Joubert syndrome (JBTS) is a rare, mostly autosomal recessively inherited developmental disorder characterized by cerebellar hypoplasia, ataxia, psychomotor delay, and an altered respiratory pattern in the neonatal period. Retinal degeneration, renal cysts, liver fibrosis, and skeletal involvement can be present. The hallmark of JBTS is a midbrain-hindbrain malformation, named “molar tooth sign” (MTS) because of its appearance on axial magnetic resonance imaging.
Primary cilia are essential for vertebrate development, and mutations affecting this organelle underlie a large group of human malformation syndromes, the ciliopathies.
Cilia are antenna-like protrusions on the cell surface, covered by the plasma membrane and containing a ring of 9 microtubule doublets. Studies in mutants with defective ciliogenesis revealed that cilia are essential for vertebrate Hedgehog (Hh) signaling, a pathway that is crucial for embryonic patterning, organogenesis, and tumor formation.
Disease characteristics. Classic Joubert syndrome is characterized by three primary findings:
A distinctive cerebellar and brain stem malformation called the molar tooth sign (MTS)
Often these findings are accompanied by episodic tachypnea or apnea and/or atypical eye movements. In general, the breathing abnormalities improve with age, truncal ataxia develops over time, and acquisition of gross motor milestones is delayed.
Cognitive abilities are variable, ranging from severe intellectual disability to normal.
The designation Joubert syndrome and related disorders (JSRD) is used to describe individuals with JS who have additional findings including retinal dystrophy, renal disease, ocular colobomas, occipital encephalocele, hepatic fibrosis, polydactyly, oral hamartomas, and endocrine abnormalities. Both intra- and interfamilial variation are seen.
Diagnosis/testing. The diagnosis of JSRD is based on the presence of characteristic clinical features and magnetic resonance images (MRI) through the junction of the midbrain and pons (isthmus region) that resemble a molar tooth. To date mutations in one of the following 18 genes are identified in about 50% of individuals with a JSRD:
NPHP1, CEP290, AHI1, TMEM67 (MKS3),RPGRIP1L, CC2D2A, ARL13B, INPP5E, OFD1, TMEM216, KIF7, TCTN1, TCTN2, TMEM237, CEP41, TMEM138, C5orf42, and TTC21B; the other genes in which mutations are causative are unknown. Molecular genetic testing is clinically available for most of the known genes.
Management. Treatment of manifestations: Infants and children with abnormal breathing may require stimulatory medications (e.g., caffeine); supplemental oxygen; mechanical support; or tracheostomy in rare cases.
Other interventions may include speech therapy for oromotor dysfunction; occupational and physical therapy; educational support, including special programs for the visually impaired; and feedings by gastrostomy tube.
Surgery may be required for polydactyly and symptomatic ptosis and/or strabismus. Nephronophthisis, end-stage renal disease, liver failure and/or fibrosis are treated with standard approaches.
Surveillance: Annual evaluations of growth, vision, and liver and kidney function; periodic neuropsychologic and developmental testing.
Agents/circumstances to avoid: Nephrotoxic medications such as nonsteroidal anti-inflammatory drugs in those with renal impairment; hepatotoxic drugs in those with liver impairment.
Genetic counseling. JSRDs are predominantly inherited in an autosomal recessive manner. JSRD caused by mutation of OFD1is inherited in an X-linked manner. Digenic inheritance has been reported.
For autosomal recessive inheritance: at conception, each sib of an affected individual has a 25% chance of being affected, a 50% chance of being an asymptomatic carrier, and a 25% chance of being unaffected and not a carrier. Carrier testing for at-risk family members and prenatal testing for pregnancies at increased risk are possible if the disease-causing mutations have been identified in the family. For pregnancies at known increased risk for Joubert syndrome prenatal diagnosis by ultrasound examination with or without fetal MRI has been successful.
Diagnostic criteria for Joubert syndrome and related disorders (JSRD) continue to evolve; most authors concur that the neuroradiologic finding of the the molar tooth sign is obligatory [Valente et al 2008, Parisi 2009, Brancati et al 2010].
