Compound heterozygous alterations in intraflagellar transport protein CLUAP1 in a child with a novel Joubert and oral-facial-digital overlap syndrome

Retinol dehydrogenase 12 (RDH12) knock out may cause hyperuricemia phenotype in mice

Hyperuricemia is a chronic metabolic disease caused by purine metabolism disorder. And several gene loci and transporter proteins that associated with uric acid transport functions have been identified. Retinol Dehydrogenase 12 (RDH12), recognized for its role in safeguarding photoreceptors, and our study investigated the potential impact of Rdh12 mutations on other organs and diseases, particularly hyperuricemia. We assessed Rdh12 mRNA expression levels in various tissues and conducted serum biochemical analyses in Rdh12-/- mice. Compared with the wild type, significant alterations in serum uric acid levels and kidney-related biochemical indicators have been revealed. Then further analysis, including quantitative RT-PCR of gene expression in the liver and kidney, highlighted variations in the expression levels of specific genes linked to hyperuricemia. And renal histology assessment exposed mild pathological lesions in the kidneys of Rdh12-/- mice. In summary, our study suggests that Rdh12 mutations impact not only retinal function but also contribute to hyperuricemia and renal disease phenotypes in mice. Our finding implies that individuals with Rdh12 mutations may be prone to hyperuricemia and gout, emphasizing the significance of preventive measures and regular examinations in daily life.

Defective airway intraflagellar transport underlies a combined motile and primary ciliopathy syndrome caused by IFT74 mutations

Ciliopathies are inherited disorders caused by defective cilia. Mutations affecting motile cilia usually cause the chronic muco-obstructive sinopulmonary disease primary ciliary dyskinesia (PCD) and are associated with laterality defects, while a broad spectrum of early developmental as well as degenerative syndromes arise from mutations affecting signalling of primary (non-motile) cilia. Cilia assembly and functioning requires intraflagellar transport (IFT) of cargos assisted by IFT-B and IFT-A adaptor complexes. Within IFT-B, the N-termini of partner proteins IFT74 and IFT81 govern tubulin transport to build the ciliary microtubular cytoskeleton. We detected a homozygous 3-kb intragenic IFT74 deletion removing the exon 2 initiation codon and 40 N-terminal amino acids in two affected siblings. Both had clinical features of PCD with bronchiectasis, but no laterality defects. They also had retinal dysplasia and abnormal bone growth, with a narrowed thorax and short ribs, shortened long bones and digits, and abnormal skull shape. This resembles short-rib thoracic dysplasia, a skeletal ciliopathy previously linked to IFT defects in primary cilia, not motile cilia. Ciliated nasal epithelial cells collected from affected individuals had reduced numbers of shortened motile cilia with disarranged microtubules, some misorientation of the basal feet, and disrupted cilia structural and IFT protein distributions. No full-length IFT74 was expressed, only truncated forms that were consistent with N-terminal deletion and inframe translation from downstream initiation codons. In affinity purification mass spectrometry, exon 2-deleted IFT74 initiated from the nearest inframe downstream methionine 41 still interacts as part of the IFT-B complex, but only with reduced interaction levels and not with all its usual IFT-B partners. We propose that this is a hypomorphic mutation with some residual protein function retained, which gives rise to a primary skeletal ciliopathy combined with defective motile cilia and PCD.

Primary cilia in skeletal development and disease

Primary cilia are non-motile, microtubule-based sensory organelle present in most vertebrate cells with a fundamental role in the modulation of organismal development, morphogenesis, and repair. Here we focus on the role of primary cilia in embryonic and postnatal skeletal development. We examine evidence supporting its involvement in physiochemical and developmental signaling that regulates proliferation, patterning, differentiation and homeostasis of osteoblasts, chondrocytes, and their progenitor cells in the skeleton. We discuss how signaling effectors in mechanotransduction and bone development, such as Hedgehog, Wnt, Fibroblast growth factor and second messenger pathways operate at least in part at the primary cilium. The relevance of primary cilia in bone formation and maintenance is underscored by a growing list of rare genetic skeletal ciliopathies. We collate these findings and summarize the current understanding of molecular factors and mechanisms governing primary ciliogenesis and ciliary function in skeletal development and disease.

