Mutations of the CEP290 gene encoding a centrosomal protein cause Meckel-Gruber syndrome

Occipital meningoencephalocele in a newborn: A case report in East Africa

INTRODUCTION

Encephalocele refers to protrusion of cranial contents through a bony skull defect. Prevalence of encephaloceles in East Africa is approximately 2 per 10,000 livebirths, with occipital encephaloceles making the least proportion of these in this region. We present a case which was diagnosed postnatally and managed surgically with good outcome and few anticipated complications.

CASE PRESENTATION

Newborn baby delivered to a 26-year-old mother at 38 weeks of gestation by spontaneous vaginal delivery, with swelling on the occipital region since birth. Physical examination revealed a mass measuring 8 cm by 6 cm over the occiput. Initial cranial ultrasound and MRI of the brain revealed an occipital myelomeningocele with part of the right cerebellar lobe, meninges, and CSF herniating through the defect in the occipital skull bone. Surgical correction was successfully done. The patient developed CSF leakage due to hydrocephalus 1-week post-surgery and VP shunt placed to relieve the increased intracranial pressure.

DISCUSSION

This case highlights a very rare neurosurgical congenital defect in East Africa that was managed as early as possible in a low resource setting with minimal post-surgical complications.

CONCLUSION

There is a need for high index of suspicion for encephalocele during antenatal ultrasound screening for prenatal diagnosis. Early surgical repair and prompt post operative follow up help to minimize complications especially in low resource settings where morbidity can be high due to high costs of managing complications.

Substitution of a single non-coding nucleotide upstream of TMEM216 causes non-syndromic retinitis pigmentosa and is associated with reduced TMEM216 expression

Genome analysis of individuals affected by retinitis pigmentosa (RP) identified two rare nucleotide substitutions at the same genomic location on chromosome 11 (g.61392563 [GRCh38]), 69 base pairs upstream of the start codon of the ciliopathy gene TMEM216 (c.-69G>A, c.-69G>T [GenBank: NM_001173991.3]), in individuals of South Asian and African ancestry, respectively. Genotypes included 71 homozygotes and 3 mixed heterozygotes in trans with a predicted loss-of-function allele. Haplotype analysis showed single-nucleotide variants (SNVs) common across families, suggesting ancestral alleles within the two distinct ethnic populations. Clinical phenotype analysis of 62 available individuals from 49 families indicated a similar clinical presentation with night blindness in the first decade and progressive peripheral field loss thereafter. No evident systemic ciliopathy features were noted. Functional characterization of these variants by luciferase reporter gene assay showed reduced promotor activity. Nanopore sequencing confirmed the lower transcription of the TMEM216 c.-69G>T allele in blood-derived RNA from a heterozygous carrier, and reduced expression was further recapitulated by qPCR, using both leukocytes-derived RNA of c.-69G>T homozygotes and total RNA from genome-edited hTERT-RPE1 cells carrying homozygous TMEM216 c.-69G>A. In conclusion, these variants explain a significant proportion of unsolved cases, specifically in individuals of African ancestry, suggesting that reduced TMEM216 expression might lead to abnormal ciliogenesis and photoreceptor degeneration.

Prenatal and Postnatal Diagnosis and Genetic Background of Corpus Callosum Malformations and Neonatal Follow-Up

INTRODUCTION

The corpus callosum is one of the five main cerebral commissures. It is key to combining sensory and motor functions. Its structure can be pathological (dysgenesis) or completely absent (agenesis). The corpus callosum dys- or agenesis is a rare disease (1:4000 live births), but it can have serious mental effects.

METHODS

In our study, we processed the data of 64 pregnant women. They attended a prenatal diagnostic center and genetic counseling from 2005 to 2019 at the Department of Obstetrics and Gynecology at Semmelweis University.

RESULTS

The pregnancies had the following outcomes: 52 ended in delivery, 1 in spontaneous abortion, and 11 in termination of pregnancy (TOP) cases (n = 64). The average time of detection with imaging tests was 25.24 gestational weeks. In 16 cases, prenatal magnetic resonance imaging (MRI) was performed. If the abnormality was detected before the 20th week, a genetic test was performed on an amniotic fluid sample obtained from a genetic amniocentesis. Karyotyping and cytogenetic tests were performed in 15 of the investigated cases. Karyotyping gave normal results in three cases (46,XX or XY). In one of these cases, postnatally chromosomal microarray (CMA) was later performed, which confirmed Aicardi syndrome (3q21.3-21.1 microdeletion). In one case, postnatally, the test found Wiedemann-Rautenstrauch syndrome. In other cases, it found X ring, Di George syndrome, 46,XY,del(13q)(q13q22) and 46,XX,del(5p)(p13) (Cri-du-chat syndrome). Edwards syndrome was diagnosed in six cases, and Patau syndrome in one case.

CONCLUSIONS

We found that corpus callosum abnormalities are often linked to chromosomal problems. We recommend that a cytogenetic test be performed in all cases to rule out inherited diseases. Also, the long-term outcome does not just depend on the disease's severity and the associated other conditions, and hence proper follow-up and early development are also key. For this reason, close teamwork between neonatology, developmental neurology, and pediatric surgery is vital.

Delivering large genes using adeno-associated virus and the CRE-lox DNA recombination system

Adeno-associated virus (AAV) is a safe and efficient gene delivery vehicle for gene therapies. However, its relatively small packaging capacity limits its use as a gene transfer vector. Here, we describe a strategy to deliver large genes that exceed the AAV's packaging capacity using up to four AAV vectors and the CRE-lox DNA recombination system. We devised novel lox sites by combining non-compatible and reaction equilibrium-modifying lox site variants. These lox sites facilitate sequence-specific and near-unidirectional recombination of AAV vector genomes, enabling efficient reconstitution of up to 16 kb of therapeutic genes in a pre-determined configuration. Using this strategy, we have developed AAV gene therapy vectors to deliver IFT140 , PCDH15 , CEP290 , and CDH23 and demonstrate efficient production of full-length proteins in cultured mammalian cells and mouse retinas. Notably, this approach significantly surpasses the trans-splicing and split-intein-based reconstitution methods in efficiency, requiring lower doses, minimizing or eliminating the production of truncated protein products, and offering flexibility in selecting splitting positions. The CRE-lox approach described here provides a simple and effective platform for producing AAV gene therapy vectors beyond AAV's packaging capacity.

Bi-allelic variants in CEP295 cause Seckel-like syndrome presenting with primary microcephaly, developmental delay, intellectual disability, short stature, craniofacial and digital abnormalities

BACKGROUND

Pathogenic variants in the centrosome protein (CEP) family have been implicated in primary microcephaly, Seckel syndrome, and classical ciliopathies. However, most CEP genes remain unlinked to specific Mendelian genetic diseases in humans. We sought to explore the roles of CEP295 in human pathology.

METHODS

Whole-exome sequencing was performed to screen for pathogenic variants in patients with severe microcephaly. Patient-derived fibroblasts and CEP295-depleted U2OS and RPE1 cells were used to clarify the underlying pathomechanisms, including centriole/centrosome development, cell cycle and proliferation changes, and ciliogenesis. Complementary experiments using CEP295 mRNA were performed to determine the pathogenicity of the identified missense variant.

