Mutation spectrum in BBS genes guided by homozygosity mapping in an Indian cohort

Inherited retinal degeneration in Malay and Indian populations of Singapore and Malaysia: a prospective multicentre study

PURPOSE

To analyze the phenotypic and genotypic characteristics of inherited retinal degeneration (IRD) patients of Malay and Indian ethnicity from Singapore and Malaysia.

METHODS

Ethnic Malay and Indian IRD patients were consecutively enrolled from retina clinics in Singapore and Malaysia. Phenotypic and genetic data were reviewed.

RESULTS

A total of 100 unrelated individuals (Malay: n = 46, Indian: n = 54) were enrolled. Sixteen distinct IRD phenotypes were identified, with nonsyndromic retinitis pigmentosa (RP) comprising 46% of all cases. Stargardt disease and cone-rod dystrophy accounted for 20% and 11% of cases, respectively. Exome sequencing yielded genotypes in 64.3% of Malay and 68.9% of Indian cases. Variants in ABCA4 were the most common cause of IRD overall. Recurrent variants were identified in ABCA4, GUCY2D, PRPH2, and TULP1 for Malays, and in ABCA4 and MFSD8 (CLN7) for Indians. Homozygosity was more frequent among Indians than Malays (58.1% vs. 19.2%; p = 0.003).

CONCLUSIONS

This study demonstrated diverse phenotypic and genotypic outcomes in Malay and Indian populations of Singapore and Malaysia, with distinct differences between them. Homozygosity was common among ethnic Indian IRD cases, explaining phenotypic diversity. These findings inform the identification of regionally relevant IRDs for developing targeted therapies in Malay and Indian patients from Southeast Asia.

Clinical Presentation and Co-Morbidities in Bardet-Biedel Syndrome: Case Series from a Single Centre

Introduction

Bardet-Biedl syndrome (BBS-OMIM 209900) is a rare genetic multi-system obesity syndrome with limited case reports from India. We describe a case series of BBS with varied clinical presentation and their co-morbidities.

Methods

BBS was diagnosed based on the clinical criteria by Beales et al. Their clinical presentations including the presence of primary and secondary features, metabolic profile, and systemic complications were examined.

Results

Eleven cases of BBS were analyzed over 9 years, of which the most common primary clinical manifestations were post-axial polydactyly and learning disabilities, noted in all individuals (100%). Retinitis pigmentosa and truncal obesity were present in 91% (10 out of 11). Clinical and biochemical features of hypogonadism and genital abnormalities were observed in 73% of individuals. Craniofacial dysmorphism and developmental delay were the more commonly observed secondary features, observed in 91%. Speech delay and brachydactyly/syndactyly were present in 73% of cases. Hyperactive behavioural disturbances and diabetes mellitus were noted in 45% and 18% of cases, respectively. Cataracts, hypertonia, dental malocclusion and cardiac anomalies (dextrocardia) were each observed in just one patient among the study population of 11 patients. The molecular genetics were analysed in two individuals.

Conclusion

BBS is an extremely rare clinical syndrome with clinical heterogeneity at presentation. The appropriate diagnosis of syndromic obesity and an early multi-disciplinary intervention may improve their quality of life.

Beyond the phenotype: Exploring inherited retinal diseases with targeted next-generation sequencing in a Turkish cohort

This research aims to compile recent clinical and genetic data from Turkish patients with inherited retinal disorders and evaluate the effectiveness of targeted Next-generation sequencing panels. The study included Turkish individuals with hereditary retinal diseases who visited the Medical Genetic Department of Erciyes University between 2019 and 2022. One proband per family was selected based on eligibility. We used Hereditary Disorder Solution (HDS) by Sophia Genetics and performed next-generation sequencing (NGS) with Illumina NextSeq-500. Bioinformatics analysis using Sophia DDM® SaaS algorithms and ACMG guidelines classified genomic changes. The study involved 354 probands. Disease-causing variants were found in 58.1% of patients, with ABCA4, USH2A, RDH12, and EYS being the most frequently implicated genes. Forty-eight novel variants were detected. This study enhances the knowledge of clinical diagnoses, symptom onset, inheritance patterns, and genetic details for Turkish individuals with hereditary retinal disease. It contributes to broader health strategies by enabling comparisons with other studies.

