Joubert syndrome-derived induced pluripotent stem cells show altered neuronal differentiation in vitro

Joubert syndrome (JS) is a recessively inherited congenital ataxia characterized by hypotonia, psychomotor delay, abnormal ocular movements, intellectual disability, and a peculiar cerebellar and brainstem malformation, the "molar tooth sign." Over 40 causative genes have been reported, all encoding for proteins implicated in the structure or functioning of the primary cilium, a subcellular organelle widely present in embryonic and adult tissues. In this paper, we developed an in vitro neuronal differentiation model using patient-derived induced pluripotent stem cells (iPSCs), to evaluate possible neurodevelopmental defects in JS. To this end, iPSCs from four JS patients harboring mutations in distinct JS genes (AHI1, CPLANE1, TMEM67, and CC2D2A) were differentiated alongside healthy control cells to obtain mid-hindbrain precursors and cerebellar granule cells. Differentiation was monitored over 31 days through the detection of lineage-specific marker expression by qRT-PCR, immunofluorescence, and transcriptomics analysis. All JS patient-derived iPSCs, regardless of the mutant gene, showed a similar impairment to differentiate into mid-hindbrain and cerebellar granule cells when compared to healthy controls. In addition, analysis of primary cilium count and morphology showed notable ciliary defects in all differentiating JS patient-derived iPSCs compared to controls. These results confirm that patient-derived iPSCs are an accessible and relevant in vitro model to analyze cellular phenotypes connected to the presence of JS gene mutations in a neuronal context.

Joubert syndrome presenting bilateral peroneal neuropathies: A case report

RATIONALE

Joubert syndrome (JS) is a rare genetic disorder that presents with various neurological symptoms, primarily involving central nervous system dysfunction. Considering the etiology of JS, peripheral nervous system abnormalities cannot be excluded; however, cases of JS accompanied by peripheral nervous system abnormalities have not yet been reported. Distinct radiological findings on brain magnetic resonance imaging were considered essential for the diagnosis of JS. However, recently, cases of JS with normal or nearly normal brain morphology have been reported. To date, there is no consensus on the most appropriate diagnostic method for JS when imaging-based diagnostic approach is challenging. This report describes the case of an adult patient who exhibited bilateral peroneal neuropathies and was finally diagnosed with JS through genetic testing.

PATIENT CONCERNS AND DIAGNOSIS

A 27-year-old man visited our outpatient clinic due to a gait disturbance that started at a very young age. The patient exhibited difficulty maintaining balance, especially when walking slowly. Oculomotor apraxia was observed on ophthalmic evaluation. During diagnostic workups, including brain imaging and direct DNA sequencing, no conclusive findings were detected. Only nerve conduction studies revealed profound bilateral peroneal neuropathies. We performed whole genome sequencing to obtain a proper diagnosis and identify the gene mutation responsible for JS.

LESSONS

This case represents the first instance of peripheral nerve dysfunction in JS. Further research is needed to explore the association between JS and peripheral nervous system abnormalities. Detailed genetic testing may serve as a valuable tool for diagnosing JS when no prominent abnormalities are detected in brain imaging studies.

Syndromic ciliopathy: a taiwanese single-center study

BACKGROUND

Syndromic ciliopathies are a group of congenital disorders characterized by broad clinical and genetic overlap, including obesity, visual problems, skeletal anomalies, mental retardation, and renal diseases. The hallmark of the pathophysiology among these disorders is defective ciliary functions or formation. Many different genes have been implicated in the pathogenesis of these diseases, but some patients still remain unclear about their genotypes.

METHODS

The aim of this study was to identify the genetic causes in patients with syndromic ciliopathy. Patients suspected of or meeting clinical diagnostic criteria for any type of syndromic ciliopathy were recruited at a single diagnostic medical center in Southern Taiwan. Whole exome sequencing (WES) was employed to identify their genotypes and elucidate the mutation spectrum in Taiwanese patients with syndromic ciliopathy. Clinical information was collected at the time of patient enrollment.

RESULTS

A total of 14 cases were molecularly diagnosed with syndromic ciliopathy. Among these cases, 10 had Bardet-Biedl syndrome (BBS), comprising eight BBS2 patients and two BBS7 patients. Additionally, two cases were diagnosed with Alström syndrome, one with Oral-facial-digital syndrome type 14, and another with Joubert syndrome type 10. A total of 4 novel variants were identified. A recurrent splice site mutation, BBS2: c.534 + 1G > T, was present in all eight BBS2 patients, suggesting a founder effect. One BBS2 patient with homozygous c.534 + 1G > T mutations carried a third ciliopathic allele, TTC21B: c.264_267dupTAGA, a nonsense mutation resulting in a premature stop codon and protein truncation.

CONCLUSIONS

Whole exome sequencing (WES) assists in identifying molecular pathogenic variants in ciliopathic patients, as well as the genetic hotspot mutations in specific populations. It should be considered as the first-line genetic testing for heterogeneous disorders characterized by the involvement of multiple genes and diverse clinical manifestations.

Novel compound heterozygous variants in ARL13B lead to Joubert syndrome

Joubert syndrome (JBTS) is a systematic developmental disorder mainly characterized by a pathognomonic mid-hindbrain malformation. All known JBTS-associated genes encode proteins involved in the function of antenna-like cellular organelle, primary cilium, which plays essential roles in cellular signal transduction and development. Here, we identified four unreported variants in ARL13B in two patients with the classical features of JBTS. ARL13B is a member of the Ras GTPase family and functions in ciliogenesis and cilia-related signaling. The two missense variants in ARL13B harbored the substitutions of amino acids at evolutionarily conserved positions. Using model cell lines, we found that the accumulations of the missense variants in cilia were impaired and the variants showed attenuated functions in ciliogenesis or the trafficking of INPP5E. Overall, these findings expanded the ARL13B pathogenetic variant spectrum of JBTS.

