The site of a missense mutation in the extracellular Ig or FN domains of L1CAM influences infant mortality and the severity of X linked hydrocephalus

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.

Severe Phenotype in Patients with X-linked Hydrocephalus Caused by a Missense Mutation in L1CAM

OBJECTIVE

The study aimed to show the clinical characteristics and prognosis of the L1 syndrome in patients with L1CAM mutations in the extracellular region.

MATERIALS AND METHODS

Three affected boys and their siblings and parents from a large family were included in this study. Genetic etiology was investigated by whole-exome sequencing in the index patient. The pathogenic variant was detected by whole-exome sequencing and was validated by Sanger sequencing in 3 patients and other family members.

RESULTS

We present 2 brothers and their cousin with prenatal onset hydrocephalus, severe developmental and speech delay, corpus callosum agenesis/hypogenesis, epilepsia, and adducted thumbs. A hemizygous missense mutation NM_024003 (c.A2351G:p.Y784C) on exon 18 of L1CAM gene was found in the 3 patients and their carrier mother. This missense mutation in the conserved region of the second fibronectin type III-like repeats located in the extracellular region of L1CAM resulted in the severe phenotype of X-linked inherited L1 syndrome in the patients.

CONCLUSION

L1 syndrome should be considered in the differential diagnosis of male children with intellectual disability, hydrocephalus, and adducted thumbs. While truncating mutations of L1CAM may cause a more severe phenotype, missense mutations cause milder forms. However, pathogenic missense mutations affecting key amino acid residues lead to severe phenotype likely.

L1CAM whole gene deletion in a child with L1 syndrome

L1 syndrome is a group of overlapping, X-linked disorders caused by mutations in L1CAM. Clinical phenotypes within L1 syndrome include X-linked hydrocephalus with stenosis of the aqueduct of sylvius (HSAS); mental retardation, adducted thumbs, shuffling gait, and aphasia (MASA) syndrome; spastic paraplegia type 1; and agenesis of the corpus callosum. Over 200 mutations in L1CAM have been reported; however, only a few large gene deletions have been observed. We report on a 4-month-old male with a de novo whole gene deletion of L1CAM presenting with congenital hydrocephalus, aqueductal stenosis, and adducted thumbs. Initial failure of L1CAM gene sequencing suggested the possibility of a whole gene deletion of L1CAM. Further investigation through chromosome microarray analysis showed a 62Kb deletion encompassing the first exon of the PDZD4 gene and the entire L1CAM gene. Investigations into genotype-phenotype correlations have suggested that mutations leading to truncated or absent L1 protein cause more severe forms of L1 syndrome. Based on the presentation of the proband and other reported patients with whole gene deletions, we provide further evidence that L1CAM whole gene deletions result in L1 syndrome with a severe phenotype, deletions of PDZD4 do not cause additional manifestations, and that X-linked nephrogenic diabetes insipidus reported in a subset of patients with large L1CAM deletions results from the loss of AVPR2.

Malformations among the X-linked intellectual disability syndromes

Malformations are significant contributions to childhood mortality and disability. Their co-occurrence with intellectual disability may compound the health burden, requiring additional evaluation and management measures. Overall, malformations of greater or lesser severity occur in at least some cases of almost half of the 153 XLID syndromes. Genitourinary abnormalities are most common, but tend to contribute little or no health burden and occur in only a minority of cases of a given XLID syndrome. Some malformations (e.g., lissencephaly, hydranencephaly, long bone deficiency, renal agenesis/dysplasia) are not amenable to medical or surgical intervention; others (e.g., hydrocephaly, facial clefting, cardiac malformations, hypospadias) may be substantially corrected.

Prenatal molecular diagnosis of a severe type of L1 syndrome (X-linked hydrocephalus)

OBJECT

The aim of this study was to evaluate the feasibility of prenatal L1CAM gene testing for X-linked hydrocephalus (XLH).

METHODS

In a nationwide study conducted in Japan between 1999 and 2009, the authors identified 51 different L1CAM gene mutations in 56 families with XLH. Of these 56 families, 9 obligate carriers requested prenatal gene mutation analysis for the fetal L1CAM gene in 14 pregnancies.

RESULTS

In 2004, new clinical guidelines for genetic testing were established by 10 Japanese genetic medicine-related societies. These guidelines stated that the genetic testing of carriers should be done only with their consent and with genetic counseling. Therefore, because females are carriers, since 2004, L1CAM gene analysis has not been performed for female fetuses. The authors report on 7 fetal genetic analyses that were performed at the request of families carrying L1CAM mutations, involving 3 female (prior to 2004) and 4 male fetuses. Of the 7 fetuses, 3 (1 male and 2 female) carried L1CAM mutations. Of these 3, 1 pregnancy (the male fetus) was terminated; in the other cases, the pregnancies continued, and 3 female and 3 male babies without the XLH phenotype were born.

CONCLUSIONS

Prenatal L1CAM gene testing combined with genetic counseling was beneficial for families carrying L1CAM mutations.

