It is generally held that most living cells spend the vast majority of their lifetime in a quiescent, non-growing state. However, there is little known about how cells in the quiescent state repair DNA to maintain genomic integrity. A great deal of information is available about mechanisms of DNA repair, but this knowledge derives almost exclusively from studies on actively growing cells, not from cells in the quiescent state. Here we propose a genetic approach using the yeast-like fungus Ustilago maydis as a model microbial system to define genes and elucidate the mechanism of DNA repair in the quiescent state. In preliminary studies we found that U. maydis cells heavily damaged with DNA clastogens could literally be resurrected from the dead if held for a prolonged period in a nutrient-free, dormant or quiescent state. Physical analysis of the DNA from cells shunted into quiescence after severe damage indicated that there was a considerable amount of processing over a several hour period. Damaged DNA is first degraded into nucleosome-sized fragments, then rapidly reassembled into chromosomal lengths. We observed that quiescence repair appears to be highly efficient and accurate, but not dependent on the essential strand exchange components of the homologous recombination system nor on the primary signaling factors of the DNA damage response system. To determine the genetic basis of DNA repair in the quiescent state, we are testing the contribution of components of known DNA double-strand-break repair pathways and evaluating other factors that could modify chromatin or regulate metabolic responses. To go deeper into the mechanism we will perform an unbiased mutant screen to identify components of the quiescence repair system. This work promises new insight into the process of DNA repair in the quiescent state. We believe that steps defined in the process could be points of intervention for guiding new therapeutic strategies aimed at preventing dormant cancer cells from re-entering into the cell cycle and causing disease.
Entry of cancer cells into a quiescent state after chemo- or radiation therapy provides a safe haven for these cells to repair DNA so that they can re-emerge at a later date and cause disease. However, there is very little known about mechanisms of DNA repair in the quiescent state. Here we propose to use a model microbial system to identify genes responsible for repair in the quiescent state and to understand the mechanistic basis of the process.