Identification of novel L1CAM mutations using fluorescence-assisted mismatch analysis

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.

Mice Mutated in the Third Fibronectin Domain of L1 Show Enhanced Hippocampal Neuronal Cell Death, Astrogliosis and Alterations in Behavior

Adhesion molecules play major roles in cell proliferation, migration, survival, neurite outgrowth and synapse formation during nervous system development and in adulthood. The neural cell adhesion molecule L1 contributes to these functions during development and in synapse formation and synaptic plasticity after trauma in adulthood. Mutations of L1 in humans result in L1 syndrome, which is associated with mild-to-severe brain malformations and mental disabilities. Furthermore, mutations in the extracellular domain were shown to cause a severe phenotype more often than mutations in the intracellular domain. To explore the outcome of a mutation in the extracellular domain, we generated mice with disruption of the dibasic sequences RK and KR that localize to position 858RKHSKR863 in the third fibronectin type III domain of murine L1. These mice exhibit alterations in exploratory behavior and enhanced marble burying activity. Mutant mice display higher numbers of caspase 3-positive neurons, a reduced number of principle neurons in the hippocampus, and an enhanced number of glial cells. Experiments suggest that disruption of the dibasic sequence in L1 results in subtle impairments in brain structure and functions leading to obsessive-like behavior in males and reduced anxiety in females.

Hydrocephalus due to multiple ependymal malformations is caused by mutations in the MPDZ gene

Congenital hydrocephalus is considered as either acquired due to haemorrhage, infection or neoplasia or as of developmental nature and is divided into two subgroups, communicating and obstructive. Congenital hydrocephalus is either syndromic or non-syndromic, and in the latter no cause is found in more than half of the patients. In patients with isolated hydrocephalus, L1CAM mutations represent the most common aetiology. More recently, a founder mutation has also been reported in the MPDZ gene in foetuses presenting massive hydrocephalus, but the neuropathology remains unknown. We describe here three novel homozygous null mutations in the MPDZ gene in foetuses whose post-mortem examination has revealed a homogeneous phenotype characterized by multiple ependymal malformations along the aqueduct of Sylvius, the third and fourth ventricles as well as the central canal of the medulla, consisting in multifocal rosettes with immature cell accumulation in the vicinity of ependymal lining early detached from the ventricular zone. MPDZ also named MUPP1 is an essential component of tight junctions which are expressed from early brain development in the choroid plexuses and ependyma. Alterations in the formation of tight junctions within the ependyma very likely account for the lesions observed and highlight for the first time that primary multifocal ependymal malformations of the ventricular system is genetically determined in humans. Therefore, MPDZ sequencing should be performed when neuropathological examination reveals multifocal ependymal rosette formation within the aqueduct of Sylvius, of the third and fourth ventricles and of the central canal of the medulla.

Neuropathological review of 138 cases genetically tested for X-linked hydrocephalus: evidence for closely related clinical entities of unknown molecular bases

L1 syndrome results from mutations in the L1CAM gene located at Xq28. It encompasses a wide spectrum of diseases, X-linked hydrocephalus being the most severe phenotype detected in utero, and whose pathophysiology is incompletely understood. The aim of this study was to report detailed neuropathological data from patients with mutations, to delineate the neuropathological criteria required for L1CAM gene screening in foetuses by characterizing the sensitivity, specificity and positive predictive value of the cardinal signs, and to discuss the main differential diagnoses in non-mutated foetuses in order to delineate closely related conditions without L1CAM mutations. Neuropathological data from 138 cases referred to our genetic laboratory for screening of the L1CAM gene were retrospectively reviewed. Fifty-seven cases had deleterious L1CAM mutations. Of these, 100 % had hydrocephalus, 88 % adducted thumbs, 98 % pyramidal tract agenesis/hypoplasia, 90 % stenosis of the aqueduct of Sylvius and 68 % agenesis/hypoplasia of the corpus callosum. Two foetuses had L1CAM mutations of unknown significance. Seventy-nine cases had no L1CAM mutations; these were subdivided into four groups: (1) hydrocephalus sometimes associated with corpus callosum agenesis (44 %); (2) atresia/forking of the aqueduct of Sylvius/rhombencephalosynapsis spectrum (27 %); (3) syndromic hydrocephalus (9 %), and (4) phenocopies with no mutations in the L1CAM gene (20 %) and in whom family history strongly suggested an autosomal recessive mode of transmission. These data underline the existence of closely related clinical entities whose molecular bases are currently unknown. The identification of the causative genes would greatly improve our knowledge of the defective pathways involved in these cerebral malformations.

