Clinical manifestations

Presentation and course

The clinical spectrum of this X-linked recessive disorder is broad, and there is notable interfamilial and intrafamilial phenotypic heterogeneity (45). Moderate to severe hydrocephalus, macrocephaly, spastic paraparesis or quadriparesis, and flexion-adduction thumb deformities are commonly present. Affected boys with severe hydrocephalus may be stillborn, or they may die shortly after birth or in childhood (04; 11; 27; 61). Other abnormalities include nystagmus with visual impairment, short stature, bilateral syndactyly of the second and third toes, hyperreflexia, gait disturbances, and intellectual disability. The degree of cognitive impairment ranges from mild to profound (40). Congenital aganglionic megacolon (Hirschsprung disease), confirmed by rectal biopsy, has also been reported in boys with X-linked hydrocephalus and L1CAM mutations (29; 41). Learning problems or mild intellectual disability may be seen in obligate female carriers (24).

In a large family with 22 known affected boys with the MASA syndrome, Kaepernick and colleagues described a wide variation of clinical features among family members (33). Some of the affected males had characteristics of the MASA syndrome, and others had features of X-linked hydrocephalus. In this large family, female carriers displayed a variety of features including adducted thumbs, cognitive impairment, and hydrocephalus.

Neuroimaging and gross pathologic studies reveal diverse characteristics in X-linked hydrocephalus, notably bilateral enlargement of the lateral ventricles, diffuse white-matter hypoplasia, absence of the corticospinal tract, agenesis of the corpus callosum, and atrophy of the anterior vermian lobe (09; 64; 05). A patent cerebral aqueduct is observed in most cases on magnetic resonance imaging (64).

Prognosis and complications

Many boys with X-linked hydrocephalus are stillborn or die in infancy or early childhood. Certain L1CAM mutations, especially those that result in a truncation of the extracellular domain of the protein, are associated with the severe phenotype of congenital hydrocephalus, marked developmental motor and cognitive disabilities, and shorter survival (65; 37). Boys harboring a truncating mutation are significantly more likely to die before three years of age (52%) than boys with a missense mutation (8%), indicating a correlation between the severity of the developmental encephalopathy and the type of mutation (57). Most L1 nonsense and frameshift mutations lead to truncation of the protein prior to the transmembrane domain eliminating L1- expression on the cell surface. Such mutations interrupt the effects of cell adhesion and interfere with the intracellular signaling pathways producing severe phenotypes. Missense mutations in the extracellular domains can lead to both severe or mild phenotypes whereas mutations in the cytoplasmic domain tend to produce the mildest form of the neurodevelopmental disorder (Weller and Gartner).

In boys with longer survival, severe motor and cognitive neurologic deficits are invariably present. Intellectual outcome is notably poorer in cases of X-linked hydrocephalus compared to non-X-linked, shunted congenital hydrocephalus. School performance is generally poorer in mothers of X-linked patients as well (24). Recurrent seizures are present in most affected boys (21). Because hydrocephalus alone does not account for seizures, the coexpression of epilepsy implies focal cerebral cortical dysgeneses or secondary neuroblast migratory disturbances that may be microscopic and below the resolution of neuroimaging.

Clinical vignette

Prenatal diagnosis of hydrocephalus and macrocephaly was made in this male fetus by ultrasound at 36 weeks’ gestation. Subsequently, a 3.8 kg boy was delivered by elective cesarean section. Occipitofrontal circumference at birth was 39.5 cm. Physical findings at birth included mild diffuse spasticity and abnormal fisting of the hands with fixed flexion deformities of the thumbs. A ventriculoperitoneal shunt was placed on the second day of life. A CT scan performed after surgery demonstrated a peripheral 1.5 cm mantel of brain parenchyma and marked hydrocephalus. Seizures developed postoperatively and were adequately managed with phenobarbital.

By one year of age the infant showed signs of severe developmental disabilities. He could support his head in the prone position and roll over, but he could not sit independently. He made babbling sounds and had a social smile to some stimuli. Spontaneous nystagmus, minimal ocular tracking, and signs of severe cerebral visual impairment were noted. A follow-up CT scan showed mildly increased cerebral parenchyma without discernible gray or white matter differentiation.