FINDINGS

Here, we report bi-allelic variants of CEP295 in four children from two unrelated families, characterized by severe primary microcephaly, short stature, developmental delay, intellectual disability, facial deformities, and abnormalities of fingers and toes, suggesting a Seckel-like syndrome. Mechanistically, depletion of CEP295 resulted in a decrease in the numbers of centrioles and centrosomes and triggered p53-dependent G1 cell cycle arrest. Moreover, loss of CEP295 causes extensive primary ciliary defects in both patient-derived fibroblasts and RPE1 cells. The results from complementary experiments revealed that the wild-type CEP295, but not the mutant protein, can correct the developmental defects of the centrosome/centriole and cilia in the patient-derived skin fibroblasts.

INTERPRETATION

This study reports CEP295 as a causative gene of the syndromic microcephaly phenotype in humans. Our study also demonstrates that defects in CEP295 result in primary ciliary defects.

FUNDING

A full list of funding bodies that contributed to this study can be found under "Acknowledgments."

A Role for Genetic Modifiers in Tubulointerstitial Kidney Diseases

With the increased availability of genomic sequencing technologies, the molecular bases for kidney diseases such as nephronophthisis and mitochondrially inherited and autosomal-dominant tubulointerstitial kidney diseases (ADTKD) has become increasingly apparent. These tubulointerstitial kidney diseases (TKD) are monogenic diseases of the tubulointerstitium and result in interstitial fibrosis and tubular atrophy (IF/TA). However, monogenic inheritance alone does not adequately explain the highly variable onset of kidney failure and extra-renal manifestations. Phenotypes vary considerably between individuals harbouring the same pathogenic variant in the same putative monogenic gene, even within families sharing common environmental factors. While the extreme end of the disease spectrum may have dramatic syndromic manifestations typically diagnosed in childhood, many patients present a more subtle phenotype with little to differentiate them from many other common forms of non-proteinuric chronic kidney disease (CKD). This review summarises the expanding repertoire of genes underpinning TKD and their known phenotypic manifestations. Furthermore, we collate the growing evidence for a role of modifier genes and discuss the extent to which these data bridge the historical gap between apparently rare monogenic TKD and polygenic non-proteinuric CKD (excluding polycystic kidney disease).

Clinical and Molecular Features of a Chinese Cohort With Syndromic and Nonsyndromic Retinal Dystrophies Related to the CEP290 Gene

PURPOSE

To reveal the clinical and genetic features of 54 Chinese pedigrees with syndromic or nonsyndromic retinal dystrophies related to CEP290 and to explore the genotype-phenotype correlation.

DESIGN

Retrospective cohort study.

METHODS

Patients diagnosed with nonsyndromic inherited retinal dystrophy (IRD) or syndromic ciliopathy (SCP) were enrolled. We identified 61 patients from 54 families carrying biallelic pathogenic CEP290 variants using next-generation sequencing, Sanger sequencing, and co-segregation validation. Genotype-phenotype correlation was evaluated.

RESULTS

This study included 37 IRD patients from 32 families and 24 patients with SCP from 22 pedigrees. Four retinal dystrophy phenotypes were confirmed: Leber congenital amaurosis (LCA, 46/61), early-onset severe retinal dystrophy (EOSRD, 4/61), retinitis pigmentosa (RP, 10/61), and cone-rod dystrophy (CORD, 1/61). The SCP phenotypes included Joubert syndrome (JS) (23/24) and Bardet-Biedl syndrome (BBS) (1/24). We detected 73 different CEP290 variants, of which 33 (45.2%) were not previously reported. Two novel copy number variations (CNVs) and 1 novel pathogenic synonymous change were identified. The most recurrent alterations in the IRD and SCP were p.Q123* (6/64, 9.4%) and p.I556Ffs*17 (10/44, 22.7%), respectively. IRD patients carried more stop-gain alleles (25/64, 39.1%), whereas SCP patients carried more frameshift alleles (23/44, 52.3%).

CONCLUSIONS

LCA was the most common retinal dystrophy phenotype, and JS was the most prevalent syndrome in CEP290 patients; RP/CORD and BBS may be present in early adulthood. The hot spot variants and distribution of genotypes were distinct between IRD and SCP. Our study expands the CEP290 variant spectrum and enhances the current knowledge of CEP290 heterogeneity.

Alternative splicing in CEP290 mutant cats results in a milder phenotype than LCACEP290 patients

PURPOSE

The rdAc cat has an intronic mutation in the centrosomal 290 kDa (CEP290) gene resulting in a frameshift and a premature stop codon (c.6960 + 9 T > G, p.Ile2321AlafsTer3) predicted to truncate the protein by 157 amino acids. CEP290 mutations in human patients cause a range or phenotypes including syndromic conditions and severe childhood loss of vision while the rdAc cat has a milder phenotype. We sought to further characterize the effect of rdAc mutation on CEP290 expression.

METHODS

TaqMan quantitative real-time polymerase chain reaction assays were used to compare wildtype and truncated transcript levels. Relative protein abundance was analyzed by Western blot. Immunohistochemistry (IHC) was performed to detect CEP290 protein.

RESULTS

CEP290 mutant cats show low-level (17.4% of wildtype cats) use of the wildtype splice site and usage of the mutant splice site. Western analysis shows retina from cats homozygous for the mutation has CEP290 protein that likely comprises a combination of both wildtype and truncated protein. IHC detects CEP290 in affected and control retina labeling the region of the interconnecting cilium.

CONCLUSIONS

The comparably milder phenotype of CEP290 mutant cats is likely due to the retained production of some full-length CEP290 protein with possible functional contributions from presence of truncated protein.

The Joubert-Meckel-Nephronophthisis Spectrum of Ciliopathies

The Joubert syndrome (JS), Meckel syndrome (MKS), and nephronophthisis (NPH) ciliopathy spectrum could be the poster child for advances and challenges in Mendelian human genetics over the past half century. Progress in understanding these conditions illustrates many core concepts of human genetics. The JS phenotype alone is caused by pathogenic variants in more than 40 genes; remarkably, all of the associated proteins function in and around the primary cilium. Primary cilia are near-ubiquitous, microtubule-based organelles that play crucial roles in development and homeostasis. Protruding from the cell, these cellular antennae sense diverse signals and mediate Hedgehog and other critical signaling pathways. Ciliary dysfunction causes many human conditions termed ciliopathies, which range from multiple congenital malformations to adult-onset single-organ failure. Research on the genetics of the JS-MKS-NPH spectrum has spurred extensive functional work exploring the broadly important role of primary cilia in health and disease. This functional work promises to illuminate the mechanisms underlying JS-MKS-NPH in humans, identify therapeutic targets across genetic causes, and generate future precision treatments. Expected final online publication date for the Annual Review of Genomics and Human Genetics, Volume 23 is October 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Control of meiotic chromosomal bouquet and germ cell morphogenesis by the zygotene cilium

A hallmark of meiosis is chromosomal pairing, which requires telomere tethering and rotation on the nuclear envelope via microtubules, driving chromosome homology searches. Telomere pulling toward the centrosome forms the "zygotene chromosomal bouquet". Here, we identified the "zygotene cilium" in oocytes. This cilium provides a cable system for the bouquet machinery, extending throughout the germline cyst. Using zebrafish mutants and live manipulations, we demonstrate that the cilium anchors the centrosome to counterbalance telomere pulling. The cilium is essential for bouquet and synaptonemal complex formation, oogenesis, ovarian development, and fertility. Thus, a cilium represents a conserved player in zebrafish and mouse meiosis, which sheds light on reproductive aspects in ciliopathies, and suggests that cilia can control chromosomal dynamics.