Pathogenic RAB34 variants impair primary cilium assembly and cause a novel oral-facial-digital syndrome

Oral-facial-digital syndromes (OFDS) are a group of clinically and genetically heterogeneous disorders characterized by defects in the development of the face and oral cavity along with digit anomalies. Pathogenic variants in over 20 genes encoding ciliary proteins have been found to cause OFDS through deleterious structural or functional impacts on primary cilia. We identified by exome sequencing bi-allelic missense variants in a novel disease-causing ciliary gene RAB34 in four individuals from three unrelated families. Affected individuals presented a novel form of OFDS (OFDS-RAB34) accompanied by cardiac, cerebral, skeletal and anorectal defects. RAB34 encodes a member of the Rab GTPase superfamily and was recently identified as a key mediator of ciliary membrane formation. Unlike many genes required for cilium assembly, RAB34 acts selectively in cell types that use the intracellular ciliogenesis pathway, in which nascent cilia begin to form in the cytoplasm. We find that the protein products of these pathogenic variants, which are clustered near the RAB34 C-terminus, exhibit a strong loss of function. Although some variants retain the ability to be recruited to the mother centriole, cells expressing mutant RAB34 exhibit a significant defect in cilium assembly. While many Rab proteins have been previously linked to ciliogenesis, our studies establish RAB34 as the first small GTPase involved in OFDS and reveal the distinct clinical manifestations caused by impairment of intracellular ciliogenesis.

CORUM: the comprehensive resource of mammalian protein complexes-2022

The CORUM database has been providing comprehensive reference information about experimentally characterized, mammalian protein complexes and their associated biological and biomedical properties since 2007. Given that most catalytic and regulatory functions of the cell are carried out by protein complexes, their composition and characterization is of greatest importance in basic and disease biology. The new CORUM 4.0 release encompasses 5204 protein complexes offering the largest and most comprehensive publicly available dataset of manually curated mammalian protein complexes. The CORUM dataset is built from 5299 different genes, representing 26% of the protein coding genes in humans. Complex information from 3354 scientific articles is mainly obtained from human (70%), mouse (16%) and rat (9%) cells and tissues. Recent curation work includes sets of protein complexes, Functional Complex Groups, that offer comprehensive collections of published data in specific biological processes and molecular functions. In addition, a new graphical analysis tool was implemented that displays co-expression data from the subunits of protein complexes. CORUM is freely accessible at http://mips.helmholtz-muenchen.de/corum/.

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.

Genotype-phenotype correlates in Joubert syndrome: A review

Joubert syndrome (JS) is a genetically heterogeneous primary ciliopathy characterized by a pathognomonic cerebellar and brainstem malformation, the “molar tooth sign,” and variable organ involvement. Over 40 causative genes have been identified to date, explaining up to 94% of cases. To date, gene‐phenotype correlates have been delineated only for a handful of genes, directly translating into improved counseling and clinical care. For instance, JS individuals harboring pathogenic variants in TMEM67 have a significantly higher risk of liver fibrosis, while pathogenic variants in NPHP1, RPGRIP1L, and TMEM237 are frequently associated to JS with renal involvement, requiring a closer monitoring of liver parameters, or renal functioning. On the other hand, individuals with causal variants in the CEP290 or AHI1 need a closer surveillance for retinal dystrophy and, in case of CEP290, also for chronic kidney disease. These examples highlight how an accurate description of the range of clinical symptoms associated with defects in each causative gene, including the rare ones, would better address prognosis and help guiding a personalized management. This review proposes to address this issue by assessing the available literature, to confirm known, as well as to propose rare gene‐phenotype correlates in JS.

The crucial role of model systems in understanding the complexity of cell signaling in human neurocristopathies

Animal models are useful to study the molecular, cellular, and morphogenetic mechanisms underlying normal and pathological development. Cell-based study models have emerged as an alternative approach to study many aspects of human embryonic development and disease. The neural crest (NC) is a transient, multipotent, and migratory embryonic cell population that generates a diverse group of cell types that arises during vertebrate development. The abnormal formation or development of the NC results in neurocristopathies (NCPs), which are characterized by a broad spectrum of functional and morphological alterations. The impaired molecular mechanisms that give rise to these multiphenotypic diseases are not entirely clear yet. This fact, added to the high incidence of these disorders in the newborn population, has led to the development of systematic approaches for their understanding. In this article, we have systematically reviewed the ways in which experimentation with different animal and cell model systems has improved our knowledge of NCPs, and how these advances might contribute to the development of better diagnostic and therapeutic tools for the treatment of these pathologies. This article is categorized under: Congenital Diseases > Genetics/Genomics/Epigenetics Congenital Diseases > Stem Cells and Development Congenital Diseases > Molecular and Cellular Physiology Neurological Diseases > Genetics/Genomics/Epigenetics.