Spectrum of pathogenic variants and high prevalence of pathogenic BBS7 variants in Russian patients with Bardet-Biedl syndrome

Introduction

Bardet-Biedl syndrome is a rare condition characterized by obesity, retinitis pigmentosa, polydactyly, development delay, and structural kidney anomalies. This syndrome has an autosomal recessive type of inheritance. For the first time, molecular genetic testing has been provided for a large cohort of Russian patients with Bardet-Biedl syndrome.

Materials and methods

Genetic testing was provided to 61 unrelated patients using an MPS panel that includes coding regions and intronic areas of all genes (n = 21) currently associated with Bardet-Biedl syndrome.

Results

The diagnosis was confirmed for 41% of the patients (n = 25). Disease-causing variants were observed in BBS1, BBS4, BBS7, TTC8, BBS9, BBS10, BBS12, and MKKS genes. In most cases, pathogenic and likely pathogenic variants were localized in BBS1, BBS10, and BBS7 genes; recurrent variants were also observed in these genes.

Discussion

The frequency of pathogenic and likely pathogenic variants in the BBS1 and BBS10 genes among Russian patients matches the research data in other countries. The frequency of pathogenic variants in the BBS7 gene is about 1.5%-2% of patients with Bardet-Biedl syndrome, while in the cohort of Russian patients, the fraction is 24%. In addition, the recurrent pathogenic variant c.1967_1968delinsC was detected in the BBS7 gene. The higher frequency of this variant in the Russian population, as well as the lack of association of this pathogenic variant with Bardet-Biedl syndrome in other populations, suggests that the variant c.1967_1968delinsC in the BBS7 gene is major and has a founder effect in the Russian population. Results provided in this article show the significant role of pathogenic variants in the BBS7 gene for patients with Bardet-Biedl syndrome in the Russian population.

Autosomal Recessive Rod-Cone Dystrophy with Mild Extra-Ocular Manifestations Due to a Splice-Affecting Variant in BBS9

Bardet-Biedl syndrome (BBS), one of the most common forms of syndromic inherited retinal diseases (IRDs), is characterized by the combination of retinal degeneration with additional extra-ocular manifestations, including obesity, intellectual disability, kidney disease, polydactyly and other skeletal abnormalities. We observed an Israeli patient with autosomal recessive apparently non-syndromic rod-cone dystrophy (RCD). Extra-ocular findings were limited to epilepsy and dental problems. Genetic analysis with a single molecule molecular inversion probes-based panel that targets the exons and splice sites of 113 genes associated with retinitis pigmentosa and Leber congenital amaurosis revealed a homozygous rare missense variant in the BBS9 gene (c.263C>T;p.(Ser88Leu)). This variant, which affects a highly conserved amino acid, is also located in the last base of Exon 3, and predicted to be splice-altering. An in vitro minigene splice assay demonstrated that this variant leads to the partial aberrant splicing of Exon 3. Therefore, we suggest that this variant is likely hypomorphic. This is in agreement with the relatively mild phenotype observed in the patient. Hence, the findings in our study expand the phenotypic spectrum associated with BBS9 variants and indicate that variants in this gene should be considered not only in BBS patients but also in individuals with non-syndromic IRD or IRD with very mild extra-ocular manifestations.

Genotype-phenotype associations in Alström syndrome: a systematic review and meta-analysis

BACKGROUND

Alström syndrome (ALMS; #203800) is an ultrarare monogenic recessive disease. This syndrome is associated with variants in the ALMS1 gene, which encodes a centrosome-associated protein involved in the regulation of several ciliary and extraciliary processes, such as centrosome cohesion, apoptosis, cell cycle control and receptor trafficking. The type of variant associated with ALMS is mostly complete loss-of-function variants (97%) and they are mainly located in exons 8, 10 and 16 of the gene. Other studies in the literature have tried to establish a genotype-phenotype correlation in this syndrome with limited success. The difficulty in recruiting a large cohort in rare diseases is the main barrier to conducting this type of study.

METHODS

In this study we collected all cases of ALMS published to date. We created a database of patients who had a genetic diagnosis and an individualised clinical history. Lastly, we attempted to establish a genotype-phenotype correlation using the truncation site of the patient's longest allele as a grouping criteria.

RESULTS

We collected a total of 357 patients, of whom 227 had complete clinical information, complete genetic diagnosis and meta-information on sex and age. We have seen that there are five variants with high frequency, with p.(Arg2722Ter) being the most common variant, with 28 alleles. No gender differences in disease progression were detected. Finally, truncating variants in exon 10 seem to be correlated with a higher prevalence of liver disorders in patients with ALMS.

