APA (Old Order Amish; Eastern Pennsylvania)

Inheritance: autosomal recessive
Genes: BBS1 (11q13)
BBS2 (16q21)

BBS1
BBS2
BBS
Bardet-Biedl syndrome 1

Obesity
Renal disease
Polydactyly
Mental retardation
Retinal dystrophy
Infertility

Bardet-Biedl Syndrome

Clinical Characteristics

General description (for patients):  

Mild obesity, extra fingers and toes, and mild mental retardation are the hallmarks of this condition.  Cystic kidney disease and evidence of sexual infantilism with infertility are also common.  Vision may be decreased and many patients are night-blind secondary to abnormalities in the retina.

Medical description:  

This disease is both etiologically and phenotypically heterogeneous.  However, the combination of mild obesity, pigmentary retinal dystrophy, polydactyly, and mental retardation is characteristic of BBS1.  It resembles Alstrom sydrome which, however, does not have the mental changes or the polydactyly.  Cystic renal disease and evidence of hypogonadism are also common.  The nephronothisis may progress to end stage renal failure. Secondary sexual characteristics do not develop and infertility results from hypogonadotropic hypogonadism.  The retinal picture resembles that of primary retinitis pigmentosa complete with reduced vision and night blindness.  Nearly half are found to have cardiac abnormalities, primarily valvular in nature, but rarely of clinical significance.  Many patients have partial or complete anosmia.  BBS1 offspring tend to be taller than their parents, compared to children in other recessive obesity/retardation syndromes such as Alstrom, Biemond, and Laurence-Moon.

Genetics:   

Bardet-Biedl is a genetically heterogeneous disorder caused by many mutations on numerous chromosomes.  At least 14 specific variants are recognized.  BBS1 occurs secondary to a mutation on chromosome 11 (11q13).  The most common mutation causing BBS1 results in a Met390Arg change which in the homozygous state causes the BBS phenotype (autosomal recessive inheritance).  It is a widespread disorder and has been found in the Amish of southeastern Pennsylvania as a result of the 1179T>G mutation.  It has been suggested that heterozygotes such as parents have an increased frequency of obesity, hypertension, diabetes mellitus, and renal disease.
A novel splice site founder mutation (c.472-2A>G) in the BBS2 gene (16q21) has been found among members of several Hutterite leuts.  Previous examples of BBS2 were identified among inbred Bedouin family members in Israel.

Treatment:  

No treatment is curative.

Prognosis:  

Generally little change throughout life except for progressive renal disease in some, and mild increased loss of vision.  Increased risk for diabetes mellitus.

Ancillary tratments and support:

Supportive care, low vision evaluation, renal care.

Specialists and specialty centers:

Pediatrician, neurologist, nephrologist.

References:

Mykytyn, K., Nishimura, D.Y., Searby, C.C., Shastri, M., Yen, H., Beck, J.S., Braun, T., Streb, L.M., Cornier, A.S., Cox, G.F., Futlton, A.B., Carmi, R., Luleci, G., Chandrasekharappa, S.C., Collins, F.S., Jacobson, S.G., Heckenlively, J.R., Weleber, R.G., Stone, E.M., and Sheffield, V.C.:  Identification of the gene (BBS1) most commonly involved in Bardet-Biedl syndrome, a complex human obesity syndrome.  Nature Genet. 31: 435-438, 2002. PubMed ID: 12118255

Beales, P.L., Warner, A.M., Hitman, G.A., Thakker, R., and Flinter, F.A.:  Bardet-Biedl syndrome: a molecular and phenotypic study of 18 families.  J. Med. Genet. 34: 92-98, 1997.  PubMed ID: 9039982

Croft, J.B., and Swift, M.:  Obesity, hypertension, and renal disease in relatives of Bardet-Biedl syndrome sibs.  Am. J. Med. Genet. 36: 37-42, 1990.  PubMed ID: 2333905

Innes, A.M., Boycott, K.M., Puffenberger, E.G., Redl, D., MacDonald, I.M., Chudley, A.E., Beaulieu, C., Perrier, R., Gillan, T., Wade, A., Parboosingh, J.S.:  A founder mutation in BBS2 is responsible for Bardet-Biedl syndrome in the Hutterite population: utillity of SNP arrays in genetically heterogeneous disorders.  E pub June 7.  Clin. Genet. 2010.  PubMed ID: 20618352

Resources:

Laurence-Moon Bardet-Biedl Association
Foundation Fighting Blindness