Prenatal diagnosis of Bardet-Biedl syndrome: a multidisciplinary approach

Abstract

Bardet-Biedl syndrome (BBS) is a rare ciliopathic human genetic disorder with mainly an autosomal recessive inheritance. BBS phenotype develops over the years and diagnosis is usually made in late childhood or early adulthood. Prenatal diagnosis is rare in the absence of family history or consanguinity. We present a prenatal case without a family history of inherited diseases or consanguinity. Mid-trimester ultrasound revealed hyperechogenic kidneys and postaxial polydactyly putting us on track of BBS. The fetopathology supported this diagnosis and the whole-exome sequencing confirmed the hypothesis. Our case illustrates how high-resolution obstetric scan, detailed observation of fetal features and application of gene sequencing technology contribute to elucidate the aetiology of rare, yet disabling and incurable disease, with the particular setting of negative family history.

Background

Bardet-Biedl syndrome (BBS) is rare ciliopathic human genetic disorder with mainly an autosomal recessive inheritance. The incidence of BBS is estimated in 1/125 000-160 000.^{1 2} In isolated societies where prevalence of consanguineous marriages is high, the incidence of BBS can reach 1/13500, as is the case of Bedouin tribes.³

The primary features of the disease include rod-cone dystrophy, postaxial polydactyly, truncal obesity, learning disabilities, renal abnormalities and hypogonadism in men or genital abnormalities in women.

Visual prognosis for children with BBS is poor, with a mean age of legal blindness of 15.5 years.⁴ Renal disease is a major cause of morbidity and mortality. A majority of the affected individuals have significant learning difficulties while a minority have severe impairment on IQ testing.

Prenatal diagnosis in the absence of family history or consanguinity is rare, but when the ultrasound (US) reveals large hyperechogenic kidneys with loss of corticomedullary differentiation in the presence of polydactyly, the diagnosis of BBS should be considered.

Karyotype and array comparative genomic hybridisation (aCGH) are frequently not helpful in the diagnosis of BBS because genomic rearrangements and large deletions are not usually present. Diagnosis is made by genomic sequencing of the coding regions of their causative genes. Molecular diagnosis during pregnancy remains a timely challenge for this genetically heterogeneous disease (25 genes are known).⁵

Case presentation

An early 30s, healthy, primigravid, with 21 weeks and 6 days gestation, was referred to our department due to US scan revealing a singleton male fetus with enlarged hyperechogenic kidneys, without corticomedular differentiation and postaxial polydactyly of the four limbs. These findings were confirmed by US in our department and are illustrated in figures 1–3. Kidneys were found to be enlarged two SD above the mean for gestational age. No central nervous system, gastrointestinal or cardiac defects were found, lens and interorbital distance were normal, as was long bones’ morphology, and there was no orofacial clefting. Syndactyly was not detected. Fetal biometry was within normal, including abdominal circumference (64th percentile). Fetal movements and amniotic fluid volume were also normal. Renal US was performed in both parents. Both had normal kidneys.

Figure 1.

Postaxial polydactyly of the hand. (A) Plan without the thumb. (B) Plan with the thumb.

Figure 2.

Postaxial polydactyly of the hand. (A) Plan without the thumb. (B) Plan with the thumb.

Figure 3.

Enlarged hyperechogenic kidneys.

No anomalies were found in the first-trimester US scan and first trimester combined screening showed low risk for the most frequent aneuploidies. The patient had an uneventful antenatal follow-up until then. The couple was non-consanguineous, neither relevant medical conditions nor family history of inherited diseases.

Investigations

An amniocentesis was performed at 22 weeks and 3 days of gestation for genetic studies. Quantitative fluorescence PCR (QF-PCR) for the common aneuploidies and aCGH were both normal. Meanwhile, a whole-exome sequencing (WES) was started.

Differential diagnosis

Hyperechogenic kidneys have a wide range of differential diagnosis. It might be related to numeric chromosomal anomalies such as trisomy 13 and numerous genetic disorders like recessive and dominant autosomal polycystic kidney disease, Beckwith-Wiedmann syndrome, Perlman syndrome, BBS, Zellweger syndrome, Meckel-Gruber syndrome (MGS) and Elejalde syndrome.⁶

When we find polydactyly on US scan associated with hyperechogenic kidneys, we are able to narrow differential diagnosis.

