Ku86 variant expression and function in multiple myeloma cells is associated with increased sensitivity to DNA damage

Ku is a heterodimer of Ku70 and Ku86 that binds to double-stranded DNA brea ks (DSBs), activates the catalytic subunit (DNA-PKcs) when DNA is bound, an d is essential in DSB repair and V(D)J recombination, Given that abnormalit ies in Ig gene rearrangement and DNA damage repair are hallmarks of multipl e myeloma (MM) cells, we have characterized Ku expression and function in h uman MM cells. Tumor cells (CD38(+)CD45RA(-)) from 12 of 14 (86%) patients preferentially express a 69-kDa variant of Ku86 (Ku86v), Immunoblotting of whole cell extracts (WCE) from MM patients shows reactivity with Abs target ing Ku86 N terminus (S1OB1) but no reactivity with Abs targeting Ku86 C ter minus (111), suggesting that Ku86v has a truncated C terminus. EMSA confirm ed a truncated C terminus in Ku86v and further demonstrated that Ku86vin MM cells had decreased Ku-DNA end binding activity. Ku86 forms complexes with DNA-PKcs and activates kinase activity, but Ku86v neither binds DNA-PKcs n or activates kinase activity. Furthermore, MM cells with Ku86v have increas ed sensitivity to irradiation, mitomycin C, and bleomycin compared with pat ient MM cells or normal bone marrow donor cells with Ku86, Therefore, this study suggests that Ku86v in MM cells may account for decreased DNA repair and increased sensitivity to radiation and chemotherapeutic agents, whereas Ku86 in MM cells confers resistance to PNA damaging agents. Coupled with a recent report that Ku86 activity correlates with resistance to radiation a nd chemotherapy, these results have implications for the potential role of Ku86 as a novel therapeutic target.