Premature condensation induces breaks at the interface of early and late replicating chromosome bands bearing common fragile sites.

Various studies suggest a tight relationship between chromosome rearrangements driving tumor progression and breaks at loci called common fragile sites. Most of these sites are induced after perturbation of the replication dynamics, notably by aphidicolin treatment. We have mapped the majority of these sites to the interface of R and G bands, which calls into question the previous assignment of aphidicolin-sensitive sites to R bands. This observation suggests that most of them correspond to loci that ensure the transition between early and late replicating domains. We show that calyculin A, which triggers chromosome condensation at any phase of the cell cycle but does not markedly impair replication, induces damage in the chromosomes of human lymphocytes treated in G(2) but not in G(1) phase. We demonstrate that these lesions colocalize with those induced by aphidicolin treatment. Hence, common fragile site stability is compromised, whether aphidicolin delays replication or calyculin A advances condensation. We also show that, in cells that go through an unperturbed S phase, completion of their replication and/or replication-associated chromatin reorganization occur all along the G(2) phase, which may explain their inability to condense properly after calyculin A treatment during this phase of the cell cycle.

Potential rapid assessment of breast cancer prognosis using induced chromosome condensation performed on cytological specimens.

We have previously shown that assessment of chromosome alteration rate by cytogenetics is well correlated with breast cancer prognosis factors. As karyotypes are usually difficult to obtain from solid tumors using conventional methods, a new approach is proposed. Metaphase-like chromosomes were directly obtained following chromosome condensation using calyculin A (okadaic acid) from cytologic specimens of breast cancers sampled by fine needle. Chromosome counts and rearrangement rates were established in a series of 45 tumors, as early as 24-48 h after sampling. A high rate of rearranged chromosomes was found to correlate with high histological grade, TNM stage and S-phase fraction, loss of estrogen receptor expression and DNA aneuploidy. The indication of genome alteration provided by this method constitutes a simple, potent and early potential prognostic factor in breast cancer directly applied on cytological specimens.