A missense mutation in CASK causes FG syndrome in an Italian family

Clinical and radiological features of Japanese patients with a severe phenotype due to CASK mutations

Heterozygous loss of function mutations of CASK at Xp11.4 in females cause severe intellectual disability (ID) and microcephaly with pontine and cerebellar hypoplasia (MICPCH). However, the longitudinal clinical and radiological course of affected patients, including patterns of postnatal growth, has not been described. Neurodevelopmental and imaging information was retrospectively accrued for 16 Japanese (15 female and 1 male) patients with ID and MICPCH associated with CASK mutations. All records were analyzed; patient age ranged from 2 to 16 years at the time of the most recent examinations. The growth pattern, neurological development, neurological signs/symptoms, and facial features were similar in the 15 female patients. Their head circumference at birth was within the normal range in about half, and their height and weight were frequently normal. This was followed by early development of severe microcephaly and postnatal growth retardation. The patients acquired head control almost normally between 3 and 6 months, followed by motor delay. More than half of the female patients had epilepsy. Their MRIs showed microcephaly, brainstem, and cerebellar hypoplasia in early infancy, and a normal or large appearing corpus callosum. The male patient showed a more severe clinical phenotype. These uniform clinical and radiological features should facilitate an early diagnosis and be useful for medical care of females with ID and MICPCH associated with CASK mutations.

CASK aberrations in male patients with Ohtahara syndrome and cerebellar hypoplasia

PURPOSE

Ohtahara syndrome (OS) is one of the most severe and earliest forms of epilepsy. STXBP1 and ARX mutations have been reported in patients with OS. In this study, we aimed to identify new genes involved in OS by copy number analysis and whole exome sequencing.

METHODS

Copy number analysis and whole exome sequencing were performed in 34 and 12 patients with OS, respectively. Fluorescence in situ hybridization, quantitative polymerase chain reaction (PCR), and breakpoint-specific and reverse-transcriptase PCR analyses were performed to characterize a deletion. Immunoblotting using lymphoblastoid cells was done to examine expression of CASK protein.

KEY FINDINGS

Genomic microarray analysis revealed a 111-kb deletion involving exon 2 of CASK at Xp11.4 in a male patient. The deletion was inherited from his mother, who was somatic mosaic for the deletion. Sequencing of the mutant transcript expressed in lymphoblastoid cell lines derived from the patient confirmed the deletion of exon 2 in the mutant transcript with a premature stop codon. Whole exome sequencing identified another male patient who was harboring a c.1A>G mutation in CASK, which occurred de novo. Both patients showed severe cerebellar hypoplasia along with other congenital anomalies such as micrognathia, a high arched palate, and finger anomalies. No CASK protein was detected by immunoblotting in lymphoblastoid cells derived from two patients.

SIGNIFICANCE

The detected mutations are highly likely to cause the loss of function of the CASK protein in male individuals. CASK mutations have been reported in patients with intellectual disability with microcephaly and pontocerebellar hypoplasia or congenital nystagmus, and those with FG syndrome. Our data expand the clinical spectrum of CASK mutations to include OS with cerebellar hypoplasia and congenital anomalies at the most severe end.

Role of the MAGUK protein family in synapse formation and function

Synaptic function is crucially dependent on the spatial organization of the presynaptic and postsynaptic apparatuses and the juxtaposition of both membrane compartments. This precise arrangement is achieved by a protein network at the submembrane region of each cell that is built around scaffold proteins. The membrane-associated guanylate kinase (MAGUK) family of proteins is a widely expressed and well-conserved group of proteins that plays an essential role in the formation and regulation of this scaffolding. Here, we review general features of this protein family, focusing on the discs large and calcium/calmodulin-dependent serine protein kinase subfamilies of MAGUKs in the formation, function, and plasticity of synapses.

Spectrum of pontocerebellar hypoplasia in 13 girls and boys with CASK mutations: confirmation of a recognizable phenotype and first description of a male mosaic patient

Background

Pontocerebellar hypoplasia (PCH) is a heterogeneous group of diseases characterized by lack of development and/or early neurodegeneration of cerebellum and brainstem. According to clinical features, seven subtypes of PCH have been described, PCH type 2 related to TSEN54 mutations being the most frequent. PCH is most often autosomal recessive though de novo anomalies in the X-linked gene CASK have recently been identified in patients, mostly females, presenting with intellectual disability, microcephaly and PCH (MICPCH).