Human L1CAM carrying the missense mutations of the fibronectin-like type III domains is localized in the endoplasmic reticulum and degraded by polyubiquitylation

Any mutations in the human neural cell adhesion molecule L1 (hL1CAM) gene might cause various types of serious neurological syndromes in humans, characterized by increased mortality, mental retardation, and various malformations of the nervous system. Such missense mutations often cause severe abnormalities or even fatalities, and the reason for this may be a disruption of the adhesive function of L1CAM resulting from a misdirection of the degradative pathway. Transfection studies using neuroblastoma N2a cells demonstrated that hL1CAM carrying the missense mutations in the fibronectin-like type III (FnIII) domains most likely is located within the endoplasmic reticulum (ER), but it is less well expressed on the cell surface. One mutant, L935P, in the fourth FnIII domain, was chosen from six mutants (K655 and G698 at Fn1, L935P and P941 at Fn4, W1036 and Y1070 at Fn5) in the FnIII domains to study in detail the functions of hL1CAM(200 kDa) , such as the intracellular traffic and degradation, because only a single band at 200 kDa was detected in the hL1CAM(L935P) -transfected cells. hL1CAM(200 kDa) is expressed predominantly in the ER but not on the cell surface. In addition, this missense mutated hL1CAM(200 kDa) is polyubiquitylated at some sites in the extracellular domain and thus becomes degraded by proteasomes via the ER-associated degradation pathway. These observations demonstrate that the missense mutations of hL1CAM in the FnIII domain may cause the resultant pathogenesis because of a loss of expression on the cell surface resulting from misrouting to the degradative pathway.

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.

Expanding the phenotypic spectrum of L1CAM-associated disease

Mutations in the L1CAM gene cause neurological abnormalities of variable severity, including congenital hydrocephalus, agenesis of the corpus callosum, spastic paraplegia, bilaterally adducted thumbs, aphasia, and mental retardation. Inter- and intrafamilial variability is a well-known feature of the L1CAM spectrum, and several patients have a combination of L1CAM mutations and Hirschsprung's disease (HSCR). We report on two siblings with a missense mutation in exon 7 (p.P240L) of the L1CAM gene. In one of the siblings, congenital dislocation of the radial heads and HSCR were present. Neither patient had hydrocephalus, adducted thumbs, or absent speech, but both had a hypoplastic corpus callosum. We suggest that L1CAM mutation testing should be considered in male patients with a positive family history compatible with X-linked inheritance and either the combination of agenesis of the CC and HSCR or the combination of agenesis of the CC and limb abnormalities, including abnormalities other than adducted thumbs.

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.

The C264Y missense mutation in the extracellular domain of L1 impairs protein trafficking in vitro and in vivo

The neural cell adhesion molecule L1, a member of the immunoglobulin superfamily, performs important functions in the developing and adult nervous system and is implicated in neuronal migration and survival, elongation, fasciculation and pathfinding of axons, and synaptic plasticity. This view is in line with the fact that mutations in the L1 gene result in severe neurological syndromes in humans. Patients with missense mutations in the extracellular domain of L1 often develop severe phenotypes. Here, we characterized in vitro and in vivo the missense mutation C264Y, which is located in the extracellular domain of L1 and causes a severe phenotype in humans. Transfection studies in vitro demonstrate that L1 carrying this missense mutation is not expressed at the cell surface but instead is located intracellularly, most likely within the endoplasmic reticulum. Lack of cell surface expression of L1 with a C264Y mutation was confirmed in a transgenic mouse line expressing the C264Y mutation under the control of the L1 promoter in an L1-deficient background. Analysis of these transgenic mice indicates that they represent functional null mutants, phenotypically indistinguishable from L1-deficient mice. These observations corroborate the view that impaired cell surface expression of mutated variants of L1 is a potential explanation for the high number of severe pathogenic mutations identified within the human L1 gene.

The C264Y missense mutation in the extracellular domain of L1 impairs protein trafficking in vitro and in vivo

The neural cell adhesion molecule L1, a member of the immunoglobulin superfamily, performs important functions in the developing and adult nervous system and is implicated in neuronal migration and survival, elongation, fasciculation and pathfinding of axons, and synaptic plasticity. This view is in line with the fact that mutations in the L1 gene result in severe neurological syndromes in humans. Patients with missense mutations in the extracellular domain of L1 often develop severe phenotypes. Here, we characterized in vitro and in vivo the missense mutation C264Y, which is located in the extracellular domain of L1 and causes a severe phenotype in humans. Transfection studies in vitro demonstrate that L1 carrying this missense mutation is not expressed at the cell surface but instead is located intracellularly, most likely within the endoplasmic reticulum. Lack of cell surface expression of L1 with a C264Y mutation was confirmed in a transgenic mouse line expressing the C264Y mutation under the control of the L1 promoter in an L1-deficient background. Analysis of these transgenic mice indicates that they represent functional null mutants, phenotypically indistinguishable from L1-deficient mice. These observations corroborate the view that impaired cell surface expression of mutated variants of L1 is a potential explanation for the high number of severe pathogenic mutations identified within the human L1 gene.