New Insights into the Neuropsychological Profile and Intellectual Quotient Variability in Joubert Syndrome Compared to Other Congenital Cerebellar Malformations

The neuropsychological characteristics of the cerebellar cognitive affective syndrome (CCAS) in congenital, non-progressive malformations of the cerebellum have been scarcely investigated, and even less is known for Joubert syndrome (JS), an inherited, non-progressive cerebellar ataxia characterized by the so-called molar tooth sign. The few studies on this topic reported inconsistent results about intellectual functioning and specific neuropsychological impairments. The aim of this research is to examine the neuropsychological profile of JS compared to other congenital cerebellar malformations (CM), considering individual variability of intellectual quotient (IQ) in the two groups. Fourteen patients with JS and 15 patients with CM aged 6-25 years were tested through a comprehensive, standardized neuropsychological battery. Their scores in the neuropsychological domains were inspected through descriptive analysis and compared by mean of MANOVA and ANOVA models, then replicated inserting IQ as covariate. The two groups showed a largely overlapping neuropsychological profile, consistent with CCAS. However, the JS group showed worse performance in visual-spatial memory compared to CM patients, although this difference was mitigated when considering IQ. These findings highlight a divergence between JS and other CM in visual-spatial memory, which might suggest a critical role of the cerebellum in recalling task-relevant memories and might inform rehabilitative interventions.

High Incidence of CPLANE1-Related Joubert Syndrome in the Products of Conceptions from Early Pregnancy Losses

Background

The fetal monogenic causes of early pregnancy losses (EPLs) are mainly unknown, with only a few articles on the subject published. In our previous study of EPLs using whole-exome sequencing analysis, we confirmed a genetic diagnosis of CPLANE1-related Joubert syndrome (JS) in three EPLs from two couples and identified a relatively common CPLANE1 allele among our population (NM_001384732.1:c.1819delT;c.7817T>A, further after referred as “complex allele”). Pathogenic variants in the CPLANE1 (C5orf42) gene are reported to cause JS type 17, a primary ciliopathy with various system defects.

Aims

To examine the hypothesis that the CPLANE1 “complex allele,” whether homozygous or compound heterozygous, is a common cause of EPLs in our population.

Study Design

Cohort study/case-control study.ontrol study.

Methods

In this study, we used polymerase chain reaction-based methods to screen for CPLANE1 “complex allele” presence among 246 euploid EPLs (< 12 gestational weeks) from families in North Macedonia. We also investigated the impact of this allele in 650 women with EPLs versus 646 women with no history of pregnancy loss and at least one livebirth, matched by ethnic origin.

Results

We found a high incidence of JS in the total study group of EPLs (2.03%), with a considerably higher incidence among Albanian families (6.25%). Although not statistically significant, women with EPLs had a higher allele frequency of the CPLANE1 “complex allele” (AF = 1.38%) than the controls (AF = 0.85%; p = 0.2). Albanian women had significantly higher frequency of the “complex allele” than the Macedonians (AF = 1.65% and 0.39%, respectively; p = 0.003).

Conclusion

To the best of our knowledge, this is the highest reported incidence of fetal monogenic disease that might cause EPLs. Targeted screening for the CPLANE1 “complex allele” would be warranted in Albanian ethnic couples because it would detect one JS in every 16 euploid EPLs. Our findings have a larger impact on the pathogenesis of pregnancy loss and contribute to a better understanding of the pathogenicity of the variants in the CPLANE1 gene.

Joubert syndrome causing mutation in C2 domain of CC2D2A affects structural integrity of cilia and cellular signaling molecules

Cilia are organelles extend from cells to sense external signals for tuning intracellular signaling for optimal cellular functioning. They have evolved sensory and motor roles in various cells for tissue organization and homeostasis in development and post-development. More than a thousand genes are required for cilia function. Mutations in them cause multisystem disorders termed ciliopathies. The null mutations in CC2D2A result in Meckel syndrome (MKS), which is embryonic lethal, whereas patients who have missense mutations in the C2 domain of CC2D2A display Joubert syndrome (JBTS). They survive with blindness and mental retardation. How C2 domain defects cause disease conditions is not understood. To answer this question, C2 domain of Cc2d2a (mice gene) was knocked down (KD) in IMCD-3 cells by shRNA. This resulted in defective cilia morphology observed by immunofluorescence analysis. To further probe the cellular signaling alteration in affected cells, gene expression profiling was done by RNAseq and compared with the controls. Bioinformatics analysis revealed that the differentially expressed genes (DEGs) have functions in cilia. Among the 61 cilia DEGs identified, 50 genes were downregulated and 11 genes were upregulated. These cilia genes are involved in cilium assembly, protein trafficking to the cilium, intraflagellar transport (IFT), cellular signaling like polarity patterning, and Hedgehog signaling pathway. This suggests that the C2 domain of CC2D2A plays a critical role in cilia assembly and molecular signaling hosted in cilia for cellular homeostasis. Taken together, the missense mutations in the C2 domain of CC2D2A seen in JBTS might have affected cilia-mediated signaling in neurons of the retina and brain.

Phenotypic variability in Joubert syndrome is partially explained by ciliary pathophysiology

INTRODUCTION

Joubert syndrome (JS) arises from defects of primary cilia resulting in potential malformations of the brain, kidneys, eyes, liver, and limbs. Several of the 35+ genes associated with JS have recognized genotype/phenotype correlations, but most genes have not had enough reported individuals to draw meaningful conclusions.

METHODS

A PubMed literature review identified 688 individuals with JS across 32 genes and 112 publications to bolster known genotype/phenotype relationships and identify new correlations. All included patients had the "molar tooth sign" and a confirmed genetic diagnosis. Individuals were categorized by age, ethnicity, sex and the presence of developmental disability/intellectual disability, hypotonia, abnormal eye movements, ataxia, visual impairment, renal impairment, polydactyly, and liver abnormalities.

RESULTS

Most genes demonstrated unique phenotypic profiles. Grouping proteins based on physiologic interactions established stronger phenotypic relationships that reflect known ciliary pathophysiology. Age-stratified data demonstrated that end-organ disease is progressive in JS. Most genes demonstrated a significant skew towards having variants with either residual protein function or no residual protein function.

CONCLUSION

This cohort demonstrates that clinically meaningful genotype/phenotype relationships exist within most JS-related genes and can be referenced to allow for more personalized clinical care.

Novel mutation in RPGRIP1L gene causing Joubert syndrome: A case report

INTRODUCTION

Joubert syndrome is a rare disease of genetic origin with autosomal recessive inheritance and extreme genetic heterogeneity with more than 40 causative genes. Joubert syndrome 7 is caused by mutations in the RPGRIP1L gene.

PATIENT CONCERNS

Our report describes a pediatric patient with clinical features compatible with JS type 7 such as hypotonia, developmental delay and aplasia of the cerebellar vermis.

