Sloppier DNA Repair Mechanism Takes Over in Aging Mice

A neuron transfected with GFP (source)

As vertebrates age, DNA damage and mutations accumulate to the point where we begin to see "functional failure", causing cancers and other age-related illnesses.  Specifically, double-stranded breaks (DSBs) are the worst type of DNA damage, because they can lead to loss of genetic information and chromosomal rearrangement.  Cells do possess mechanisms to correct any damaged DNA, but when the repair machinery can't keep up with the DNA damage, the cells stop dividing (senescence) or they commit suicide (apoptosis).  But what is it that prevents DNA repair processes from keeping up as we age?  Researchers Vera Gorbunova and Andrei Seluanov of the University of Rochester (New York, USA) have discovered one reason (open access): the machinery ages too, and it is replaced with slower, less accurate parts.

In vertebrates, DSBs are repaired by non-homologous end-joining (NHEJ), a process that sticks the broken ends back together.  When NHEJ is impaired in humans and mice, we see premature and accelerated aging including cardiovascular disease, osteoporosis, cancer, atrophy, and alopecia.  Because of these studies we know that NHEJ is involved in aging, but we don't quite know how it is affected by aging.

Using mice, the researchers inserted NHEJ machinery fused to green fluorescent protein (GFP), which glowed every time the DSBs were repaired.  The team was able to calculate NHEJ efficiency by measuring the number of green signals in different tissues.  Comparing the tissues of young (5 month old) and old (25 month old) mice, the researchers found that NHEJ efficiency declined with age (by 1.8- to 3.8-fold).  The highest age-related decline in NHEJ efficiency was found in skin (right, top).

Since NHEJ efficiency decreases with age, the authors wanted to find out which of the two NHEJ pathways was primarily affected.  There are two NHEJ pathways: canonical NHEJ (cNHEJ) and microhomology-mediated end joining (MMEJ).  The authors found that the use of MMEJ to repair DSBs actually increases with age in several mouse tissues, possibly to compensate for the decline in cNHEJ function (right, bottom).  MMEJ glues the broken ends back together, but create an overlap between homologous sequences - something that NHEJ does not do.  The downside is that the MMEJ pathway is more error-prone, which leads to the accumulation of mutations that ultimately age the cell and cause cell death.  Interestingly, many cancer tissues exhibit high levels of MMEJ, including bladder cancer and myeloid leukemia.

Using the genetically modified line of mice from this study, the researchers now plan to look at the effects of diet, medicines, and genetic factors affect aging in mice.  They hope to provide information that will help us fight age-related illnesses in the future.