Shared and unique consequences of Joubert Syndrome gene dysfunction on the zebrafish central nervous system.

Joubert Syndrome (JBTS) is a neurodevelopmental ciliopathy defined by a highly specific midbrain-hindbrain malformation, variably associated with additional neurological features. JBTS displays prominent genetic heterogeneity with >40 causative genes that encode proteins localising to the primary cilium, a sensory organelle that is essential for transduction of signalling pathways during neurodevelopment, among other vital functions. JBTS proteins localise to distinct ciliary subcompartments, suggesting diverse functions in cilium biology. Currently, there is no unifying pathomechanism to explain how dysfunction of such diverse primary cilia-related proteins results in such a highly specific brain abnormality. In order to identify the shared consequence of JBTS gene dysfunction, we carried out transcriptomic analysis using zebrafish mutants for the JBTS-causative genes cc2d2aw38, cep290fh297, inpp5ezh506, talpid3i264 and togaram1zh510and the Bardet-Biedl syndrome-causative gene bbs1k742. We identified no commonly dysregulated signalling pathways in these mutants and yet all mutants displayed an enrichment of altered gene sets related to central nervous system function. We found that JBTS mutants have altered primary cilia throughout the brain, however do not display abnormal brain morphology. Nonetheless, behavioural analyses revealed reduced locomotion and loss of postural control which, together with the transcriptomic results, hint at underlying abnormalities in neuronal activity and/or neuronal circuit function. These zebrafish models therefore offer the unique opportunity to study the role of primary cilia in neuronal function beyond early patterning, proliferation and differentiation.

Spatial and cell type transcriptional landscape of human cerebellar development.

The human neonatal cerebellum is one-fourth of its adult size yet contains the blueprint required to integrate environmental cues with developing motor, cognitive and emotional skills into adulthood. Although mature cerebellar neuroanatomy is well studied, understanding of its developmental origins is limited. In this study, we systematically mapped the molecular, cellular and spatial composition of human fetal cerebellum by combining laser capture microscopy and SPLiT-seq single-nucleus transcriptomics. We profiled functionally distinct regions and gene expression dynamics within cell types and across development. The resulting cell atlas demonstrates that the molecular organization of the cerebellar anlage recapitulates cytoarchitecturally distinct regions and developmentally transient cell types that are distinct from the mouse cerebellum. By mapping genes dominant for pediatric and adult neurological disorders onto our dataset, we identify relevant cell types underlying disease mechanisms. These data provide a resource for probing the cellular basis of human cerebellar development and disease.

Loss of CBY1 results in a ciliopathy characterized by features of Joubert syndrome.

Ciliopathies are clinically and genetically heterogeneous diseases. We studied three patients from two independent families presenting with features of Joubert syndrome: abnormal breathing pattern during infancy, developmental delay/intellectual disability, cerebellar ataxia, molar tooth sign on magnetic resonance imaging scans, and polydactyly. We identified biallelic loss-of-function (LOF) variants in CBY1, segregating with the clinical features of Joubert syndrome in the families. CBY1 localizes to the distal end of the mother centriole, contributing to the formation and function of cilia. In accordance with the clinical and mutational findings in the affected individuals, we demonstrated that depletion of Cby1 in zebrafish causes ciliopathy-related phenotypes. Levels of CBY1 transcript were found reduced in the patients compared with controls, suggesting degradation of the mutated transcript through nonsense-mediated messenger RNA decay. Accordingly, we could detect CBY1 protein in fibroblasts from controls, but not from patients by immunofluorescence. Furthermore, we observed reduced ability to ciliate, increased ciliary length, and reduced levels of the ciliary proteins AHI1 and ARL13B in patient fibroblasts. Our data show that CBY1 LOF-variants cause a ciliopathy with features of Joubert syndrome.

Interpreting the clinical significance of combined variants in multiple recessive disease genes: systematic investigation of Joubert syndrome yields little support for oligogenicity.

