Byline: Chiara. Di Resta, Giovanni. Pipitone, Paola. Carrera, Maurizio. Ferrari
Next generation sequencing is currently a cornerstone of genetic testing in routine diagnostics, allowing for the detection of sequence variants with so far unprecedented large scale, mainly in genetically heterogenous diseases, such as neurological disorders. It is a fast-moving field, where new wet enrichment protocols and bioinformatics tools are constantly being developed to overcome initial limitations. Despite the as yet undiscussed advantages, however, there are still some challenges in data analysis and the interpretation of variants. In this review, we address the current state of next generation sequencing diagnostic testing for inherited human disorders, particularly giving an overview of the available high-throughput sequencing approaches; including targeted, whole-exome and whole-genome sequencing; and discussing the main critical aspects of the bioinformatic process, from raw data analysis to molecular diagnosis.
So far, more than 7000 rare mendelian disorders are described and half of them affect the peripheral and central nervous system. The inherited neurological disorders are a group of heterogenous diseases from clinical and genetic point of view, often characterized by progressive and severe disability (Warman Chardon et al., 2015). The wide range of clinical manifestation includes ataxias, encephalopathies, genetic form of brain malformations, myopathies and muscular dystrophies, neuropathies and form of dementia. There is a significant phenotypic overlap between different forms of neurological diseases. For example, Emery-Dreyfuss muscular dystrophy can present similar proximal muscular weakness than the limb girdle muscular dystrophies, making sometimes difficult the precise diagnosis. From genetic point of view, the identification of the causative mutation can be very challenging, due to the heterogenous genetic nature of these disorders (Vgontzas and Renthal, 2019). For example, more than 300 genes are associated with ataxia or more than 50 genes are causative of the hereditary spastic paraplegia. On the other hand, there are rare neurological diseases for which the genetic basis is still unknown. Moreover this heterogenous picture is often characterized by reduced penetrance, variable onset and variable expressivity (Fogel, 2018). For that reasons, many patients affected by rare neurological disorders spend many years before receiving a molecular diagnosis or remain genetically undiagnosed (Adams and Eng, 2018).
For patients with rare neurological disorders, the rapid and correct diagnosis can reduce the time from onset of symptoms to medical treatment, reducing multiple specialists' visits, number of different clinical exams and avoiding ineffective medical treatments.
In this scenario, a precise molecular diagnosis may have several benefits on patient care. For example, the accurate genetic characterization can be useful for the prevention harmful immunosuppressant therapy in patients affected by progressive muscular dystrophies presenting like an inflammatory myopathy (Adams and Eng, 2018). Other example is the importance of the detection of causative mutations in the GAA gene associated with Pompe disease, that allow a timely enzyme replacement therapy that can significantly improve muscle strength and reduce mortality (Chan et al., 2017). Therefore, an early and accurate molecular diagnosis can be fundamental to initiating the timely and optimal treatment for patients affected by rare neurological disease....