The gold standard for diagnosing PD relies on the presence of substantia nigra (SN) pars compacta degeneration and Lewy pathology, as confirmed by post-mortem pathological examination[20, 21]. Pathologically, PD is a slowly progressing neurological disease that begins years before a diagnosis[22]. Diagnostic examinations that allow for clear diagnosis during the initial phases of the disease, in particular, do not exist. Therefore, there is a need to develop suitable diagnostic approaches for early diagnosis. Through bioinformatics analyses, we aimed at identifying diagnostic genes that are related to the disease.
Based on GSE7621, GSE20141, GSE20159, GSE20163, and GSE20164, RRA and batch correction were performed to identify the consensus DEGs, which were 80 in number. Among them, 71 genes were downregulated while nine genes were upregulated. Functional enrichment analysis revealed that the enriched GO terms were mainly in nerve conduction, DA metabolism, and DA biosynthesis. Pathologically, PD is associated with both neuronal dysfunction and inflammation of the central nervous system, consistent with nerve conduction disorders[23]. Dysregulated DA is more likely to play a crucial role in early onset of PD, thus, early identification of dysregulated DA should be a priority[24]. The enriched KEGG terms were also mainly in nerve conduction. Analysis of the PPI network revealed 40 hub genes, which were visualized by Cytoscape. The LASSO Cox regression model was used to assess the diagnostic genes with a high accuracy, which were SLC18A2, TAC1, PCDH8, KIAA0319, PDE6H, AXIN1, and AGTR1. CIBERSORT was used to assess immune cell infiltrations in PD. Plasma cells were found to be differentially expressed between HC and PD patients.
SLC18A2, the vesicular monoamine transporter 2, is important in neurotransmitter transportation. It packages histamine into vesicles in preparation for neurotransmitter release from the presynaptic neuron[25]. The gene, which is important in the monoaminergic signaling pathway, has been extensively researched on. In the absence of this monoaminergic transporter, histamine immunoreactivity is significantly suppressed in neuronal cell bodies of the brain[26]. Reduced histamine metabolism in the central nervous system is a preventative measure against PD onset[27]. Further elucidation of the involved mechanisms will contribute to a better understanding of the disease. Substance P (SP) dysregulation is associated with the etiology of PD. Mice lacking endogenous SP (TAC1-/-) exhibited greater resistance to nigral dopaminergic neurodegeneration than wild-type controls. The neuroinflammatory and dopaminergic neurodegenerative effects of SP are mediated by microglial NOX2[28], thus, they may shed new light on PD pathophysiology. Protocadherins, which contain PCDH8, are calcium-dependent adhesion molecules that have drawn interest for their potential functions in development of neural circuits and their potential effects on neurological illnesses. Physiologically, PCDH8 is involved in development and maintenance of intrahippocampal circuits[29]. However, the association between PCDH8 and PD has yet to be established. KIAA0319 is associated with extracellular signaling pathways[30, 31]. Extracellular signaling pathways and endocytosis are both necessary to control neurogenesis[32]. As a pivotal upstream gatekeeper, KIAA0319 plays crucial roles in neurogenesis by arresting cellular progression at the neural progenitor cell stage. Cell cycle progression is deregulated in PD, and key regulators of the G1/S transition checkpoint are significantly altered[33]. Moreover, the cell cycle is enriched in GO terms. Even though there is no conclusive proof that KIAA0319 is directly associated with PD, our findings shed light on the subject. PDE6H is associated with changes in circadian rhythms that are involved in aging[34], however, the association between PDE6H and PD has not been determined. AXIN1 was overexpressed in hippocampus tissues and cells from MPTP-lesioned mice models of PD. AXIN1 suppression in PD suppressed hippocampus neuron apoptosis. AXIN1 downregulation suppresses DA neuron death via miR-128[35]. These outcomes suggest a therapeutic potential for AXIN1 in treatment of PD. AGTR1 encodes the angiotensin II type 1 receptor (AT1R), whose expressions decreases in dopaminergic neurons of the SN as PD advances[36]. In contrast, AT1R upregulation induces the release of pro-inflammatory cytokines, leading to inflammation that culminates in dopaminergic cell death and dysfunction. Blocking AT1R with its antagonist can attenuate neurotoxin-induced degeneration of dopaminergic neurons in the SN[37, 38]. These findings underscore the complex nature of AGTR1's role in PD and highlight the need for further research.
The GO function analysis revealed that the DEGs were primarily enriched in nerve conduction, DA metabolism, and DA biosynthesis. Nerve conduction is divided into sensory nerve conduction and motor nerve conduction[39]. Regarding nerve conduction, Toth reported that individuals with PD exhibited slower motor conduction velocities, when compared to HC. Toth hypothesized that peripheral neuropathy in PD could be attributed to levodopa exposure and elevated levels of methylmalonic acid[40]. Thus, motor nerve transmission abnormalities maybe present in PD patients[41]. Pathologically, PD is characterized by degeneration of the nigrostriatal dopaminergic system. In 1988, Gotham et al. proposed the 'dopamine overdose' hypothesis[42]. The hypothesis, which proposes an impact on cognitive functions in PD, suggests that the medication doses required to restore DA functions in the most severely affected regions may be too high for less affected areas. According to this theory, the ventral striatum, which remains relatively intact, can become excessively stimulated when dopamine replacement therapy is administered. Subsequently, this overstimulation affects the limbic system, leading to impaired executive functions mediated by the limbic and orbitofrontal systems, such as learning and risk-taking. Empirical evidence from literature and clinical observations, particularly in relation to dopamine agonists, provides support for this theory and the emergence of impulse control disorders[43]. The enriched KEGG terms also included the PD and cycle of nerve conduction. The diagnostic gene, SLC18A2, is also associated with neurotransmitter transport, synaptic vesicle cycle, and dopaminergic synapses. In terms of neurotransmitter transport, mutations in SLC18A2 will impact the transmission of monoamine neurotransmitters, leading to a phenotype that shares characteristics with all monoamine-related disorders[44]. In terms of the relationship between SLC18A2 and DA, SLC18A2 is a vesicular monoamine transporter that is essential in DA regulation. Suppressed SLC18A2 activities might reduce DA release[45].
Then, CIBERSORT was used to assess immune cell infiltrations in PD. Plasma cells were found to be differentially expressed between HC and PD patients. Studies have reported on significantly changed B cell subpopulation structures in PD[46]. Given that plasma cells are derived from B cells, the abundance of plasma cells is associated with PD development. Plasma cells may affect PD pathogenesis by influencing the immune microenvironment. However, the relationship between plasma cells and PD has not been fully defined. Systematic studies should be performed to elucidate on possible mechanisms of plasma cells in PD.
This study has some limitations. First, even though we combined the datasets due to the lower number of samples in PD, studies with bigger sample sizes should be performed to confirm our findings. Second, we utilized RT-qPCR to validate the conclusions in blood samples of PD patients. Other experimental verification methods should be used to verify our results.
In conclusion, the developed diagnostic model provides new insights for early stage PD diagnosis. Plasma cells were found to be differentially expressed between HC and PD patients. Elucidation of the genetic and immunological mechanisms that underlie the initial signs of PD will unlock new therapeutic avenues. These insights will empower clinicians to effectively intervene with innovative or repurposed anti-inflammatory and immunomodulatory treatments, with the aim of slowing down the progression of this disease.