The Bardet-Biedl syndrome is a a syndrome characterized mainly by obesity, pigmentary retinopathy, polydactyly, mental retardation, hypogonadism, and renal failure in fatal cases.

The syndrome is named after Georges Bardet and Arthur Biedl.

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.

Major features

Eyes: Pigmentary retinopathy.
Hand and foot: Polydactyly.
Cardiovascular system: Hypertrophy of interventricular septum and left ventricle and dilated cardiomyopathy.
Gastrointestinal system: Fibrosis.
Urogenital system: Hypogonadism, renal failure, urogenital sinuses, ectopic urethra, uterus duplex, septate vagina, and hypoplasia of the uterus, ovaries, and fallopian tubes.
Growth and development: Mental and growth retardation.
Behavior and performance: Poor visual acuity and blindness.
Heredity: The syndrome is familial and is transmitted as an autosomal recessive trait. 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.
Additional features: Obesity.

Cause

The detail biochemical mechanism that leads to BBS is still unclear. Recently, eight genes (BBS1 to BBS8) that are responsible for the disease when mutated have been cloned, and most of the gene products encoded by these BBS genes are located in the basal body and cilia of the cell. It has been postulated that these BBS gene products might involve in the cell signaling pathway in the cilia, and these signaling systems play an essential role in the normal development so that a malfunction in these systems causes the diverse pathological effects of the Syndrome.

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Mutations Cause Eye Disorder - enhanced S-cone syndrome - Brief Article
From Applied Genetics News, 2/1/00

The cause of an eye disorder called enhanced S-cone syndrome (ESCS) has been discovered. Affected individuals are sensitive to blue light and develop night blindness at an early age. A research team led by Val C. Sheffield, a Howard Hughes investigator at the University of Iowa College of Medicine, discovered that 94% of DNA samples from ESCS-affected individuals showed mutations in NR2E3, a photoreceptor gene that is also known as PNR (photoreceptor- specific nuclear receptor). Their work is published in the February issue of Nature Genetics.

Sheffield's lab, which focuses on identifying genes that cause human hereditary diseases--especially hereditary blindness-- discovered the ESCS mutations while studying another retinal disorder called Bardet Biedl syndrome (BB). However, they could not find any PNR mutations in patients with BB. They did, however, think PNR a good candidate for other eye diseases, so they screened nearly 400 DNA samples from people with other eye disorders, and found two samples that possibly contained a PNR mutation. Both patients suffered from ESCS.

To increase their sample size, they tested 35 ESCS-affected individuals from 29 families, obtaining samples from collaborators. "Lo and behold we found that nearly every sample had a mutation," remarks Sheffield. The research team also found that expression of NR2E3 is specific to the nuclear layer of the retina, which is lined by the nuclei of photoreceptors.

Humans have two types of photoreceptors, called rods and cones. Rods mediate black and white vision and are useful mainly at night. During the day, however, humans depend on cones for color vision. Cones come in three types--red, green, and blue--that are sensitive to different wavelengths of light. Discovering the molecular signals that determine why one cell becomes a rod or a cone, and how cone cells become blue-, green-, or red-sensitive are important questions in the study of eye development.

One hypothesis is that cone cells become blue by default unless they receive a signal to become red or green. Another hypothesis is that NR2E3 mutations alter a photoreceptor's fate, causing cells that would normally develop as rods to become cones instead.

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