CONCLUSION

Pathogenic variants in exon 10 of the ALMS1 gene were associated with a higher prevalence of liver disease. However, the location of the variant in the ALMS1 gene does not have a major impact on the phenotype developed by the patient.

Mutation profile of Bardet-Biedl syndrome patients from India: Implicative role of multiallelic rare variants and oligogenic inheritance pattern

Bardet-Biedl syndrome (BBS), a rare primary form of ciliopathy, with heterogeneous clinical and genetic presentation is characterized by rod cone dystrophy, obesity, polydactyly, urogenital abnormalities, and cognitive impairment. Here, we delineate the genetic profile in a cohort of 108 BBS patients from India by targeted gene sequencing-based approach for a panel of ciliopathy (including BBS) and other inherited retinal disease genes. We report here a higher frequency of BBS10 and BBS1 gene variations. A different spectrum of variations including a putatively novel gene TSPOAP1, for BBS was identified. Increased percentage frequency of digenic variants (36%) in the disease cohort, role of modifiers in familial cases are some of the salient observations in this work. This study appends the knowledge of BBS genetics pertaining to patients from India. We observed a different molecular epidemiology of BBS patients in this study cohort compared to other reports, which emphasizes the need for molecular testing in affected patients.

McCormick E, Elden L, Aleman TS, O’Neil EC, J. Falk M, Hakonarson H. Multiple Independent Gene Disorders Causing Bardet-Biedl Syndrome, Congenital Hypothyroidism, and Hearing Loss in a Single Indian Patient

We report a 20-year-old, female, adopted Indian patient with over 662 Mb regions of homozy-gosity who presented with intellectual disability, ataxia, schizophrenia, retinal dystrophy, moder-ate-to-severe progressive sensorineural hearing loss (SNHL), congenital hypothyroidism, cleft mi-tral valve with mild mitral valve regurgitation, and dysmorphic features. Exome analysis first on a clinical basis and subsequently on research reanalysis uncovered pathogenic variants in three nu-clear genes following two modes of inheritance that were causal to her complex phenotype. These included (1) compound heterozygous variants in BBS6 potentially causative for Bardet-Biedl syn-drome 6; (2) a homozygous, known pathogenic variant in the stereocilin (STRC) gene associated with nonsyndromic deafness; and (3) a homozygous variant in dual oxidase 2 (DUOX2) gene asso-ciated with congenital hypothyroidism. A variant of uncertain significance was identified in a fourth gene, troponin T2 (TNNT2), associated with cardiomyopathy but not the cleft mitral valve, with mild mitral regurgitation seen in this case. This patient was the product of an apparent first-degree relationship, explaining the multiple independent inherited findings. This case high-lights the need to carefully evaluate multiple independent genetic etiologies for complex pheno-types, particularly in the case of consanguinity, rather than presuming unexplained features are expansions of known gene disorders.

Bardet-Biedl Syndrome: Current Perspectives and Clinical Outlook

The Bardet Biedl syndrome (BBS) is a rare inherited disorder considered a model of non-motile ciliopathy. It is in fact caused by mutations of genes encoding for proteins mainly localized to the base of the cilium. Clinical features of BBS patients are widely shared with patients suffering from other ciliopathies, especially autosomal recessive syndromic disorders; moreover, mutations in cilia-related genes can cause different clinical ciliopathy entities. Besides the best-known clinical features, as retinal degeneration, learning disabilities, polydactyly, obesity and renal defects, several additional clinical signs have been reported in BBS, expanding our understanding of the complexity of its clinical spectrum. The present review aims to describe the current knowledge of BBS i) pathophysiology, ii) clinical manifestations, highlighting both the most common and the less described features, iii) current and future perspective for treatment.

A Japanese boy with Bardet-Biedl syndrome caused by a novel homozygous variant in the ARL6 gene who was initially diagnosed with retinitis punctata albescens: A case report

PURPOSE

Bardet-Biedl Syndrome (BBS) is an autosomal recessive systemic disorder characterized by retinitis pigmentosa, polydactyly, obesity, intellectual disability, renal impairments, and hypogonadism. The purpose of this study was to determine the ocular characteristics of a boy with BBS caused by a novel homozygous variant in the ARL6 (alternative named BBS3) gene who had been originally diagnosed with retinitis punctata albescens.

