TMEM218 dysfunction causes ciliopathies, including Joubert and Meckel syndromes

The Meckel-Gruber Syndrome Gene, MKS3, Is Mutated in Joubert Syndrome

Joubert syndrome is an autosomal-recessive disorder characterized by cerebellar hypoplasia, hypotonia, developmental delay, abnormal respiratory patterns, and abnormal eye movements. The biochemical and genetic basis of Joubert syndrome is unknown and a specific chromosomal locus for this disorder has not been identified. Review of this disorder and related syndromes suggests that (1) hypoplasia of the cerebellar vermis in Joubert syndrome is frequently associated with a complex brain stem malformation represented as the "molar tooth sign" on magnetic resonance imaging, (2) the "molar tooth sign" could be present in association with the Dandy-Walker malformation and occipital encephalocele, (3) cerebellar hypoplasia is present in conditions related to Joubert syndrome such as Arima syndrome; Senior-Loken syndrome; cerebellar vermian hypoplasia, oligophrenia, congenital ataxia, coloboma, and hepatic fibrosis syndrome; and juvenile nephronophthisis due to NPH1 mutations, and (4) the brainstem-vermis malformation spectrum is probably caused by at least two and probably several genetic loci. We have ascertained previously a cohort of 50 patients with a putative diagnosis of Joubert syndrome in order to evaluate the presence of associated malformations, and to initiate studies leading to the identification of genes causing Joubert and related syndromes. Among the associated malformations found in patients ascertained as having Joubert syndrome, 8% of patients had polydactyly, 4% had ocular colobomas, 2% had renal cysts, and 2% had soft-tissue tumors of the tongue. The WNT1 gene has been tested as a candidate gene for Joubert syndrome based on its expression in the developing cerebellum and an associated mutation in the swaying mouse. A search for mutations in WNT1 in a series of patients with Joubert syndrome did not detect mutations at this locus. This analysis suggested that mutations in WNT1 might not have a significant role in Joubert syndrome, and other functional candidate genes related to development of the cerebellum need to be examined. A genome-wide linkage analysis carried out in 10 Joubert syndrome pedigrees did not identify a specific chromosomal locus for this disorder. This observation, along with those from clinical studies, provides further evidence that Joubert and related syndromes are genetically heterogeneous.

Human ciliopathies are hereditary conditions caused by defects of proteins expressed at the primary cilium. Among ciliopathies, Joubert syndrome and related disorders (JSRD), Meckel syndrome (MKS) and nephronophthisis (NPH) present clinical and genetic overlap, being allelic at several loci. One of the most interesting gene is TMEM67, encoding the transmembrane protein meckelin. We performed mutation analysis of TMEM67 in 341 probands, including 265 JSRD representative of all clinical subgroups and 76 MKS fetuses. We identified 33 distinct mutations, of which 20 were novel, in 8/10 (80%) JS with liver involvement (COACH phenotype) and 12/76 (16%) MKS fetuses. No mutations were found in other JSRD subtypes, confirming the strong association between TMEM67 mutations and liver involvement. Literature review of all published TMEM67 mutated cases was performed to delineate genotype-phenotype correlates. In particular, comparison of the types of mutations and their distribution along the gene in lethal versus non lethal phenotypes showed in MKS patients a significant enrichment of missense mutations falling in TMEM67 exons 8 to 15, especially when in combination with a truncating mutation. These exons encode for a region of unknown function in the extracellular domain of meckelin.

Joubert syndrome and related disorders (JSRD) are clinically and genetically heterogeneous ciliopathies sharing a peculiar midbrain-hindbrain malformation known as the 'molar tooth sign'. To date, 19 causative genes have been identified, all coding for proteins of the primary cilium. There is clinical and genetic overlap with other ciliopathies, in particular with Meckel syndrome (MKS), that is allelic to JSRD at nine distinct loci. We previously identified the INPP5E gene as causative of JSRD in seven families linked to the JBTS1 locus, yet the phenotypic spectrum and prevalence of INPP5E mutations in JSRD and MKS remain largely unknown. To address this issue, we performed INPP5E mutation analysis in 483 probands, including 408 JSRD patients representative of all clinical subgroups and 75 MKS fetuses. We identified 12 different mutations in 17 probands from 11 JSRD families, with an overall 2.7% mutation frequency among JSRD. The most common clinical presentation among mutated families (7/11, 64%) was Joubert syndrome with ocular involvement (either progressive retinopathy and/or colobomas), while the remaining cases had pure JS. Kidney, liver and skeletal involvement were not observed. None of the MKS fetuses carried INPP5E mutations, indicating that the two ciliopathies are not allelic at this locus.