DIAGNOSIS

The clinical features and the MRI of the head and neck which showed alterations at the level of the posterior fossa, with absence of the vermis and horizontal disposition of the cerebellar peduncles, were compatible with Joubert syndrome. Whole exome sequencing detected the variants RPGRIP1L (NM_015272.2) c.697A > T (p. Lys233Ter) and RPGRIP1L (NM_015272.2) c.3545 del (p.Pro1182LeufsTer25).

INTERVENTIONS

Resection was performed to correct the polydactyly. At age 2 years umbilical hernia, adenoid surgery and ventilatory tubes surgery were performed. Renal biopsy confirmed interstitial fibrosis and focally accentuated mild tubular atrophy with focal tubular hypertrophy, compatible with the clinical suspicion of Joubert syndrome. Congenital hip dislocation surgery was performed. The patient underwent surgery for correction of concomitant divergent strabismus and continued with glasses for astigmatism and hyperopia.

OUTCOMES

Sanger sequencing confirmed the patient´s results and the father was found to be a carrier of RPGRIP1L (NM_015272.2) c.697A > T (p. Lys233Ter) and the mother and maternal grandmother as carriers of RPGRIP1L (NM_015272.2) c.3545del (p.Pro1182LeufsTer25). RPGRIP1L:c.3545del novel variant is a deletion which changes the reading frame, altering the RPGR1_C terminal domain and giving rise to an incomplete protein whose functions will be altered.

CONCLUSION

This is the first genetically confirmed case of JS in Colombia, the first carrier of biallelic RPGRIP1L gene mutations with hip dislocation and incomplete glottic closure and the first report of the novel c.3545del likely pathogenic variant causing JS.

A novel mutation of the RPGRIP1L gene in a Chinese boy with Joubert syndrome with oculorenal involvement

BACKGROUND

Joubert syndrome (JS) is a rare genetically heterogeneous primary ciliopathy characterized by a pathognomonic cerebellar and brainstem malformation, the "molar tooth sign", and variable organ involvement (such as eye, kidney, liver, and skeleton). Here, we present a case of JS in a Chinese boy.

CASE PRESENTATION

An 11-year-old Chinese boy presented with neonatal asphyxiation and hypoxia, strabismus, subsequent developmental delay, ataxia and end-stage kidney disease (ESKD). Routine blood tests showed severe anemia, increasing blood urea nitrogen and creatinine, elevated parathyroid hormone, hypocalcemia, hypokalemia and metabolic acidosis. Urine tests showed mild proteinuria. Ultrasound showed two small kidneys. Brain magnetic resonance imaging (MRI) showed dysplasia of the cerebellar vermis and extension of the upper cerebellar feet with the "molar tooth sign". Genetic analysis showed novel compound heterozygous mutations in the RPGRIP1L gene [p.L447fs*7(p.Leu447fsTer7) and p.G908V (p.Gly908Val)].

CONCLUSION

In the present study, we identified novel compound heterozygous mutations in the RPGRIP1L gene in a Chinese boy. The clinical and genetic findings of this study will expand the understanding of JS.

Recurrent, founder and hypomorphic variants contribute to the genetic landscape of Joubert syndrome

BACKGROUND

Joubert syndrome (JS) is a neurodevelopmental ciliopathy characterised by a distinctive mid-hindbrain malformation, the 'molar tooth sign'. Over 40 JS-associated genes are known, accounting for two-thirds of cases.

METHODS

While most variants are novel or extremely rare, we report on 11 recurring variants in seven genes, including three known 'founder variants' in the Ashkenazi Jewish, Hutterite and Finnish populations. We evaluated variant frequencies in ~550 European patients with JS and compared them with controls (>15 000 Italian plus gnomAD), and with an independent cohort of ~600 JS probands from the USA.

RESULTS

All variants were markedly enriched in the European JS cohort compared with controls. When comparing allele frequencies in the two JS cohorts, the Ashkenazim founder variant (TMEM216 c.218G>T) was significantly enriched in American compared with European patients with JS, while MKS1 c.1476T>G was about 10 times more frequent among European JS. Frequencies of other variants were comparable in the two cohorts. Genotyping of several markers identified four novel European founder haplotypes.Two recurrent variants (MKS1 c.1476T>G and KIAA0586 c.428delG), have been detected in homozygosity in unaffected individuals, suggesting they could act as hypomorphic variants. However, while fibroblasts from a MKS1 c.1476T>G healthy homozygote showed impaired ability to form primary cilia and mildly reduced ciliary length, ciliary parameters were normal in cells from a KIAA0586 c.428delG healthy homozygote.

CONCLUSION

This study contributes to understand the complex genetic landscape of JS, explain its variable prevalence in distinct geographical areas and characterise two recurrent hypomorphic variants.

TOPORS as a novel causal gene for Joubert syndrome

Joubert syndrome (JBTS) is a Mendelian disorder of the primary cilium defined by the clinical triad of hypotonia, developmental delay, and a distinct cerebellar malformation called the molar tooth sign. JBTS is inherited in an autosomal recessive, autosomal dominant, or X-linked recessive manner. Though over 40 genes have been identified as causal for JBTS, molecular diagnosis is not made in 30%-40% of individuals who meet clinical criteria. TOPORS encodes topoisomerase I-binding arginine/serine-rich protein, and homozygosity for a TOPORS missense variant (c.29C > A; p.(Pro10Gln)) was identified in individuals with the ciliopathy oral-facial-digital syndrome in two families of Dominican descent. Here, we report an additional proband of Dominican ancestry with JBTS found by exome sequencing to be homozygous for the identical p.(Pro10Gln) TOPORS missense variant. Query of the Mount Sinai BioMe biobank, which includes 1880 individuals of Dominican ancestry, supports a high carrier frequency of the TOPORS p.(Pro10Gln) variant in individuals of Dominican descent. Our data nominates TOPORS as a novel causal gene for JBTS and suggests that TOPORS variants should be considered in the differential of ciliopathy-spectrum disease in individuals of Dominican ancestry.

[Clinical features and genetic analysis of two Chinese pedigrees affected with Joubert syndrome]

OBJECTIVE

To explore the clinical characteristics and genetic basis of two Chinese pedigrees affected with Joubert syndrome.

METHODS

Clinical data of the two pedigrees was collected. Genomic DNA was extracted from peripheral blood samples and subjected to high-throughput sequencing. Candidate variants were verified by Sanger sequencing. Prenatal diagnosis was carried out for a high-risk fetus from pedigree 2.