Evaluation of novel compound variants of CEP290 in prenatally suspected case of Meckel syndrome through whole exome sequencing

Background

Meckel syndrome (MKS) is a fatal disease characterized by multisystem fibrosis during the prenatal or perinatal period. It has an autosomal recessive genetic pattern and is characterized by meningo occipital encephalocele, polycystic kidney dysplasia, polydactyly, and hepatobiliary ductal plate malformation. Germline variations in CEP290 have been shown to cause MKS4.

Methods

In this study, a 23‐year‐old Chinese woman who was 18 weeks pregnant was examined. The pregnancy was terminated due to occipital meningocele and enlarged cystic dysplastic kidney revealed by ultrasonography. In addition, the patient had a history of adverse pregnancy whereby the fetus presented with double kidney enlargement. Karyotype analysis and chromosomal microarray examination (CMA) were carried out using amniotic fluid samples. Whole exome sequencing (WES) was performed using tissue specimens of the aborted fetus.

Results

Karyotype and CMA analyses showed normal results. However, compound heterozygous mutations of CEP290 c.3175dup and CEP290 c.1201dup were detected through WES. CEP290 c.1201dup is a novel heterozygous mutation of CEP290 that has not been reported previously.

Conclusions

The findings of this study provide information on the correlation between MKS phenotype and genotype in CEP290. In addition, these findings indicate that WES is an effective method for detecting genetic causes of multiple structural defects especially those showing normal karyotype and CMA results.

Super-resolution microscopy reveals photoreceptor-specific subciliary location and function of ciliopathy-associated protein CEP290

Mutations in the cilium-associated protein CEP290 cause retinal degeneration as part of multiorgan ciliopathies or as retina-specific diseases. The precise location and the functional roles of CEP290 within cilia and, specifically, the connecting cilia (CC) of photoreceptors, remain unclear. We used super-resolution fluorescence microscopy and electron microscopy to localize CEP290 in the CC and in the primary cilia of cultured cells with subdiffraction resolution and to determine effects of CEP290 deficiency in 3 mutant models. Radially, CEP290 localizes in close proximity to the microtubule doublets in the region between the doublets and the ciliary membrane. Longitudinally, it is distributed throughout the length of the CC whereas it is confined to the very base of primary cilia in human retinal pigment epithelium-1 cells. We found Y-shaped links, ciliary substructures between microtubules and membrane, throughout the length of the CC. Severe CEP290 deficiencies in mouse models did not prevent assembly of cilia or cause obvious mislocalization of ciliary components in early stages of degeneration. There were fewer cilia and no normal outer segments in the mutants, but the Y-shaped links were clearly present. These results point to photoreceptor-specific functions of CEP290 essential for CC maturation and stability following the earliest stages of ciliogenesis.

Genetic compensation for cilia defects in cep290 mutants by upregulation of cilia-associated small GTPases

Mutations in CEP290 (also known as NPHP6), a large multidomain coiled coil protein, are associated with multiple cilia-associated syndromes. Over 130 CEP290 mutations have been linked to a wide spectrum of human ciliopathies, raising the question of how mutations in a single gene cause different disease syndromes. In zebrafish, the expressivity of cep290 deficiencies were linked to the type of genetic ablation: acute cep290 morpholino knockdown caused severe cilia-related phenotypes, whereas deficiencies in a CRISPR/Cas9 genetic mutant were restricted to photoreceptor defects. Here, we show that milder phenotypes in genetic mutants were associated with the upregulation of genes encoding the cilia-associated small GTPases arl3, arl13b and unc119b. Upregulation of UNC119b was also observed in urine-derived renal epithelial cells from human Joubert syndrome CEP290 patients. Ectopic expression of arl3, arl13b and unc119b in cep290 morphant zebrafish embryos rescued Kupffer's vesicle cilia and partially rescued photoreceptor outer segment defects. The results suggest that genetic compensation by upregulation of genes involved in a common subcellular process, lipidated protein trafficking to cilia, may be a conserved mechanism contributing to genotype-phenotype variations observed in CEP290 deficiencies. This article has an associated First Person interview with the first author of the paper.

High rate of hypomorphic variants as the cause of inherited ataxia and related diseases: study of a cohort of 366 families

PURPOSE

Diagnosis of inherited ataxia and related diseases represents a real challenge given the tremendous heterogeneity and clinical overlap of the various causes. We evaluated the efficacy of molecular diagnosis of these diseases by sequencing a large cohort of undiagnosed families.

METHODS

We analyzed 366 unrelated consecutive patients with undiagnosed ataxia or related disorders by clinical exome-capture sequencing. In silico analysis was performed with an in-house pipeline that combines variant ranking and copy-number variant (CNV) searches. Variants were interpreted according to American College of Medical Genetics and Genomics/Association for Molecular Pathology (ACMG/AMP) guidelines.

RESULTS

We established the molecular diagnosis in 46% of the cases. We identified 35 mildly affected patients with causative variants in genes that are classically associated with severe presentations. These cases were explained by the occurrence of hypomorphic variants, but also rarely suspected mechanisms such as C-terminal truncations and translation reinitiation.

CONCLUSION

A significant fraction of the clinical heterogeneity and phenotypic overlap is explained by hypomorphic variants that are difficult to identify and not readily predicted. The hypomorphic C-terminal truncation and translation reinitiation mechanisms that we identified may only apply to few genes, as it relies on specific domain organization and alterations. We identified PEX10 and FASTKD2 as candidates for translation reinitiation accounting for mild disease presentation.

Animals Models of Inherited Retinal Disease

Inherited retinal diseases (IRDs) are an important cause of blindness worldwide. Over 270 genes have been associated with IRD. Genetic testing can determine the cause of the clinical disease in the majority of patients. However, at least 25-50% of patients with clinical diagnosis of IRD remain unsolved even after whole genome sequencing. Animal models of IRD can be useful for expanding the set of established IRD genes, to gain biological understanding of the function of these genes in the retina, and to test advanced therapeutics prior to human clinical trials. In this chapter some small and large animal models of IRD are discussed including some of the advantages and limitations of each for various forms of retinopathy.

