Several microRNAs in the blood of people with Parkinson’s disease differ from healthy controls, correlate with disease progression, and are associated with hallmarks of the disorder, a large-scale study reported.
These findings support the possible use of microRNAs as biomarkers in diagnosing Parkinson’s and in predicting a patient’s likely outcomes (prognosis), the scientists said.
The study, “Deep sequencing of sncRNAs reveals hallmarks and regulatory modules of the transcriptome during Parkinson’s disease progression,” was published in the journal Nature Aging.
MicroRNAs (miRNAs) are short, stable RNA segments found in the blood that play a regulatory role in translating genetic information. Research indicates they also may be valuable biomarkers for various diseases, including Parkinson’s.
“Because microRNAs are stable in the bloodstream, contain diverse information for diagnosis and prognosis, and their influence on an organism’s genes has been well studied, we consider them promising candidates for robust biomarkers, also in the context of Parkinson’s disease,” Fabian Kern, the study’s first author, said in a press release.
In previous large-scale studies, this research team showed that microRNAs can serve as diagnostic biomarkers for Alzheimer’s disease and lung cancer.
The researchers, based at Saarland University in Germany and working with scientists at several sites in the U.S., now analyzed 5,450 blood samples collected from 1,614 people participating in the global Parkinson’s Progression Markers Initiative (PPMI).
Advanced biomarker studies “require carefully designed cohorts and only a few large-scale PD [Parkinson’s disease] studies … fulfill stringent requirements. Among them, the PPMI,” the researchers wrote.
PPMI groups include healthy controls, people with sporadic (non-familial) Parkinson’s, those who carry disease-related mutations (affected or not by the disease), early stage patients (prodromal), and individuals showing brain scan evidence without low dopamine (SWEDD) levels, the hallmark of Parkinson’s.
miRNA was isolated from all cells in blood samples, sequenced, and analyzed by bioinformatic methods — a computer technique used to examined large amounts of biological data with artificial intelligence.
A comparison between total Parkinson’s samples (sporadic plus genetic) and controls (healthy plus unaffected) identified five miRNAs with large differences in patients, with a trend toward lower production (downregulation). A more detailed analysis found two miRNAs upregulated (produced at higher levels) in affected genetic carriers, and one downregulated.
Within samples from people with non-familial Parkinson’s, 24 miRNAs were downregulated and two were upregulated, suggesting “the effects on miRNAs seem to be more diverse and stronger in sporadic cases than for genetic cases,” the team wrote.
To test whether miRNAs show increasing effects over time and disease progression, patients were assessed in up to four follow-up visits. Results were compared with measures taken at the beginning of the PPMI study (baseline) to each visit. Over time, many miRNAs were found to be downregulated.
These downregulated miRNAs were found in red blood cells, T-helper immune cells (a type of white blood cell), and exosomes, large molecular structures released from cells. In contrast, upregulated miRNAs were identified in blood serum — the clear liquid portion without clotting factors or blood cells — and immune cells called neutrophils.
“These findings are consistent with an observed increase in neutrophils and decrease in [white blood cells] identified in the PPMI blood cell count records,” the researchers wrote.
Further investigation found the pool of white blood cells to be a primary source of time-dependent depleted miRNAs in Parkinson’s patients.
Global analysis for miRNA production across all groups, their genetic status, clinical visits, age, and disease severity confirmed that miRNAs had diagnostic and prognostic potential “but also point at a general age-dependent variability of miRNA biomarkers,” they added.
Notably, the number of miRNAs changes (deregulated mRNAs) in Parkinson’s patients, compared with controls, was age-dependent, with two ages of change onset: the 30s and starting in the late 60s.
“The indication for this effect is the increased concentration of deregulated microRNAs that we found in blood samples of the corresponding study cohorts,” said Andreas Keller, the study’s lead author and head of the research group for clinical bioinformatics at Saarland University.
miRNA constantly declined in both sporadic and genetic cases, with a similar pattern in SWEDD patients. In contrast, prodromal patients had a late onset for deregulated miRNAs, and the number of miRNA changes increased with age. Severe downregulation of miRNAs occurred at different ages of onset.
To validate these findings, the team analyzed a group from the Luxembourg Parkinson’s Study (NCER-PD), which comprised 1,440 blood samples from 988 donors, each with up to four clinical visits. Of these participants, 440 were diagnosed with sporadic disease, 81 with parkinsonism (atypical forms of Parkinson’s), and 485 were age-matched healthy controls.
A similarity was seen between the PPMI and NCER-PD groups, with the deregulated miRNAs highest in the third and seventh decade of life. Substantial differences between the number of upregulated and downregulated miRNAs were not observed, except for more downregulation with older age.
“The clear diagnostic patterns that are largely consistent between the two cohorts open the question as to whether the biomarker candidates share similar molecular functions,” the researchers said.
The most significant downregulated miRNAs in Parkinson’s were found in mitochondria, which supply energy to cells, as well as other cells, tissues, and signaling pathways known to be impacted by the disease.
“[M]itochondrial dysfunction seems to play a role in the disease, although it is still unclear whether it is part of the cause or a consequence of the disease,” the researchers wrote.
To further assess miRNAs as a prognostic marker, levels were correlated with functional assessments as determined by the Unified Parkinson’s Disease Rating Scale (UPDRS), which rates severity of symptoms and determines disease stage.
In PPMI participants with at least three visits, 71 miRNAs were positively correlated in progressing patients but negatively correlated in non-progressing patients. Conversely, 71 miRNAs were negatively correlated with progressing individuals and positively correlated in non-progressing cases.
“Based on the presented data, we propose interesting candidates as diagnostic and prognostic biomarkers suitable for downstream validation,” the authors wrote. “Given the careful cohort design and statistical power facilitated by PPMI, effects are considerable and highly significant.”
“In the future, we want to analyze the blood down to the single-cell level, which will allow us to make much more precise statements,” Keller added.
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