Delineating the mechanisms of cerebellar degeneration in paediatric and adult primary mitochondrial disease

Cerebellar ataxia is a frequent, debilitating neurological manifestation of primary mitochondrial disease and is associated with extensive neurodegeneration of the cerebellar cortical circuitry. However, the precise neuropathological mechanisms resulting in cerebellar degeneration in paediatric and adult forms of mitochondrial disease remain unclear. We therefore sought to perform a comparative neuropathological study using post-mortem cerebellar tissues from 28 paediatric and adult patients with pathogenic bi-allelic POLG variants and pathogenic mitochondrial DNA variants (m.3243A > G, m.8344A > G, m.13094T > C, and m.14709T > C), in addition to 18 neurologically normal control cases. We also sought to assess the prevalence and progression of cerebellar ataxia in an adult mitochondrial disease patient clinical cohort (n = 310) harbouring the same pathogenic variants as the post-mortem cases. Analysis of the clinical patient cohort revealed that at least 23.5-39.7% of adult patients with primary mitochondrial disease had predominantly cerebellar ataxia, with disease progression evident in 38.8% of patients. In the mitochondrial disease post-mortem tissue cohort, there was clear evidence of selective loss of inhibitory Purkinje cells, with corresponding oxidative phosphorylation protein deficiencies, which were more severe in comparison to mainly excitatory neuronal populations of the granule cell layer and dentate nucleus. Remaining Purkinje cells also demonstrated an increased expression of mitophagy-related proteins, including LC3B and BNIP3. Focal necrotic cerebellar cortical lesions, identified in eight patients, were characterised by decreased parvalbumin immunoreactivity, and sporadic c-Fos immunoreactivity was observed throughout the cerebellar cortices of 14 patients, suggestive of cerebellar cortical hyperactivity. Overall, these neuropathological features were more severe in the early onset POLG-related disease group and patients who had epilepsy. Our findings provide an important insight to the pathological mechanisms contributing to the degeneration of the cerebellar cortex in paediatric and adult forms of primary mitochondrial disease, highlighting an increased burden of pathology in early onset POLG-related disease which may have important prognostic and therapeutic implications.