Bardet-Biedl Syndrome
Incidence
The syndrome is familial and is transmitted as an autosomal recessive trait. Among the non-consanguineous populations of Northern Europe and America, the prevalence ranges from one in 100,000 (North America). Among the Bedouin peoples of Kuwait, where consanguinity is frequent, the prevalence is estimated at one in 13,500. Chromosome 3 locus appears to be linked to polydactyly of all four limbs, whereas chromosome 15 is associated with early-onset morbid obesity and is mostly confined to the hands, and chromosome 16 represents the "leanest" form.
Clinical Characteristics
A syndrome characterized mainly by obesity, pigmentary retinopathy, polydactyly, mental retardation, hypogonadism, and renal failure in fatal cases. Bardet-Biedl syndrome is characterized by rod-cone dystrophy, truncal obesity, post axial polydactyly, cognitive impairment, male hypogonadotrophic hypogonadism, complex female genitourinary malformations, and renal dysfunction, which is a major cause of morbidity and mortality. The visual prognosis for children with Bardet-Biedl syndrome is poor. Night blindness is usually evident by seven to eight years of age. The mean age at which patients become legally blind is 15.5 years. Birth weight is usually normal. Significant weight gain begins within the first year and becomes a lifelong issue for most patients. A majority of patients have significant learning difficulties, but only a minority have severe impairment on IQ testing. Two forms have been identified: Bardet-Biedl syndrome 1 (BBS1) has no linkage to chromosome 16 Bardet-Biedl syndrome 2 (BBS2) is mapped to markers on chromosome 16. Laurence-Moon-Biedl syndrome and Laurence-Moon-Biedl-Bardet syndrome are no longer considered as valid terms in that patients of Laurence and Moon had paraplegia but no polydactyly and obesity which are the key elements of the Bardet-Biedl the syndrome. Laurence-Moon syndrome is a separate entity.
Precipitants
None.
Provocation Tests
None.
Diagnostic Procedures
The diagnosis of Bardet-Biedl syndrome is established by clinical findings. The diagnosis of Bardet-Biedl syndrome is established by clinical findings. Beales et al (1999 , 2001) have suggested that the presence of four primary features or three primary features plus two secondary features are diagnostic. Five genes are known to be associated with Bardet-Biedl syndrome: BBS1, BBS2, BBS4, MKKS, and BBS7. Two additional loci (BBS3 and BBS5) are known to be linked to Bardet-Biedl syndrome. Molecular genetic testing is available on a clinical basis for M390R, the common mutation in BBS1. For more information, visit Genetest.org.