PurposeNext-generation sequencing (NGS) often identifies multiple rare predicted-deleterious variants (RDVs) in different genes associated with a recessive disorder in a given patient. Such variants have been proposed to contribute to digenicity/oligogenicity or "triallelism" or to act as genetic modifiers.MethodsUsing the recessive ciliopathy Joubert syndrome (JBTS) as a model, we investigated these possibilities systematically, relying on NGS of known JBTS genes in a large JBTS and two control cohorts.Results65% of affected individuals had a recessive genetic cause, while 4.9% were candidates for di-/oligogenicity, harboring heterozygous RDVs in two or more genes, compared with 4.2-8% in controls (P = 0.66-0.21). Based on Exome Aggregation Consortium (ExAC) allele frequencies, the probability of cumulating RDVs in any two JBTS genes is 9.3%. We found no support for triallelism, as no unaffected siblings carried the same biallelic RDVs as their affected relative. Sixty percent of individuals sharing identical causal RDVs displayed phenotypic discordance. Although 38% of affected individuals harbored RDVs in addition to the causal mutations, their presence did not correlate with phenotypic severity.ConclusionOur data offer little support for triallelism or digenicity/oligogenicity as clinically relevant inheritance modes in JBTS. While phenotypic discordance supports the existence of genetic modifiers, identifying clinically relevant modifiers remains challenging.

Mutations in ARMC9, which Encodes a Basal Body Protein, Cause Joubert Syndrome in Humans and Ciliopathy Phenotypes in Zebrafish.

Joubert syndrome (JS) is a recessive neurodevelopmental disorder characterized by hypotonia, ataxia, abnormal eye movements, and variable cognitive impairment. It is defined by a distinctive brain malformation known as the "molar tooth sign" on axial MRI. Subsets of affected individuals have malformations such as coloboma, polydactyly, and encephalocele, as well as progressive retinal dystrophy, fibrocystic kidney disease, and liver fibrosis. More than 35 genes have been associated with JS, but in a subset of families the genetic cause remains unknown. All of the gene products localize in and around the primary cilium, making JS a canonical ciliopathy. Ciliopathies are unified by their overlapping clinical features and underlying mechanisms involving ciliary dysfunction. In this work, we identify biallelic rare, predicted-deleterious ARMC9 variants (stop-gain, missense, splice-site, and single-exon deletion) in 11 individuals with JS from 8 families, accounting for approximately 1% of the disorder. The associated phenotypes range from isolated neurological involvement to JS with retinal dystrophy, additional brain abnormalities (e.g., heterotopia, Dandy-Walker malformation), pituitary insufficiency, and/or synpolydactyly. We show that ARMC9 localizes to the basal body of the cilium and is upregulated during ciliogenesis. Typical ciliopathy phenotypes (curved body shape, retinal dystrophy, coloboma, and decreased cilia) in a CRISPR/Cas9-engineered zebrafish mutant model provide additional support for ARMC9 as a ciliopathy-associated gene. Identifying ARMC9 mutations as a cause of JS takes us one step closer to a full genetic understanding of this important disorder and enables future functional work to define the central biological mechanisms underlying JS and other ciliopathies.

Mortality in Joubert syndrome.

Joubert syndrome (JS) is a rare, recessively inherited neurodevelopmental disorder characterized by a distinctive mid-hindbrain malformation. Little is known about mortality in affected individuals. Identifying the timing and causes of death will allow for development of healthcare guidelines for families and providers and, thus, help to prolong and improve the lives of patients with JS. We evaluated information on 40 deceased individuals with JS to characterize age and cause of death. We compared this population with 525 living individuals with JS to estimate associations between risk of death and extra-neurological features. Genetic causes were examined in both groups. Mean age of death in this cohort was 7.2 years, and the most prevalent causes of death were respiratory failure (35%), particularly in individuals younger than 6 years, and kidney failure (37.5%), which was more common in older individuals. We identified possible associations between risk of death and kidney disease, liver fibrosis, polydactyly, occipital encephalocele, and genetic cause. This work highlights factors (genetic cause, extra-neurological organ involvement, and other malformations) likely to be associated with higher risk of mortality in JS, which should prompt increased monitoring for respiratory issues, kidney disease, and liver fibrosis.

Abnormal glycosylation in Joubert syndrome type 10.

