N6-Methyladenine Progressively Accumulates in Mitochondrial DNA during Aging

Abstract

<i>N⁶</i>-methyladenine (6mA) in the DNA is a conserved epigenetic mark with various cellular, physiological and developmental functions. Although the presence of 6mA was discovered a few years ago in the nuclear genome of distantly related animal taxa and just recently in mammalian mitochondrial DNA (mtDNA), accumulating evidence at present seriously questions the presence of <i>N⁶</i>-adenine methylation in these genetic systems, attributing it to methodological errors. In this paper, we present a reliable, PCR-based method to determine accurately the relative 6mA levels in the mtDNA of <i>Caenorhabditis elegans</i>, <i>Drosophila melanogaster</i> and dogs, and show that these levels gradually increase with age. Furthermore, <i>daf-2(-)</i>-mutant worms, which are defective for insulin/IGF-1 (insulin-like growth factor) signaling and live twice as long as the wild type, display a half rate at which 6mA progressively accumulates in the mtDNA as compared to normal values. Together, these results suggest a fundamental role for mtDNA <i>N⁶</i>-adenine methylation in aging and reveal an efficient diagnostic technique to determine age using DNA.