Up to a third of adults attending the Queen Square UK NHS Specialised Service for Rare Mitochondrial Disease out-patient clinic remain genetically undetermined. This thesis describes research which aimed to establish the molecular basis of mitochondrial disease in these patients and evaluate genotype/phenotype correlations. Both novel clinical syndromes and molecular causes of disease were identified. Furthermore, new insights into the respiratory chain (RC) protein structure were elucidated. Novel clinical phenotypes: Three previously unrecognised mitochondrial disease phenotypes were characterised. First, m.9185T>C in MT-ATP6, encoding subunit 6 of ATP synthase (complex V), was detected in a pedigree exhibiting matrilineal inheritance. Presentation was with axonal Charcot-Marie-Tooth (CMT2) disease. Further screening of 270 patients with genetically unclassified CMT2 demonstrated three additional families harbouring the same mutation, thus proving a causal link between reduced complex V activity and impaired axonal function. Second, a severe distal myopathy was observed in two unrelated patients with de novo dominant POLG mutations. Finally, COX10 mutations were linked to adult cytochrome c oxidase (COX) deficiency. Despite a complex multisystem phenotype comprising short stature, proximal myopathy, fatigue, sensorineural hearing loss, pigmentary maculopathy, renal Fanconi syndrome and premature ovarian failure, the patient’s clinical severity was considerably milder than fatal COX10-related infantile disease. Nuclear gene mutations: Three major experimental strategies were employed to locate mutations in the nuclear genes of adults with clinically and/or biochemically suspected mitochondrial disease. First, a candidate gene approach identified RRM2B mutations in 2/33 patients with multiple mitochondrial DNA deletions. Second, whole-exome sequencing confirmed COX10 mutations can cause adult mitochondrial disease. Finally, combined homozygosity mapping/whole-exome sequencing in a consanguineous family led to the discovery that NDUFA4 mutations cause COX-deficient Leigh syndrome. This example of ‘back-translation’ led to the discovery that NDUFA4, previously considered to be a complex I subunit, is actually an important component of the COX enzyme complex