BACKGROUND: The discovery of disease pathogenesis requires systematic agnostic screening of multiple homeostatic processes that may become deregulated. We illustrate this principle in the evaluation and diagnosis of a 5-year-old boy with Joubert syndrome type 10 (JBTS10). He carried the OFD1 mutation p.Gln886Lysfs*2 (NM_003611.2: c.2656del) and manifested features of Joubert syndrome. METHODS: We integrated exome sequencing, MALDI-TOF mass spectrometry analyses of plasma and cultured dermal fibroblasts glycomes, and full clinical evaluation of the proband. Analyses of cilia formation and lectin staining were performed by immunofluorescence. Measurement of cellular nucleotide sugar levels was performed with high-performance anion-exchange chromatography with pulsed amperometric detection. Statistical analyses utilized the Student's and Fisher's exact t tests. RESULTS: Glycome analyses of plasma and cultured dermal fibroblasts identified abnormal N- and O-linked glycosylation profiles. These findings replicated in two unrelated males with OFD1 mutations. Cultured fibroblasts from affected individuals had a defect in ciliogenesis. The proband's fibroblasts also had an abnormally elevated nuclear sialylation signature and increased total cellular levels of CMP-sialic acid. Ciliogenesis and each glycosylation anomaly were rescued by expression of wild-type OFD1. CONCLUSIONS: The rescue of ciliogenesis and glycosylation upon reintroduction of WT OFD1 suggests that both contribute to the pathogenesis of JBTS10.

Molecular genetic findings and clinical correlations in 100 patients with Joubert syndrome and related disorders prospectively evaluated at a single center.

PURPOSE: Joubert syndrome (JS) is a genetically and clinically heterogeneous ciliopathy characterized by distinct cerebellar and brainstem malformations resulting in the diagnostic "molar tooth sign" on brain imaging. To date, more than 30 JS genes have been identified, but these do not account for all patients. METHODS: In our cohort of 100 patients with JS from 86 families, we prospectively performed extensive clinical evaluation and provided molecular diagnosis using a targeted 27-gene Molecular Inversion Probes panel followed by whole-exome sequencing (WES). RESULTS: We identified the causative gene in 94% of the families; 126 (27 novel) unique potentially pathogenic variants were found in 20 genes, including KIAA0753 and CELSR2, which had not previously been associated with JS. Genotype-phenotype correlation revealed the absence of retinal degeneration in patients with TMEM67, C5orf52, or KIAA0586 variants. Chorioretinal coloboma was associated with a decreased risk for retinal degeneration and increased risk for liver disease. TMEM67 was frequently associated with kidney disease. CONCLUSION: In JS, WES significantly increases the yield for molecular diagnosis, which is essential for reproductive counseling and the option of preimplantation and prenatal diagnosis as well as medical management and prognostic counseling for the age-dependent and progressive organ-specific manifestations, including retinal, liver, and kidney disease.Genet Med advance online publication 26 January 2017.

Recessive Inactivating Mutations in TBCK, Encoding a Rab GTPase-Activating Protein, Cause Severe Infantile Syndromic Encephalopathy.

Infantile encephalopathies are a group of clinically and biologically heterogeneous disorders for which the genetic basis remains largely unknown. Here, we report a syndromic neonatal encephalopathy characterized by profound developmental disability, severe hypotonia, seizures, diminished respiratory drive requiring mechanical ventilation, brain atrophy, dysgenesis of the corpus callosum, cerebellar vermis hypoplasia, and facial dysmorphism. Biallelic inactivating mutations in TBCK (TBC1-domain-containing kinase) were independently identified by whole-exome sequencing as the cause of this condition in four unrelated families. Matching these families was facilitated by the sharing of phenotypic profiles and WES data in a recently released web-based tool (Geno2MP) that links phenotypic information to rare variants in families with Mendelian traits. TBCK is a putative GTPase-activating protein (GAP) for small GTPases of the Rab family and has been shown to control cell growth and proliferation, actin-cytoskeleton dynamics, and mTOR signaling. Two of the three mutations (c.376C>T [p.Arg126(∗)] and c.1363A>T [p.Lys455(∗)]) are predicted to truncate the protein, and loss of the major TBCK isoform was confirmed in primary fibroblasts from one affected individual. The third mutation, c.1532G>A (p.Arg511His), alters a conserved residue within the TBC1 domain. Structural analysis implicated Arg511 as a required residue for Rab-GAP function, and in silico homology modeling predicted impaired GAP function in the corresponding mutant. These results suggest that loss of Rab-GAP activity is the underlying mechanism of disease. In contrast to other disorders caused by dysregulated mTOR signaling associated with focal or global brain overgrowth, impaired TBCK function results in progressive loss of brain volume.