Methods

Fourteen patients (12 females and two males; aged 16 months-14 years) presenting with PCH at neuroimaging and with clinical characteristics unsuggestive of PCH1 or PCH2 were included. The CASK gene screening was performed using Array-CGH and sequencing. Clinical and neuroradiological features were collected.

Results

We observed a high frequency of patients with a CASK mutation (13/14). Ten patients (8 girls and 2 boys) had intragenic mutations and three female patients had a Xp11.4 submicroscopic deletion including the CASK gene. All were de novo mutations. Phenotype was variable in severity but highly similar among the 11 girls and was characterized by psychomotor retardation, severe intellectual disability, progressive microcephaly, dystonia, mild dysmorphism, and scoliosis. Other signs were frequently associated, such as growth retardation, ophthalmologic anomalies (glaucoma, megalocornea and optic atrophy), deafness and epilepsy. As expected in an X-linked disease manifesting mainly in females, the boy hemizygous for a splice mutation had a very severe phenotype with nearly no development and refractory epilepsy. We described a mild phenotype in a boy with a mosaic truncating mutation. We found some degree of correlation between severity of the vermis hypoplasia and clinical phenotype.

Conclusion

This study describes a new series of PCH female patients with CASK inactivating mutations and confirms that these patients have a recognizable although variable phenotype consisting of a specific form of pontocerebellar hypoplasia. In addition, we report the second male patient to present with a severe MICPCH phenotype and a de novo CASK mutation and describe for the first time a mildly affected male patient harboring a mosaic mutation. In our reference centre, CASK related PCH is the second most frequent cause of PCH. The identification of a de novo mutation in these patients enables accurate and reassuring genetic counselling.

Comparative analysis of Neph gene expression in mouse and chicken development

Neph proteins are evolutionarily conserved members of the immunoglobulin superfamily of adhesion proteins and regulate morphogenesis and patterning of different tissues. They share a common protein structure consisting of extracellular immunoglobulin-like domains, a transmembrane region, and a carboxyl terminal cytoplasmic tail required for signaling. Neph orthologs have been widely characterized in invertebrates where they mediate such diverse processes as neural development, synaptogenesis, or myoblast fusion. Vertebrate Neph proteins have been described first at the glomerular filtration barrier of the kidney. Recently, there has been accumulating evidence suggesting a function of Neph proteins also outside the kidney. Here we demonstrate that Neph1, Neph2, and Neph3 are expressed differentially in various tissues during ontogenesis in mouse and chicken. Neph1 and Neph2 were found to be amply expressed in the central nervous system while Neph3 expression remained localized to the cerebellum anlage and the spinal cord. Outside the nervous system, Neph mRNAs were also differentially expressed in branchial arches, somites, heart, lung bud, and apical ectodermal ridge. Our findings support the concept that vertebrate Neph proteins, similarly to their Drosophila and C. elegans orthologs, provide guidance cues for cell recognition and tissue patterning in various organs which may open interesting perspectives for future research on Neph1-3 controlled morphogenesis.

Liprin-mediated large signaling complex organization revealed by the liprin-α/CASK and liprin-α/liprin-β complex structures

Liprins are highly conserved scaffold proteins that regulate cell adhesion, cell migration, and synapse development by binding to diverse target proteins. The molecular basis governing liprin/target interactions is poorly understood. The liprin-α2/CASK complex structure solved here reveals that the three SAM domains of liprin-α form an integrated supramodule that binds to the CASK kinase-like domain. As supported by biochemical and cellular studies, the interaction between liprin-α and CASK is unique to vertebrates, implying that the liprin-α/CASK interaction is likely to regulate higher-order brain functions in mammals. Consistently, we demonstrate that three recently identified X-linked mental retardation mutants of CASK are defective in binding to liprin-α. We also solved the liprin-α/liprin-β SAM domain complex structure, which uncovers the mechanism underlying liprin heterodimerizaion. Finally, formation of the CASK/liprin-α/liprin-β ternary complex suggests that liprins can mediate assembly of target proteins into large protein complexes capable of regulating numerous cellular activities.