METHODS

This was an observational case study. The patient underwent ophthalmological examinations, electroretinography, and genetic analyses using whole-exome sequencing.

RESULTS

A 7-year-old boy was examined in our hospital with complaints of a progressive reduction of his visual acuity and night blindness in both eyes. There was no family history of eye diseases and no consanguineous marriage. Fundus examinations showed numerous white spots in the deep retina and retinal pigment epithelium. Fundus autofluorescence showed hypofluorescence consistent with these spots. Both the scotopic and photopic components of the full-field electroretinographies were non-detectable. Based on these clinical findings, this boy was suspected to have retinitis punctata albescens. Subsequent genetic testing using whole-exome sequencing revealed a novel homozygous variants in the ARL6/BBS3 gene (NM_001278293.3:c.528G>A, (p.Trp176Ter)). A systemic examination by the pediatric department revealed that this boy had a history of a surgical excision of polydactyly on his left foot when he was born, and that he was mildly obese. There were no prominent intellectual or gonadal dysfunctions, no craniofacial or dental abnormalities, no congenital heart disease, and no hearing impairment. He was then clinically and genetically diagnosed with BBS.

CONCLUSION AND IMPORTANCE

In children with night blindness and progressive visual dysfunction, it is important for ophthalmologists to consult clinical geneticists and pediatricians to rule out the possibility of systemic diseases such as BBS.

Rare eye diseases in India: A concise review of genes and genetics

Rare eye diseases (REDs) are mostly progressive and are the leading cause of irreversible blindness. The disease onset can vary from early childhood to late adulthood. A high rate of consanguinity contributes to India’s predisposition to RED. Most gene variations causing REDs are monogenic and, in some cases, digenic. All three types of Mendelian inheritance have been reported in REDs. Some of the REDs are related to systemic illness with variable phenotypes in affected family members. Approximately, 50% of the children affected by REDs show associated phenotypes at the early stages of the disease. A precise clinical diagnosis becomes challenging due to high clinical and genetic heterogeneity. Technological advances, such as next-generation sequencing (NGS), have improved genetic and genomic testing for REDs, thereby aiding in determining the underlying causative gene variants. It is noteworthy that genetic testing together with genetic counseling facilitates a more personalized approach in the accurate diagnosis and management of the disease. In this review, we discuss REDs identified in the Indian population and their underlying genetic etiology.

A case of Bardet-Biedl syndrome caused by a recurrent variant in BBS12: A case report

Bardet-Biedl syndrome (BBS) is a clinically and genetically heterogenous disorder that manifests as a result of primary cilia impairment. Cilia are present on most cell types, thus BBS is a multisystemic condition involving the majority of organ systems. The core features of the syndrome include retinal degeneration, obesity, polydactyly, cognitive impairment, renal anomalies and urogenital malformations. To date, pathogenic variants in 26 genes have been shown to be involved in the molecular basis of this rare ciliopathy. Of these causal loci, BBS12 accounts for ~8% of all cases. In this case report, an individual with BBS caused by a rare recurrent variant in BBS12 (NM_152618.3: c.1063C>T; p.Arg355^*) is described and compared with others with the same DNA variant, placing this finding in the context of the current literature.

Renal agenesis-related genes are associated with Herlyn-Werner-Wunderlich syndrome

OBJECTIVE

To explore the genetic causes of Herlyn-Werner-Wunderlich syndrome (HWWS) using whole-exome sequencing.

DESIGN

Retrospective genetic study.

SETTING

Academic medical center.

PATIENT(S)

Twelve patients with HWWS.

INTERVENTION(S)

Whole-exome sequencing was performed for each patient. Sanger sequencing was used to confirm the potential causative genetic variants. In silico analysis and American College of Medical Genetics and Genomics guidelines were used to classify the pathogenicity of each variant.

MAIN OUTCOME MEASURE(S)

Rare sequence variants associated with müllerian duct development and renal agenesis were identified and included in subsequent analyses.