Hydrocephalus and intestinal aganglionosis: is L1CAM a modifier gene in Hirschsprung disease?

Congenital hydrocephalus associated with aqueductal stenosis and/or agenesis of the corpus callosum has been described in newborn males with mutations in L1CAM, a gene that encodes a neural cell adhesion molecule. These males usually have severe mental retardation and may have spastic paraplegia and adducted thumbs. In contrast, Hirschsprung disease, or absence of ganglion cells in the distal gut, has rarely been described in such individuals. We report a male infant who had severe hydrocephalus identified in the prenatal period with evidence of aqueductal stenosis and adducted thumbs at birth. He developed chronic constipation, and rectal biopsy confirmed the diagnosis of Hirschsprung disease. Molecular testing of the L1CAM gene revealed a G2254A mutation, resulting in a V752M amino acid substitution. A common polymorphism in RET, but no mutation, was identified. Our patient represents the third example of coincident hydrocephalus and Hirschsprung disease in an individual with an identified L1CAM mutation. We hypothesize that L1CAM-mediated cell adhesion may be important for the ability of ganglion cell precursors to populate the gut, and that L1CAM may modify the effects of a Hirschsprung disease-associated gene to cause intestinal aganglionosis.

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.

Splitting and lumping in the nosology of XLMR

Although it is assumed that genes that influence cognitive function are ubiquitous in the human genome, to date, more such genes have been found on the X chromosome than on any other comparable segment of the autosomes. This is in large measure because of the power of hemizygosity in exposing mutations of X-linked genes in males. Clinical manifestations, mapping of gene loci by linkage analysis or chromosome rearrangements, and gene identification by positional cloning or mutational analysis of candidate genes have permitted extensive lumping and splitting within the large and heterogeneous category of X-linked mental retardation (XLMR). Approximately 130 XLMR syndromes have been identified, 25 gene loci have been mapped and cloned, and 55 other loci have been mapped but not cloned. Well-recognized syndromes (e.g., Fragile X and Coffin-Lowry syndromes) and syndromes represented by only a single family (e.g., Arena and monoamine oxidase-A syndromes) are among these more or less well-defined entities. In addition, more than 75 families with nonsyndromal XLMR have been regionally mapped and 7 causative genes have been identified.

Stereological analysis of regional brain volumes and neuron numbers in rats displaying a spontaneous hydrocephalic condition

Stereological methods were employed to investigate a novel spontaneously occurring brain mutation in an inbred colony of Wistar rats. These mutants displayed changes (enlarged cerebral ventricles and malformed hippocampi) similar to those seen in H-Tx hydrocephalic rats. Mutant and control rats were studied at three postnatal ages: 4, 7, and 13 weeks. Brain weight in the mutant animals was significantly (P < 0.05) increased when compared to age-matched controls. Using systematic random sampling and the Cavalieri principle we estimated the volumes of various brain compartments, including the cerebral ventricles, forebrain, and cerebral cortex. We found that ventricular volume (P < 0.001) and forebrain volume (P < 0.05) were significantly increased in mutant rats when compared to control rats. Total numbers of nucleoli, estimated using the physical fractionator, were obtained for neurons in the cerebral cortex and granule cells in the dentate gyrus. Numbers were not altered significantly in mutant rats. Nor were mean soma volumes as estimated from total volumes and numbers. The changes in brain and ventricle volumes provide quantitative evidence that these animals display a hydrocephalic condition. This condition appears not to compromise cell number or mean soma size in the brain regions examined.

Severe hydrocephalus in L1-deficient mice

The neural adhesion molecule L1, a member of the immunoglobulin superfamily of cell recognition molecules, performs important functions in the developing and adult nervous system. This view is confirmed by the fact that mutations in the human L1 gene cause a severe neurological disease, termed CRASH (acronym for: corpus callosum hypoplasia, retardation, adducted thumbs, spastic paraplegia, and hydrocephalus). X-linked hydrocephalus is certainly the most prominent symptom of CRASH syndrome. Mouse mutants deficient in L1 also develop enlarged ventricles. Here, we report that ventricular dilation in L1-deficient mice is not correlated with stenosis of the aqueduct of Sylvius nor with ultrastructural abnormalities of ependymal cells lining the lateral ventricles or the aqueduct. However, a few L1 mutant mice displayed severe hydrocephalus, characterized by a significant enlargement of the skull and an almost complete atrophy of the cerebral cortex. The aqueduct of these severely affected animals was completely closed. Since mutant animals from two independently generated L1-deficient mouse lines displayed a similar phenotype, we consider severe hydrocephalus as a specific consequence of L1-deficiency. However, results of the present study also indicate that severe hydrocephalus represents a secondary rather than a primary defect of the L1 mutation; our combined data suggest that deformations of the brain as a result of massively enlarged ventricles secondarily cause stenosis of the aqueduct and subsequently high pressure hydrocephalus.