HYPERSENSITIVITY OF KU-DEFICIENT CELLS TOWARD THE DNA TOPOISOMERASE-II INHIBITOR ICRF-193 SUGGESTS A NOVEL ROLE FOR KU ANTIGEN DURING THE G(2)-PHASE AND M-PHASE OF THE CELL-CYCLE

Ku antigen is a heterodimer, Comprised of 86- and 70-kDa subunits, whi ch binds preferentially to free DNA ends, Ku is associated with a cata lytic subunit of 450 kDa in the DNA-dependent protein kinase (DNA-PK), which plays a crucial role in DNA double-strand break (DSB) repair an d V(D)J recombination of immunoglobulin and T-cell receptor genes. We now demonstrate that Ku86 (86-kDa subunit)-deficient Chinese hamster c ell lines are hypersensitive to ICRF-193, a DNA topoisomerase II inhib itor that does not produce DSB in DNA, Mutant cells were blocked in G( 2) at drug doses which had no effect on wild-type cells. Moreover, byp ass of this G(2) block by caffeine revealed defective chromosome conde nsation in Ku86-deficient cells. The hypersensitivity of Ku86-deficien t cells toward ICRF-193 was not due to impaired in vitro decatenation activity or altered levels of DNA topoisomerase II alpha or -beta. Rat her, wild-type sensitivity was restored by transfection of a Ku86 expr ession plasmid into mutant cells. In contrast to cells deficient in th e Ku86 subunit of DNA-PK, cells deficient in the catalytic subunit of the enzyme neither accumulated in G(2)/M nor displayed defective chrom osome condensation at lower doses of ICRF-193 compared to wild-type ce lls. Our data suggests a novel role for Ku antigen in the G(2) and M p hases of the cell cycle, a role that is not related to its role in DNA -PK-dependent DNA repair.