RESULT(S)

A total of 11 variants were identified in 10 of 12 patients (83.3%) and were considered to constitute a molecular genetic diagnosis of HWWS. These 11 variants were related to 9 genes: CHD1L, TRIM32, TGFBR3, WNT4, RET, FRAS1, FAT1, FOXF1, and PCSK5. All variants were heterozygous and confirmed by Sanger sequencing. The changes included one frameshift variant, one splice-site variant, and eight missense variants. All of the identified variants were absent or rare in Genome Aggregation Database East Asian populations. One of the 11 variants (9.1%) was classified as a pathogenic variant according to the American College of Medical Genetics and Genomics guidelines, and 8 of the 11 variants (72.7%) were classified as variants of uncertain significance.

CONCLUSION(S)

To our knowledge, this is the first report of the genetic causes of HWWS. Renal agenesis-related genes, such as CHD1L, TRIM32, RET, and WNT4, may be associated with HWWS. Identification of these variants can not only help us understand the etiology of HWWS and the relationship between reproductive tract development and urinary system development, but additionally improve the level of genetic counseling for HWWS.

Whole-exome sequencing identifies two novel ALMS1 mutations in Indian patients with Leber congenital amaurosis

Leber congenital amaurosis (LCA) is a severe autosomal recessive retinal degenerative disease. The current study describes exome sequencing results for two unrelated Indian LCA patients carrying novel nonsense p.(Glu636*) and frameshift p.(Pro2281Leufs*63) mutations in the ALMS1 gene. Although ALMS1 gene mutations are associated with Alstrom syndrome (AS), the current patients did not exhibit typical syndromic features of AS. These data suggest that ALMS1 should be included in the candidate gene panel for LCA to improve diagnostic efficiency.

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

BACKGROUND

Bardet-Biedl syndrome (BBS) is a rare autosomal recessive ciliopathy disorder. Many BBS disease-causing genetic variants have been identified due to the advancement of molecular diagnostic tools. We report on a novel pathogenic variant in a consanguineous Pakistani family with an affected child.

CASE PRESENTATION

Clinical exome sequencing was used to search for BBS causing variants in the affected individual and identified a novel homozygous splice-site variant in the BBS9 gene (c.702 + 1del). Sanger sequencing was performed for variant validation and segregation studies. Expression analysis using mRNA levels to assess the functional impact of the novel variant demonstrated skipping of exon 7 in the affected alleles, suggesting a truncating effect. Three-dimensional structural modelling was used to predict pathogenicity of the variant residue and the alteration leads to a partial deletion of the PHTB1_N domain and a total deletion of the PHTB1_C domain.

CONCLUSION

The study of this case expands the spectrum of biallelic variants in the BBS9 gene associated with BBS and increased the knowledge on the molecular consequences of splicing variation c.702 + 1del.

Ophthalmic genetics practice and research in India: Vision in 2020

Ophthalmic genetics is a much needed and growing area in India. Ethnic diversity, with a high degree of consanguinity, has led to a high prevalence of genetic disorders in the country. As the second most populous country in the world, this naturally results in a significant number of affected people overall. Practice involves coherent association between ophthalmologists, genetic counselor and pediatricians. Eye genetics in India in recent times has witnessed advanced research using cutting edge diagnostics, next generation sequencing (NGS) approaches, stem cell therapies, gene therapy and genomic editing. This article will highlight the studies reporting genetic variations in the country, challenges in practice, and the latest advances in ophthalmic genetic research in India.

Meta-analysis of genotype-phenotype associations in Bardet-Biedl syndrome uncovers differences among causative genes

Bardet-Biedl syndrome (BBS) is a recessive genetic disease causing multiple organ anomalies. Most patients carry mutations in genes encoding for the subunits of the BBSome, an octameric ciliary transport complex, or accessory proteins involved in the BBSome assembly or function. BBS proteins have been extensively studied using in vitro, cellular, and animal models. However, the molecular functions of particular BBS proteins and the etiology of the BBS symptoms are still largely elusive. In this study, we applied a meta-analysis approach to study the genotype-phenotype association in humans using our database of all reported BBS patients. The analysis revealed that the identity of the causative gene and the character of the mutation partially predict the clinical outcome of the disease. Besides their potential use for clinical prognosis, our analysis revealed functional differences of particular BBS genes in humans. Core BBSome subunits BBS2, BBS7, and BBS9 manifest as more critical for the function and development of kidneys than peripheral subunits BBS1, BBS4, and BBS8/TTC8, suggesting that incomplete BBSome retains residual function at least in the kidney.

