Background. Bardet–Biedl syndrome (BBS) is a rare multisystem developmental disorder. In this study, we report the genetic causes and clinical manifestations in two Chinese families with BBS. Materials and Methods. Two families were recruited in this study. Family A was a four-generation family with four affected and 15 unaffected members participating in the study, and family B was a consanguineous family with one affected and three unaffected members participating. Whole exome sequencing was performed in the two families, followed by a multistep bioinformatics analysis. Sanger sequencing was used to verify the variants and to perform a segregation analysis. Comprehensive ocular and systemic examinations were also conducted. Results. Novel compound heterozygous variants c.235T > G (p.T79P) and c.534 + 1G > T were detected in the BBS2 gene in family A, and known homozygous variant c.748G > A (p.G250R) was detected in the MKKS gene in family B. Both families presented with retinitis pigmentosa; however, except for polydactyly, all other systemic manifestations were different. All of the affected family members in family A were overweight with a high body mass index (range from 26.5 to 41.9) and high blood pressure. Family A also presented with a delay in the onset of secondary sex characteristics and genital anomalies, while other systemic abnormalities were absent in family B. Conclusions. This study presents one family with two novel BBS2 variants, expanding the variant spectrum of BBS, and one family with a known homozygous MKKS variant. The different phenotypes seen between the families with BBS2 and MKKS variants will contribute to the literature and our overall understanding of BBS.
Bardet–Biedl syndrome (BBS; MIM: 209900) is a rare multisystem developmental disorder with a prevalence of 1 : 13,500 to 1 : 160,000 [[
According to the Retinal Information Network (RetNet; https://sph.uth.edu/retnet/sum-dis.htm),a total of 26 genes, including ADIPOR1 (MIM: 607945) [[
In this study, we report on two Chinese families with BBS, including their gene variants and clinical manifestations.
Two families were recruited from the Zhongshan Ophthalmic Center (Guangzhou, China). Nineteen participants, including four affected and 15 unaffected, were included from four-generation family A. Four participants, including one affected and three unaffected, were recruited from consanguineous family B. Written informed consent was obtained from each participating individual or a guardian prior to the study. This study was approved by the Institutional Review Board of the Zhongshan Ophthalmic Center. Genomic DNA was prepared from venous leukocytes for all 23 family members using a previously described method [[
Whole exome sequencing (WES) was performed in six family members in family A: III : 4, III : 5, IV : 1, IV : 5, IV : 8, and IV : 10 and one family member in family B: IV : 2. The WES was performed using the Illumina MiSeq platform (Illumina, Madison, WI, USA) and average sequencing depth was set to 100-fold. Strand NGS software version 2.0 (Strand Scientific Intelligence Inc., LA, USA) was used to set the sequencing reads to University of California Santa Cruz hg19. The Human Gene Mutation Database (HGMD;
Detailed clinical data was collected, including age, gender, height, weight, blood pressure, and heart rate. Visual acuity was measured with a Snellen visual chart. The eyes were examined with a hand-held slit lamp. Fundus photography was taken with a hand-held nonmydriatic digital fundus camera (Optomed Oy, Oulu, Finland). The hearing test was performed using a hand-held device (Interacoustics, Middelfart, Denmark). Ultrasound (GE Healthcare, Chicago, USA) was performed on the heart, kidney, and genitals. Full field electroretinogram (RETIport, Roland Consult, Brandenburg, Germany) was performed in the probands of families A and B.
