Interplay of p53 and DNA-repair protein XRCC4 in tumorigenesis, genomic stability and development
Yijie Gao, David O. Ferguson, Wei Xie, John P. Manis, JoAnn Sekiguchi, Karen M. Frank, Jayanta Chaudhuri, James Horner, Ronald A. DePinho & Frederick W. Alt
XRCC4 is a non-homologous end-joining protein employed in
DNA double strand break repair and in V(D)J recombination1,2. In mice, XRCC4-de®ciency causes a pleiotropic phenotype, which includes embryonic lethality and massive neuronal apoptosis2. When DNA damage is not repaired, activation of the cell cycle checkpoint protein p53 can lead to apoptosis3. Here we show that p53-de®ciency rescues several aspects of the XRCC4-de®cient phenotype, including embryonic lethality, neuronal apoptosis, and impaired cellular proliferation. However, there was no sig- ni®cant rescue of impaired V(D)J recombination or lymphocyte development. Although p53-de®ciency allowed postnatal survival of XRCC4-de®cient mice, they routinely succumbed to pro-B-cell lymphomas which had chromosomal translocations linking ampli®ed c-myc oncogene and IgH locus sequences. Moreover, even XRCC4-de®cient embryonic ®broblasts exhibited marked genomic instability including chromosomal translocations. Our ®ndings support a crucial role for the non-homologous end- joining pathway as a caretaker of the mammalian genome, a role required both for normal development and for suppression of tumours.
DNA repair ability is reduced in a variety of pathologic conditions. In addition, in some of these diseases a disturbance in cellular Ca homeostasis occurs or cytosolic [Ca2÷] responses to various stimuli are impaired. The leading environ- mental cause for genomal DNA damage is ultraviolet (UV) irradiation. The aims of the present study were (I) to evaluate a possible dependence of UV-induced DNA repair ability on cytosolic Ca2÷ in human lymphocytes and (2) to assess the direct effect of UV irradiation on Ca 2÷ homeostasis in these cells. UV-induced DNA repair ability in lymphocytes was maximal at I mmol/L CaCI2 in the me- dium. Suppression of DNA repair ability occurred after elevation or reduction of cellular [Ca2÷] when various methods were used, including changes in Ca2÷ concentration in the medium, cellular Ca2÷ depletion by ethyleneglycol-bis- (l~aminoethylether)-N,N,N’,N’-tetraacetic acid, excessive Ca2+ concentration induced by ionophore, and shortening of Ca2÷ presence time during repair synthesis. UV irradiation caused an immediate and significant rise in cytosolic [Ca2÷] that was the result of both enhanced Ca2÷ uptake and inhibition of plasma membrane Ca-adenosine triphosphatase activity. The tyrosine kinase inhibitor genistein inhibited both UV-induced DNA repair and UV-induced cyto- solic [Ca2÷] elevation. These results emphasize the importance of a precise cellular Ca 2÷ level regulation for the optimal DNA repair process. UV irradiation, by inducing cellular Ca2÷ rise, may activate DNA repair as soon as DNA is damaged. (J Lab Clin Med 1997;130:33-41)