Bardet–Biedl Syndrome

Bardet–Biedl syndrome (BBS) is primarily characterized by early-onset retinal dystrophy that is typically a rod–cone dystrophy but may present initially as a maculopathy. Other features include truncal obesity, postaxial polydactyly (▶ Fig. 19.1), renal abnormalities, hypogonadism, and learning difficulties. Birth weight is typically normal in these patients, but weight gain starts within the first year. Most patients do not present with all findings. BBS is considered a clinically and genetically heterogeneous ciliopathy, with significant intra- and interfamilial phenotypic variability. Prevalence ranges from 1:100,000 to 1:160,000, although in populations with known consanguinity, frequency may be as high as 1:13,000.

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Fig. 19.1 Polydactyly in a patient with Bardet–Biedl syndrome.

Retinal involvement occurs in more than 90% of patients with BBS. Most patients present a rod–cone pattern, but sometimes have a cone–rod pattern. Initial examination may be normal, but rarely maculopathy can be seen in late childhood. Most individuals with BBS present with nyctalopia and abnormal visual fields by the age of 10 years. At the end of the second decade of life, they usually present with a central island of vision. After that, macular involvement progresses, and visual acuity falls into legal blindness, which affects up to 75% of patients with BBS. Other ophthalmic findings may include nystagmus, strabismus, refractive errors, and cataract.

Renal malformations and renal dysfunction that lead to end-stage renal disease are seen in approximately 50 to 80% of affected patients. Up to 45% of individuals with BBS have structural renal abnormalities, including cysts, scarring, agenesis, or dysplasia.

Many patients with BBS are delayed in reaching developmental milestones including speech delay, motor skills, and psychosocial skills. Approximately one-third of children with BBS also show behavioral or psychiatric disorders. Some patients may have ataxia, mild hypertonia, or anosmia. A minority of patients have severe intellectual disability. Structural brain anomalies include ventriculomegaly, cortical thinning, reduced size of the corpus striatum, and hippocampal dysgenesis. Up to 50% of individuals develop a subclinical sensorineural hearing loss.

Polydactyly is seen in up to 80% of cases. Other findings, such as brachydactyly, partial syndactyly, and clinodactyly can be found. Up to half of patients have cardiac anomalies including most commonly valvular stenosis or atrial/ventricular septal defects. BBS patients commonly show hypertension and hyperlipidemia. Liver involvement includes fibrosis, small bile ducts, biliary cirrhosis, portal hypertension, and congenital cystic dilatation of both the intrahepatic and extrahepatic biliary tract. Non–insulin-dependent diabetes mellitus is usually seen in adolescence or early adulthood. Dental crowding, hypodontia, and high-arched palate have also been described. Craniofacial defects include brachycephaly, macrocephaly, narrow forehead, frontal balding, large ears, short and narrow palpebral fissures, a long smooth philtrum, depressed nasal bridge, short nose, midface retrusion, and retrognathia.

19.2 Molecular Genetics

BBS is typically an autosomal recessive (AR) condition. At least 20 genetic subtypes have been described (▶ Table 19.1). BBS1, BBS10, and BBS8 account for most cases (~23, 20, and 8%, respectively). BBS6, BBS9, BBS12, and BBS13 each represent 5%. Other subtypes are less frequent. Although there are no clear genotype–phenotype correlations, patients with BBS1 seem to have less severe ophthalmologic involvement than those with other genotypes (▶ Table 19.1).

Table 19.1 Genes associated with Bardet–Biedl syndrome

Subtype

Gene (locus)

BBS1

BBS1 (11q13)

BBS2

BBS2 (16q13)

BBS3

ARL6 (3q11)

BBS4

BBS4 (15q22)

BBS5

BBS5 (2q31)

BBS6

MKKS (20p12)

BBS7

BBS7 (4q27)

BBS8

TTC8 (14q32)

BBS9

BBS9 (7p14)

BBS10

BBS10 (12q)

BBS11

TRIM32 (9q33)

BBS12

BBS12 (4q27)

BBS13

MKS1 (17q23)

BBS14

CEP290 (12q21)

BBS15

C2ORF86 (2p15)

BBS16

SDCCAG8 (1q43)

BBS17

LZTFL1 (3p21)

BBS18

BBIP1 (10q25)

BBS19

IFT27 (22q12)

BBS20

IFT172 (2p23)

BBS21

C8orf37 (8q22)

CCDC28B, MKS1, MKS3, and C2orf86 genes can modify the expression of BBS phenotypes in individuals who have pathogenic variants in other BBS genes. There are reports of unaffected individuals who carry two heterozygous mutations in the same BBS gene, which supports the notion that BBS can be more complex molecularly, requiring three mutant alleles to manifest the phenotype. This is called triallelic inheritance.

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Apr 7, 2019 | Posted by in OPHTHALMOLOGY | Comments Off on Bardet–Biedl Syndrome

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