Mitochondria are subcellular organelles whose major function is to generate energy by coupling through oxidation of nutrient substrates with ATP synthesis, via ADP phosphorylation. This process, known as oxidative phosphorylation, is carried out by the mitochondrial respiratory chain, a pathway consisting of five multi-subunit complexes, four of which take contribution from genes located in two separate compartments, the nuclear chromosomes, and a genome found in mitochondria themselves, mitochondrial DNA (mtDNA). Defects affecting either genome give rise to mitochondrial dysfunction, causing disease that often affects the brain and in particular the cerebellum. Mitochondrial disorders can give rise to pure cerebellar, spinocerebellar, or sensory ataxia, usually as part of a multisystem (and multisymptom) disorder. In this chapter we divide the diseases into those caused by mtDNA defects and those due to mutations involving nuclear genes. With more than 100 mutations in mtDNA and new nuclear genes being described all the time, we have focused on the commonest disorders and used these as examples of the different types of mitochondrial ataxia.
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