The role of mitochondrial DNA-copy number in infectious disease: A bidirectional two-sample Mendelian randomization study

• Genetic evidence suggests that pneumonia significantly increases blood cellular mtDNA-CN. • Suggestive genetic evidence links sepsis to increased and COVID-19 to decreased blood cellular mtDNA-CN. • Blood cellular mtDNA-CN shows promise as a dynamic biomarker for specific infections, illuminating the complex role of mitochondrial function. The causal relationship between blood cellular mitochondrial DNA copy number (mtDNA-CN) and infectious diseases remains to be elucidated. We conducted a bidirectional two-sample MR study using the summary statistics of blood cellular mtDNA-CN correlations from a meta-analysis of large-scale genome-wide association studies (GWAS) at the UK Biobank (n=395718) with the summary statistics of genetic associations for the 9 common infectious diseases. Causal associations between mtDNA-CN and infectious diseases were examined mainly by inverse variance weighted (IVW), MR‒Egger, simple mode, weighted median, and weighted mode methods. Mendelian randomization pleiotropy residual sum and outlier (MR-PRESSO) was used to detect, exclude, and correct for horizontal gene pleiotropy, and Cochran's Q statistic was used to quantify the heterogeneity of instrumental variables. Leave-one-out sensitivity analysis was used to detect the existence of dominant instrumental variables. Moreover, we performed reverse MR analysis of infectious diseases with mtDNA-CN with the above methods, and applied the robust MR-RAPS method to evaluate the reliability of the reverse causal estimates. The IVW results revealed that mtDNA-CN was not associated with the risk of sepsis (P=0.773), COVID-19 infection (P=0.395), urinary tract infection (P=0.414), pneumonia (P=0.537), intestinal infection (P=0.635), viral hepatitis (P=0.551), candidiasis (P=0.126), upper respiratory tract infection (P=0.753) or skin and subcutaneous tissue infection (P=0.622). According to the results of the inverse MR analysis after Bonferroni correction, only pneumonia demonstrated a trend toward significantly increasing mtDNA-CN (β= 0.037, 95% CI 0.012 to 0.062; P=0.006). Nominally significant associations were observed for sepsis with increased mtDNA-CN (β = 0.009, 95% CI 0.001 to 0.017; P=0.015) and for COVID-19 with decreased mtDNA-CN (β = -0.018, 95% CI -0.034 to -0.002; P=0.035). Sensitivity analyses using MR-RAPS yielded consistent results for these associations. No significant heterogeneity or horizontal pleiotropy was found for the instrumental variables. The forward analysis results of this two-sample MR study did not reveal a causal relationship between blood cellular mtDNA-CN and the development of infectious diseases. Nevertheless, our reverse MR analysis provides suggestive genetic evidence that pneumonia may increase mtDNA-CN, and also highlights potential links for sepsis and COVID-19, albeit with weaker statistical support. These findings underscore the potential of mtDNA-CN as a dynamic clinical biomarker, particularly in pneumonia, and illuminate the complex role of mitochondrial function in infection.