Ginevra Zanni

Oligophrenin 1 mutations frequently cause X-linked mental retardation with cerebellar hypoplasia

Background: Mutations of oligophrenin 1, one of the first genes identified in nonspecific X-linked mental retardation (MRX), have been described in patients with moderate to severe cognitive impairment and predominant cerebellar hypoplasia, in the vermis. Objective: To further delineate the phenotypic and mutational spectrum of the syndrome, by screening oligophrenin 1 in two cohorts of male patients with mental retardation (MR) with or without known posterior fossa anomalies. Methods: Clinical examination, cognitive testing, MRI studies, and mutational analysis (denaturing gradient gel electrophoresis and direct sequencing) on blood lymphocytes were performed in 213 unrelated affected individuals: 196 patients classified as MRX and 17 patients with MR and previously detected cerebellar anomalies. Results: Four novel oligophrenin 1 mutations were identified. In the MRX group, two nonsense mutations were detected. In the MR group, two mutations were found: a deletion of exons 16 to 17 and a splice site mutation. All patients shared characteristic clinical, radiologic, and distinctive features with a degree of intrafamilial variability in motor and cognitive deficits. Conclusions: Oligophrenin 1 mutations were found in 12% (2/17) of individuals with mental retardatin and known cerebellar anomalies and in 1% (2/196) of the X-linked mental retardation group.

Mutation of plasma membrane Ca2+ ATPase isoform 3 in a family with X-linked congenital cerebellar ataxia impairs Ca2+ homeostasis

Ca2+ in neurons is vital to processes such as neurotransmission, neurotoxicity, synaptic development, and gene expression. Disruption of Ca2+ homeostasis occurs in brain aging and in neurodegenerative disorders. Membrane transporters, among them the calmodulin (CaM)-activated plasma membrane Ca2+ ATPases (PMCAs) that extrude Ca2+ from the cell, play a key role in neuronal Ca2+ homeostasis. Using X-exome sequencing we have identified a missense mutation (G1107D) in the CaM-binding domain of isoform 3 of the PMCAs in a family with X-linked congenital cerebellar ataxia. PMCA3 is highly expressed in the cerebellum, particularly in the presynaptic terminals of parallel fibers–Purkinje neurons. To study the effects of the mutation on Ca2+ extrusion by the pump, model cells (HeLa) were cotransfected with expression plasmids encoding its mutant or wild-type (wt) variants and with the Ca2+-sensing probe aequorin. The mutation reduced the ability of the PMCA3 pump to control the cellular homeostasis of Ca2+. It significantly slowed the return to baseline of the Ca2+ transient induced by an inositol-trisphosphate (InsP3)-linked plasma membrane agonist. It also compromised the ability of the pump to oppose the influx of Ca2+ through the plasma membrane capacitative channels.

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.

BackgroundPontocerebellar 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).MethodsFourteen 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.ResultsWe 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.ConclusionThis 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.

Mutations in the AP1S2 gene encoding the sigma 2 subunit of the adaptor protein 1 complex are associated with syndromic X-linked mental retardation with hydrocephalus and calcifications in basal ganglia

Fried syndrome, first described in 1972, is a rare X-linked mental retardation that has been mapped by linkage to Xp22. Clinical characteristics include mental retardation, mild facial dysmorphism, calcifications of basal ganglia and hydrocephalus. A large four-generation family in which the affected males have striking clinical features of Fried syndrome were investigated for linkage to X-chromosome markers; the results showed that the gene for this condition lies within the interval DXS7109–DXS7593 in Xp22.2. In total, 60 candidate genes located in this region, including AP1S2, which was recently shown to be involved in mental retardation, were screened for mutations. A mutation in the third intron of AP1S2 was found in all affected male subjects in this large French family. The mutation resulted in skipping of exon 3, predicting a protein with three novel amino-acids and with termination at codon 64. In addition, the first known large Scottish family affected by Fried syndrome was reinvestigated, and a new nonsense mutation, p.Gln66X, was found in exon 3. Using CT, both affected patients from the French family who were analysed had marked calcifications of the basal ganglia, as previously observed in the first Scottish family, suggesting that the presence of distinctive basal ganglia calcification is an essential parameter to recognise this syndromic disorder. It may be possible to use this feature to identify families with X-linked mental retardation that should be screened for mutations in AP1S2.

