Beyond the exome: utility of long-read whole genome sequencing in exome-negative autosomal recessive diseases.

BACKGROUND: Long-read whole genome sequencing (lrWGS) has the potential to address the technical limitations of exome sequencing in ways not possible by short-read WGS. However, its utility in autosomal recessive Mendelian diseases is largely unknown. METHODS: In a cohort of 34 families in which the suspected autosomal recessive diseases remained undiagnosed by exome sequencing, lrWGS was performed on the Pacific Bioscience Sequel IIe platform. RESULTS: Likely causal variants were identified in 13 (38%) of the cohort. These include (1) a homozygous splicing SV in TYMS as a novel candidate gene for lethal neonatal lactic acidosis, (2) a homozygous non-coding SV that we propose impacts STK25 expression and causes a novel neurodevelopmental disorder, (3) a compound heterozygous SV in RP1L1 with complex inheritance pattern in a family with inherited retinal disease, (4) homozygous deep intronic variants in LEMD2 and SNAP91 as novel candidate genes for neurodevelopmental disorders in two families, and (5) a promoter SNV in SLC4A4 causing non-syndromic band keratopathy. Surprisingly, we also encountered causal variants that could have been identified by short-read exome sequencing in 7 families. The latter highlight scenarios that are especially challenging at the interpretation level. CONCLUSIONS: Our data highlight the continued need to address the interpretation challenges in parallel with efforts to improve the sequencing technology itself. We propose a path forward for the implementation of lrWGS sequencing in the setting of autosomal recessive diseases in a way that maximizes its utility.

Further delineation of HIDEA syndrome.

Recently, the genetic cause of HIDEA syndrome (hypotonia, hypoventilation, intellectual disability, dysautonomia, epilepsy, and eye abnormalities) was identified as biallelic pathogenic variants in P4HTM, which encodes an atypical member of the prolyl 4-hydroxylases (P4Hs) family of enzymes. We report seven patients from four new families in whom HIDEA was only diagnosed after whole-exome sequencing (WES) revealed novel disease-causing variants in P4HTM. We note the variable phenotypic expressivity of the syndrome except for cognitive impairment/developmental delay, and hypotonia, which seem to be consistent findings. One patient only presented with hypotonia, developmental delay, and abnormal eye movements, which highlights the challenge in diagnosing milder cases with this new syndrome. Other notable features include mild facial dysmorphism, obesity, and brain dysmyelination and atrophy. We conclude that HIDEA is a highly variable syndrome and suspect that a large fraction of patients will be diagnosed via reverse phenotyping after recessive P4HTM variants are identified by agnostic genomic sequencing assays.

The morbid genome of ciliopathies: an update.

PURPOSE: Ciliopathies are highly heterogeneous clinical disorders of the primary cilium. We aim to characterize a large cohort of ciliopathies phenotypically and molecularly. METHODS: Detailed phenotypic and genomic analysis of patients with ciliopathies, and functional characterization of novel candidate genes. RESULTS: In this study, we describe 125 families with ciliopathies and show that deleterious variants in previously reported genes, including cryptic splicing variants, account for 87% of cases. Additionally, we further support a number of previously reported candidate genes (BBIP1, MAPKBP1, PDE6D, and WDPCP), and propose nine novel candidate genes (CCDC67, CCDC96, CCDC172, CEP295, FAM166B, LRRC34, TMEM17, TTC6, and TTC23), three of which (LRRC34, TTC6, and TTC23) are supported by functional assays that we performed on available patient-derived fibroblasts. From a phenotypic perspective, we expand the phenomenon of allelism that characterizes ciliopathies by describing novel associations including WDR19-related Stargardt disease and SCLT1- and CEP164-related Bardet-Biedl syndrome. CONCLUSION: In this cohort of phenotypically and molecularly characterized ciliopathies, we draw important lessons that inform the clinical management and the diagnostics of this class of disorders as well as their basic biology.

Phenotypic delineation of the retinal arterial macroaneurysms with supravalvular pulmonic stenosis syndrome.

