Dottorato in NEUROSCIENZE CLINICO-SPERIMENTALI E PSICHIATRIA

Titolo della tesi: Semiological and Etiopathogenetic description of a cohort of subjects affected by ataxia and cerebellar-related disorders: the impact of Next Generation Sequencing techniques in the diagnostic work-up and expansion of phenotypic spectra

Background: Hereditary ataxia (HA) is a group of disease characterized by clinical and genetic heterogeneity. Coordination disorder is the most common clinical sign, that can be isolated or part of a neurodevelopmental disorder. Seizures, movement disorder, intellectual disability, extra-neurological involvement, can be present. Cerebellar dysgenesis and/or spinocerebellar traits involvment are neuroimaging features often associated with clinical phenotype. In other cases, cerebellar dysgenesis is detected without obvious ataxic disorder. Next Generation Sequencing (NGS) techniques have broadened knowledge on the genetic causes of HA. Studies evaluating NGS efficacy in diagnosing hereditary ataxia reported a diagnostic rate between 11-82% (mean value 19.4%) for targeted panels, 20-57% (mean value 34.6%) for exome sequencing. Aim of the study: to characterize clinical phenotype of subjects with undiagnosed ataxia and cerebellar related-disorders, to define molecular diagnosis, and to evaluate the diagnostic role of Next Generation Sequencing techniques. Patients and methods: we collected 39 subjects (22 males and 17 females; mean age 16.7± 10.7, range 2-52 years) with undiagnosed ataxia and cerebellar related disorders, underwent to Next Generation Sequencing in the past three years. For each subject other neurological signs (developmental delay, seizures, movement disorder, intellectual disability) and extraneurological involvment were evaluated. In patients with a specific clinical phenotype targeted panel was used; in the more complex phenotypes or in cases in which the targeted panels had given negative result, exome sequencing was performed. Results 39 subjects were analysed (22 males and 17 females); the results are available for 34 patients; 27 patients received a definitive genetic diagnosis (mean age at diagnosis 15,8± 11,7, range 2-49 years); 26 subjects received molecular diagnosis with NGS techniques (26/34, 76%), and a patient received the definitive diagnosis after further review of array-CGH, which was initially reported negative. DNA repair related disorders (6/27) and Spinocerebellar Ataxia (7/27) are the two main categories of diagnosed disorders. In the first category are included patients with atypical features compared to the classical phenotype described in literature, in the second one developmental delay is the onset recurrent clinical sign. In both categories, diagnosis by a single-gene approach would have been difficult. Conclusions NGS approach significantly facilitates molecular diagnosis of hereditary ataxias, and cerebellar-related disorder. Patients selection, functional studies of variants of uncertain significance, type of approach used (targeted panel, WES, CES, WGS), number of genes included in targeted panels, re-assessing the variants identified, influence diagnostic result of NGS techniques. This approach can expand clinical and mutational spectrum of known genes and attribute a pathogenetic role to new genotypes.