Detection of L1 CAM mutation in a male child with mental retardation

Recent studies have presented evidence for the involvement of L1CAM gene mutations in various X-linked mental retardation syndromes. The neural cell adhesion molecule, L1CAM is a transmembrane protein belonging to the super family of the immunoglobulins that play a key role in embryonic development of the nervous system and is involved in memory and learning. No studies were carried out from India on L1 CAM gene in X-linked mental retardation syndromes. Hence, an investigation was taken up to delineate the role of L1CAM gene in mental retardation.Two families (Family I and Family II) having only two members affected with mental retardation in each family were studied for mutations in L1CAM gene. In family II, the younger sibling showed deletion involving region between the nucleotide 13,773 (intron 25) and 14,158 (intron 27) region. The mutation what we observed in younger sibling of the family II is a novel mutation which was not hitherto reported in the world literature.

Pathomechanistic characterization of two exonic L1CAM variants located in trans in an obligate carrier of X-linked hydrocephalus

Mutations in the gene encoding the neural cell adhesion molecule L1CAM cause several neurological disorders collectively referred to as L1 syndrome. We report here a family case of X-linked hydrocephalus in which an obligate female carrier has two exonic L1CAM missense mutations in trans substituting amino acids in the first (p.W635C) or second (p.V768I) fibronectin-type III domains. We performed various biochemical and cell biological in vitro assays to evaluate the pathogenicity of these variants. Mutant L1-W635C protein accumulates in the endoplasmic reticulum (ER), is not transported into axons, and fails to promote L1CAM-mediated cell-cell adhesion as well as neurite growth. Immunoprecipitation experiments show that L1-W635C associates with the molecular ER chaperone calnexin and is modified by poly-ubiquitination. The mutant L1-V768I protein localizes at the cell surface, is not retained in the ER, and promotes neurite growth similar to wild-type L1CAM. However, the p.V768I mutation impairs L1CAM-mediated cell-cell adhesion albeit less severe than L1-W635C. These data indicate that p.W635C is a novel loss-of-function L1 syndrome mutation. The p.V768I mutation may represent a non-pathogenic variant or a variant associated with low penetrance. The poly-ubiquitination of L1-W635C and its association with the ER chaperone calnexin provide further insights into the molecular mechanisms underlying defective cell surface trafficking of L1CAM in L1 syndrome.

The intracellular interactions of the L1 family of cell adhesion molecules

The L1 family of CAMs (cell adhesion molecules) has long aroused the interest of researchers, but primarily the extracellular interactions of these proteins have been elucidated. More recently, attention has turned to the intracellular signalling potentiated by transmembrane proteins and the cytoplasmic proteins with which they can interact. The present review brings up to date the current body of published knowledge for the intracellular interactions of L1-CAM family proteins and the potential importance of these interactions for the mechanisms of L1-CAM action.

Molecular mechanisms and neuroimaging criteria for severe L1 syndrome with X-linked hydrocephalus

OBJECT

Mutations in the gene that codes for the human neural cell adhesion molecule L1 (L1CAM), are known to cause a wide variety of anomalies, now understood as phenotypic expressions of L1 syndrome. The correlations between genotype and phenotype, however, are not fully established. The authors report the results of a nationwide investigation of L1CAM gene mutations that was performed to improve the understanding of L1-mediated molecular mechanisms of X-linked hydrocephalus and to establish neurorimaging criteria for this severe form of L1 syndrome.