Background:Nephronophthisis (NPHP), a rare recessive cystic kidney disease, is the most frequent genetic cause of chronic renal failure in children and young adults. Mutations in nine genes (NPHP1-9) have been...

Joubert (JBTS) and Meckel-Gruber (MKS) syndromes are recessive neurodevelopmental conditions caused by mutations in proteins that are structural or functional components of the primary cilium. In this review we provide an overview of their clinical diagnosis, management and molecular genetics. Both have variable phenotypes, extreme genetic heterogeneity, and display allelism both with each other and other ciliopathies. Recent advances in genetic technology have significantly improved diagnosis and clinical management of ciliopathy patients, with the delineation of some general genotype-phenotype correlations. We highlight those that are most relevant for clinical practice, including the correlation between TMEM67 mutations and the JBTS variant phenotype of COACH syndrome. The subcellular localization of the known MKS and JBTS proteins is now well-described, and we discuss some of the contemporary ideas about ciliopathy disease pathogenesis. Most JBTS and MKS proteins localize to a discrete ciliary compartment called the transition zone (TZ), and act as structural components of the so-called “ciliary gate” to regulate the ciliary trafficking of cargo proteins or lipids. Cargo proteins include enzymes and transmembrane proteins that mediate intracellular signaling. The disruption of TZ function may contribute to the ciliopathy phenotype by altering the composition of the ciliary membrane or axoneme, with impacts on essential developmental signaling including the Wnt and Shh pathways as well as the regulation of secondary messengers such as inositol-1,4,5-trisphosphate (InsP3) and cAMP. However, challenges remain in the interpretation of the pathogenic potential of genetic variants of unknown significance, and in the elucidation of the molecular mechanisms of phenotypic variability in JBTS and MKS. The further genetic and functional characterization of these conditions is essential to prioritize patients for new targeted therapies.

Disclosure: N. Mojahed: None. S.W. Athul: None. B.S. Ellsworth: None. Pituitary gland deficiency can cause infertility, metabolic dysfunction, and growth insufficiency. Many gene mutations that cause pituitary gland deficiency also cause severe developmental brain defects. During development, sonic hedgehog (SHH)signaling is crucial for the specification and differentiation of pituitary cells, with primary cilia acting as hubs for processing developmental signals. While organellescalled primary cilia play an essential role in SHH signaling, their specific influence on pituitary development remain unknown. Over the last two decades, ciliopathies have been identified as a novel category of pleiotropic diseases, characterized by overlapping phenotypes caused by dysfunction in primary cilia. Using ARL13B, a small GTPase involved in ciliary membrane dynamics and signaling, and pericentrin, a marker of centrioles, as markers of primary cilia, we explored the spatial and temporal patterns of primary cilia during pituitary gland development. Immunohistochemistry results revealed the presence of primary cilia as early as embryonic day (e)10.5. To determine if primary cilia are expressed in distinct anterior pituitarycell types, we performed co-immunohistochemistry. These studies revealed the presence of ARL13B in a subset of somatotropes, lactotropes, thyrotropes, corticotropes, and gonadotropes. These findingshighlight the potential role of primary cilia in facilitating sensory functions that may influence hormonesecretion. To investigate the role of the ciliopathy gene, Mks1, in pituitary gland development, we used the Mks1 gene trap mouse model. Mutations in MKS1 causes ciliopathies such as Bardet-Biedl, Joubert, and Meckel syndromes. The MKS1 gene is located on chromosome 17q22 in humans and encodes the MKS1 protein, which is a key factor in the transition zone of primary cilia. We examined e12.5 and e14.5 wild-type and Mks1-null mouse embryos and found that in null embryos, the pituitary gland was extremely hypoplastic and the ventral diencephalon was dysmorphic. Interestingly expression of critical transcription factors, including LHX3, PITX2, and POU1F1, which are essential for cell lineage specification and pituitary differentiation, was markedly absent in null embryos, indicating a failure of oral ectoderm to adapt apituitary fate. These findings highlight the essential role of MKS1 during pituitary morphogenesis and development. The severe morphological, molecular and cellular defects observed in Mks1-null embryos demonstrate the critical dependence of pituitary development on functional primary cilia. Our studyprovides insights into how ciliopathies, such as Bardet-Biedl and Meckel syndromes, may disrupt endocrine organogenesis through impaired ciliary function. Presentation: Saturday, July 12, 2025

Pathogenic KIAA0586/TALPID3 variants are associated with defects in primary and motile cilia.

