The public generally correlates DNA damage with aging. However, the effect of mutation accumulation in the genome has not been answered. These ideas, among others, are the focuses of Assistant Professor Patrick Maxwell’s experiments. It is well established that as organisms age, there is an accumulation of mutations, but these mutations have not been shown as a casual factor for aging.
Maxwell’s lab uses single-celled baker’s yeast as its test subjects. The unicellular organisms accumulate oxidative damage and other mutations similar to humans.
“We have a very reduced approach,” said Maxwell. “It’s not the sort of thing you would automatically think of for aging research, but a unicellular organism has similar cell-level changes as we would have when we get older.”
With these single-celled organisms, huge sample sizes can easily be obtained. In order to genetically modify the yeast, they amplify a sequence of DNA using a technique called polymerase chain reaction. This particular DNA sequence will replace a gene that is in the yeast genome. Typically, the altered gene will give the yeast a detectable characteristic. For example, Maxwell’s lab can replace a normal yeast gene with an antibiotic resistant gene.
Established in early 2011, Maxwell’s lab can genetically engineer a strain to reduce its ability to carry out a certain type of DNA repair or reduce its ability to recognize DNA damage. With the right treatments and conditions, a fraction of a population of yeast cells will undergo a DNA repair process where they replace the normal yeast gene with an introduced amplified sequence.
“We can take 50 or 100 million yeast cells, and this [modification] might happen in 50 of them,” said Maxwell. “It might happen in only one in one million cells, but when you work with a microorganism, you can easily have hundreds of millions of cells.”
Then, the lab will grow the modified cells in a population and check the DNA of the population, making sure the original gene has been replaced. Several independent cells generally acquire the change, so they test multiple independent strains of the original yeast strains to ensure that there isn’t another random, unintended change.
The lab also studies mobile DNA sequences. Similar to gene sequences in terms of length and function, these DNA sequences, called retrotransposons, code for proteins that will make a new copy of the sequence. Then, that sequence will be inserted into a genome.
“It’s a sequence that can duplicate itself, and that duplicate can go anywhere else. It’s potentially a disruptive process,” said Maxwell. “By definition, their activity changes something about the genome.”
So far, results show the copy number of the mobile DNA sequences seems to affect the life span of the cells.
Overall, Maxwell remains open to other ideas. “We have to be open to the possibility that accumulating mutations isn’t a driving force in aging,” said Maxwell. “We’re not always seeing a correlation between a change in a DNA repair activity and a change in life span.”