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We recently have shown that mice deficient for the 86-kDa component (Ku80) of the DNA-dependent protein kinase exhibit growth retardation and a profound deficiency in V(D)J (variable, diversity, and joining) recombination. These defects may be related to abnormalities in DNA metabolism that arise from the inability of Ku80 mutant cells to process DNA double-strand breaks. To further characterize the role of Ku80 in DNA double-strand break repair, we have generated embryonic stem cells and pre-B cells and examined their response to ionizing radiation. Ku80 ؊/؊ embryonic stem cells are more sensitive than controls to ␥-irradiation, and pre-B cells derived from Ku80 mutant mice display enhanced spontaneous and ␥-ray-induced apoptosis. We then determined the effects of ionizing radiation on the survival, growth, and lymphocyte development in Ku80-deficient mice. Ku80 ؊/؊ mice display a hypersensitivity to ␥-irradiation, characterized by loss of hair pigmentation, severe injury to the gastrointestinal tract, and enhanced mortality. Exposure of newborn Ku80 ؊/؊ mice to sublethal doses of ionizing radiation enhances their growth retardation and results in the induction of T cell-specific differentiation. However, unlike severe combined immunodeficient mice, radiation-induced T cell development in Ku80 ؊/؊ mice is not accompanied by extensive thymocyte proliferation. The response of Ku80-deficient cell lines and mice to DNA-damaging agents provides important insights into the role of Ku80 in growth regulation, lymphocyte development, and DNA repair.

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