A founder CEP120 mutation in Jeune asphyxiating thoracic dystrophy expands the role of centriolar proteins in skeletal ciliopathies

KIAA0556 is a novel ciliary basal body component mutated in Joubert syndrome

BACKGROUND

Joubert syndrome (JBTS) and related disorders are defined by cerebellar malformation (molar tooth sign), together with neurological symptoms of variable expressivity. The ciliary basis of Joubert syndrome related disorders frequently extends the phenotype to tissues such as the eye, kidney, skeleton and craniofacial structures.

RESULTS

Using autozygome and exome analyses, we identified a null mutation in KIAA0556 in a multiplex consanguineous family with hallmark features of mild Joubert syndrome. Patient-derived fibroblasts displayed reduced ciliogenesis potential and abnormally elongated cilia. Investigation of disease pathophysiology revealed that Kiaa0556 (-/-) null mice possess a Joubert syndrome-associated brain-restricted phenotype. Functional studies in Caenorhabditis elegans nematodes and cultured human cells support a conserved ciliary role for KIAA0556 linked to microtubule regulation. First, nematode KIAA0556 is expressed almost exclusively in ciliated cells, and the worm and human KIAA0556 proteins are enriched at the ciliary base. Second, C. elegans KIAA0056 regulates ciliary A-tubule number and genetically interacts with an ARL13B (JBTS8) orthologue to control cilium integrity. Third, human KIAA0556 binds to microtubules in vitro and appears to stabilise microtubule networks when overexpressed. Finally, human KIAA0556 biochemically interacts with ciliary proteins and p60/p80 katanins. The latter form a microtubule-severing enzyme complex that regulates microtubule dynamics as well as ciliary functions.

CONCLUSIONS

We have identified KIAA0556 as a novel microtubule-associated ciliary base protein mutated in Joubert syndrome. Consistent with the mild patient phenotype, our nematode, mice and human cell data support the notion that KIAA0556 has a relatively subtle and variable cilia-related function, which we propose is related to microtubule regulation.

Mutations in human homologue of chicken talpid3 gene (KIAA0586) cause a hybrid ciliopathy with overlapping features of Jeune and Joubert syndromes

BACKGROUND

In chicken, loss of TALPID3 results in non-functional cilia and short-rib polydactyly syndrome. This phenotype is caused by a frameshift mutation in the chicken ortholog of the human KIAA0586 gene, which encodes a novel coiled-coil domain protein essential for primary ciliogenesis, suggesting that KIAA0586 can be associated with ciliopathy in human beings.

METHODS

In our patients with ciliopathy (http://www.clinicaltrials.gov: NCT00068224), we have collected extensive clinical and neuroimaging data from affected individuals, and performed whole exome sequencing on DNA from affected individuals and their parents. We analysed gene expression on fibroblast cell line, and determined the effect of gene mutation on ciliogenesis in cells derived from patients.

RESULTS

We identified biallelic mutations in the human TALPID3 ortholog, KIAA0586, in six children with findings of overlapping Jeune and Joubert syndromes. Fibroblasts cultured from one of the patients with Jeune-Joubert syndrome exhibited more severe cilia defects than fibroblasts from patients with only Joubert syndrome; this difference was reflected in KIAA0586 RNA expression levels. Rescue of the cilia defect with full-length wild type KIAA0586 indicated a causal link between cilia formation and KIAA0586 function.

CONCLUSIONS

Our results show that biallelic deleterious mutations in KIAA0586 lead to Joubert syndrome with or without Jeune asphyxiating thoracic dystrophy. Furthermore, our results confirm that KIAA0586/TALPID3 is essential in cilia formation in human beings, expand the KIAA0586 phenotype to include features of Jeune syndrome and provide a pathogenetic connection between Joubert and Jeune syndromes, based on aberrant ciliogenesis.

New frontiers: discovering cilia-independent functions of cilia proteins

In most vertebrates, mitotic spindles and primary cilia arise from a common origin, the centrosome. In non-cycling cells, the centrosome is the template for primary cilia assembly and, thus, is crucial for their associated sensory and signaling functions. During mitosis, the duplicated centrosomes mature into spindle poles, which orchestrate mitotic spindle assembly, chromosome segregation, and orientation of the cell division axis. Intriguingly, both cilia and spindle poles are centrosome-based, functionally distinct structures that require the action of microtubule-mediated, motor-driven transport for their assembly. Cilia proteins have been found at non-cilia sites, where they have distinct functions, illustrating a diverse and growing list of cellular processes and structures that utilize cilia proteins for crucial functions. In this review, we discuss cilia-independent functions of cilia proteins and re-evaluate their potential contributions to "cilia" disorders.