Retinal arterial macroaneurysms with supravalvular pulmonic stenosis (RAMSVPS), also known as Familial Retinal Arterial Macroaneurysms (FRAM) syndrome, is a very rare multisystem disorder. Here, we present a case series comprising ophthalmologic and systemic evaluation of patients homozygous for RAMSVPS syndrome causative IGFBP7 variant. New clinical details on 22 previously published and 8 previously unpublished patients are described. Age at first presentation ranged from 1 to 34 years. The classical feature of macroaneurysms and vascular beading involving the retinal arteries was universal. Follow up extending up to 14 years after initial diagnosis revealed recurrent episodes of bleeding and leakage from macroaneurysms in 55% and 59% of patients, respectively. The majority of patients who underwent echocardiography (18/23) showed evidence of heart involvement, most characteristically pulmonary (valvular or supravalvular) stenosis, often requiring surgical correction (12/18). Four patients died in the course of the study from complications of pulmonary stenosis, cerebral hemorrhage, and cardiac complications. Liver involvement (usually cirrhosis) was observed in eight patients. Cerebral vascular involvement was observed in one patient, and stroke was observed in two. We conclude that RAMSVPS is a recognizable syndrome characterized by a high burden of ocular and systemic morbidity, and risk of premature death. Recommendations are proposed for early detection and management of these complications.

Congenital glaucoma and CYP1B1: an old story revisited.

Primary congenital glaucoma is a trabecular meshwork dysgenesis with resultant increased intraocular pressure and ocular damage. CYP1B1 mutations remain the most common identifiable genetic cause. However, important questions about the penetrance of CYP1B1-related congenital glaucoma remain unanswered. Furthermore, mutations in other genes have been described although their exact contribution and potential genetic interaction, if any, with CYP1B1 mutations are not fully explored. In this study, we employed modern genomic approaches to re-examine CYP1B1-related congenital glaucoma. A cohort of 193 patients (136 families) diagnosed with congenital glaucoma. We identified biallelic CYP1B1 mutations in 80.8% (87.5 and 66.1% in familial and sporadic cases, respectively, p < 0.0086). The large family size of the study population allowed us to systematically examine penetrance of all identified alleles. With the exception of c.1103G>A (p.R368H), previously reported pathogenic mutations were highly penetrant (91.2%). We conclude from the very low penetrance and genetic epidemiological analyses that c.1103G>A (p.R368H) is unlikely to be a disease-causing recessive mutation in congenital glaucoma as previously reported. All cases that lacked biallelic CYP1B1 mutations underwent whole exome sequencing. No mutations in LTBP2, MYOC or TEK were encountered. On the other hand, mutations were identified in genes linked to other ophthalmic phenotypes, some inclusive of glaucoma, highlighting conditions that might phenotypically overlap with primary congenital glaucoma (SLC4A4, SLC4A11, CPAMD8, and KERA). We also encountered candidate causal variants in genes not previously linked to human diseases: BCO2, TULP2, and DGKQ. Our results both expand and refine the genetic spectrum of congenital glaucoma with important clinical implications.

Expanding the phenome and variome of skeletal dysplasia.

PURPOSE: To describe our experience with a large cohort (411 patients from 288 families) of various forms of skeletal dysplasia who were molecularly characterized. METHODS: Detailed phenotyping and next-generation sequencing (panel and exome). RESULTS: Our analysis revealed 224 pathogenic/likely pathogenic variants (54 (24%) of which are novel) in 123 genes with established or tentative links to skeletal dysplasia. In addition, we propose 5 genes as candidate disease genes with suggestive biological links (WNT3A, SUCO, RIN1, DIP2C, and PAN2). Phenotypically, we note that our cohort spans 36 established phenotypic categories by the International Skeletal Dysplasia Nosology, as well as 18 novel skeletal dysplasia phenotypes that could not be classified under these categories, e.g., the novel C3orf17-related skeletal dysplasia. We also describe novel phenotypic aspects of well-known disease genes, e.g., PGAP3-related Toriello-Carey syndrome-like phenotype. We note a strong founder effect for many genes in our cohort, which allowed us to calculate a minimum disease burden for the autosomal recessive forms of skeletal dysplasia in our population (7.16E-04), which is much higher than the global average. CONCLUSION: By expanding the phenotypic, allelic, and locus heterogeneity of skeletal dysplasia in humans, we hope our study will improve the diagnostic rate of patients with these conditions.

Correction to: Expanding the genetic heterogeneity of intellectual disability.

Variant nomenclature discrepancy was identified in the article.

Expanding the genetic heterogeneity of intellectual disability.

