A rare mutant of OFD1 gene responsible for Joubert syndrome with significant phenotype variation

The genetic basis of hydrocephalus: genes, pathways, mechanisms, and global impact

Hydrocephalus (HC) is a heterogenous disease characterized by alterations in cerebrospinal fluid (CSF) dynamics that may cause increased intracranial pressure. HC is a component of a wide array of genetic syndromes as well as a secondary consequence of brain injury (intraventricular hemorrhage (IVH), infection, etc.) that can present across the age spectrum, highlighting the phenotypic heterogeneity of the disease. Surgical treatments include ventricular shunting and endoscopic third ventriculostomy with or without choroid plexus cauterization, both of which are prone to failure, and no effective pharmacologic treatments for HC have been developed. Thus, there is an urgent need to understand the genetic architecture and molecular pathogenesis of HC. Without this knowledge, the development of preventive, diagnostic, and therapeutic measures is impeded. However, the genetics of HC is extraordinarily complex, based on studies of varying size, scope, and rigor. This review serves to provide a comprehensive overview of genes, pathways, mechanisms, and global impact of genetics contributing to all etiologies of HC in humans.

Novel variants identified in five Chinese families with Joubert Syndrome: a case report

BACKGROUND

Joubert syndrome (JS) is a group of rare ciliopathies, mainly characterized by cerebellar dysplasia representing the "molar tooth sign (MTS)" on neuroimaging, hypotonia, and developmental delay. Having a complicated genotype-phenotype correlation due to its rich genetic heterogeneity, JS is usually combined with other organic defects affecting the retina, kidney, and liver. This report aimed to present new cases and novel variants of JS.

CASE PRESENTATION

Five unrelated patients who were diagnosed with JS, with or without typical clinical characteristics, received integrated examinations, including whole-exome sequencing (WES) and Sanger sequencing. We identified nine pathogenic variants in the TCTN2, CPLANE1, INPP5E, NPHP1, and CC2D2A genes.

CONCLUSION

Four novel pathogenic mutations in the TCTN2, CPLANE1, and INPP5E genes were reported. The findings broadened the genotypic spectrum of JS and contributed to a better understanding of genotype-phenotype correlation.

Brain mosaicism of hedgehog signalling and other cilia genes in hypothalamic hamartoma

Hypothalamic hamartoma (HH) is a rare benign developmental brain lesion commonly associated with a well characterized epilepsy phenotype. Most individuals with HH are non-syndromic without additional developmental anomalies nor a family history of disease. Nonetheless, HH is a feature of Pallister-Hall (PHS) and Oro-Facial-Digital Type VI (OFD VI) syndromes, both characterized by additional developmental anomalies. Initial genetic of analysis HH began with syndromic HH, where germline inherited or de novo variants in GLI3, encoding a central transcription factor in the sonic hedgehog (Shh) signalling pathway, were identified in most individuals with PHS. Following these discoveries in syndromic HH, the hypothesis that post-zygotic mosaicism in related genes may underly non-syndromic HH was tested. We discuss the identified mosaic variants within individuals with non-syndromic HH, review the analytical methodologies and diagnostic yields, and explore understanding of the functional role of the implicated genes with respect to Shh signalling, and cilia development and function. We also outline future challenges in studying non-syndromic HH and suggest potential novel strategies to interrogate brain mosaicism in HH.

A novel non-sense variant in the OFD1 gene caused Joubert syndrome

Background: Joubert syndrome (JBS) is a rare neurodevelopmental disorder associated with progressive renal, liver, and retinal involvement that exhibits heterogeneity in both clinical manifestations and genetic etiology. Therefore, it is difficult to make a definite prenatal diagnosis. Methods: Whole-exome sequencing and Sanger sequencing were performed to screen the causative gene variants in a suspected JBS family. RNA-seq and protein model prediction were performed to clarify the potential pathogenic mechanism. A more comprehensive review of previously reported cases with OFD1 variants is presented and may help to establish a genotype-phenotype. Results: We identified a novel non-sense variant in the OFD1 gene, OFD1 (NM_003611.3): c.2848A>T (p.Lys950Ter). Sanger sequencing confirmed cosegregation among this family. RNA-seq confirmed that partial degradation of mutant transcripts, which was predicted to be caused by the non-sense-mediated mRNA decay (NMD) mechanism, may explain the reduction in the proportion of mutant transcripts. Protein structure prediction of the non-sense variant transcript revealed that this variant may lead to a change in the OFD1 protein structure. Conclusion: The genetic variation spectrum of JBS10 caused by OFD1 was broadened. The novel variants further deepened our insight into the molecular mechanism of the disease.

