Novel biallelic variant in BBS9 causative of Bardet-Biedl syndrome: expanding the spectrum of disease-causing genetic alterations

Genotype-phenotype correlations for 17 Chinese families with inherited retinal dystrophies due to homozygous variants

In this study, patients with inherited retinal dystrophies (IRDs) who visited Ningxia Eye Hospital from January 2015 to September 2023 were analyzed. Through Whole Exome Sequencing (WES) and Sanger verification, 17 probands carrying homozygous variants were detected. The association between the genotype and clinical phenotype of patients with homozygous variants was analyzed. Among all the patients, 3 patients (17.6%) had a family history of consanguineous marriage, and the onset age of 5 patients(29.4%) was less than 10 years. According to 12 patients (70.6% ), they had the best corrected visual acuity (monocular) < 0.3. 3 were blind, 9 with moderate to severe visual impairment, and 2 with mild visual impairment. 16 homozygous variants were detected in 9 different genes, of which 7 were novel homozygous variants, including frameshift variants, missense variants, and a copy number variant. These variants are related to clinical phenotypes such as Usher syndrome type II (USH2), Stargardt disease (STGD), retinitis pigmentosa (RP), Leber congenital amaurosis (LCA), and Bardet-Biedl syndrome (BBS) respectively. The results of the study indicate that more than 80% of persons with homozygous variant originated from non-consanguineous families, emphasizing the significance of genetic screening for individuals who lack a family history of consanguineous marriage and no obvious clinical phenotypes, but who may carry genetic pathogenic variants for genetic diseases. Furthermore, by analyzing the genotypes and clinical phenotypes of IRD patients from these 17 Chinese families, we have expanded the spectrum of variants in known pathogenic genes for IRDs and the range of clinical phenotypes associated with variants in these genes. We have identified couples at high risk of having affected offspring and individuals with moderate to severe IRDs, providing a basis for genetic counseling, reproductive decision-making, disease prevention, and management. Our findings highlight the association between homozygous variants and more severe clinical phenotypes within these families, thus laying the groundwork for future genetic screening and intervention strategies.

Whole Genome Sequencing Solves an Atypical Form of Bardet-Biedl Syndrome: Identification of Novel Pathogenic Variants of BBS9

Bardet-Biedl syndrome (BBS) is a rare recessive multisystem disorder characterized by retinitis pigmentosa, obesity, postaxial polydactyly, cognitive deficits, and genitourinary defects. BBS is clinically variable and genetically heterogeneous, with 26 genes identified to contribute to the disorder when mutated, the majority encoding proteins playing role in primary cilium biogenesis, intraflagellar transport, and ciliary trafficking. Here, we report on an 18-year-old boy with features including severe photophobia and central vision loss since childhood, hexadactyly of the right foot and a supernumerary nipple, which were suggestive of BBS. Genetic analyses using targeted resequencing and exome sequencing failed to provide a conclusive genetic diagnosis. Whole-genome sequencing (WGS) allowed us to identify compound heterozygosity for a missense variant and a large intragenic deletion encompassing exon 12 in BBS9 as underlying the condition. We assessed the functional impact of the identified variants and demonstrated that they impair BBS9 function, with significant consequences for primary cilium formation and morphology. Overall, this study further highlights the usefulness of WGS in the diagnostic workflow of rare diseases to reach a definitive diagnosis. This report also remarks on a requirement for functional validation analyses to more effectively classify variants that are identified in the frame of the diagnostic workflow.

Gene Therapy in Hereditary Retinal Dystrophies: The Usefulness of Diagnostic Tools in Candidate Patient Selections

PURPOSE

Gene therapy actually seems to have promising results in the treatment of Leber Congenital Amaurosis and some different inherited retinal diseases (IRDs); the primary goal of this strategy is to change gene defects with a wild-type gene without defects in a DNA sequence to achieve partial recovery of the photoreceptor function and, consequently, partially restore lost retinal functions. This approach led to the introduction of a new drug (voretigene neparvovec-rzyl) for replacement of the RPE65 gene in patients affected by Leber Congenital Amaurosis (LCA); however, the treatment results are inconstant and with variable long-lasting effects due to a lack of correctly evaluating the anatomical and functional conditions of residual photoreceptors. These variabilities may also be related to host immunoreactive reactions towards the Adenovirus-associated vector. A broad spectrum of retinal dystrophies frequently generates doubt as to whether the disease or the patient is a good candidate for a successful gene treatment, because, very often, different diseases share similar genetic characteristics, causing an inconstant genotype/phenotype correlation between clinical characteristics also within the same family. For example, mutations on the RPE65 gene cause Leber Congenital Amaurosis (LCA) but also some forms of Retinitis Pigmentosa (RP), Bardet Biedl Syndrome (BBS), Congenital Stationary Night Blindness (CSNB) and Usher syndrome (USH), with a very wide spectrum of clinical manifestations. These confusing elements are due to the different pathways in which the product protein (retinoid isomer-hydrolase) is involved and, consequently, the overlapping metabolism in retinal function. Considering this point and the cost of the drug (over USD one hundred thousand), it would be mandatory to follow guidelines or algorithms to assess the best-fitting disease and candidate patients to maximize the output. Unfortunately, at the moment, there are no suggestions regarding who to treat with gene therapy. Moreover, gene therapy might be helpful in other forms of inherited retinal dystrophies, with more frequent incidence of the disease and better functional conditions (actually, gene therapy is proposed only for patients with poor vision, considering possible side effects due to the treatment procedures), in which this approach leads to better function and, hopefully, visual restoration. But, in this view, who might be a disease candidate or patient to undergo gene therapy, in relationship to the onset of clinical trials for several different forms of IRD? Further, what is the gold standard for tests able to correctly select the patient? Our work aims to evaluate clinical considerations on instrumental morphofunctional tests to assess candidate subjects for treatment and correlate them with clinical and genetic defect analysis that, often, is not correspondent. We try to define which parameters are an essential and indispensable part of the clinical rationale to select patients with IRDs for gene therapy. This review will describe a series of models used to characterize retinal morphology and function from tests, such as optical coherence tomography (OCT) and electrophysiological evaluation (ERG), and its evaluation as a primary outcome in clinical trials. A secondary aim is to propose an ancillary clinical classification of IRDs and their accessibility based on gene therapy's current state of the art.

MATERIAL AND METHODS

OCT, ERG, and visual field examinations were performed in different forms of IRDs, classified based on clinical and retinal conditions; compared to the gene defect classification, we utilized a diagnostic algorithm for the clinical classification based on morphofunctional information of the retina of patients, which could significantly improve diagnostic accuracy and, consequently, help the ophthalmologist to make a correct diagnosis to achieve optimal clinical results. These considerations are very helpful in selecting IRD patients who might respond to gene therapy with possible therapeutic success and filter out those in which treatment has a lower chance or no chance of positive results due to bad retinal conditions, avoiding time-consuming patient management with unsatisfactory results.