RESULTS

The proband of pedigree 1 was a fetus at 23⁺⁵ weeks gestation, for which both ultrasound and MRI showed "cerebellar vermis malformation" and "molar tooth sign". No apparent abnormality was noted in the fetus after elected abortion. The fetus was found to harbor c.812+3G>T and c.1828G>C compound heterozygous variants of the INPP5E gene, which have been associated with Joubert syndrome type 1. The proband from pedigree 2 had growth retardation, mental deficiency, peculiar facial features, low muscle tone and postaxial polydactyly of right foot. MRI also revealed "cerebellar dysplasia" and "molar tooth sign". The proband was found to harbor c.485C>G and c.1878+1G>A compound heterozygous variants of the ARMC9 gene, which have been associated with Joubert syndrome type 30. Prenatal diagnosis found that the fetus only carried the c.485C>G variant. A healthy infant was born, and no anomalies was found during the follow-up.

CONCLUSION

The compound heterozygous variants of the INPP5E and ARMC9 genes probably underlay the disease in the two pedigrees. Above finding has expanded the spectrum of pathogenic variants underlying Joubert syndrome and provided a basis for genetic counseling and prenatal diagnosis.

Joubert syndrome: Molecular basis and treatment

Joubert syndrome (JS; MIM PS213300) is a rare genetic autosomal recessive disease characterized by cerebellar vermis hypoplasia, a distinctive malformation of the cerebellum and the so-called "molar tooth sign." Other characteristic features are hypotonia with lateral ataxia, intellectual disability/mental retardation, oculomotor apraxia, retinal dystrophy, abnormalities in the respiratory system, renal cysts, hepatic fibrosis, and skeletal changes. Such pleiotropic characteristics are typical of many disorders involving primary cilium aberrations, providing a significant overlap between JS and other ciliopathies such as nephronophthisis, Meckel syndrome, and Bardet-Biedl syndrome. This review will describe some characteristics of JS associated with changes in 35 genes, and will also address subtypes of JS, clinical diagnosis, and the future of therapeutic developments.

Systematic analysis of physical examination characteristics of 94 individuals with Joubert syndrome: Keys to suspecting the diagnosis

Joubert syndrome (JS) is a neurodevelopmental disorder characterized by hypotonia and developmental delay, as well as the obligatory molar tooth sign on brain imaging. Since hypotonia and developmental delay are nonspecific features, there must be a high level of clinical suspicion of JS so that the diagnostic brain imaging and/or molecular testing for the >38 genes associated with JS is/are obtained. The goal of this study was to analyze clinical photographs of a cohort of patients with JS to define a list of physical examination features that should prompt investigation for JS. Analysis of photographs from 94 individuals with JS revealed that there is a recognizable pattern of facial features in JS that changes over time as individuals age. Macrocephaly, head tilting even when looking straight ahead, eye movement abnormalities (oculomotor apraxia, nystagmus, strabismus), and ptosis are common in those with JS. Distinctive features in younger children include triangular-shaped open mouth with tongue protrusion; in older children and adults, mandibular prognathia and prominent nasal bridge are common.

Compound heterozygous ADAMTS9 variants in Joubert syndrome-related disorders without renal manifestation

BACKGROUND

Ciliopathies are the outcomes of defects of primary cilia structures and functions which cause multisystemic developmental disorders, such as polycystic kidney disease, nephronophthisis, retinitis pigmentosa, Joubert syndrome (JS), and JS-related disorders (JSRD) with additional organ involvement including oral-facial-digital syndrome and so on. They often share common and unexpected phenotypic features.

CASE PRESENTATION

We report a 4-year-old-boy case with compound heterozygous variants of ADAMTS9. Unlike the cases with ADAMTS9 variants in the previous report, which identified that homozygous variants of ADAMTS9 were responsible for nephronophthisis-related ciliopathies in two cases, the current case did not have nephronophthisis nor renal dysfunction, and his clinical features, such as oculomotor apraxia, hypotonia, developmental delay, bifid tongue, and mild hypoplasia of cerebellar vermis indicated JSRD.

CONCLUSIONS

The case suggested a possible association between the clinical presentation of JSRD and ADAMTS9-related disease, and it shows a wide spectrum of ADAMTS9 phenotype.

[Phenotype and genotype analysis of a pedigree affected with Joubert syndrome due to variant of TMEM237 gene]

OBJECTIVE

To explore the pathogenesis of two siblings (including a fetus) from a pedigree affected with Joubert syndrome.

METHODS

Peripheral blood samples of the proband and his parents as well as amniotic fluid and abortion tissues of the fetus were collected. Part of the samples were used for the extraction of DNA, and whole exome sequencing (WES) was carried out to screen potential variants in the proband and his parents. Suspected variants were subjected to bioinformatics analysis with consideration of the clinical phenotype, and were verified by Sanger sequencing of the proband, fetus and their parents.The remainders were used for the extraction of RNA, and the mechanism of splicing variant was validated by reverse transcription-PCR (RT-PCR).

RESULTS

WES showed that both patients have carried c.175C>T (p.R59X) and c.553+1G>A compound heterozygous variants of the TMEM237 gene. Among these, c.175C>T was a nonsense mutation inherited from the asymptomatic mother, while c.553+1G>A was an alternative splicing mutation inherited from the asymptomatic father. RT-PCR showed that this variant has resulted in aberrant splicing by exon skipping.

CONCLUSION

The compound heterozygous variants of the TMEM237 gene probably underlay the etiology of Joubert syndrome in this pedigree. Above finding has enriched the phenotype and variant spectrum of the TMEM237 gene, and facilitated genetic counseling and prenatal diagnosis for the family.

Prenatal diagnosis of rhombencephalosynapsis: neuroimaging features and severity of vermian anomaly

OBJECTIVES

To describe the prenatal neuroimaging spectrum of rhombencephalosynapsis (RES) and criteria for its classification according to the severity of vermian anomaly.

METHODS

In this multicenter retrospective study of fetuses with RES between 2002 and 2020, the medical records and brain ultrasound and magnetic resonance images were evaluated comprehensively to determine the severity of the vermian anomaly and the presence of associated brain findings. RES was classified, according to the pattern of vermian agenesis and the extent of the fusion of the hemispheres, as complete RES (complete absence of the vermis) or partial RES (further classified according to the part of the vermis that was missing and, consequently, the region of hemispheric fusion, as anterior, posterior, severe or mixed RES). Findings were compared between cases with complete and those with partial RES.