The diagnosis of "classic" or “pure” Joubert syndrome is based on the presence of the following three primary criteria:
The molar tooth sign. The MRI appearance of hypoplasia of the cerebellar vermis and accompanying brain stem abnormalities in an axial plane through the junction of the midbrain and pons (isthmus region) [Maria et al 1997, Maria et al 1999b, Quisling et al 1999]. The molar tooth sign comprises an abnormally deep interpeduncular fossa; prominent, straight, and thickened superior cerebellar peduncles; and hypoplasia of the vermis, the midline portion of the cerebellum (Figures 1A, 1B) [Maria et al 1999b].
Hypotonia in infancy with later development of ataxia
Developmental delays/intellectual disability
Additional features often identified in individuals with Joubert syndrome include:
Abnormal breathing pattern (alternating tachypnea and/or apnea);
Abnormal eye movements, typically oculomotor apraxia or difficulty in smooth visual pursuit and jerkiness in gaze and tracking [Saraiva & Baraitser 1992, Steinlin et al 1997, Maria et al 1999b, Tusa & Hove 1999]
The term “Joubert syndrome and related disorders” (JSRD) refers to those individuals with JS who have additional findings including retinal dystrophy, renal disease, ocular colobomas, occipital encephalocele, hepatic fibrosis, polydactyly, oral hamartomas, and other abnormalities.
In reality, a significant proportion of individuals diagnosed with classic JS in infancy or early childhood will manifest additional findings that represent a JSRD over time.
Molecular Genetic Testing
Genes. The 18 genes in which mutations are known to cause Joubert syndrome and related disorders are: NPHP1, AHI1,CEP290 (NPHP6), TMEM67 (MKS3), RPGRIP1L,
CC2D2A, ARL13B, INPP5E, OFD1, TMEM216, KIF7, TCTN1, TCTN2, TMEM237, CEP41, TMEM138, C5orf42, and TTC21B.
Evidence for additional locus heterogeneity. It is likely that additional loci are involved:
Overall, about 50% of individuals with a JSRD have mutations identified in one of the identified genes.
The JSRD phenotype in many families is not linked to any of the genes identified to date.
Clinical utility gene card for: Joubert syndrome 2013
Melissa C. Humberta,b, Katie Weihbrechta,b, Charles C. Searbyb,c, Yalan Lid, Robert M. Poped, Val C. Sheffieldb,c, and Seongjin Seoa,1
aDepartment of Ophthalmology and Visual Sciences,
bDepartment of Pediatrics,
cHoward Hughes Medical Institute, and
dProteomics Facility, University of Iowa, Iowa City, IA 52242
Edited by Kathryn V. Anderson, Sloan-Kettering Institute, New York, NY, and approved October 19, 2012 (received for review June 28, 2012); Proceedings of the National Academy of Sciences of the United States of America (PNAS)
Mutations affecting ciliary components cause a series of related genetic disorders in humans, including nephronophthisis (NPHP), Joubert syndrome (JBTS), Meckel-Gruber syndrome (MKS), and Bardet-Biedl syndrome (BBS), which are collectively termed “ciliopathies.” Recent protein–protein interaction studies combined with genetic analyses revealed that ciliopathy-related proteins form several functional networks/modules that build and maintain the primary cilium. However, the precise function of many ciliopathy-related proteins and the mechanisms by which these proteins are targeted to primary cilia are still not well understood. Here, we describe a protein–protein interaction network of inositol polyphosphate-5-phosphatase E (INPP5E), a prenylated protein associated with JBTS, and its ciliary targeting mechanisms. INPP5E is targeted to the primary cilium through a motif near the C terminus and prenyl-binding protein phosphodiesterase 6D (PDE6D)-dependent mechanisms. Ciliary targeting of INPP5E is facilitated by another JBTS protein, ADP-ribosylation factor-like 13B (ARL13B), but not by ARL2 or ARL3. ARL13B missense mutations that cause JBTS in humans disrupt the ARL13B–INPP5E interaction. We further demonstrate interactions of INPP5E with several ciliary and centrosomal proteins, including a recently identified ciliopathy protein centrosomal protein 164 (CEP164). These findings indicate that ARL13B, INPP5E, PDE6D, and CEP164 form a distinct functional network that is involved in JBTS and NPHP but independent of the ones previously defined by NPHP and MKS proteins.
