Different mutations responsible for the elevated sister-chromatid exchange frequencies in Bloom syndrome and X-irradiated B-lymphoblastoid cell lines originating from acute leukemia.

Cell hybridization and co-cultivation protocols have been used to determine whether the increased rates of sister-chromatid exchanges (SCEs) exhibited by Bloom syndrome (BS) and a human mutant cell line (CCRF-SB-T1), originating from an X-irradiated acute leukemia-derived B-lymphoblastoid cell line, have the same or different bases. Cell fusion of CCRF-SB-T1 with each of 4 different BS B-lymphoblastoid cell lines (LCLs), retaining a high-SCE character, exhibited low (normal level) numbers of SCEs, signifying complementation. Co-cultivation of CCRF-SB-T1 and BS B-LCLs also resulted in a significant reduction in SCE level, from 70 to 35, in BS cells, lowered the BrdU concentrations necessary for sister-chromatid differential staining (SCD) from 15 micrograms/ml (0.05 mM) to 2.0 micrograms/ml (0.01 mM) and resulted in a completely normal level of SCE in CCRF-SB-T1 cells. This strongly suggests that the defects in the 2 cell types are different. In the assay of cell extracts, the 4 BS cell lines appear to have lost thymidylate (TMP) synthetase activity (about 50% reduction from that of normal cells), whereas CCRF-SB-T1 cells show a 20% increase of TMP synthetase activity compared to normal cells.