The boy died unexpectedly in sleep shortly after his second birthday. Postmortem neuropathologic assessment showed aqueductal stenosis, irregularly dilated lateral and third ventricles, absence of septum pellucidum, marked atrophy of the corpus callosum, marked cerebral white matter hypoplasia, diffuse polymicrogyria, hypoplastic and poorly differentiated hippocampi, and marked hypoplasia of medullary pyramids.

Biological basis

Etiology and pathogenesis

X-linked hydrocephalus is associated with mutations in the gene for the neural cell adhesion molecule L1, which is located on the subchromosomal region Xq28 (32). L1 is a transmembrane glycoprotein involved in cell adhesion, axonal guidance, fasciculation, and synapse formation (15; 07; 20). Various human L1CAM mutations can be responsible for a wide spectrum of neurologic abnormalities with intellectual disability including X-linked hypogammaglobulinemia, MASA syndrome, SPG1, and X-linked agenesis of the corpus callosum (18; 17). Phenotypic heterogeneity occurs within individual families.

Over 115 different L1CAM gene mutations have been reported (http://www.L1CAMmutationdatabase.info/references.aspx); however, this online database was last updated on October 19, 2012. A registry of specific small deletions, insertions and indels, nucleotide substitutions (nonsense and missense, regulatory, and splicing), gross deletions, and a single gross duplication is obtainable with references via the Internet in the Human Gene Mutation Database, in association with Celera. Approximately half of the currently known mutations cause disruption of the overall L1 structure, and the remaining mutations are missense mutations (18). Milder neurologic disabilities in obligate female carriers may be related to nonrandom X-chromosome inactivation (33).

SPG1 and other hereditary spastic paraplegias are clinically diverse, but all have characteristic leg spasticity and motor dyspraxia caused by either abnormal development or progressive degeneration of corticospinal tracts. A large and rapidly expanding list of at least 83 hereditary spastic paraplegias genes are known to cause this heterogeneous condition (46). The molecular genetic diagnosis of a patient suspected of having hereditary spastic paraplegias may require genetic sequencing using a targeted gene panel, whole-exome sequencing, or whole-genome sequencing, depending on the clinical circumstances and level of diagnostic suspicion.

L1 syndrome is caused by the loss of L1-protein structure and function during the development of the nervous system, which impairs neuronal cell migration, neurite elongation, and axon fasciculation. Congenital hydrocephalus generally occurs by obstruction of the cerebrospinal fluid circulation. In some cases of X-linked hydrocephalus, the obstruction of the cerebral aqueduct may result from a primary CNS malformation. Communicating hydrocephalus followed by secondary aqueductal stenosis was initially postulated to be a cause (63; 34).

X-linked hydrocephalus and two clinically milder disorders, MASA syndrome and spastic paraplegia type I, are associated with mutations of the L1 gene. L1 is a member of the immunoglobulin super family of cell adhesion molecules. Neural cell adhesion proteins play an important role in neural development (07; 20). Mutations in L1 are thought to result in disruption of neuroblast migration, axonal guidance, and axon-fascicle formation (06). Homophilic interactions between L1 family proteins are essential for neural system development through signal transduction-activated cell growth signaling. Various models for homophilic interaction have been proposed for cell adhesion proteins, including zipper, domain-swapped monomer, domain-swapped multimer, 2-fold symmetry-related surface interaction, and 2-fold symmetry-related edge interaction models based on the structures of axonin-1, hemolin, Dscam, and TAG-1/neurofascin (an L1 family protein) (59). It is believed that some cancer progression may involve the homophilic interaction of L1.

The L1-protein is a transmembrane glycoprotein with six immunoglobulin-like and five fibronectine type III domains on the extracellular surface, a single-pass transmembrane segment, and a short but highly conserved cytoplasmic C terminal region. Although the natural distribution of mutations over the functional L1 protein domains are variable, mutations are most common in the second, third, and fourth immunoglobulin-like domain and in the second fibronectin domain (65).

