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Bardet-Biedl syndrome: A case report

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Bardet-Biedl syndrome: A case report
Ali Karaman MD
Dermatology Online Journal 14 (1): 9

Department of Genetics, State Hospital, Erzurum, Turkey.

Abstract

Bardet-Biedl syndrome (BBS) is a genetically heterogeneous autosomal recessive disorder characterized by progressive retinal dystrophy, polydactyly, obesity, hypogonadism, mental retardation, and renal dysfunction. Other manifestations include diabetes mellitus, heart disease, hepatic fibrosis, neurological features, and multiple pigmented nevi. To date, twelve BBS genes have been cloned (BBS1-BBS12). Herein we discussed a patient with BBS who had multiple pigmented nevi.



Clinical synopsis

History – Our patient was born in 1987 to non-consanguineous Turkish parents. At term, birth weight was 3500 g, and length was 50 cm. Polydactyly of both hands and syndactyly of the left hand was noted. At the age of 10 years, obesity and myopia (2 diopters) were recorded. The family history was unremarkable.


Figure 1Figure 2

Physical examination revealed the patient to be mildly dysmorphic with a long face, long philtrum, thin upper lip, micrognathia, and short neck (Fig. 1). There were multiple pigmented nevi on the face (Fig. 1). Polydactyly (six fingers) of both hands, partial syndactyly (between fourth and fifth fingers) of the left hand, fifth finger clinodactyly of both hands, brachydactyly of both hands and feet were noted (Fig. 2). Ophthalmic examination revealed characteristic retinitis pigmentosa with low visual acuity (1 to 2/10) and myopia (6 diopters). Ultrasonographic investigations showed hypoplastic fallopian tubes and uterus, and bilateral ovarian cysts. There was no renal disease. She was 150 cm tall and weighed 90 kg. Her IQ was 70. She experienced normal maturation at puberty with the onset of regular menses. Basal gonadotrophins and steroid hormones were normal.

Based on the history and clinical presentation, she was determined to have Bardet-Biedl syndrome.


Comment

The variable manifestations of Bardet-Biedl syndrome (BBS) were initially described by Bardet and Biedl in the 1920s [1]. It is clearly different from a condition reported by Lourence and Moon in 1865. Bardet-Biedl syndrome occurs throughout the world with varying frequencies. Prevalence rates in North America and Europe range from 1:140,000 to 1:160,000 live births. However, in Kuwait and Newfoundland the rate is much higher, with an estimated incidence of 1:13,500 and 1:17,500, respectively, suggesting a founder effect [2].

Bardet-Biedl syndrome is genetically heterogeneous, with 12 BBS genes (BBS1–12) identified to date [3-10]. Although the cellular mechanisms that underlie BBS remain unclear, it is now evident that all of the known BBS proteins are components of the centrosome and/or basal body and have an impact on ciliary transport [11]. The BBS1, BBS2, BBS3, and BBS4 genes contribute to ocular phenotype [5, 6]. The BBS10 gene encodes a vertebrate-specific chaperonin-like protein [7]. The BBS5-9 and BBS11 genes are expressed in adipose tissue [8, 9]. The BBS12 gene is vertebrate specific and, together with the BBS6 and BBS10 genes, defines a novel branch of the type-II chaperonin superfamily [10].

Bardet-Biedl syndrome is characterized by the following associations: early-onset retinal dystrophy, obesity, limb-abnormalities, mental retardation, hypogonadism, and renal disease. Retinal dystrophy (100%) is the first major feature of the disorder. It is found occasionally in the first decade but present in almost all patients by the second decade [12]. The appearance of the fundus does not predict vision and the defect has been described as an atypical pigmentary retinal dystrophy of the photoreceptors with early macular involvement [1, 13].

Obesity is the second major feature of BBS, with a frequency of 72-96 percent depending on measurement criteria. Obesity usually begins in childhood and the severity increases with age, with the majority of cases exhibiting symptoms within the first year of life [14].

Limb-abnormalities are the third major feature of BBS. Limb deformities have been reported at varying frequencies [1, 14]. Of these, post-axial polydactyly, polydactyly, and brachydactyly of both hands and feet are most common. Partial syndactyly, fifth finger clinodactyly, and a prominent gap between the first and second toes are sometimes associated.

Mental retardation is a more disputed feature of BBS. Recently, objective IQ tests determined that only a minority of patients are mentally retarded. An IQ of 79 or below is found in 44 per cent of BBS patients. The decrease in IQ level correlates with the presence of visual handicap [1,14].

Hypogenitalism is reportedly more frequently in BBS males than females [14]. In BBS females, genital abnormalities encompass a wide range, including hypoplastic fallopian tubes, uterus, and ovaries, partial and complete vaginal atresia, absent vaginal orifice, and absent urethral orifice [15,16]. Bardet-Biedl syndrome males have small penis and testes (88%) [1].

Renal dysfunction has been recognized only recently to be a component of the BBS clinical phenotype. Renal malformations in BBS had been reported infrequently, although a high frequency of structural abnormalities were observed postmortem. In one study, 26/57 patients (46%) had renal structural abnormalities. However, only 5 percent had functional impairment at the time of assessment [1,14].

In addition to the major diagnostic features of BBS, multiple minor features have been also documented in patients at varying frequencies. These include developmental delay, speech and language deficit, psychosis, facial dysmorphism, multiple pigmented nevi (ten patients, of whom eight were female, had multiple widespread pigmented nevi), neurological abnormalities, hearing loss, metabolic and endocrine disturbances including diabetes mellitus, cardiovascular anomalies, disturbances of dentition and liver function, atresia ani and Hirschprung disease [1,14]. Consequently, the multiple pigmented nevi found in our patient and other previous studies [14] suggests that this might be considered a secondary feature for diagnosis.

References

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12. Fulton AB, Hansen RM, Glynn RJ. Natural course of visual functions in the Bardet-Biedl syndrome. Arch. Ophthalmo 1993; 111: 1500-1506.

13. Runge P, Calver D, Marshall J, Taylor D. Histopathology of mitochondrial cytopathy and the Laurence-Moon-Biedl syndrome. Br J Ophthalmo1 1986; 70: 782-796.

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15. Mehrotra N, Taub S, Covert RF. Hydrometrocolpos as a neonatal manifestation of the Bardet-Biedl syndrome. Am J Med Genet 1997;69: 220.

16. Stoler JM, Herrin JT, Holmes LB. Genital abnormalities in females with Bardet-Biedl syndrome. Am J Med Genet 1995;55: 276-278.

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