Meckel-Gruber Syndrome: Clinical and Molecular Genetic Profiles in Two Fetuses and Review of the Current Literature

Background: Meckel-Gruber syndrome (MKS; OMIM No. 249000) is a rare, in utero lethal disease characterized by occipital encephalocele, polycystic kidneys, and polydactyly. Methodology and Results: In this study, two fetuses diagnosed as having MKS in the prenatal period were evaluated on the basis of ultrasonographic findings, postmortem autopsy findings, and molecular genetic analyses. Using exome sequencing analyses a novel homozygous frameshift variant (NM_015631: c.530delA, p.Lys177Argfs*47) was detected at exon 4 of TCTN3 gene in case 1, and a novel homozygous synonymous variant (NM_025114: c.180G>A, p Lys60Lys) was detected at exon 3 of CEP290 gene in case 2. Case 1 is the first reported case in the literature, which showed the typical MKS clinical feature with a novel frameshift variation in the TCTN3 gene. The variant in case 2 is the first reported synonymous variant of CEP290 gene in the literature, which has been shown to affect splicing in a functional study at the RNA level. Conclusion: TCTN3 gene variants that were rarely associated with the typical MKS phenotype and all cases with these variations have been discussed in the context of genotype-phenotype. The detection of the first synonymous variant of CEP290 gene and the demonstration of its effect on splicing by a functional study are likely to contribute to the molecular etiology of MKS.

Diagnosis of Inherited Retinal Diseases

Inherited retinal diseases are a frequent cause of severe visual impairment or blindness in children and adults of working age. Across this group of diseases, there is great variability in the degree of visual impairment, the impact on everyday life, disease progression, and the suitability to therapeutic intervention. Therefore, an early and precise diagnosis is crucial for patients and their families. Characterizing inherited retinal diseases involves a detailed medical history, clinical examination with testing of visual function, multimodal retinal imaging as well as molecular genetic testing. This may facilitate a distinction between different inherited retinal diseases, as well as a differentiation from monogenic systemic diseases with retinal involvement, and from mimicking diseases.

CEP55 promotes cilia disassembly through stabilizing Aurora A kinase

Primary cilia protrude from the cell surface and have diverse roles during development and disease, which depends on the precise timing and control of cilia assembly and disassembly. Inactivation of assembly often causes cilia defects and underlies ciliopathy, while diseases caused by dysfunction in disassembly remain largely unknown. Here, we demonstrate that CEP55 functions as a cilia disassembly regulator to participate in ciliopathy. Cep55-/- mice display clinical manifestations of Meckel-Gruber syndrome, including perinatal death, polycystic kidneys, and abnormalities in the CNS. Interestingly, Cep55-/- mice exhibit an abnormal elongation of cilia on these tissues. Mechanistically, CEP55 promotes cilia disassembly by interacting with and stabilizing Aurora A kinase, which is achieved through facilitating the chaperonin CCT complex to Aurora A. In addition, CEP55 mutation in Meckel-Gruber syndrome causes the failure of cilia disassembly. Thus, our study establishes a cilia disassembly role for CEP55 in vivo, coupling defects in cilia disassembly to ciliopathy and further suggesting that proper cilia dynamics are critical for mammalian development.

A LINE-1 insertion situated in the promoter of IMPG2 is associated with autosomal recessive progressive retinal atrophy in Lhasa Apso dogs

Background

Canine progressive retinal atrophies are a group of hereditary retinal degenerations in dogs characterised by depletion of photoreceptor cells in the retina, which ultimately leads to blindness. PRA in the Lhasa Apso (LA) dog has not previously been clinically characterised or described in the literature, but owners in the UK are advised to have their dog examined through the British Veterinary Association/ Kennel Club/ International Sheep Dog Society (BVA/KC/ISDS) eye scheme annually, and similar schemes that are in operation in other countries. After the exclusion of 25 previously reported canine retinal mutations in LA PRA-affected dogs, we sought to identify the genetic cause of PRA in this breed.

Results

Analysis of whole-exome sequencing data of three PRA-affected LA and three LA without signs of PRA did not identify any exonic or splice site variants, suggesting the causal variant was non-exonic. We subsequently undertook a genome-wide association study (GWAS), which identified a 1.3 Mb disease-associated region on canine chromosome 33, followed by whole-genome sequencing analysis that revealed a long interspersed element-1 (LINE-1) insertion upstream of the IMPG2 gene. IMPG2 has previously been implicated in human retinal disease; however, until now no canine PRAs have been associated with this gene. The identification of this PRA-associated variant has enabled the development of a DNA test for this form of PRA in the breed, here termed PRA4 to distinguish it from other forms of PRA described in other breeds. This test has been used to determine the genotypes of over 900 LA dogs. A large cohort of genotyped dogs was used to estimate the allele frequency as between 0.07–0.1 in the UK LA population.

Conclusions

Through the use of GWAS and subsequent sequencing of a PRA case, we have identified a LINE-1 insertion in the retinal candidate gene IMPG2 that is associated with a form of PRA in the LA dog. Validation of this variant in 447 dogs of 123 breeds determined it was private to LA dogs. We envisage that, over time, the developed DNA test will offer breeders the opportunity to avoid producing dogs affected with this form of PRA.

Rapid Ex-Vivo Ciliogenesis and Dose-Dependent Effect of Notch Inhibition on Ciliogenesis of Respiratory Epithelia

Background: Cilia are actin based cellular protrusions conserved from algae to complex multicellular organisms like Homo sapiens. Respiratory motile cilia line epithelial cells of the tracheobronchial tree, beat in a synchronous, metachronal wave, moving inhaled pollutants and pathogens cephalad. Their role in both congenital disorders like primary ciliary dyskinesia (PCD) to acquired disorders like chronic obstructive pulmonary disease (COPD) continues to evolve. In this current body of work we outline a protocol optimized to reciliate human nasal epithelial cells and mouse tracheal cells in vitro. Using this protocol, we knocked down known cilia genes, as well as use a small molecule inhibitor of Notch, N-[N-(3,5-Difluorophenacetyl)-L-alanyl]-S-phenylglycine t-butyl Ester (DAPT), to assess the effect of these on ciliogenesis in order to show the validity of our protocol. Methods: Tracheas were harvested from wild-type, adult C57B6 mice, pronase digested and sloughed off epithelial cells grown to confluence in stationary culture on rat-tail collagen coated wells. Upon reaching confluence, collagen was digested and cells placed suspension culture protocol to reciliate the cells. Using this suspension culture protocol, we employed siRNA gene knockdown to assay gene functions required for airway ciliogenesis. Knock down of Dynein axonemal heavy chain 5 (Dnah5), a ciliary structural protein, was confirmed using immunostaining. Mouse tracheal cells were treated in suspension with varying doses of DAPT, an inhibitor of Notch, with the purpose of evaluating its effect and dose response on ciliogenesis. The optimum dose was then used on reciliating human nasal epithelial cells. Results: siRNA knockdown of Foxj1 prevented ciliation, consistent with its role as a master regulator of motile cilia. Knockdown of Dnai1 and Dnah5 resulted in immotile cilia, and Cand1 knockdown, a centrosome protein known to regulate centrosome amplification, inhibited airway ciliogenesis. Dnah5 knockdown was confirmed with significantly decreased immunostaining of cilia for this protein. Inhibiting Notch signaling by inhibiting gamma secretase with DAPT enhanced the percentage of ciliation, and resulted in longer cilia that beat with higher frequency in both mouse and human airway epithelia. Conclusions: Modifying existing reciliation protocols to suit both human nasal epithelial and mouse tracheal tissue, we have shown that knockdown of known cilia-related genes have the expected effects. Additionally, we have demonstrated the optimal dosage for significantly improving reciliation of airway epithelia using DAPT. Given that cilia length and function are significantly compromised in COPD, these findings open up interesting avenues for further exploration.

