Clinical and Genetic Characterization of Inherited Myopathies and Mitochondrial Diseases

Inherited myopathies and mitochondrial diseases are rare genetic disorders leading to premature death and morbidity. Until recently the diagnostic approach used a combination of clinical assessment and muscle biopsy analysis to guide Sanger sequencing of individual candidate genes; however the high number of causative genes and the broad spectrum of phenotypes have made the diagnostic process slow and challenging. The advent of next generation sequencing (NGS) has revolutionised the diagnostic approach for inherited conditions allowing the screening of multiple genes in parallel. Despite the potential utility of broad NGS screening there are still some limitations, in particular the difficulty in the interpretation of the large amount of data obtained with multiple genetic variants of unknown significance (VUS) per patient. This thesis describes research that aimed to better understand the genetic basis of inherited myopathies and mitochondrial diseases and their correlation with the clinical manifestations. First, I used NGS technology to screen a broad cohort of inherited myopathies and I evaluated the frequency of genetic diagnoses, clinical and pathological factors most likely to be associated with a positive diagnosis and clinical pitfalls. Second, I screened undiagnosed mitochondrial cases attending Mitochondrial Service at National Hospital for Neurology and Neurosurgery using NGS and I identified MRPS25 gene mutation as a potential new cause of mitochondrial disease. Consequently I performed complex functional studies to demonstrate an impairment of mitochondrial translation caused by MRSP25 gene mutation. Finally, I studied new genotype-phenotype correlations that can guide future genetic analysis in clinical practice. These include ITGA7 mutation associated with cardiac arrhythmia, RNASEH1-related mitochondrial disease and encephalomyopathy caused by MT-ATP6 truncating mutations