RESULTS

Included in the study were 62 fetuses with a gestational age ranging between 12 and 37 weeks. Most had complete absence of the vermis (complete RES, 77.4% of cases), a 'round-shaped' cerebellum on axial views (72.6%) and a transverse cerebellar diameter (TCD) < 3^rd centile (87.1%). Among the 22.6% of cases with partial RES, 6.5% were classified as severe partial, 6.5% as partial anterior, 8.1% as partial mixed and 1.6% as partial posterior. Half of these cases presented with normal or nearly normal cerebellar morphology and 28.5% had a TCD within the normal limits. Infratentorially, the fourth ventricle was abnormal in 88.7% of cases overall, and anomalies of the midbrain and pons were frequent (93.5% and 77.4%, respectively). Ventriculomegaly was observed in 80.6% of all cases, being more severe in cases with complete RES than in those with partial RES, with high rates of parenchymal and septal disruption.

CONCLUSIONS

This study provides prenatal neuroimaging criteria for the diagnosis and classification of RES, and identification of related features, using ultrasound and magnetic resonance imaging. According to our findings, a diagnosis of RES should be considered in fetuses with a small TCD (severe cerebellar hypoplasia) and/or a round-shaped cerebellum on axial views, during the second or third trimester, especially when associated with ventriculomegaly. Partial RES is more common than previously thought, but presents an extreme diagnostic challenge, especially in cases with normal or nearly-normal cerebellar morphobiometric features. © 2021 International Society of Ultrasound in Obstetrics and Gynecology.

Expanding the Phenotype of the FAM149B1-Related Ciliopathy and Identification of Three Neurogenetic Disorders in a Single Family

Biallelic truncating FAM149B1 variants result in cilia dysfunction and have been reported in four infants with Joubert syndrome and orofaciodigital syndrome type VI, respectively. We report here on three adult siblings, 18 to 40 years of age, homozygous for the known FAM149B1 c.354_357delinsCACTC (p.Gln118Hisfs*20) variant. Detailed clinical examinations were performed including ocular and gait analyses, skeletal- and neuroimaging. All three patients presented with neurological and oculomotor symptoms since birth and mild skeletal dysplasia in infancy resulting in characteristic gait abnormalities. We document mild skeletal dysplasia, abnormal gait with increased hip rotation and increased external foot rotation, ataxia, variable polydactyly, ocular Duane syndrome, progressive ophthalmoplegia, nystagmus, situs inversus of the retinal vessels, olfactory bulb aplasia, and corpus callosal dysgenesis as novel features in FAM149B1-ciliopathy. We show that intellectual disability is mild to moderate and retinal, renal and liver function is normal in these affected adults. Our study thus expands the FAM149B1-related Joubert syndrome to a mainly neurological and skeletal ciliopathy phenotype with predominant oculomotor dysfunction but otherwise stable outcome in adults. Diagnosis of FAM149B1-related disorder was impeded by segregation of multiple neurogenetic disorders in the same family, highlighting the importance of extended clinical and genetic studies in families with complex phenotypes.

Head titubation and irritability as early symptoms of Joubert syndrome with a homozygous NPHP1 variant

BACKGROUND

Joubert syndrome is an autosomal recessive or X-linked genetic disease with a cerebellar vermis defect or hypoplasia, hypotonia, ocular dyskinesia, and mental retardation. In neonates, respiratory problems such as apnea and tachypnea are notable.

CASE REPORT

We report a patient Joubert syndrome with a homozygous NPHP1 variant, who had head titubation with irritability, including exaggerated jitteriness and a marked Morrow reflex appeared soon after birth without neonatal respiratory problems. These symptoms decreased gradually and disappeared until 1 year.

CONCLUSION

Irritability with head titubation may be an early clinical clue for the clinician to suspect Joubert syndrome.

Prenatal Versus Postnatal Diagnosis of Meckel-Gruber and Joubert Syndrome in Patients with TMEM67 Mutations

Renal cystic diseases are characterized by genetic and phenotypic heterogeneity. Congenital renal cysts can be classified as developmental disorders and are commonly diagnosed prenatally using ultrasonography and magnetic resonance imaging. Progress in molecular diagnostics and availability of exome sequencing procedures allows diagnosis of single-gene disorders in the prenatal period. Two patients with a prenatal diagnosis of polycystic kidney disease are presented in this article. TMEM67 mutations were identified in both fetuses using a whole-exome sequencing (WES) study. In one of them, the phenotypic syndrome diagnosed prenatally was different from that diagnosed in the postnatal period.

A Homozygous Synonymous Variant Likely Cause of Severe Ciliopathy Phenotype

Joubert syndrome (OMIM #213300) is a rare neurodevelopmental disease characterized by abnormal breathing patterns, intellectual impairment, ocular findings, renal cysts, and hepatic fibrosis. It is classified as a ciliopathy disease, where cilia function or structure in various organs are affected. Here, we report a 17-year-old male whose main clinical findings are oculomotor apraxia and truncal ataxia. Magnetic resonance imaging revealed the characteristic molar tooth sign of Joubert syndrome. He also has obsessive–compulsive disorder concomitantly, which is not a known feature of Joubert syndrome. Molecular genetic analysis revealed a homozygous c.2106G>A (p.(Thr702=)) variation in the Abelson helper integration 1 (AHI1) gene and another homozygous c.1739C>T (p.Thr580Ile) variation in the coiled-coil and C2 domain-containing protein 1A (CC2D1A) gene. Even though certain AHI1 variations were previously associated with Joubert syndrome (JS), c.2106G>A (p.(Thr702=)) was only reported in one patient in trans with another known pathogenic JS variant. The CC2D1A c.1739C>T (p.Thr580Ile) variation, on the other hand, has been reported to cause autosomal recessive nonsyndromic mental retardation, but there are conflicting interpretations about its pathogenicity. Overall, to our knowledge, this is the first patient representing a severe ciliopathy phenotype caused by a homozygous synonymous AHI1 variation. Further investigations should be performed to determine any involvement of the CC2D1A gene in ciliopathy phenotypes such as Joubert syndrome.

