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DNA-Dependent Protein Kinase Is a Therapeutic Target and an Indicator of Poor Prognosis in B-Cell Chronic Lymphocytic Leukemia

Authors' Affiliations: ¹ Northern Institute for Cancer Research, ² Department of Hematology, ³ Institute of Human Genetics, and ⁴ Institute for Cell and Molecular Biosciences, Newcastle University, Newcastle upon Tyne, United Kingdom and ⁵ Department of Hematology, University of Liverpool, Liverpool, United Kingdom

Requests for reprints: Elaine Willmore, Northern Institute for Cancer Research, Newcastle University, Newcastle upon Tyne NE2 4HH, United Kingdom. Phone: 44-191-246-4447; Fax: 44-191-246-4301; E-mail: Elaine.Willmore{at}ncl.ac.uk.

Purpose: del(17p), del(11q), and associated p53 dysfunction predict for short survival and chemoresistance in B-cell chronic lymphocytic leukemia (CLL). DNA-dependent protein kinase (DNA-PK) is activated by DNA damage and mediates DNA double-strand break repair. We hypothesized that inhibiting DNA-PK would sensitize CLL cells to drug-induced DNA damage and that this approach could increase the therapeutic index of agents used to treat CLL.

Experimental Design: Fifty-four CLL cases were characterized for poor prognosis markers [del(17p), del(11q), CD38, and ZAP-70]. In selected cases, DNA-PK catalytic subunit (DNA-PKcs) expression and activity and p53 function were also measured. Ex vivo viability assays established sensitivity to fludarabine and chlorambucil and also tested the ability of a novel DNA-PK inhibitor (NU7441) to sensitize CLL cells to these drugs. The effects of NU7441 on fludarabine-induced DNA damage repair were also assessed (Comet assays and detection of H2AX).

Results: DNA-PKcs levels correlated with DNA-PK activity and varied 50-fold between cases but were consistently higher in del(17p) (P = 0.01) and del(11q) cases. NU7441 sensitized CLL cells to chlorambucil and fludarabine, including cases with del(17p), del(11q), p53 dysfunction, or high levels of DNA-PKcs. NU7441 increased fludarabine-induced double-strand breaks and abrogated drug-induced autophosphorylation of DNA-PKcs at Ser²⁰⁵⁶. High DNA-PK levels predicted for reduced treatment-free interval.

Conclusions: These data validate the concept of targeting DNA-PKcs in poor risk CLL, and demonstrate a mechanistic rationale for use of a DNA-PK inhibitor. The novel observation that DNA-PKcs is overexpressed in del(17p) and del(11q) cases indicates that DNA-PK may contribute to disease progression in CLL.