Holden Higginbotham1, Tae-Yeon Eom1, Laura E. Mariani2, 3, Amelia Bachleda1, Joshua Hirt1, Vladimir Gukassyan1, Corey L. Cusack1, Cary Lai4, Tamara Caspary3, , , E.S. Anton1, ,
1 UNC Neuroscience Center and the Department of Cell Biology and Physiology, University of North Carolina School of Medicine, Chapel Hill, NC 27599, USA
2 Neurosciences Graduate Program, Laney Graduate School, Emory University, Atlanta, GA 30322, USA
3 Department of Human Genetics, Emory University School of Medicine, Atlanta, GA 30322, USA
4 The Gill Center for Biomolecular Science, Indiana University, Bloomington, IN 47405, USA
Coordinated migration and placement of interneurons and projection neurons lead to functional connectivity in the cerebral cortex; defective neuronal migration and the resultant connectivity changes underlie the cognitive defects in a spectrum of neurological disorders. Here we show that primary cilia play a guiding role in the migration and placement of postmitotic interneurons in the developing cerebral cortex and that this process requires the ciliary protein, Arl13b. Through live imaging of interneuronal cilia, we show that migrating interneurons display highly dynamic primary cilia and we correlate cilia dynamics with the interneuron's migratory state. We demonstrate that the guidance cue receptors essential for interneuronal migration localize to interneuronal primary cilia, but their concentration and dynamics are altered in the absence of Arl13b. Expression of Arl13b variants known to cause Joubert syndrome induce defective interneuronal migration, suggesting that defects in cilia-dependent interneuron migration may in part underlie the neurological defects in Joubert syndrome patients.
Here we mapped a new locus for Joubert syndrome (JBTS)3, which we have designated as JBTS15.Depletion of CEP41 causes ciliopathy-related phenotypes and results in glutamylation defects in the ciliary axoneme. Our data identify CEP41 mutations as a cause of JBTS and implicate tubulin post-translational modification in the pathogenesis of human ciliary dysfunction.
Joubert syndrome (JS) and related disorders (JSRD) are autosomal recessive and X-linked disorders characterized by hypoplasia of the cerebellar vermis with a characteristic ‘molar tooth sign’ on brain imaging and accompanying neurological symptoms including episodic hyperpnoea, abnormal eye movements, ataxia and intellectual disability. JSRD are clinically and genetically heterogeneous, and, to date, a total of 17 causative genes are known. We applied whole-exome sequencing (WES) to five JSRD families and found mutations in all: either CEP290, TMEM67 or INPP5E was mutated. Compared with conventional Sanger sequencing, WES appears to be advantageous with regard to speed and cost, supporting its potential utility in molecular diagnosis.
Joubert syndrome and related disorders (JSRD) are a genetically and phenotypically heterogeneous group of autosomal or X-linked recessive conditions united by the pathognomonic
midbrain-hindbrain malformation defined as the molar tooth sign (MTS) on magnetic resonance imaging (MRI). Affected individuals also exhibit developmental delay, ataxia, hypotonia,
irregular breathing and eye movements. Joubert syndrome is variably associated with extra-neural manifestations including postaxial polydactyly, liver fibrosis, NPHP, cystic renal
dysplasia, ocular colobomas and retinal dystrophy. Mutations in 17 genes with established roles in cilia structure, function or trafficking have been linked to Joubert
Joubert syndrome and related diseases (JSRD) are a group of inherited ciliopathies, characterised by a cerebello-retinal-renal phenotype. The brain phenotype is a developmental midbrain malformation leading to cerebellar vermis hypoplasia or aplasia, and seen in brain MRI imaging as a “molar tooth sign”. Other structural brain defects including hypoplasia of the corpus callosum and occipital meningoencephalocele have been reported.