Homoplasmy of the m. 8993 T>G variant in a patient without MRI findings of Leigh syndrome, ataxia or retinal abnormalities

Leigh syndrome is a progressive neurodegenerative syndrome caused by multiple mitochondrial DNA and nuclear DNA pathological variants. Patients with Leigh syndrome consistently have distinct brain lesions found on MRI scanning involving abnormal signal in the basal ganglia, brainstem and/or cerebellum. Other clinical findings vary depending on the genetic etiology and epigenetic factors. Mitochondrial DNA-derived Leigh syndrome phenotype is thought to be modulated by heteroplasmy level. The classic example is the clinical expression of the pathological variant, m. 8993 T>G. At heteroplasmy levels above 90%, the resulting phenotype is Leigh syndrome, but at levels 70-90% patients present with a syndrome of neuropathy, ataxia and retinitis pigmentosa. We describe a 15-year old girl with homoplasmic variant in m.8993 T>G and clinical and biochemical findings consistent with Leigh syndrome but with normal brain MRI findings and without retinal abnormalities or ataxia.

Upregulation of the PPAR signaling pathway and accumulation of lipids are related to the morphological and structural transformation of the dragon-eye goldfish eye

Goldfish comprise around 300 different strains with drastically altered and aesthetical morphologies making them suitable models for evolutionary developmental biology. The dragon-eye strain is characterized by protruding eyes (analogous to those of Chinese dragons). Although the strain has been selected for about 400 years, the mechanism of its eye development remains unclear. In this study, a stable dragon-eye goldfish strain with a clear genetic background was rapidly established and studied. We found that upregulation of the PPAR signaling pathway accompanied by an increase in lipid accumulation might trigger the morphological and structural transformation of the eye in dragon-eye goldfish. At the developmental stage of proptosis (eye protrusion), downregulation of the phototransduction pathway was consistent with the structural defects and myopia of the dragon-eye strain. With the impairment of retinal development, cytokine-induced inflammation was activated, especially after proptosis, similar to the pathologic symptoms of many human ocular diseases. In addition, differentially expressed transcription factors were significantly enriched in the PAX and homeobox families, two well-known transcription factor families involved in eye development. Therefore, our findings reveal the dynamic changes in key pathways during eye development in dragon-eye goldfish, and provide insights into the molecular mechanisms underlying drastically altered eyes in goldfish and human ocular 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.

Toxicity evaluation of triphenyltin in zebrafish larvae by embryonic malformation, retinal development, and GH/IGF axis

The adverse influences of triphenyltin (TPT) on the aquatic system have been of great concern due to their widespread use and ubiquity in water environment, although it has been prohibited as antifouling coatings. In the present study, we investigated the developmental toxicity of TPT on zebrafish embryos by exposure to different concentrations (0, 1, 10, and 100 ng/l) from 2-h post-fertilization (hpf). Some parameters of developmental abnormalities (hatching, survival, body length, and malformation) were recorded, as well as the expression of several genes involved in the retinal development and growth hormone/insulin-like growth factor (GH/IGF) axis. Our results showed that TPT exposure induced developmental toxicity, including growth inhibition, malformation, and the dysregulation of gene expression levels related to the retinal development and GH/IGF axis. Thus, our data indicated that environmental exposure of TPT could induce developmental toxicity in zebrafish embryos, and those parameters could extend our understanding of the adverse effects of TPT on aquatic organisms.

Epiglottic aplasia in an infant with Joubert syndrome

Congenital aplasia of the epiglottis is a rare condition with variable presentation ranging from respiratory distress requiring surgical airway to an asymptomatic finding. Epiglottic aplasia is presumed to be caused by arrest of development of laryngeal structures and is most commonly associated with syndromic conditions, though isolated episodes of aplasia of the epiglottis do exist. In this report, we present a term infant with multiple congenital anomalies who was noted to have a hoarse cry prompting laryngoscopy. This showed complete absence of the epiglottis. Subsequent genetic testing showed mutations in the CPLANE1 gene that is associated with Joubert syndrome. Our patient was able to be discharged home on a thickened formula diet and is eating and gaining weight appropriately. Here, we present a review of the currently available literature of other cases of congenital epiglottic aplasia or hypoplasia discussing the presentation, management and outcomes in these cases.

Low availability of choline in utero disrupts development and function of the retina

Adequate supply of choline, an essential nutrient, is necessary to support proper brain development. Whether prenatal choline availability plays a role in development of the visual system is currently unknown. In this study, we addressed the role of in utero choline supply for the development and later function of the retina in a mouse model. We lowered choline availability in the maternal diet during pregnancy and assessed proliferative and differentiation properties of retinal progenitor cells (RPCs) in the developing prenatal retina, as well as visual function in adult offspring. We report that low choline availability during retinogenesis leads to persistent retinal cytoarchitectural defects, ranging from focal lesions with displacement of retinal neurons into subretinal space to severe hypocellularity and ultrastructural defects in photoreceptor organization. We further show that low choline availability impairs timely differentiation of retinal neuronal cells, such that the densities of early-born retinal ganglion cells, amacrine and horizontal cells, as well as cone photoreceptor precursors, are reduced in low choline embryonic d 17.5 retinas. Maintenance of higher proportions of RPCs that fail to exit the cell cycle underlies aberrant neuronal differentiation in low choline embryos. Increased RPC cell cycle length, and associated reduction in neurofibromin 2/Merlin protein, an upstream regulator of the Hippo signaling pathway, at least in part, explain aberrant neurogenesis in low choline retinas. Furthermore, we find that animals exposed to low choline diet in utero exhibit a significant degree of intraindividual variation in vision, characterized by marked functional discrepancy between the 2 eyes in individual animals. Together, our findings demonstrate, for the first time, that choline availability plays an essential role in the regulation of temporal progression of retinogenesis and provide evidence for the importance of adequate supply of choline for proper development of the visual system.-Trujillo-Gonzalez, I., Friday, W. B., Munson, C. A., Bachleda, A., Weiss, E. R., Alam, N. M., Sha, W., Zeisel, S. H., Surzenko, N. Low availability of choline in utero disrupts development and function of the retina.

Persistent Avascular Retina in Infants With a History of Type 2 Retinopathy of Prematurity: To Treat or Not to Treat?

PURPOSE

To investigate persistent avascular retina in infants with type 2 retinopathy of permaturity (ROP) that persisted after 45 weeks' post-menstrual age when regular ROP screening ceased.

METHODS

A prospective observational study where fundus fluorescein angiography (FFA) was completed on consecutive infants who had a history of type 2 ROP and avascular retina during ROP screening that persisted after 45 weeks' post-menstrual age.

