Mitochondrial DNA copy number is not associated with fatigue status in Primary Sjögren’s Syndrome^*

Background: Primary Sjögren’s syndrome (pSS) is a heterogeneous disease characterized by lymphocytic infiltrates to the exocrine glands, causing sicca symptoms and other manifestations. Fatigue is one of the most prominent symptom in pSS; up to 70% suffer from chronic fatigue. Fatigue has been shown to be common and severe in patients suffering from primary mitochondrial DNA (mtDNA) disease. In a number of chronic diseases, mitochondrial DNA copy number (mtDNAcn) has been reported to be altered. Purpose: The aim of the study was to examine if mtDNAcn was altered in fatigued versus non-fatigued pSS. Methods: We quantified mtDNAcn using quantitative polymerase chain reaction (qPCR) with mitochondrial ND2 as a target gene normalized to nuclear HβB (mtDNA:nDNA). Results: In 204 participants, 108 fatigued and 96 non-fatigued, relative mtDNAcn did not distinguish fatigue status (p = 0.7) nor was it correlated with severity of fatigue (p = 0.21). Conclusions: MtDNAcn is not altered with fatigue status in blood in pSS. Our analysis suggests that when conducting mtDNAcn analysis with large sample numbers over multiple days a two-way-ANOVA should be used in preference to a one-way-ANOVA or t-test to allow detection of batch effects.