Visual acuities and dark-adapted thresholds of children with Bardet-Biedl syndrome

Inherited causes of combined vision and hearing loss: clinical features and molecular genetics

Combined vision and hearing loss, also known as dual sensory impairment, can occur in several genetic conditions, including ciliopathies such as Usher and Bardet-Biedl syndrome, mitochondrial DNA disorders and systemic diseases, such as CHARGE, Stickler, Waardenburg, Alport and Alstrom syndrome. The retinal phenotype may point to the diagnosis of such disorders. Herein, we aim to provide a comprehensive review of the molecular genetics and clinical features of the most common non-chromosomal inherited disorders to cause dual sensory impairment.

Phenotypic expression of Bardet-Biedl syndrome in patients homozygous for the common M390R mutation in the BBS1 gene

PURPOSE

To characterize the phenotype of Bardet-Biedl syndrome (BBS) patients homozygous for the BBS1 M390R mutation.

METHODS

Three patients [PT1, F, 27 years old (yo) at last examination, 14-year follow-up (F/U) PT2, F, 15-yo PT3, M, 15-yo, both 1-year F/U] underwent eye exams, Goldmann visual fields (GVFs), dark- (DA) and light-adapted (LA) electroretinograms (ERGs), spectral domain optical coherence tomography (SD-OCT) and fundus autofluorescence (FAF). Vision and systemic history were also collected.

RESULTS

All patients had night blindness, hyperopic astigmatism, ptosis or mild blepharospasm, foot polydactyly, 5th finger clinodactyly, history of headaches, and variable, diet-responsive obesity. Two had asthma, PT1 was developmentally delayed, PT2 had Asperger-like symptoms, and PT3 had normal cognition. At age 14, acuity was 20/100 in PT1, who had nystagmus since age 2, 20/40 in PT2 and 20/30 in PT3. By 27yo PT1 progressed to 20/320, by 15 yo PT2 was 20/60 and PT3 remained stable. PT1 had well preserved peripheral GVFs, with minimal progression over 10 years of F/U. PT2 and PT3 presented with ring scotomas and I4e<5°. All patients had severe generalized visual sensitivity depression. ERGs were consistently recordable (also rod ERG in PT3 after 60 min DA), but progressed to non-recordable in PT1. Mixed DA ERGs exhibited electronegativity. In PT3, this was partly due to a bleaching effect during bright-flash DA averaging, partly to ON≫OFF LA response compromise. PT2 and 3 had, on SD-OCTs, generalized macular thinning, normal retinal lamination, and widespread photoreceptor outer/inner segment attenuation except foveally, and multiple rings of abnormal FAF configuring a complex bull's eye-pattern. PT1 had macular atrophy. All patients also had peripapillary nerve fiber layer thickening.

CONCLUSIONS

The observed phenotype matches very closely that reported in patients by Azari et al. (IOVS 2006) and in the Bbs1-M390R knock-in mouse model, and expands it to the characterization of important ERG response characteristics that provide insight in the pathogenesis of retinopathy in these patients. Our findings confirm the consistent pathogenicity of the BBS1 M390R mutation.

Retinal dystrophy in Bardet-Biedl syndrome and related syndromic ciliopathies

Primary cilia are almost ubiquitously expressed in eukaryotic cells where they function as sensors relaying information either from the extracellular environment or between two compartments of the same cell, such as in the photoreceptor cell. In ciliopathies, a continuously growing class of genetic disorders related to ciliary defects, the modified primary cilium of the photoreceptor, also known as the connecting cilium, is frequently defective. Ciliary dysfunction involves disturbances in the trafficking and docking of specific proteins involved in its biogenesis or maintenance. The main well-conserved ciliary process, intraflagellar transport (IFT), is a complex process carried out by multimeric ciliary particles and molecular motors of major importance in the photoreceptor cell. It is defective in a growing number of ciliopathies leading to retinal degeneration. Retinitis pigmentosa related to ciliary dysfunction can be an isolated feature or a part of a syndrome such as Bardet-Biedl syndrome (BBS). Research on ciliopathies and BBS has led to the discovery of several major cellular processes carried out by the primary cilium structure and has highlighted their genetic heterogeneity.

