Review of the phenotypes and genotypes of Bardet-Biedl syndrome from China

Objective: To analyze the phenotypes, genotypes, and the relationship of phenotypes and genotypes for Chinese patients with Bardet-Biedl syndrome (BBS). Methods: The Chinese Wanfang and Weipu data, and PubMed were searched up to December 2022. Patients with detailed clinical feature data were involved in the analysis. Results: A total of 153 Chinese patients, including 87 males, 53 females, and 12 unknown, were enrolled. Their ages ranged from 1.2 to 44 years old with a mean of 16.70 ± 9.90 years old. Among these patients, 80 (52.29%) were reported by ophthalmologists, and only 24 (15.68%) reported by pediatricians. Most patients (132/137, 96.35%) had visual problems; 131/153 (85.62%) had polydactyly; 124/132 (93.93%) were overweight or obese; 63/114 (55.26%) had renal abnormalities; kidney dysfunction was found in 33 (21.57%); 83/104 (79.81%) had hypogonadism and/or genital hypoplasia; and 111/136 (81.62%) had mental retardation. In this series, genetic analysis was performed in 90 (58.82%) patients, including 22 BBS7 (24.71%), 20 BBS2 (22.73%), and 10 BBS10 (11.24%) patients. Moreover, 11 fetuses were diagnosed prenatally in the last 4 years except for one patient in 2004 year. It was noted that BBS7 had higher penetrance. BBS2 had higher hearing impairment and lower renal abnormality penetrance. BBS10 also had lower renal abnormality penetrance as well. Conclusion: Misdiagnosis or miss diagnosis of BBS may be common in China. In patients with polydactyly, visual impairment, obesity, renal abnormalities, hypogonadism, and mental retardation, or in fetuses with polydactyly and/or renal abnormalities, BBS should be considered in the differential diagnosis. Other deformities should be evaluated carefully and genetic analysis should be performed as early as possible.

BBS is a ciliopathy and has been shown to be closely related to dysfunction of immotile cilia.To date, 28 genes have been reported to be associated with the BBS phenotypes, including 2 candidate genes (SCLT1 and SCAPER) and 2 contributors (NPHP1 and TTC21B) (Niederlova et al., 2019;Khan et al., 2023;Gnanasekaran et al., 2023).In comparison, pathogenic variants in the BBS genes involved in encoding the BBSome complex, including BBS1 (OMIM 209901), BBS2 (OMIM 606151), BBS4 (OMIM 600374), BBS5 (OMIM 603650), BBS7 (OMIM 607590), BBS8/TTC8 (OMIM 615985), and BBS9/PTHB1 (OMIM 607968), were the most common, followed by BBS genes involved in encoding BBSome complex "chaperone-like" proteins, including BBS6/MKKS (OMIM 604896), BBS10 (OMIM 610148), and BBS12 (OMIM 610683) (Seongjin et al., 2009;Dai et al., 2022;Melluso et al., 2023).BBS proteins are needed for the maintenance of ciliary structure and function.The BBSome complex is an essential component of cilia differentiation.It mediates the transport of proteins to the membrane structure of cilia and participates in their structure formation and function (Seongjin et al., 2009).BBSome complex chaperone-like proteins are involved in the regulation of the BBSome complex, which together with members of the Rab family of proteins to promote intraflagellar transport (Yan and Shen, 2022;Melluso et al., 2023).Molecular diagnosis can be clarified by genetic testing in about 80% of patients.Most studies reported that BBS1 and BBS10 are the most frequently implicated genes (Mujahid et al., 2018).However, still about 20% of patients currently do not have a definitive molecular diagnosis currently (Forsythe et al., 2018), presenting a significant challenge for diagnosis and genetic counseling.
We aim to highlight the genotype-phenotype relationship and the diagnosis of BBS by summering the BBS patients reported from China.Studies in Chinese and English from China were all reviewed.

Data collection
According to the PRISMA guidelines, we reviewed the Wanfang and Weipu data in Chinese using "Bardet-Biedl syndrome", "Laurence-Moon-Biedl syndrome", "Laurence-Moon-Bardet-Biedl syndrome", or "Polydactyly-Obesity-Renal-Ocular syndrome" and in Chinese or English, also above keyword and "China or Chinese" in PubMed up to December 2022.Only patients with detailed clinic feature data were involved in the following analysis.

Diagnostic criteria
In children, overweight and obesity were defined according to "Body mass index growth curves for Chinese children and adolescents aged 0-18 years" (Li et al., 2009a).In adults, overweight was defined as a BMI between 24.0 and 27.99 kg/m 2 while obesity was defined as a BMI ≥28.00 kg/m 2 .
In children, hypertension was defined according to the "updating blood pressure references for Chinese children aged 3-17 years" (Fan et al., 2017).In adults, hypertension was defined as a diastolic blood pressure >90 mmHg and/or a systolic blood pressure >140 mmHg.
Diabetes was defined as fasting glucose ≥7.0 mmol/L and/or random blood glucose ≥11.1 mmol/L.
Short stature was defined according to the "height and weight standardized growth chats for Chinese children and adolescents aged 0-18 years" (Li et al., 2009b).