Large-scale targeted sequencing comparison highlights extreme genetic heterogeneity in nephronophthisis-related ciliopathies.

BACKGROUND: The term nephronophthisis-related ciliopathies (NPHP-RC) describes a group of rare autosomal-recessive cystic kidney diseases, characterised by broad genetic and clinical heterogeneity. NPHP-RC is frequently associated with extrarenal manifestations and accounts for the majority of genetically caused chronic kidney disease (CKD) during childhood and adolescence. Generation of a molecular diagnosis has been impaired by this broad genetic heterogeneity. However, recently developed high-throughput exon sequencing techniques represent powerful and efficient tools to screen large cohorts for dozens of causative genes. METHODS: Therefore, we performed massively multiplexed targeted sequencing using the modified molecular inversion probe strategy (MIPs) in an international cohort of 384 patients diagnosed with NPHP-RC. RESULTS: As a result, we established the molecular diagnoses in 81/384 unrelated individuals (21.1%). We detected 127 likely disease-causing mutations in 18 of 34 evaluated NPHP-RC genes, 22 of which were novel. We further compared a subgroup of current findings to the results of a previous study in which we used an array-based microfluidic PCR technology in the same cohort. While 78 likely disease-causing mutations were previously detected by the array-based microfluidic PCR, the MIPs approach identified 94 likely pathogenic mutations. Compared with the previous approach, MIPs redetected 66 out of 78 variants and 28 previously unidentified variants, for a total of 94 variants. CONCLUSIONS: In summary, we demonstrate that the modified MIPs technology is a useful approach to screen large cohorts for a multitude of established NPHP genes in order to identify the underlying molecular cause. Combined application of two independent library preparation and sequencing techniques, however, may still be indicated for Mendelian diseases with extensive genetic heterogeneity in order to further increase diagnostic sensitivity.

MKS1 regulates ciliary INPP5E levels in Joubert syndrome.

BACKGROUND: Joubert syndrome (JS) is a recessive ciliopathy characterised by a distinctive brain malformation 'the molar tooth sign'. Mutations in >27 genes cause JS, and mutations in 12 of these genes also cause Meckel-Gruber syndrome (MKS). The goals of this work are to describe the clinical features of MKS1-related JS and determine whether disease causing MKS1 mutations affect cellular phenotypes such as cilium number, length and protein content as potential mechanisms underlying JS. METHODS: We measured cilium number, length and protein content (ARL13B and INPP5E) by immunofluorescence in fibroblasts from individuals with MKS1-related JS and in a three-dimensional (3D) spheroid rescue assay to test the effects of disease-related MKS1 mutations. RESULTS: We report MKS1 mutations (eight of them previously unreported) in nine individuals with JS. A minority of the individuals with MKS1-related JS have MKS features. In contrast to the truncating mutations associated with MKS, all of the individuals with MKS1-related JS carry ≥ 1 non-truncating mutation. Fibroblasts from individuals with MKS1-related JS make normal or fewer cilia than control fibroblasts, their cilia are more variable in length than controls, and show decreased ciliary ARL13B and INPP5E. Additionally, MKS1 mutant alleles have similar effects in 3D spheroids. CONCLUSIONS: MKS1 functions in the transition zone at the base of the cilium to regulate ciliary INPP5E content, through an ARL13B-dependent mechanism. Mutations in INPP5E also cause JS, so our findings in patient fibroblasts support the notion that loss of INPP5E function, due to either mutation or mislocalisation, is a key mechanism underlying JS, downstream of MKS1 and ARL13B.

The Ciliopathy Protein CC2D2A Associates with NINL and Functions in RAB8-MICAL3-Regulated Vesicle Trafficking.

