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Bardet-Biedl syndrome

Also known as: Laurence-Moon-Bardet-Biedl syndrome, Laurence-Moon-Biedl syndrome

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Background

Bardet-Biedl syndrome (BBS) is a rare inherited condition. It is characterised by visual difficulties, obesity, additional fingers and/or toes, undeveloped genitals, learning difficulty (see entry Learning Disability) and renal (kidney) problems. BBS occurs more frequently in populations where there is marriage between family members and children born as a result of that (consanguineous marriage). It is also more common in historically isolated communities, such as Newfoundland.

Credits

Medical text written January 1996 by Dr PL Beales. Last updated March 2012 by Professor PL Beales, Professor of Medical and Molecular Genetics, Wellcome Trust Senior Research Fellow in Clinical Science, Consultant in Clinical Genetics, Molecular Medicine Unit, Institute of Child Health, London, UK.

What are the symptoms?

Primary features:

  • rod/cone dystrophy - a progressive eye condition, which can lead to blindness
  • obesity, particular around the trunk (middle of body)
  • polydactyly (extra fingers and/or toes)
  • hypogenitalism (underdeveloped genitals)
  • mild-to-severe learning difficulties
  • renal abnormalities and impaired renal function.

Secondary features may include:

  • speech problems
  • developmental delay (see entry Global Developmental Delay)
  • behavioural abnormalities
  • eye abnormalities, including strabismus (cross-eyed), cataracts, and astigmatism (abnormally shaped eyeball)
  • brachydactyly (short fingers and/or thumbs)
  • syndactyly (digits joined together)
  • diabetes mellitus
  • dental and roof of mouth abnormalities
  • cardiovascular anomalies (high blood pressure, abnormal heart valves)
  • hepatic (liver) problems
  • olfactory dysfunction (lack of ability to smell)

In children with BBS, night-blindness usually occurs by seven to eight years of age and total blindness may occur by mid to late-adolescence.

What are the causes?

Mutations in at least 16 genes are known to be associated with BBS. Not all people with BBS have mutations in these known genes, so more remain to be discovered. It is now known that these mutations result in abnormal cilia function. These are finger-like projections that are important for cells to sense their surrounding environment and communicate with one another.

How is it diagnosed?

Diagnosis is made on the basis of clinical features initially. A diagnosis can be confirmed by testing DNA to see if the mutation(s) causing the condition lie in one of the 16 genes already identified.

How is it treated?

There is no cure for BBS. Treatment aims to reduce the symptoms experienced by a person with BBS. Regular eye check-ups by an ophthalmologist will highlight any existing problems. Visual aids and educational programmes can assist with visual difficulties. Obesity is managed with diet, exercise and behavioural therapies. Hypercholesterolemia (high cholesterol) and diabetes mellitus may be treated with the appropriate medications and changes to diet. Early intervention, special education provision and speech and language therapy can help with learning.

In cases of severe renal abnormality, renal transplantation may be required. Genital abnormalities, for example hypospadias, may be surgically corrected. Hormone replacement therapy can be initiated for hypogonadism (where sex glands produce little or no sex hormones). Cardiac abnormalities need to be monitored and may be corrected surgically. Surgery to remove extra digits prevents difficulties walking and balancing and poor fitting of footwear.

Inheritance patterns and prenatal diagnosis

Inheritance patterns
For the majority of cases, inheritance is autosomal recessive (two mutations required to manifest the disease). Although in a few cases, more than two mutations in two BBS genes may be required to display symptoms of the condition (this is known as 'triallelic inheritance').

Prenatal diagnosis
A number of features can be determined on high-definition ultrasound scanning of the fetus. This, however, is not a test of exclusion of the condition. If two gene mutations can be identified in a BBS family then a molecular genetic DNA test can be offered prenatally.

Is there support?

LMBB Society

Tel: 01633 718 415
Email: chris.humphreys4@ntlworld.com
www.lmbbs.org.uk

The Society is a Registered Charity in England and Wales No. 1027384. It provides information and support for people with Laurence-Moon-Bardet-Biedl Syndrome and their families and carers, and contact with other families where possible. It holds an annual family conference.

Group details last updated November 2013.

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