Intellectual disability (ID) is a common morbid condition with a wide range of etiologies. The list of monogenic forms of ID has increased rapidly in recent years thanks to the implementation of genomic sequencing techniques. In this study, we describe the phenotypic and genetic findings of 68 families (105 patients) all with novel ID-related variants. In addition to established ID genes, including ones for which we describe unusual mutational mechanism, some of these variants represent the first confirmatory disease-gene links following previous reports (TRAK1, GTF3C3, SPTBN4 and NKX6-2), some of which were based on single families. Furthermore, we describe novel variants in 14 genes that we propose as novel candidates (ANKHD1, ASTN2, ATP13A1, FMO4, MADD, MFSD11, NCKAP1, NFASC, PCDHGA10, PPP1R21, SLC12A2, SLK, STK32C and ZFAT). We highlight MADD and PCDHGA10 as particularly compelling candidates in which we identified biallelic likely deleterious variants in two independent ID families each. We also highlight NCKAP1 as another compelling candidate in a large family with autosomal dominant mild intellectual disability that fully segregates with a heterozygous truncating variant. The candidacy of NCKAP1 is further supported by its biological function, and our demonstration of relevant expression in human brain. Our study expands the locus and allelic heterogeneity of ID and demonstrates the power of positional mapping to reveal unusual mutational mechanisms.

Characterizing the morbid genome of ciliopathies.

BACKGROUND: Ciliopathies are clinically diverse disorders of the primary cilium. Remarkable progress has been made in understanding the molecular basis of these genetically heterogeneous conditions; however, our knowledge of their morbid genome, pleiotropy, and variable expressivity remains incomplete. RESULTS: We applied genomic approaches on a large patient cohort of 371 affected individuals from 265 families, with phenotypes that span the entire ciliopathy spectrum. Likely causal mutations in previously described ciliopathy genes were identified in 85% (225/265) of the families, adding 32 novel alleles. Consistent with a fully penetrant model for these genes, we found no significant difference in their "mutation load" beyond the causal variants between our ciliopathy cohort and a control non-ciliopathy cohort. Genomic analysis of our cohort further identified mutations in a novel morbid gene TXNDC15, encoding a thiol isomerase, based on independent loss of function mutations in individuals with a consistent ciliopathy phenotype (Meckel-Gruber syndrome) and a functional effect of its deficiency on ciliary signaling. Our study also highlighted seven novel candidate genes (TRAPPC3, EXOC3L2, FAM98C, C17orf61, LRRCC1, NEK4, and CELSR2) some of which have established links to ciliogenesis. Finally, we show that the morbid genome of ciliopathies encompasses many founder mutations, the combined carrier frequency of which accounts for a high disease burden in the study population. CONCLUSIONS: Our study increases our understanding of the morbid genome of ciliopathies. We also provide the strongest evidence, to date, in support of the classical Mendelian inheritance of Bardet-Biedl syndrome and other ciliopathies.

Lack of Correlation Between Diabetic Macular Edema and Thickness of the Peripapillary Retinal Nerve Fibre Layer.

INTRODUCTION: We compared the thickness of the peripapillary retinal nerve fiber layer (RNFL) in patients with diabetic macular edema (DME) and/against the thickness in the normal population. METHODS: This cross-sectional study compared the RNFL thickness in patients with DME (DME group) using optical coherence tomography (OCT) to a comparable group of healthy (nondiabetic) patients (control group). Measurements were performed in different/the four peripapillary quadrants and in the macula region for the fovea, parafoveal, and perifoveal areas. The mean RNFL thickness was compared between both groups. RESULTS: There were fifty eyes of fifty nonglaucomatous diabetic patients with DME (29 with nonproliferative diabetic retinopathy [PDR] and 21 with PDR), and fifty eyes in the control group. The macular regions were significantly thicker in the DME group compared to the control group. The central foveal thickness was 149 µ thicker in eyes with DME compared to the control group (P < 0.001). The difference in total RNFL thickness between groups was not significant (4.4 µ [95% confidence interval: -3.1 to +12]). The between-group differences in peripapillary RNFL thickness by age group, glycemic control, history of intravitreal treatments, and refractive errors were not statistically significant (P > 0.05, all comparisons). CONCLUSION: Peripapillary RNFL thickness measurements were not significantly influenced by DME. Hence, OCT parameters could be used to monitor/early detect glaucomatous eyes even in the presence of DME.

Argon Laser Photoablation for Treating Benign Pigmented Conjunctival Nevi.

