Disruption of the complex processes underlying central nervous system development leads to a broad spectrum of brain malformations and neurodevelopmental disorders, often with a genetic cause. Here, we report bi-allelic pathogenic variants in fibronectin type III and SPRY domain-containing 1-like (FSD1L), encoding a protein of unknown function, in eleven individuals, including five fetuses from six unrelated families. The phenotype ranges from severe hydrocephalus, corpus callosum agenesis, and absent pyramid decussation to a neurodevelopmental syndrome characterized by severe intellectual disability, spastic tetraparesis, reduced vision, and epilepsy, associated with corpus callosum agenesis/hypoplasia, mild ventricular dilation, optic nerve hypoplasia, and white matter reduction. This phenotype closely resembles that observed in L1 syndrome, caused by pathogenic variants in L1CAM, encoding a neural adhesion molecule. The knockdown of Fsd1l in mouse embryos recapitulated the ventricular dilation observed in affected fetuses. Immunohistochemical studies in human control fetuses revealed that FSD1L localized to neurons with commissural fate and projection neurons during human development. Induced pluripotent stem cell (iPSC)-derived neural progenitor cells from affected individuals failed to differentiate into premature neurons and to properly form neurospheres while undergoing increased cell death. In neural progenitors, FSD1L localized with microtubules of the mitotic spindle during M phase and to the transition zone and along the axoneme of the primary cilium during interphase. In line with this, fibroblasts from affected individuals exhibited marked alterations of the mitotic spindle and reduced ciliogenesis and ciliary length compared to control cells. Our findings define FSD1L as a microtubule-associated protein implicated in neuronal differentiation, axon guidance, and fasciculation.

L1CAM protein plays a crucial role during early development and mutations in L1CAM cause L1 syndrome. L1 syndrome demonstrates a highly variable presentation within and between families. The clinical symptoms of L1 syndrome include mental retardation, hydrocephalus, spasticity, aphasia, and adducted thumb. Mutations in L1CAM gene were found to affect structurally essential key residues in extracellular region of L1 leading to changes in protein binding properties. In most cases, these mutations create unexpected phenotypes which need to be understood thoroughly. The L1 syndrome patients were identified by various phenotypes like mental retardation, hydrocephalus, aphasia, spasticity, adducted thumb, etc., and the patients or mental retardation (MR) children who had more than three symptoms. This study aimed to screen mutations in multiple exons by Sanger sequencing. The present study employed primers which are designed for specific exons of L1CAM gene to amplify and sequence the amplified product to detect the mutations in L1 syndrome patients by the Sanger sequencing. Chi-square test was used to determine the mutation detection rate with the number of L1 syndrome phenotypes and several in silico programs were used to investigate potential effects of the variants. The nine different mutations in six patients. The mutation detection rate was high (83.33%) in patients with more than one L1 syndrome phenotype and in patients with more than one affected member in a family compared to patients with single phenotypes and negative family history (16.6%). The mutation detection rate was related to the presence of typical L1 syndrome phenotypes and the family history. Screening of L1CAM gene mutations in the Indian population is much needed to analyze the mutations and understand the mechanism underlying L1 disease. The present study has identified some novel mutations which are implicated in alterations in various biological functions during development leading to pathogenesis of L1 syndrome.

Loss-of-function mutations of AVPR2 and L1CAM result in nephrogenic diabetes insipidus (NDI) and L1 syndrome. These diseases are inherited in an X-linked recessive manner. Females with heterozygous variants can be affected owing to skewed X-chromosome inactivation (XCI). A 3-year-old girl with normal development was presented with polydipsia and polyuria, and diagnosed of NDI through an improper response to water restriction and desmopressin administration. A targeted genome capture sequencing for X chromosome confirmed Xq28 microdeletion involving AVPR2 and L1CAM, derived from mother with mosaicism of the deletion. No pathogenic variants were identified in the paternal X allele nor in AQP2, another causative gene of NDI. XCI was exclusively skewed toward the maternal X chromosome in hair, oral mucosa, and blood, while it was random in nails and renal tubular epithelial cells (RTECs). Both AVPR2 and L1CAM mRNA expression in the patient's RTECs were significantly reduced compared to those of controls, with AVPR2 showing a more pronounced decrease. Thus, we demonstrated for the first time that NDI can develop in a female with a AVPR2 deletion despite of random XCI. Moreover, the absence of L1 syndrome in the female patient was caused most probably through organ-dependent skewed XCI in the deletion allele.

L1 syndrome, a neurological disorder with an X-linked inheritance pattern, mainly results from mutations occurring in the L1 cell adhesion molecule (L1CAM) gene. The L1CAM molecule, belonging to the immunoglobulin (Ig) superfamily of neurocyte adhesion molecules, plays a pivotal role in facilitating intercellular signal transmission across membranes and is indispensable for proper neuronal development and function. This study identified a rare missense variant (c.1759G>C; p.G587R) in the L1CAM gene within a male fetus presenting with hydrocephalus. Due to a lack of functional analysis, the significance of the L1CAM mutation c.1759G>C (p.G587R) remains unknown. We aimed to perform further verification for its pathogenicity. Blood samples were obtained from the proband and his parents for trio clinical exome sequencing and mutation analysis. Expression level analysis was conducted using western blot techniques. Immunofluorescence was employed to investigate L1CAM subcellular localization, while cell aggregation and cell scratch assays were utilized to assess protein function. The study showed that the mutation (c.1759G>C; p.G587R) affected posttranslational glycosylation modification and induced alterations in the subcellular localization of L1-G587R in the cells. It resulted in the diminished expression of L1CAM on the cell surface and accumulation in the endoplasmic reticulum. The p.G587R altered the function of L1CAM protein and reduced homophilic adhesion capacity of proteins, leading to impaired adhesion and migration of proteins between cells. Our findings provide first biological evidence for the association between the missense mutation (c.1759G>c; p.G587R) in the L1CAM gene and L1 syndrome, confirming the pathogenicity of this missense mutation.

Heterozygous L1CAM variants cause L1 syndrome with hydrocephalus and aplasia/hypoplasia of the corpus callosum. L1 syndrome usually has an X-linked recessive inheritance pattern; however, we report a rare case occurring in a female child. The patient's family history was unremarkable. Fetal ultrasonography revealed enlarged bilateral ventricles of the brain and hypoplasia of the corpus callosum. The patient was born at 38 weeks and 4 days of gestation. Brain MRI performed on the 8th day of life revealed enlargement of the brain ventricles, marked in the lateral and third ventricles with irregular margins, and hypoplasia of the corpus callosum. Exome sequencing at the age of 2 years and 3 months revealed a de novo heterozygous L1CAM variant (NM_000425.5: c.2934_2935delp. (His978Glnfs * 25). X-chromosome inactivation using the human androgen receptor assay revealed that the pattern of X-chromosome inactivation in the patients was highly skewed (96.6 %). The patient is now 4 years and 11 months old and has a mild developmental delay (developmental quotient, 56) without significant progression of hydrocephalus. In this case, we hypothesized that the dominant expression of the variant allele arising from skewed X inactivation likely caused L1 syndrome. Symptomatic female carriers may challenge the current policies of prenatal and preimplantation diagnoses.