Relationship of amplified dihydrofolate reductase genes to double minute chromosomes in unstably resistant mouse fibroblast cell lines.

Doubleminutechromosomes (DMs)are small, often paired, extrachromosomal elements whichhavebeenfoundina widevariety of permanent cell lines ofhuman(1,2,24)and mouse(8, 18)origin, aswell asinbiopsied tumor tissue ofhumanorigin (2,19,25). Although no functions wereassigned toDMs intheabovecited studies, thepersistence ofthese chromosomesinpermanent cell lines andtumorssuggests thattheyconfer aselective advantage to cells. Previously, wereported theoccurrence of DMsincells whichwereselected forhigh levels ofresistance tomethotrexate (MTX). Inmouse cell lines S-180andL5178Y, theexistence of DMs isassociated withunstable MTX resistance, andwehavefoundpreliminary evidence whichsuggests thattheDMscontain amplified dihydrofolate reductase (DHFR)genes (13). Inthisworkweextended ourstudies totwo MTX-resistant murinefibroblast cell lines derived from3T6and3T3cells. We present additional datawhichsubstantiate thelocalization ofunstably amplified DHFR genes toDMs.