Recurrent insertional polydactyly and situs inversus in a Bardet-Biedl syndrome family

LZTFL1, a rare cause of Bardet-Biedl syndrome: A new patient with severe short stature and moderate intellectual disability, more than casual associations?

Bardet-Biedl syndrome (BBS) is an inherited ciliopathy affecting multiple organs and systems with wide clinical and genetic heterogeneity. To date, biallelic variants of the LZTFL1 gene have been reported only in six patients with BBS. We identified a homozygous LZTFL1 nonsense variant in a boy presenting with classical BBS features. In addition, he showed a more pronounced cognitive impairment than previously reported subjects and severe short stature, matching the phenotype displayed by some other patients with LZTFL1 variants and lztfl1 knock-out mice. This case report contributes to a better understanding of the clinical spectrum associated with LZTFL1 pathogenic variants, and highlights possible genotype-phenotype correlations.

Clinical and exome sequencing findings in seven children with Bardet-Biedl syndrome from Turkey

BACKGROUND

Bardet-Biedl syndrome (BBS) is a very-rare autosomal recessive genetic disorder with severe multisystem manifestations. Genetic testing plays an important role in the early diagnosis of the disease. In this study, while trying to elucidate the genetic etiology of seven individuals with clinical BBS diagnosis from six different families, we also aimed to examine the distribution of BBS variations in this region of Turkey.

METHODS AND MATERIALS

Exome sequencing analysis is performed for clinically diagnosed patients with BBS in the present study followed by parental segregation. The unreported and previously described clinical features are presented.

RESULTS

Homozygous variants, four of which are unreported, in BBS-related genes (BBS5 [c.682-2A > G], MKKS [c.775del], BBS7 [c.849+1G > T], BBS9 [c.965G > A], BBS10 [c.145C > T], LZTFL1[c.384G > A]) are detected for all the seven individuals included in the study. The most common clinical finding is polydactyly followed by renal anomalies. The clinical features not previously described are correlated to the unreported variant.

CONCLUSIONS

In this study, exome sequencing findings are discussed and four previously unreported disease-associated variants are described including the fifth BBS-implicated LZTFL1 change and possible genotype-phenotype correlation is described.

Genetic basis of human left-right asymmetry disorders

Humans and other vertebrates exhibit left-right (LR) asymmetric arrangement of the internal organs, and failure to establish normal LR asymmetry leads to internal laterality disorders, including situs inversus and heterotaxy. Situs inversus is complete mirror-imaged arrangement of the internal organs along LR axis, whereas heterotaxy is abnormal arrangement of the internal thoraco-abdominal organs across LR axis of the body, most of which are associated with complex cardiovascular malformations. Both disorders are genetically heterogeneous with reduced penetrance, presumably because of monogenic, polygenic or multifactorial causes. Research in genetics of LR asymmetry disorders has been extremely prolific over the past 17 years, and a series of loci and disease genes involved in situs inversus and heterotaxy have been described. The review highlights the classification, chromosomal abnormalities, pathogenic genes and the possible mechanism of human LR asymmetry disorders.

Functional modelling of a novel mutation in BBS5

Background

Bardet-Biedl syndrome (BBS) is an autosomal recessive ciliopathy disorder with 18 known causative genes (BBS1-18). The primary clinical features are renal abnormalities, rod-cone dystrophy, post-axial polydactyly, learning difficulties, obesity and male hypogonadism.

Results

We describe the clinical phenotype in three Saudi siblings in whom we have identified a novel mutation in exon 12 of BBS5 (c.966dupT; p.Ala323CysfsX57). This single nucleotide duplication creates a frame shift results in a predicted elongated peptide. Translation blocking Morpholino oligonucleotides were used to create zebrafish bbs5 morphants. Morphants displayed retinal layering defects, abnormal cardiac looping and dilated, cystic pronephric ducts with reduced cilia expression. Morphants also displayed significantly reduced dextran clearance via the pronephros compared to wildtype embryos, suggesting reduced renal function in morphants. The eye, kidney and heart defects reported in morphant zebrafish resemble the human phenotype of BBS5 mutations. The pathogenicity of the novel BBS5 mutation was determined. Mutant mRNA was unable to rescue pleiotropic phenotypes of bbs5 morphant zebrafish and in cell culture we demonstrate a mislocalisation of mutant BBS5 protein which fails to localise discretely with the basal body.

Conclusions

We conclude that this novel BBS5 mutation has a deleterious function that accounts for the multisystem ciliopathy phenotype seen in affected human patients.

Potential amelioration of morbidity in patients with chromosomal anomalies: relevance to Bardet-Biedl syndrome

Given the genetic basis of their disease, children with major chromosomal abnormalities including Bardet-Biedl syndrome (BBS) are generally considered to have a guarded prognosis with persistence or progression of disease manifestations. Although various therapeutic interventions are commonly used to control signs and symptoms of illness, parents of BBS children are usually cautioned against hoping for sustained improvement. A case of a 21-month-old girl, diagnosed with BBS, manifesting signs of worsening visual impairment, obesity, irascible and disordered behaviour, as well as developmental delay, is presented. After initial evaluation suggested specific biochemical deficiencies, nutritional status correction was undertaken and the patient's signs and symptoms subsequently resolved over the course of several months. To the authors' knowledge, this is the first case report of sustained resolution of all disease manifestations in the face of previously deteriorating health in a young child with this major chromosomal abnormality. It appears that biochemical imbalances and insufficiencies resulting from abnormal metabolism and excretion are potentially amenable to extraordinary dietary supplementation, with partial or complete resolution of clinical abnormalities. It is recommended that all children with chromosomal abnormalities have biochemical and nutritional status evaluation with correction of disordered biochemistry as is possible.