Unmasking the ciliopathies: craniofacial defects and the primary cilium

Over the past decade, the primary cilium has emerged as a pivotal sensory organelle that acts as a major signaling hub for a number of developmental signaling pathways. In that time, a vast number of proteins involved in trafficking and signaling have been linked to ciliary assembly and/or function, demonstrating the importance of this organelle during embryonic development. Given the central role of the primary cilium in regulating developmental signaling, it is not surprising that its dysfunction results in widespread defects in the embryo, leading to an expanding class of human congenital disorders known as ciliopathies. These disorders are individually rare and phenotypically variable, but together they affect virtually every vertebrate organ system. Features of ciliopathies that are often overlooked, but which are being reported with increasing frequency, are craniofacial abnormalities, ranging from subtle midline defects to full-blown orofacial clefting. The challenge moving forward is to understand the primary mechanism of disease given the link between the primary cilium and a number of developmental signaling pathways (such as hedgehog, platelet-derived growth factor, and WNT signaling) that are essential for craniofacial development. Here, we provide an overview of the diversity of craniofacial abnormalities present in the ciliopathy spectrum, and reveal those defects in common across multiple disorders. Further, we discuss the molecular defects and potential signaling perturbations underlying these anomalies. This provides insight into the mechanisms leading to ciliopathy phenotypes more generally and highlights the prevalence of widespread dysmorphologies resulting from cilia dysfunction.

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 three pre-mRNA processing factors (PRPF6, PRPF8 and PRPF31) mutated in autosomal dominant retinitis pigmentosa. The PRPFs localise 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/CEP90 and C21orf2/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.

TCTEX1D2 mutations underlie Jeune asphyxiating thoracic dystrophy with impaired retrograde intraflagellar transport

The analysis of individuals with ciliary chondrodysplasias can shed light on sensitive mechanisms controlling ciliogenesis and cell signalling that are essential to embryonic development and survival. Here we identify TCTEX1D2 mutations causing Jeune asphyxiating thoracic dystrophy with partially penetrant inheritance. Loss of TCTEX1D2 impairs retrograde intraflagellar transport (IFT) in humans and the protist Chlamydomonas, accompanied by destabilization of the retrograde IFT dynein motor. We thus define TCTEX1D2 as an integral component of the evolutionarily conserved retrograde IFT machinery. In complex with several IFT dynein light chains, it is required for correct vertebrate skeletal formation but may be functionally redundant under certain conditions.

Severe congenital development defects such as Jeune syndrome can result from the malfunction of primary cilia and dynein. Here Schmidts et al. report unique biallelic null mutations in a gene encoding a dynein light chain, helping to explain the nature of ciliopathies in human patients.

Respiratory motile cilia dysfunction in a patient with cranioectodermal dysplasia

Ciliopathies such as cranioectodermal dysplasia, Sensenbrenner syndrome, short-rib polydactyly, and Jeune syndrome are associated with respiratory complications arising from rib cage dysplasia. While such ciliopathies have been demonstrated to involve primary cilia defects, we show motile cilia dysfunction in the airway of a patient diagnosed with cranioectodermal dysplasia. While this patient had mild thoracic dystrophy not requiring surgical treatment, there was nevertheless newborn respiratory distress, restrictive airway disease with possible obstructive airway involvement, repeated respiratory infections, and atelectasis. High-resolution videomicroscopy of nasal epithelial biopsy showed immotile/dyskinetic cilia and nasal nitric oxide was reduced, both of which are characteristics of primary ciliary dyskinesia, a sinopulmonary disease associated with mucociliary clearance defects due to motile cilia dysfunction in the airway. Exome sequencing analysis of this patient identified compound heterozygous mutations in WDR35, but no mutations in any of the 30 known primary ciliary dyskinesia genes or other cilia-related genes. Given that WDR35 is only known to be required for primary cilia function, we carried out WDR35 siRNA knockdown in human respiratory epithelia to assess the role of WDR35 in motile cilia function. This showed WDR35 deficiency disrupted ciliogenesis in the airway, indicating WDR35 is also required for formation of motile cilia. Together, these findings suggest patients with WDR35 mutations have an airway mucociliary clearance defect masked by their restrictive airway disease.

Advances in Skeletal Dysplasia Genetics

Skeletal dysplasias result from disruptions in normal skeletal growth and development and are a major contributor to severe short stature. They occur in approximately 1/5,000 births, and some are lethal. Since the most recent publication of the Nosology and Classification of Genetic Skeletal Disorders, genetic causes of 56 skeletal disorders have been uncovered. This remarkable rate of discovery is largely due to the expanded use of high-throughput genomic technologies. In this review, we discuss these recent discoveries and our understanding of the molecular mechanisms behind these skeletal dysplasia phenotypes. We also cover potential therapies, unusual genetic mechanisms, and novel skeletal syndromes both with and without known genetic causes. The acceleration of skeletal dysplasia genetics is truly spectacular, and these advances hold great promise for diagnostics, risk prediction, and therapeutic design.

The more we know, the more we have to discover: an exciting future for understanding cilia and ciliopathies

The Cilia 2014 conference was organised by four European networks: the Ciliopathy Alliance, the Groupement de Recherche CIL, the Nordic Cilia and Centrosome Network and the EU FP7 programme SYSCILIA. More than 400 delegates from 27 countries gathered at the Institut Pasteur conference centre in Paris, including 30 patients and patient representatives. The meeting offered a unique opportunity for exchange between different scientific and medical communities. Major highlights included new discoveries about the roles of motile and immotile cilia during development and homeostasis, the mechanism of cilium construction, as well as progress in diagnosis and possible treatment of ciliopathies. The contributions to the cilia field of flagellated infectious eukaryotes and of systems biology were also presented.