Mutation profile of BBS genes in patients with Bardet-Biedl syndrome: an Italian study

BACKGROUND

Bardet-Biedl syndrome (BBS) is a rare inherited multisystemic disorder with autosomal recessive or complex digenic triallelic inheritance. There is currently no treatment for BBS, but some morbidities can be managed. Accurate molecular diagnosis is often crucial for the definition of appropriate patient management and for the development of a potential personalized therapy.

METHODS

We developed a next-generation-sequencing (NGS) protocol for the screening of the 18 most frequently mutated genes to define the genotype and clarify the mutation spectrum of a cohort of 20 BBS Italian patients.

RESULTS

We defined the causative variants in 60% of patients; four of those are novel. 33% of patients also harboured variants in additional gene/s, suggesting possible oligogenic inheritance. To explore the function of different genes, we looked for correlations between genotype and phenotype in our cohort. Hypogonadism was more frequently detected in patients with variants in BBSome proteins, while renal abnormalities in patients with variations in BBSome chaperonin genes.

CONCLUSIONS

NGS is a powerful tool that can help understanding BBS patients' phenotype through the identification of mutations that could explain differences in phenotype severity and could provide insights for the development of targeted therapy. Furthermore, our results support the existence of additional BBS loci yet to be identified.

Screening for mutation hotspots in Bardet-Biedl syndrome patients from India

Background & objectives:

Bardet–Biedl syndrome (BBS) is a genetically heterogeneous autosomal recessive disorder characterized by multiple organ defects involving retina, kidney, liver and brain. Disease-causing mutations in BBS genes narrowed down by homozygosity mapping in small consanguineous and non-consanguineous pedigrees were reported in 80 per cent of the study population. This study was aimed to screen these genes (BBS3, BBS10) and specific exons of BBS genes (BBS1, BBS5, MKKS, BBS9, BBS11 and BBS12) for recurrent mutations in a selected sample of BBS patients.

Methods:

The recurrent mutations in BBS genes were screened in the BBS affected individuals by PCR based direct sequencing. The pathogenicity of the observed mutations were confirmed by co-segregation analysis, screening of healthy unrelated controls and in silico analysis.

Results:

In the 64 BBS patients (44 males, 20 females) were studied, mutations were predominant in BBS10 and ARL6 genes; the c.272T>C; p.(I91T) mutation in ARL6 gene was a recurrent mutation. One novel non-sense mutation c.425T>G; p(L142^*) was obtained in BBS5 gene (family BSI-31).

Interpretation & conclusions:

BBS10 gene mutations clustered in exon 2 of the gene suggesting the exon as a probable hotspot for mutations in Indian population. A cost- and time-effective strategy for the molecular diagnosis of BBS was designed based on these results.

Genetics of Obesity in Consanguineous Populations: Toward Precision Medicine and the Discovery of Novel Obesity Genes

OBJECTIVE

Consanguinity has been instrumental in the elucidation of many Mendelian genetic diseases. Here, the unique advantage of consanguineous populations was considered in the quest for genes causing obesity.

METHODS

PubMed was searched for articles relevant to consanguinity and obesity published between 1995 and 2016. Some earlier articles of interest were also consulted.

RESULTS

Although obesity is the most heritable disorder, even in outbred populations, only 2% to 5% of severe obesity cases have so far been proven to be caused by single gene mutations. In some highly consanguineous populations, a remarkably higher proportion of obesity cases because of known and novel monogenic variants has been identified (up to 30%).

CONCLUSIONS

Combining the power conferred by consanguinity with current large-capacity sequencing techniques should bring new genetic factors and molecular mechanisms to the fore, unveiling a large part of the yet-elusive neurohumoral circuitry involved in the regulation of energy homeostasis and appetite. Importantly, the undertaking of such initiatives is destined to unfold novel targets for the development of precision medicine relevant to different forms of obesity.

Diagnosis of lethal or prenatal-onset autosomal recessive disorders by parental exome sequencing

OBJECTIVE

Rare genetic disorders resulting in prenatal or neonatal death are genetically heterogeneous, but testing is often limited by the availability of fetal DNA, leaving couples without a potential prenatal test for future pregnancies. We describe our novel strategy of exome sequencing parental DNA samples to diagnose recessive monogenic disorders in an audit of the first 50 couples referred.

