High catabolism of BrdU may explain unusual sister chromatid differentiation and replication banding patterns in cancer cells.

Two T-cell acute lymphoblastic leukemia (ALL) cell lines, PEER and CCRF-CEM, were studied by various chromosome banding techniques, including 5-bromodeoxyuridine (BrdU) incorporation methods. Although of very similar origin, these two cell lines behave quite differently. In particular, CEM cell line exhibited an abnormal replication banding pattern (RBP) and poor sister chromatid differentiation (SCD). Study of their thymidylate synthase and thymidine kinase activities indicated that CEM had a more active salvage pathway for thymidylate synthesis than did PEER cell line, which may suggest an efficient BrdU incorporation and its fast decrease in culture medium, resulting in the observed peculiarities. However, this was contradictory to the fact that CEM need a higher dose of BrdU than do PEER cells to induce SCD and RBP. Finally, the radioactivity from 3H-thymidine decreased in the culture medium much faster for PEER cell line than for CEM cell line, and about 50% of the remaining radioactivity was due to 3H-thymidine for CEM cell line. Thus, the abnormal SCD and RBP are explained by an active catabolism of thymidine and BrdU in CEM cell line.