Article Review: Exercise-induced increases in cell free DNA in human plasma originate predominantly from cells of the hematopoietic lineage

Tug S, Helmig S, Deichmann ER, Schmeier-Jurchott A, Wagner E, Zimmerman T et al. Exercise-induced increases in cell free DNA in human plasma originate predominantly from cells of the hematopoietic lineage. Exerc Immunol Rev, 2015;21: 164-173.

Article Summary:

The authors utilize sex-mismatched transplantation patients to analyze the origin of cell-free DNA (cfDNA) increases after a treadmill exercise test. During normal physiologic states, roughly 1-50ng of fragmented DNA can be found extracellularly per milliliter of plasma. During exercise, this value increases, upwards of 20-fold. Recently, cfDNA has gained popularity as a biomarker for chromosomal abnormalities during pregnancy, as the fetus and placenta both contribute to cfDNA. It has also been suggested as a biomarker for cancers and autoimmune diseases, as both instances lead to a raise in total cfDNA. While it is known from transfusion studies that under normal conditions, most cfDNA is of hematopoietic origin, there currently is no good method to determine the origin of cfDNA.

Many studies use cfDNA collected from subjects after exercise, since they are able to collect higher quantities. However, since we do not know if the origin of this DNA is the same as at normal conditions, this collection technique could potentially bias cfDNA studies. Tug et al. recruited five female HSCT patients who received from male donors, two male HSCT patients who received from female donors, five females who received a liver transplant from a male donor, and three male and female controls. The researchers collected blood from the antecubital vein before, immediately after, and 90 minutes after an incremental treadmill test, and measured nuclear and Y chromosomal cfDNA via qPCR.

In the male patients with female donors, only +5.5ng/ml and -2.6ng/ml changes were found in Y-chromosomal DNA, suggesting that most of the increase was from the transplanted hematopoietic cells. However, in the five female patients with male donors, the data only accounts for about half of the total cfDNA. They suggest that this could be due to lack of sensitivity of their qPCR assay or that it could reflect the success of grafting. Regardless, while the data support that the rise in cfDNA is likely due to DNA from hematopoietic lineage, it is far from conclusive.

Questions for Discussion:

  1. This study was conducted in accordance with a short incremental treadmill test. While this test has been shown to lead to increases in cfDNA, much larger increases have been identified during endurance exercise such as marathon running and cycling. Will the hematopoietic origin hold true during these endurance events? During marathons, there is evidence of liver and gall bladder damage (Wu 2004), kidney dysfunction (Neyiackas 1981), vascular damage (Fagerhol 2005) and brain damage (Marchi 2003). As novel methods for analyzing cfDNA are developed, it would be interesting to determine whether these systems are contributing to the increase during endurance exercise.
  2. How could the authors have improved their quantification assay? While only accounting for <50% of cfDNA, it is hard to make the claim that most of the cfDNA comes from one lineage. A better assay for copy number variation in Y-chromosomal and nuclear DNA could help to greatly increase sensitivity.
  3. This is an amazing patient cohort for addressing the question of cfDNA origin. What additional types of experiments could be conducted to better address the question? For example, could the study be repeated with strength vs. endurance exercise, as they are suggested to have different mechanisms?

Written By: Jason Klein

Reviewed By: Paul Nelson, PT