Functional analysis of BBS3 A89V that results in non-syndromic retinal degeneration

Bardet–Biedl syndrome (BBS) is a syndromic form of retinal degeneration. Recently, homozygosity mapping with a consanguineous family with isolated retinitis pigmentosa identified a missense mutation in BBS3, a known BBS gene. The mutation in BBS3 encodes a single amino acid change at position 89 from alanine to valine. Since this amino acid is conserved in a wide range of vertebrates, we utilized the zebrafish model system to functionally characterize the BBS3 A89V mutation. Knockdown of bbs3 in zebrafish alters intracellular transport, a phenotype observed with knockdown of all BBS genes in the zebrafish, as well as visual impairment. Here, we find that BBS3 A89V is sufficient to rescue the transport delays induced by the loss of bbs3, indicating that this mutation does not affect the function of BBS3 as it relates to syndromic disease. BBS3L A89V, however, was unable to rescue vision impairment, highlighting a role for a specific amino acid within BBS3 that is necessary for visual function, but dispensable in other cell types. These data aid in our understanding of why patients with the BBS3 A89V missense mutation only present with isolated retinitis pigmentosa.

Identification and functional analysis of the vision-specific BBS3 (ARL6) long isoform

Bardet-Biedl Syndrome (BBS) is a heterogeneous syndromic form of retinal degeneration. We have identified a novel transcript of a known BBS gene, BBS3 (ARL6), which includes an additional exon. This transcript, BBS3L, is evolutionally conserved and is expressed predominantly in the eye, suggesting a specialized role in vision. Using antisense oligonucleotide knockdown in zebrafish, we previously demonstrated that bbs3 knockdown results in the cardinal features of BBS in zebrafish, including defects to the ciliated Kupffer's Vesicle and delayed retrograde melanosome transport. Unlike bbs3, knockdown of bbs3L does not result in Kupffer's Vesicle or melanosome transport defects, rather its knockdown leads to impaired visual function and mislocalization of the photopigment green cone opsin. Moreover, BBS3L RNA, but not BBS3 RNA, is sufficient to rescue both the vision defect as well as green opsin localization in the zebrafish retina. In order to demonstrate a role for Bbs3L function in the mammalian eye, we generated a Bbs3L-null mouse that presents with disruption of the normal photoreceptor architecture. Bbs3L-null mice lack key features of previously published Bbs-null mice, including obesity. These data demonstrate that the BBS3L transcript is required for proper retinal function and organization.

Retinitis pigmentosa (RP), a disorder of retinal degeneration resulting in blindness, occurs due to mutations in dozens of different genes encoding proteins with highly diverse functions. To date, there are no effective therapies to delay or arrest retinal degeneration. RP places a large burden on affected families and on society as a whole. We have studied a syndromic form of RP known as Bardet-Biedl Syndrome (BBS), which leads to degeneration of the photoreceptor cells and is associated with non-vision abnormalities including obesity, hypertension, diabetes, and congenital abnormalities of the kidney, heart, and limbs. In this study we utilized two model systems, the zebrafish and mouse, to evaluate the function of a specific form of BBS (BBS3). We have identified a novel protein product of the BBS3 gene and demonstrated that functional and structural abnormalities of the eye occur when this form of BBS3 is absent. This finding is of significance because it indicates that BBS3 mutations can lead to non-syndromic blindness, as well as blindness associated with other clinical features. This work also indicates that treatment of BBS3 blindness will require replacement of a specific form of the BBS3 gene.

[Update on Bardet-Biedl syndrome]

Until recently, Bardet-Biedl syndrome was considered as a classic autosomal recessive condition. The disorder is defined by the association of the following clinical features: retinitis pigmentosa, polydactyly, obesity, hypogonadism, and possible mental retardation. This syndrome leads to multiple handicaps (visual impairment, complications of obesity, kidney failure, endocrine dysfunction). This condition, apparently clearly defined from a clinical point of view, appears to be genetically heterogenous. To date, six different genes have been identified: BBS1, BBS2, BBS4, BBS6, BBS7 and BBS8. Interestingly, this condition has recently been linked to a failure of cellular ciliogenesis. Moreover, this disorder is characterized by an additional degree of complexity, as it is the first example of triallelic inheritance described in human beings. However, this new finding appears to be less frequent than expected in this syndrome.

