What the papers say: where is the somatic mutation that causes aging?

It has been proposed that somatic mutations make major contributions to aging. The first paper, based on a gene knock-in mouse, supports a contributory role for mutation in mtDNA in aging, but does not support a damaged-mtDNA-producing-more-damaged-mtDNA hypothesis. The second paper indicates some GC-rich sequences in the nuclear DNA are more sensitive to oxidative damage than mtDNA. As a result, key genes involved in brain function and mitochondrial function are progressively inactivated with age. Failure in these nucleus-encoded mitochondrial genes may be a primary reason for mitochondrial failure in old age.