Ciliopathies are a group of human disorders caused by dysfunction of primary cilia, ubiquitous microtubule-based organelles involved in transduction of extra-cellular signals to the cell. This function requires the concentration of receptors and channels in the ciliary membrane, which is achieved by complex trafficking mechanisms, in part controlled by the small GTPase RAB8, and by sorting at the transition zone located at the entrance of the ciliary compartment. Mutations in the transition zone gene CC2D2A cause the related Joubert and Meckel syndromes, two typical ciliopathies characterized by central nervous system malformations, and result in loss of ciliary localization of multiple proteins in various models. The precise mechanisms by which CC2D2A and other transition zone proteins control protein entrance into the cilium and how they are linked to vesicular trafficking of incoming cargo remain largely unknown. In this work, we identify the centrosomal protein NINL as a physical interaction partner of CC2D2A. NINL partially co-localizes with CC2D2A at the base of cilia and ninl knockdown in zebrafish leads to photoreceptor outer segment loss, mislocalization of opsins and vesicle accumulation, similar to cc2d2a-/- phenotypes. Moreover, partial ninl knockdown in cc2d2a-/- embryos enhances the retinal phenotype of the mutants, indicating a genetic interaction in vivo, for which an illustration is found in patients from a Joubert Syndrome cohort. Similar to zebrafish cc2d2a mutants, ninl morphants display altered Rab8a localization. Further exploration of the NINL-associated interactome identifies MICAL3, a protein known to interact with Rab8 and to play an important role in vesicle docking and fusion. Together, these data support a model where CC2D2A associates with NINL to provide a docking point for cilia-directed cargo vesicles, suggesting a mechanism by which transition zone proteins can control the protein content of the ciliary compartment.

An siRNA-based functional genomics screen for the identification of regulators of ciliogenesis and ciliopathy genes.

Defects in primary cilium biogenesis underlie the ciliopathies, a growing group of genetic disorders. We describe a whole-genome siRNA-based reverse genetics screen for defects in biogenesis and/or maintenance of the primary cilium, obtaining a global resource. We identify 112 candidate ciliogenesis and ciliopathy genes, including 44 components of the ubiquitin-proteasome system, 12 G-protein-coupled receptors, and 3 pre-mRNA processing factors (PRPF6, PRPF8 and PRPF31) mutated in autosomal dominant retinitis pigmentosa. The PRPFs localize to the connecting cilium, and PRPF8- and PRPF31-mutated cells have ciliary defects. Combining the screen with exome sequencing data identified recessive mutations in PIBF1, also known as CEP90, and C21orf2, also known as LRRC76, as causes of the ciliopathies Joubert and Jeune syndromes. Biochemical approaches place C21orf2 within key ciliopathy-associated protein modules, offering an explanation for the skeletal and retinal involvement observed in individuals with C21orf2 variants. Our global, unbiased approaches provide insights into ciliogenesis complexity and identify roles for unanticipated pathways in human genetic disease.

Recognizable cerebellar dysplasia associated with mutations in multiple tubulin genes.

Mutations in alpha- and beta-tubulins are increasingly recognized as a major cause of malformations of cortical development (MCD), typically lissencephaly, pachygyria and polymicrogyria; however, sequencing tubulin genes in large cohorts of MCD patients has detected tubulin mutations in only 1-13%. We identified patients with a highly characteristic cerebellar dysplasia but without lissencephaly, pachygyria and polymicrogyria typically associated with tubulin mutations. Remarkably, in seven of nine patients (78%), targeted sequencing revealed mutations in three different tubulin genes (TUBA1A, TUBB2B and TUBB3), occurring de novo or inherited from a mosaic parent. Careful re-review of the cortical phenotype on brain imaging revealed only an irregular pattern of gyri and sulci, for which we propose the term tubulinopathy-related dysgyria. Basal ganglia (100%) and brainstem dysplasia (80%) were common features. On the basis of in silico structural predictions, the mutations affect amino acids in diverse regions of the alpha-/beta-tubulin heterodimer, including the nucleotide binding pocket. Cell-based assays of tubulin dynamics reveal various effects of the mutations on incorporation into microtubules: TUBB3 p.Glu288Lys and p.Pro357Leu do not incorporate into microtubules at all, whereas TUBB2B p.Gly13Ala shows reduced incorporation and TUBA1A p.Arg214His incorporates fully, but at a slower rate than wild-type. The broad range of effects on microtubule incorporation is at odds with the highly stereotypical clinical phenotype, supporting differential roles for the three tubulin genes involved. Identifying this highly characteristic phenotype is important due to the low recurrence risk compared with the other (recessive) cerebellar dysplasias and the apparent lack of non-neurological medical issues.

