Utilisation du séquençage à haut débit dans l’identification des gènes prédisposant à l’épilepsie et aux syndromes neurocutanés

Epilepsy is a common neurological disorder affecting 1-3 % of the population. It is characterized by transitory neurological symptoms caused by excessive or synchronized abnormal neuronal activity—a seizure. Many factors can contribute to the etiology of epilepsy. It can be caused by genetic factors, morphological abnormalities, altered levels of biochemical molecules, excessive immune response in the brain, or by infections. Importantly, these factors are not mutually exclusive and can be caused by genetic aberrations. Thus, epilepsy associated with genetic mutations can be part of a complex symptomatology including several monogenic disorders such as progressive myoclonus epilepsies and neurocutaneous syndromes. The first objective of this thesis is to use modern sequencing methods in order to identify genetic causes of rare forms of epilepsies and neurocutaneous syndrome. Firstly, we studied Kufs disease, an adult form of neuronal ceroid lipofuscinosis, part of the progressive myoclonus epilepsies. The study of two American families and one sporadic case led us to identify two causative mutations (p.L116del, p.L115R) in the gene DNAJC5. Other families with this rare but fatal syndrome were reported with mutations in that gene, corroborating our findings. The gene DNAJC5 encodes for the cysteine string protein alpha (CSPα), a presynaptic protein involved in neurodegeneration. Secondly, using the same methods, we discovered the causative mutation for the Giroux-Barbeau syndrome, a neurocutaneous syndrome characterized by spinocerebellar ataxia and erythrokeratodermia variabilis. Thus, following the study of a large French Canadian family, we found the p.L168F mutation in the gene ELOVL4. This gene encodes an enzyme involved in the metabolism of very long chain fatty acid. These fatty acids participate in a wide variety of physiological functions, including skin barrier formation and peroxisome β-oxidation. The second objective of this work is to use whole exome sequencing in the context of molecular diagnostics. To achieve that, we studied a large family presenting skin lesions at the extremities following exposure to cold. Initially diagnosed as a primary form of cryofibrinogenemia, our work showed that these patients were affected with an atypical form of chilblain lupus. In turn, the phenotypical resemblance as well as the presence of the p.D18N mutation in the gene TREX1 led to that new diagnosis. This study shows the role of whole exome sequencing in the diagnosis of rare disease and widen the phenotypic spectrum associated with TREX1 mutations. The last objective of this research is to apply whole exome sequencing in the discovery of genetic mechanisms predisposing to genetic generalized epilepsy (GGE). For this study, we recruited 7 large families with GGE. We showed that variants found in the genes GLI2, MBD5, CELSR2, CELSR3, TNIK and CACNA1G could participate in the genetic etiology in our families. However, because of the numbers of variants identified, other factors could contribute to the epileptic phenotype in our families. Other large scale studies are necessary in order to validate the implication of those genes in epilepsy.

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