Gene discovery and cellular modelling of rare autosomal recessive neurodevelopmental conditions

Neurodevelopmental disorders (NDD) are a group of heterogenous conditions characterised by global developmental delay with additional neurological defects. While some NDDs display non-specific brain phenotypes, some NDDs may have particular distinguishing neurological phenotypes, such as agenesis of the corpus callosum or hypomyelination. This project aimed to delineate novel genetic causes of neurodevelopmental conditions using next generation sequencing (NGS) technologies in a family-based approach. As part of this study, thirteen families were recruited, with at least three individuals per family sequenced using whole exome sequencing (WES). Standardized bioinformatics pipeline analysis was carried out, prioritising variants based on segregation, in silico pathogenicity prediction tools and biological relevance. Known and candidate genes were successfully identified in the majority of the cases. A nonsense variant was identified in HERC2 in a family with severe global developmental delay. This finding provided a molecular diagnosis for this patient and expanded the known phenotype-genotype correlation associated with HERC2 mutations. In addition, a de novo variant in TUBA1A was discovered in a family with a clinical diagnosis of a Complex Moebius syndrome and perisylvian polymicrogyria and slight callosal dysmorphism. This finding further expanded the known phenotype caused by TUBA1A mutations. This study was able to establish new genotype-phenotype correlations that will be beneficial for future clinical diagnosis and patient care. Novel genetic causations for NDDs were established for the first time by the discovery of mutations in two disease genes. A nonsense variant in KLHL7 was identified as a cause of NDD. Specifically, the affected individuals presented with a phenotype similar to 4H syndrome with additional features of myopathy, stoke-like episodes, microcephaly and abnormal sweating. The second genetic discovery was a missense variant in MAL identified in a family with hypomyelinating leukodystrophy similar to Pelizaeus- Merzbacher disease. The variants in HERC2, KLHL7 and MAL were modelled in vitro and functionally characterized as pathogenic by using cellular and biochemical approaches. The functional studies have provided novel insights into the disease mechanisms of NDD. For instance, the functional characterization of the variant in MAL determined its role in the process of myelination. This finding expands our current knowledge of the mechanisms and proteins involved in myelin development. In the post-genomic era, it is crucial to characterise variants identified in rare autosomal recessive conditions, as they offer us a unique opportunity to gain further understanding of disease mechanisms and biological processes that would otherwise remain ambiguous.