Up-regulation of SNCA gene expression: implications to synucleinopathies

Abstract

Synucleinopathies are a group of neurodegenerative diseases that share a common pathological lesion of intracellular protein inclusions largely composed by aggregates of alpha-synuclein protein. Accumulating evidence, including genome wide association studies, has implicated alpha-synuclein (SNCA) gene in the etiology of synucleinopathies. However, the precise variants within SNCA gene that contribute to the sporadic forms of Parkinson's disease (PD), dementia with Lewy bodies (DLB), multiple system atrophy (MSA), and other synucleinopathies and their molecular mechanisms of action remain elusive. It has been suggested that SNCA expression levels are critical for the development of these diseases. Here, we review several model systems that have been developed to advance the understanding of the role of SNCA expression levels in the etiology of synucleinopathies. We also describe different molecular mechanisms that regulate SNCA gene expression and discuss possible strategies for SNCA down-regulation as means for therapeutic approaches. Finally, we highlight some examples that underscore the relationships between the genetic association findings and the regulatory mechanisms of SNCA expression, which suggest that genetic variability in SNCA locus is directly responsible, at least in part, to the changes in gene expression and explain the reported associations of SNCA with synucleinopathies. Future studies utilizing induced pluripotent stem cells (iPSCs)-derived neuronal lines and genome editing by CRISPR/Cas9, will allow us to validate, characterize, and manipulate the effects of particular cis-genetic variants on SNCA expression. Moreover, this model system will enable us to compare different neuronal and glial lineages involved in synucleinopathies representing an attractive strategy to elucidate-common and specific-SNCA-genetic variants, regulatory mechanisms, and vulnerable expression levels underlying synucleinopathy spectrum disorders. This forthcoming knowledge will support the development of precision medicine for synucleinopathies.

Keywords: Gene expression; Genetic regulation; SNCA; Synucleinopathies.

Figure 1. Generation of cholinergic neurons from SNCA triplication and control iPSCs lines

Induced pluripotent stem cells (iPSCs) derived from a Parkinson's disease patient with the triplication of the human SNCA genomic locus (SNCA-Tri) and from a healthy individual (Control) were differentiated to basal forebrain cholinergic neurons (BFCNs). Following forebrain patterning, neural precursors cells (NPCs) were passaged at day 16+. BFCNs were next maturated for 5-7 weeks. (a) SNCA-mRNA expression levels at different stages of the neuronal differentiation. Levels of SNCA-mRNA were measured by realtime RT-PCR and calculated relatively to the geometric mean of GAPDH- and PPIA- mRNAs reference control using the 2^−Δ_CT method. The blue bars represent SNCA-mRNA fold expression levels measured in the control cell-lines and the red bars represent SNCA-mRNA fold expression levels measured in the SNCATri lines. Immunolabeling for α-syn protein (Alexa-488) was performed in matured cholinergic neurons (BFCNs, day 35) from control (b) and SNCA-Tri (c). Immunocytochemistry was analyzed by confocal microscopy. (d) Phase-contrast micrograph of representative mature cholinergic neurons (day 50) differentiated from SNCA-Tri iPSC. The arrows indicate the neuritis outgrowth.

Figure 2. A schematic model of the contribution of SNCA expression to synucleinopathies

Changes in the SNCA expression can be attributed to different factors. The upper panel shows the cis-genetic variants across the SNCA genomic locus and their interactions with trans-acting factors, including transcription and splicing machineries and microRNAs. The middle panel presents plausible factors affecting the endpoint cellular level of α-syn protein. These factors include, but not limited to, genetic, epigenetic, environment, gender, and age; and may also interact with one another and cross-influence each other. The threshold among physiological and pathological levels of SNCA, designated in gray vertical lines, needs to be determined. The thresholds of SNCA expression levels leading to the different synucleinopathy diseases could be cell-type and disease specific. Dopaminergic neurons, cortical neurons (mainly cholinergic) and oligodendrocytes, indicated in the lower panel, may exhibit different vulnerability to α-syn overexpression. In addition, interactions with other causal genes may determine the particular disease path. Collectively, the model suggests that common and distinct regulatory mechanisms and vulnerability thresholds of SNCA gene expression underlie the etiology of synucleinopathies.