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 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 twofold increased presence 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, we used immunofluorescence for γH2AX in a subset of experiments. The proportion of γH2AX-positive cells was significantly higher in MSA than controls overall (4.9% vs 2.5%, p = 0.01), in oligodendrocytes (9.7% vs 2.5%, p = 0.024) and in other cells (3.2% vs 1.1%; p = 0.048). The proportion of γH2AX-positive cells was also higher in preferentially affected regions (6.3% vs 3.5%, p = 0.004) and 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.