Proximity Mapping of CCP6 Reveals Its Association with Centrosome Organization and Cilium Assembly

The cytosolic carboxypeptidase 6 (CCP6) catalyzes the deglutamylation of polyglutamate side chains, a post-translational modification that affects proteins such as tubulins or nucleosome assembly proteins. CCP6 is involved in several cell processes, such as spermatogenesis, antiviral activity, embryonic development, and pathologies like renal adenocarcinoma. In the present work, the cellular role of CCP6 has been assessed by BioID, a proximity labeling approach for mapping physiologically relevant protein-protein interactions (PPIs) and bait proximal proteins by mass spectrometry. We used HEK 293 cells stably expressing CCP6-BirA* to identify 37 putative interactors of this enzyme. This list of CCP6 proximal proteins displayed enrichment of proteins associated with the centrosome and centriolar satellites, indicating that CCP6 could be present in the pericentriolar material. In addition, we identified cilium assembly-related proteins as putative interactors of CCP6. In addition, the CCP6 proximal partner list included five proteins associated with the Joubert syndrome, a ciliopathy linked to defects in polyglutamylation. Using the proximity ligation assay (PLA), we show that PCM1, PIBF1, and NudC are true CCP6 physical interactors. Therefore, the BioID methodology confirms the location and possible functional role of CCP6 in centrosomes and centrioles, as well as in the formation and maintenance of primary cilia.

Case report and a brief review: Analysis and challenges of prenatal imaging phenotypes and genotypes in Joubert syndrome

Prenatal imaging phenotypes and genotypes were analyzed in 13 cases prenatally diagnosed with Joubert syndrome (JS), all of which underwent magnetic resonance imaging (MRI), ultrasound, and genetic testing. Prenatal MRI diagnosed 10 cases as JS with a typical molar tooth sign (MTS), while prenatal ultrasound diagnosed or suspiciously diagnosed 11 cases as JS with typical or mild MTS in 10 cases. Mutations in JS-related genes and other prenatal JS imaging phenotypes were identified in 10 cases, including OFD1 in two cases [cerebellar vermis (CV) absence, posterior fossa dilation, ventriculomegaly, polydactyly, malformations of cortical development (MCD), and persistent left superior vena cava], TMEM67 in two cases (CV absence, polydactyly, hyperechoic kidneys or polycystic kidneys, posterior fossa dilation, and ventriculomegaly), CC2D2A in two cases (CV absence, polydactyly, MCD, agenesis of the corpus callosum, encephalocele and hydrocephalus, ventriculomegaly, and posterior fossa dilation), RPGRIP1L in one case (CV absence), TCTN3 in one case (CV absence, polydactyly, MCD, and posterior fossa dilation), CEP290 in one case (CV absence and polycystic kidney), and NPHP1 in one case (CV absence). The prenatal diagnosis of JS presents a number of challenges, including the variants of unknown significance, the lack of functional assessment in prenatal imaging, unclear phenotype-genotype relationships in prenatal evaluation, and the incorrect identification of the JS hallmark, the MTS, in prenatal imaging, especially on ultrasound. Although combined MRI, ultrasound, and exome sequencing could help improve the prenatal diagnosis of JS, there still exist significant challenges.

Sonic Hedgehog Signaling in Cerebellar Development and Cancer

The sonic hedgehog (SHH) pathway regulates the development of the central nervous system in vertebrates. Aberrant regulation of SHH signaling pathways often causes neurodevelopmental diseases and brain tumors. In the cerebellum, SHH secreted by Purkinje cells is a potent mitogen for granule cell progenitors, which are the most abundant cell type in the mature brain. While a reduction in SHH signaling induces cerebellar structural abnormalities, such as hypoplasia in various genetic disorders, the constitutive activation of SHH signaling often induces medulloblastoma (MB), one of the most common pediatric malignant brain tumors. Based on the existing literature on canonical and non-canonical SHH signaling pathways, emerging basic and clinical studies are exploring novel therapeutic approaches for MB by targeting SHH signaling at distinct molecular levels. In this review, we discuss the present consensus on SHH signaling mechanisms, their roles in cerebellar development and tumorigenesis, and the recent advances in clinical trials for MB.

OFD1: One gene, several disorders

The OFD1 protein is necessary for the formation of primary cilia and left–right asymmetry establishment but additional functions have also been ascribed to this multitask protein. When mutated, this protein results in a variety of phenotypes ranging from multiorgan involvement, such as OFD type I (OFDI) and Joubert syndromes (JBS10), and Primary ciliary dyskinesia (PCD), to the engagement of single tissues such as in the case of retinitis pigmentosa (RP23). The inheritance pattern of these condition differs from X‐linked dominant male‐lethal (OFDI) to X‐linked recessive (JBS10, PCD, and RP23). Distinctive biological peculiarities of the protein, which can contribute to explain the extreme clinical variability and the genetic mechanisms underlying the different disorders are discussed. The extensive spectrum of clinical manifestations observed in OFD1‐mutated patients represents a paradigmatic example of the complexity of genetic diseases. The elucidation of the mechanisms underlying this complexity will expand our comprehension of inherited disorders and will improve the clinical management of patients.

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.