X-linked disorders with cerebellar dysgenesis

X-linked disorders with cerebellar dysgenesis (XLCD) are a genetically heterogeneous and clinically variable group of disorders in which the hallmark is a cerebellar defect (hypoplasia, atrophy or dysplasia) visible on brain imaging, caused by gene mutations or genomic imbalances on the X-chromosome. The neurological features of XLCD include hypotonia, developmental delay, intellectual disability, ataxia and/or other cerebellar signs. Normal cognitive development has also been reported. Cerebellar dysgenesis may be isolated or associated with other brain malformations or multiorgan involvement. There are at least 15 genes on the X-chromosome that have been constantly or occasionally associated with a pathological cerebellar phenotype. 8 XLCD loci have been mapped and several families with X-linked inheritance have been reported. Recently, two recurrent duplication syndromes in Xq28 have been associated with cerebellar hypoplasia. Given the report of several forms of XLCD and the excess of males with ataxia, this group of conditions is probably underestimated and families of patients with neuroradiological and clinical evidence of a cerebellar disorder should be counseled for high risk of X-linked inheritance.

Targeted deletion of CASK-interacting nucleosome assembly protein causes higher locomotor and exploratory activities

CASK-interacting nucleosome assembly protein (CINAP) has been shown to interact with the calcium/calmodulin-dependent serine kinase (CASK) and the T-box transcription factor T-brain-1 (Tbr1) thus modulating the expression of N-methyl-D-aspartic acid receptor subunit 2b (NMDAR2b) in cultured hippocampal neurons. To explore the physiological significance of CINAP in vivo, CINAP knockout mice were generated and subjected to biochemical, anatomical, and behavioral analyses. Unexpectedly, CINAP deletion did not impact NMDAR2b expression, and these CINAP knockout mice were consistently comparable to wild-type littermates in terms of immediate memory (assessed with the Y maze) and associative memory (evaluated by conditioned taste aversion and contextual and auditory fear conditioning). Although CINAP deletion did not obviously influence learning and memory behaviors, CINAP knockout mice exhibited higher locomotor and exploratory activities. Compared with wild-type littermates, the horizontal and vertical movements of the CINAP knockout mice were higher in a novel environment; in home cages, rearing, sniffing, and jumping also occurred more frequently in CINAP knockout mice. These observations suggest that although CINAP deletion in mice does not influence learning and memory behaviors, CINAP is required for restriction of locomotor and exploratory activities.

A novel mutation in MED12 causes FG syndrome (Opitz-Kaveggia syndrome)

Opitz-Kaveggia syndrome is a rare X-linked multiple congenital anomalies and intellectual disability disorder caused by the recurrent p.R961W mutation in the MED12 gene. Twenty-three affected males from 10 families with this mutation in the MED12 gene have been described so far. Here we report on a new family with three affected cousins, in which we identified a novel MED12 mutation (p.G958E). This is the first demonstration that other mutations in this gene can also lead to Opitz-Kaveggia syndrome. The clinical phenotype of these three new cases is reviewed in detail and compared with the previous reported cases.

Natural history of Christianson syndrome

Christianson syndrome is an X-linked mental retardation syndrome characterized by microcephaly, impaired ocular movement, severe global developmental delay, hypotonia which progresses to spasticity, and early onset seizures of variable types. Gilfillan et al.2008] reported mutations in SLC9A6, the gene encoding the sodium/hydrogen exchanger NHE6, in the family first reported and in three others. They also noted the clinical similarities to Angelman syndrome and found cerebellar atrophy on MRI and elevated glutamate/glutamine in the basal ganglia on MRS. Here we report on nonsense mutations in two additional families. The natural history is detailed in childhood and adult life, the similarities to Angelman syndrome confirmed, and the MRI/MRS findings documented in three affected boys.