Statistical analysis
Statistical analyses were conducted using SPSS software (version 22).The Pearson's chi-square test and Fisher's exact test were used to measure enumeration data between subgroups.Quantitative data with a normal distribution were expressed as the means ± SDs and analyzed by the independent t-test.Quantitative data with skewed distributions were expressed as medians (minimums-maximums).Differences were considered statistically significant at p < 0.05.
In this series, 11 fetuses were diagnosed prenatally from 2019 to 2022 years (Li et al., 2019;Qiang et al., 2019;Li et al., 2020;Jing et al., 2021;Cai et al., 2022;Dong et al., 2022;Yan et al., 2022) except for one patient reported in 2004 (Hou, 2004).All fetuses presented abnormal kidney images and/or bilateral enlarged hyperechogenic kidneys that implied renal cysts, and one presented hydrometrocolpos and postaxial polydactyly with vaginal atresia (Hou, 2004).Oligohydramnios was noted in one, and ventricular defect combined with a single atrium, persistent left superior vena cava and ascites were noted in one patient.

Genotype-phenotype relationship
We compared the phenotypes among BBS2, BBS7, and BBS10 patients.It was noted that BBS7 had higher penetrance.BBS2 had higher hearing impairment and lower renal abnormality penetrance.

FIGURE 1
The genotypes of BBS in the current series (number).Moreover, BBS2 had lower mental retardation and polydactyly penetrance with marginal differneces.BBS10 had higher penetrance except for the lower renal abnormality as well (Table 3).

