THE XPB AND XPD DNA HELICASES ARE COMPONENTS OF THE P53-MEDIATED APOPTOSIS PATHWAY

The molecular pathway of p53-dependent apoptosis (programmed cell deat h) is poorly understood. Because p53 binds to the basal transcription- repair complex TFIIH and modulates its DNA helicase activities, we hyp othesized that TFIIH DNA helicases XPB and XPD are members of the p53- mediated apoptotic pathway. Whereas transfer of a wild-type p53 expres sion vector by microinjection or retroviral infection into primary nor mal human fibroblasts resulted in apoptosis, primary fibroblasts from individuals with xeroderma pigmentosum (XP), who are deficient in DNA repair and have germ-line mutations in the XPB or XPD gene, but not in the XPA or XPC gene, have a deficiency in the apoptotic response. Thi s deficiency can be rescued by transferring the wild-type XPB or XPD g ene into the corresponding mutant cells. XP-D lymphocytes also have a decreased apoptotic response to DNA damage by adriamycin, indicating a physiologically relevant deficiency. The XP-B or XP-D mutant cells un dergo a normal apoptotic response when microinjected with the Ich-1(L) and ICE genes. Analyses of p53 mutants and the effects of microinject ed anti-p53 antibody, Pab421, indicate that the carboxyl terminus of p 53 may be required for apoptosis. Direct microinjection of the p53 car boxy-terminal-derived peptide (amino acid residues 319-393) resulted i n apoptosis of primary normal human fibroblasts. These results disclos e a novel pathway of p53-induced apoptosis.