mtDNA heteroplasmy level and copy number indicate disease burden in m.3243A>G mitochondrial disease

• Main Authors: John P Grady, Sarah J Pickett, Yi Shiau Ng, Charlotte L Alston, Emma L Blakely, Steven A Hardy, Catherine L Feeney, Alexandra A Bright, Andrew M Schaefer, Gráinne S Gorman, Richard JQ McNally, Robert W Taylor, Doug M Turnbull, Robert McFarland
• Format: Article
• Language: English
• Published: Wiley 2018-06-01
• Series: EMBO Molecular Medicine
• Subjects: m.3243A>G; MELAS; mitochondrial disease; mtDNA copy number; mtDNA heteroplasmy
• Online Access: https://doi.org/10.15252/emmm.201708262

Abstract Mitochondrial disease associated with the pathogenic m.3243A>G variant is a common, clinically heterogeneous, neurogenetic disorder. Using multiple linear regression and linear mixed modelling, we evaluated which commonly assayed tissue (blood N = 231, urine N = 235, skeletal muscle N = 77) represents the m.3243A>G mutation load and mitochondrial DNA (mtDNA) copy number most strongly associated with disease burden and progression. m.3243A>G levels are correlated in blood, muscle and urine (R2 = 0.61–0.73). Blood heteroplasmy declines by ~2.3%/year; we have extended previously published methodology to adjust for age. In urine, males have higher mtDNA copy number and ~20% higher m.3243A>G mutation load; we present formulas to adjust for this. Blood is the most highly correlated mutation measure for disease burden and progression in m.3243A>G‐harbouring individuals; increasing age and heteroplasmy contribute (R2 = 0.27, P < 0.001). In muscle, heteroplasmy, age and mtDNA copy number explain a higher proportion of variability in disease burden (R2 = 0.40, P < 0.001), although activity level and disease severity are likely to affect copy number. Whilst our data indicate that age‐corrected blood m.3243A>G heteroplasmy is the most convenient and reliable measure for routine clinical assessment, additional factors such as mtDNA copy number may also influence disease severity.