KIAA0586 is Mutated in Joubert Syndrome.

Joubert syndrome (JS) is a recessive neurodevelopmental disorder characterized by a distinctive mid-hindbrain malformation. JS is part of a group of disorders called ciliopathies based on their overlapping phenotypes and common underlying pathophysiology linked to primary cilium dysfunction. Biallelic mutations in one of 28 genes, all encoding proteins localizing to the primary cilium or basal body, can cause JS. Despite this large number of genes, the genetic cause can currently be determined in about 62% of individuals with JS. To identify novel JS genes, we performed whole exome sequencing on 35 individuals with JS and found biallelic rare deleterious variants (RDVs) in KIAA0586, encoding a centrosomal protein required for ciliogenesis, in one individual. Targeted next-generation sequencing in a large JS cohort identified biallelic RDVs in eight additional families for an estimated prevalence of 2.5% (9/366 JS families). All affected individuals displayed JS phenotypes toward the mild end of the spectrum.

Rapid identification and recovery of ENU-induced mutations with next-generation sequencing and Paired-End Low-Error analysis.

BACKGROUND: Targeting Induced Local Lesions IN Genomes (TILLING) is a reverse genetics approach to directly identify point mutations in specific genes of interest in genomic DNA from a large chemically mutagenized population. Classical TILLING processes, based on enzymatic detection of mutations in heteroduplex PCR amplicons, are slow and labor intensive. RESULTS: Here we describe a new TILLING strategy in zebrafish using direct next generation sequencing (NGS) of 250 bp amplicons followed by Paired-End Low-Error (PELE) sequence analysis. By pooling a genomic DNA library made from over 9,000 N-ethyl-N-nitrosourea (ENU) mutagenized F1 fish into 32 equal pools of 288 fish, each with a unique Illumina barcode, we reduce the complexity of the template to a level at which we can detect mutations that occur in a single heterozygous fish in the entire library. MiSeq sequencing generates 250 base-pair overlapping paired-end reads, and PELE analysis aligns the overlapping sequences to each other and filters out any imperfect matches, thereby eliminating variants introduced during the sequencing process. We find that this filtering step reduces the number of false positive calls 50-fold without loss of true variant calls. After PELE we were able to validate 61.5% of the mutant calls that occurred at a frequency between 1 mutant call:100 wildtype calls and 1 mutant call:1000 wildtype calls in a pool of 288 fish. We then use high-resolution melt analysis to identify the single heterozygous mutation carrier in the 288-fish pool in which the mutation was identified. CONCLUSIONS: Using this NGS-TILLING protocol we validated 28 nonsense or splice site mutations in 20 genes, at a two-fold higher efficiency than using traditional Cel1 screening. We conclude that this approach significantly increases screening efficiency and accuracy at reduced cost and can be applied in a wide range of organisms.

Mutations in LAMA1 cause cerebellar dysplasia and cysts with and without retinal dystrophy.

Cerebellar dysplasia with cysts (CDC) is an imaging finding typically seen in combination with cobblestone cortex and congenital muscular dystrophy in individuals with dystroglycanopathies. More recently, CDC was reported in seven children without neuromuscular involvement (Poretti-Boltshauser syndrome). Using a combination of homozygosity mapping and whole-exome sequencing, we identified biallelic mutations in LAMA1 as the cause of CDC in seven affected individuals (from five families) independent from those included in the phenotypic description of Poretti-Boltshauser syndrome. Most of these individuals also have high myopia, and some have retinal dystrophy and patchy increased T2-weighted fluid-attenuated inversion recovery (T2/FLAIR) signal in cortical white matter. In one additional family, we identified two siblings who have truncating LAMA1 mutations in combination with retinal dystrophy and mild cerebellar dysplasia without cysts, indicating that cysts are not an obligate feature associated with loss of LAMA1 function. This work expands the phenotypic spectrum associated with the lamininopathy disorders and highlights the tissue-specific roles played by different laminin-encoding genes.

Mutations in CSPP1 cause primary cilia abnormalities and Joubert syndrome with or without Jeune asphyxiating thoracic dystrophy.

