Investigation of somatic mutations in human brains targeting genes associated to Parkinson's disease

Somatic mutations in neurons may underlie vulnerability to degeneration or initiate a 'prion-like' spread of proteins causing aggregation and disease. The role of somatic mutations in synucleinopathies is still unknown. Our work aimed to identify disease-relevant low-level somatic single nucleotide variants (SNVs) in brains with synucleinopathies. Most of the cases were sporadic, trying to exclude as many cases with late-onset or long disease as possible, and including a monozygotic twin carrying a LRRK2 mutation, whose phenotypic discordance could be explained by somatic variation. We included different brain regions from 27 Parkinson's disease (PD), 12 control, 1 Incidental Lewy body and 3 multiple system atrophy cases. For variant detection, exons of genes associated with PD and neurodegeneration were sequenced at high-coverage using molecular barcodes to improve accuracy. We selected for validation 21 potentially pathological variants at 0.33-5% allele frequencies, using deeper sequencing at the variant sites. Two variants validated by this method were tested by droplet digital PCR assays. We could not detect somatic SNVs in PD-associated genes, however we cannot exclude their presence in cells dying at earlier stages of degeneration. Another possibility is that variants exclusively present in dopaminergic neurons from degenerated brains may be present at even lower levels than 0.33% in bulk-tissue. To address this possibility, single-cell sequencing in sorted dopaminergic neurons would be adequate. Our results further support studies suggesting that somatic SNVs in genes associated with neurodegeneration can be rare, however other types of somatic variation with pathological consequences may occur in synucleinopathies.