María Álvarez-Satta

Exploring genotype-phenotype relationships in Bardet-Biedl syndrome families

Background Bardet-Biedl syndrome (BBS) is a pleiotropic autosomal recessive ciliopathy that displays retinal dystrophy, obesity, polydactyly, cognitive impairment, urogenital anomalies and renal abnormalities as primary clinical features. To date, 19 causative genes (BBS1-19) have been involved, whose mutations would explain over 80% of patients. The overlapping phenotypes among ciliopathies, in addition to the high intrafamilial and interfamilial variability in clinical presentation, further complicate the diagnosis of this syndrome. Thus, the main purpose of this study was to elucidate some genotype-phenotype trends that could be helpful to focus the molecular diagnosis of patients with BBS. Methods Thirty-seven families (52 cases) with mutations in BBS1 or chaperonin-like BBS genes (BBS6, BBS10, BBS12) from our Spanish cohort were enrolled. Systemic and ocular features were documented as comprehensively as possible. Results Comparing BBS1 versus chaperonin-like genes phenotypes we found more severe clinical features in the second group, since they displayed higher prevalence of all primary features, remarkable being the frequency of cognitive impairment (75%) in BBS12 and urogenital anomalies (83%) in patients with BBS10. With regards to p.(Met390Arg) cases, homozygotes showed a relatively more severe ocular phenotype than compound heterozygotes, since more severe fundus alterations and higher frequency of cataracts and dyschromatopsia (not previously described) were documented in the first group. The phenotypes observed frequently overlapped with Alström syndrome and, in the case of chaperonin-like genes, McKusick-Kauffman syndrome overlapping was detected. Conclusions We provide the first evidence of BBS12 mutations related to severe phenotypes as previously described for patients with BBS10, while BBS1 ocular phenotype should not be considered as mild as generally reported when compared with other BBS phenotypes.

Alström syndrome: current perspectives

Alström syndrome (ALMS) is a rare genetic disorder that has been included in the ciliopathies group, in the last few years. Ciliopathies are a growing group of diseases associated with defects in ciliary structure and function. The development of more powerful genetic approaches has been replaced the strategies to follow for getting a successful molecular diagnosis for these patients, especially for those without the typical ALMS phenotype. In an effort to deepen the understanding of the pathogenesis of ALMS disease, much work has been done, in order to establish the biological implication of ALMS1 protein, which is still being elucidated. In addition to its role in ciliary function and structure maintenance, this protein has been implicated in intracellular trafficking, regulation of cilia signaling pathways, and cellular differentiation, among others. All these progresses will lead to identifying therapeutic targets, thus opening the way to future personalized therapies for human ciliopathies.

Overview of Bardet–Biedl syndrome in Spain: identification of novel mutations in BBS1, BBS10 and BBS12 genes

To the Editor : Bardet–Biedl syndrome (BBS; OMIM #209900) is a rare disease that has as primary features: retinal dystrophy, obesity, polydactyly, hypogonadism, intellectual disability and renal dysfunction (1). BBS belongs to the group of ciliopathies that share partial-overlapping phenotypes and common genes (1). At present, 18 genes are known to be involved in BBS (BBS1–18 ) (1–3), explaining about 80% of the clinically diagnosed BBS patients. In northern European individuals, p.(Met390Arg) in BBS1 and p.(Cys91Leufs*4) in BBS10 are the most common alleles (1), and several studies show that BBS1, BBS10 and BBS12 account for 75% of the identified mutational load in the series analysed (4, 5). Here, we report the first study of a Spanish BBS cohort by sequencing the three BBS genes with high contribution (BBS1, BBS10 and BBS12 ) in 53 probands, mainly of Spanish origin. They were

Whole exome sequencing as a diagnostic tool for patients with ciliopathy-like phenotypes

