Elevated DNA double strand breaks and apoptosis in the CNS of scid mutant mice

Genetic approaches have provided evidence that DNA end-joining problems ser ve an essential role in neuronal survival during development of mammalian e mbryos. In the present study, we tested whether the DNA repair enzyme, DNA dependent protein kinase, plays an important role in the survival of cerebr al cortical neurons in mice. DNA-PK is comprised of a DNA-binding subunit c alled Ku and a catalytic subunit called DNA-PKcs. In mice with the scid mut ation, DNA-PKcs is truncated near the kinase domain, which causes loss of k inase activity. We compared the spatial and temporal aspects of neuronal ce ll death in scid versus isogenic wildtype embryos and found a significant i ncrease in dying cells in scid mice, as assessed by nuclear changes, DNA fr agmentation and caspase-3 activity. Additional biochemical and immunocytoch emical studies indicated that of several DNA repair enzymes investigated, o nly PARP was increased in scid mice, possibly in response to elevated DNA s trand breaks.