly to monitor kidney function, detect and treat diabetes, as well as common lipid disorders.
Endocrinological assessment must include assessment for any signs and symptoms of diabetes mellitus with subsequent oral glucose tolerance testing if appropriate. Assessment of thyroid function, lipid profile and the development of secondary sexual characteristics is important. If appropriate, further pituitary function testing can be done and hormone replacement therapy instigated. During adolescence, blood tests will detect and prompt common sex hormone secretion disorders (testosterone or oestrogen).
Genetic testing: Since BBS can be caused by mutations in different genes and symptoms may suggest other syndromes also associated with the malfunction of cilia, molecular methods of genetic testing, including gene oriented research (multigene panel) or comprehensive genomic testing (exon sequencing) are recommended.
Genomic studies may reveal pathogenic variants in known genes that have not yet been included in gene panels ornew pathogenetic variants in genes previously known to be BBS-related.
Currently, genetic mutations in more than 24 genes, that cause this syndrome, have been discovered. All these mutations lead to the abnormal structure and/or impaired action of the primary cilia.
The most common mutations involve the following genes:
BBS 1 – 23.4% of all BBS patients
BBS10 – 14.5% of all BBS patients
BBS 2 – 9.6% of all BBS patients
BBS12 – 6.4% of all BBS patients
Can we confuse BBS with other diseases and how do we tell the difference?
Yes, Bardet-Biedl syndrome can be confused with other syndromes with similar symptoms. It is not always easy to make a diagnosis quickly. Patients often present signs and symptoms common to different syndromes.
Laurence-Moon syndrome, long confused with Bardet-Biedl syndrome, among other symptoms causes neurological disorders (balance disorders or lack of coordination and / or paralysis of the legs) and polydactyly. The genes responsible are different from those that cause Bardet-Biedl syndrome.
Alström syndrome is characterized by abnormalities of the retina, obesity, progressive hearing loss, kidney abnormalities, diabetes and poor sexual organ development (hypogonadism) in boys. Some of these symptoms and manifestations coincide with Bardet-Biedl syndrome, however, there is no polydactyly or learning difficulties. The gene responsible for Alström syndrome has been identified and is different from those involved in Bardet-Biedl syndrome.
Cohen’s syndrome combines retinopathy with myopia (nearsightedness), obesity and specific dental abnormalities. The gene in question is also identified and is different from those involved in Bardet-Biedl syndrome.
McKusick-Kaufman syndrome causes genital abnormalities, supernumerary fingers, and heart defects. However, it does not include retinopathy pigmentosa, which distinguishes it from Bardet-Biedl syndrome.
A properly made diagnosis in early childhood can support the effective management of Bardet-Biedl syndrome. Research is ongoing to develop a drug for BBS, but so far, no causal treatment has been developed.
The life expectancy of the patient is not reduced by the syndrome, but people with BBS require specialist care and, in many cases, are dependent on the help of other people in their everyday lives.
The medical interventions for BBS patients consist in alleviating the symptoms and side effects of the condition, but most of them are difficult to treat. And although treatment usually focuses on specific symptoms in a person, patients require multidisciplinary care involving a nephrologist, ophthalmologist, endocrinologist, and genetics specialist. Patients and their families also need learning and psychological support.
Likewise, the development of children may be significantly delayed and limited by the syndrome. Anxiety disorders can also occur. Not infrequently, BBS may lead to psychological disorders and depression in relatives or parents of those with BBS.
BBS patients may be significantly limited in their everyday lives and suffer from a greatly reduced visual field. The blindness itself can lead to severe psychological discomfort or even depression.
BBS may also lead to behavioral problems, and children especially may experience bullying or teasing as a result.
Recent studies have demonstrated that the common point for all BBS symptoms and cilia disfunction is the dysregulation of the glycosphingolipid (GSL) metabolism.
Research is exploring how this metabolic defect may be targeted, to maintain cilia structure and signalling, and therefore lead to an improvement of pathology in multiple organs. This therapy option is only at experimental stages and is yet to be proven.