RESULTS

FFA was completed on 72 eyes of 36 infants (53% male), with a mean gestational age of 26.0 ± 2.2 weeks and a mean birth weight of 834.6 ± 216.3 grams. The mean age at discharge from ROP screening was 47.6 weeks' post-menstrual age. All patients had type 2 ROP at the worst stage of their disease, with predominantly stage 2 disease. FFA was performed at a mean age of 18.8 ± 10.3 months post-menstrual age. All patients had detectable avascular retina in peripheral zone II or III on FFA. Peripheral vessel leakage was present in 3 eyes of 2 infants (5.5%), who both subsequently received peripheral laser treatment.

CONCLUSIONS

Premature infants with type 2 ROP may have persistent peripheral avascular retina with unknown long-term ocular complications, which can present management dilemmas. Retinal FFA is recommended to determine retinal ischemia and aid decision making for treatment in these cases. [J Pediatr Ophthalmol Strabismus. 2019;56(4):222-228.].

Bi-allelic Mutations in FAM149B1 Cause Abnormal Primary Cilium and a Range of Ciliopathy Phenotypes in Humans

Ciliopathies are clinical disorders of the primary cilium with widely recognized phenotypic and genetic heterogeneity. In two Arab consanguineous families, we mapped a ciliopathy phenotype that most closely matches Joubert syndrome (hypotonia, developmental delay, typical facies, oculomotor apraxia, polydactyly, and subtle posterior fossa abnormalities) to a single locus in which a founder homozygous truncating variant in FAM149B1 was identified by exome sequencing. We subsequently identified a third Arab consanguineous multiplex family in which the phenotype of Joubert syndrome/oral-facial-digital syndrome (OFD VI) was found to co-segregate with the same founder variant in FAM149B1. Independently, autozygosity mapping and exome sequencing in a consanguineous Turkish family with Joubert syndrome highlighted a different homozygous truncating variant in the same gene. FAM149B1 encodes a protein of unknown function. Mutant fibroblasts were found to have normal ciliogenesis potential. However, distinct cilia-related abnormalities were observed in these cells: abnormal accumulation IFT complex at the distal tips of the cilia, which assumed bulbous appearance, increased length of the primary cilium, and dysregulated SHH signaling. We conclude that FAM149B1 is required for normal ciliary biology and that its deficiency results in a range of ciliopathy phenotypes in humans along the spectrum of Joubert syndrome.

Fourth ventricle index: sonographic marker for severe fetal vermian dysgenesis/agenesis

OBJECTIVE

Prenatal diagnosis of midbrain-hindbrain (MB-HB) malformations relies primarily on abnormal size and shape of the cerebellum and retrocerebellar space, particularly 'open fourth ventricle' (4V), the most common indicator of MB-HB malformations. The aim of this study was to present the fourth ventricle index (4VI), and to evaluate its role as a marker for severe vermian dysgenesis/agenesis in cases without open 4V.

METHODS

This was a prospective cross-sectional study of patients with singleton low-risk pregnancy at 14 + 1 to 36 + 6 gestational weeks presenting between May 2016 and November 2017 for routine ultrasound examination. Axial images of the fetal 4V were obtained and the 4VI was calculated as the ratio between the laterolateral and the anteroposterior diameters. Reference ranges were constructed and retrospectively collected values from 44 fetuses with confirmed anomalies involving severe vermian dysgenesis/agenesis (Joubert syndrome and related disorders, rhombencephalosynapsis, cobblestone malformations and cerebellar hypoplasia) but without open 4V were compared with the normal values.

RESULTS

In total, 384 healthy fetuses were enrolled into the study, from which reference ranges were produced, and 44 cases were collected retrospectively. The 4VI in the normal fetuses was always > 1. In affected fetuses, it was always below mean -2 SD and < 1.

CONCLUSIONS

The 4VI is a sonographic marker for severe fetal vermian dysgenesis/agenesis in the absence of an open 4V. It may be incorporated easily into the routine brain scan; 4VI < 1 indicates a need for dedicated fetal neuroimaging for diagnosis and prenatal counseling. Copyright © 2018 ISUOG. Published by John Wiley & Sons Ltd.

ARL13B, a Joubert Syndrome-Associated Protein, Is Critical for Retinogenesis and Elaboration of Mouse Photoreceptor Outer Segments

Mutations in the Joubert syndrome-associated small GTPase ARL13B are linked to photoreceptor impairment and vision loss. To determine the role of ARL13B in the development, function, and maintenance of ciliated photoreceptors, we generated a pan-retina knock-out (Six3-Cre) and a rod photoreceptor-specific inducible conditional knock-out (Pde6g-CreERT2) of ARL13B using murine models. Embryonic deletion of ARL13B led to defects in retinal development with reduced cell proliferation. In the absence of ARL13B, photoreceptors failed to develop outer segment (OS) membranous discs and axonemes, resulting in loss of function and rapid degeneration. Additionally, the majority of photoreceptor basal bodies did not dock properly at the apical edge of the inner segments. The removal of ARL13B in adult rod photoreceptor cells after maturation of OS resulted in loss of photoresponse and vesiculation in the OS. Before changes in photoresponse, removal of ARL13B led to mislocalization of rhodopsin, prenylated phosphodiesterase-6 (PDE6), and intraflagellar transport protein-88 (IFT88). Our findings show that ARL13B is required at multiple stages of retinogenesis, including early postnatal proliferation of retinal progenitor cells, development of photoreceptor cilia, and morphogenesis of photoreceptor OS discs regardless of sex. Last, our results establish a need for ARL13B in photoreceptor maintenance and protein trafficking.SIGNIFICANCE STATEMENT The normal development of photoreceptor cilia is essential to create functional, organized outer segments with stacked membrane discs that house the phototransduction proteins necessary for sight. Our study identifies a complex role for ARL13B, a small GTPase linked to Joubert syndrome and visual impairment, at various stages of photoreceptor development. Loss of ARL13B led to defects in retinal proliferation, altered placement of basal bodies crucial for components of the cilium (transition zone) to emanate, and absence of photoreceptor-stacked discs. These defects led to extinguished visual response and dysregulated protein trafficking. Our findings show the complex role ARL13B plays in photoreceptor development, viability, and function. Our study accounts for the severe retinal impairment observed in ARL13B-linked Joubert syndrome patients.

Subthreshold micropulse laser photocoagulation therapy in a case of bilateral retinal astrocytic hamartomas with tuberous sclerosis complex: A case report

RATIONALE

Report a case of bilateral multiple retinal hamartomas (RAHs) in a patient with tuberous sclerosis complex (TSC) and introduced a new method (subthreshold micropulse laser photocoagulation) for the treatment of RAHs.

