Investigating Genome Instability and Retrotransposition during Replicative Aging in Saccharomyces cerevisiae.

摘要

Recent evidence has linked the activity of retrotransposons, mobile genetic elements that replicate through an RNA intermediate, with age-related pathologies in higher eukaryotes, such as mice and humans. Retrotransposons show increased activity with age in several model organisms from yeast to mammals. Yeast has many homologous metabolic, DNA damage and aging pathways making it a useful model organism to elucidate the potential contribution of retrotransposition to aging and age-related pathologies. During aging, organisms experience an increase in genome instability but it is not clear if this a direct result of aging or simply a by-product of replication. We developed a method to calculate expected mutation frequency in replicatively aged yeast cells to determine if the increase in genome instability was an age-related effect. Although we did not see an age-related effect for mutation frequency, we were able to demonstrate that S. cerevisiae Ty1 retrotransposon mobility was elevated above predicted values in replicatively aged cells and that these insertions occurred preferentially in mother cells compared to daughters. This increase in retrotransposition was correlated with significantly higher levels of Ty1 cDNA in mother cells compared with daughters. Ty1 cDNA can be mutagenic by insertion into the genome or by being used inappropriately for repair of DNA damage. Interestingly, mother cells with new insertions had a corresponding increase in gross chromosomal rearrangements compared to daughters with insertions. GCRs were particularly high in mother cells that contained tandem arrays of Ty1 elements. Normally, Ty1 inserts into genes transcribed by RNA polymerase III with the majority of inserts going to the region upstream of tRNA genes, where effect on gene function is minimal. In replicatively aged mother cells, insertion into this preferred region was significantly reduced. There was a corresponding increase in insertions into the rDNA where insertions normally occur at very low levels, potentially contributing to the increase in instability seen in this region in old yeasts cells. Further work could identify the mechanisms responsible for and the ramifications of increased retrotransposon-related genome instability during aging. Since retrotransposon regulation and consequences are very similar between yeast and mammals, future work could identify mechanisms that could be targeted to improve health and lifespan in humans.