In mice that do not express L1 (L1-Y hemizygous males), there are defects in the guidance of corticospinal tract axons. In this mouse model, the pathway to the caudal medulla appears normal, but a major portion of axons fail to cross the midline to the opposite dorsal column. These defects in medullary or cervical decussation and axon guidance arise during early stages of development (10).

In other experimental models, a novel L1 syndrome mouse with an extracellular L1 mutation and a cell surface-exposed L1 can be remediated through L1 mimetic compounds such as duloxetine, crotamiton, and trimebutine, which rescue impaired cell migration and neuritogenesis in culture (35).

L1CAM receptors are a subfamily of the immunoglobulin superfamily of transmembrane receptors, which include L1, CHL1 (close homologue of L1), NrCAM (neuron-glia-related CAM), and neurofascin. At the cellular level, the L1-CAM family is also expressed in non-neuronal tissues, often as different isoforms. For example, L1 lacking the RSLE motif is found in a variety of glial cells, and the 185 kDa isoform of CHL1 is found in astrocytes, Schwann cells, and oligodendrocyte precursors and progenitors (54; 26).

Under physiological conditions, L1 expression is absent in epithelial cells, but L1 is detectable in precursor lesions, or epithelial cells under inflammatory conditions. L1-CAM proteins expressed outside of the nervous system have also been heavily implicated and are involved in tumor initiation, progression, and migration, as well as in chemoresistance in diseases, including cancers of the lung, pancreas, kidney, and colon (25; 02). L1 expression is a poor prognostic biomarker for cancer patients and is regarded as a potential target for cancer therapy (14).

In humans, postmortem neuropathologic examination reveals polymicrogyria, pachygyria and leptogyria, hypoplasia of the medullary pyramids, hypoplasia of the corpus callosum, small anterior commissure, and poorly differentiated hippocampi. Despite the extensive cerebral malformations, the cortex shows normal-appearing laminar cortical neuronal architecture, further suggesting an underlying disturbance of neuronal connectivity, fasciculation, and synapse formation rather than abnormal neuroblast migration or secondary changes from aqueductal stenosis and increased intracranial pressure (21; 48).

Some nonneuronal cells express a short isoform of L1, in contrast to the complete L1 gene expressed exclusively in neurons (55). L1CAM-mediated cell adhesion may also alter ganglion cell precursors in the development of intestinal aganglionosis (29; 41).

Epidemiology

Isolated congenital hydrocephalus not associated with neural tube defects is estimated to occur in 1 in 1250 to 1 in 2500 live and stillborn births. Approximately three-fifths of the cases occur in boys (24).

X-linked hydrocephalus is the most common form of inherited hydrocephalus and is associated with severe neurologic deficits and premature death. Between 7% and 15% of all cases of congenital hydrocephalus of uncertain cause were estimated to be due to X-linked hydrocephalus based on clinical findings and autopsy studies prior to the identification of the L1CAM gene (24; 56). In the L1CAM gene-testing era of the 1990s, similar rates of L1CAM mutations were detected in populations of male fetuses and boys with hydrocephalus. Finckh and colleagues found 46 pathogenic mutations out of 153 cases (30% detection rate) of prenatally or postnatally suspected L1 syndrome. L1CAM mutation rates were higher (74%) when there were at least two additional cases in the family and lower (16%) when there was no family history (16). The estimated incidence of L1 syndrome is 1:30,000 male births (http://www.L1CAMmutationdatabase.info/). Over 200 different pathogenic L1CAM mutations have been reported; 130 were reviewed by Weller and Gartner (60). Fifty-two new L1CAM mutations were reported in an online L1CAM mutations databank (58). A broad range of individual phenotypes are associated with different pathogenic L1CAM gene sequence variants (28).

Prevention

Prenatal diagnosis of affected males by ultrasonographic detection of hydrocephalus is not completely reliable because hydrocephalus may be absent antenatally. Furthermore, carrier detection in mothers is not always possible because they are usually asymptomatic (62).