Ciliopathies are a group of rare disorders characterized by a high genetic and phenotypic variability, which complicates their molecular diagnosis. Hence the need to use the latest powerful approaches to faster identify the genetic defect in these patients. We applied whole exome sequencing to six consanguineous families clinically diagnosed with ciliopathy-like disease, and for which mutations in predominant Bardet-Biedl syndrome (BBS) genes had previously been excluded. Our strategy, based on first applying several filters to ciliary variants and using many of the bioinformatics tools available, allowed us to identify causal mutations in BBS2, ALMS1 and CRB1 genes in four families, thus confirming the molecular diagnosis of ciliopathy. In the remaining two families, after first rejecting the presence of pathogenic variants in common cilia-related genes, we adopted a new filtering strategy combined with prioritisation tools to rank the final candidate genes for each case. Thus, we propose CORO2B, LMO7 and ZNF17 as novel candidate ciliary genes, but further functional studies will be needed to confirm their role. Our data show the usefulness of this strategy to diagnose patients with unclear phenotypes, and therefore the success of applying such technologies to achieve a rapid and reliable molecular diagnosis, improving genetic counselling for these patients. In addition, the described pipeline also highlights the common pitfalls associated to the large volume of data we have to face and the difficulty of assigning a functional role to these changes, hence the importance of designing the most appropriate strategy according to each case.

Functional analysis by minigene assay of putative splicing variants found in Bardet–Biedl syndrome patients

Bardet–Biedl syndrome (BBS) and Alström syndrome (ALMS) are rare diseases belonging to the group of ciliopathies. Although mutational screening studies of BBS/ALMS cohorts have been extensively reported, little is known about the functional effect of those changes. Thus, splicing variants are estimated to represent 15% of disease‐causing mutations, and there is growing evidence that many exonic changes are really splicing variants misclassified. In this study, we aimed to analyse for the first time several variants in BBS2, ARL6/BBS3, BBS4 and ALMS1 genes predicted to produce aberrant splicing by minigene assay. We found discordance between bioinformatics analysis and experimental data when comparing wild‐type and mutant constructs. Remarkably, we identified nonsense variants presumably resistant to nonsense‐mediated decay, even when a premature termination codon would be introduced in the second amino acid (p.(G2*) mutation in ARL6/BBS3 gene). As a whole, we report one of the first functional studies of BBS/ALMS1 variants using minigene assay, trying to elucidate their role in disease. Functional studies of variants identified in BBS and ALMS patients are essential for their proper classification and subsequent genetic counselling and could also be the start point for new therapeutic approaches, currently based only on symptomatic treatment.

Bardet-Biedl syndrome: A rare genetic disease

Bardet-Biedl syndrome (BBS) is a rare multisystem genetic disease, with high phenotypic and genetic heterogeneity. Rod-cone dystrophy, obesity, polydactyly, hypogonadism, cognitive impairment and renal abnormalities have been established as primary features. There are 17 BBS genes (BBS1-BBS17) described to date, which explain 70-80% of the patients clinically diagnosed, therefore more BBS genes remain to be identified. BBS belongs to a group of diseases known as ciliopathies. In general, ciliopathies and BBS in particular share a partial overlapping phenotype that makes them complicated to diagnose. We present an up-to-date review including clinical, epidemiologic and genetic aspects of the syndrome.

DNA Methylation and Ciliopathies: a way to be explored

Results Regarding to bioinformatic analysis, all genes harboured at least one CpG island. Some of them included one or more sequences compatible with x-box motifs, which can be recognized by transcription factors of RFX family that are known to be involved in the regulation of ciliary genes transcription. We selected a CpG island in the ALMS1 gene containing 67 cytosine residues potentially methylated for performing MS-qPCR. A mean efficiency ranging from 90 to 96% was reached for each amplicon in which the CpG island was divided. Unfortunately, no methylation was detected in the enrolled patients. Conclusion Although the results of this preliminary study were negative, limitations due to sample size, sample type and experimental approach have to be taken into account. However, we consider it worth exploring this mechanism in BBS and ALMS using different techniques such as methylation arrays, which could provide more accurate data.

Performing whole-exome sequencing in Bardet-Bield syndrome

Objective Bardet-Biedl syndrome (BBS) is a rare disease characterized by a high genetic heterogeneity, accounting for 75% of affected families. As part of the next-generation technology, whole-exome sequencing (WES) allows all exons of the genome to be sequenced at once. Here we show the use of WES as a useful approach in BBS families in which mutations in predominant genes have been discarded.