Bardet-Bield Syndrome (BBS): Overview

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Table of Contents

Description
Etiology
Incidence
Pathological Process
Symptoms
Associated problems
Diagnosis
Prognosis
Conclusion
References

Description

Bardet-Bield syndrome (BBS) is one of the world’s most rare autosomal recessive disorders which is accompanied by both clinical and genetic heterogeneity (Ghr.nlm.nih.com. 2011). This disorder is also associated with clinical variability, precisely observed within families; BBS syndrome sometimes can affect the brains resulting to multiple physical disorders, including deterioration of the intellect and neurological dysfunction (Ghr.nlm.nih.com. 2011). Nevertheless, the syndrome can also proceed further and affect the eyesight (the rod-cone dystrophy) by eating the light sensitive cells called the retina (Ghr.nlm.nih.com. 2011). Extra toe or fingers (polydactyl disorder), mental retardation, obesity, hypogornadism and renal abnormalities are also some of the associated abnormalities that accompany BBS (Ghr.nlm.nih.com. 2011).

Etiology

The etiology of BBS is considered to be a recessive inherited disorder; in some cases, the parents may never exhibit the symptoms of the syndrome despite carrying the recessive genes which eventually transmits the disease to their offspring’s (Ghr.nlm.nih.com. 2011). Considerably, the reason as to why the syndrome is rare is because this recessive gene is approximately carried by 1 out of 179 people who are unlikely to conceive a child with another person who also possesses the same recessive gene (Beales & Waters, 2010). This means the odd of having both parents with this particular causative gene is extremely hard.

Incidence

Not surprising then, in northern America and Europe the prevalence and the incidence of the disease range from 1 in 140,000 to 1 in 160,000 newborns as is also the case in the island of Newfoundland (Canada), where it affects an estimate of 1 in 17,000 newborns (Beales & Waters, 2010). But depending with regions, this prevalence also varies and might be high or low.

Pathological Process

The biochemical mechanism which leads to BBS involves twelve genes that are responsible for the syndrome when mutated; these genes are; “BBS1, BBS2, BBS4, BBS5, BBS7, TTC8/BBS8, BBS9, BBS6, BBS10, BBS12, ARL6/BBS3 and TRIM32/BBS11” (Beales & Waters, 2010). The BBS proteins located at the basal body and cilia of the cell are the one involved in a process called intraflagella transport (IFT) within the cilia and also within the ciliary shaft (Beales & Waters, 2010). Eventually, the BBS proteins become’s assembled into a complex multiple proteins that are collectively referred as Bbsome (Beales & Waters, 2010). It is this BBsome protein that is responsible for transportation of intracellular vesicles to the base of the cilia and any abnormalities of these cilia will automatically result to BBS. It’s now accepted globally that mutated BBS gene has capabilities of affecting normal cilia function resulting to BBS (Beales & Waters, 2010).

Symptoms

The major clinical symptoms associated with the condition include pigmentary retinopathy of the eyes due to the impairment of photoreceptor transport mechanism in the retina of the eye (Beales & Waters, 2010).

Polydactyly is another visual clinical symptom where the victim has extra digits on the fingers or toes which sometimes may be webbed, usually described as syndactyly (Beales & Waters, 2010). Other classical symptoms of BBS condition include “obesity, mental growth and development retardation, hypogornadism, renal failure, ectopic urethra, septet vagina, urogenital sinuses and hypoplasia of the female uterus” (Beales & Waters, 2010).

Associated problems

Due to the impairment of photoreceptor, the patient may develop poor vision and in some cases complete blindness, in addition some patients exhibit reduced or extra sensitive sense of smell (Beales & Waters, 2010).

Diagnosis

Molecular genetic testing is one of the diagnostic methods employed during BBS diagnosis; the method involves identifying mutation of the 14 genes namely: “BBS1, BBS2, ARL6/BBS3, BBS4, BBS5, MKKS/BBS6, BBS7, TTC8/BBS8, B1/BBS9, BBS10, TRIM32/BBS11, BBS12, MKS1/BBS13, and CEP290/BBS14” which are responsible for causing BBS (Beales & Waters, 2010). The mutated genes procedural diagnostic tests include; sequence analysis which seeks to identify gene sequence variant, deletion/duplication analysis which tends to checks the genetic codes if they are deleted or inserted causing abnormality and target mutation analysis which tends to reveal any mutation present in the particular genes (Beales & Waters, 2010).

Prenatal diagnosis and preimplantation genetic diagnosis (PGD) are also other diagnosis methods done to identify the specific mutated gene causing the disease (Beales & Waters, 2010). Family history is also another fundamental method used to predict presence of the recessive gene causing BBS (Beales & Waters, 2010). Hydrometrocolpos and polydactyl diagnosed during the prenatal period or during the early childhood stage also tend to investigate presence of mutated BBS6 gene (Beales & Waters, 2010).

Clinical diagnosis is used to determine presence of primary and secondary features, which in most individual may always be delayed as a result of slow emergence of the visual variable expressions (signs and symptoms) (Beales & Waters, 2010).

This poses difficulty in clinical diagnosis since an obese children with mental retardation and difficulty in learning without polydactyly for instance has very broad range of symptoms (Beales & Waters, 2010). The other additional BBS symptoms which appear during the early childhood stage include; retinitis pigmentosa, learning disabilities and progressed renal dysfunction which are all clear symptoms of BBS that can be diagnosed clinically (Beales & Waters, 2010).

Ophthalmologic assessment is one of the management tools established to measure the extent at which BBS has spread; it’s done to determine visual acuity or field deficits which should be filed for later comparison (Beales & Waters, 2010). Genitalia examination done for both sexes is very important as well as ultrasound photographs of ovaries, fallopian tubes, uterus and vagina area which should be examined to identify if BBS disorder exists (Beales & Waters, 2010). Calculation of body mass index is medically significant for obesity diagnosis since the dietary evaluation of obesity is BMI>30 (Beales & Waters, 2010). Finally, otoacoustic emissions (OAE) and audiometry are other relevant assessments conducted for hearing abnormality evaluation (Beales & Waters, 2010).

Prognosis

There is no therapy present for any progressive visual loss, but periodic evaluation by a specialist always facilitates improvement of vision (Beales & Waters, 2010).

Diet, exercise and behavioral therapies are considered to be the best educational and dietary measures to control overweight among individuals in their early years (Beales & Waters, 2010). Speech therapy can be offered to identify signs of speech impairment; additionally, as children approach puberty stages, sex and gonardotropin hormones levels tests should be perfomed to detect abnormality (Beales & Waters, 2010).

Conclusion

Genetic counseling should be provided to individuals and families of BBS patients to help them make informed decisions. The risk of the family of an affected child and whose parents are obligate heterozygous comes with each conception, where each sibling has a 25% chance of being asymptomatic carrier, 50% of being uninfected and 25% of being infected (Beales & Waters, 2010).

References

Beales, P. & Waters, A. (2010). Bardet-Bield Syndrome. Web.

Ghr.nlm.nih.com. (2011). Genetic Conditions: Bardet-Bield syndrome. Web.

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