The FG syndrome from a pathological perspective

We report on a case of FG syndrome in an almost 6-year-old boy, diagnosed post-mortem. The description of the intellectual and behavior phenotype provided by the mother, together with the evidence gathered at autopsy, were sufficient to reach a clinical diagnosis. The mother had mild manifestations, including a symptomatic tethered cord, which established her as a carrier of the putative mutation causing the syndrome in the son. The propositus' phenotype did not suggest involvement of the MED12 gene.

Bcl11A/CTIP1 mediates the effect of the glutamate receptor on axon branching and dendrite outgrowth

While neuronal activity regulates neurite outgrowth and branching, the details of the underlying mechanisms are still largely unclear. This study investigated the effect of the glutamate receptors on neuritogenesis using cultured hippocampal neurons. At 3-4 days in vitro, NMDA treatment promoted axon branching but not primary axon extension. In contrast, blockade of the NMDA receptor (NMDAR) by AP5 treatment enhanced primary axon extension. NMDAR activation also increased dendrite number and total dendrite length. These results suggest that NMDAR controls axon and dendrite morphogenesis. A previous study demonstrated that knockdown of the zinc finger transcription factor B cell lymphoma 11A-long (Bcl11A-L) reduces deleted in colorectal cancer (DCC) and microtubule-associated protein (MAP) 1b expression, thereby promoting axon branching. Here, glutamate stimulation down-regulated the levels of the Bcl11A-L, DCC, MAP1b, and MAP2c proteins. Over-expression of either Bcl11A-L or DCC countered the effect of NMDA or glutamate on axon branching and dendrite outgrowth, indicating that the Bcl11A-L/DCC pathway is an important downstream effector of glutamate receptors in neurite arborization. Because knockdown of Bcl11A-L did not down-regulate MAP2c, our results suggest that glutamate receptors also use a Bcl11A-L-independent pathway to control dendrite outgrowth. To summarize, this study reveals novel pathways downstream of glutamate receptors that regulate axon and dendrite arborization.

FG syndrome, an X-linked multiple congenital anomaly syndrome: the clinical phenotype and an algorithm for diagnostic testing

FG syndrome is a rare X-linked multiple congenital anomaly-cognitive impairment disorder caused by the p.R961W mutation in the MED12 gene. We identified all known patients with this mutation to delineate their clinical phenotype and devise a clinical algorithm to facilitate molecular diagnosis. We ascertained 23 males with the p.R961W mutation in MED12 from 9 previously reported FG syndrome families and 1 new family. Six patients are reviewed in detail. These 23 patients were compared with 48 MED12 mutation-negative patients, who had the clinical diagnosis of FG syndrome. Traits that best discriminated between these two groups were chosen to develop an algorithm with high sensitivity and specificity for the p.R961W MED12 mutation. FG syndrome has a recognizable dysmorphic phenotype with a high incidence of congenital anomalies. A family history of X-linked mental retardation, deceased male infants, and/or multiple fetal losses was documented in all families. The algorithm identifies the p.R961W MED12 mutation-positive group with 100% sensitivity and 90% specificity. The clinical phenotype of FG syndrome defines a recognizable pattern of X-linked multiple congenital anomalies and cognitive impairment. This algorithm can assist the clinician in selecting the patients for testing who are most likely to have the recurrent p.R961W MED12 mutation.

CASK phosphorylation by PKA regulates the protein-protein interactions of CASK and expression of the NMDAR2b gene

Calcium/calmodulin-dependent serine kinase (CASK), a causative gene in X-linked mental retardation, acts as a multi-domain scaffold protein and interacts with more than 20 cellular proteins in different subcellular regions of neurons. It is of interest, therefore, to explore whether post-translational modification regulates CASK's protein-protein interactions. Here, we provide evidence that CASK is phosphorylated by protein kinase A (PKA), identifying residue S562 in the PSD-95-Dlg-ZO-1 domain and residue T724 in the guanylate kinase domain as PKA sites by an in vitro PKA kinase reaction and site-directed mutagenesis. Although the role of S562 phosphorylation is not clear, T724 phosphorylation up-regulates the interaction between CASK and T-box transcription factor T-brain-1 (Tbr-1). NMDAR2b, a downstream target of the CASK-Tbr-1 complex, was then used to explore the significance of CASK phosphorylation by PKA. In cultured cortical neurons, the PKA pathway stimulates both the protein expression and the promoter activity of NMDAR2b. Deletion of the Tbr-1-binding sites greatly reduces the 3'-5'-cyclic AMP responsiveness of the NMDAR2b promoter, and the CASK T724A mutation does not promote the 3'-5'-cyclic AMP responsiveness of NMDAR2b. In conclusion, our data provide evidence that PKA phosphorylates CASK, regulates the nuclear function of CASK, and consequently modulates NMDAR2b expression.

