Chronic Obstructive Pulmonary Disease (COPD) is a progressive lung disease and leading cause of death and disability worldwide. Characterized by chronic airway obstruction, COPD has a complex etiology influenced by genetic risk factors and environmental exposure to smoke. Increased COPD severity is associated with extra-pulmonary manifestations such as cachexia. The mitochondrion, a cellular organelle with its own genome (mtDNA), is responsible for cellular metabolism and redox homeostasis. MtDNA variants differ from those in the nucleus, and are categorized as either homoplasmic or heteroplasmic, based on whether the variant is or is not present in all mtDNA copies in the same sample. The aim of this study was to test the association of homoplasmic mtDNA variants with cachexia in individuals with COPD.

We analyzed whole genome sequencing (WGS) data generated from blood samples taken from 1,936 participants with COPD from the Evaluation of COPD Longitudinally to Identify Predictive Surrogate End-points (ECLIPSE) cohort. mtDNA variants were called with Mitochondrial High-Performance Caller (MitoHPC) using WGS data. Homoplasmic variants were classified using a variant allele frequency threshold of 3%. Specifically for a given homoplasmic site participants with less than 3% alternate allele reads were coded as 0 and those greater than 97% were coded as 1. Whereas mtDNA SNVs in participant samples with alternate allele frequencies between 3% and 97% were classified as heteroplasmic and excluded from analyses. Homoplasmic variants present in 5% or more of participants were included in analyses. Cachexia was defined as weight loss of 5% or more in 12 months or less. Logistic regression models assessing the association of each homoplasmic variant with cachexia as the outcome were adjusted for sex, age, and pack years of smoking.

62 homoplasmic variants were considered common, 5 of the variants were nominally significantly associated with the development of cachexia (p-value < 0.05) and found in regions of the ND5, ND1, and COX3 genes, or the D-loop. In particular, the chrM13708G>A variant in ND5 (OR=1.46, SE=0.17, p= 0.022) is a missense variant. ND5 encodes NADH-ubiquinone oxidoreductase chain 5, a protein involved in the electron transport chain. Variants in this gene have been linked to Leigh syndrome, mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes, and mitochondrial complex I deficiency.

Our planned future analyses will expand our research to include other TOPMed cohorts with participants with COPD. This study provides suggestive evidence for the relationship between mtDNA variation and COPD cachexia.

BACKGROUND
Asthma is a disease characterized by chronic inflammation, due in part to oxidative stress and mitochondrial dysfunction. Mitochondrial DNA copy number (mtDNA-CN) is an indirect biomarker of mitochondrial function. The relationship between mtDNA-CN and asthma risk or severity is unknown.
METHODS
Mitochondrial DNA copy number was estimated from a combination of whole exome sequencing data and whole genome sequencing data in participants from the UK Biobank (UKB) in the UK and the Severe Asthma Research Program (SARP) in the US. Exclusion criteria for both studies include current smokers, prevalent cancer, and other lung, kidney, liver, and heart diseases. We investigated whether mitochondrial DNA copy number is associated with asthma diagnosis in the UKB and/or with asthma severity, oxidative stress, antioxidant markers, and asthma exacerbation frequency in SARP.
RESULTS
In the UKB, asthmatics (N = 29,775) have significantly lower mtDNA-CN in buffy coat compared to non-asthmatics (N = 239,204) (P < 0.0001). In SARP, severe asthmatics (N = 580) have significantly lower mtDNA-CN in whole blood than non-severe asthmatics (N = 703) (P = 0.001), and mtDNA-CN is positively associated with lung function FEV1/FVC (P = 0.003). In asthma, fractional exhaled nitric oxide, a biomarker of asthma, is positively correlated with mtDNA-CN, while antioxidants superoxide dismutase and glutathione peroxidase are inversely correlated with mtDNA-CN (all P < 0.05). Lower mtDNA-CN identifies a greater risk of occurrence of 3+ vs 0 exacerbations in the following year.
CONCLUSIONS
Asthma is associated with mitochondria dysfunction, and mtDNA-CN serves as a biomarker for severity and exacerbation risk.