DNA Methyltransferase1 (DNMT1) Isoform3 methylates mitochondrial genome and modulates its biology.

Here we demonstrate localization of the isoform3 of DNA Methyltransferase1 (DNMT1) enzyme to mitochondria, instead of isoform1 as reported earlier. The fused DNMT1-isoform1, reported earlier to localize in mitochondria, surprisingly showed its exclusive presence inside the nucleus after its ectopic expression; and failed to localize in mitochondria. On the other hand, ectopically expressed DNMT1-isoform3 targeted itself to mitochondria and subsequently methylated CpG regions in the mitochondrial genome. In addition, overexpression of DNMT1-isoform3 affected mitochondrial biology and regulated its function. Under different conditions of oxidative and nutritional stress, this isoform was down-regulated, resulting in hypomethylation of mitochondrial genome. Our study reveals how DNMT1-isoform3, instead of isoform1, is responsible for mtDNA methylation, influencing its biology.