CASK mutations are frequent in males and cause X-linked nystagmus and variable XLMR phenotypes

Mutations of the calcium/calmodulin-dependent serine protein kinase (CASK) gene have recently been associated with X-linked mental retardation (XLMR) with microcephaly, optic atrophy and brainstem and cerebellar hypoplasia, as well as with an X-linked syndrome having some FG-like features. Our group has recently identified four male probands from 358 probable XLMR families with missense mutations (p.Y268H, p.P396S, p.D710G and p.W919R) in the CASK gene. Congenital nystagmus, a rare and striking feature, was present in two of these families. We screened a further 45 probands with either nystagmus or microcephaly and mental retardation (MR), and identified two further mutations, a missense mutation (p.Y728C) and a splice mutation (c.2521-2A>T) in two small families with nystagmus and MR. Detailed clinical examinations of all six families, including an ophthalmological review in four families, were undertaken to further characterise the phenotype. We report on the clinical features of 24 individuals, mostly male, from six families with CASK mutations. The phenotype was variable, ranging from non-syndromic mild MR to severe MR associated with microcephaly and dysmorphic facial features. Carrier females were variably affected. Congenital nystagmus was found in members of four of the families. Our findings reinforce the CASK gene as a relatively frequent cause of XLMR in females and males. We further define the phenotypic spectrum and demonstrate that affected males with missense mutations or in-frame deletions in CASK are frequently associated with congenital nystagmus and XLMR, a striking feature not previously reported.

Calcium/calmodulin-dependent serine protein kinase and mental retardation

Calcium/calmodulin-dependent serine protein kinase (CASK) belongs to the membrane-associated guanylate kinase protein family. The members of this protein family function as multiple domain adaptor proteins originally identified at cell junctions and synapses. Insertional mutations or targeted disruption of the CASK gene in mice results in neonatal lethality, indicating an important role for CASK in development. Recently, several reports have also indicated that mutations in the human CASK gene result in X-linked malformations of the brain and mental retardation. At the molecular level, many studies indicate that CASK is critical for synapse formation at both presynaptic and postsynaptic junctions, and in the regulation of gene expression. The known molecular functions of CASK explain, at least partially, mental retardation and brain developmental defects in patients. In this review, recent findings about CASK are summarized and discussed.

Post-translational modification of CASK leads to its proteasome-dependent degradation

CASK is a member of the membrane-associated guanylate kinase family. In mammals it is an essential protein, as CASK knockout mice die after birth and its deletion in humans has developmental consequences. CASK plays a role in the transcription of genes required for forebrain development, and in the nervous systems of Drosophila and C. elegans, it participates in receptor localization at the plasma membrane. This role in organizing supramolecular protein complexes to appropriate subcellular regions is shared in mammals and is regulated by phosphorylation. CASK is a kinase and regulator of cell proliferation and adhesion, which adds to an expanding list of roles. In this study we report for the first time that CASK is degraded in a characteristic fashion in mammalian cells. We found that CASK is a long-lived protein despite the fact that it contains three putative PEST sequences. Finally, we provide detailed evidence that CASK degradation is mediated through a ubiquitin-proteasome pathway and this is phosphorylation-dependent. Together, these results provide evidence that post-translational modifications to CASK are major regulatory steps leading to its proteasomal degradation. This regulation not only has important implications on how CASK participates in its many disparate roles, but highlights how altering this regulation may contribute to the pathogenesis of human disease.