The Role of Wnt Signalling in Chronic Kidney Disease (CKD)

Chronic kidney disease (CKD) encompasses a group of diverse diseases that are associated with accumulating kidney damage and a decline in glomerular filtration rate (GFR). These conditions can be of an acquired or genetic nature and, in many cases, interactions between genetics and the environment also play a role in disease manifestation and severity. In this review, we focus on genetically inherited chronic kidney diseases and dissect the links between canonical and non-canonical Wnt signalling, and this umbrella of conditions that result in kidney damage. Most of the current evidence on the role of Wnt signalling in CKD is gathered from studies in polycystic kidney disease (PKD) and nephronophthisis (NPHP) and reveals the involvement of β-catenin. Nevertheless, recent findings have also linked planar cell polarity (PCP) signalling to CKD, with further studies being required to fully understand the links and molecular mechanisms.

Whole exome sequencing identified a homozygous novel variant in CEP290 gene causes Meckel syndrome

Meckel syndrome (MKS) is a pre‐ or perinatal multisystemic ciliopathic lethal disorder with an autosomal recessive mode of inheritance. Meckel syndrome is usually manifested with meningo‐occipital encephalocele, polycystic kidney dysplasia, postaxial polydactyly and hepatobiliary ductal plate malformation. Germline variants in CEP290 cause MKS4. In this study, we investigated a 35‐years‐old Chinese female who was 17+1 weeks pregnant. She had a history of adverse pregnancy of having foetus with multiple malformations. We performed ultrasonography and identified the foetus with occipital meningoencephalocele and enlarged cystic dysplastic kidneys. So, she decided to terminate her pregnancy and further genetic molecular analysis was performed. We identified the aborted foetus without postaxial polydactyly. Histological examination of foetal kidney showed cysts in kidney and thinning of the renal cortex with glomerular atrophy. Whole exome sequencing identified a novel homozygous variant (c.2144T>G; p.L715^*) in exon 21 of the CEP290 in the foetus. Sanger sequencing confirmed that both the parents of the foetus were carrying this variant in a heterozygous state. This variant was not identified in two elder sisters of the foetus as well as in the 100 healthy individuals. Western blot analysis showed that this variant leads to the formation of truncated CEP290 protein with the molecular weight of 84 KD compared with the wild‐type CEP290 protein of 290 KD. Hence, it is a loss‐of‐function variant. We also found that the mutant cilium appears longer in length than the wild‐type cilium. Our present study reported the first variant of CEP290 associated with MKS4 in Chinese population.

Trio-whole-exome sequencing and preimplantation genetic diagnosis for unexplained recurrent fetal malformations

Whole-exome sequencing (WES) is widely used to detect genetic mutations that cause Mendelian diseases, and has been successfully applied in combination with preimplantation genetic diagnosis (PGD) to avoid the transmission of genetic defects. We investigated 40 nonconsanguineous families with unexplained, recurrent fetal malformations (two or more malformed fetuses) from May 2016 to December 2018. Using Trio-WES, we identified 32 disease-associated variants in 40 families (80% positive rate), which were subsequently verified. Known Mendelian diseases were identified in 12 families (30%), highly suspected Mendelian diseases in 12 families (30%), variants with uncertain significance in 8 families (20%), and no noticeable variants for 8 families (20%). Further analysis showed variants in 22 genes may cause fetal malformations. Four gene variants were detected in fetuses for the first time, which expanded the spectrum of the disease phenotype. Two novel candidate genes may be related to fetal malformations. Of 26 couples receiving PGD on disease-associated genes, 3 healthy newborns were delivered, and 4 couples are undergoing pregnancies. We reported the fetal data and developed an optimized genetic testing strategy. Our finding strongly suggests the presence of single gene Mendelian disorders in 60% of those families, and PGD services for couples to have healthy babies.

The myosin-tail homology domain of centrosomal protein 290 is essential for protein confinement between the inner and outer segments in photoreceptors

Mutations in the centrosomal protein 290 (CEP290) gene cause various ciliopathies involving retinal degeneration. CEP290 proteins localize to the ciliary transition zone and are thought to act as a gatekeeper that controls ciliary protein trafficking. However, precise roles of CEP290 in photoreceptors and pathomechanisms of retinal degeneration in CEP290-associated ciliopathies are not sufficiently understood. Using conditional Cep290 mutant mice, in which the C-terminal myosin-tail homology domain of CEP290 is disrupted after the connecting cilium is assembled, we show that this domain is essential for protein confinement between the inner and the outer segments. Upon disruption of the myosin-tail homology domain, inner segment plasma membrane proteins, including syntaxin 3 (STX3), synaptosome-associated protein 25 (SNAP25), and interphotoreceptor matrix proteoglycan 2 (IMPG2), rapidly accumulated in the outer segment. In contrast, localization of endomembrane proteins was not altered. Trafficking and confinement of most outer segment-resident proteins appeared to be unaffected or only minimally affected in Cep290 mutant mice. One notable exception was rhodopsin (RHO), which severely mislocalized to inner segments during the initial stage of degeneration. Similar mislocalization phenotypes were observed in Cep290^rd16 mice. These results suggest that a failure of protein confinement at the connecting cilium and consequent accumulation of inner segment membrane proteins in the outer segment, along with insufficient RHO delivery, is part of the disease mechanisms that cause retinal degeneration in CEP290-associated ciliopathies. Our study provides insights into the pathomechanisms of retinal degenerations associated with compromised ciliary gates.

Deubiquitylase USP9X maintains centriolar satellite integrity by stabilizing pericentriolar material 1 protein

Centriolar satellites are small cytoplasmic granules that play important roles in regulating the formation of centrosomes and primary cilia. Ubiquitylation of satellite proteins, including the core satellite scaffold protein pericentriolar material 1 (PCM1), regulates centriolar satellite integrity. Currently, deubiquitylases that control centriolar satellite integrity have not been identified. In this study, we find that the deubiquitylase USP9X binds PCM1, and antagonizes PCM1 ubiquitylation to protect it from proteasomal degradation. Knockdown of USP9X in human cell lines reduces PCM1 protein levels, disrupts centriolar satellite particles and causes localization of satellite proteins, such as CEP290, to centrosomes. Interestingly, knockdown of mindbomb 1 (MIB1), a ubiquitin ligase that promotes PCM1 ubiquitylation and degradation, in USP9X-depleted cells largely restores PCM1 protein levels and corrects defects caused by the loss of USP9X. Overall, our study reveals that USP9X is a constituent of centriolar satellites and functions to maintain centriolar satellite integrity by stabilizing PCM1.

The ciliopathy gene product Cep290 is required for primary cilium formation and microtubule network organization

The mammalian centrosome/cilium complex is composed of the centrosome, the primary cilium, and the centriolar satellites, which together function in key cellular processes including signaling. Defective assembly, maintenance, and function of the centrosome/ cilium complex cause the human genetic diseases known as ciliopathies, which are characterized by a multitude of developmental syndromes including retinal degeneration and kidney cysts. The molecular mechanisms underlying pathogenesis in ciliopathies remain poorly understood, which requires structural and functional characterization of the mutated ciliopathy proteins at the cellular level. To this end, we elucidated the function and regulation of Cep290, which is the most frequently mutated gene in ciliopathies and importantly its functions remain poorly understood. First, we generated Cep290-null cells using the CRISPR/Cas9 genome editing approach. Using functional assays, we showed that Cep290-null cells do not ciliate and that they have defects in centriolar satellites dynamics and interphase microtubule organization. The centriolar satellites were tightly clustered around the centrosome in Cep290-null cells, and the interphase microtubule network lost its radial organization. Our results provide phenotypic insight into the disease mechanisms of Cep290 ciliopathy mutations and also the tools for studying genotype/phenotype relationships in ciliopathies.

