LAUSR

The study by Whalen et al has delineated the association between circulating cellfree DNA (ccfDNA) and metabolic derangements in idiopathic pulmonary fibrosis (IPF). They examined healthy controls compared with slow and rapid progressors. The latter was defined as subjects who met any part of a composite outcome (death, acute exacerbation of IPF, relative decline in forced vital capacity of at least 10% or diffusing capacity for carbon monoxide of 15%) during an 80week followup. The authors found that ccfDNA levels are elevated in rapid progressors compared with healthy controls and slow progressors with significant associations. ccfDNA is usually released during cell death, such as apoptosis, necrosis, pyroptosis, ferroptosis or extracellular trapassociated cell death (termed ETosis). Notably, the release of ccfDNA in cells in culture correlated with per cent in G1 phase rather than with cell death. ccfDNA is usually doublestranded nuclear DNA and mitochondrial DNA that is generally released into the circulation as multiple distinct species such as long and fragmented forms of DNA. ccfDNA is commonly packaged within proteins, and together this is called a nucleosome. In addition, packaging may occur in microparticles, small membranebound vesicles, organelles and exosomes that likely interact with different DNA recognition systems. The estimated halflife of ccfDNA in circulating blood varies from several minutes, around 4 min posthaemodialysis cessation to 1–2 hours. ccfDNA was initially thought to be a ‘waste product’ but increasingly is now found to mediate processes at distant sites such as tumour cell proliferation. The authors have correlated ccfDNA levels with 79 differentially expressed circulatory metabolites mapped to Kyoto Encyclopedia of Genes and Genomes pathways. The 15 pathways (10 amino acid pathways, 2 energy pathways, 2 nucleic acid pathways and 1 lipid pathway) have associations with increased collagen synthesis, glycolysis and antioxidants in IPF and reduced sphingolipid production in this condition. These metabolic findings are in line with the pathogenesis of IPF. Although this study did not investigate mechanisms, the authors suggest that intracellular metabolic changes regulate doublestranded DNAdependent immune responses through cytosolic DNA sensors, including absent in melanoma 2 (AIM2) inflammasome and cyclic GMPAMP synthasestimulator of interferon genes (cGASSTING). Both AIM2 and cGASSTING normally detect cytosolic DNA that has leaked from the nucleus, bacteria, doublestranded viruses and tumours and directs an immune response against these pathogens. A technical issue in this study is the time between sample collection (>24 hours) and processing since standard protocols recommend processing within 2 hours of collection. This delay could favour increased cell breakdown and release of DNA and metabolites that would confound the data. A gap in this study is the mechanism by which intracellular metabolites in IPF will cause the DNA leak and activation of these sensors. Alternatively, ccfDNA may influence cytosolic metabolism at distal sites. More likely, ccfDNA and metabolic changes are likely separate compartments of the same pathogenic process. Cell injury leads to altered metabolism and release their constituents with ccfDNA leading to associations between metabolites and ccfDNA. Future directions for this study include the identification of the origin of ccfDNA in IPF. This provides the mechanistic bridge to metabolic derangement and reveals sources of tissue damage. The techniques to achieve this include the identification of tissuespecific patterns of promoter methylation, analysis of tissuespecific modifications in circulating nucleosome, and identification of tissuespecific DNA fragmentation patterns or nucleosome occupancy. This will increase insights into the pathogenesis of IPF and provide viable circulatory biomarkers for the progression of this condition.