WES was performed in six family members (III : 4, III : 5, IV : 1, IV : 5, IV : 8, and IV : 10) of family A. Bioinformatics analysis revealed novel compound heterozygous variants c.235T > G (p.T79P) and c.534 + 1G > T in the BBS2 gene in family A. Sanger sequencing was performed in all of the available family members, which confirmed that all the affected members had the compound heterozygous variants and the unaffected members had no more than one of the two variants (Figure 1). Variant c.235T > G (p.T79P) was transmitted from the father (III : 4), and variant c.534 + 1G > T was transmitted from the mother (III : 5). Variant c.235T > G causes a substitution of residual 79 in BBS2 from threonine (Thr) to proline (Pro). Thr79 is a conserved residue among vertebrates (Figure 2(a)). Hydrophilic Thr changes to hydrophobic Pro, causing the protein structure to change (Figure 2(b)). The c.534 + 1G > T variant was predicted to change the splice site by HSF. WES was performed in IV:2 of family B, and a known homozygous variant, the c.748G > A (p.G250R) variant in the MKKS gene, was identified [[
Graph: Figure 1 The pedigree and DNA sequencing of family (a) A. The pedigree of family A: M1 stands for variant c.235T > G (p.T79P) in the BBS2 gene, and M2 stands for variant c.534 + 1G > T in the BBS2 gene. (b) The Sanger sequencing of this family. The left column is the sequencing of variant c.235T > G, and the right column is the sequencing of variant c.534 + 1G > T. The arrows indicate where the variant is.
Graph: (b)
Graph: Figure 2 Conservation analysis and protein structure changes with variant p.T79P in the BBS2 gene. (a) The threonine located at position 79 is conserved residue among vertebrates. (b) The hydrophilic threonine was substituted to hydrophilic proline, causing the protein structure change.
Graph: (b)
Graph: Figure 3 The pedigree and DNA sequencing of family B (a). The pedigree of family B.(b) III : 1 and III : 2 had a heterozygous c.748G > A variant in the MKKS gene, IV : 1 had none of the mutant allele, and proband IV : 2 had a homozygous c.748G > A variant.
Graph: (b)
In family A, IV : 10 was the proband whose poor vision, night blindness, and seeking of medical help brought the entire family to the hospital. After an examination of all of the family members, it was found that all four of those affected by BBS had night blindness, poor visual acuity, and horizontal nystagmus. No light perception was detected in IV : 1 and IV : 2, and finger counting at 1.5 meters for the right eye and at 2.0 meters for the left eye was detected in IV : 3 (because of poor vision and illiteracy, she could not read the visual acuity chart). The electroretinogram of IV : 10 showed undistinguishable photopic and scotopic responses. The ocular findings of this family are summarized in Table 1. The fundus examination showed vascular attenuation as well as disk pallor (Figure 4). Considering the symptoms and ocular examinations, a diagnosis of retinitis pigmentosa was given to this family.
Table 1 Clinical features of families with BBS.