PURPOSE: To evaluate the outcomes of argon laser photoablation of benign conjunctival pigmented nevi with different clinical presentations. PATIENTS AND METHODS: This interventional case series was conducted between July 2014 and January 2015. Patients presenting with benign conjunctival nevi were included. Data were collected on the clinical features at presentation, argon laser photoablation, and follow-up at 8 and 24 weeks. Postoperative photography allowed recording of the success of each case and the overall success rate. Complete removal of conjunctival pigments was considered an absolute success. Partial pigmentation requiring repeat laser treatment was considered a qualified success. RESULTS: There were 14 eyes (four right eyes and ten left eyes) with benign pigmented conjunctival nevi. There were three males and eight females in the study sample. The median age was 36 (25% percentile: 26 years). Three patients had bilateral lesions. The nevi were located temporally in nine eyes, nasally in three eyes, and on the inferior bulbar conjunctiva in two eyes. The mean horizontal and vertical diameters of nevi were 5 ± 2 mm and 4 ± 2.7 mm, respectively. The mean follow-up period was 5 months. Following laser treatment, no eyes had subconjunctival hemorrhage, infection, scarring, neovascularization, recurrence, or corneal damage. The absolute success rate of laser ablation was 79%. Three eyes with elevated nevi had one to three sessions of laser ablation resulting in a qualified success rate of 100%. CONCLUSIONS: Argon laser ablation was a safe and effective treatment for the treatment of selective benign pigmented conjunctival nevi in Arab patients.

Treatment of retinitis pigmentosa due to MERTK mutations by ocular subretinal injection of adeno-associated virus gene vector: results of a phase I trial.

MERTK is an essential component of the signaling network that controls phagocytosis in retinal pigment epithelium (RPE), the loss of which results in photoreceptor degeneration. Previous proof-of-concept studies have demonstrated the efficacy of gene therapy using human MERTK (hMERTK) packaged into adeno-associated virus (AAV2) in treating RCS rats and mice with MERTK deficiency. The purpose of this study was to assess the safety of gene transfer via subretinal administration of rAAV2-VMD2-hMERTK in subjects with MERTK-associated retinitis pigmentosa (RP). After a preclinical phase confirming the safety of the study vector in monkeys, six patients (aged 14 to 54, mean 33.3 years) with MERTK-related RP and baseline visual acuity (VA) ranging from 20/50 to <20/6400 were entered in a phase I open-label, dose-escalation trial. One eye of each patient (the worse-seeing eye in five subjects) received a submacular injection of the viral vector, first at a dose of 150 µl (5.96 × 10(10)vg; 2 patients) and then 450 µl (17.88 × 10(10)vg; 4 patients). Patients were followed daily for 10 days at 30, 60, 90, 180, 270, 365, 540, and 730 days post-injection. Collected data included (1) full ophthalmologic examination including best-corrected VA, intraocular pressure, color fundus photographs, macular spectral domain optical coherence tomography and full-field stimulus threshold test (FST) in both the study and fellow eyes; (2) systemic safety data including CBC, liver and kidney function tests, coagulation profiles, urine analysis, AAV antibody titers, peripheral blood PCR and ASR measurement; and (3) listing of ophthalmological or systemic adverse effects. All patients completed the 2-year follow-up. Subretinal injection of rAAV2-VMD2-hMERTK was associated with acceptable ocular and systemic safety profiles based on 2-year follow-up. None of the patients developed complications that could be attributed to the gene vector with certainty. Postoperatively, one patient developed filamentary keratitis, and two patients developed progressive cataract. Of these two patients, one also developed transient subfoveal fluid after the injection as well as monocular oscillopsia. Two patients developed a rise in AAV antibodies, but neither patient was positive for rAAV vector genomes via PCR. Three patients also displayed measurable improved visual acuity in the treated eye following surgery, although the improvement was lost by 2 years in two of these patients. Gene therapy for MERTK-related RP using careful subretinal injection of rAAV2-VMD2-hMERTK is not associated with major side effects and may result in clinical improvement in a subset of patients.

Expanding the clinical, allelic, and locus heterogeneity of retinal dystrophies.

