Lateral Meningocele Syndrome (LMS), a disorder associated with NOTCH3 pathogenic variants, presents with neurological, craniofacial and skeletal abnormalities. Mouse models of the disease exhibit osteopenia that is ameliorated by the administration of Notch3 antisense oligonucleotides (ASO) targeting either Notch3 or the Notch3 mutation. To determine the consequences of LMS pathogenic variants in human cells and whether they can be targeted by ASOs, induced pluripotent NCRM1 and NCRM5 stem (iPS) cells harboring a NOTCH36692-93insC insertion were created. Parental iPSCs, NOTCH36692-93insC and isogenic controls, free of chromosomal aberrations as determined by human CytoSNP850 array, were cultured under conditions of neural crest, mesenchymal and osteogenic cell differentiation. The expected cell phenotype was confirmed by surface markers and a decline in OCT3/4 and NANOG mRNA. NOTCH36692-93insC cells displayed enhanced expression of Notch target genes HES1, HEY1, 2 and L demonstrating a NOTCH3 gain-of-function. There was enhanced osteogenesis in NOTCH36692-93insC cells as evidenced by increased mineralized nodule formation and ALPL, BGLAP and BSP expression. ASOs targeting NOTCH3 decreased both NOTCH3 wild type and NOTCH36692-93insC mutant mRNA by 40% in mesenchymal and 90% in osteogenic cells. ASOs targeting the NOTCH3 insertion decreased NOTCH36692-93insC by 70-80% in mesenchymal cells and by 45-55% in osteogenic cells and NOTCH3 mRNA by 15-30% and 20-40%, respectively. In conclusion, a NOTCH3 pathogenic variant causes a modest increase in osteoblastogenesis in human iPS cells in vitro and NOTCH3 and NOTCH3 mutant specific ASOs downregulate NOTCH3 transcripts associated with LMS.

Lateral meningoceles are defined as protrusions of the arachnoid and dura mater through the intra- and intervertebral foramina of the spine. Dural dysplasia, such as dural ectasia, contributes to their occurrence. Lateral meningocele syndrome (LMS) is an extremely rare genetic connective tissue disorder. A 3-year-old boy began to frequently slip and fall. At 5 years of age, he developed progressive intermittent claudication with radiating pain along the lateral aspect of the right thigh. Physical examination revealed bilateral blepharophimosis, downslanting palpebral fissures, hypertelorism, micrognathia, retrognathia, low-set ears, and a right-sided paraspinal protrusion. Spinal magnetic resonance imaging (MRI) at 6 years of age demonstrated multiple lateral meningoceles of the thoracic and lumbar spine with posteriorly scalloped vertebral bodies. Computed tomography (CT) revealed severe scalloping of vertebral bodies and pedicles from T8 to L4 with widening of the spinal canal. Two cyst-subarachnoid (C-S) shunts were placed, at the superior aspect of the meningocele (T8) and the inferior aspect (L5-S1). His symptoms improved immediately, and the meningocele gradually regressed postoperatively. LMS is a rare genetic disorder associated with craniofacial anomalies, developmental delay, intellectual disability, hypotonia, and decreased muscle mass. Although C-S shunting successfully reduced intracystic pressure in this case, long-term neurological and radiological follow-up is required to determine whether shunting can halt the progression of lateral meningoceles.

NOTCH3, one of the four mammalian Notch receptors, acts as a transcriptional activator in a variety of tissues. Variants in NOTCH3 lead to distinct phenotypes, depending on variant type and location. Truncating variants in the last exon generate a protein lacking the PEST domain, responsible for degradation, leading to a gain-of-function effect and causing Lateral Meningoceles syndrome (LMS), characterized by dysmorphisms and variable cardiac, skeletal, and connective tissue abnormalities; motor delay may occur, but the cognitive function is usually normal. We report the first case of prenatal molecular diagnosis of LMS, which was made using prenatal exome sequencing after an ultrasound with findings of fetal cystic hygroma, mild bilateral ventriculomegaly, and facial dysmorphisms. After birth, magnetic resonance imaging confirmed the presence of lateral meningoceles. A complete clinical evaluation was performed and unexpected biliary anomalies were found. The occurrence of biliary anomalies has not been previously reported in LMS but may have biological plausibility. Expression of NOTCH3 has been demonstrated in biliary development and is thought to play a role in the differentiation of hepatoblasts into biliary epithelial cells, and also in liver regeneration and repair. We hypothesize that the findings reported here might expand the phenotype of LMS.

Lateral meningocele syndrome is a rare skeletal syndrome caused by truncating variants in the final exon of the NOTCH3 gene. It is characterized by multiple lateral meningoceles that may result in neurological sequelae. A wider systemic phenotype has been demonstrated, including musculoskeletal abnormalities, feeding difficulties, structural cardiac and renal anomalies, and facial dysmorphism. We describe the clinical details of a child who was initially diagnosed with Copenhagen syndrome (progressive non-infectious anterior vertebral body fusion), based on radiological findings, in the context of kyphosis and back pain. Later, a novel de novo c.6723_6736del p.(Glu2241AspfsTer8) NOTCH3 variant was identified from the 100,000 Genomes Project, in keeping with a genetic diagnosis of lateral meningocele syndrome. Without the context of additional features that may point toward an underlying syndrome, radiological findings-when reviewed in isolation-may be suggestive of alternate diagnoses. In this case, the radiological finding of anterior vertebral fusion suggested Copenhagen syndrome, whereas the identification of dural ectasia prompted further investigation into Ehlers-Danlos syndrome subtypes. Recognition of dysmorphology prompted wider investigation by Whole Genome Sequencing. Features of lateral meningocele syndrome significantly overlap with those of connective tissue disorders including EDS, Marfan syndrome, and Loeys-Dietz syndrome. We describe the clinical features of the here-reported proband with a novel NOTCH3 variant, and compare the phenotypes of these differential diagnoses.