[show abstract][hide abstract] ABSTRACT: Defects in the mitochondrial DNA replication enzyme, polymerase γ, are an important cause of mitochondrial disease with ∼25% of all adult diagnoses attributed to mutations in the POLG gene. Peripheral neuronopathy is often part of the clinical syndrome and can represent the most disabling feature. In spite of this, the molecular mechanisms underlying the neuronopathy remain to be elucidated and treatment strategies are limited. In the present study, we use a combined approach comprising clinical, electrophysiological, neuropathological and molecular genetic investigations to unravel the mechanisms underpinning peripheral neuronopathy in autosomal recessive polymerase γ-related disease. Electrophysiological assessments documented a dorsal root ganglionopathy in all 11 cases. Of the 11 cases, eight also showed changes consistent with motor fibre loss. Detailed neuropathological investigation of two patients confirmed the electrophysiological findings, revealing atrophy of posterior columns and striking neuronal cell loss from the dorsal root ganglia, which was accompanied by severe mitochondrial biochemical abnormalities involving respiratory chain complexes I and IV due to clonally-expanded mitochondrial DNA deletions and a significant reduction in mitochondrial DNA copy number in affected neurons. We propose that the respiratory chain defects, secondary to mitochondrial DNA deletion and depletion, are likely to be responsible for pathology observed in the dorsal root ganglion and the sensory ganglionopathy documented electrophysiologically.