Family ID Gene DNA change G Age Eye UCVA RCD Other P GA RA Obesity BP HR SD DA BS AP AH HD (yrs.) NB OD OS H (cm) W (Kg) BMI (mm Hg) A II : 4 BBS2 c.[=]; [=] M 67 N 6/6 6/6 N N N N N NA NA NA NA NA N N N N N N A II : 5 BBS2 c.[=]; [534 + 1G > T] F 60 N NA NA N N N N N 162 61.9 23.6 123/69 92 N N N N N N A II : 3 BBS2 c.[=]; [=] F 62 N 6/6 6/7.5 N N N N N 166 85.2 30.9 139/83 73 N N N N N N A III : 2 BBS2 c.[235T > G]; [=] M 57 N 6/4.8 6/4.8 N N N N N 170 70.0 24.2 129/84 72 N N N N N N A III : 4 BBS2 c.[235T > G]; [=] M 43 N 6/4.8 6/4.8 N N N N N 170 76.4 26.4 124/78 93 N N N N N N A III : 6† BBS2 c.[=]; [=] F 42 N 6/60 6/60 N HM N N N 162 64.5 24.6 106/70 71 N N N N N N A III : 5‡ BBS2 c.[=]; [534 + 1G > T] F 39 N 6/60 6/60 N HM N N N 158 69.7 27.9 120/81 81 N N N N N N A III : 3 BBS2 c.[=]; [=] F 51 N 6/4.8 6/4.8 N N N N N 156 40.6 16.7 114/71 95 N N N N N N A IV : 1 BBS2 c.[235T > F 20 Y NLP NLP Y Nys Y Y N 160 71.7 28.0 132/90 95 N N Y Y Y N A IV : 2 BBS2 c.[235T > G]; [534 + 1G > F 18 Y NLP NLP Y Nys Y Y N 157 103.4 41.9 135/95 119 N N Y Y NA N A IV : 3 BBS2 c.[235T > G]; [534 + 1G > T] F 16 Y FC/1.5m FC/2m Y Nys Y Y N 167 84.1 30.2 184/114 94 N N Y Y Y Y A IV : 5 BBS2 c.[=]; [534 + 1G > T] M 11 N 6/4.8 6/6 N N N N N 144 34.1 16.4 103/73 91 N N N N N N A IV : 6 BBS2 c.[=]; [534 + 1G > T] F 9 N 6/3.8 6/6 N N N N N 129 26.4 15.9 110/68 86 N N N N N N A IV : 7 BBS2 c.[235T > G]; [=] F 7 N 6/4.8 6/4.8 N N N N N 113 20.4 16.0 NA NA N N N N N N A IV : 8 BBS2 c.[=]; [534 + 1G > T] F 5 N 6/7.5 6/9.5 N N N N N 108 18.4 15.8 NA NA N Y N N N N A IV : 9 BBS2 c.[=]; [=] F 3 N NA NA N NA N N N 90 13.4 16.5 NA NA N N N N N N A IV : 10 BBS2 c.[235T > G]; [534 + 1G > M 2 Y NA NA Y Nys Y Y N 82 17.8 26.5 NA NA Y N Y NA NA NA A IV : 11 BBS2 c.[235T > G]; [=] M 25 N 6/4.8 6/6 N N N N N 173 66.3 22.2 122/82 65 N N N N N N A IV : 12 BBS2 c.[235T > G]; [=] F 23 N 6/6 6/6 N N N N N 157 40.2 16.3 107/75 89 N N N N N N B IV : 2 MKKS c.[748G > A]; [748G > A] F 19 Y FC/5cm HM/20cm Y Nys Y N N 155 57.5 23.9 111/63 80 N N N N N N
1 G, gender; NB, night blindness; UCVA, uncorrected visual acuity; BP, blood pressure; HR, heart rate; BMI, body mass index; Y, yes; N, No; NA, not available; RCD, rod-cone dystrophy; † and ‡: the III:6 and III:5 are with high myopia, the visual acuity was not corrected by spectacles; M, male; F, female; No, normal; Age, age at examination; OD, right eye; OS, left eye; HM, high myopia; NLP, no light perception; Nys, nystagmus; P, polydactyly; GA, genital anomalies; RA, renal anomalies; H, height; W, weight; SD, speech delay; DM, diabetes mellitus; DA, dental anomalies; BS, brachydactyly/ syndactyly; AP, ataxia/poor coordination; AH, anosmia/hyposmia; HD; hearing defect. Yrs., years old.
Graph: Figure 4 Fundus photos of family A and family B (a). Fundus photo of IV : 2 in family A (affected) showing vascular attenuation, retinal pigment epithelium (PRE) disturbance, and disk pallor. (b) Normal fundus photo of III : 4 in family A (unaffected). (c) Fundus photo of IV : 2 in family B showing vascular attenuation, disc pallor, and retinal pigment epithelium disturbance. (d) Normal fundus photo of IV : 1 in family B.
Graph: (b)
Graph: (c)
Graph: (d)
In family B, the proband IV : 2 had decreased vision with night blindness in both eyes since early childhood. Her visual acuity was finger counting at 5 centimeters for the right eye and hand movement at 20 centimeters with horizontal nystagmus. The fundus examination revealed vascular attenuation as well as disc pallor and retinal pigment epithelium changes (Figure 4). The electroretinogram showed dramatically reduced photopic and scotopic responses, confirming the diagnosis of retinitis pigmentosa.
