Objective: To investigate the presence of genetic mosaicism due to somatically acquired copy number variants (CNVs) affecting PD genes in PD brain.

Background: The cause of most sporadic PD cases remains unknown. Inherited mutations, including CNVs, in several genes can cause PD. Mosaicism, the presence of cells with genetic differences in an organism, can arise through post-zygotic somatic mutations. We have hypothesized that such mutations arising in known PD genes may have a role in sporadic PD (Proukakis et al, Mov Disord 2013, 28:705–12). The evidence for existence of mosaicism in healthy and diseased brain is increasing rapidly. We have so far not found any evidence of coding alpha-synuclein (SNCA) point mutations in PD brain (Proukakis et al, Mov Disord 2014, 29:1060–4), and have now focused on CNV detection.

Methods: Sporadic cases with relatively young onset and short disease duration were selected, and aged controls with negative alpha-synuclein histochemistry were studied in parallel. Brain DNA from up to 3 regions (frontal cortex, midbrain, cerebellum) was analysed using: (1) custom array comparative genomic hybridisation (aCGH), focused on PD genes; (2) droplet digital PCR (ddPCR) for SNCA copy number, with repeat if appropriate using a different reference gene and new DNA extraction. We have demonstrated that the aCGH can detect 15% change in copy number (eg a heterozygous gain in 30% of cells, equivalent to copy number 2.3), and expect similar or better sensitivity for ddPCR. In addition, preliminary fluorescent in situ hybridisation (FISH) using bacterial artificial chromosome probes for SNCA on frozen brain nuclear suspensions was performed.

Results: DNA analysis of at least one brain region in 26 PD cases (including midbrain in 22) and 13 controls (including midbrain in 10) was negative for CNV mosaicism in all PD genes by aCGH, and SNCA by ddPCR. FISH of midbrain suspensions from 8 cases and 5 controls showed occasional nuclei with possible SNCA gains.

Conclusions: We have found no evidence of high level CNV mosaicism in brain using aCGH for PD genes, and additionally ddPCR for SNCA. Our results do not exclude lower level mosaicism, which could still be biologically relevant, loss of susceptible neurons carrying somatic CNVs, or sampling bias within the brains tested. Preliminary FISH data suggest possible very low level SNCA CNV mosaicism, but further technical improvements are required for a definitive conclusion.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/investigation-of-mosaicism-for-copy-number-variants-in-pd-brain/