Bardet-Biedl syndrome: main causes and symptoms

April 22, 2024

Ciliopathies are diseases of genetic origin that cause morphological and functional alterations in many parts of the body. In this article we will describe the symptoms and causes of Bardet-Biedl syndrome , one of the most frequent ciliopathies, which causes a progressive loss of vision and renal alterations, among other problems.

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What is Bardet-Biedl syndrome?

Bardet-Biedl syndrome is a type of ciliopathy , a group of genetic diseases that involve alterations in the primary cilia of cells. Other common ciliopathies are polycystic kidney disease, nephronophthisis, Joubert's syndrome, Leber congenital amaurosis, orofaciodigital syndrome, asphyxiating thoracic dystrophy and Alström syndrome.

Among the signs of ciliopathies are the degeneration of retinal tissue, the presence of brain anomalies and the development of kidney diseases. Obesity, diabetes and skeletal dysplasia (morphological abnormalities in bone cells) are also common consequences of genetic defects in the synthesis of ciliary proteins .

The Bardet-Biedl syndrome affects approximately 1 in 150 thousand people. Its specific manifestations vary depending on the case because of the great genetic variability of the defects that cause this disease.

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Causes of this disorder

So far they have identified at least 16 genes that are associated with Bardet-Biedl disease . In most of the cases the alterations are caused by mutations in several of the genes involved; this makes the signs vary a lot. Defects in the MKS gene appear to result in especially severe variants of the disorder.

However, about a fifth of the cases do not present mutations in any of these genes, which suggests that in the future they will find others related to the same signs and symptoms.

These genetic defects make proteins that allow the synthesis and proper functioning of primary cilia, structures that are found in different cells. This causes malformations and problems in the eyes, kidneys, face or genitals, among other parts of the body.

The Bardet-Biedl syndrome is associated with McKusick-Kaufman, which is transmitted by autosomal recessive inheritance and it is characterized by the presence of postaxial polydactyly, congenital heart disease and hydrometrocolpos (cystic dilation of the vagina and uterus that causes accumulation of fluids). The differential diagnosis between both disorders can be complex.

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Symptoms and main signs

There are many alterations that can appear as a result of defects in the cilia. We will focus on the most characteristic and relevant, although we will mention most of the

1. Dystrophy of cones and canes (retinitis pigmentosa)

Retinitis or pigmentary retinopathy is one of the most characteristic signs of ciliopathies; It occurs in more than 90% of cases. It implies a progressive loss of vision It starts with lack of visual acuity and night blindness and is maintained after childhood, reaching in some cases the lack of vision in adolescence or adulthood.

2. Postaxial polydactyly

Another of the key signs is the presence of an additional finger on the hands or feet , which occurs in 69% of people with Bardet-Biedl syndrome. In some cases the fingers are shorter than normal (brachydactyly) and / or are fused together (syndactyly).

3. Trunk obesity

In more than 70% of cases obesity occurs after two or three years of age; it is common for them to develop hypertension, hypercholesterolemia and diabetes mellitus type 2 due. This type of obesity is also considered a key sign.

4. Renal alterations

Bardet-Biedl syndrome increases the risk of various diseases of the kidneys: polycystic kidneys, renal dysplasia, nephronophthisis (or tubular cystic disease) and segmental focal glomerulosclerosis. Occasionally, alterations in the lower part of the urinary tract also appear, such as dysfunctions of the bladder detrusor.

5. Hypogonadism

Hypogonadism consists of the deficient production of sex hormones , which means that sexual characteristics do not develop in a normative way. This alteration also causes sterility and is more frequent in males with the syndrome.

6. Developmental disorders

It is common for them to delays in development, language, speech and motor skills , as well as cognitive deficits (usually moderate) as a consequence of ciliary dysfunctions. These problems usually cause alterations of variable severity in learning, social interaction and self-management.

7. Other symptoms

There are many other symptoms and signs common in Bardet-Biedl syndrome, although they are not considered fundamental features of the disease. Among these we find behavioral and psychological alterations such as sudden attacks of anger and lack of emotional maturity.

Alterations in the size of the head and malformations of the teeth and face , especially in the area of the nose. The partial or total loss of smell (anosmia) also appears in some cases.

Hirschsprung's disease frequently appears as a consequence of Bardet-Biedl's disease. It is a disorder of the large intestine that hinders intestinal transit causing constipation, diarrhea and vomiting. It can sometimes cause severe gastrointestinal problems, especially in the colon.

Bibliographic references:

Beales, P., Elcioglu, N., Woolf, A., Parker, D. & Flinter, F. (1999). New criteria for improved diagnosis of Bardet-Biedl syndrome: results of a population survey. Journal of Medical Genetics, 36 (6): 437-46.