Nephronophthisis: A review of genotype-phenotype correlation

Nephronophthisis is an autosomal recessive cystic kidney disease and one of the most common genetic disorders causing end‐stage renal disease in children. Nephronophthisis is a genetically heterogenous disorder with more than 25 identified genes. In 10%–20% of cases, there are additional features of a ciliopathy syndrome, such as retinal defects, liver fibrosis, skeletal abnormalities, and brain developmental disorders. This review provides an update of the recent advances in the clinical features and related gene mutations of nephronophthisis, and novel approaches for therapy in nephronophthisis patients may be needed.

Nephronophthisis (NPHP) is a renal ciliopathy affecting children and young adults. This review gives an update on the recent advances in the clinical features and related gene mutations of NPHP.

Special Issue Introduction: Inherited Retinal Disease: Novel Candidate Genes, Genotype-Phenotype Correlations, and Inheritance Models

Inherited retinal diseases (IRDs) are genetically and clinically heterogeneous disorders.[...].

Photoreceptor Cilia and Retinal Ciliopathies

Photoreceptors are sensory neurons designed to convert light stimuli into neurological responses. This process, called phototransduction, takes place in the outer segments (OS) of rod and cone photoreceptors. OS are specialized sensory cilia, with analogous structures to those present in other nonmotile cilia. Deficient morphogenesis and/or dysfunction of photoreceptor sensory cilia (PSC) caused by mutations in a variety of photoreceptor-specific and common cilia genes can lead to inherited retinal degenerations (IRDs). IRDs can manifest as isolated retinal diseases or syndromic diseases. In this review, we describe the structure and composition of PSC and different forms of ciliopathies with retinal involvement. We review the genetics of the IRDs, which are monogenic disorders but genetically diverse with regard to causality.

Central retinal preservation in rdAc cats

OBJECTIVE

Children with Leber congenital amaurosis (LCA) due to CEP290 mutations show characteristic macular preservation. Spectral domain-optical coherence tomography (SD-OCT) is a noninvasive technique to investigate retinal structural changes. Loss of integrity of the ellipsoid zone (EZ) on OCT in people with retinal disease has been associated with loss of visual function and is a useful measure of retinal disease progression. We hypothesized that rdAc felines with Cep290 mutation would have a similar pattern of degeneration, with relative central retinal preservation associated with maintenance of the EZ.

PROCEDURES

Fundus imaging, confocal scanning laser ophthalmoscopy, and SD-OCT cross-sectional imaging was performed on 11 rdAc cats ranging from 6 months to 10 years of age. Images were collected from the area centralis, visual streak, and the mid-superior and mid-inferior retina. Receptor plus (REC+, encompassing the entire length of photoreceptors) thicknesses were measured. Regional rates of degeneration were determined by regression analysis and compared using unpaired t-tests. The EZ was evaluated for the presence, absence, or loss of definition.

RESULTS

RdAc cats showed REC+ thinning over time in all regions. The area centralis and visual streak had a slower rate of thinning than the mid-peripheral retina. There was loss of integrity of the EZ initially in the more peripheral regions, while its integrity was maintained in the area centralis and visual streak at all ages studied.

CONCLUSIONS

rdAc cats show preservation of the central retina with maintenance of EZ integrity, which recapitulates findings in human patients.

A Rare Form of Retinal Dystrophy Caused by Hypomorphic Nonsense Mutations in CEP290

PURPOSE

To identify the gene defect and to study the clinical characteristics and natural course of disease in a family originally diagnosed with oligocone trichromacy (OT), a rare congenital cone dysfunction syndrome.

METHODS

Extensive clinical and ophthalmologic assessment was performed on two siblings with OT and long-term follow up data were analyzed. Subsequently, whole exome sequencing (WES) and Sanger sequence analysis of CEP290 was performed in the two siblings. Additionally, the identified CEP290 mutations were analyzed in persons with achromatopsia (ACHM) (n = 23) and autosomal recessive or isolated cone dystrophy (CD; n = 145).

RESULTS

In the first decade of life, the siblings were diagnosed with OT based on low visual acuity, photophobia, nystagmus, and absent cone response on electroretinography , but with normal color discrimination. Over time, the phenotype of OT evolved to a progressive degenerative disease without any CEP290-associated non-ocular features. In both siblings, two nonsense mutations (c.451C>T; p.(Arg151*) and c.4723A>T; p.(Lys1575*)) in CEP290 were found. Previously, p.(Arg151*) was demonstrated to induce nonsense-mediated alternative splicing events leading to intact open reading frames of the resulting mRNA products (p.(Leu148_Glu165del) and p.(Leu148_Lys172del)). mRNA analysis for p.(Lys1575*) confirmed a suspected hypomorphic character, as exon 36 skipping was observed in a small fraction of CEP290 mRNA, resulting in a 36 aa in-frame deletion (p.(Glu1569_Trp1604del)). No additional cases carrying these variants were identified in the ACHM and CD cohorts.

CONCLUSIONS

Compound heterozygous hypomorphic mutations in CEP290 may lead to a rare form of cone-dominated retinal dystrophy, a novel phenotype belonging to the CEP290-associated spectrum of ciliopathies. These findings provide insight into the effect of CEP290 mutations on the clinical phenotype.

Human DNA helicase, RuvBL1 and its Chlamydomonas homologue, CrRuvBL1 plays an important role in ciliogenesis

Several nuclear and nucleic acid-binding proteins were detected in the proteomic analyses of ciliary fractions from various organisms. Yet very little is known about the role of these proteins in ciliogenesis and ciliary signaling. In an attempt to characterize the role of these nuclear proteins, we identified a hypothetical protein from Chlamydomonas reinhardtii, CrRuvBL1, which is homologous to human DNA helicase, HsRuvBL1. CrRuvBL1 localizes to flagella and nucleus in vegetative Chlamydomonas cells. It accumulates in the nucleus specifically during initial stages of flagellar assembly and cell division indicating its role in these processes. Mammalian counterpart of this protein, HsRuvBL1, was found to be present at the basal bodies and in the primary cilium of quiescent Retinal Pigment Epithelial (RPE1) cells. In interphase cells, HsRuvBL1 is present at centrioles while the protein localizes on spindle fibers, spindle poles and midbodies, which are important structures formed during different phases of cell division. Depletion of HsRuvBL1 by using siRNAs leads to complete loss of primary cilia in RPE1 cells. Together these results suggest that nuclear proteins play an important role in ciliogenesis.