PURPOSE: Retinal dystrophies (RD) are heterogeneous hereditary disorders of the retina that are usually progressive in nature. The aim of this study was to clinically and molecularly characterize a large cohort of RD patients. METHODS: We have developed a next-generation sequencing assay that allows known RD genes to be sequenced simultaneously. We also performed mapping studies and exome sequencing on familial and on syndromic RD patients who tested negative on the panel. RESULTS: Our panel identified the likely causal mutation in >60% of the 292 RD families tested. Mapping studies on all 162 familial RD patients who tested negative on the panel identified two novel disease loci on Chr2:25,550,180-28,794,007 and Chr16:59,225,000-72,511,000. Whole-exome sequencing revealed the likely candidate as AGBL5 and CDH16, respectively. We also performed exome sequencing on negative syndromic RD cases and identified a novel homozygous truncating mutation in GNS in a family with the novel combination of mucopolysaccharidosis and RD. Moreover, we identified a homozygous truncating mutation in DNAJC17 in a family with an apparently novel syndrome of retinitis pigmentosa and hypogammaglobulinemia. CONCLUSION: Our study expands the clinical and allelic spectrum of known RD genes, and reveals AGBL5, CDH16, and DNAJC17 as novel disease candidates.Genet Med 18 6, 554-562.

Exome-based case-control association study using extreme phenotype design reveals novel candidates with protective effect in diabetic retinopathy.

UNLABELLED: Diabetic retinopathy (DR) is a common clinical expression of diabetes mellitus-induced vasculopathy and is a major cause of vision loss. Significant gaps remain in our understanding of the molecular pathoetiology of DR, and it is hoped that human genetic approaches can reveal novel targets especially since DR is a heritable trait. Previous studies have focused on genetic risk factors of DR but their results have been mixed. In this study, we hypothesized that the use of the extreme phenotype design will increase the power of a genomewide search for "protective" genetic variants. We enrolled a small yet atypical cohort of 43 diabetics who did not develop DR a decade or more after diagnosis (cases), and 64 diabetics with DR (controls), all of similar ethnic background (Saudi). Whole-exome sequencing of the entire cohort was followed by statistical analysis employing combined multivariate and collapsing methods at the gene level, to identify genes that are enriched for rare variants in cases vs. CONTROLS: Three genes (NME3, LOC728699, and FASTK) reached gene-based genome-wide significance at the 10(-08) threshold (p value = 1.55 × 10(-10), 6.23 × 10(-10), 3.21 × 10(-08), respectively). Our results reveal novel candidate genes whose increased rare variant burden appears to protect against DR, thus highlighting them as attractive candidate targets, if replicated by future studies, for the treatment and prevention of DR. Extreme phenotype design when implemented in sequencing-based genome-wide case-control studies has the potential to reveal novel candidates with a smaller cohort size compared to standard study designs.

Cataract Surgery Audit at a Private Hospital in Saudi Arabia.

BACKGROUND: To assess the visual outcomes following cataract surgeries at a Private Eye Hospital in Riyadh, Saudi Arabia. METHODS: This was a cohort study of cataract surgeries performed from January to June 2014. Preoperative data were collected on patient demographics presenting and best corrected distance visual acuity (BCVA) and ocular comorbidity. Data were also collected on the type of surgery, type of intraocular lens (IOLs) implanted, and complications. BCVA and refractive status at 6-8 weeks postoperatively were noted. The predictors of vision ≥ 6/18 were identified. RESULTS: Four hundred eyes of 400 patients underwent cataract surgery. There were 235 (59%) males. Presenting preoperative vision was < 6/60 in 52 (13%) eyes. There were 395 (99%) eyes that underwent IOL implantation following phacoemulsification and 4 eyes received a sulcus fixated IOL. A single piece aspheric IOL was implanted in 358 (90%) eyes and a toric IOL was implanted in 31 (8%) eyes. Postoperative BCVA was classified as a "good outcome" (≥ 6/18) in 320 (80%) and a "poor outcome" (< 6/60) in 24 (6%) eyes. Young age (adjusted odds ratio (OR) = 0.97, P = 0.01), male (adjusted OR = 2.4, P = 0.002), and ocular co-morbidities (adjusted OR = 0.2, P < 0.001) were predictors of vision ≥ 6/18. Complications included a dropped nucleus and a posterior capsular tear in 2 eyes each. Two hundred and fifty-two (63%) eyes were emmetropic or intentionally myopic for distance. Astigmatism < 2 D was present in 264 (66%) eyes and astigmatism > 2 D was present in 33 (8%) eyes. CONCLUSION: The recent trend of intentional overcorrection in one eye following modern cataract surgery in order to provide some functional near vision indicates that benchmark for success in getting "good visual outcomes" postoperatively (vision of ≥ 6/18) may need to be revised.