TXNDC15 encodes thioredoxin domain-containing protein 15, a protein disulfide isomerase that plays a role in ciliogenesis. Biallelic TXNDC15 variants have been reported in six individuals of Meckel syndrome (MKS) with perinatal lethal phenotypes, but have not been reported in patients with Joubert syndrome (JS). Here, we describe a 1-year-old female patient with compound heterozygous TXNDC15 variants demonstrating cerebellar vermis hypoplasia with the molar tooth sign, mild holoprosencephaly, and cortical abnormalities. She had severe developmental delay and epilepsy. Her clinical features were similar to those of JS, but distinctive forebrain abnormalities were also noted including mild holoprosencephaly and cortical abnormalities, which have been reported in a severe form of ciliopathy. Biallelic TXNDC15 variants manifest as overlapping phenotypes of JS and MKS, including the molar tooth sign, cortical dysgenesis, and mild holoprosencephaly. This report supports the hypothesis that JS and MKS are spectrum ciliopathy disorders with overlapping causative genes and hypomorphic TXNDC15 variants might contribute to JS.

The Primary cilium, a non-motile organelle, was long underestimated but has recently been recognized as a pivotal signaling platform in nervous system development. This review summarizes the structural features, biogenesis, and dynamic regulation of primary cilia, and systematically examines their roles in neural stem cells fate determination, neurogenesis, neuronal migration, axon guidance, and synaptogenesis. By integrating multiple developmental signaling pathways, including Sonic hedgehog, Wnt, Notch, and mTOR, primary cilia orchestrate the precise spatiotemporal patterning of the nervous system. Dysfunction of primary cilia is closely linked to a wide spectrum of neurodevelopmental disorders, ranging from classical ciliopathies such as Meckel-Gruber syndrome, Joubert syndrome, and Bardet-Biedl syndrome, to complex conditions such as autism spectrum disorder, schizophrenia, and bipolar disorder, all of which can be traced to ciliary signaling imbalances. This paper introduces the concept of the continuous spectrum of Ciliogenesis, in which phenotypes from severe structural malformations to subtle functional abnormalities can be attributed to defects in specific ciliary modules. Future interventions, including organoid models, super-resolution imaging, and cilia-targeted therapeutic strategies, hold promise for advancing pathological insights and developing novel treatments for neurodevelopmental disorders.

AimsGoldston syndrome (GS) is a rare disorder characterised by a combination of Dandy-Walker malformation (DWM) with autosomal recessive polycystic kidney disease (ARPKD). Features may also include oligohydramnios or hepatic fibrosis....

Of Cilia and Cysts: Modeling Pancreatic Polycystic Disease

BackgroundThe widespread adoption of high-throughput sequencing technologies by genetic diagnostic laboratories has enabled significant expansion of their testing portfolios. Rare autosomal recessive conditions have been a particular focus of many new services. Here we report a cohort of 26 patients referred for genetic analysis of Joubert (JBTS) and Meckel-Gruber (MKS) syndromes, two clinically and genetically heterogeneous neurodevelopmental conditions that define a phenotypic spectrum, with MKS at the severe end.MethodsExome sequencing was performed for all cases, using Agilent SureSelect v5 reagents and Illumina paired-end sequencing. For two cases medium-coverage (9×) whole genome sequencing was subsequently undertaken.ResultsUsing a standard analysis pipeline for the detection of single nucleotide and small insertion or deletion variants, molecular diagnoses were confirmed in 12 cases (4 %). Seeking to determine whether our cohort harboured pathogenic copy number variants (CNV), in JBTS- or MKS-associated genes, targeted comparative read-depth analysis was performed using FishingCNV. These analyses identified a putative intragenic AHI1 deletion that included three exons spanning at least 3.4 kb and an intergenic MPP4 to TMEM237 deletion that included exons spanning at least 21.5 kb. Whole genome sequencing enabled confirmation of the deletion-containing alleles and precise characterisation of the mutation breakpoints at nucleotide resolution. These data were validated following development of PCR-based assays that could be subsequently used for “cascade” screening and/or prenatal diagnosis.ConclusionsOur investigations expand the AHI1 and TMEM237 mutation spectrum and highlight the importance of performing CNV screening of disease-associated genes. We demonstrate a robust increasingly cost-effective CNV detection workflow that is applicable to all MKS/JBTS referrals.Electronic supplementary materialThe online version of this article (doi:10.1186/s12881-015-0265-z) contains supplementary material, which is available to authorized users.