Discussion
BBS is a main cause of syndromic forms of obesity although it is a rare disease.According to the prevalence (about 1/125,000~1/ 160,000) reported (Moore et al., 2005;Tsang et al., 2018), these are currently about 10,000 patients in China (about 1.4 billion people) now.However, fewer than 150 patients have been reported to date.Moreover, most cases were reported from the Eastern region.This finding implied that misdiagnosis or miss diagnosis of BBS was common in China, especially in the Midwest region, although it may also be because of the non-register system of BBS in China.
Although it was an autosomal recessive disease, we noted that BBS was predominantly male with the male to female ratio of 1.64:1 (87/53).This result was similar to that in most other studies by Beales et al. (1999) (1.3:1) and Klein and Ammann (1969) (1.4:1), but the reason is still unclear.Whether this is associated with the fact that dysplasia of the vagina, uterus, and ovaries may be more apt to be ignored than cryptorchidism and micropenis needs further investigation.Although the age at diagnosis may be younger than those in other studies from Canada in 2005 (Moore et al., 2005) and the United Kingdom in 2018 (Mujahid et al., 2018), the mean age of these Chinese patients was 16.7 years old.This implies that the delayed diagnosis is still common.It was also notable that while only 15.68% of patients were reported by pediatricians, most (52.29%) were reported by ophthalmologists.In fact, polydactyly, a main feature and early clue of BBS, was noted in 85.62% of patients in this series.Hence, in addition to visual impairment, obesity, and genitourinary system abnormalities, BBS should be considered in infants (even fetuses) with polydactyly by pediatricians and pediatric surgeons.
The phenotype of BBS is heterogeneous.In our Chinese series, the incidences of visual impairment, polydactyly, obesity, genital anomalies, renal anomalies, and mental retardation were higher than those in most previous reports, although the age ass not older than those in previous reports (Beales et al., 1999;Moore et al., 2005;Mujahid et al., 2018).This implies that some atypical patients with few main clinical features may be misdiagnosis in China.It was notable that the incidence of some non-main clinical features (e.g., hearing loss, dental anomalies, anosmia/hyposmia, hydrometrocolpos, vaginal dysplasia, polycystic ovary syndrome in females) and complications (e.g., hypertriglyceridemia, hyperglycemia, diabetes millitus, and hypertension) were significantly lower than those in previous reports (Beales et al., 1999;Moore et al., 2005;Mujahid et al., 2018).These results suggest that careful evaluate of various deformities and congratulations on defects is needed for Chinese patients.
To date, 28 genes have been reported to be associated with BBS phenotypes (Niederlova et al., 2019;Gnanasekaran et al., 2023;Khan et al., 2023).Unlike Caucasian patients with higher proportions of BBS1 and BBS10 (Niederlova et al., 2019;Melluso et al., 2023), we noted that BBS7 was the prominent genotype, followed by BBS2, BBS10, BBS12, and BBS1 in these Chinese seies.This difference may imply the different genotype prominence in different geographic areas.This may also be associated with the fact that poor clinical diagnosis as BBS1 patients tend to have a milder pattern of disease (Niederlova et al., 2019;Guardiola et al., 2021).The analysis for the relationship between genotype and phenotype is difficult, as this rare disease.The BBSome chaperone-like protein is involved in the early synthesis of BBSome, so patients with BBS6/MKKS, BBS10, and BBS12 have more severe symptoms, especially BBS10 (Forsythe et al., 2017;Niederlova et al., 2019).Moreover, patients with BBS2 were reported to have severe symptoms (Florea et al., 2021).The proportion of renal abnormalities in patients with BBS7 types is relatively high (>60%) and relatively lower in BBS2 and BBS10 types (Niederlova et al., 2019;Florea et al., 2021).Patients with BBS2 and BBS4 types have a higher proportion of polydactyly (Niederlova et al., 2019), and BBS10 and BBS12 types are apt to obese (Forsythe and Beales, 2013;Dai et al., 2022;Melluso et al., 2023).We noted that BBS7 had higher penetrance.BBS2 had higher hearing impairment and lower renal abnormality penetrance.BBS10 had higher penetrance except for the lower enal abnormality.It was notable that hearing impairment was reported in BBS2, but not in BBS7 and BBS10.The relationship between genotype and phenotype must be observed with a larger sample size.
The diagnosis of BBS is some difficult due to the heterogeneity of phenotypes.Beales et al. (1999) summarized and amended that BBS clinical diagnosis should meet 4 major symptoms, or 3 major symptoms and 2 secondary symptoms.Fortunately, genetic diagnosis has been improved in recent years in China, and 89 patients had comfirmed diagnosis and genotyping by sequencing in recent years.Unlike reports from the United States (Guardiola et al., 2021), the variant sites are more dispersed in our Chinese series.As most patients have point variants, sequencing, but not karyotype and array comparative genomic hybridization (aCGH), is suggested as the first line genetic analysis.Moreover, we also used next-generation sequencing for newborn screening in China, which may improve the early diagnosis for some rare genetic diseases (Tong et al., 2022).
The diagnosis of BBS based on prenatal findings is still some difficult, as it cannot identify visual impairment, learning difficulties, or obesity in utero.Fortunately, prenatal diagnosis war comfirmed by sequencing for 10 fetuses after renal anomalies and/or polydactyly found by ultrasound in the past 4 years.Hence, in fetuses with genitourinary abnormalities, polydactyly, and/or hydrometrocolpos, BBS should be considered.Further careful evaluation for various deformities and genomic sequencing should be suggested.
Genetic counseling for the family of the proband is needed.As BBS is an autosomal recessive disease, consanguineous marriage should be avoided.The consanguinity rate in our series was 17.65%, which was lower than that in other studies by Beales et al. (1999) (39%) and Klein and Ammann (1969) (48%), and may be associated with more dispersed variant sites in our series.However, this reflected the lack of awareness of the dangers of consanguineous marriage in these Chinese families.
There is still no specific treatment for BBS.Congenital structural abnormalities (e.g., digestive tract abnormalities), obesity and metabolic syndrome, chronic kidney disease, and retinitis pigmentosa are the main influencing factors for the quality of life and longevity of BBS patients.Congenital structural abnormality correction, diet and lifestyle interventions to prevent obesity and metabolic syndrome, relieve chronic kidney disease, and slow down the retinitis pigmentosa progression are important for BBS patients.Drugs (e.g., setmelanotide) for the obesity (Tauber, 2022;Forsythe et al., 2023), and gene therapy for retinitis pigmentosa are in clinical trails (Xie et al., 2021b;Hsu et al., 2023).There were several limitations.First, some patients were reported having "Laurence-Moon-Biedl syndrome" or "Laurence-Moon-Bardet-Biedl syndrome".It may not be accurate to exclude some patients without genetic analysis data.Second, as a review analysis, some clinical data of some patients were not provided.Moreover, genetic analysis was not performed in all patients, and most patients reported before 2017 years.Hence, the genotypephenotype relationship analysis may not be accurate.
In summary, misdiagnosis or miss diagnosis of BBS may be common in China.In patients with polydactyly, visual impairment, obesity, renal abnormalities, hypogonadism, and mental retardation, or in fetuses with polydactyly and/or renal abnormalities, BBS should be considered in the differential diagnosis.Other deformities should be evaluated carefully and genetic analysis should be performed as early as possible.

TABLE 1
Clinical features in the current series and previous reports (number, %).
a One died after18 months.bRaised fasting plasma glucose.c

TABLE 2
The genotypes in the current series and several previous reports (number, %).
a Only BBS1 to BBS8/TTC8 were analyzed.
b One fetus.c One patient with renal calculus.d One patient with central precocious puberty.