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Mitochondrial DNA Damage May Serve as Biomarker of Parkinson's Disease

NEW YORK — An international team of researchers has developed a PCR-based assay to detect mitochondrial DNA (mtDNA) damage in blood, an indicator of Parkinson's disease. The test may help diagnose the condition before the nervous system of patients is excessively damaged, they said, and could be used to measure response to certain drugs in development to treat the disease. 

The investigators, led by a team at Duke University, built the assay based on several previous studies that had linked mitochondrial dysregulation to Parkinson's and had noted the accumulation of damaged mtDNA in brain tissue of deceased patients. 

"A simple blood test would allow us to diagnose the disease earlier and start therapies sooner," corresponding author Laurie Sanders, a researcher at the Duke Center for Neurodegeneration and Neurotherapeutics, said in a statement.

As they described in a study published in Science Translational Medicine on Wednesday, the researchers used PCR to quantify levels of damaged mitochondrial DNA damage in peripheral blood cells collected from Parkinson's patients and compared them to those of healthy controls.

The assay, called Mito DNADX, allowed for the real-time quantification of DNA damage in distinct loci in a 96-well platform and produced results within 24 hours, the authors wrote. It relies on the fact that that less PCR product is produced when mtDNA damage or lesions block the ability of the DNA polymerase to replicate.

Next, the investigators compared their assay to conventional methods of detecting DNA damage and found that the latter could not detect mtDNA damage with great sensitivity and were often contaminated with nuclear DNA. Moreover, the authors noted that existing methods required cycle number optimization, whereas Mito DNADX collected the amplification data from cycles 0 to 40, eliminating that requirement.

Meanwhile, they found that blood samples of individuals who harbor mutations in the leucine-rich repeat kinase 2 (LRRK2) gene have increased damaged mtDNA. These mutations, in particular G2019S, have previously been associated with elevated mtDNA damage and are among the most common known causes of PD, making LRRK2 a promising therapeutic target, the authors wrote. The test, however, could also detect mtDNA damage in Parkinson's disease cases without LRRK2 mutations, as well as in LRRK2 mutation carriers with no diagnosis of the disease, they added.

Next, they developed LRRK2 G2019S knock-in mice, which displayed increased mtDNA damage, whereas LRRK2 knockout mice showed fewer mtDNA lesions in the ventral midbrain compared to wild-type mice, corroborating their findings.

Subsequently, they sought to determine whether LRRK2 inhibitors could reduce mtDNA damage. For this, they treated idiopathic Parkinson's patient-derived lymphoblastoid cell lines with a high concentration of LRRK2 kinase inhibitor, which restored mtDNA damage to baseline levels within 24 hours without changing mtDNA copy number. However, future studies must explore whether these findings will extend to in vivo Parkinson's models, the authors cautioned.

"Our data provide evidence to support the inclusion of mtDNA damage as a blood-based candidate marker for [Parkinson's disease] in future clinical trials," they concluded.

Highlighting the limitations of Mito DNADX, the authors wrote that the test cannot identify the specific molecular type of mtDNA damage and gives a relative, not absolute, quantification of mtDNA damage, though future iterations of the assay may overcome these shortcomings.

As next steps, Sanders said in a statement, her team plans to use the assay to test samples from patients with the earliest stages of the disease, before symptoms develop.