Elevated DNA double strand breaks and apoptosis in the CNS of scid mutant mice. 
Genetic approaches have provided evidence that DNA end-joining problems serve an essential role in neuronal survival during development of mammalian embryos.  In the present study, we tested whether the DNA repair enzyme, DNA dependent protein kinase, plays an important role in the survival of cerebral cortical neurons in mice.  DNA-PK is comprised of a DNA-binding subunit called Ku and a catalytic subunit called DNA-PKcs.  In mice with the scid mutation, DNA-PKcs is truncated near the kinase domain, which causes loss of kinase activity.  We compared the spatial and temporal aspects of neuronal cell death in scid versus isogenic wild-type embryos and found a significant increase in dying cells in scid mice, as assessed by nuclear changes, DNA fragmentation and caspase-3 activity.  Additional biochemical and immunocytochemical studies indicated that of several DNA repair enzymes investigated, only PARP was increased in scid mice, possibly in response to elevated DNA strand breaks.