Accurate prenatal diagnosis is feasible by L1 gene mutation analysis (49; 47). Jouet and Kenwrick first demonstrated a mutation in the L1 gene in two cases of sporadic hydrocephalus through definitive prenatal diagnosis at 10 weeks' gestation in the subsequent pregnancies using a chorionic villous sample for DNA analysis (31). L1CAM gene sequence analysis is now readily available and applicable for prenatal diagnosis.

Differential diagnosis

Numerous developmental disorders may present with hydrocephalus or ventriculomegaly, such as 13 or 18 trisomy, arrhinencephaly, Apert syndrome, oro-facio-digital syndrome, Hurler syndrome, achondroplasia, thanatophoric dysplasia, osteogenesis imperfecta, and incontinentia pigmenti (51).

Strain and colleagues described a separate form of X-linked hydrocephalus in a family that displayed the typical clinical features of X-linked hydrocephalus but lacked the linkage to region Xq28 markers (53). In their family, linkage to the FRAXA region of Xq27.3 was demonstrated. Thus, clinical criteria alone are not sufficient in discriminating between genetic types of X-linked hydrocephalus.

Other less common X-linked and autosomal genetic disorders may cause congenital hydrocephalus. Pathological variants in the X-linked AP1S2 gene cause Fried and Pettigrew syndromes, which include congenital hydrocephalus in their phenotype spectrums (19; 08). The AP1S2 gene encodes the sigma-2 subunit of the heterotetrameric adaptor protein 1 (AP1) complex found in the cytosolic side of coated vesicle in the Golgi compartment and mediates the recruitment of clathrin and the recognition of sorting signals of transmembrane receptors.

Autosomal gene variants associated with congenital hydrocephalus include the CCDC88C, MPDZ, EML1, and WDR81 genes. In 2010, the first autosomal recessive disorder causing congenital hydrocephalus secondary to a pathologic CCDC88C gene variant was reported in a consanguineous multiplex family (13). In 2013, a founder homozygous truncating variant in the MPDZ gene was the second reported autosomal recessive disorder causing congenital hydrocephalus (01). Exome sequencing, genome-wide autozygome analysis, and “Mendeliome assays” have identified additional autosomal recessive genes associated with congenital hydrocephalus including the EML1 and WDR81 genes (50).

Macrocephaly is defined as an occipitofrontal circumference (OFC) greater than the 98th percentile for age. Macrocephaly without hydrocephalus is easily diagnosed by all neuroimaging modalities. Macrocephaly without hydrocephalus occurs for many reasons, including megalencephaly, cerebral edema, neoplasia, structural anomalies, and certain neurodevelopmental disorders. Syndromic macrocephaly may also exhibit somatic overgrowth such as in Cowden syndrome (PTEN hamartoma tumor syndrome) or Sotos syndrome (NSD1-related genetic disorder).

Confusing conditions

By definition, in congenital hydrocephalus the condition should be present at birth, and, thus, a developmental etiology is implied. If the condition is diagnosed postnatally, other forms of hydrocephalus caused by overproduction of CSF, defective reabsorption, or obstruction of the CSF pathway should be considered. Overproduction of CSF may occur with increased function of choroid plexus tissue. Underabsorption of CSF most commonly occurs with obstruction of venous return, such as in venous sinus thrombosis. Blockage of the CSF pathway at the cerebral aqueduct or the foramina of Magendie and of Luschka will result in dilatation of the proximal ventricular system. Intraventricular hemorrhage is a frequent complication of prematurity and may result in blockage of the aqueduct from a blood clot. Obstruction of CSF flow secondary to prenatal or perinatal CNS infections may result in inflammatory or gliotic blocks at multiple sites.

Other identifiable developmental disorders are associated with "complicated" hydrocephalus where it is secondary to a primary malformation, such as a neural tube defect. In myelomeningocele, approximately 90% of infants will require ventriculoperitoneal shunt placement to manage hydrocephalus secondary to the Chiari II malformation.