IFT27, encoding a small GTPase component of IFT particles, is mutated in a consanguineous family with Bardet-Biedl syndrome.

Bardet-Biedl syndrome (BBS) is an autosomal recessive ciliopathy with multisystem involvement. So far, 18 BBS genes have been identified and the majority of them are essential for the function of BBSome, a protein complex involved in transporting membrane proteins into and from cilia. Yet defects in the identified genes cannot account for all the BBS cases. The genetic heterogeneity of this disease poses significant challenge to the identification of additional BBS genes. In this study, we coupled human genetics with functional validation in zebrafish and identified IFT27 as a novel BBS gene (BBS19). This is the first time an intraflagellar transport (IFT) gene is implicated in the pathogenesis of BBS, highlighting the genetic complexity of this disease.

Mutations in LRPAP1 are associated with severe myopia in humans.

Myopia is an extremely common eye disorder but the pathogenesis of its isolated form, which accounts for the overwhelming majority of cases, remains poorly understood. There is strong evidence for genetic predisposition to myopia, but determining myopia genetic risk factors has been difficult to achieve. We have identified Mendelian forms of myopia in four consanguineous families and implemented exome/autozygome analysis to identify homozygous truncating variants in LRPAP1 and CTSH as the likely causal mutations. LRPAP1 encodes a chaperone of LRP1, which is known to influence TGF-ß activity. Interestingly, we observed marked deficiency of LRP1 and upregulation of TGF-ß in cells from affected individuals, the latter being consistent with available data on the role of TGF-ß in the remodeling of the sclera in myopia and the high frequency of myopia in individuals with Marfan syndrome who characteristically have upregulation of TGF-ß signaling. CTSH, on the other hand, encodes a protease and we show that deficiency of the murine ortholog results in markedly abnormal globes consistent with the observed human phenotype. Our data highlight a role for LRPAP1 and CTSH in myopia genetics and demonstrate the power of Mendelian forms in illuminating new molecular mechanisms that may be relevant to common phenotypes.

Autozygome-guided exome sequencing in retinal dystrophy patients reveals pathogenetic mutations and novel candidate disease genes.

Retinal dystrophy (RD) is a heterogeneous group of hereditary diseases caused by loss of photoreceptor function and contributes significantly to the etiology of blindness globally but especially in the industrialized world. The extreme locus and allelic heterogeneity of these disorders poses a major diagnostic challenge and often impedes the ability to provide a molecular diagnosis that can inform counseling and gene-specific treatment strategies. In a large cohort of nearly 150 RD families, we used genomic approaches in the form of autozygome-guided mutation analysis and exome sequencing to identify the likely causative genetic lesion in the majority of cases. Additionally, our study revealed six novel candidate disease genes (C21orf2, EMC1, KIAA1549, GPR125, ACBD5, and DTHD1), two of which (ACBD5 and DTHD1) were observed in the context of syndromic forms of RD that are described for the first time.

In search of triallelism in Bardet-Biedl syndrome.

Bardet-Biedl syndrome (BBS) is a model disease for ciliopathy in humans. The remarkable genetic heterogeneity that characterizes this disease is consistent with accumulating data on the interaction between the proteins encoded by the 14 BBS genes identified to date. Previous reports suggested that such interaction may also extend to instances of oligogenic inheritance in the form of triallelism which defies the long held view of BBS as an autosomal recessive disease. In order to investigate the magnitude of triallelism in BBS, we conducted a comprehensive analysis of all 14 BBS genes as well as the CCDC28B-modifier gene in a cohort of 29 BBS families, most of which are multiplex. Two in trans mutations in a BBS gene were identified in each of these families for a total of 20 mutations including 12 that are novel. In no instance did we observe two mutations in unaffected members of a given family, or observe the presence of a third allele that convincingly acted as a modifier of penetrance and supported the triallelic model of BBS. In addition to presenting a comprehensive genotype/phenotype overview of a large set of BBS mutations, including the occurrence of nonsyndromic retinitis pigmentosa in a family with a novel BBS9 mutation, our study argues in favor of straightforward autosomal recessive BBS in most cases.