DNA strand breaks in murine lymphocytes: induction by purine and pyrimidine analogues.

At the onset of culture of mouse splenic lymphocytes with concanavalin A (Con A), a 6 h pulse with 5-fluorouracil (5-FU) or 8-azaguanine (8-AG), under conditions previously shown to lead to an irreversible block of the stimulated cells in the G1 phase of the cell cycle (1,2), causes extensive DNA strand breakage. Breaks induced by the analogues early in culture were largely unrepaired even after 48 h culture. Analogues that did not block the proliferative response did not cause DNA strand breakage. Unrepaired DNA strand breaks, induced by the purine and pyrimidine analogues, provide a mechanism that can account for the block of the stimulated lymphocytes before S phase. Many strand breaks were found to exist in the DNA of normal, resting splenic lymphocytes; these were rapidly repaired within 2 h of stimulation with Con A, unlike those induced by the analogues.