Whole exome sequencing of patients with neuronal migration disorders

Neuronal migration is a phenomenon that underlies the formation of a human brain. Each neuron is born in the proliferative ventricular zones, but must journey to its final location either radially or tangentially. If this migration is impaired there are devastating consequences; cortical malformations, accompanied by epilepsy and/or mental retardation. Diseases such as, lissencephaly, double-cortex syndrome, periventricular nodular heterotopia (PVNH), and polymicrogyria are all a consequence of impairments in neuronal migration. In addition some focal cortical dysplasias (FCDs) are also thought to be due abnormalities in the migration of neurons. Initially the genes responsible for these disorders (e.g. DCX, LIS1, FLNA) were identified by positional cloning, however, this is becoming increasingly difficult. Few large pedigrees remain to be mapped and most of the unexplained cases are sporadic in nature. Moreover, it is clear that many more causative genes remain to be identified. Only 25% of cases of sporadic PVNH are attributable to mutations in FLNA; for 80% of polymicrogyria cases the genetic lesion is unknown; and at most 5% of FCDs have been associated with a genetic disruption in the tuberous sclerosis complex. The central hypothesis of this research project is that the application of whole exome sequencing will permit the identification of new de novo mutations in genes that cause NMDs. It is further hypothesised that Type-1 focal cortical dysplasias are a consequence of somatic mutations in known or unidentified NMD genes and this results in a clonal lineage with a defect in neuronal migration. To test these ideas our international network of clinical geneticists, paediatric neurologists, neurosurgeons; and molecular geneticists will: 1. Collect DNA samples and detailed clinical and neuroimaging data from 100 patients with NMDs and 30 patients with Type-1 FCDs; 2. Undertake whole exome sequencing of 20 patient-parent trios where the affected individual presents with a sporadic NMD; 3. Investigate if somatic genomic lesions cause Type-1 FCDs by performing whole exome sequencing on affected brain tissue from 20 patients who have undergone epilepsy surgery; 4. Undertake bioinformatic and functional investigations of genes in which we find de novo mutations; 5. Establish robust genotypic-phenotypic correlations for these new genes by identifying and characterising additional patients through existing international collaborative networks. In total we expect to identify between 2-5 new genes that cause NMDs which will aid genetic diagnosis and enhance patient care. In addition we expect our work will yield novel insights in the genetic and pathophysiological mechanisms that cause NMDs, which will provide a rational basis for drug design and innovative treatment approaches in the future.

Neurodevelopmental disorders like autism, epilepsy, and intellectual disability affect millions, are associated with large socio-economic costs, and dramatically impair the quality of life of affected individuals. For instance more than 6 million European citizens suffer from epilepsy, with 400,000 new cases each year, at a cost of more than 20 billion euros per a year. These diseases can result from defects in the proliferation, migration and maturation of neurons at early stages of human development due to genetic mutations. Some of the causative genes have been identified (e.g. DCX, LIS1, TUBA1A and FLNA) but in many cases the underlying mutation is unknown. In this study we collected DNA samples from patients with neurodevelopmental disorders, and compared their DNA sequence to their unaffected parents. Working with collaborators around this globe, we were able to identify a number of new genes that cause epilepsy and intellectual impairment that are associated with abnormalities in neuronal migration and survival. These genes include DENR, C6orf70, NEDD4L, TUBB5 and MAST1. By studying these genes in mice we were able to gain insight into how mutations in these genes cause disease. This work will assist with the diagnosis of patients and lays the groundwork for developing treatments in the future.