Ataxia, Intellectual Disability, and Ocular Apraxia with Cerebellar Cysts: A New Disease?

Cerebellar cysts are rare findings in pediatric neuroimaging and rather characteristic for dystroglycanopathies and GPR56-related encephalopathy. We aim to report on seven children with cerebellar cysts showing absence of weakness and ruling out mutations within eight dystroglycanopathy genes and GPR56. Data about neurological and ophthalmological features, outcome, and creatine kinase values were collected from clinical histories and follow-up examinations. All MR images were qualitatively evaluated for infra- and supratentorial abnormalities. A SNP 6.0-Array was performed in three children. The POMT1, POMT2, POMGnT1, FKRP, FKTN, LARGE, ISPD, B3GALNT2, and GPR56 genes were screened in all patients by Sanger sequencing. Seven children from five families were studied. Ataxia, intellectual disability, and language impairment were found in all patients, ocular motor apraxia in five, and severe myopia in three. None of the patients had weakness, only three a minimally increased creatine kinase value. Qualitative neuroimaging evaluation showed cerebellar cysts and dysplasia in the cerebellar hemispheres and vermis in all children. Additional findings were an enlarged fourth ventricle in all children, vermian hypoplasia and brain stem morphological abnormalities in five. The SNP array showed no pathogenetic imbalances in all children evaluated. In all patients, no mutations were found in POMT1, POMT2, POMGnT1, FKRP, FKTN, LARGE, ISPD, B3GALNT2, and GPR56. The peculiar combination of the same clinical and neuroimaging findings in our patients highly suggests that this phenotype may represent a novel entity, possibly falling within the spectrum of dystroglycanopathies.

A Novel Mutation in Isoform 3 of the Plasma Membrane Ca2+ Pump Impairs Cellular Ca2+ Homeostasis in a Patient with Cerebellar Ataxia and Laminin Subunit 1α Mutations

Background: Mutations in plasma membrane Ca2+-ATPase (PMCA) isoform 3 and in laminin subunit 1α have previously been linked to ataxic phenotypes. Results: A novel PMCA3 missense mutation co-occurring with a compound heterozygous mutation in laminin subunit 1α impaired cellular Ca2+ homeostasis. Conclusion: The two mutations could work synergistically to generate the disease phenotype. Significance: A digenic mechanism could be responsible for this case of cerebellar ataxia. The particular importance of Ca2+ signaling to neurons demands its precise regulation within their cytoplasm. Isoform 3 of the plasma membrane Ca2+ ATPase (the PMCA3 pump), which is highly expressed in brain and cerebellum, plays an important role in the regulation of neuronal Ca2+. A genetic defect of the PMCA3 pump has been described in one family with X-linked congenital cerebellar ataxia. Here we describe a novel mutation in the ATP2B3 gene in a patient with global developmental delay, generalized hypotonia and cerebellar ataxia. The mutation (a R482H replacement) impairs the Ca2+ ejection function of the pump. It reduces the ability of the pump expressed in model cells to control Ca2+ transients generated by cell stimulation and impairs its Ca2+ extrusion function under conditions of low resting cytosolic Ca2+ as well. In silico analysis of the structural effect of the mutation suggests a reduced stabilization of the portion of the pump surrounding the mutated residue in the Ca2+-bound state. The patient also carries two missense mutations in LAMA1, encoding laminin subunit 1α. On the basis of the family pedigree of the patient, the presence of both PMCA3 and laminin subunit 1α mutations appears to be necessary for the development of the disease. Considering the observed defect in cellular Ca2+ homeostasis and the previous finding that PMCAs act as digenic modulators in Ca2+-linked pathologies, the PMCA3 dysfunction along with LAMA1 mutations could act synergistically to cause the neurological phenotype.

Exome sequencing in a family with intellectual disability, early onset spasticity, and cerebellar atrophy detects a novel mutation in EXOSC3

Whole exome sequencing in two-generational kindred from Bangladesh with early onset spasticity, mild intellectual disability, distal amyotrophy, and cerebellar atrophy transmitted as an autosomal recessive trait identified the following two missense mutations in the EXOSC3 gene: a novel p.V80F mutation and a known p.D132A change previously associated with mild variants of pontocerebellar hypoplasia type 1. This study confirms the involvement of RNA processing proteins in disorders with motor neuron and cerebellar degeneration overlapping with spinocerebellar ataxia 36 and rare forms of hereditary spastic paraplegia with cerebellar features.

