Synuclein Proteins in Cancer Development and Progression

Abstract

Synucleins are a family of small, soluble proteins mainly expressed in neural tissue and in certain tumors. Since their discovery, tens of thousands of scientific reports have been published about this family of proteins as they are associated with severe human diseases. Although the physiological function of these proteins is still elusive, their relationship with neurodegeneration and cancer has been clearly described over the years. In this review, we summarize data connecting synucleins and cancer, going from the structural description of these molecules to their involvement in tumor-related processes, and discuss the putative use of these proteins as cancer molecular biomarkers.

Keywords: aggregation; biomarkers; cancer; pathways; synucleins.

Figure 1

Structure of synucleins, phylogeny, and expression in human tissues. (A) Multiple sequence alignment of human synucleins by Clustal W. Sequences were obtained from NCBI (accession numbers CAG33339.1; CAG33308.1 and CAG46587.1). “*” indicates identical amino acids; “:” and “.” indicate conserved and semi-conserved residues, respectively. Each synuclein is organized in a tripartite arrangement, with the N-terminal region (light violet), the central NAC region (dark blue), and the C-terminal region (light green). Amino acids involved in aggregation are marked in light blue. 11-mer repeats are underlined. Amino acid numbers are displayed at right. (B) Human synuclein structures obtained by Pymol. αS full-length protein structure was obtained from PDB (Protein Data Bank) with the accession code 1XQ8 (https://doi.org/10.2210/pdb1XQ8/pdb (accessed on 19 April 2023)), the unique membrane-bound structure of synuclein family known. The UniProt accession codes Q16143 and Q6FHG5 were used to predict βS and γS full-length protein structures using AlphaFold that draw on structural models from previously determined structures (meaning that these models are not the most accurate) and obtained finally in PDB format, which was used then to exemplify βS and γS structure in Pymol. Different regions of the proteins are highlighted in colors, according to (A). (C) Evolutionary tree of synucleins. αS, βS, and γS found in different branches of jawed vertebrates are shown. As synucleins were not reported in Caudata (amphibians) and Sphenodon (reptilia) genders, they are not represented. (D) Mammal synuclein genomic tree. The common node between αS (pink) and βS (green) is shown in blue. The γS (violet) clade is shown in black. Homo Sapiens taxa are highlighted in red. FASTA files were downloaded from NCBI. Sequence alignment was generated with Molecular Evolutionary Genetics Analysis software (https://www.megasoftware.net/ (accessed on 16 April 2023)). The 311 sequences aligned were uploaded to the IQTREE WEB SERVER (http://iqtree.cibiv.univie.ac.at/ (accessed on 21 April 2023)); the best fit model according to AICc (Second-order Akaike’s information criterion) JTT+G4 was used. The tree was generated using FigTree software (http://tree.bio.ed.ac.uk/software/figtree/ (accessed on 22 April 2023)). (E,F) Protein (E) and mRNA (F) synucleins expression in human tissues. Expression information was obtained from The Human Protein Atlas (https://www.proteinatlas.org/ (accessed on 10 April 2023)) and eighteen tissues were selected for plotting. Neuronal tissues are highlighted in violet. H: high; M: medium; L: low; ND: not detected; nTPM: normalized transcript per million.

Figure 2

Synuclein expression in cancer. (A) To identify synuclein-associated gene clusters, a search was conducted in the NCBI database, limited to Homo sapiens genes. A total of 323, 22, and 49 genes related to αS, βS, and γS, respectively, available in the NCBI database (accessed on 27 April 2023), were downloaded. Using these gene clusters, an enrichment analysis was performed, and a word cloud was generated to display the most common terms found in the results. Word size in the clouds is proportional to the frequency of occurrence in the over-representation analysis (p > 0.05), and the thickness of the lines connecting clouds represents the number of shared terms. (B–D) Expression level of synucleins in tumor samples and noncancerous (normal) samples through GEPIA2 database (http://gepia2.cancer-pku.cn/ (accessed on 10 April 2023)). Light gray represents normal tissues (N) and light pink tumor tissues (T). The expression levels on the Y axis are expressed as log2(TPM + 1), TPM: transcript per million. The statistical analysis performed is t-test. * p < 0.01. (COAD: Colon adenocarcinoma; LUAD: Lung adenocarcinoma; PAAD: Pancreatic adenocarcinoma; SKCM: Skin Cutaneous Melanoma; GBM: Glioblastoma multiforme; LGG: Brain Lower Grade Glioma; BRCA: Breast invasive carcinoma; and OV: Ovarian serous cystadenocarcinoma). (n = number).

Figure 3

Synucleins are involved in several cancer-related cellular processes. Graphic scheme illustrating key cancer-related cellular processes described for synucleins. The figure shows the main processes involved in a central cell and two neighbors. Light pink boxes represent processes involving αS; light violet boxes represent γS processes; light orange boxes represent processes in which both proteins are involved. αS involvement in autophagy, mitochondrial metabolism, and the generation of ROS are well-established. Additionally, different aggregation states of αS play important roles in cancer. γS levels in tumor cells regulate microtubule dynamics, cell–cell adhesion, apoptosis, and influence signaling pathways involving ERK and AKT. Both synucleins were reported to be involved in cell proliferation and migration. In addition, both proteins can be secreted and transmitted to neighboring cells as monomer or oligomer through different mechanisms (by exosomes release, cross-cell membrane by membrane diffusion, and/or attaching proteins that function as membrane receptors), inducing the aggregation and accumulation of cytosolic synuclein in the proximal cell via prion-like properties, which could have important implications in cancer progression. Depicted organelles and structures include mitochondria, autolysosomes, exosomes, membrane proteins, adhesion proteins, and the cytoskeleton (tubulin and actin filaments). (MMP: extracellular matrix metalloproteinases, ROS: mitochondrial reactive oxygen species).

Figure 4

Correlation between synuclein expression and overall survival. (A–C) Association between synuclein expression and overall survival (OS) using TCGA (The Cancer Genome Atlas) mRNA expression datasets for different tumors. Kaplan–Meier curves for OS of cancer patients with low (gray line) versus high (violet line) expressions of αS (A), βS (B), and γS (C) were generated using Survminer R package (version 0.4.9) (p < 0.05) and compared by log-rank tests. The number of patients for each case is described in the figure (n). The x-axis depicts time in days. (COAD: Colon adenocarcinoma; LUAD: Lung adenocarcinoma; SKCM: Skin Cutaneous Melanoma; LGG: Brain Lower Grade Glioma; BRCA: Breast invasive carcinoma; PCPG: Pheochromocytoma and Paraganglioma; UCEC: Cervical squamous cell carcinoma and endocervical adenocarcinoma).