All of the detailed clinical data are summarized in Table 1. In family A, all the affected participants were overweight with a body mass index (BMI) ranging from 26.5 to 41.9, while the BMI ranged from 15.8 to 16.5 in the unaffected siblings. The blood pressure of III : 1, III : 2, and III : 3 was 132/90 mmHg, 135/95 mmHg, and 184/114 mmHg, respectively (Table 1). All of those affected had polydactyly (six fingers and six toes bilaterally) but were without speech delay and dental anomalies (Figure 4). The extra fingers were surgically removed at birth (Figure 5). IV : 1, IV : 2, and IV : 3 had delays in the onset of secondary sex characteristics and menarche, and genital anomalies were detected by B scan in IV : 1, IV : 2, and IV : 3 with small-sized ovaries and uteruses. IV : 10 had micropenis detected at birth and small-volume testes detected by B scan. Meanwhile, IV : 1, IV : 2, and IV : 3 had poor coordination, IV : 1 and IV : 3 had hyposmia, and IV : 3 had a hearing defect (Table 1). In family B, the proband had polydactyly with a blood pressure of 113/63 mmHg and BMI of 23.9. A blood test revealed normal glucose. B scans of the cardiovascular, urinary, and reproductive systems showed normal function. However, B scan showed the gallbladder polyps and thyroid nodules, and blood test detected high total cholesterol, high low-density lipoprotein cholesterol, high apolipoprotein B, and high uric acid in the proband of family B.
Graph: Figure 5 The hands and toes of affected and unaffected participants from family A and family B (a). Hands of IV :2 in family B showing polydactyly. (b) Hand of IV : 2 in family A with surgically treated extra finger. (c) Brachydactyly and polydactyly (six toes) of IV : 2 in family A (d). Brachydactyly and polydactyly (six toes) of IV : 10 in family A (e). (f) Normal hands and toes of IV : 6 in family A.
Graph: (b)
Graph: (c)
Graph: (d)
Graph: (e)
Graph: (f)
In this study, we report one novel compound heterozygous variant in the BBS2 gene in a four-generation Chinese family and a known homozygous variant in the MKKS gene as well as on the comprehensive ocular manifestations and systemic features.
The primary features of BBS include rod-cone dystrophy, polydactyly, obesity, genital anomalies, renal anomalies, and learning difficulties, and the secondary features include speech delay, developmental delay, diabetes mellitus, dental anomalies, congenital heart disease, brachydactyly or syndactyly, ataxia or poor coordination, and anosmia or hyposmia. However, phenotypes vary according to the different causative genes. In family A, the patient with the compound heterozygous BBS2 variants had all the primary features except for the renal anomalies, which was confirmed by the B scan. Among the secondary features, the patient had developmental delay, brachydactyly, poor coordination, and anosmia. The clinical features within the family were exactly the same. However, the patient with the homozygous MKKS variants presented ocular abnormalities with polydactyly and without other systemic features. The phenotype variations between genes have been illustrated in previous studies. Carmi et al. illustrated differences in the limb distribution of postaxial polydactyly and the extent and age-association of BMI among patients mapped to loci BBS2, BBS3, and BBS4 [[
Referred to the diagnostic criteria of BBS, clinical diagnosis is made by the presence of either four major features or three major features and two minor features [[
Several studies have reported on the triallelic inheritance [[
This study presented one family with two novel BBS2 variants, expanding the variant spectrum of BBS, and one family with a known homozygous MKKS variant. The phenotypic similarity in family A with the BBS2 variant, and the phenotypic difference between family B and the family in the previous study with the same MKKS variant, will contribute to improved understanding of BBS.
The data that support the findings of this study are available from the corresponding author (Xiaoyan Ding) upon reasonable request.
The authors alone are responsible for the content and writing of this article.
The authors report no conflicts of interest.
The authors are grateful to the patients for their participation. This work was supported by research funds of Sun Yat-Sen University (Grant no. 15ykjxc22d), Science and Technology Program Guangzhou, China (Grant no. 201803010031), National Natural Science Foundation of China (Grant no. 81700879), Natural Science Foundation of Guangdong Province, China (Grant no. 2017A030310275), and Medical Scientific Research Foundation of Guangdong Province (Grant no. 2016118152947764).
By Li Huang; Limei Sun; Zhirong Wang; Songshan Li; Chonglin Chen; Xiaoling Luo and Xiaoyan Ding
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