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 rearrangements were involved in the induction of hydroxyurea (HU) resistance in mouse SEWA cells, we undertook detailed cytogenetic analyses of three independently selected lines during the long-term treatment with HU. We found that cells with trisomy 12 had selective advantage during early steps of HU treatment. Subsequently, numerous rearrangements of chromosome 12 took place in each of the HU-resistant cell lines. More specifically, the proximal end of chromosome 12 (band A3) was frequently involved in breaks and fusions generating multicentric marker chromosomes. In situ hybridization showed that the functional Rrm2 gene was located in this particular region of chromosome 12. Furthermore, amplification 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 model of gene amplification.