From disease modelling to personalised therapy in patients with CEP290 mutations

Mutations that give rise to premature termination codons are a common cause of inherited genetic diseases. When transcripts containing these changes are generated, they are usually rapidly removed by the cell through the process of nonsense-mediated decay. Here we discuss observed changes in transcripts of the centrosomal protein CEP290 resulting not from degradation, but from changes in exon usage. We also comment on a landmark paper (Drivas et al. Sci Transl Med. 2015) where modelling this process of exon usage may be used to predict disease severity in CEP290 ciliopathies, and how understanding this process may potentially be used for therapeutic benefit in the future.

Many Genes-One Disease? Genetics of Nephronophthisis (NPHP) and NPHP-Associated Disorders

Nephronophthisis (NPHP) is a renal ciliopathy and an autosomal recessive cause of cystic kidney disease, renal fibrosis, and end-stage renal failure, affecting children and young adults. Molecular genetic studies have identified more than 20 genes underlying this disorder, whose protein products are all related to cilia, centrosome, or mitotic spindle function. In around 15% of cases, there are additional features of a ciliopathy syndrome, including retinal defects, liver fibrosis, skeletal abnormalities, and brain developmental disorders. Alongside, gene identification has arisen molecular mechanistic insights into the disease pathogenesis. The genetic causes of NPHP are discussed in terms of how they help us to define treatable disease pathways including the cyclic adenosine monophosphate pathway, the mTOR pathway, Hedgehog signaling pathways, and DNA damage response pathways. While the underlying pathology of the many types of NPHP remains similar, the defined disease mechanisms are diverse, and a personalized medicine approach for therapy in NPHP patients is likely to be required.

Murine Cep290 phenotypes are modified by genetic backgrounds and provide an impetus for investigating disease modifier alleles

The study of primary cilia is of broad interest both in terms of disease pathogenesis and the fundamental biological role of these structures. Murine models of ciliopathies provide valuable tools for the study of these diseases. However, it is important to consider the precise phenotype of murine models and how dependant it is upon genetic background. Here we compare and contrast murine models of Cep290, a frequent genetic cause of Joubert syndrome in order to refine our concept of genotype-phenotype correlations.

Bardet-Biedl syndrome: Is it only cilia dysfunction?

Bardet-Biedl syndrome (BBS) is a genetically heterogeneous, pleiotropic disorder, characterized by both congenital and late onset defects. From the analysis of the mutational burden in patients to the functional characterization of the BBS proteins, this syndrome has become a model for both understanding oligogenic patterns of inheritance and the biology of a particular cellular organelle: the primary cilium. Here we briefly review the genetics of BBS to then focus on the function of the BBS proteins, not only in the context of the cilium but also highlighting potential extra-ciliary roles that could be relevant to the etiology of the disorder. Finally, we provide an overview of how the study of this rare syndrome has contributed to the understanding of cilia biology and how this knowledge has informed on the cellular basis of different clinical manifestations that characterize BBS and the ciliopathies.

Nephronophthisis and related syndromes

PURPOSE OF REVIEW

Nephronophthisis (NPHP) is an autosomal recessive cystic kidney disease and is one of the most common genetic disorders causing end-stage renal disease (ESRD) in children and adolescents. NPHP is a genetically heterogenous disorder with 20 identified genes. NPHP occurs as an isolated kidney disease, but approximately 15% of NPHP patients have additional extrarenal symptoms affecting other organs [e.g. eyes, liver, bones and central nervous system (CNS)]. The pleiotropy in NPHP is explained by the finding that almost all NPHP gene products share expression in primary cilia, a sensory organelle present in most mammalian cells. If extrarenal symptoms are present in addition to NPHP, these disorders are classified as NPHP-related ciliopathies (NPHP-RC). This review provides an update about recent advances in the field of NPHP-RC.

RECENT FINDINGS

The identification of novel disease-causing genes has improved our understanding of the pathomechanisms contributing to NPHP-RC. Multiple interactions between different NPHP-RC gene products have been published and outline the interconnectivity of the affected proteins and shared pathways.

SUMMARY

The significance of recently identified genes for NPHP-RC is discussed and the complex role and interaction of NPHP proteins in ciliary function and cellular signalling pathways is highlighted.

Species-dependent splice recognition of a cryptic exon resulting from a recurrent intronic CEP290 mutation that causes congenital blindness

A mutation in intron 26 of CEP290 (c.2991+1655A>G) is the most common genetic cause of Leber congenital amaurosis (LCA), a severe type of inherited retinal degeneration. This mutation creates a cryptic splice donor site, resulting in the insertion of an aberrant exon (exon X) into ~50% of all CEP290 transcripts. A humanized mouse model with this mutation did not recapitulate the aberrant CEP290 splicing observed in LCA patients, suggesting differential recognition of cryptic splice sites between species. To further assess this phenomenon, we generated two CEP290 minigene constructs, with and without the intronic mutation, and transfected these in cell lines of various species. RT-PCR analysis revealed that exon X is well recognized by the splicing machinery in human and non-human primate cell lines. Intriguingly, this recognition decreases in cell lines derived from species such as dog and rodents, and it is completely absent in Drosophila. In addition, other cryptic splicing events corresponding to sequences in intron 26 of CEP290 were observed to varying degrees in the different cell lines. Together, these results highlight the complexity of splice site recognition among different species, and show that care is warranted when generating animal models to mimic splice site mutations in vivo.

Morphological and functional aspects of progenitors perturbed in cortical malformations

In this review, we discuss molecular and cellular mechanisms important for the function of neuronal progenitors during development, revealed by their perturbation in different cortical malformations. We focus on a class of neuronal progenitors, radial glial cells (RGCs), which are renowned for their unique morphological and behavioral characteristics, constituting a key element during the development of the mammalian cerebral cortex. We describe how the particular morphology of these cells is related to their roles in the orchestration of cortical development and their influence on other progenitor types and post-mitotic neurons. Important for disease mechanisms, we overview what is currently known about RGC cellular components, cytoskeletal mechanisms, signaling pathways and cell cycle characteristics, focusing on how defects lead to abnormal development and cortical malformation phenotypes. The multiple recent entry points from human genetics and animal models are contributing to our understanding of this important cell type. Combining data from phenotypes in the mouse reveals molecules which potentially act in common pathways. Going beyond this, we discuss future directions that may provide new data in this expanding area.

Retinitis pigmentosa caused by mutations in the ciliary MAK gene is relatively mild and is not associated with apparent extra-ocular features

PURPOSE

Defects in MAK, encoding a protein localized to the photoreceptor connecting cilium, have recently been associated with autosomal recessive retinitis pigmentosa (RP). The aim of this study is to describe our detailed clinical observations in patients with MAK-associated RP, including an assessment of syndromic symptoms frequently observed in ciliopathies.

METHODS

In this international collaborative study, 11 patients carrying nonsense or missense mutations in MAK were clinically evaluated, including extensive assessment of the medical history, slit-lamp biomicroscopy, ophthalmoscopy, kinetic perimetry, electroretinography (ERG), spectral-domain optical coherence tomography (SD-OCT), autofluorescence imaging and fundus photography. Additionally, we used a questionnaire to evaluate the presence of syndromic features and tested the olfactory function.

