P53-DEPENDENT CELL-CYCLE ARRESTS ARE PRESERVED IN DNA-ACTIVATED PROTEIN KINASE-DEFICIENT MOUSE FIBROBLASTS

p53 is involved in at least three cell cycle checkpoints in normal cel ls: two in G(1), activated by either DNA damage or by ribonucleotide p ool depletion in the absence of damage, and one in metaphase/anaphase activated by an incomplete mitotic spindle. We tested whether any of t hese checkpoints require the DNA-activated protein kinase (DNAPK), sin ce data indicate that it is activated by damaged DNA to modify p53 in cultured cells and in cell-free systems. Fibroblasts isolated from mic e with severe combined immune deficiency (SCID) were used because the sole genetic defect underlying this syndrome lies within the DNAPX gen e. This report shows that age-matched SCID and isogenic wild-type embr yonic fibroblasts arrested in response to DNA damage, ribonucleoside t riphosphate depletion, and spindle poisons, whereas p53-/-fibroblasts failed to do so. Therefore, DNAPK-deficient scid cells preserve normal p53-dependent cell cycle checkpoints. The data provide one explanatio n of why scid mice are not tumor prone though they are deficient in do uble-strand break repair.