Given its interest in developing treatments for Alzheimer's disease (AD), Japanese drugmaker Eisai is exploring the potential of plasma-based microRNAs to serve as biomarkers for the disease.
The effort has yielded promising results thus far, including the identification of a miRNA signature that appears to differentiate between AD patients and unaffected individuals. However, the project is still early stage and validation work remains ongoing, a company researcher noted.
While an estimated 5 million Americans have AD, the disease's etiology remains unclear. Further, diagnosis can be difficult — requiring either expensive imaging scans or invasive spinal taps — and is not always accurate.
As such, "there is a big need for accurate molecular diagnostics for Alzheimer's" based on easily accessible biomarkers," Pavan Kumar, a molecular biomarker research scientist at Eisai, told Gene Silencing News.
Given the success others have had using miRNAs as biomarkers for various cancers, Kumar and his colleagues have been exploring whether the non-coding RNAs have diagnostic potential in AD.
Most recently, their work yielded a collection of seven miRNAs that have thus far been effective in identifying AD patients. As described in a 2013 paper in PLOS One, the scientists obtained plasma samples from 11 AD patients and 20 age- and sex-matched normal controls, and searched them for differentially expressed miRNAs using Nanostring's nCounter technology. Of the 654 total miRNAs measured, 12 were differentially expressed in the AD samples.
The downregulation of seven of these — 7d-5p, let-7g-5p, miR-15b-5p, miR-142-3p, miR-191-5p, miR-301a-3p, and miR-545-3p — were confirmed using singleplex qPCR. This signature was then validated in an independent cohort of 20 AD patients and 17 controls, with up to 95 percent prediction accuracies in both cohorts.
"The technology platform you use will really influence the signature you get," Kumar noted. "It's critical that any signature gets validated across multiple platforms. The fact that the [seven-miRNA] signature was working across two very different technologies gave us the confidence that this is something biological and real," he said.
To further validate the signature in larger cohorts and across different ethnicities, Kumar and his colleagues then conducted a follow-up validation study in a significantly larger Asian cohort. (The initial discovery and validation experiments were run in samples from Caucasians.)
Data from the Asian study, which were presented at last year's Alzheimer's Association International Conference, were not available as they are being considered for publication. But Kumar said they were "very encouraging," with accuracy for some signatures approaching up to 85 percent.
Buoyed by these results, the team is now looking to replicate its findings in additional Asian and Caucasian cohorts, specifically looking to see whether the miRNA signature can be used to differentiate AD from other forms of dementia — currently a major challenge in diagnosis.
In an effort to keep things simplified and cost effective, Kumar and his collaborators are using TaqMan qPCR analysis in their validation studies. However, they are also testing out a digital PCR approach because of its "simplicity and absolute numbers," Kumar said.
Although he is generally optimistic about the program, Kumar stressed that a blood-based miRNA diagnostic for AD remains years away, partly due to logistical issues.
Large-scale validation experiments, for instance, require "very well-annotated samples," which can be difficult to obtain as AD diagnosis is imperfect and oftentimes subjective, he explained.
It is also particularly challenging to use plasma taken with a simple blood draw in the diagnosis of a brain-specific disease. "You have to understand how that biomarker potentially could have made its way from the brain all the way to the bloodstream."