Functional interaction of p53 and BLM DNA helicase in apoptosis

The Bloom syndrome (BS) protein, BLM, is a member of the RecQ DNA helicase family that also includes the Werner syndrome protein, WRN. Inherited mutat ions in these proteins are associated with cancer predisposition of these p atients. We recently discovered that cells from Werner syndrome patients di splayed a deficiency in p53-mediated apoptosis and WRN binds to p53. Here, we report that analogous to WRN, BLM also binds to p53 in vivo and in vitro , and the C-terminal domain of p53 is responsible for the interaction. p53- mediated apoptosis is defective in BS fibroblasts and can be rescued by exp ression of the normal BLM gene. Moreover, lymphoblastoid cell lines (LCLs) derived from BS donors are resistant to both gamma -radiation and doxorubic in-induced cell killing, and sensitivity can be restored by the stable expr ession of normal BLM. In contrast, BS cells have a normal Fas-mediated apop tosis, and in response to DNA damage normal accumulation of p53, normal ind uction of p53 responsive genes, and normal G(1)-S and G(2)-M Cell cycle arr est. BLM localizes to nuclear foci referred to as PAM nuclear bodies (NBs). Cells from Li-Fraumeni syndrome patients carrying p53 germline mutations a nd LCLs lacking a functional p53 have a decreased accumulation of BLM in NB s, whereas isogenic lines with functional p53 exhibit normal accumulation. Certain BLM mutants (C1055S or Delta 133-237) that have a reduced ability t o localize to the NBs when expressed in normal cells can impair the localiz ation of wild type BLM to NBs and block p53-mediated apoptosis, suggesting a dominant-negative effect. Taken together, our results indicate both a nov el mechanism of p53 function by which p53 mediates nuclear trafficking of B LM to NBs and the cooperation of p53 and BLM to induce apoptosis.