In vitro neurogenesis: development and functional implications of iPSC technology

Neurogenesis is the developmental process regulating cell proliferation of neural stem cells, determining their differentiation into glial and neuronal cells, and orchestrating their organization into finely regulated functional networks. Can this complex process be recapitulated in vitro using induced pluripotent stem cell (iPSC) technology? Can neurodevelopmental and neurodegenerative diseases be modeled using iPSCs? What is the potential of iPSC technology in neurobiology? What are the recent advances in the field of neurological diseases? Since the applications of iPSCs in neurobiology are based on the capacity to regulate in vitro differentiation of human iPSCs into different neuronal subtypes and glial cells, and the possibility of obtaining iPSC-derived neurons and glial cells is based on and hindered by our poor understanding of human embryonic development, we reviewed current knowledge on in vitro neural differentiation from a developmental and cellular biology perspective. We highlight the importance to further advance our understanding on the mechanisms controlling in vivo neurogenesis in order to efficiently guide neurogenesis in vitro for cell modeling and therapeutical applications of iPSCs technology.

The use of muscle biopsy in the diagnosis of undefined ataxia with cerebellar atrophy in children

Childhood cerebellar ataxias, and particularly congenital ataxias, are heterogeneous disorders and several remain undefined. We performed a muscle biopsy in patients with congenital ataxia and children with later onset undefined ataxia having neuroimaging evidence of cerebellar atrophy. Significant reduced levels of Coenzyme Q10 (COQ10) were found in the skeletal muscle of 9 out of 34 patients that were consecutively screened. A mutation in the ADCK3/Coq8 gene (R347X) was identified in a female patient with ataxia, seizures and markedly reduced COQ10 levels. In a 2.5-years-old male patient with non syndromic congenital ataxia and autophagic vacuoles in the muscle biopsy we identified a homozygous nonsense mutation R111X mutation in SIL1 gene, leading to early diagnosis of Marinesco-Sjogren syndrome. We think that muscle biopsy is a valuable procedure to improve diagnostic assesement in children with congenital ataxia or other undefined forms of later onset childhood ataxia associated to cerebellar atrophy at MRI.

TBCE Mutations Cause Early-Onset Progressive Encephalopathy with Distal Spinal Muscular Atrophy

Tubulinopathies constitute a family of neurodevelopmental/neurodegenerative disorders caused by mutations in several genes encoding tubulin isoforms. Loss-of-function mutations in TBCE, encoding one of the five tubulin-specific chaperones involved in tubulin folding and polymerization, cause two rare neurodevelopmental syndromes, hypoparathyroidism-retardation-dysmorphism and Kenny-Caffey syndrome. Although a missense mutation in Tbce has been associated with progressive distal motor neuronopathy in the pmn/pmn mice, no similar degenerative phenotype has been recognized in humans. We report on the identification of an early-onset and progressive neurodegenerative encephalopathy with distal spinal muscular atrophy resembling the phenotype of pmn/pmn mice and caused by biallelic TBCE mutations, with the c.464T>A (p.Ile155Asn) change occurring at the heterozygous/homozygous state in six affected subjects from four unrelated families originated from the same geographical area in Southern Italy. Western blot analysis of patient fibroblasts documented a reduced amount of TBCE, suggestive of rapid degradation of the mutant protein, similarly to what was observed in pmn/pmn fibroblasts. The impact of TBCE mutations on microtubule polymerization was determined using biochemical fractionation and analyzing the nucleation and growth of microtubules at the centrosome and extracentrosomal sites after treatment with nocodazole. Primary fibroblasts obtained from affected subjects displayed a reduced level of polymerized α-tubulin, similarly to tail fibroblasts of pmn/pmn mice. Moreover, markedly delayed microtubule re-polymerization and abnormal mitotic spindles with disorganized microtubule arrangement were also documented. Although loss of function of TBCE has been documented to impact multiple developmental processes, the present findings provide evidence that hypomorphic TBCE mutations primarily drive neurodegeneration.