METHODS

Ninety-six genomic DNA samples from members of 57 families were obtained from the Congenital Hydrocephalus Research Committee. By using polymerase chain reaction and direct DNA sequencing, the authors identified 25 different L1CAM gene mutations, 20 of them novel, in 26 families with X-linked hydrocephalus. All the mutations were L1CAM loss-of-function mutations, and all the patients had severe hydrocephalus and severe mental retardation. In all cases, specific abnormalities were visible on neuroimaging: a rippled ventricular wall after shunt placement, an enlarged quadrigeminal plate, a large massa intermedia, and hypoplasia of the cerebellar vermis (anterior or total). The patients also had adducted thumbs, spastic paraplegia, and hypoplasia of the corpus callosum, which are characteristic of L1 syndrome.

CONCLUSIONS

The L1CAM loss-of-function mutations cause a severe form of L1 syndrome, unlike the milder form produced by mutations in the L1CAM cytoplasmic domain. We also identified neurorimaging criteria for this severe form of L1 syndrome. These criteria can be used to predict loss-of-function mutations in patients with X-linked hydrocephalus and to help in diagnosing this syndrome.

Genetic test indications and interpretations in patients with hereditary angioedema

Patients with hereditary angioedema (HAE) present with recurrent, circumscribed, and self-limiting episodes of tissue or mucous membrane swelling caused by C1-inhibitor (CI-INH) deficiency. The estimated frequency of HAE is 1:50,000 persons. Distinguishing HAE from acquired angioedema (AAE) facilitates therapeutic interventions and family planning or testing. Patients with HAE benefit from treatment with attenuated androgen, antifibrinolytic agents, and C1-INH concentrate replacement during acute attacks. HAE is currently recognized as a genetic disorder with autosomal dominant transmission. Other forms of inherited angioedema that are not associated with genetic mutations have also been identified. Readily available tests are complement studies, including C4 and C1-esterase inhibitor, both antigenic and functional C1-INH. These are the most commonly used tests in the diagnosis of HAE. Analysis of C1q can help differentiate between HAE and AAE caused by C1-INH deficiency. Genetic tests would be particularly helpful in patients with no family history of angioedema, which occurs in about half of affected patients, and in patients whose C1q level is borderline and does not differentiate between HAE and AAE. Measuring autoantibodies against C1-INH also would be helpful, but the test is available in research laboratories only. Simple complement determinations are appropriate for screening and diagnosis of the disorder.

Radioresistance in carcinoma of the breast

Radiotherapy plays an important role in the management of breast cancer. Whilst its role in achieving local control following surgery in patients with early stage cancer is well established, there is still unclear evidence to explain the factors governing radioresistance in patients who develop recurrences both in the breast and axilla. Radiotherapy induces damage to the DNA. Various cell cycle damage check points and DNA damage repair pathways have been demonstrated. Ataxia telangiectasia mutant (ATM) kinase, which is a member of phosphatidylinositol-3 kinase (PI-3K) family appears to play a central role in DNA damage check point pathways. Over-expression of Insulin like growth factor-I receptor (IGF-IR), Human Epidermal Growth factor receptors (HERS), Vascular Endothelial growth factor (VEGF) on the cell surface and increased concentration of Epidermal Growth factor in the extracellular fluid have been associated with radioresistance. Specific genes such as p53, BRCA, Bcl-2 and chromosomal characteristics like telomere lengths have also been identified as playing significant roles in radiation responsiveness of a cell. This article reviews the current data on general principles of radiotherapy, the cellular mechanisms that operate in response to radiation damage and various molecular markers, intranuclear and extranuclear which have been demonstrated to influence radiation sensitivity in breast cancer cells.