Ocular phenotypes of three genetic variants of Bardet-Biedl syndrome

Bardet-Biedl syndrome is a genetically heterogeneous multisystem disorder that causes severe visual impairment. Retinitis pigmentosa (RP), hypogonadism, digit and renal anomalies, obesity, and a variable degree of mental retardation characterize the disorder. Eight different loci have been identified on 2q31(BBS5), 3p13 (BBS3), 4q27 (BBS7), 11q13 (BBS1), 14q32 (BBS8), 15q22.3 (BBS4), 16q21 (BBS2), and 20p12 (BBS6). The ocular manifestations of Bardet-Biedl syndrome include an early and severe rod-cone dystrophy causing legal blindness in the second decade. Features of systemic phenotypic variability were proposed to distinguish patients mapped to either the BBS2, BBS3, or BBS4 loci but no phenotype-genotype correlation has been established for the ocular phenotype. We studied the three original families used for the identification of BBS2, BBS3, and BBS4 loci to define the ocular phenotypes of patients (n = 34) and obligate carriers (n = 32) using clinical examination and electroretinography (ERG). RP was severe and early in all cases. Myopia was associated with BBS3 and BBS4, but not BBS2. One patient with Bardet-Biedl syndrome also had iris and chorioretinal colobomata, features suggestive of Biemond syndrome.

A review of the literature of Bardet-Biedl disease and report of three cases associated with metabolic syndrome and diagnosed after the age of fifty

Bardet-Biedl syndrome (BBS) is a genetic autosomal-recessive disease (formerly grouped with Laurence-Moon-Biedl syndrome but considered today as a separate entity) characterized by abdominal obesity, mental retardation, dysphormic extremities (syndactyly, brachydactyly or polydactyly), retinal dystrophy or pigmentary retinopathy, hypogonadism or hypogenitalism (limited to male patients) and kidney structural abnormalities or functional impairment. The expression and severity of the various clinical BBS features show inter- and intrafamilial variability. This study focuses on three cases of familial BBS--two sisters and one brother (66, 64 and 51 years of age, respectively)--with the main cardinal findings of the disease plus a classic 'metabolic syndrome' (characterized by abdominal obesity, atherogenic dyslipidaemia, raised blood pressure, insulin resistance with or without glucose intolerance, and prothrombotic risk and proinflammatory states). One female patient (not affected by reproductive dysfunction) had three healthy offspring, while the other two patients were unmarried. Another severely affected brother died at 70 years of age; two other brothers are lean but affected by nephropathy, retinopathy, slight mental retardation, polydactyly, hypertension and thrombotic diseases, and had healthy offspring. BBS is a rather rare but severe syndrome that is often mis- or undiagnosed. Ophthalmologists, endocrinologists and nephrologists should be aware of BBS because of its adverse prognosis--early onset of blindness, associated findings of metabolic syndrome and increased vascular risk, and severe renal impairment (the most frequent cause of reduced survival and death early in life).

Nystagmus mimicking spasmus nutans as the presenting sign of Bardet-Biedl syndrome

PURPOSE

To investigate the nystagmus of twin brothers presenting with spasmus nutans later diagnosed as Bardet-Biedl syndrome.

METHODS

The twins presented at the age of 14 months with a presumed diagnosis of spasmus nutans. They were followed clinically and with quantitative electro-oculographic eye movement recordings until the age of 6 years.

RESULTS

Polydactyly, truncal obesity, mild delay in cognitive development, visual acuity of 20/100, attenuated retinal vessels and pale disks, and bilaterally almost extinguished scotopic and photopic electroretinograms were found in both brothers. They had fine, fast, pendular, disconjugate, intermittent, oblique nystagmus. No head nodding was observed.

CONCLUSION

As described in patients with other retinal diseases such as achromatopsia and congenital stationary night blindness, nystagmus of patients with Bardet-Biedl syndrome can mimic spasmus nutans.

Alström syndrome. Report of 22 cases and literature review

OBJECTIVE

The authors report 22 cases of Alström syndrome (AS), which is the largest series to date. Only 37 cases have been reported in the world literature since 1959. The authors review the clinical features and compare these with the overlapping condition of Bardet-Biedl syndrome. Their aim is to clarify the AS phenotype and to increase awareness of the early features.

DESIGN

A retrospective case series.

PARTICIPANTS

All patients (22) with a diagnosis of AS admitted to the authors' hospital in the past 10 years were included in this review.

INTERVENTION

This is principally a review of ocular features, but other features are recorded and discussed.

MAIN OUTCOME MEASURES

Features noted included age at onset of visual symptoms, presence of photophobia, visual acuity, and electroretinogram findings. Nonocular features recorded included cardiac status, weight and height, hearing, and presence of diabetes mellitus.