Functional Evaluation of Splicing for Variants of Uncertain Significance in Patients with Inherited Retinal Diseases

Inherited retinal diseases (IRD) comprise a heterogeneous set of clinical and genetic disorders that lead to blindness. Given the emerging opportunities in precision medicine and gene therapy, it has become increasingly important to determine whether DNA variants with uncertain significance (VUS) are responsible for patients’ IRD. This research was performed to assess the functional consequence of six VUS identified in patients with IRD. Clinical assessments included an ophthalmic examination, best-corrected visual acuity, and kinetic perimetry. Imaging was acquired with the Optos ultra-widefield camera and spectral domain optical coherence tomography (SD-OCT). Genetic testing was performed by Molecular Vision Laboratories. VUS that were predicted to alter splicing were analyzed with a minigene assay, which revealed that VUS in the genes OPA1, CNGB1, and CLUAP1 altered spicing mechanisms. Due to emerging gene and cell therapies, these results expand the genotype-phenotype correlations for patients diagnosed with an IRD.

Expanding the genetic landscape of oral-facial-digital syndrome with two novel genes

Oral‐facial‐digital syndromes (OFDS) are a heterogeneous and rare group of Mendelian disorders characterized by developmental abnormalities of the oral cavity, face, and digits caused by dysfunction of the primary cilium, a mechanosensory organelle that exists atop most cell types that facilitates organ patterning and growth. OFDS is inherited both in an X‐linked dominant, X‐linked recessive, and autosomal recessive manner. Importantly, though many of the causal genes for OFDS have been identified, up to 40% of OFD syndromes are of unknown genetic basis. Here we describe three children with classical presentations of OFDS including lingual hamartomas, polydactyly, and characteristic facial features found by exome sequencing to harbor variants in causal genes not previously associated with OFDS. We describe a female with hypothalamic hamartoma, urogenital sinus, polysyndactyly, and multiple lingual hamartomas consistent with OFDVI with biallelic pathogenic variants in CEP164, a gene associated with ciliopathy‐spectrum disease, but never before with OFDS. We additionally describe two unrelated probands with postaxial polydactyly, multiple lingual hamartomas, and dysmorphic features both found to be homozygous for an identical TOPORS missense variant, c.29 C>A; (p.Pro10Gln). Heterozygous TOPORS pathogenic gene variants are associated with autosomal dominant retinitis pigmentosa, but never before with syndromic ciliopathy. Of note, both probands are of Dominican ancestry, suggesting a possible founder allele.

Mendelian pathway analysis of laboratory traits reveals distinct roles for ciliary subcompartments in common disease pathogenesis

Rare monogenic disorders of the primary cilium, termed ciliopathies, are characterized by extreme presentations of otherwise common diseases, such as diabetes, hepatic fibrosis, and kidney failure. However, despite a recent revolution in our understanding of the cilium's role in rare disease pathogenesis, the organelle's contribution to common disease remains largely unknown. Hypothesizing that common genetic variants within Mendelian ciliopathy genes might contribute to common complex diseases pathogenesis, we performed association studies of 16,874 common genetic variants across 122 ciliary genes with 12 quantitative laboratory traits characteristic of ciliopathy syndromes in 452,593 individuals in the UK Biobank. We incorporated tissue-specific gene expression analysis, expression quantitative trait loci, and Mendelian disease phenotype information into our analysis and replicated our findings in meta-analysis. 101 statistically significant associations were identified across 42 of the 122 examined ciliary genes (including eight novel replicating associations). These ciliary genes were widely expressed in tissues relevant to the phenotypes being studied, and eQTL analysis revealed strong evidence for correlation between ciliary gene expression levels and laboratory traits. Perhaps most interestingly, our analysis identified different ciliary subcompartments as being specifically associated with distinct sets of phenotypes. Taken together, our data demonstrate the utility of a Mendelian pathway-based approach to genomic association studies, challenge the widely held belief that the cilium is an organelle important mainly in development and in rare syndromic disease pathogenesis, and provide a framework for the continued integration of common and rare disease genetics to provide insight into the pathophysiology of human diseases of immense public health burden.

