THE ROLE OF DOUBLE MINUTE CHROMOSOMES IN UNSTABLE METHOTREXATE RESISTANCE

ABSTRACT Murine 3T6 cells resistant to 50 pM MTX and having approximately 50–100 DHFR gene copies per cell lose these amplified sequences and cellular resistance to MTX with prolonged growth in MTX-free medium. Coincident with these losses are the losses of DMs and DHFR sequences in fraction enriched in DMs by filtration. Additionally, the correlation between DHFR enzyme content, gene copy number and DM content was established by analysis of populations of cells prelabeled with a fluorescein derivative of MTX and sorted for fluorescence by fluorescence activated cell sorting.