Combined polydactyly and large or displasic kidneys, when isolated, are the main prenatal characteristics of BBS.⁷

These two features may be present in trisomy 13, but this diagnosis was excluded by QF-PCR.

MGS, MIM 249000 is a classic hypothesis but is associated with larger kidneys with pronounced corticomedullary differentiation and frequently occipital encephalocele.^{6 8}

Other good hypotheses would be Beckwith-Wiedemann syndrome (BWS, MIM 130650), Simpson-Golabi-Behmel syndrome (SGBS, MIM 312870) or Pallister-Hall syndrome (PHS, MIM 146510). Omphalacele is common in BWS phenotype, but most cases do not have it, macrossomia and hydramnios should be present but only in later pregnancy weeks, the absent macroglossia is the main impediment to this frequent rare syndrome. Polydactyly is infrequent, being present in 4% of BWS cases.⁹ SGBS is quite similar to BWS in the prenatal setting, although polydactyly is more frequent. The distinctive facies could not be diagnosed by US at 22 weeks. PHS usually has hypothalamic hamartoma and imperforated anus, but these features can be overlooked on US scan, so PHS may be incorrectly diagnosed as isolated postaxial polydactyly. Renal abnormalities are possible in PHS and include cystic malformations.

In our case, no features other than enlarged hyperechogenic kidneys and postaxial polydactyly were found. BBS was our main diagnosis.

Outcome and follow-up

Despite no familial history of genetic disease or consanguinity, US findings were very suggestive of BBS. Other possible diagnoses also had poor prognosis. The possible diagnoses were discussed with the couple after multidisciplinary approach with geneticist, paediatrician and obstetrician. The consultant decided for medical termination of pregnancy, performed at 24 weeks and 2 days of gestational age, in accordance with Portuguese law.

Two weeks after the procedure, WES revealed a likely pathogenic (LP) variant and a pathogenic (P) variant in heterozygosity on exon 3 of BBS12 gene, supporting the diagnosis of BBS. This gene is responsible for around 5% of patients with BBS.⁴

The NM_152618.2: c.1375C>T p.(Gln459*), described in gnomAD database (0.0032%; one heterozygotic individual reported) and in ClinVar (ID: 531820) is a non-sense variant, a truncated protein is predicted. The variant is classified as LP. The NM_152618.2: c1531_1539del p.(Gln511_Gln 513 del) is found in the literature associated to BBS (PMID: 17160889), in ClinVar (ID: 434492) and in gnomAD (0.0040%, 12 heterozygotic individuals reported). It is an in frame deletion located in a non-repetitive region, giving origin to a deletion of three amino acids. Functional studies support its pathogenicity (PMID: 20498079). The variant is classified as P.

To establish that the variants found in the WES were in two different alleles (trans), the couple was studied for the two variants found, using Sanger sequencing. Each is heterozygous for one of the variants.

The autopsy of the fetus showed a male fetus with urinary system, limb and craniofacial abnormalities, characterised in table 1 and shown in figure 4. Those findings are consistent with the diagnosis of BBS.

Table 1.

Autopsy findings

Urinary system	Enlarged kidneys; loss of corticomedullary differentiation associated with bilateral renal microcystic dysplasia;
Limb	Postaxial polydactyly of both hands and feet, left-hand syndactyly of the fifth and sixth digit
Craniofacial	Small mouth with thin labia, palpebral oedema, microretrognathia
Central nervous system	No anomalies
Cardiovascular system	No anomalies
Abdominal wall defects	No anomalies

Figure 4.

Autopsy (A) male foetus with micrognathia (arrowhead). (B and C) Gross sagittal section of kidneys with abnormal renal appearance in a 24 w+2 d, multiple cysts within deep renal cortex (inset). (D–G) Hand and foot bilateral postaxial polydactyly with syndactyly between the fifth and sixth fingers at left hand.

