Neurodevelopmental disorders: From clinical phenotypes and genome analysis to gene function

Neurodevelopmental disorders (NDDs) are a clinically heterogeneous group of diseases characterized by impaired brain development, which can result in intellectual disability, impaired motor and speech development, and epilepsy. On the more severe end of the phenotypic spectrum NDDs can be accompanied with a brain malformation of cortical development (MDD), characterized by an abnormal formation of the cerebral cortex. NDDs with or without MCD are relatively common disease with a high disease burden for the affected individual and their family. A substantial proportion of NDDs and/or MCDs has a monogenetic origin. However, the majority of individuals suspected for a monogenetic NDD do not receive a conclusive molecular diagnosis. This is essential since a genetic diagnosis is required for accurate outcome prediction, risk recurrence counseling and therapy options, which has substantial benefits for the affected individual and their family. This thesis aims to obtain a better understanding of genetic causes and mechanisms underlying NDDs through novel clinical, diagnostic, and research approaches, including pattern recognition of MCD on brain MRI, RNA-sequencing as a novel diagnostic tools, and the use of zebrafish as a model organism.

Next-generation sequencing, particularly exome sequencing, has improved the identification of potential pathogenic variants in NDDs. For example, the MACF1 gene, which plays a role in linking filamentous- actin with microtubules, was studied in a cohort of individuals with heterogeneous NDDs and variants in this gene. De novo missense variants located in the microtubule-binding EF-hand domains or GAR domain, result in a recognizable MCD on brain MRI characterized by lissencephaly with axonal midline crossing defects. Functional in vitro studies showed that these variants result in increased microtubule-binding, suggesting that impaired microtubule organization by MACF1 is causal for the distinctive MCD. Additionally, RNA-seq was explored as a novel diagnostic tool, proving effective in identifying splicing abnormalities and other genetic alterations that are not easily detected by traditional DNA diagnostics. This approach increased diagnostic yield and aided in discovering new disease-related genes, such as NDC1, associated with a triple-A-like syndrome.

The thesis also highlights the utility of zebrafish models in studying genetic NDDs. Zebrafish were used to explore the effects of genetic mutations, such as those in CLEC16A and TMX2, on brain development, revealing mechanisms like disrupted vesicle trafficking (CLEC16A) and calcium dysregulation (TMX2) that contribute to these disorders. Overall, this research emphasizes the importance of multidisciplinary approaches in clinical genetics, combining advanced molecular techniques with model organisms to improve diagnosis and potentially identify new therapeutic targets for NDDs.