The levels of circular RNAs in the brains of patients with Alzheimer’s disease are linked to several traits and clinical measures of disease severity, a study shows.
Because these circular RNAs also can be detected in cerebrospinal fluid and blood, they may hold the potential to be used as biomarkers to detect Alzheimer’s before symptoms occur.
The study, “An atlas of cortical circular RNA expression in Alzheimer disease brains demonstrates clinical and pathological associations,” was published in the journal Nature Neuroscience.
All genetic information contained within genes (DNA) ultimately is translated into proteins. However, several complex steps occur before a protein can be produced. Ribonucleic acid, or RNA, is a linear molecule that carries the necessary information to produce proteins from the corresponding genes.
Circular RNAs (circRNAs) comprise a large class of RNA molecules that, unlike their linear counterparts, carry no information for the production of proteins. Instead, circRNAs seem to control the activity of certain genes.
These circular molecules are abundant in the nervous system, particularly synapses — the junctions between two nerve cells that allow them to communicate — and have been observed accumulating in the brains of mice and flies as the animals age.
So far, the most well-established role of circRNAs is in the regulation of another set of RNA molecules — microRNAs — that also are capable of controlling the activity of certain genes. However, the role of circRNAs in Alzheimer’s remains largely unknown, with one study reporting the low abundance of a single circRNA in the brains of patients with sporadic Alzheimer’s disease.
To explore the role of circRNAs in the context of Alzheimer’s, researchers at the Washington University School of Medicine in St. Louis, Missouri, analyzed brain cortex samples of 83 Alzheimer’s patients and 13 individuals without the disease (controls). The cortex is a brain region responsible for complex cognitive functions.
In this first group of patients and controls, called the discovery cohort, researchers sought to identify circRNAs that could be associated with two quantitative traits of Alzheimer’s severity — the Braak score and the clinical dementia rating (CDR) — and those that could differentiate between those with Alzheimer’s and healthy controls.
The Braak score assesses the severity of Alzheimer’s by measuring the number and distribution of neurofibrillary tau tangles throughout the brain. It ranges from zero (absence of tau tangles) to six (presence of severe, extensive tau tangles). CDR is a clinical measure of cognitive impairment that ranges from zero (no dementia) to three (severe dementia).
The analysis revealed that 31 circRNAs significantly correlated with CDR, nine with the Braak score, and nine with the Alzheimer’s versus control status.
Researchers then confirmed these findings in a second set of brain samples (replication group) from the Mount Sinai/JJ Peters VA Medical Center Brain Bank (MSBB–Mount Sinai NIH Neurobiobank) cohort, which holds more than 2,040 well-characterized brains.
These samples were obtained from a region within the brain’s frontal cortex called Brodmann area-44 (BM44). The frontal cortex is a brain region responsible for controlling cognitive behavior, personality expression, decision-making and moderating social behavior.
The replication group consisted of 195 brain samples, which included 40 controls, 89 patients with a definite diagnosis of Alzheimer’s, 31 from individuals who probably had Alzheimer’s, and 35 from possible Alzheimer’s cases.
The analysis confirmed that 27 of the 31 CDR-associated circRNAs continued to correlate with CDR scores. Moreover, five of the nine circRNAs showing a correlation with the Braak score and Alzheimer’s versus control status in the discovery group were maintained in the MSBB cohort.
The results of the discovery and replication datasets identified a total of 164 circRNAs that were significantly associated with at least one Alzheimer’s trait, and 28 that were linked to all three traits.
“Altogether, these results support a consistent, replicable and highly significant association between changes in circRNA expression [levels] and AD traits,” the researchers wrote.
They also confirmed that most of the changes in circRNAs linked with Alzheimer’s traits were independent of changes in other RNA molecules, called messenger RNAs — the templates cells use to make proteins — and that these changes were consistent across the brain’s cortex, a region commonly affected in Alzheimer’s disease.
Researchers then focused on a small number of individuals with pre-symptomatic Alzheimer’s (six from the discovery group and 18 from the replication group), including those with very mild dementia (CDR lower than 0.5) to look for changes in the levels of circRNAs.
The analysis showed that changes in circRNAs were apparent early on in several regions of the cortex prior to the onset of Alzheimer’s symptoms.
The team also analyzed data from 21 brains donated by individuals with autosomal dominant Alzheimer’s (ADAD), an early-onset form of Alzheimer’s caused by mutations in the genes PSEN1, PSEN2, and APP, who were enrolled in the Dominantly Inherited Alzheimer Network (DIAN) study.
The analysis identified changes in the levels of 236 circRNAs, which were more striking in ADAD patients compared to individuals from the Alzheimer’s discovery group.
Further analysis showed that changes in the levels of certain circRNAs contributed more to the variation seen in some quantitative traits of Alzheimer’s, particularly the CDR and Braak scores, than well-established risk factors. Those include the number of ApoE4 alleles — the most common genetic risk factor for Alzheimer’s — and the estimated proportion of neurons (EstNeuron), which also is thought to differ between Alzheimer’s patients and healthy individuals. (An allele is a different form of the same gene.)
Computational analysis showed the activity of Alzheimer’s-associated circRNAs was linked with that of known Alzheimer’s-genes, including APP, PSEN1, PSEN2 and SNCA.
Moreover, researchers found that Alzheimer’s-associated circRNAs may potentially regulate Alzheimer’s-associated pathways and genes by targeting specific microRNAs.
Overall, these results suggest the levels of circRNAs had a “strong predictive ability for [Alzheimer’s] case status, even in the absence of demographic or APOE4 risk factor data,” the researchers wrote.
Moreover, since circRNAs are relatively stable in body fluids such as blood and cerebrospinal fluid — the liquid that circulates in the brain and spinal cord — they “will probably have use as peripheral biomarkers of pre-symptomatic and symptomatic [Alzheimer’s] and potentially other neurodegenerative diseases,” the researchers concluded.
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