RESULTS

MAK-associated RP is not associated with syndromic features, not even with subclinical dysfunction of the olfactory apparatus. All patients experienced typical RP symptoms of night blindness followed by visual field constriction. Symptoms initiated between childhood and the age of 43 (mean: 23 years). Although some patients experienced vision loss, the visual acuity remained normal in most patients. ERG and ophthalmoscopy revealed classic RP characteristics, and SD-OCT demonstrated thinning of the overall retina, outer nuclear layer and photoreceptor-pigment epithelium complex.

CONCLUSION

Nonsense and missense mutations in MAK give rise to a non-syndromic recessive RP phenotype without apparent extra-ocular features. When compared to other retinal ciliopathies, MAK-associated RP appears to be relatively mild and shows remarkable resemblance to RP1-associated RP, which could be explained by the close functional relation of these proteins.

Mutations in IFT172 cause isolated retinal degeneration and Bardet-Biedl syndrome

Primary cilia are sensory organelles present on most mammalian cells. The assembly and maintenance of primary cilia are facilitated by intraflagellar transport (IFT), a bidirectional protein trafficking along the cilium. Mutations in genes coding for IFT components have been associated with a group of diseases called ciliopathies. These genetic disorders can affect a variety of organs including the retina. Using whole exome sequencing in three families, we identified mutations in Intraflagellar Transport 172 Homolog [IFT172 (Chlamydomonas)] that underlie an isolated retinal degeneration and Bardet-Biedl syndrome. Extensive functional analyses of the identified mutations in cell culture, rat retina and in zebrafish demonstrated their hypomorphic or null nature. It has recently been reported that mutations in IFT172 cause a severe ciliopathy syndrome involving skeletal, renal, hepatic and retinal abnormalities (Jeune and Mainzer-Saldino syndromes). Here, we report for the first time that mutations in this gene can also lead to an isolated form of retinal degeneration. The functional data for the mutations can partially explain milder phenotypes; however, the involvement of modifying alleles in the IFT172-associated phenotypes cannot be excluded. These findings expand the spectrum of disease associated with mutations in IFT172 and suggest that mutations in genes originally reported to be associated with syndromic ciliopathies should also be considered in subjects with non-syndromic retinal dystrophy.

Disruption of the basal body protein POC1B results in autosomal-recessive cone-rod dystrophy

Exome sequencing revealed a homozygous missense mutation (c.317C>G [p.Arg106Pro]) in POC1B, encoding POC1 centriolar protein B, in three siblings with autosomal-recessive cone dystrophy or cone-rod dystrophy and compound-heterozygous POC1B mutations (c.199_201del [p.Gln67del] and c.810+1G>T) in an unrelated person with cone-rod dystrophy. Upon overexpression of POC1B in human TERT-immortalized retinal pigment epithelium 1 cells, the encoded wild-type protein localized to the basal body of the primary cilium, whereas this localization was lost for p.Arg106Pro and p.Gln67del variant forms of POC1B. Morpholino-oligonucleotide-induced knockdown of poc1b translation in zebrafish resulted in a dose-dependent small-eye phenotype, impaired optokinetic responses, and decreased length of photoreceptor outer segments. These ocular phenotypes could partially be rescued by wild-type human POC1B mRNA, but not by c.199_201del and c.317C>G mutant human POC1B mRNAs. Yeast two-hybrid screening of a human retinal cDNA library revealed FAM161A as a binary interaction partner of POC1B. This was confirmed in coimmunoprecipitation and colocalization assays, which both showed loss of FAM161A interaction with p.Arg106Pro and p.Gln67del variant forms of POC1B. FAM161A was previously implicated in autosomal-recessive retinitis pigmentosa and shown to be located at the base of the photoreceptor connecting cilium, where it interacts with several other ciliopathy-associated proteins. Altogether, this study demonstrates that POC1B mutations result in a defect of the photoreceptor sensory cilium and thus affect cone and rod photoreceptors.

From the cytoplasm into the cilium: bon voyage

The primary cilium compartmentalizes a tiny fraction of the cell surface and volume, yet many proteins are highly enriched in this area and so efficient mechanisms are necessary to concentrate them in the ciliary compartment. Here we review mechanisms that are thought to deliver protein cargo to the base of cilia and are likely to interact with ciliary gating mechanisms. Given the immense variety of ciliary cytosolic and transmembrane proteins, it is almost certain that multiple, albeit frequently interconnected, pathways mediate this process. It is also clear that none of these pathways is fully understood at the present time. Mechanisms that are discussed below facilitate ciliary localization of structural and signaling molecules, which include receptors, G-proteins, ion channels, and enzymes. These mechanisms form a basis for every aspect of cilia function in early embryonic patterning, organ morphogenesis, sensory perception and elsewhere.

Unexpected CEP290 mRNA splicing in a humanized knock-in mouse model for Leber congenital amaurosis

Leber congenital amaurosis (LCA) is the most severe form of retinal dystrophy with an onset in the first year of life. The most frequent genetic cause of LCA, accounting for up to 15% of all LCA cases in Europe and North-America, is a mutation (c.2991+1655AG) in intron 26 of CEP290. This mutation generates a cryptic splice donor site resulting in the insertion of an aberrant exon (exon X) containing a premature stop codon to CEP290 mRNA. In order to study the pathophysiology of the intronic CEP290 mutation, we generated two humanized knock-in mouse models each carrying ~6.3 kb of the human CEP290 gene, either with or without the intronic mutation. Transcriptional characterization of these mouse models revealed an unexpected splice pattern of CEP290 mRNA, especially in the retina. In both models, a new cryptic exon (coined exon Y) was identified in ~5 to 12% of all Cep290 transcripts. This exon Y was expressed in all murine tissues analyzed but not detected in human retina or fibroblasts of LCA patients. In addition, exon x that is characteristic of LCA in humans, was expressed at only very low levels in the retina of the LCA mouse model. Western blot and immunohistochemical analyses did not reveal any differences between the two transgenic models and wild-type mice. Together, our results show clear differences in the recognition of splice sites between mice and humans, and emphasize that care is warranted when generating animal models for human genetic diseases caused by splice mutations.

Joubert syndrome and related disorders

Joubert syndrome (JS) is a rare autosomal recessive condition characterized by a peculiar midbrain-hindbrain malformation, known as the molar tooth sign (MTS). The neurological presentation of JS includes hypotonia that evolves into ataxia, developmental delay, abnormal eye movements, and neonatal breathing abnormalities. This picture is often associated with variable multiorgan involvement, mainly of the retina, kidneys, and liver, defining a group of conditions termed Joubert syndrome and related disorders (JSRDs), that share the MTS. To date, 16 causative genes have been identified, all encoding for proteins expressed in the primary cilium or its apparatus. Indeed, JSRD present clinical and genetic overlap with a growing field of disorders due to mutations in ciliary proteins, that are collectively known as "ciliopathies." These include isolated nephronophthisis, Senior-Løken syndrome, Bardet-Biedl syndrome and, in particular, Meckel syndrome, which is allelic at JSRD at seven distinct loci. Significant genotype-phenotype correlates are emerging between specific clinical presentations and mutations in JSRD genes, with relevant implications in terms of molecular diagnosis, clinical follow-up, and management of mutated patients. Moreover, the identification of mutations allows early prenatal diagnosis in couples at risk, while fetal neuroimaging may remain uninformative until the late second trimester of pregnancy.