L1CAM variants cause two distinct imaging phenotypes on fetal MRI

A novel splicing variation in L1CAM is responsible for recurrent fetal hydrocephalus

BACKGROUND

The L1 cell adhesion molecule (L1CAM, OMIM 308840) gene is primarily expressed in the nervous system and encodes the L1 adhesion molecule protein. Variations in L1CAM cause a wide spectrum of X-linked neurological disorders summarized as the L1 syndrome.

METHODS

We report a 29-year-old pregnant woman who experienced multiple adverse pregnancy outcomes due to recurrent fetal hydrocephalus with an X-linked recessive inheritance. Genomic DNA was extracted from the third aborted male fetus and analyzed via trio whole-exome sequencing (WES). Total RNA was isolated from the pregnant woman to assess splicing variation at the mRNA level, and amniotic fluid was extracted from the woman for prenatal diagnosis on her fourth fetus.

RESULTS

All four male fetuses were affected by severe hydrocephalus. We identified a maternally derived hemizygous splicing variation NM_000425.5:[c.3046 + 1G > A]; NP_000416.1 p.(Gly1016AspfsTer6) (chrX:153130275) in Intron 22 of the L1CAM. This variation disrupts the donor splice site and causes the retention of Intron 22, which results in frame shift and a premature termination codon at position 1021 with the ability to produce a truncated protein without the fifth fibronectin-repeat III, transmembrane, and cytoplasmic domains or to induce the degradation of mRNAs by nonsense-mediated mRNA decay. The same hemizygous variant was also detected in the amniocytes.

CONCLUSION

This report enhances our knowledge of genetic and phenotypic characteristics of X-linked fetal hydrocephalus, providing a new genetic basis for prenatal diagnosis and pre-implantation prenatal diagnosis.

X-Linked Hydrocephalus with New L1CAM Pathogenic Variants: Review of the Most Prevalent Molecular and Phenotypic Features

Introduction

The underlying molecular defects of congenital hydrocephalus are heterogeneous and many isolated forms of hydrocephalus remain unsolved at the molecular level. Congenital hydrocephalus in males associated with agenesis of the corpus callosum is a notable characteristic of L1CAM gene which is by far the most common genetic etiology of congenital hydrocephalus.

Methods and Results

Sequencing of the L1CAM gene on 25 male patients/fetuses who had been presented with hydrocephalus revealed 6 patients and two fetuses with different hemizygous pathogenic variants. Our study identified 4 novel variants and 4 previously reported. The detection rate was 32%, and all the variants were shown to be maternally inherited. Nonsense variants were detected in 3 patients, while missense variants were detected in 2 patients. Frameshift, silent, and splicing variant, each was detected in 1 patient. The clinical manifestations of the patients are in line with those frequently observed including communicating hydrocephalus and agenesis of the corpus callosum. Moreover, rippled ventricles with subdural collection and asymmetry of ventricles after shunt operation were seen in 1 patient and 2 patients, respectively. In addition, abnormal basal ganglia were found in 4 patients which seems to be an additional distinct new finding. We also describe a patient with novel nonsense variant with the rare association of Hirschsprung's disease. This patient displayed additionally multiple porencephalic cysts and encephalomalacia secondary to hemorrhage due to repeated infections after shunt operation. The patients with the missense variants showed long survival, while those with truncating variants showed poor prognosis.

Conclusion

This report adds knowledge of novel pathogenic variants to the L1CAM variant database. Furthermore, we evaluated the clinical and imaging data of these patients.

"Floppy brain" in congenital hydrocephalus

Hydrocephalus is classically considered to be a disorder of altered cerebrospinal fluid (CSF) circulation, leading to the dilation of cerebral ventricles. Here, we report a clinical case of a patient who presented with fetal-onset hydrocephalus with diffusely reduced cortical and white matter volumes resulting from a genetic mutation in L1CAM, a well-known hydrocephalus disease gene involved in neuronal cell adhesion and axon development. After CSF was drained from the ventricle intraoperatively, the patient's cortical mantle collapsed and exhibited a "floppy" appearance on neuroimaging, suggesting an inability of the hydrocephalic brain to maintain its structural integrity. The case provides clinical support for altered brain biomechanical properties in human hydrocephalus and adds to the emerging hypothesis that altered brain development with secondary impact on brain structural stability may contribute to ventricular enlargement in some subsets of hydrocephalus.

Second trimester fetal MRI of the brain: Through the ground glass

Fetal MRI is an important tool for the prenatal diagnosis of brain malformations and is often requested after second-trimester ultrasonography reveals a possible abnormality. Despite the immature state of the fetal brain at this early stage, early suggestive signs of the presence of brain malformations can be recognized. To differentiate between the normal dynamics of the growing brain and the developing pathological conditions can be challenging and requires extensive knowledge of normal central nervous system developmental stages and their neuroradiological counterparts at those different stages. This article reviews the second-trimester appearances of some commonly encountered brain malformations, focusing on helpful tricks and subtle signs to aid in the diagnosis of such conditions as rhombencephalosynapsis, various causes of vermian rotation, molar tooth spectrum anomalies, diencephalic-mesencephalic junction dysplasia, ganglionic eminence anomalies, and the most common malformations of cortical development.

PRENATAL IMAGING FEATURES RELATED TO RAC3 PATHOGENIC VARIANT AND DIFFERENTIAL DIAGNOSES

This article is protected by copyright. All rights reserved. This article is protected by copyright. All rights reserved. What's already known about this topic? Six individuals between 5- and 19-year-old were reported with pathogenic variants in RAC3 responsible for brain malformations involving midline What does this study add? RAC3-fetopathy is associated with fetal akinesia deformation sequence, and complex brain malformations including corpus callosum agenesis, diencephalosynapsis, kinked brainstem, and vermian hypoplasia The differential diagnoses with prenatal kinked brainstem include fetopathies related to KIAA1109, L1CAM, Walker-Warburg syndrome and tubulinopathies. The present report adds the RAC3 related condition to this list.

Diagnostic Approach to Macrocephaly in Children

Macrocephaly affects up to 5% of the pediatric population and is defined as an abnormally large head with an occipitofrontal circumference (OFC) >2 standard deviations (SD) above the mean for a given age and sex. Taking into account that about 2-3% of the healthy population has an OFC between 2 and 3 SD, macrocephaly is considered as "clinically relevant" when OFC is above 3 SD. This implies the urgent need for a diagnostic workflow to use in the clinical setting to dissect the several causes of increased OFC, from the benign form of familial macrocephaly and the Benign enlargement of subarachnoid spaces (BESS) to many pathological conditions, including genetic disorders. Moreover, macrocephaly should be differentiated by megalencephaly (MEG), which refers exclusively to brain overgrowth, exceeding twice the SD (3SD-"clinically relevant" megalencephaly). While macrocephaly can be isolated and benign or may be the first indication of an underlying congenital, genetic, or acquired disorder, megalencephaly is most likely due to a genetic cause. Apart from the head size evaluation, a detailed family and personal history, neuroimaging, and a careful clinical evaluation are crucial to reach the correct diagnosis. In this review, we seek to underline the clinical aspects of macrocephaly and megalencephaly, emphasizing the main differential diagnosis with a major focus on common genetic disorders. We thus provide a clinico-radiological algorithm to guide pediatricians in the assessment of children with macrocephaly.