Retinal dysplasia and degeneration occur in a proportion of patients and may lead to progressive blindness. Ocular coloboma may also be a feature. Renal disease is variable, with nephronophthisis and multicystic dysplasia as reported phenotypes, which may lead to end stage renal failure. Consistent with JSRD as a ciliopathy are the findings of polydactaly and liver fibrosis.
In keeping with the clinical heterogeneity of JSRD, 16 causal genes, have been identified in patients. These include INPP5E, TMEM2169, AHI1, NPHP1, NPHP6 (CEP290), TMEM67, RPGRIP1L, ARL13B, CC2D2A, CXORF5, TTC21B, KIF7, TCTN1, TMEM237, CEP41 and TMEM138. Defects in these genes produce phenotypes that may be termed ciliopathies, given that the protein products encoded by all of these genes have been localised in the basal body, centrosome or primary cilium, a highly conserved cellular organelle, central to the regulation of cellular signalling pathways.
Mutations in AHI1 and CEP290 are both a frequent causes of Joubert syndrome and genetic variants in both genes may act as modifier alleles, especially in regard to a retinal and CNS phenotype. Mutations in AHI1 (Abelson-helper integration site-1) are the most common genetic cause of JSRD, accounting for 12% of cases and 20% of individuals with Joubert syndrome and Leber's congenital amaurosis.
A novel finding is that both AHI1 and CEP290 demonstrate strong expression within the developing choroid plexus, a ciliated structure important for central nervous system development. We found no obvious differences in Cep290 localisation in the presence or absence of Ahi1, suggesting that, while Ahi1 and Cep290 may function together in the whole organism, they are not interdependent for localisation within a single cell. Taken together these data support a role for AHI1 and CEP290 in multiple organs throughout development and we suggest that this accounts for the wide phenotypic spectrum of AHI1 and CEP290 mutations in man.
Joubert syndrome (JBTS) is an inherited ciliopathy leading to a cerebellum-retinal-renal syndrome. Recent genetic advances have allowed positional cloning and identification of numerous JBTS
genes. CEP290, one of the JBTS genes identified, (alias NPHP6) encodes a centrosomal protein and accounts for 7% of patients with Joubert syndrome.
A Simple Cell-Based Assay Reveals That Diverse Neuropsychiatric Risk Genes Converge on Primary Cilia
Human disease genetics are presently making important inroads to investigating the biological basis of severe neuropsychiatric disorders. As progress in this exciting field continues to advance, a major next challenge is to understand biological mechanisms underlying the influence of identified genes on pathophysiology or disease vulnerability. This requires defining normal cellular structures or functions that are impacted by the respective disease-linked genes.
A remarkable observation that has emerged is that the same locus, the same gene or even the same allele can affect risk for phenotypically different syndromes , , , , , . Many genes implicated in schizophrenia (SCZ), for example, are also implicated in bipolar affective disorder (BAD) , , , ,. The same can be said for genes implicated in autism spectrum disorder (ASD) and intellectual disability (ID) . Such extensive genetic crossover suggests that common biological vulnerabilities underlie the pathophysiology of, or determine susceptibility to, multiple neuropsychiatric syndromes. Identifying these shared ‘nodes’ of vulnerability is not a simple task because the genetic architecture of neuropsychiatric illness is complex. Indeed, most genes exhibiting such phenotypic crossover do not encode products that are known to physically interact or function in a shared biochemical pathway. This suggests that apparently disparate risk genes may be related at a higher level of biological integration, through convergent effects on a common cellular structure or process.
One such shared cellular node, which is already well recognized, is the synapse. Numerous genes associated with severe neuropsychiatric disorders affect synaptic structure or function , . Even though their respective gene products are not related by direct interaction, or through any single biochemical pathway, these genes converge in supporting the integrated operation of synapses in intercellular signaling. Only a subset of neuropsychiatric risk genes are known to affect synapses, however. Moreover, synapses are specific to neural cell types, whereas neuropsychiatric disorders have systemic manifestations and many disease-implicated genes are expressed also outside the nervous system . This raises the question of whether there exist additional cellular nodes at which disease-linked genes converge, and if such convergence of disease-linked genes can be observed in non-neural cells.