Introduction: Rapid-onset obesity, hypoventilation, hypothalamic dysfunction and autonomic dysregulation (ROHHAD) is a rare syndrome beginning at 3-6 years of age with approximately 150 cases described. Additional features include eye abnormalities, neurobehavioral dysfunction and paraneoplastic tumors. The etiology of the complex phenotype remains unknown. Methods: This study aims to investigate the genetic landscape of this complex phenotype by whole exome sequencing (WES) and copy number variation (CNV) analysis. We recruited 33 families (27 trios, 1 duo and 5 singletons) with a proband with ROHHAD syndrome (Ize-Ludlow 2007, Pediatrics). WES of 89 individuals was performed at the Center for Mendelian Genomics, Broad Institute. The Illumina platform with a mean coverage of ~100X (> 90% targets 20x) and Infinium Global Screening Array BeadChip 24v1.0 were used. Results: This report includes 28 probands (female = 18, 64%) with rapid onset obesity (100%), hypoventilation (88%), hypothalamic dysfunction (69%), eye disorders (62%) and neurobehavioral abnormalities (76%). Neuroendocrine tumor, ganglioneuroblastoma, was present in 38% (n=13). No unifying causative single gene or CNV was identified, but a number of sequence variants are prioritized. ARNT2, which encodes for a helix-loop-helix transcription factor, plays a role in the development of the hypothalamic-pituitary axis, postnatal brain growth, and visual and renal function. The de novo monoallelic missense variant was found in a 14-year old white girl (BMIz +3.25) with extreme obesity and a neurobehavioral phenotype. OCRL1, a multi-domain protein involved in cytoskeleton-plasma membrane adhesion, endosomal trafficking and in primary cilium assembly. Mutations in this gene have also been known to cause Lowe syndrome. A hemizygous X-linked frameshift variant in a 5-year old white boy with extreme obesity (BMIz +5.48), central hypoventilation neurobehavioral dysfunction and ganglioneuroblastoma. A monoallelic missense variant in NSD1, a transcriptional intermediary factor acting as a histone methyltransferase, was identified in a 8-year old Hispanic girl with severe obesity (BMIz +2.91), neurobehavioral disorder, pituitary and eye dysfunction and ganglioneuroblastoma. NSD1 is known to cause Sotos and Beckwith-Wiedemann. Compound heterozygous variants in KIF7, a key component of the Hedgehog signaling pathway, were identified in a 14-year old white girl with severe obesity (BMIz +3.00), autistic behavior, pituitary dysfunction and central hypoventilation. This gene is known to cause autosomal recessive hydrolethalis and acroscallosal syndromes with mutations also noted in Bardet-Biedl, Meckel and Joubert syndromes. Conclusion: While no unifying genetic cause has been identified in ROHHAD syndrome, it is possible that the phenotype represents a collection of complex genetic syndromes.

Bardet-Biedl syndrome (BBS) belongs to the group of ciliopathies and has an autosomal-recessive inheritance with 22 genes known already. In the child it is associated with retinal dystrophy, obesity, neurological and psychic disabilities, hypogonadism, postaxial polydactyly and renal dysfunction. There are overlapping with other ciliopathies. Prenatal report of BBS is rather scarce. We report the features of two prenatally confirmed cases of BBS. Case 1: On screening US at 22 weeks (G2-P1) in a non-consanguineous patient enlarged, fetal hyperechogenic kidneys were detected. The detailed examination showed a hexadactyly at one foot, with no other malformations. A BBS was suspected and amniocentesis revealed a homozygotic mutation in the BBS1 gene. Both parents were found to be heterozygote carriers. The parents opted for pregnancy termination. Case 2: In the primigravida non-consanguineous 30y old patient first trimester screening revealed a polydactyly of both hands and feet with no additional anomalies. CVS showed a male karyotype. A follow up at 18 weeks showed in addition large hyperechogenic kidneys with no other signs of a ciliopathy. Bardet-Biedl syndrome was suspected and a deletion of exons 14-17 of the BBS-1 was found. Both parents were found to be heterozygote carriers. Patient opted for pregnancy termination. Prenatal diagnosis of BBS remains difficult but these two cases show that the presence of hyperechogenic kidneys and polydactyly only should prompt the careful targeted examination for a BBS. The additional presence of intracerebral, skeletal or other malformations can be the hint for other ciliopathies, including Joubert or Meckel syndrome. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.