The DNA-dependent protein kinase (DNA-PK) is a mammalian serine/threonine kinase that is implicated in the repair of DNA double-strand breaks, DNA replication, transcription, and V(D)J recombination. To determine the role of the DNA-binding subunit of DNA-PK in vivo, we targeted Ku80 in mice. In mutant mice, T and B lymphocyte development is arrested at early progenitor stages and there is a profound deficiency in V(D)J rearrangement. Although Ku80-/- mice are viable and reproduce, they are 40-60% of the size of littermate controls. Consistent with this growth defect, fibroblasts derived from Ku80-/- embryos showed an early loss of proliferating cells, a prolonged doubling time, and intact cell-cycle checkpoints that prevented cells with damaged DNA from entering the cell-cycle. The unexpected growth phenotype suggests a new and important link between Ku80 and growth control.

Ku is a complex of two proteins, Ku70 and Ku80, and functions as a heterodimer to bind DNA double-strand breaks (DSB) and activate DNA-dependent protein kinase. The role of the Ku70 subunit in DNA DSB repair, hypersensitivity to ionizing radiation, and V(D)J recombination was examined in mice that lack Ku70 (Ku70 −/−). Like Ku80 −/− mice, Ku70 −/− mice showed a profound deficiency in DNA DSB repair and were proportional dwarfs. Surprisingly, in contrast to Ku80−/− mice in which both T and B lymphocyte development were arrested at an early stage, lack of Ku70 was compatible with T cell receptor gene recombination and the development of mature CD4+CD8− and CD4−CD8+ T cells. Our data shows, for the first time, that Ku70 plays an essential role in DNA DSB repair, but is not required for TCR V(D)J recombination. These results suggest that distinct but overlapping repair pathways may mediate DNA DSB repair and V(D)J recombination.