DRUG-SPECIFIC REARRANGEMENTS OF CHROMOSOME-12 IN HYDROXYUREA-RESISTANT MOUSE SEWA CELLS - SUPPORT FOR CHROMOSOMAL BREAKAGE MODEL OF GENE AMPLIFICATION

In order to investigate whether specific, nonrandom chromosome rearran gements were involved in the induction of hydroxyurea (HU) resistance in mouse SEWA cells, we undertook derailed cytogenetic analyses of thr ee independently selected lines during the long-term treatment with HU . We found that cells with trisomy 12 had selective advantage during e arly steps of HU treatment. Subsequently, numerous rearrangements of c hromosome 12 took place in each of the HU-resistant cell lines. More s pecifically, the proximal end of chromosome 12 (band A3) was frequentl y involved in breaks and fusions generating multicentric marker chromo somes. In situ hybridization showed that the functional Rrm2 gene was located in this particular region of chromosome 12. Furthermore, ampli fication and rearrangements of the structural gene Rrm2 were detected both at the chromosomal and at the molecular level. As discussed, the results of the cytogenetic analyses support the chromosomal breakage m odel of gene amplification.