Deficiency of U4atac snRNA secondarily results in ciliary defects

In the human genome, about 700 genes contain usually one intron excised by the minor spliceosome. This spliceosome comprises its own set of snRNAs, among which U4atac. Its non-coding gene, RNU4ATAC, has been found mutated in Taybi-Linder (MOPD1/TALS), Roifman (RFMN) and Lowry-Wood syndromes (LWS). These rare developmental disorders, whose physiopathological mechanisms remain unsolved, associate ante- and post-natal growth retardation, microcephaly, skeletal dysplasia, intellectual disability, retinal dystrophy and immunodeficiency. Here, we report a homozygous RNU4ATAC mutation in the Stem II domain, n.16G>A, in two unrelated patients presenting with both typical traits of the Joubert syndrome (JBTS), a well-characterized ciliopathy, and of TALS/RFMN/LWS, thus widening the clinical spectrum of RNU4ATAC-associated disorders and indicating ciliary dysfunction as a mechanism downstream of minor splicing defects. This finding is supported by alterations of primary cilium function in TALS and JBTS/RFMN fibroblasts, as well as by u4atac zebrafish model, which exhibit ciliopathy-related phenotypes and ciliary defects. Altogether, our data indicate that alteration of cilium biogenesis is part of the physiopathological mechanisms of TALS/RFMN/LWS, secondarily to defects of minor intron splicing.

ATXN10 Is Required for Embryonic Heart Development and Maintenance of Epithelial Cell Phenotypes in the Adult Kidney and Pancreas

Atxn10 is a gene known for its role in cytokinesis and is associated with spinocerebellar ataxia (SCA10), a slowly progressing cerebellar syndrome caused by an intragenic pentanucleotide repeat expansion. Atxn10 is also implicated in the ciliopathy syndromes nephronophthisis (NPHP) and Joubert syndrome (JBTS), which are caused by the disruption of cilia function leading to nephron loss, impaired renal function, and cerebellar hypoplasia. How Atxn10 disruption contributes to these disorders remains unknown. Here, we generated Atxn10 congenital and conditional mutant mouse models. Our data indicate that while ATXN10 protein can be detected around the base of the cilium as well as in the cytosol, its loss does not cause overt changes in cilia formation or morphology. Congenital loss of Atxn10 results in embryonic lethality around E10.5 associated with pericardial effusion and loss of trabeculation. Similarly, tissue-specific loss of ATXN10 in the developing endothelium (Tie2-Cre) and myocardium (cTnT-Cre) also results in embryonic lethality with severe cardiac malformations occurring in the latter. Using an inducible Cagg-CreER to disrupt ATXN10 systemically at postnatal stages, we show that ATXN10 is also required for survival in adult mice. Loss of ATXN10 results in severe pancreatic and renal abnormalities leading to lethality within a few weeks post ATXN10 deletion in adult mice. Evaluation of these phenotypes further identified rapid epithelial-to-mesenchymal transition (EMT) in these tissues. In the pancreas, the phenotype includes signs of both acinar to ductal metaplasia and EMT with aberrant cilia formation and severe defects in glucose homeostasis related to pancreatic insufficiency or defects in feeding or nutrient intake. Collectively, this study identifies ATXN10 as an essential protein for survival.

A Novel Homozygous Variant of TMEM231 in a Case With Hypoplasia of the Cerebellar Vermis and Polydactyly

Background: Transmembrane protein 231 (TMEM231) is a component of the B9 complex that participates in the formation of the diffusion barrier between the cilia and plasma membrane. Mutations in TMEM231 gene may contribute to the Joubert syndrome (JBTS) or Meckel–Gruber syndrome (MKS). However, reports on JBTS or MKS caused by TMEM231 mutations are comparatively rare.Method: We describe a Chinese fetus with unexplained hypoplasia of the cerebellar vermis and polydactyly, detected by ultrasound imaging. The fetus was primarily diagnosed with JBTS/MKS. The parents of this fetus were non-consanguineous and healthy. Whole-exome sequencing (WES) and bioinformatics strategies were employed to explore the genetic lesion of this family.Results: An unknown missense variant (c.19C>T;p.R7W) of TMEM231 gene was detected. The variant was predicted as pathogenic and was absent in our 200 healthy controls.Conclusion: WES was employed to explore the genetic lesion of a fetus with unexplained hypoplasia of the cerebellar vermis and polydactyly. A novel variant in TMEM231 gene was identified. Our study not only provided data for genetic counseling and prenatal diagnosis to this family but also broadened the spectrum of TMEM231 mutations.

Molecular diagnoses in the congenital malformations caused by ciliopathies cohort of the 100,000 Genomes Project

BackgroundPrimary ciliopathies represent a group of inherited disorders due to defects in the primary cilium, the ‘cell’s antenna’. The 100,000 Genomes Project was launched in 2012 by Genomics England (GEL), recruiting National Health Service (NHS) patients with eligible rare diseases and cancer. Sequence data were linked to Human Phenotype Ontology (HPO) terms entered by recruiting clinicians.MethodsEighty-three prescreened probands were recruited to the 100,000 Genomes Project suspected to have congenital malformations caused by ciliopathies in the following disease categories: Bardet-Biedl syndrome (n=45), Joubert syndrome (n=14) and ‘Rare Multisystem Ciliopathy Disorders’ (n=24). We implemented a bespoke variant filtering and analysis strategy to improve molecular diagnostic rates for these participants.ResultsWe determined a research molecular diagnosis for n=43/83 (51.8%) probands. This is 19.3% higher than previously reported by GEL (n=27/83 (32.5%)). A high proportion of diagnoses are due to variants in non-ciliopathy disease genes (n=19/43, 44.2%) which may reflect difficulties in clinical recognition of ciliopathies. n=11/83 probands (13.3%) had at least one causative variant outside the tiers 1 and 2 variant prioritisation categories (GEL’s automated triaging procedure), which would not be reviewed in standard 100,000 Genomes Project diagnostic strategies. These include four structural variants and three predicted to cause non-canonical splicing defects. Two unrelated participants have biallelic likely pathogenic variants in LRRC45, a putative novel ciliopathy disease gene.ConclusionThese data illustrate the power of linking large-scale genome sequence to phenotype information. They demonstrate the value of research collaborations in order to maximise interpretation of genomic data.

CEP120-mediated KIAA0753 recruitment onto centrioles is required for timely neuronal differentiation and germinal zone exit in the developing cerebellum

Joubert syndrome (JS) is a recessive ciliopathy in which all affected individuals have congenital cerebellar vermis hypoplasia. Here, we report that CEP120, a JS-associated protein involved in centriole biogenesis and cilia assembly, regulates timely neuronal differentiation and the departure of granule neuron progenitors (GNPs) from their germinal zone during cerebellar development. Our results show that depletion of Cep120 perturbs GNP cell cycle progression, resulting in a delay of cell cycle exit in vivo. To dissect the potential mechanism, we investigated the association between CEP120 interactome and the JS database and identified KIAA0753 (a JS-associated protein) as a CEP120-interacting protein. Surprisingly, we found that CEP120 recruits KIAA0753 to centrioles, and that loss of this interaction induces accumulation of GNPs in the germinal zone and impairs neuronal differentiation. Importantly, the replenishment of wild-type CEP120 rescues the above defects, whereas expression of JS-associated CEP120 mutants, which hinder KIAA0753 recruitment, does not. Together, our data reveal a close interplay between CEP120 and KIAA0753 for the germinal zone exit and timely neuronal differentiation of GNPs during cerebellar development, and mutations in CEP120 and KIAA0753 may participate in the heterotopia and cerebellar hypoplasia observed in JS patients.