Circulating Mitochondrial DNA as Predictor of Mortality in Critically Ill Patients: A Systematic Review of Clinical Studies

Abstract

Background: Despite numerous publications on mitochondrial DNA (mtDNA) in the last decade it remains to be seen whether mtDNA can be used clinically. We conducted a systematic review to assess circulating cell-free mtDNA as a biomarker of mortality in critically ill patients.

Methods: This systematic review was registered with PROSPERO (CRD42016046670). PubMed, CINAHL, the Cochrane Library, Embase, Scopus, and Web of Science, and reference lists of retrieved articles were searched. Studies measuring circulating cell-free mtDNA and reporting on all-cause mortality in critically ill adult and pediatric patients were included. The primary and secondary outcomes were mortality and morbidity, respectively.

Results: Of the 1,566 initially retrieved publications, 40 studies were included, accounting for 3,450 critically ill patients. Substantial differences between studies were noted in how mtDNA was isolated and measured. Sixteen of the 40 included studies (40%) explored the association between mtDNA levels and mortality; of those 16 studies, 11 (68.8%) reported a statistically significant association. The area under the receiver operating characteristic (AUROC) curve for mtDNA and mortality was calculated for 10 studies and ranged from 0.61 to 0.95.

Conclusions: There is growing interest in mtDNA as a predictor of mortality in critically ill patients. Most studies are small, lack validation cohorts, and utilize different protocols to measure mtDNA. When reported, AUROC analysis usually suggests a statistically significant association between mtDNA and mortality. Standardization of mtDNA protocols and the completion of a large, prospective, multicenter trial may be warranted to firmly establish the clinical usefulness of mtDNA.

Keywords: biomarkers; circulating cell-free DNA; critical illness; mitochondrial DNA; mortality.

Figure 1

Production and release of extracellular mitochondrial DNA. Pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMPs) are released in the setting of infection or cellular injury, respectively. Both PAMPs and DAMPs can stimulate pattern recognition receptors, leading to increased production of mitochondrial reactive oxygen species. This causes fragmentation of the mitochondrial genome and decreased mitochondrial membrane transition permeability, allowing mitochondrial DNA (mtDNA) to enter the cytosol. mtDNA is a potent DAMP; it can stimulate inflammasomes or the Toll-like receptor 9 (TLR-9)/nuclear factor-κB pathway, leading to the production of proinflammatory cytokines (IL-1β, IL-18, tumor necrosis factor-α, IL-6). Cytosolic mtDNA can also enter the extracellular environment via necroptosis, NETosis, or platelet activation, where it can propagate the initial inflammatory response through further recognition as a DAMP by immune and nonimmune cells expressing TLR-9. RBCs have been shown to scavenge extracellular mtDNA through binding to TLR-9. MTP = membrane transition permeability; NF-κB = nuclear factor-κB; NETosis = formation of neutrophil extracellular traps (NETs); TNF-α = tumor necrosis factor-α.

Figure 2

Flow diagram illustrating study selection process. CINAHL = Cumulative Index to Nursing and Allied Health Literature.

Figure 3

Forest plot illustrating the area under the receiver operating characteristic curve with CIs for mtDNA and all-cause mortality in critically ill patients. The data presented here were either readily available within the articles or made available by the authors on request. The studies are organized according to discipline, with medicine patients near the top and surgery patients near the bottom of the plot. The size of each square denotes the proportional size of each cohort. *Data derived from the Brigham and Women’s Hospital cohort. †Data derived from the sepsis subgroup. ‡Data collected from use of NADH as a primer. §Data derived from the trauma subgroup.