RESULTS

Cardiomyopathy presenting in infancy has only been recognized recently to be a feature of AS. Of the authors' cases, 18 of 22 had infantile cardiomyopathy. In the authors' tertiary referral institution, there is an ascertainment bias toward younger patients and especially those with pathology that is other than ocular pathology. In addition, AS is difficult to recognize in childhood without the development of infantile cardiomyopathy. Alström syndrome often is not recognized until diabetes mellitus develops in the second or third decade. Initially, a diagnosis of cone-rod dystrophy, achromatopsia, Leber's congenital amaurosis, or Bardet-Biedl syndrome may be made. In AS, there is a severe infantile retinal dystrophy. The electroretinogram is absent or attenuated with better preserved rod than cone function. The retinal dystrophy is progressive with the patient's visual acuity of 6/60 or less by 10 years of age and no light perception by 20 years of age.

CONCLUSIONS

A diagnosis of AS should be considered in infantile cone and rod retinal dystrophy, particularly if the weight is above the 90th percentile (16 of 18 cases) or if there is an infantile cardiomyopathy (18 of 22 cases).

The ocular phenotype of the Bardet-Biedl syndrome. Comparison to non-syndromic retinitis pigmentosa

PURPOSE

To investigate 20 patients affected with Bardet-Biedl (BB) syndrome and compare them to an age-matched group of 70 non-syndromic patients with retinitis pigmentosa (RP) to identify hallmarks peculiar to the BB phenotype.

METHODS

Patients were examined clinically and with functional tests (color vision, kinetic perimetry, electroretinography, ocular motility tests). Fundus findings were numerically graded for statistical purposes.

RESULTS

Recurrent ocular features in BB patients were early and severe reduction of visual acuity, constantly altered color vision, high incidence of strabismus and nystagmus, mild-to-severe atrophic changes of the optic disc, and frequently absent or minimal pigmentary retinal changes. Visual acuity was more closely correlated to optic disc than to macular conditions. These findings were remarkably different from non-syndromic RP.

CONCLUSIONS

This investigation further suggests that retinopathy in BB syndrome has features distinctive from those in non-syndromic RP. The early occurrence of optic disc atrophy in the BB syndrome, even in those patients with healthy maculas, suggests that optic atrophy could often be primary in nature and might play a major role in decreasing central vision in BB patients. Variability of some findings is in line with the documented heterogeneity of the BB syndrome.

Full-field electroretinograms in individuals with the Laurence-Mood-Bardet-Biedl syndrome

PURPOSE

To evaluate rod and cone function in individuals with the Laurence-Moon-Bardet-Biedl syndrome.

METHODS

We obtained a full-field electroretinograms in 36 patients. If responses less than 10 microV were recorded with single white flashes a special techniques with narrow band filter and computer averaging was used.

RESULTS

No rod responses to dim blue light could be obtained in any of the patients. Residual cone flicker responses were measurable in 28 of the individuals. Those with amplitudes < 0.05 microV were significantly older than those with amplitudes > 1.00 microV. The ERG pattern was consistent within affected pairs of siblings in 8 families.

CONCLUSION

The retinal dystrophy in Laurence-Moon-Bardet-Biedl syndrome is primarily a rod-cone dystrophy, but even cone flicker amplitudes are severely reduced with further progression with age. There is no intrafamilial variability of the electroretinograms in affected siblings.

The importance of renal impairment in the natural history of Bardet-Biedl syndrome

