Nicholas Sears

Introduction. Parkinson’s Disease (PD) is a progressive neurodegenerative disease characterized by loss of dopaminergic innervation in the substantia nigra pars compacta (SNc).^1,2 The initial manifestations of early PD are subtle and are often overlooked; however, as the disease progresses, symptoms of PD gradually worsen until eventually they become so debilitating that the patient is no longer able to remain functionally independent. Clinical diagnosis of PD is extraordinarily difficult as the disease often mimics other neurological disorders, and there are very few (if any) reputable biomarkers to aid in diagnosis. Indeed, it is estimated that as many as 25-30% of PD cases are misdiagnosed upon postmortem autopsy. Although routine imaging studies (such as CT or MRI) may be used to rule out other neurologic conditions, radiographic changes of the brain in PD are not seen until end-stage progression. Alternatively, a PET scan using fluorodopa radiotracers may be used to analyze nigrostriatal dopamine function in order to confirm the diagnosis of suspected PD, however this is not routinely done due to cost and/or accessibility.¹  Early diagnosis of PD is crucial in beginning treatment early and limiting disease progression, and if not identified in the initial stages, the disease may advance past the point of current therapy. This provides a need for seeking early diagnostic and therapeutic strategies to delay the onset and slow the exacerbation of disease symptoms.² To aid in early diagnosis of PD, several potential biomarkers have been proposed. One such proposal is the assessment of circRNA expression patterns.^1,3 CircRNA is a class of non-coding RNA molecules that form chemically-stable circular structures via by a process called backsplicing, in which the 3′ and 5′ ends of the RNA strand become covalently linked together to form a loop-like shape. Because they do not contain free 3′ or 5′ ends, circRNAs are resistant to exonuclease degradation and are thus stable enough to be found circulating in various bodily fluids, such as plasma, saliva, and urine.^1,2,5 CircRNAs are believed to be involved in post-transcriptional regulation of gene expression through a variety of mechanisms and have been implicated in a number of different diseases. As they are highly stable compared to linear RNA molecules, circRNAs serve as promising candidates for studying patterns of dysregulated gene expression and causes of cellular dysfunction in diseased neurons.^1,2,3,4,5  Methods. RNA samples were extracted from patients previously diagnosed with PD and replicated with RT-qPRC, using various circRNA-specific primers. Those samples were then analyzed using microarray assays and compared to samples from healthy, non-PD control groups.  Results. Various circRNA levels were found to be differentially expressed in patients with PD compared to those without. Many of the parent genes found to be dysregulated in PD patients are involved in cellular processes vital to neuron functioning, such as cell signaling, membrane trafficking, protein crosslinking, axon guidance, and regulation of membrane action potentials. It was found that as the course of PD progressed, the number of dysregulated genes and abnormal circRNA transcripts increased dramatically. Of the thousands of different genes analyzed, six circRNA molecules were found to be abnormally expressed amongst all five stages of the disease, and four of these six transcripts rapidly increased with disease progression. Interestingly, circRNA levels were found to be substantially increased in males with PD compared to females with PD, yet there was no statistically significant difference in circRNA expression patterns between different age groups at time of diagnosis.  Conclusion. Although the use of circRNAs assays in detecting diseases is still in early infancy, it has shown excellent promise as a potential biomarker in the diagnosis and pathogenesis of early PD. Unlike protein biomarkers (e.g. alpha-synuclein), some circRNAs have been shown to reflect disease progression over time, and the use of these assays do not require invasive procedures (such as a lumbar puncture) to obtain. In addition to serving as early diagnostic biomarkers, the use of circRNAs can further our understanding of dysregulated cellular/molecular functions and abnormal gene expression involved in the pathogenesis of PD. Analysis of circRNA and linear mRNA transcripts and their translated gene products may, in turn, lead to more targeted treatments compared to what is currently available.

1. Ravanidis, Stylianos et al. “Differentially Expressed Circular RNAs in Peripheral Blood Mononuclear Cells of Patients with Parkinson’s Disease.” Movement disorders : official journal of the Movement Disorder Society vol. 36,5 (2021): 1170-1179. doi:10.1002/mds.28467
2. Zhong L, Ju K, Chen A, Cao H. Circulating CircRNAs Panel Acts as a Biomarker for the Early Diagnosis and Severity of Parkinson’s Disease. Frontiers in Aging Neuroscience. 2021;13. doi:10.3389/fnagi.2021.684289
3. Hanan M, Simchovitz A, Yayon N, et al. A Parkinson’s disease CircRNAs Resource reveals a link between circSLC8A1 and oxidative stress. EMBO Mol Med. 2020;12(11):e13551. doi:10.15252/emmm.202013551
4. Jia E, Zhou Y, Liu Z, et al. Transcriptomic Profiling of Circular RNA in Different Brain Regions of Parkinson’s Disease in a Mouse Model. Int J Mol Sci. 2020;21(8):3006. Published 2020 Apr 24. doi:10.3390/ijms21083006
5. Wang M, Hou J, Müller-McNicoll M, Chen W, Schuman EM. Long and Repeat-Rich Intronic Sequences Favor Circular RNA Formation under Conditions of Reduced Spliceosome Activity. iScience. 2019;20:237-247. doi:10.1016/j.isci.2019.08.058