One year later, a new pregnancy spontaneously occurred. Due to the previous pregnancy diagnosis of BBS, chorionic villous sample was obtained for genetic testing. NGS of the foetus revealed only a pathogenic variant (inherited from the father) on BBS12 gene, which is compatible with asymptomatic carrier state. This pregnancy proceeded uneventful. A healthy male baby was born at 39 weeks gestation.

Discussion

BBS is a rare inherited multisystemic ciliopathy clinically presenting several features, mainly progressive retinal degeneration, renal dysfunction, post-axial polydactyly, obesity and cognitive impairment. Multiple genes (25 genes)⁵ are currently known to be associated with BBS. Recent genetic testing such as next-generation sequencing (NGS) progressively allowed the identification of the increasing number of genes associated with BBS. NGS testing proved to be an effective way in diagnosing diseases such as BBS, with many possible genes as cause. Parallel to the advances in genetic testing, prenatal US imaging progressed as well, permitting the evaluation of increasingly subtle fetal anatomic details.

Prenatal diagnosis of BBS without family or consanguinity history is challenging, as S. Garcia-Tizon Larroca et al and Ashkinadze et al underlined in their case reports.^{10 11}

Diagnosis of BBS based on prenatal findings requires a multidisciplinary approach with obstetrics, genetics and anatomopathology joining efforts towards diagnosis.

Although renal anomalies are usually diagnosed in US scan in BBS, polydactyly represents a more challenging US diagnosis. The aetiology of enlarged hyperechogenic kidneys is a major issue given the numerous differential diagnosis. Therefore, the detection of polydactyly in the US scan can narrow the differential diagnosis and should be actively searched in the presence of enlarged hyperechogenic kidneys.

QF-PCR for aneuploidies and array CGH are not suited to detect BBS genetic variants but were performed to exclude chromosomic anomalies and were chosen for their fastest result compared with NGS at the time.

DNA sequencing method is needed to achieve the genetic diagnosis of BBS. In 2017, Lei et al found WES to have a detection rate of 25% for congenital anomalies of the kidney and urinary tract when associated to other anomalies, in cases of normal karyotype and chromosome microarray analysis.¹² An exome is less costly to sequence than an entire genome because the exome represents only 1%–2% of the genome but harbours approximately 85% of the mutations with large effects on disease-related traits.¹² In our case, a WES for BBS genetic diagnosis was performed having found a LP and a pathogenic BBS12 variants in compound heterozygosity. This confirms the clinical hypothesis. Later the fetopathology excluded other relevant anomalies, which supported the BBS diagnosis.

The option of pregnancy termination was explained to the consultant, based on the high suspicion level of BBS. This syndrome and most of the differential diagnosis are associated with severe and incurable disease.

Our case illustrates how high-resolution obstetric scan, detailed observation of fetal features and application of gene sequencing technology contribute to elucidate the aetiology of rare, yet disabling diseases, with the particular setting of no previous known genetic disease in the family. The knowledge of the condition also had a great impact in the possible choices for the management of the next pregnancy and could be relevant to other relatives in the preconception or prenatal period, cascade studies were offered.

Learning points.

• Bardet-Biedl syndrome (BBS) is a rare, mainly autosomal recessive disease with difficult prenatal diagnosis, particularly when there is no family or consanguinity history. Yet, with a tenacious and multidisciplinary approach, a prenatal diagnosis of BBS is possible in this context, with great impact on patient’s life.

• Ultrasound findings of fetal large hyperechogenic kidneys and polydactyly should raise suspicion of BBS.

• Karyotype and array comparative genomic hybridisation will not diagnose Bardet-Biedl syndrome, genomic sequencing of the coding regions of the causative genes will. Growing use of target panels and whole-exome sequencing in the diagnosis of genetic heterogeneity diseases will definitely contribute to increase the number of successful diagnosis.

• Combined imagological and genetic technologies can produce huge impact in prenatal diagnosis clarifying the aetiology of fetal anomalies with uncertain prognosis.

• Fetal phenotype will likely be important to validate new diagnostic test evaluation and to comparative efficacy trials.