Rate of sister chromatid exchanges in Bloom syndrome fibroblasts reduced by co-cultivation with normal fibroblasts.

Six strains of Bloom syndrome (BlS) fibroblasts responded to co-cultivation with normal fibroblasts at a 1:2 ratio by a reduced rate of sister chromatid exchanges (SCE's) from a mean of 67.5 (range = 59--78) to 28.4 (range = 21--35). The response was dose-dependent in one strain tested at 1:2, 1:1, and 2:1 ratios. In addition, quadriradial exchange figures and other signs of increased chromosomal instability were not found in BlS cells following co-cultivation with control cells. Control cells did not respond to BlS cells and maintained a normal rate of SCEs. Culture medium conditioned for 48 hrs by normal fibroblasts could also reduce the rate of SCEs in BlS fibroblasts, but less than in co-cultivation. We suggest that the reduced rate of SCEs and the lack of chromosomal instability in BlS cells following co-cultivation represent a corrective effect that is related to the basic defect and not dependent on cell-to-cell contact.