Congenital idiopathic intestinal pseudo-obstruction and hydrocephalus with stenosis of the aqueduct of sylvius

We present the first report of an association between hydrocephalus with stenosis of the aqueduct of Sylvius (HSAS) and a specific form of congenital idiopathic intestinal pseudo-obstruction (CIIP) in an infant. Diagnosis of HSAS was suspected during the neonatal period because of a severely dilated ventricular system associated with bilateral adducted thumbs, and was confirmed by demonstration of a mutation in the gene encoding L1 cell adhesion molecule (L1CAM). L1CAM mutations cause a variable clinical spectrum. This gene is located at Xq28 and encodes a transmembrane glycoprotein involved in neurite outgrowth and neuronal migration. Hirschprung disease has been reported to involve an L1CAM mutation that manifests as a quantitative defect in the migration of neural crest cells in distal segments of the gut. We report an association that suggests that alterations of L1CAM may cause another type of intestinal pseudo-obstruction distension with a qualitative defect in differentiated Cajal's cells in the anterior part of the gut. This observation suggests that L1CAM has a role in the developmental regulation of multiple systems. Further clinical descriptions of gastroenterological and neuropathological data are required to extend our understanding of the mechanisms underlying L1CAM functions.

Prenatal diagnosis of a rhodopsin mutation using chemical cleavage of the mismatch

OBJECTIVE

Mutations of the rhodopsin gene are responsible for autosomal dominant or recessive retinitis pigmentosa (RP). The present study reports the first prenatal diagnosis performed on chorionic villi biopsy of a pregnant woman affected by a severe form of autosomal dominant transmitted RP, due to the Arg135Trp substitution.

METHODS

The rhodopsin gene was analysed by automated direct sequencing and, for the first time, by fluorescence-assisted mismatch analysis (FAMA). The latter is an inexpensive, rapid and particularly sensitive method, based on the chemical cleavage of the mismatch in heteroduplex DNA molecules marked with strand-specific fluorophores.

RESULTS

FAMA is a feasible procedure for prenatal molecular diagnosis of rhodopsin mutations. The redundancy of signals obtained by FAMA and its sensitivity make it suitable for identifying exactly the position of the mutation and the nucleotide substitution.

CONCLUSIONS

An association is proposed between FAMA and automated direct sequencing procedures, in order to achieve optimal results in terms of reliability for prenatal diagnosis of rhodopsin mutations.

Genetic and clinical aspects of X-linked hydrocephalus (L1 disease): Mutations in the L1CAM gene

L1 disease is a group of overlapping clinical phenotypes including X-linked hydrocephalus, MASA syndrome, spastic paraparesis type 1, and X-linked agenesis of corpus callosum. The patients are characterized by hydrocephalus, agenesis or hypoplasia of corpus callosum and corticospinal tracts, mental retardation, spastic paraplegia, and adducted thumbs. The responsible gene, L1CAM, encodes the L1 protein which is a member of the immunoglobulin superfamily of neuronal cell adhesion molecules. The L1 protein is expressed in neurons and Schwann cells and seems to be essential for nervous system development and function. The patients' gene mutations are distributed over the functional protein domains. The exact mechanisms by which these mutations cause a loss of L1 protein function are unknown. There appears to be a relationship between the patients' clinical phenotype and the genotype. Missense mutations in extracellular domains or mutations in cytoplasmic regions cause milder phenotypes than those leading to truncation in extracellular domains or to non-detectable L1 protein. Diagnosis of patients and carriers, including prenatal testing, is based on the characteristic clinical picture and DNA mutation analyses. At present, there is no therapy for the prevention or cure of patients' neurological disabilities.