Joubert syndrome (JBTS) is a recessive ciliopathy in which a subset of affected individuals also have the skeletal dysplasia Jeune asphyxiating thoracic dystrophy (JATD). Here, we have identified biallelic truncating CSPP1 (centrosome and spindle pole associated protein 1) mutations in 19 JBTS-affected individuals, four of whom also have features of JATD. CSPP1 mutations explain ∼5% of JBTS in our cohort, and despite truncating mutations in all affected individuals, the range of phenotypic severity is broad. Morpholino knockdown of cspp1 in zebrafish caused phenotypes reported in other zebrafish models of JBTS (curved body shape, pronephric cysts, and cerebellar abnormalities) and reduced ciliary localization of Arl13b, further supporting loss of CSPP1 function as a cause of JBTS. Fibroblasts from affected individuals with CSPP1 mutations showed reduced numbers of primary cilia and/or short primary cilia, as well as reduced axonemal localization of ciliary proteins ARL13B and adenylyl cyclase III. In summary, CSPP1 mutations are a major cause of the Joubert-Jeune phenotype in humans; however, the mechanism by which these mutations lead to both JBTS and JATD remains unknown.

The Joubert syndrome-associated missense mutation (V443D) in the Abelson-helper integration site 1 (AHI1) protein alters its localization and protein-protein interactions.

BACKGROUND: Missense mutations in AHI1 result in the neurodevelopmental ciliopathy called Joubert syndrome. RESULTS: Mutations in AHI1 decrease cilia formation, alter its localization and stability, and change its binding to HAP1 and NPHP1. CONCLUSION: Mutations in AHI1 affect ciliogenesis, AHI1 protein localization, and AHI1-protein interactions. SIGNIFICANCE: This study begins to describe how missense mutations in AHI1 can cause Joubert syndrome. Mutations in AHI1 cause Joubert syndrome (JBTS), a neurodevelopmental ciliopathy, characterized by midbrain-hindbrain malformations and motor/cognitive deficits. Here, we show that primary cilia (PC) formation is decreased in fibroblasts from individuals with JBTS and AHI1 mutations. Most missense mutations in AHI1, causing JBTS, occur in known protein domains, however, a common V443D mutation in AHI1 is found in a region with no known protein motifs. We show that cells transfected with AHI1-V443D, or a new JBTS-causing mutation, AHI1-R351L, have aberrant localization of AHI1 at the basal bodies of PC and at cell-cell junctions, likely through decreased binding of mutant AHI1 to NPHP1 (another JBTS-causing protein). The AHI1-V443D mutation causes decreased AHI1 stability because there is a 50% reduction in AHI1-V443D protein levels compared with wild type AHI1. Huntingtin-associated protein-1 (Hap1) is a regulatory protein that binds Ahi1, and Hap1 knock-out mice have been reported to have JBTS-like phenotypes, suggesting a role for Hap1 in ciliogenesis. Fibroblasts and neurons with Hap1 deficiency form PC with normal growth factor-induced ciliary signaling, indicating that the Hap1 JBTS phenotype is likely not through effects at PC. These results also suggest that the binding of Ahi1 and Hap1 may not be critical for ciliary function. However, we show that HAP1 has decreased binding to AHI1-V443D indicating that this altered binding could be responsible for the JBTS-like phenotype through an unknown pathway. Thus, these JBTS-associated missense mutations alter their subcellular distribution and protein interactions, compromising functions of AHI1 in cell polarity and cilium-mediated signaling, thereby contributing to JBTS.

Multiplex targeted sequencing identifies recurrently mutated genes in autism spectrum disorders.

Exome sequencing studies of autism spectrum disorders (ASDs) have identified many de novo mutations but few recurrently disrupted genes. We therefore developed a modified molecular inversion probe method enabling ultra-low-cost candidate gene resequencing in very large cohorts. To demonstrate the power of this approach, we captured and sequenced 44 candidate genes in 2446 ASD probands. We discovered 27 de novo events in 16 genes, 59% of which are predicted to truncate proteins or disrupt splicing. We estimate that recurrent disruptive mutations in six genes-CHD8, DYRK1A, GRIN2B, TBR1, PTEN, and TBL1XR1-may contribute to 1% of sporadic ASDs. Our data support associations between specific genes and reciprocal subphenotypes (CHD8-macrocephaly and DYRK1A-microcephaly) and replicate the importance of a ß-catenin-chromatin-remodeling network to ASD etiology.