Ciliopathies: Coloring outside of the lines

Ciliopathy syndromes are a diverse spectrum of disease characterized by a combination of cystic kidney disease, hepatobiliary disease, retinopathy, skeletal dysplasia, developmental delay, and brain malformations. Though generally divided into distinct disease categories based on the pattern of system involvement, ciliopathy syndromes are known to display certain phenotypic overlap. We performed next-generation sequencing panel testing, clinical exome sequencing, and research-based exome sequencing reanalysis on patients with suspected ciliopathy syndromes with additional features. We identified biallelic pathogenic variants in BBS1 in a child with features of cranioectodermal dysplasia, and biallelic variants in BBS12 in a child with the clinical stigmata of Bardet-Biedl syndrome, but also with anal atresia. We additionally identified biallelic pathogenic variants in WDR35 and DYNC2H1 in children with predominant liver disease and ductal plate malformation without skeletal dysplasia. Our study highlights the phenotypic and genetic diversity of ciliopathy syndromes, the importance of considering ciliopathy syndromes as a disease-spectrum and screening for all associated complications in all patients, and describes exclusive extra-skeletal manifestations in two classical skeletal dysplasia syndromes.

Phen2Gene: rapid phenotype-driven gene prioritization for rare diseases

Human Phenotype Ontology (HPO) terms are increasingly used in diagnostic settings to aid in the characterization of patient phenotypes. The HPO annotation database is updated frequently and can provide detailed phenotype knowledge on various human diseases, and many HPO terms are now mapped to candidate causal genes with binary relationships. To further improve the genetic diagnosis of rare diseases, we incorporated these HPO annotations, gene-disease databases and gene-gene databases in a probabilistic model to build a novel HPO-driven gene prioritization tool, Phen2Gene. Phen2Gene accesses a database built upon this information called the HPO2Gene Knowledgebase (H2GKB), which provides weighted and ranked gene lists for every HPO term. Phen2Gene is then able to access the H2GKB for patient-specific lists of HPO terms or PhenoPacket descriptions supported by GA4GH (http://phenopackets.org/), calculate a prioritized gene list based on a probabilistic model and output gene-disease relationships with great accuracy. Phen2Gene outperforms existing gene prioritization tools in speed and acts as a real-time phenotype-driven gene prioritization tool to aid the clinical diagnosis of rare undiagnosed diseases. In addition to a command line tool released under the MIT license (https://github.com/WGLab/Phen2Gene), we also developed a web server and web service (https://phen2gene.wglab.org/) for running the tool via web interface or RESTful API queries. Finally, we have curated a large amount of benchmarking data for phenotype-to-gene tools involving 197 patients across 76 scientific articles and 85 patients' de-identified HPO term data from the Children's Hospital of Philadelphia.

Healthcare recommendations for Joubert syndrome

Joubert syndrome (JS) is a recessive neurodevelopmental disorder defined by a characteristic cerebellar and brainstem malformation recognizable on axial brain magnetic resonance imaging as the "Molar Tooth Sign". Although defined by the neurological features, JS is associated with clinical features affecting many other organ systems, particularly progressive involvement of the retina, kidney, and liver. JS is a rare condition; therefore, many affected individuals may not have easy access to subspecialty providers familiar with JS (e.g., geneticists, neurologists, developmental pediatricians, ophthalmologists, nephrologists, hepatologists, psychiatrists, therapists, and educators). Expert recommendations can enable practitioners of all types to provide quality care to individuals with JS and know when to refer for subspecialty care. This need will only increase as precision treatments targeting specific genetic causes of JS emerge. The goal of these recommendations is to provide a resource for general practitioners, subspecialists, and families to maximize the health of individuals with JS throughout the lifespan.

Ciliogenesis-coupled accumulation of IFT-B proteins in a novel cytoplasmic compartment

Cluap1/IFT38 is a ciliary protein that belongs to the IFT-B complex and is required for ciliogenesis. In this study, we have examined the behaviors of Cluap1 protein in nonciliated and ciliated cells. In proliferating cells, Cluap1 is located at the distal appendage of the mother centriole. When cells are induced to form cilia, Cluap1 is found in a novel noncentriolar compartment, the cytoplasmic IFT spot, which mainly exists once in a cell. Other IFT-B proteins such as IFT46 and IFT88 are colocalized in this spot. The cytoplasmic IFT spot is present in mouse embryonic fibroblasts (MEFs) but is absent in ciliogenesis-defective MEFs lacking Cluap1, Kif3a or Odf2. The cytoplasmic IFT spot is also found in mouse embryos but is absent in the Cluap1 mutant embryo. When MEFs are induced to form cilia, the cytoplasmic IFT spot appears at an early step of ciliogenesis but starts to disappear when ciliogenesis is mostly completed. These results suggest that IFT-B proteins such as Cluap1 accumulate in a previously undescribed cytoplasmic compartment during ciliogenesis.