Impaired renal function:
Polydactyly / genital abnormalities:
What will happen to my child in the future? What are their prospects?
Disease documentation and test results:
Being diagnosed with kidney disease can feel difficult, but understanding your or your child’s condition is the first step to taking control of its symptoms. Some of these common terms might be mentioned by the doctors. Here’s what they mean:
|CKD (Chronic Kidney Disease)||A progressive and irreversible damage to the kidneys which, over the course of months or years can lead to kidney (renal) failure. There is no cure for CKD, but there are treatments that can significantly slow the progression of the disease if started early.|
|Creatinine||A normal body waste product. Sometimes the level of creatinine in your urine is checked and used to determine how well your kidneys are filtering waste from your blood.|
|Crohn’s Disease||A type of inflammatory bowel disease (IBD), autoimmunological disorder, occurring not rarely in BBS pts. It causes inflammation of your digestive tract, which can lead to abdominal pain, severe diarrhoea, fatigue, weight loss and malnutrition.|
|Dialysis||The process of artificially removing waste and excess fluid from your blood. There are two main kinds of dialysis-haemodialysis and peritoneal dialysis. With haemodialysis, your blood is filtered by an external machine called a dialyser. Peritoneal dialysis works by passing dialysis solution into your abdomen through a catheter to filter your blood in your body.|
|ESRD (End-Stage Renal Disease)||The final stage and most severe form of kidney disease. It signifies that your kidneys have failed, and at this point dialysis or a kidney transplant is needed.|
|GFR (Glomerular Filtration Rate)||The rate at which your kidneys (specifically the glomeruli in them) filter waste from your blood. The test for it is called estimated glomerular filtration rate test eGFR.|
|Hirschsprungs Disease||Describes an absence of the nerves, normally found in the colon, that control the innate motion of the colon and move food along the tract. This disorders was observed in BBS patients.|
|Insulin||Is a hormone produced by the pancreas that regulates the transport of glucose (sugar) to cells and its use for energy production. The secretion is known to be stimulated by the production of glucose.|
|Insulinresistance||Body’s cells do not respond adequately to insulin stimulation, and as a result glucose don`t penetrate into the cells.|
|Nycturia||Purposeful urination at night, after waking from sleep; typically caused by nocturnal urine volumen.|
|Polydipsia||Is an excessive thirst. People with this condition tend to drink too much, may never feel sated and end up consuming much more water than they should.|
|Polyuria||Is excessive or an abnormally large production or output of urine (>than 3L over 24h in adults).|
|Renoprotection||Measures taken to prevent damage to the kidney from any cause, minimise adverse effects – especially due to oxidative stress – on renal vasculature.|
9. Husson H, Bukanov NO, Moreno S, Smith M, Richards B et all. Correction of cilia structure and function alleviates multi-organ pathology in Bardet–Biedl syndrome mice Human Molecular Genetics, 2020, Vol. 29, 2508–2522
10. Khan Sa, MuhaGenetics of human Bardet-Biedl syndrome, an updates Clin Genet 2016 Jul;90(1):3-15.
11. Meng X, Long Y, Ren J, Wang G, Yin X, Li S. Ocular Characteristics of Patients With Bardet-Biedl Syndrome Caused by Pathogenic BBS Gene Variation in a Chinese Cohort. Front Cell Dev Biol. 2021
12. Tsyklauri O, Niederlova V, Forsythe E et all. .Bardet–Biedl Syndrome ciliopathy is linked to altered hematopoiesis and dysregulated self-tolerance. EMBO Rep. 2021 Feb 3; 22(2): e50785. Published online 2021 Jan 11. doi: 10.15252/embr.202050785
13. Weihbrecht K, Goar WA, Pak T, et al. Keeping an Eye on Bardet-Biedl Syndrome: A Comprehensive Review of the Role of Bardet-Biedl Syndrome Genes in the Eye. Med Res Arch. 2017; 5(9):https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5814251/.
14. https:/ rarediseases.org/rare-diseases/bardet-biedl-syndrome/