Diagnostic workup

In utero ultrasonography, performed after 20 to 24 weeks’ gestation, will reveal the fetus with hydrocephalus in most cases. Although X-linked hydrocephalus has a variable presentation of ventriculomegaly and hydrocephalus, fetal ultrasound and fetal MRI remains a useful tools for prenatal diagnosis (23). Postnatally, computerized tomography or magnetic resonance imaging provides morphologic detail needed to identify other distinct CNS malformations. Because the neuroanatomy of X-linked hydrocephalus secondary to L1CAM gene mutations includes the distinct finding of brainstem corticospinal tract hypoplasia, diffusion-weighted imaging of the brainstem can be used to demonstrate lack of anisotropy in the corticospinal tracts of the basis pontis. Thus, diffusion-weighted imaging may be useful in screening boys thought to have L1 syndrome (22).

Despite the distinct phenotype of L1 syndrome congenital hydrocephalus, its prenatal and neonatal presentation may suggest a broader differential diagnosis. Advances in genetic technologies have led to higher expectations and standards in clinical practice. L1CAM gene mutation analysis is indicated in boys, and chromosomal microarray (chromosomal microarray analysis or array comparative genome hybridization, [aCGH]) should be strongly considered in every patient with congenital hydrocephalus—especially syndromic hydrocephalus (56).

Clinical recognition of phenotypes along with a series of “first-line” and “second-line” genetic tests provide an approach to diagnose both the type and cause of various cerebral malformations. Chromosomal microarray analysis (including both microarray-based comparative genomic hybridization and single nucleotide polymorphism microarray) has become the first-line genetic test in the etiological evaluation of CNS malformations identified pre- or postnatally (52). The widespread use and high diagnostic yield of chromosomal microarray analysis provides the awareness of the unpredictability of human genomic gains and losses as a major cause of cerebral malformations, including those disorders initially described as “developmental delay”, microcephaly, intellectual disability, and multiple congenital anomalies that often include ventriculomegaly. With wide application of next-generation sequencing methods, genes related to cerebral malformations have been identified and more discoveries will be anticipated. In current practice, the combination of “first-line” chromosomal microarray analysis followed by next-generation sequencing should provide an etiological diagnosis in approximately 30% to 40% of major cerebral malformations. Variability in diagnostic rates depends on the differences in the types of patients included in different studies (39). Thus, chromosomal microarray analysis followed by next-generation sequencing testing technology might enable the “genotype first” screening approach ahead of the traditional “phenotype first” patient-examination approach, but pretesting patient selection is essential to exclude patients who are not likely to have a genetic condition.

Obligate female carriers have no reliable clinical marker. Accurate prenatal diagnosis is feasible by L1 gene mutation analysis (49; 47; 31). Using genomic DNA obtained from a blood sample, all coding exons and intron/exon boundaries of the L1CAM gene can be screened by bi-directional sequence analysis. L1CAM gene testing is currently by commercial laboratories in North America and Europe.

Management

Placement of a ventriculoperitoneal shunt will correct increased intracranial pressure and halt the progressive macrocephaly but does not appear to alter neurologic outcome (64). Supportive care may include nutritional supplements, gastrostomy tube placement, antiepileptic drug therapy, airway management, equipment for mobility, hand splints for severe flexion-adduction thumb deformities, and physical therapy.

Special considerations

Pregnancy

Prenatal diagnosis should be provided, especially if there is a family history of congenital hydrocephalus. Depending on the certainty and severity of the hydrocephalus, parents may elect either a termination of pregnancy or a cesarean section to minimize the risk to the fetus and the mother. Prenatal craniocentesis remains controversial and usually leads to death in utero.

Accurate prenatal exclusion of X-linked hydrocephalus in subsequent pregnancies with male fetuses from obligate carrier mothers is reported using closely linked Xq28 DNA markers for L1 gene mutations (49; 31). If a mutation has been determined in an affected family member, haplotyping offers a fast but indirect test with several limitations (38). Denaturing gradient gel electrophoresis and direct sequence analysis have been used to identify mutations in L1CAM (42; 57).