The Frog Xenopus as a Model to Study Joubert Syndrome: The Case of a Human Patient With Compound Heterozygous Variants in PIBF1

Joubert syndrome (JS) is a congenital autosomal-recessive or—in rare cases–X-linked inherited disease. The diagnostic hallmark of the so-called molar tooth sign describes the morphological manifestation of the mid- and hind-brain in axial brain scans. Affected individuals show delayed development, intellectual disability, ataxia, hyperpnea, sleep apnea, abnormal eye, and tongue movements as well as hypotonia. At the cellular level, JS is associated with the compromised biogenesis of sensory cilia, which identifies JS as a member of the large group of ciliopathies. Here we report on the identification of novel compound heterozygous variants (p.Y503C and p.Q485^*) in the centrosomal gene PIBF1 in a patient with JS via trio whole exome sequencing. We have studied the underlying disease mechanism in the frog Xenopus, which offers fast assessment of cilia functions in a number of embryological contexts. Morpholino oligomer (MO) mediated knockdown of the orthologous Xenopus pibf1 gene resulted in defective mucociliary clearance in the larval epidermis, due to reduced cilia numbers and motility on multiciliated cells. To functionally assess patient alleles, mutations were analyzed in the larval skin: the p.Q485^* nonsense mutation resulted in a disturbed localization of PIBF1 to the ciliary base. This mutant failed to rescue the ciliation phenotype following knockdown of endogenous pibf1. In contrast, the missense variant p.Y503C resulted in attenuated rescue capacity compared to the wild type allele. Based on these results, we conclude that in the case of this patient, JS is the result of a pathogenic combination of an amorphic and a hypomorphic PIBF1 allele. Our study underscores the versatility of the Xenopus model to study ciliopathies such as JS in a rapid and cost-effective manner, which should render this animal model attractive for future studies of human ciliopathies.

Review of Ocular Manifestations of Joubert Syndrome

Joubert syndrome is a group of rare disorders that stem from defects in a sensory organelle, the primary cilia. Affected patients often present with disorders involving multiple organ systems, including the brain, eyes, and kidneys. Common symptoms include breathing abnormalities, mental developmental delays, loss of voluntary muscle coordination, and abnormal eye movements, with a diagnostic “molar tooth” sign observed by magnetic resonance imaging (MRI) of the midbrain. We reviewed the ocular phenotypes that can be found in patients with Joubert syndrome. Ocular motor apraxia is the most frequent (80% of patients), followed by strabismus (74%) and nystagmus (72%). A minority of patients also present with ptosis (43%), chorioretinal coloboma (30%), and optic nerve atrophy (22%). Although mutations in 34 genes have been found to be associated with Joubert syndrome, retinal degeneration has been reported in only 38% of patients. Mutations in AHI1 and CEP290, genes critical to primary cilia function, have been linked to retinal degeneration. In conclusion, Joubert syndrome is a rare pleiotropic group of disorders with variable ocular presentations.

INTU-related oral-facial-digital syndrome type VI: A confirmatory report

Oral-facial-digital (OFD) syndromes are a subgroup of ciliopathies distinguished by the co-occurrence of hamartomas and/or multiple frenula of the oral region and digital anomalies. Several clinical forms of OFD syndromes are distinguished by their associated anomalies and/or inheritance patterns, and at least 20 genetic types of OFD syndromes have been delineated. We describe here a child with preaxial and postaxial polydactyly, lingual hamartoma, a congenital heart defect, delayed development and cerebellar peduncles displaying the molar tooth sign. Whole-exome sequencing and SNP array identified compound heterozygous variants in the INTU gene, which encodes a protein involved in the positioning of the ciliary basal body. INTU is a subunit of the CPLANE multiprotein complex essential for the assembly of IFT-A particles and intraflagellar transport. This report of a second patient with INTU-related OFD syndrome and the further delineation of its neuroimaging and skeletal phenotype now allow INTU-related OFD syndromes to be classified within the OFD syndrome type VI group. Patients display a phenotype similar to that of mice with a hypomorphic mutation of Intu, but with the addition of a heart defect.