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XP14BR is a cell line derived from a xeroderma pigmentosum (XP) patient from complementation group C. The patient was unusual in presenting with an angiosarcoma of the scalp, treated by surgical excision and radiotherapy. Following 38 Gy in 19 fractions with 6 MEV electrons, a severe desquamation and necrosis of the underlying bone ensued, and death followed 4 years later. The cell line was correspondingly hypersensitive to the lethal effects of gamma irradiation. We had previously shown that this sensitivity could be discriminated from that seen in ataxia-telangiectasia (A-T). The cellular response to ultraviolet radiation below 280 nm (UVC) was characteristic of XP cells, indicating the second instance, in our experience, of dual cellular UVC and ionizing radiation hypersensitivity in XP. We then set out to evaluate any defects in repair of ionizing radiation damage and to verify any direct contribution of the XPC gene. The cells were defective in repair of a fraction of double strand breaks, with a pattern reminiscent of A-T. The cell line was immortalized with the vector pSV3neo and the XPC cDNA transfected in to correct the defect. The progeny derived from this transfection showed the presence of the XPC gene product, as measured by immunoblotting. A considerable restoration of normal UVC, but not ionizing radiation, sensitivity was observed amongst the clones. This differential correction of cellular sensitivity is strong evidence for the presence of a defective radiosensitivity gene, distinct from XPC, which is responsible for the clinical hypersensitivity to ionizing radiation. It is important to resolve how widespread ionizing radiation sensitivity is amongst XP patients. [ABSTRACT FROM AUTHOR]

BRCA1 is a central component of the DNA damage response mechanism and defects in BRCA1 confer sensitivity to a broad range of DNA damaging agents. BRCA1 is required for homologous recombination and DNA damage-induced S and G2/M phase arrest. We show here that BRCA1 is required for ATM- and ATR-dependent phosphorylation of p53, c-Jun, Nbs1 and Chk2 following exposure to ionizing or ultraviolet radiation, respectively, and is also required for ATM phosphorylation of CtIP. In contrast, DNA damage-induced phosphorylation of the histone variant H2AX is independent of BRCA1. We also show that the presence of BRCA1 is dispensable for DNA damage-induced phosphorylation of Rad9, Hus1 and Rad17, and for the relocalization of Rad9 and Hus1. We propose that BRCA1 facilitates the ability of ATM and ATR to phosphorylate downstream substrates that directly influence cell cycle checkpoint arrest and apoptosis, but that BRCA1 is dispensable for the phosphorylation of DNA-associated ATM and ATR substrates. [ABSTRACT FROM AUTHOR]