Screening and application of unstable genetically resistant strains in fission yeast.

Reversible alterations at DNA sequence or epigenetic levels can result in phenotypes that are unstably inherited. The reversibility of these inheritable changes might be uniquely beneficial for adaption to possible fluctuations in environment. However, unstable changes are always ignored for the genetic instability in traditional studies, especially in the cause of drug resistance. In this study, we conduct a specific genetic screen in fission yeast using rapamycin (+caffeine) and obtain 173 resistant isolates. In contrast to the common strategy of isolating stable genetic mutants, we passage the cell culture with rapamycin resistance on drug free condition and test the resistance of offspring every five days, and obtain 14 strains that exhibit unstable resistance to rapamycin (the drug resistance is lost randomly among the cell progenies without drug selection pressure). Further studies show that the unstable genetic resistance of some strains is regulated by reversible DNA sequence alterationat the <i>ssp1</i> gene locus. This study provides new insights and relevant scientific basis for the regulatory mechanism of unstable drug resistance in the process of rapamycin as a clinical anti-tumor drug, and a new possible target for solving the problem of drug resistance.