METHOD

Exome sequencing was carried out in a consecutive series of 50 couples who had 1 or more pregnancies affected with a lethal or prenatal-onset disorder. In all cases, there was insufficient DNA for exome sequencing of the affected fetus. Heterozygous rare variants (MAF < 0.001) in the same gene in both parents were selected for analysis. Likely, disease-causing variants were tested in fetal DNA to confirm co-segregation.

RESULTS

Parental exome analysis identified heterozygous pathogenic (or likely pathogenic) variants in 24 different genes in 26/50 couples (52%). Where 2 or more fetuses were affected, a genetic diagnosis was obtained in 18/29 cases (62%). In most cases, the clinical features were typical of the disorder, but in others, they result from a hypomorphic variant or represent the most severe form of a variable phenotypic spectrum.

CONCLUSION

We conclude that exome sequencing of parental samples is a powerful strategy with high clinical utility for the genetic diagnosis of lethal or prenatal-onset recessive disorders. © 2017 The Authors Prenatal Diagnosis published by John Wiley & Sons Ltd.

Bardet-Biedl syndrome: Genetics, molecular pathophysiology, and disease management

Primary cilia play a key role in sensory perception and various signaling pathways. Any defect in them leads to group of disorders called ciliopathies, and Bardet–Biedl syndrome (BBS, OMIM 209900) is one among them. The disorder is clinically and genetically heterogeneous, with various primary and secondary clinical manifestations, and shows autosomal recessive inheritance and highly prevalent in inbred/consanguineous populations. The disease mapped to at least twenty different genes (BBS1-BBS20), follow oligogenic inheritance pattern. BBS proteins localizes to the centerosome and regulates the biogenesis and functions of the cilia. In BBS, the functioning of various systemic organs (with ciliated cells) gets deranged and results in systemic manifestations. Certain components of the disease (such as obesity, diabetes, and renal problems) when noticed earlier offer a disease management benefit to the patients. However, the awareness of the disease is comparatively low and most often noticed only after severe vision loss in patients, which is usually in the first decade of the patient's age. In the current review, we have provided the recent updates retrieved from various types of scientific literature through journals, on the genetics, its molecular relevance, and the clinical outcome in BBS. The review in nutshell would provide the basic awareness of the disease that will have an impact in disease management and counseling benefits to the patients and their families.

Bardet-Biedl Syndrome

Bardet-Biedl syndrome (BBS) is a rare autosomal recessive genetic disorder. It is characterized by heterogeneous clinical manifestations including primary features of the disease (rod-cone dystrophy, polydactyly, obesity, genital abnormalities, renal defects, and learning difficulties) and secondary BBS characteristics (developmental delay, speech deficit, brachydactyly or syndactyly, dental defects, ataxia or poor coordination, olfactory deficit, diabetes mellitus, congenital heart disease, etc.); most of these symptoms may not be present at birth but appear and progressively worsen during the first and second decades of life. At least 20 BBS genes have already been identified, and all of them are involved in primary cilia functioning. Genetic diagnosis of BBS is complicated due to lack of gene-specific disease symptoms; however, it is gradually becoming more accessible with the invention of multigene sequencing technologies. Clinical management of BBS is largely limited to a symptomatic treatment. Mouse experiments demonstrate that the most debilitating complication of BBS, blindness, can be rescued by topical gene therapy. There is a published case report describing the delay of BBS symptoms by nutritional compensation of the disease-related biochemical deficiencies. Progress in DNA testing technologies is likely to rapidly resolve all limitations in BBS diagnosis; however, much slower improvement is expected with regard to BBS treatment.

Alström Syndrome: Mutation Spectrum of ALMS1

Alström Syndrome (ALMS), a recessive, monogenic ciliopathy caused by mutations in ALMS1, is typically characterized by multisystem involvement including early cone-rod retinal dystrophy and blindness, hearing loss, childhood obesity, type 2 diabetes mellitus, cardiomyopathy, fibrosis, and multiple organ failure. The precise function of ALMS1 remains elusive, but roles in endosomal and ciliary transport and cell cycle regulation have been shown. The aim of our study was to further define the spectrum of ALMS1 mutations in patients with clinical features of ALMS. Mutational analysis in a world-wide cohort of 204 families identified 109 novel mutations, extending the number of known ALMS1 mutations to 239 and highlighting the allelic heterogeneity of this disorder. This study represents the most comprehensive mutation analysis in patients with ALMS, identifying the largest number of novel mutations in a single study worldwide. Here, we also provide an overview of all ALMS1 mutations identified to date.