Here we describe a cell culture-based approach for addressing these questions, and provide evidence suggesting that the primary cilium represents such a node. Specifically, we identify 20 genes previously linked to diverse neuropsychiatric disorders -including SCZ, BAD, ASD and ID - which converge in supporting proper cilium formation or maintenance in a simplified non-neural model system.
In the present study we identified 20 genes linked to phenotypically diverse clinical syndromes including SCZ, BAD, ASD and ID, which disrupt primary cilia when knocked down in a simple cell culture model, and three for which altering cellular expression increases the frequency of ciliation and cilia length. Three of the genes identified- NEK4, SDCCAG8 and SYNE1- were previously reported to function in ciliogeneisis ,, , supporting the validity of the present screening approach and revealing a ciliary phenotype for 20 additional candidates. This is a remarkably high hit rate relative to that observed in RNAi screens not focusing on neuropsychiatric risk genes, and we note that screening the druggable genome collection for ciliary defects achieved <1% hit rate using similar methods and scoring criteria . With the exception of DISC interactors and CHD4/CHD5 , we are not aware of previous evidence for physical or functional interaction between the hits identified in the present study. Accordingly, we propose that the present cell-based screen has the potential to detect additional relationship(s) between disease genes, and that primary cilia may represent a common cellular ‘node’ at which the effects of diverse neuropsychiatric risk genes converge.
We pursued a simple loss-of-function approach, based on RNAi-mediated knockdown in a non-neural cellular model, and thus focused on detecting conserved normal cellular functions of the selected candidate genes. We did not attempt to test specific disease-linked alleles, or disease-relevant neuronal populations, although doing so could represent an interesting future direction. Accordingly, the present results do not indicate that neuropsychiatric disorders represent ciliopathies per se. Indeed this seems unlikely, as major neuropsychiatric syndromes are not associated with classical manifestations of gross cilia loss or dysfunction. Rather, the present results suggest that primary cilia represent a common cellular node at which diverse disease-linked genes impact proper formation, maintenance or regulation. Thus we would propose that neuropsychiatric pathology could involve relatively subtle defects in the structure or function of primary cilia, rather than a complete disruption, much like disease-linked genes affecting the synapse do not simply abrogate neurotransmission in affected individuals.
The primary cilium represents a complex and dynamic cellular structure that is conserved in neurons and non-neural cell types, and which mediates diverse and important functions in cellular signaling . Primary cilia display numerous receptors and downstream signaling components that are important both in development and adult tissue function, including D1 and D2 dopamine receptors which are affected by drugs used currently in the management of neuropsychiatric illness , . Moreover, while classical ciliopathies and primary neuropsychiatric disorders are discrete clinical entities, many ciliopathies have behavioral manifestations . Thus we think that ciliary convergence of diverse neuropsychiatric risk genes, as suggested by the present analysis, is plausible with regard to previous knowledge in the area.
The present study took a simplified cell culture-based approach that was motivated by 1) extensive conservation of primary cilia across cell types, 2) the fact that many of genes implicated in neuropsychiatric disease are expressed widely in neural as well as non-neural tissues, and 3) previous evidence showing that NIH3T3 cells reliably detect DISC1-dependent ciliation effects. A possible implication of our results is that primary cilia contribute directly to neuropsychiatric pathophysiology and/or determine disease vulnerability. Alternatively, because primary cilia are themselves complex structures that are functionally associated with numerous cellular processes, our results could reflect a more distant convergence of disease-linked genes, . In either case, our results establish a starting point for elucidating the cellular function of genes determining neuropsychiatric disease risk. They may also provide a useful approach for identifying biochemical events occurring downstream of disease-linked gene convergence, and thus reveal new targets for therapeutic consideration.