Prenatal diagnosis of hydrocephalus-stenosis of the aqueduct of Sylvius by ultrasound in the first trimester of pregnancy. Report of two cases

Hydrocephalus-stenosis of the acqueduct of Sylvius sequence (HSAS) is characterized by hydrocephalus, macrocephaly, adducted thumbs, spasticity, agenesis of the corpus callosum and mental retardation. X-linked hydrocephalus is known to be due to mutations in the gene coding for the neural cell adhesion molecule L1 (L1-CAM) and diagnosis is made by identification of a mutation in the L1-CAM gene. Prenatal diagnosis of HSAS is usually suggested on ultrasound examination showing hydrocephalus in a male fetus associated with bilateral adducted thumbs. Mutation screening of the L1-CAM gene is indicated when neuropathological examination shows hypoplasia of the corticospinal tract associated with aqueductal stenosis. We report here two cases of HSAS diagnosed within the same family by ultrasound examination in the first trimester of pregnancy when bilateral adducted thumbs were the only early ultrasound marker.

Cytoplasmic domain mutations of the L1 cell adhesion molecule reduce L1-ankyrin interactions

The neural adhesion molecule L1 mediates the axon outgrowth, adhesion, and fasciculation that are necessary for proper development of synaptic connections. L1 gene mutations are present in humans with the X-linked mental retardation syndrome CRASH (corpus callosum hypoplasia, retardation, aphasia, spastic paraplegia, hydrocephalus). Three missense mutations associated with CRASH syndrome reside in the cytoplasmic domain of L1, which contains a highly conserved binding region for the cytoskeletal protein ankyrin. In a cellular ankyrin recruitment assay that uses transfected human embryonic kidney (HEK) 293 cells, two of the pathologic mutations located within the conserved SFIGQY sequence (S1224L and Y1229H) strikingly reduced the ability of L1 to recruit 270 kDa ankyrinG protein that was tagged with green fluorescent protein (ankyrin-GFP) to the plasma membrane. In contrast, the L1 missense mutation S1194L and an L1 isoform lacking the neuron-specific sequence RSLE in the cytoplasmic domain were as effective as RSLE-containing neuronal L1 in the recruitment of ankyrin-GFP. Ankyrin binding by L1 was independent of cell-cell interactions. Receptor-mediated endocytosis of L1 regulates intracellular signal transduction, which is necessary for neurite outgrowth. In rat B35 neuroblastoma cell lines stably expressing L1 missense mutants, antibody-induced endocytosis was unaffected by S1224L or S1194L mutations but appeared to be enhanced by the Y1229H mutation. These results suggested a critical role for tyrosine residue 1229 in the regulation of L1 endocytosis. In conclusion, specific mutations within key residues of the cytoplasmic domain of L1 (Ser(1224), Tyr(1229)) destabilize normal L1-ankyrin interactions and may influence L1 endocytosis to contribute to the mechanism of neuronal dysfunction in human X-linked mental retardation.

Rapid detection by fluorescent multiplex PCR of exon deletions and duplications in the C1 inhibitor gene of hereditary angioedema patients

Hereditary angioedema (HAE) is due to a variety of defects in the C1 inhibitor gene (C1NH gene), including approximately 20% of partial deletions/duplications whose boundaries are usually within Alu repeats. To ensure complete molecular characterization of C1 inhibitor deficiencies a fluorescent multiplex assay was constructed to amplify simultaneously five exons of C1NH and an exon of the BRCA1 gene. PCR protocols were optimized for these amplicons (size range between 300 and 700 bp). Forward and reverse chimeric primers that carry strand-specific 5' tags of 16 nucleotides were used to ensure similar levels of PCR products for each amplicon in the multiplex. Data were analyzed by superposing fluorescent profiles of test and control DNA and by visually comparing the normalized peak levels of corresponding amplicons, rather than by calculating the ratios of peak areas. Tests on a collection of known defects, including five different Alu-mediated deletions and a partial duplication have validated this approach. In a study of 19 sporadic cases of HAE, of which four had failed to reveal mutations upon screening all exons by fluorescent chemical cleavage, three de novo deletions were diagnosed by using this multiplex PCR approach: a deletion of exon 4, a deletion of exons 5 and 6, and an apparently complete gene deletion. Besides being suitable for the initial DNA screening of the C1NH gene in HAE patients prior to screening for point mutations, this method can be easily adapted to complex genes for the screening of rearrangements.