Bardet-Biedl syndrome is a rare autosomal recessive disease characterized by dysphormic extremities, retinal dystrophy, obesity, hypogenitalism in males, and renal structural abnormalities. Because the clinical outcome of these patients is not well known, 21 families with Bardet-Biedl syndrome (BBS) were studied to determine the natural history of the disease. In a prospective cohort study, 38 patients with the syndrome and 58 unaffected siblings were identified. Patients were studied in 1987 and again in 1993. Age of onset of blindness, hypertension, diabetes, renal impairment, and death was determined. The prevalence of obesity, gonadal dysfunction, and renal structural abnormalities was assessed. All but 5 BBS patients (86%) were legally blind, 26% being blind by the age of 13 years and 50% by 18 years. Eighty-eight percent were above the 90th percentile for height and weight. Twenty-five (66%) patients had hypertension, 25% of BBS patients by age 26 years, and 50% by age 34 years, whereas in the unaffected group, 25% had hypertension by age 49 years (P < 0.0001). Twelve (32%) BBS patients developed diabetes mellitus, compared with none of the unaffected group. Only 2 patients were insulin dependent. Twenty-five percent of BBS patients had diabetes by the age of 35 years. In 12 women of reproductive age, 1 (8%) had primary gonadal failure. In 10 men, 4 had primary testicular failure. Nine (25%) patients developed renal impairment, with 25% of the BBS group affected by the age of 48 years. Imaging procedures of the kidney were performed in 25 patients with normal renal function. Whereas fetal lobulation and calyceal cysts/diverticula/clubbing were characteristic, occurring in 96% of patients, 20% (n = 5) had diffuse and 4% (n = 1) focal cortical loss. Eight patients with BBS died, 3 with end-stage renal failure and 3 with chronic renal failure. On life-table analysis, 25% of BBS patients had died by 44 years, whereas at that age 98% of unaffected siblings were still alive (P < 0.0001). Bardet-Biedl syndrome has an adverse prognosis, with early onset of blindness, obesity, hypertension, and diabetes mellitus. Renal impairment is frequent and an important cause of death. Survival is substantially reduced.

Clinical pathology and retinal vascular structure in the Bardet-Biedl syndrome

A comparative study of clinical pathology and retinal vascular structure is described as studied by vascular casting in an eye of a patient with the Bardet-Biedl syndrome. At the time of examination the eye had been almost blind for at least 4 years. The histopathological examination showed a largely uniform loss of the outer retinal layers. The gross pathological examination of the cast ocular fundus showed three distinct zones, an inner zone inside the temporal vascular arcades where retinal vessels had been cast, a mid peripheral zone with bone spicules, and a peripheral zone with neither cast vessels nor bone spicules. The findings are discussed in relation to possible pathophysiological mechanisms involved in the development of retinal dystrophy in the Bardet-Biedl syndrome.

Longitudinal study of the early electroretinographic changes in Alström's syndrome

We obtained serial electroretinograms in four patients aged between 6 months and 5 years with Alström's syndrome and studied the early stages of the severe retinopathy that is characteristic of that disease. The weak electroretinographic signals found at age 6 months demonstrate a severe early cone dysfunction; one year later the cone activity is undetectable. The rod component of the electroretinogram is initially normal but can rapidly deteriorate to become undetectable as early as 5 years of age. These unusual electroretinographic findings are pathognomonic of Alström's syndrome and different from other cone-rod dystrophies or other syndromes with similar phenotypes such as Bardet-Biedl, Laurence-Moon, and Cohen syndromes.

Prevalence of retinitis pigmentosa and allied disorders in Denmark. II. Systemic involvement and age at onset

A population survey of retinitis pigmentosa and allied disorders in the Danish population disclosed 1301 cases prevalent in Denmark per January 1, 1988. Crude prevalence rates and World Standardized prevalences of systemic and non-systemic RP are presented. Sixty-four per cent of all RP-cases were non-systemic. A preponderance of males was encountered in both systemic and non-systemic RP. Usher disease was diagnosed in 12%, Bardet-Biedl syndrome constituted 5%, and the frequency of Spielmeyer-Vogt disease was 1% of all prevalent RP-cases. Other defined syndromes occurred in 1% of the cases. The Danish figures with respect to Usher disease are compared with standardized prevalence rates from Sweden and Finland. Nineteen per cent of patients affected by non-systemic RP had an onset later than 30 years of age. In patients affected by systemic RP the eye disease tended to an earlier debut with only a few per cent beginning after the age of 30 years.

Patterns of rod and cone dysfunction in Bardet-Biedl syndrome

We studied visual function in 16 patients with the Bardet-Biedl syndrome. Visual acuity, kinetic perimetry, and electroretinography results indicated a severe loss of central and peripheral vision and rod and cone function by the second or third decade of life. Light- and dark-adapted static perimetry in patients 10 to 15 years of age with early involvement showed a parallel and marked loss of rod and cone sensitivity across the visual field. Patients with more advanced disease and no measurable peripheral visual field showed different patterns of central visual dysfunction: an island of only cone function centered in a bull's-eye lesion; patches of rod function surrounding geographic atrophy; or a central island of excellent rod sensitivity but severely impaired cones. In the two least-affected patients, a 13-year-old boy and the asymptomatic 45-year-old sibling of a patient, there were more rod than cone abnormalities as determined by electroretinography and static perimetry.