Oligodendroglial somatic SNCA copy number gains are associated with inclusions and disease onset in multiple system atrophy

Multiple system atrophy (MSA) is a rapidly progressive synucleinopathy of unknown aetiology with neuronal and oligodendroglial α-synuclein inclusions. We previously reported somatic copy number variants (CNVs), specifically gains, of SNCA (encoding α-synuclein) in MSA brains. Here, we expand on this work by combining fluorescent in situ hybridisation for SNCA on isolated nuclei with α-synuclein and SOX10 immunofluorescence, to assess oligodendrocyte-specific SNCA gains and losses, and their relationship with inclusions across differentially affected regions in two MSA subtypes: striatonigral degeneration (SND) and olivopontocerebellar atrophy (OPCA). Analysis of 13 SND, 12 OPCA, and 15 control brains demonstrated significantly higher somatic SNCA CNVs, both gains and losses, in MSA oligodendrocytes compared with controls (gains: 6.3% vs 2.0%; losses: 12.5% vs 7.4%, p < 0.0001 for both). Oligodendrocyte SNCA gains were high in the preferentially affected regions (putamen in SND, cerebellum in OPCA), where they were associated with a two-fold increased relative risk of α-synuclein inclusions in the same cell (p < 0.0001). Higher gain burden correlated with earlier disease onset (rho = -0.45, p = 0.03). Oligodendrocyte SNCA losses, conversely, showed less regional predilection, limited association with inclusions, and no correlation with onset age. As double strand DNA breaks have been reported in Lewy body diseases, and may cause deletions, we used immunofluorescence for γH2AX to explore their prevalence in MSA in a subset of experiments. The proportion of γH2AX-positive cells was significantly higher in MSA than controls, both overall (6.0% vs 2.2%, p = 0.049) and in oligodendrocytes (14.4% vs 5.5%, p = 0.02), and also in inclusion-bearing cells (22.2% vs 14.9%, p = 0.02). These findings define the oligodendrocyte-specific patterns of somatic SNCA CNVs in MSA, support a role for gains in MSA pathogenesis, and demonstrate the presence of SNCA losses and DNA double strand breaks which require further investigation.