PATIENT CONCERNS

A 20-year-old man with TSC complained of decreased vision and metamorphosia in both eyes for 2 months. At presentation, visual acuity (VA) was 20/32 in the right eye and 20/40 in the left eye. Fundus photographs, optical coherence tomography, fundus fluorescein angiography (FFA), and indocyanine green angiography indicated multiple RAHs in both eyes.

DIAGNOSES

Bilateral retinal astrocytic hamartomas.

INTERVENTIONS

In the right eye, 577 nm photocoagulation was adopted to treat the RAHs with obvious fluorescein leakage in FFA. The paramacular RAHs were treated by subthreshold micropulse mode to minimize the damage to macula. Photocoagulation therapy was administrated in the left eye after 1 dose of intravitreal ranibizumab treatment.

OUTCOMES

After photocoagulation therapy (including subthreshold micropulse laser photocoagulation for the paramacular RAHs in both eyes), the VA improved to 20/25 OD and 20/32 OS with no recurrence of exudation.

LESSONS

About 577 nm photocoagulation for the peripheral RAHs in combination with subthreshold micropulse laser photocoagulation for RAHs in the macular zone is a good option for multiple RAHs in patients with TSC.

ARL3 Mutations Cause Joubert Syndrome by Disrupting Ciliary Protein Composition

Joubert syndrome (JBTS) is a genetically heterogeneous autosomal-recessive neurodevelopmental ciliopathy. We investigated further the underlying genetic etiology of Joubert syndrome by studying two unrelated families in whom JBTS was not associated with pathogenic variants in known JBTS-associated genes. Combined autozygosity mapping of both families highlighted a candidate locus on chromosome 10 (chr10: 101569997-109106128, UCSC Genome Browser hg 19), and exome sequencing revealed two missense variants in ARL3 within the candidate locus. The encoded protein, ADP ribosylation factor-like GTPase 3 (ARL3), is a small GTP-binding protein that is involved in directing lipid-modified proteins into the cilium in a GTP-dependent manner. Both missense variants replace the highly conserved Arg149 residue, which we show to be necessary for the interaction with its guanine nucleotide exchange factor ARL13B, such that the mutant protein is associated with reduced INPP5E and NPHP3 localization in cilia. We propose that ARL3 provides a potential hub in the network of proteins implicated in ciliopathies, whereby perturbation of ARL3 leads to the mislocalization of multiple ciliary proteins as a result of abnormal displacement of lipidated protein cargo.

Axoneme polyglutamylation regulated by Joubert syndrome protein ARL13B controls ciliary targeting of signaling molecules

Tubulin polyglutamylation is a predominant axonemal post-translational modification. However, if and how axoneme polyglutamylation is essential for primary cilia and contribute to ciliopathies are unknown. Here, we report that Joubert syndrome protein ARL13B controls axoneme polyglutamylation, which is marginally required for cilia stability but essential for cilia signaling. ARL13B interacts with RAB11 effector FIP5 to promote cilia import of glutamylase TTLL5 and TTLL6. Hypoglutamylation caused by a deficient ARL13B-RAB11-FIP5 trafficking pathway shows no effect on ciliogenesis, but promotes cilia disassembly and, importantly, impairs cilia signaling by disrupting the proper anchoring of sensory receptors and trafficking of signaling molecules. Remarkably, depletion of deglutamylase CCP5, the predominant cilia deglutamylase, effectively restores hypoglutamylation-induced cilia defects. Our study reveals a paradigm that tubulin polyglutamylation is a major contributor for cilia signaling and suggests a potential therapeutic strategy by targeting polyglutamylation machinery to promote ciliary targeting of signaling machineries and correct signaling defects in ciliopathies.

Nystagmus And Beyond: A Rare Ocular Motility Disorder

Joubert syndrome is a rare autosomal recessive disorder predominantly involving the cerebellar vermis and brain stem. It is characterized clinically by global developmental delay, abnormal ocular movements, hypotonia, ataxia, intellectual disability and neonatal breathing abnormalities. Due to its uncommon and unconventional presentation, its diagnosis is usually delayed. Diagnosis of this atypical disease essentially relies upon the atypical finding of the "molar tooth" sign on Magnetic Resonance Imaging (MRI). We report a case of a 5-year-old boy who presented with abnormal eye movements, regression of milestones and developmental delay. MRI investigation revealed the distinctive molar tooth sign and bat wing shaped 4th ventricle. It requires high levels of clinical suspicion and holistic approach to such children who present with delayed milestones and abnormal eye movements, to reach at early detection and diagnosis of such rare pathologies.

New Insights into Cystic Kidney Diseases

Hereditary cystic kidney diseases are considered as "ciliopathies" caused by abnormalities of the "primary cilia" situated on the tubules. As a result of dysplasia and dysfunction of cilia, formation of cysts occurs at various stages of life. Although occurring at a low incidence, hereditary cystic kidney diseases that develop from the fetal stage to childhood are diverse and are often associated with systemic disorders. The incidence of autosomal dominant polycystic kidney disease, which is the only adult-onset hereditary cystic kidney disease, is the highest among hereditary renal disorders.

The Joubert syndrome protein ARL13B binds tubulin to maintain uniform distribution of proteins along the ciliary membrane

Cilia-mediated signal transduction involves precise targeting and localization of selected molecules along the ciliary membrane. However, the molecular mechanism underlying these events is unclear. The Joubert syndrome protein ARL13B is a membrane-associated G-protein that localizes along the cilium and functions in protein transport and signaling. We identify tubulin as a direct interactor of ARL13B and demonstrate that the association occurs via the G-domain and independently from the GTPase activity of ARL13B. The G-domain is necessary for the interaction of ARL13B with the axoneme both in vitro and in vivo We further show that exogenously expressed mutants lacking the tubulin-binding G-domain (ARL13B-ΔGD) or whose GTPase domain is inactivated (ARL13B-T35N) retain ciliary localization, but fail to rescue ciliogenesis defects of null Arl13bhnn mouse embryonic fibroblasts (MEFs). However, while ARL13B-ΔGD and the membrane proteins Smoothened (SMO) and Somatostatin receptor-3 (SSTR3) distribute unevenly along the cilium of Arl13bhnn MEFs, ARL13B-T35N distributes evenly along the cilium and enables the uniform distribution of SMO and SSTR3. Thus, we propose a so far unknown function of ARL13B in anchoring ciliary membrane proteins to the axoneme through the direct interaction of its G-domain with tubulin.